The solutions to electric vehicle air conditioning ...

Renewable and Sustainable Energy Reviews 91 (2018) 443–463

Contents lists available at ScienceDirect

Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

The solutions to electric vehicle air conditioning systems: A review Zhenying Zhang a b

a,b,⁎

a

a

a

a,b

, Jiayu Wang , Xu Feng , Li Chang , Yanhua Chen

, Xingguo Wang

T a,b

Institute of Architecture and Civil Engineering, North China University of Science and Technology, 21 Bohai Road, Caofeidian Xincheng, Tangshan 063210, China Hebei earthquake engineering research center, Tangshan 063210, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Air conditioning Electric vehicle Heat pump Refrigeration Thermal management system

The air conditioning (AC) system provides cool, heating and ventilation in the cabin of the electric vehicles (EVs). It is necessary to control the interior thermal environments of the vehicle and ensure safety in visibility. Because AC systems are electrically powered, vehicle range is reduced drastically when the AC system is operating. EVs present a particular challenge to the development of more efficient AC systems for automotive applications. In this paper, the state of the art for various AC system solutions to EVs was critically reviewed. The investigations of alternative solutions are continuing along many parallel routes, e.g. vapor compression refrigeration-dedicated heater AC systems, reversible vapor compression heat pump AC systems, non-vapor compression AC systems and integrated thermal management system combined AC and battery pack. The characteristics and particular applications of each solution have been extensively discussed. Finally, a comparison listing the various pros and cons of the different available solutions was presented.

1. Introduction Recently, electric vehicles (EVs) have earned considerable attention as a promising solution to global greenhouse gas emissions [1–3]. There is a great potential to substitute EVs for internal combustion engine vehicles (ICEVs) in the coming years. The EVs include hybrid electric vehicles (HEVs, including full hybrid, mild hybrid, plug-in hybrid), pure electric vehicles (PEVs) and fuel cell electric vehicle (FCEVs). EVs discharge little air pollutants at the place where they are operated. They also typically generate less noise pollution than the ICEVs. From the energy aspect, electricity as an energy vector for vehicle propulsion offers the feasibility to replace oil with a diversity of elementary energy sources. This could ensure security of energy supply and a wide use of renewable energy sources. Furthermore, EVs will be more intelligent to improve traffic safety and road utilization, and will have a great impact on energy, environment and transportation as well as hi-tech promotion, new industry creation and economic development [4]. The air temperature and humidity in the cabin are the two crucial factors of the comfort perceived by passengers [5]. How comfortable the cabin environment is to the driver is also an influential factor of driving safety [6–8]. The AC system provides cool, heating and ventilation to the cabin of the EVs, which is necessary to control the interior thermal environments (including temperature, relative humidity and air velocity) of the vehicle and ensure safety in visibility (defogging and deicing) [9]. Besides, the batteries of the EVs are worked within a limited temperature window. Thermal management of the battery pack ⁎

is necessary to prevent premature aging and subsequent loss of capacity. The AC system is the only heat sink/source that allows a sufficient cooling/heating at high/low ambient temperatures. Also, the integrated thermal management system combined interior environment and battery pack is a great challenge for the AC system. Therefore, it is essential to find an innovative AC solution to the EVs [10]. The AC systems present the highest power consumption of the auxiliary components of the EVs [11]. The exclusive available energy for EVs propulsion is the electricity stored in the battery pack, so any additional power consumption implies a reduction of the driving range. Generally, AC systems cause about 30–40% average decrease in driving range depending on the size of AC and the driving cycle for EVs [12,13]. Pino et al. [14] numerically found that an increment of hydrogen consumption is between 3% and 12.1% when the AC system is operated in a FCEV. The driving range must be improved to boost EVs use by reducing battery power consumption, which requires the development of a highly efficient AC system. Thus, the AC system is crucial to the development of EVs. The AC system solutions as well as integrated thermal management systems combined AC and battery pack to EVs are critically reviewed in this paper. The review is divided into four main parts that each have sub-sections and covers all of the main AC system solutions to EVs presented in the literature. In the first part, AC systems of conventional vehicles are described. In the second part, a review of AC systems based on vapor compression cycle of EVs is presented. This section is divided into three subsections, i.e. vapor compression refrigeration - dedicated

Correspondence to: North China University of Science and Technology, Tangshan 063210, China. E-mail addresses: [email protected], [email protected] (Z. Zhang).

https://doi.org/10.1016/j.rser.2018.04.005 Received 10 August 2015; Received in revised form 27 February 2018; Accepted 2 April 2018 1364-0321/ © 2018 Elsevier Ltd. All rights reserved.


Z. Zhang et al.

Nomenclature AC AMRR BV COP DH DB EH EVs ERV ECOP EVI EXVs 4wV FCEVs FH GWP HX

HP HEVs ITM ICEVs ME ODP PCM PHEVs PEVs PTC RV TES TE 3wV TEWI VC WB WHD

Air conditioning Active magnetic regenerator refrigerator Bypass valve Coefficients of performance Dedicated heater Dry bulb temperature Electric heating Electric vehicles Energy recovery ventilator Equivalent COP Economized vapor injection Expansion valves Four-way valve Fuel cell electric vehicle Fuel Heating Global warming potential Heat exchanger

Heat pump Hybrid electric vehicles Integrated thermal management Internal combustion engine vehicles Magnetic effect Ozone depletion potential Phase change material Plug-in hybrid electric vehicles Pure electric vehicles Positive temperature coefficient Refrigerant valve Thermal Energy storage Thermoelectric Three-way valve Total equivalent warming impact Vapor compression Wet bulb temperature Waste heat driven

alternative for AC systems of EVs. However, the compressor used in the AC system for EVs is an electricity-driven compressor, and thus the speed of electricity-driven compressor can be adjusted independently of the vehicle speed to meet the cooling and heating loads. Also, different heating schemes and sources in order to heat their vehicle cabin are required since the waste heat gained from the motors and battery is far less than the demand of heating for EVs due to their higher efficiency.

heater (VCR-DH) systems, reversible vapor compression heat pump (VC-HP) systems and AC systems using low GWP refrigerants or natural refrigerant. In the third part, the non-vapor compression AC systems, such as thermal energy storage (TES) systems, AC systems using magnetic (ME) or thermoelectric effect (TE), waste heat driven (WHD) systems and so on, are analyzed. In the fourth part, a review focused on integrated thermal management (ITM) system combined AC and battery pack is presented. Finally, a comparison listing the various pros and cons of the different available solutions is presented, suggesting the inherent mechanisms of the AC systems and the information that is needed for proper design of AC systems.

3. AC systems based on vapor compression (VC) cycle 3.1. Vapor compression refrigeration - Dedicated heater (VCR-DH) AC systems

2. AC systems of conventional vehicles This concept is interesting owing to the few variations compared with the conventional ICEVs. The modifications are that the compressor is driven electrically instead of mechanically and heating is achieved by electric or fuel-operated heater instead of hot coolant heater core [9]. The heaters can be divided into two types: direct and indirect heating principles. Direct heating signifies to heat up directly the air to condition the cabin. Indirect heating signifies to heat up over a secondary working fluid (e.g. water, coolant, oil etc.), which transfers the heat to the air and further to the cabin. The combination of different heating concepts is applicable to gain the most comfortable heating behavior of the passenger cabin.

The vapor compression system is dominant in automobile AC systems [15]. In Fig. 1, a typical AC system of the conventional ICEVs is outlined. The AC systems usually cool the air in the cabin using the engine driving power to directly drive the A/C compressor. Since the compressor is a belt-driven device coupled to the engine crank shaft, its cycling rate is directly related to the vehicle speed. The losses of the AC system increase with increased vehicle speed, and thus with high compressor cycling. In conventional vehicles, it is well known that around 30 percent of the energy in the fuel burned is converted to mechanical energy. The left significant waste heat is either exhausted out of the tail pipe or moved to the radiator by coolant tubes where it is then dumped to the atmosphere. Thus the AC system warms up the air in the cabin using the waste heat radiated from the engine without the need to burn any additional fuel. The coolant is pumped around the engine block by means of a mechanical or electrical pump, collecting waste heat which is then sent through a heat exchanger (HX), called a heater core. Air is heated as it passes through the heater core and is distributed to the cabin air vents. This process keeps the engine temperature down and heats the cabin up which is ideal in low temperature environments. However in scenarios where the cabin does not require heat (warm ambient conditions), the coolant is diverted to a radiator in the front of the vehicle in order to dissipate heat to the environment and prevent over-heating. This type of vehicle architecture provides more than enough heat for cold weather conditions. However, some ICEVs and HEVs have become so efficient that additional auxiliary electrical heaters are installed to assist with vehicle heating in extremely cold conditions. In view of convenient substitution, low cost and serviceability, the automobile industry yearns for a direct transition from conventional ICEVs to EVs. Thus the vapor compression system is a preferred

3.1.1. Vapor compression refrigeration - Electric heating (VCR-EH) systems The electric heaters are commercially available and could thus be an economical feasible option. It requires only one fuel to thoroughly operate the EVs. It is also advantageous for the EVs owing to the characteristics of low-weight, small space, swift response, and environment protection. This concept need not route the high-voltage Heater core

Evaporator

Compressor

Radiator

Condenser

Engine Pump

Fig. 1. Typical AC system of the conventional ICEVs. 444


Z. Zhang et al.

warm-up phase than the comparable device with PTC technology. With a weight of 1.9 kg and a continuously variable power output of 0.2–5.0 kW, it has a power density of 3.2 W/cm3. Comparable PTC solutions achieve power densities of 1.8 W/cm3 and weigh more than 3 kg. Shin et al. [24] improved the sintering process to enhance surface uniformity in the manufacturing of PTC elements. An electrode production process using thin-film sputtering deposition was applied. The allowable voltage and surface heat temperature of the high-voltage PTC elements with thin-film electrodes were 800 V and 172 °C respectively. The electrode layer thickness was uniform at approximately 3.8 µm, approximately 69% less electrode materials were required compared to that before process improvement. Furthermore, a heater for the EV heating system was manufactured using the developed high-voltage PTC elements to verify performance and reliability. The results of vibration and thermal shock testing were within the standard range, confirming that the developed heater was adequately reliable for use in EV vehicles. Bauml et al. [25] presented an EVs heating system based on a special coating of vehicle interior objects. This coating generates infrared radiation and warms up the passengers directly instead of the cabin air. Thus, lower air temperatures in the cabin are required while preserving or even increasing the passenger comfort. Due to the higher efficiency of this infrared radiation heating, the energy consumption of the vehicle's heating system can be reduced by 50%. Due to the insufficient heat-up capabilities of the cabin air by radiation alone, only a combined infrared and convective heating system is applicable.

cables into the vehicle cabin if the electric water heater is mounted outside, and hence a measure of passenger safety during a crash can be incorporated into the vehicle design. Umezu and Noyama [16] introduced the AC system of EVs called “iMiEV” developed by Mitsubishi Motors Corporation. The schematic of the AC system is shown in Fig. 2. The system consists of a refrigerant cycle with an electrically driven compressor for cooling and a coolant cycle with a positive temperature coefficient (PTC) heater for heating. The PTC heater has four layers: control board part, upper coolant passage part, PTC elements part, and lower coolant passage part. The heater is installed under hood area of the vehicle. Thus the high voltage cable is avoided in the cabin. The vehicle test results showed that the cooling performance and heating performance of “i-MiEV” are better than that of the baseline vehicle that is a conventional engine vehicle. However, the driving range decreases by about 15% and 45% when cooling system and heater are operated respectively. In regard to VCR systems, a two-phase ejector was used in the 2010 Toyota Prius to improve the system efficiency [17,18]. The ejector is a fluid pump that recovers expansion energy which is typically wasted in the conventional refrigeration cycle expansion process, and converts the recovered expansion energy into pressure energy. Consequently, the ejector system can improve energy efficiency of the refrigeration cycle. Most of the recent investigations of the ejector-expansion refrigeration cycle have been concentrated on transcritical R744 cycles because of the large improvement potential owing to the high throttling loss [19]. Fig. 3 shows an outline of this cycle. The evaporator consists of two parts, the upwind evaporator and the downwind evaporator. The ejector is located in the refrigerant reservoir of the downwind evaporator. The ejector was integrated into the evaporator to reduce the volume of the device. The ejector reduces compressor power consumption by 11% at 25 °C, 18% at 35 °C, and 24% at 40 °C for the Prius EVs. For the indirect EH systems, Sun et al.[20] designed a heating system used PTC heater for an EV of the DongFeng motor corporation. The heating system is similar to the coolant cycle shown in Fig. 2. The experiment results indicated that it is about 20 min for defrosting when the ambient temperature is −18 °C, 10 mins for demisting when the ambient temperature is −3 °C. They concluded that the heating system can meet the conditions of the vehicle defrosting and demisting system. Nemesh et al. [21] developed a heating system for providing cabin heating and windshield defrost to EVs. The system includes a high voltage PTC coolant heater, electric coolant pump, and coolant control valve. It has two operating modes: Bypass-Mode, where coolant flowing through the cabin heating system is isolated from the engine, and LinkMode, where coolant flowing through the cabin heating system is shared with the engine. The experimental results showed that the new system initially warms up similar to conventional vehicles and faster than conventional hybrid vehicle in Bypass-mode. The new system has a faster warm up than conventional hybrid vehicle and is able to cycle the engine on/off much earlier and more frequently than a conventional hybrid vehicle in Link-mode. For the direct EH systems, Apfelbeck and Barthel [22] presented that the direct electric air heaters can be divided into two classes: high voltage air heaters and low voltage air heaters. The high voltage air heaters using the high battery voltage up to 450 or 500 V in EVs have the advantages such as large heating power, quick heating and small size. But the costs are a lot higher than the conventional low voltage system due to the much higher safety requirements of high voltage systems. The power consumption and heating power of low voltage air heaters using 12 or maybe 48 V in the future are lower than the high voltage variants. But it is feasible to heat each seat individually. Webasto corporation developed a high voltage heater based on layer technology for voltages between 250 and 450 V [23]. In the layer heater, an electrically conductive layer is attached to a carrier. When electricity flows through it, the layer is heated and transfers the heat to the carrier. The energy consumption of the heater was 18% lower in the

3.1.2. Vapor compression refrigeration - Fuel Heating (VCR-FH) systems The fuel-fired heater is most widely used as the automobile auxiliary heater and is well known to have high thermal efficiency [26]. A fuelfired heating system that only uses a small amount of fuel necessary for the climate control system and does not rely on the main battery of the electric vehicle can be a transitional solution. Mimuro and Takanashi [27] performed field operation tests on fueloperated heaters retrofitted into mass-produced EVs. The pros and cons of fuel-operated heaters was discussed in comparison with PTC and heat pump heaters from the viewpoints of energy efficiency, carbon dioxide emissions, heating performance, driving range influence, and usability. Apfelbeck and Barthel [22] discussed that the air outlet temperature of the fuel-operated heaters ranges from 80 °C to 120 °C. There is a risk to destroy neighboring parts, nuzzles or the wind shield owing to the high temperatures. Therefore, the most appropriate application for this heater type was suggested to being as an auxiliary heater in vans with a large passenger cabin for the rear seats. A bioethanol-operated water heater was developed for the EVs [28].

Fig. 2. AC system configuration of i-MiEV [16]. 445


Z. Zhang et al.

Receiver

energy consumption of two kinds of air conditioning system (the single cooling with PTC heater system and the heat pump air conditioning system) across 30 cities in China and concluded that except for the tropical cities, the heat pump system can achieve an average energy saving of 41.3% compared to PTC heater. Obviously, the heat pump system seems to be a practical and promising solution to the AC systems in EVs [33].

Condenser

Compressor

Flow control valve

Air flow Ejector

3.2.1. The basic VC-HP AC systems The basic VC-HP AC system of EV is similar with the residential heat pump AC system. It composed of one condenser, one evaporator, one throttling device, one compressor and one 4 wV. In 2013, Li et al. [34] designed a heat pump AC system with R134a as refrigerant for EVs. The heating/cooling mode is controlled by a 4 wV. The system cooling and heating capacities under 35 °C and −15 °C ambient temperatures were designed to be 2.95 kW and 2.63 kW respectively. The tested results showed that the temperature of the passengers’ compartment reached 18 °C in the heating mode after 10 min, 6 min, 4 min when the ambient temperature was 1.6 °C, 5.5 °C, 6.6 °C respectively. But the performance of this system is seriously affected by the ambient temperature. In 2014, Peng et al. [35] designed a VC-HP experimental system using R134a as refrigerant for EVs. The test results showed that the COPs of the heat pump AC system were 5.74, 5.3 and 4.55 when the ambient temperature were 5, −1 and −5 °C, respectively. In 2016, Wang et al. [36] developed a VC-HP system for EVs. The heating COP was reported to be about 2.3 using R407C as refrigerant at the outdoor temperature of −10 °C. Compared with R134a system, the heating capacity and the compressor power of R407C system increased and the system energy efficiency decreased. It was also found that the compressor speed has a limited influence on increasing the heating capacity, especially under the low outdoor temperature. Nevertheless, the rising of compressor speed results in the increase of the power consumption and lowering of the system energy efficiency. That same year, Lee and Lee [37] investigated the steady state performance and the transient temperature of a R134a VC-HP system of EVs under various experimental conditions. The heating COP and capacity was reported to be 3.26 and 3.10 kW at an ambient temperature of −10 °C. But the system was insufficient to satisfy the cabin heating load of passenger EVs.

Upwind evaporator

Downwind evaporator

Capillary tube ECS evaporator

Fig. 3. Configuration cycle [18].

of

Toyota

Prius

ejector-expansion

refrigeration

The test results indicated that with this heater 4.1 kWh of electrical energy from the battery can be saved compared to an electrical heater. This saved energy can extend the driving range by about 30% based on the investigated conditions. The CO2 emission could be reduced by more than 50% compared to an electrical heater with an equivalent performance. Chongpyo et al. [29] proposed a hybrid heating system that can perform both cabin and battery heating functions to overcome the defect of available battery capacity reduction in low outdoor temperatures. It is a fire-operating heating system using liquid petroleum gas. The schematic diagram of the hybrid heating system is shown in Fig. 4. 3.2. Reversible vapor compression heat pump (VC-HP) systems This solution is based on the vapor compression cycle, which provides both cooling and heating capacities by adopting an inverter driven compressor and a four-way valve (4 wV) that reverses the direction of refrigerant flow. Judging by law of thermodynamics, the coefficients of performance (COP) of the heat pump system is larger than 1.0. Thus it is more energy-efficient than the independent heater AC systems. Clodic et al. [30] presented that if a reversible vapor compression heat pump is applied, the driving range of the vehicle can be decreased by 10% in winter and by 15% in summer compared with a vehicle with no indoor environment control, under typical weather conditions. Zhang et al. [31] found that a heat pump system with an average heating COP of 1.7 will extend the range by 7.6–21.1% compared with the PTC heater. Zhang et al. [32] also calculated the annual

3.2.2. The improved VC-HP system with demisting Though simple in structure and low in cost, the basic VC-HP AC system is not widely used in EVs owing to the defect that it is unable to perform the modes of demisting and dehumidifying. Instead, the improved VC-HP system with demisting is suggested generally [38]. In 1996, Iritani and Suzuki [39,40] developed a R134a heat pump AC system for EVs where one 4 wV, two expansion valves (EXVs) and

Fig. 4. Schematic of the hybrid FH system proposed by Chongpyo et al. [29]. 446


Z. Zhang et al.

several check valves were used to control the refrigerant flow direction during the cooling and heating mode changes. The schematic of the system is shown in Fig. 5. This system could perform cooling, heating, demisting and dehumidifying. In order to prevent the windscreen frosted, it has two HXs in the air duct. When the system operated in the demisting mode, the refrigerant flowed through all three HXs. Air is desiccated by cooling through the interior evaporator, and then heated through the interior condenser. The experimental results indicated that the system COPs were 2.9 and 2.3 under 40 °C and −10 °C ambient temperatures respectively. In 2011, Kondo et al. [41] compares heat pump systems using two possible heat sources: thermal energy in the outside air and thermal energy from the ventilation air in the cabin. In consequence, the system using outside air was selected for further investigation owing to its simple structure and possible cost reduction. The schematic of the system is shown in Fig. 6. The test results indicated that compared with that of the electric water heater system, this system can reduce electric power consumption by 20% at an outside temperature of 0 °C, and by 60% at 10 °C in the heating mode. The performance test of the windshield defroster confirmed that this system satisfies domestic regulations regarding defrosters and the experiment verifies the feasibility of continuous defroster operation. In 2014, Qin et al.[42] investigated the heating performance of the improved VC-HP system with three HXs for EVs in low ambient temperature. Based on demand of moisture condensation control on the window screen of EVs, the heating performances are tested on both all fresh air supply and various indoor air inlet temperature conditions, under low environmental temperature −10 °C, −15 °C and −20 °C. The experimental results show that AC systems for EVs has big differences to buildings or household AC systems due to its big ratio of fresh air, even at −20 °C, the system's COP under the largest heat capacity is over 1.7. In 2016, Feng and Hrnjak [43] presented an improved VC-HP system based on the currently commercially available Nissan Leaf EVs. It consists of a compressor, an accumulator, three HXs, two electronic EXVs, a three-way valve (3 wV), a bypass valve (BV), and an air-side door, as shown in Fig. 7. In cooling mode, the inside condenser is bypassed by blocking air flow with the air door, the heating EXV is not used with the BV opened, and the exterior HX serves as a normal condenser. When converting to heating mode, the air door is opened to allow air flowing through the inside condenser, while the cooling EXV and evaporator are bypassed from refrigerant side by converting the 3 wV, in the meantime, the BV is cut off to force refrigerant to flow through the heating EXV. When dehumidification of cabin air is required, both the evaporator and inside condenser can be turned on by controlling the 3 wV, air door, and BV. The test indicated that refrigerant charge needed for cooling operation was more than that for heating and the necessity of a secondary heater should be reserved for extremely low ambient temperature.

Fig. 6. Schematic of Mitsubishi heat pump AC system [27].

In 2017, Xuan et al. [44] found that the heating capacity of the improved VC-HP system with three HXs is increased by 28%, 25% and 19%, the COP of the improved system is increased by 15.0%, 16.5% and 18.2% at the ambient temperature of 7 °C, 1 °C and −5 °C respectively compared with the basic VC-HP system. Furthermore, the time of defrosting and demisting is also decreased. 3.2.3. Development of efficient VC-HP systems All the aforementioned researches show that the VC-HP system is able to utilize less energy to make better performance and drivability. However, its heating performance of the VC-HP system will decline dramatically in low outdoor ambient temperature, which is opposite to the variation trend of heating load. As a result, the operation of a VC-HP system is restricted to mildly cold conditions. So far, the ambient temperature of the VC-HP system being researched for EVs is normally around at 0 °C, and rare researches are taking under −5 °C. Therefore, developing high efficiency heat pump system to save battery power and raise EVs driving range for low ambient temperature have become a hot research topic. Several technologies, such as hybrid systems with an electric heater, economized vapor injection (EVI) and method of using additional waste heat have been studied to improve the heating performance at low ambient temperatures [45]. 3.2.3.1. The hybrid VC-HP systems with an electric heater. Auxiliary electric heater such as PTC electric heater can be used as an additional heat source under low outdoor ambient temperatures. Kim et al. [46] investigated a combined system consisting of a heat pump and a PTC heater as a heating unit in EVs. Compared to the standard of the PTC heater at an indoor temperature of 20 °C, the heating capacity was increased by 59% for the combined system, and the COP was

Fig. 5. Schematic of R134a VC-HP system proposed by Iritani and Suzuki [39]. 447


Outdoor HX

Z. Zhang et al.

EEV for cooling

[49]. In 2015, Qin et al. [50] also designed and tested an air-source heat pump applying EVI for EVs. The system employs two electric three-way valves (3wV1 and 3wV2) to switch the cooling mode or heating mode by bypassing the in-car condenser or in-car evaporator. The results show that the heating capacity of the system with EVI can be increased up to 31% at 0 °C in-car inlet air temperature and it can meet the demand of heating alone in −20 °C without electric heating assessment with 50% return air. But when the in-car inlet air temperature is at −20 °C, COP on the injection mode is lower than that on no-injection mode. Tang and Zuo [51] developed a flash tank vapor injected heat pump AC system using scroll compressor for EVs as shown in Fig. 9. The experimental results showed that the capability of the system could be met the comfort requirements of 5 seat EVs in summer and winter. However, the unique function of demisting/defrosting a windshield of the automotive AC system was not factored in for the aforementioned EVI VC-HP systems. The EVI for the improved VC-HP system with three HXs was investigated in recent years. In 2016, Zhang et al. [52] built and tested a R134a heat pump AC system with EVI, the schematic of the system is shown in Fig. 10. It was found that an improvement of 14.4% in heating COP and 13.6% in heating capacity can be achieved with a better-designed economizer. In 2017, Kwon et al. [53] presented a configuration of an EVI heat pump AC system as shown in Fig. 11 where two separate indoor HXs are arranged to realize the functions of cooling, heating and demisting/defrosting. A non-injection heat pump and an EVI heat pump for EV has been modeled. The heating capacity improvement of 14–44% was obtained by the EVI technique for cold ambient regions under the EV operational conditions. To investigate the heat performance and effect of injection porthole shapes on the HP system, Qin et al. [54] built a test bench of heat pump for EVs. The experimental results showed that the heating capacity of the system with EVI was raised up by 28.6% compared with the traditional system. But COP of EVI systems was lower than no-EVI system. The larger injection porthole resulted in the increase of the heating capacity when in-car inlet air temperature was higher. COP of the single porthole system was much lower than that of three interlinked portholes system. In 2017, Choi et al. [55] evaluated the optimum injection position of the scroll compressor and intermediate pressure ratio that maximizes heating capability and COP of the EVI heat pump system for EV in cold startup condition. For the raising of heating capability, the optimum injecting port position was obtained in a specific value of about 300°. As the opening of the main EXV was decreased, the performance of the EVI system generally got better but the system had much restricted intermediate pressure ratio in which the performance was drastically decreased. As a result, the optimal intermediate pressure ratio occurred in specific value below 0.25 in startup condition. The obtained maximum COP was about 2.65, corresponding to an improvement of about 10% compared with the conventional heat pump system with PTC heater. In 2017, Jung et al. [56] developed a simulation model for a VC-HP with EVI and validated it by thermodynamic

Evap

3wV EXV

Compressor

BV

Low side receiver

Outdoor HX

a.Cooling mode EEV for cooling

Air door IC Inner condenser

Evap

3wV EXV

Compressor

BV Low side receiver

Air door IC Inner condenser

b.Heating mode Fig. 7. Schematic of VC-HP system based on Nissan Leaf EVs [42].

increased by 100% for the heat pump system. The conclusions showed that the heat pump cycle should be always operated for better efficiency, and the PTC heater should be controlled for better performance. Therefore the PTC heater and heat pump combined system is an optional AC system for EVs, especially in extremely cold weather conditions. In 2016, Lee and Lee [37] also suggested a hybrid heating method with an electric PTC heater can be used as a possible option for realization of longer driving ranges and thermal comfort for passengers under extremely cold weather conditions. In 2017, Zhang et al. [47] presented a concept of using a continuous anti-fogging air curtain for front windshield glass to realize the maximum-return-air utilization in winter. The results indicated that the heating demand can be decreased by 46.4–62.1% compared with the all-fresh-air condition when the ambient temperature is 5–20 °C. Auxiliary electric PTC heater was suggested when the ambient temperature is −20 °C owing to the insufficient heating output of heat pump. Equivalent COP (ECOP) was applied to evaluate the integrated performance of the system. The maximum ECOP was found to be 1.57 which is 12.1% higher than that of all-fresh-air condition. Accordingly, the highest energy saving is 40.6% compared to all-fresh-air condition.

4wV Exterior HX

3.2.3.2. The VC-HP systems with EVI. The vapor injection technology has many virtues such as the use of a small compressor operating at double stages, higher efficiency with less work, longer compressor life owing to lower discharge temperature, easier adjusting of capacity etc. This technique has been studied actively in recent years for AC systems of EVs. Li et al. [48,49] designed and tested an air-source heat pump based on a scroll compressor operating with EVI cycle for EVs. The experiment flow chart of the system is shown in Fig. 8. The simulation results showed that the heating load and the heating COP of the new system increased about 28% and 70% respectively compared with the conventional one-stage system [48]. The experimental results indicated that the system COP is about 1.5 and the heating capacity of the system increases more than 20% when the ambient air temperature is −20 °C

Check valve1

Check valve3

Economizer Compressor

Check valve2

Solenoid valve

Expansion valve

Check valve4 Interior HX Cooling

Separator

Heating

Fig. 8. Schematic of vapor compression heat pump with EVI [48]. 448


Z. Zhang et al.

effectively used, it not only can keep the system working in a relatively stable environment temperature to make EVs running safety, but also can be used to drive the EVs AC system to meet the needs of temperature and humidity in the vehicle cabin. In 2007, Kim et al. [58] investigated the impact of arrangement of the radiator and outdoor HXs on the performance of R744 heat pump system in FCEVs. The experimental results indicated that, for the new radiatorfront arrangement, the improvements of heating capacity and COP were up to 54% and 22% respectively as the cold ambient air was heated by the radiator initially. Nevertheless, for the arrangement, the cooling capacity and COP were reduced by 40–60% and 43–65% respectively under the cooling mode as rising air import temperature deteriorated the outdoor HX performance. In 2012, Lee et al. [59] investigated the performance of a R744 heat pump that used the stack coolant as the heat source for FCEVs under cold weather conditions. The experimental results indicated that the heating capacity and heating COP of the heat pump satisfied the heating load of the FCEVs under cold weather conditions. The heating capacity of the experimental system is adequately reached over 5.0 kW at a coolant flow rate of 5.0 l/min under extremely cold weather conditions of −20 °C. In 1997, Promme [60] suggested an improved heat pump system with an additional external heat source which utilizes the waste heat of the batteries, electric motor and power control unit, etc. The test results of the improved system demonstrated that the heating capacity of the system can be attained to be 2.5 kW at the ambient temperature of −10 °C, in which about 25% was recovered from the battery. Under this circumstance, the decrease of the energy consumption was predicted to be about 15% compared with PTC heater system. Moreover, the improved heat pump system was less sensitive with frost creation and more stable in winter conditions compared with the traditional system. In 2012, Cho et al. [61] designed a R134a heat pump AC system for heating a passengers’ cabin by using wasted heat from the coolant of electric equipments (e.g. motors and inverters of the electric bus) as the heat source. The heating performance of the system was investigated experimentally. The results indicated that heating COP was about 3.0 at the ambient temperature of 0 °C. The average air temperature at the outlet of the heater core was 25.0 °C after 15 min and increased to a peak value of 45.0 °C. The water temperature of the condenser and the air temperature at the outlet of the heater core were on average 55.4 °C and 41.9 °C, respectively. In 2013, Lee et al. [62] studied the performance of a R134a heat pump AC system with a waste heat of the electric equipments for heating and an air source for cooling. Under the experimental condition, the cooling capacity at all compressor frequencies was more than 23.0 kW, which satisfied the cooling load of an electric bus worked on a refrigerant charge of 8200 g in ambient and indoor temperatures of 27.0 °C and 35.0 °C, respectively. An adequate heating capacity of over 23.0 kW was attained at all experimental compressor frequencies and ambient temperatures. The heating COP reached 2.4 at the ambient temperature of 10.0 °C. In 2014, Ahn et al. [63] developed a dual source heat pump AC system using both air and waste heat in EVs. The working fluid was R134a. Through the analysis of experiment results, the dual source system outperformed the air source-only and waste heat-only systems in the heating mode. But when the ambient temperature was −10 °C, the heating performance of the dual source system was greatly dependent on the quantity of waste heat. The reason is that the absorbed heat from the ambient air is inappreciable when the outside temperature is low. To overcome this problem, an alternating single mode operation of air source-only and waste heat-only modes was suggested. Compared with the dual heat source mode, the COP and heating capacity of the single mode were increased by 4.3% and 10.5% respectively when the ambient temperature was −10 °C and the waste heat amount was 1.5 kW. In 2016, Qian et al. [64] also conducted an experiment study of the dual source heat pump AC system using both ambient air and waste heat of the motor as the low temperature heat sources in EVs. It was found that

4wV Exterior HX 4wV Compressor

Expansion valve Flash tank Check valve

Interior HX

Heating

Cooling

Fig. 9. Schematic of VC-HP system with flash tank [51].

Fig. 10. Schematic of VC-HP system with EVI proposed by Zhang et al. [52]. Cooling BV Inner condenser

Economizer

EXV Outdoor HX

EXV Compressor

accumulator

Heating BV

Inner evaporator

EXV Heating Mode

Cooling Mode

Fig. 11. Schematic of VC-HP system with EVI proposed by Kwon et al. [53].

analysis with geometrical information. Single-injection and dual-injection ports were investigated to optimize the COP and isentropic efficiency by regulating the injection mass flow rate. The numerical results showed that the optimal angles of the single and dual-injection ports were 440° and 535°/355° respectively. Consequently, the corresponding COP was improved by 7.5% and 9.8% over the reference VC-HP with non-EVI at an outdoor temperature of −10 °C. 3.2.3.3. The VC-HP systems using waste heat. For FCEVs, about 50% of energy is lost as heat [57]. If this part of the waste heat can be 449


Z. Zhang et al.

the heating capacity and COP were increased by 24.16% and 10.81% respectively compared with the conventional air source system. The compressor accounted for most exergy losses of up to 46.64% −50.07% while the condenser reached 30.09–33.40%. In 2015, Ahn et al. [65] suggested a dual-evaporator heat pump combined with a heater as effective dehumidifying and heating units using waste heat recovery from the dehumidifying process. The result showed that specific moisture extraction rate and COP are 53% and 62% higher respectively than those of the conventional heat pump system at the indoor air wet bulb temperature of 13 °C. In 2016, Ahn et al. [66] investigated the performance improvement of a dehumidifying heat pump using an additional waste heat source in EVs with low occupancy. The heating capacity and COP of the dual source dehumidifying heat pump was found to increase by 75.8% and 5.2% respectively over the air source dehumidifying heat pump. In 2015, Suh et al. [67] developed an integrated AC system for an electric bus equipped with a dynamic wireless charging capability. The cooling and heating cycle of the proposed AC system is shown in Fig. 12(a) and Fig. 12(b) respectively. In the heating cycle, the heat generated by coolant flow inside the driving motor system is recovered through the HX. The coolant PTC heater next to the motor, as an auxiliary heating system, is for rapid heating when the vehicle is started and the motor is cold. It was found that the energy consumption by cooling and heating was below 20% and 25% of the total electrical energy consumption of the electric bus respectively. In 2018, Bellocchi et al. [68] found that power consumption of AC system was as high as 32% of the energy required for propulsion, which

Fig. 13. VC-HP system equipped with an ERV proposed by Bellocchi et al. [68].

can decrease the driving range from 94 km to 72 km for Italian cities. For pre-conditioning and hygrometric comfort improvement, a VC-HP system equipped with an energy recovery ventilator (ERV) was proposed. The ERV is an air-to-air regenerative HX that exchanges both latent and sensible heat between the external air flow and the exhausting air from the cabin by means of a separating membrane. The schematic of the system is shown in Fig. 13. The refrigerant flow is controlled by means of two 4wVs which switch between cooling and heating mode. The system was proved to lessen the driving range

Fig. 12. Heat pump systems using waste heat proposed by Suh et al. [67]. 450


Z. Zhang et al.

reduction by 2–6%. The replacement of PTC with the system for cabin heating was reported to reduce the energy consumption by 17–52% depending on the geographical context. Similarly, Aceves and Smith [69] presented that the heating load of the EVs can be decreased by the desiccant dehumidifier. The dehumidifier can adsorb the water vapor produced by the passengers, thereby avoiding water condensation on the windows without requiring a high external air ventilation rate or a high window temperature. The results showed that the steady-state heating load can be decreased by 60% or more through the use of the dehumidifier. The reduction in heating load means that waste heat may be sufficient to supply the required heating under most outdoor conditions. The system dimensions and weight are also applicable for packaging inside EVs. 3.2.4. The defrosting of the VC-HP systems In the heating mode, the exterior HX works as the evaporator for VC-HP systems. At low outdoor temperatures, the evaporating temperature can dip below 0 °C, which leads to a surface temperature of the evaporator lower than the freezing point of water. This is the precondition for the frost formation on the exterior HX. Owing to low thermal conductivity, the growing frost layer decreases the performance of the exterior HX, which results in a decline of the heating capacity as well as the COP of the AC system. The frost formation on the exterior HX lowers the system efficiency in the heating mode for the air source heat pump AC system. Antonijevic and Heckt [70] experimentally found that there was a very thick frost creation on exterior HX with the decrease of ambient temperature and they predicted that it will lower system performance. Li [71] experimentally found that in the normal heating mode and constant fan power, the airflow rate across the exterior HX is reduced about 36.7%, the heating capacity decreased about 34.7%, and the system COP decreased about 31.2% owing to the frost creation on the exterior HX. The heating performance will be further degraded with extra energy for defrosting, which also leads to safety problems in driving process. Dong et al. [72] presented that the defrosting energy consumption accounts for 10% of the total energy consumption of a motionless air source heat pump in the air heating mode. Therefore, defrosting of the exterior HX is necessary at intervals and the defrosting method is crucial to the energy consumption and normal operation of the system. In 2013, Steiner and Rieberer [73] stated that the reverse cycle defrosting method is more applicable to the EVs compared with the method of hot gas defrosting. The test and simulation results of a reverse cycle defrosting process on a heat pump AC system using R744 as refrigerant for an EVs under the chosen operating condition are discussed. The reverse cycle defrosting process for the investigated system occurs at subcritical operating conditions. The test results indicated that the defrosting under the experimental conditions took less than 2 min with reverse cycle defrosting. The simulation results indicated the effect of different throttle openings on the defrosting process and a best performing valve-opening. The calculation of an average COP indicated that a 30% reduction throttle opening would result in an average COP 47% decrease than the COP in heating mode, whereas with the best valve-opening the reduction would only be 13%. In 2015, Li [71] experimentally found that the frost on the exterior HX can be melt thoroughly within 120 s by reverse cycle defrosting in the test condition of DB7°C, WB 6 °C. The author also found that the unevaporated water on the fin of exterior HX during defrosting process easily leads to the secondary freezing, which results in system performance degradation. Thus the solution methods such as lowering the airflow rate of exterior HX or covering part of the exterior HX surface were suggested. In 2017, Zhou et al. [74] proposed a defrosting method relying on increasing temperature and enhancing gas injection in reverse circulation. The test showed that the method can obviously shorten the defrosting time through raising the indoor temperature, the revolving speed of compressor, draught fan air output of the cabin and reducing fan delivery outside the vehicle and controlling proper air supplement rate, and then

Fig. 14. Schematic of VC-HP system with hot gas defrosting [75].

adequately improve the performance of AC system. Experimental results showed that instant defrosting time at fully defrosted air-cooled HX outside the vehicle can be controlled within 100 s under the ambient condition of −20 °C and 80% relative humidity. In 2015, Steiner and Rieberer [75] presented that hot gas defrosting method can avoid the above-mentioned defect of reverse cycle defrosting. The suggested schematic diagram of the heat pump with the hot gas defrosting is shown in Fig. 14. Two solenoid valves are added in order to shift between heating and defrosting operation. They set up a transient simulation model that can conduct frosting and defrosting using the Modelica “Air Conditioning” library to optimize the performance of a reversible R744 heat pump system. The simulation results showed that there is an optimum point of time to conduct defrosting operation at certain operating conditions in order to optimize the average COP including the frosting and defrosting period. In 2016, Huang et al. [76] designed a heat pump AC system with hot gas bypass defrosting and spray liquid cooling for EVs. In the system, the compressor exhaust is bypassed to the compressor suction port through a BV so as to realize defrosting function. The spray device is employed to decrease the compressor suction/exhaust temperature for the system operating safely and stably. The experimental results showed that the right defrosting period is 5 min in the case of the system runs 25 min and the supply air temperature can remain about 33.1 °C during the defrosting. In 2017, Chen et al. [77] also designed an AC system with hot gas bypass defrosting for EVs. The change trends of the cabin interior HX, the indoor air temperature, evaporation pressure and the compressor suction temperature with the bypass valve opening were analyzed. It was found that there was an optimum bypass valve opening where the system performed best. In 2017, Lin et al. [78] proposed a composite defrosting method for heat pump AC system of EVs. During the defrosting, the bypass defrosting is firstly operated, then it shifts the reverse cycle defrosting timely according to the defrost state. The experimental results indicated that compared with the reverse cycle defrosting, the defrosting energy consumption of the composite method decreased by 8.13%, and the compressor shock and the indoor temperature fluctuation was decreased. Compared with the hot gas bypass defrosting, the defrosting time was decreased by 60 s and the defrosting energy consumption decreased by 6.56%. In 2014, Yan et al. [79] investigated the effect of different types of 451


Z. Zhang et al.

complies with the conditions set by European environmental regulations. R1234yf is not toxic and decomposes in the air even faster than R134a. R1234yf has a mild flammability as listed in the “A2″ classification. R1234yf has similar thermodynamic properties to the R134a currently used as an automotive refrigerant, so it can be easily applied to typical automotive air conditioners [84]. The studies were mostly associated with drop-in replacement using R1234yf as the substitute in the traditional R134a system for the automotive AC systems. Basically, the R1234yf system performance, in terms of COP and cooling capacity, are slightly lower than those of R134a system [84–90]. Hence, some improvements, such as internal HX, adjustment of thermal EXV, introduction of expander or ejector are also considered. These improvements can notably reduced COP differences between R1234yf and R134a systems. However, the heat pump system performance of R1234yf concerning EVs is very limited in open literatures. R744 is an alternative to conventional refrigerants in AC system since it has zero ODP and negligible GWP. Furthermore, R744 systems have special advantages in heat pump mode, since high capacity and COP can be achieved at low ambient temperature and with high air supply temperature to the passenger compartment [91]. However, the operating pressure of the R744 system is 7–10 times greater than that of the conventional subcritical systems, so all components in the system must be able to withstand these high pressures [92]. Because the leakage risk becomes higher with increased pressure, components initially will need to be made of thicker, stronger materials, adding weight and manufacturing costs. Furthermore, the service and maintenance procedures, as well as the procedures for handling R744 and related AC systems, must be standardized. Diagnostic, service and maintenance equipment must be developed before such an AC system can enter the marketplace. In 2002, DENSO developed a R744 heat pump AC system supplied it for Toyota's FCHV-4 [93]. Fig. 15 shows the configuration of the AC system. The heating mode and the cooling mode of the system are switched by opening and closing BVs 1 and 2. Specifically, at the cooling mode, the BV1 is opened and the BV2 is closed. Further, air mix dampers of an interior gas cooler are fully closed. At the heating mode, the BV1 is closed and the BV2 is opened. Then, the air mix dampers of the interior gas cooler open. Accordingly, high-pressure and high-temperature R744 refrigerant discharged from the compressor exchanges heat with air to heat the air while flowing into the interior gas cooler. As needed, the system can provide dehumidifying by closing the BV2 and controlling the opening degree of the EXV2. The compressor for this system is a scroll type and is driven by an electric motor. The compressor and the motor are hermetically integrated together. This results in good sealing performance, a simple structure and easy installation in a vehicle. The experimental results showed the system heating capacity was as much as 5 kW under −20 °C ambient temperature. In 2012, Lee et al. [94] developed an electrical

evaporator on the performance of the heat pump of EVs. The experimental results indicated that the small diameter tube and fin type can reach even surpass the micro-channel type on capacity and COP in heating mode. The frost/defrost experimental results indicated that the capacity of micro-channel type decreases after several frost/defrost periods. The capacity of small diameter tube and fin type would recover to the initial value ultimately since the condensate water was easier to be drained. Table 1 summarizes and compares the above-mentioned studies of defrosting schemes of EVs. By analyzing the characteristics of the above mentioned defrosting methods in Table 1, it is concluded that the electrical heating method demands extra electricity during the defrosting process while thermal storage defrosting and desiccant defrosting are complicated and high-cost. For EVs, the reverse cycle defrosting method has the advantages of higher heating (defrosting) performance because the ambient air or cabin air (if there is an appropriate mechanism in the AC box) can be used as heat source, whereas the hot gas defrosting uses only the energy from the compressor. Further, the reversible cooling and heating system does not need additional valves or BVs to conduct reverse cycle defrosting. The defect of this method applied in a vehicle is the probable water condensation or frost creation on the interior HX during defrosting, which facilitates flash fogging when it is shifted back to heating operation and the cold air exiting the interior HX, which cannot be blown into the vehicle cabin without dramatically reducing thermal comfort [80]. Therefore, an additional mechanism in the AC box is necessary to pass the exiting air of the interior HX to the ambient. From the viewpoint of energy saving and feasible, reverse cycle defrosting method is a more promising defrosting method for heat pump systems of EVs in practical applications.

3.3. AC systems using low GWP refrigerants or natural refrigerants At present, R134a is the dominant refrigerant in automotive AC systems. Although R134a has zero ozone depletion potential (ODP), its global warming potential (GWP) is up to 1300. The Kyoto and Montreal protocols have prohibited or limited the use of halogenated refrigerants for future environmental considerations. Similarly, the European Union has passed regulations to restrict the use of refrigerants with a GWP higher than 150 in automobile AC systems by 2017 [81]. Therefore, the investigations on the alternative refrigerants are energetically underway. The potential candidates with GWP below 150 are being selected to replace R134a can be classified into two groups, the natural refrigerants such as R744, R717, hydrocarbons etc., the new environmental protection refrigerants, such as R1234yf, R1234ze, R152a. R1234yf is a synthetic refrigerant with GWP of 4 and ODP of 0 [82,83], making it an environmentally friendly refrigerant that Table 1 Comparison of different defrosting schemes of AC systems for EVs. Defrosting methods

Additional equipment

Advantages

Disadvantages/Limitations

References

Electric heating defrosting

Electrical heater

Simplicity

Reverse cycle defrosting

–

[71,73,74,80]

Hot-gas bypass defrosting

Solenoid valve

Thermal storage defrosting

Thermal storage system

Desiccant defrosting

Dehumidifier

No additional valves or bypass valve Energy-efficient Fast defrost Little impact on indoor temperature Little pressure fluctuation in the system Energy-efficient Continues high performance running Little impact on indoor temperature Can be used as frost prevention

Extra electricity demand Long time defrost Not economical Indoor temperature fluctuation Indoor thermal comfort deteriorate Compressor shock at the beginning of the reverse cycle Long time defrost

452

Space-consuming, cumbersome Not enough experimental results are available under longperiod operation Complex system Desiccant regeneration needs to be solved

[75–77]


Z. Zhang et al.

Exterior gas cooler

Accumulator tank

R152a and RC270 was 11% and 9% higher than that of R134a system respectively under real driving conditions. The secondary circuit system also showed an excellent performance in cooling and heating, but COP of R152a secondary circuit was 5–12% lower than that of R134a system.

BV 2

Evaporator EXV2 Innerheat exchanger BV1

3.4. Summary and discussion Interior gas cooler

Currently, the most popular solution to AC systems of EVs is VCRDH systems. It is interesting owing to the few variations compared with the conventional ICE vehicle. Electric heating is the simplest solution for EV battery and cabin heating. However, this solution suffers the defect of drastic driving range reductions caused by large electricity consumption of heating, especially in cold districts. Table 2 summaries the effect of AC system on EV's one-charge driving range. The fueloperated heaters are advantageous for the EVs owing to its reliability, simplicity and economy. It does not have a significant influence on the driving range. However, two different fuels are needed to entirely operate the EVs. Accordingly the recharging of the EVs and purchasing of oil generally proceed at different positions. It is impractical for the EVs owner. Furthermore, the environmental problem is also a defect of this solution. A summary of the previous mentioned studies of the reversible vapor compression heat pump AC systems is presented in Table 3. The heat pump AC system is interesting owing to the prospects of energy saving and system compact. However, the practical application of heat pump AC systems in the EVs is still a long way to go before some obstacles are overcome. The obstacles include decrements of heating capacity and the energy efficiency in cold weather, components and the system optimization, defrosting of exterior HX and so on. To solve these problems, many authors have proposed a number of interesting configurations and technologies, such as EVI, utilization of waste heat, innovative defrosting methods and so on. These technologies are proved to be energy-efficient, reliable and flexible in operation. However, there is a lack in experimental works on the system optimization considering double mode (cooling and heating) performance of the VCHP systems. The cooling performances of VC-HP systems with EVI need to be evaluated. The waste heat is favorable in the heating mode, but it is adverse in the cooling mode for the VC-HP systems. The novel system is needed to be exploited considering this challenge. Moreover, considering the atrocious vehicle operation conditions, the sliding block of the 4wV may be malfunction due to long-term vibration and external strike [9]. When VC-HP system is switched between heating mode and cooling mode, the mass flow distribution and the pressure bearing ability of the HXs should be paid more attentions. Further studies should clarify the on-design and off-design performance of the VC-HP system with the 4wV and develop models that can consider transient mechanism of each component of the system. A summary of potential candidates including advantages and

Compressor

EXV1

Fig. 15. Schematic of DENSO R744 heat pump AC system [93].

AC system using R744 and tested it under various operating conditions. The compressor was driven by an inverter. The experimental results showed that the cooling capacity and the cooling COP increased by 36.8% up to 6.4 kW and 30.3% up to 2.5, respectively, with the rise of the gas cooler inlet pressure from 92.0 bar to 102.0 bar under the condition of the gas cooler inlet temperature of 35.0 °C and the compressor speed of 4000 rev/min. The cooling capacity of this system was sufficient, over 5.0 kW, at the outside temperature over 35 °C and the compressor speed over 4500 rev/min. Compared with the R134a AC system, the cooling COP of this system using R744 was on average 24.3% higher than it at any compressor speeds. But the heating mode of the system was not mentioned in the paper. In 2017, Wang et al. [95] investigated the heating performance of a R744 heat pump AC system for EV in a cold climate. An electrical rotary type compressor was applied in the system, and an electrical EXV was employed to regulate the working fluid mass flow rate and high pressure in the supercritical region. The experimental results indicated that the COP of the system was 1.7 when the outdoor, indoor air inlet and outlet temperature were −20 °C, 20 °C and 40 °C respectively. Additionally, a new secondary loop heat pump shown as Fig. 16 was also investigated, and a reduction of 19% for COP was obtained compared with the conventional heat pump. R717 has been used in industrial applications since the 1930s and is generally acknowledged as being the most efficient refrigerant [96]. R717 is environmentally friendly, having zero ODP and zero GWP. However, it is a toxic and slightly flammable refrigerant as listed in the “B2L” classification. In 2016, Tan [97] presented that compared with the conventional ICE vehicles, the refrigerant is less likely to combustion owing to no fuel systems for EVs, which make it possible for the R717 to be used in AC systems of EVs. The AC system using R717 as the working fluid and its components are designed. The configuration of the AC system is shown in Fig. 17. The system consists of four subsystems: compression refrigeration circulation subsystem, the passenger compartment thermal subsystem, the front cooling subsystem and the security subsystem. The theoretical cooling COP of the system was predicted to be 4.09, and this system is expected to be more safe and reliable than the R1234yf system owing to the secondary circuit design. The hydrocarbons are naturally occurring, inexpensive, and have zero ODP and a very low GWP. Additionally, hydrocarbons are more efficient conductors of heat than halogenated refrigerants [98]. When a hydrocarbon refrigerant is employed, the refrigerating effect and system COP will be higher than that of the R134a system [99,100]. However, the hydrocarbons are in the category A3 based on the classification of safety level. It means that the hydrocarbons are non-toxic but highly prone to flammability properties. The use of these substances, especially regarding its flammability properties, a regulation standard has been established to prevent harm things to occur. Ghodbane [101] studied the possibility of R152a and hydrocarbons (R290, R600a and RC270) as a substitute to halogenated refrigerants in automobile AC systems. The results indicated that COPs of the systems with

Outdoor

Indoor

Internal HX Pump EXV

Evaporator

Accumalutor

Heater Core Plate HX

Compressor

Fig. 16. Schematic of secondary loop R744 heat pump AC system [95]. 453


Z. Zhang et al.

Compressor III

I

EXV

Pump

certain amount of heating or cooling energy through the regenerator to meet all or a portion of the cabin cooling/heating demand. Table 5 summarized the performances of the energy storage systems. It can be seen that compared with EES, TES has advantages in cycle life, overall efficiency, control procedure and so on. The energy density of EES is higher than that of TES, but it is admitted that electricity has a higher energy grade than heat and it will cost significantly more. Besides, the range decrease in severe weather could be overcome by adding more batteries which are to be used strictly for AC system. Thus it is generally believe that TES was a more practical solution compared with EES. But if the battery cost was a non-issue, EES would be the obvious option. According to the storage materials, the TES systems can be divided into sensible heat storage and latent heat storage. All materials regardless of their phase can store sensible heat. The sensible heat energy is the product of material mass, specific heat and the temperature difference. The material with a high specific heat is preferred for high storage energy density. Storing sensible heat is restricted by the freezing point and boiling point of the material where further temperature variations would cause a transition of phase. Latent heat energy is absorbed or released by a phase change material (PCM) during the process of melting or solidification. The phase change temperature and amount of energy released per kg will differ based on the PCM. The EVs of Tohoku Electric Power designed for cold climatic conditions included a latent heat TES cell, whose total energy storage capacity was about 19 kWh electrical energy equivalent [10]. It included 9.5 kWh latent heat of fusion and 9.5 kWh sensible heat of the solid PCM. The total mass of the cell was 180 kg. The cell can maintain the battery and passenger cabin at about 15 °C. In 2012, Shahidinejad [114] introduced passive TES system using PCM to maintain the temperature of the vehicle's compartment for comfort. While the EV is plugged in, PCM absorbs heat generated by an electric heater which is also connected to the electric grid. Based on the simulation results, inclusion of PCM in the seat cushions can help to maintain the cabin temperature constant at 15 °C for an increase of the electric range up to 21%. In 2017, Wang et al. [115,116] designed an AC system with TES, as shown in Fig. 18. The system was designed to store heat using power from the electric grid and release heat to warm up vehicle cabin during driving in low temperature ambient conditions. It can achieve three modes of heating: PCM discharging, energy recovery, cabin HX and PTC heating. The PCM materials and PCM HX are optimized. The PCM was reported to have up to 50% more latent heat during phase change compared with PCMs on the current market. Furthermore, the PCM was encased in a high performance insulation system to minimize thermal losses when the EV is parked for extended periods. The system was designed to provide enough thermal energy to heat the EV's cabin for approximately 46 min, covering the entire daily commute of a typical driver in the U.S. The test of the system indicated a range improvement

Safety Valve

C

II

IV

Indoor HX

T

Evaporator

P

Condenser

Outdoor HX

Secure shell

Pump

I

Compression refrigeration circulation subsystem

III

Front cooling subsystemt

II

Passenger compartment thermal subsystem

IV

Security subsystem

Fig. 17. Schematic of AC system using R717 proposed by Tan [97].

disadvantages/ limitations for AC systems of EVs is shown in Table 4. The similar thermophysical properties make R1234yf a priority to replace R134a in automobile AC applications. But few investigations about the VC-HP system performance of R1234yf concerning EVs can be found in open literatures. R744 is another promising option owing to the virtues of excellent heating performance under cold ambient conditions. But the cooling performance is often much lower than that of the halogenated systems. Some efficiency improvements, such as internal HX [108,109], two-stage compression [110,111], introduction of an expander [112] or ejector [113] need to be further studied for AC systems of EVs. In addition, the economic feasibility of this option is also a problem owing to the high operating pressures. The secondary loop systems are applicable to the solutions concerning R717 and hydrocarbons for maximum security. Nevertheless, the drawbacks of the secondary circuit are the slower response to load variations, the complex system connections and the accessorial cost in components and maintenance. Despite the substitution of refrigerants is urgent, there is very limited amount of investigations and little experimental data is available for AC systems of EVs at present. The AC prototypes should be built and tested for investigating the characteristics of the system under variable working conditions. Besides, efforts are needed to develop the applicable systems using low GWP refrigerants or natural refrigerants and optimize the performance of the systems. 4. Non-VC AC systems 4.1. Thermal Energy Storage (TES) AC system The energy storage systems can be divided into two sub-categories: electric energy storage (EES) and thermal energy storage. The EES system stores the energy by electric batteries. The TES system stores a Table 2 Effect of AC system on EV's one-charge driving range. Ref.

[16] [102] [103] [104,105] [106]

[31,107]

Types of AC

Vehicle

VCR EH VCR EH VCR VCR EH VCR EH

Mitsubishi i-MiEV

VCR PTC HP

-

NISSAN LEAF FORD ESCAPE FORD FOCUS Mid-size sedan

Ambient

30 °C 0 °C 35 °C −7 °C 27 °C 27 °C −5 °C 8 °C −2 °C −12 °C 35 °C −20 °C −20 °C

Driving cycles UDDS

HWFET

US06

SC03

NEDC

10–15 mode

-

-

-

-

-

−18%

−4%

−2%

-

-

−15% −45% -

−22% −37% −47% −40% −49% −53% -

−14% −16% −23%

−20% −20% -

−37% -

-

-

-

-

-

-

−17.2%-−37.1% −17%- −54% −10.3- −40.7%

-

454


Z. Zhang et al.

Table 3 Summary of studies of the VC-HP systems for EVs. Ref

Refrigerant

Ambient

COP

Relative COP

Relative capacity

Features and improvements

[35]

R134a

–

Basic VC-HP AC system.

R407C

–

–

Basic VC-HP AC system.

[37] [39,40]

R134a R134a

– –

– –

Basic VC-HP AC system. Improved VC-HP system with demisting.

[42] [44]

R134a R134a

Improved VC-HP system with demisting. Improved VC-HP system with demisting. Relative COP based on basic VC-HP system.

R134a

– 15.0% 16.5% 18.2% 70%

– –

[48]

+28%

Basic VC-HP AC system with EVI. Relative COP and capacity based on basic VC-HP system.

[49]

R134a

−1.5%

+20%

[50]

R134a

5.74 5.3 4.55 2.6 2.45 2.2 3.26 2.9 2.3 1.7 2.92 2.83 2.73 3.1 2.48 1.9 1.69 3.2 1.5 –

–

[36]

5 °C −1 °C −5 °C 0 °C −5 °C −10 °C −10 °C 40 °C −10 °C −20 °C 7 °C 1 °C −5 °C 10 °C 0 °C −10 °C −15 °C 10 °C − 20 °C 0 °C

–

+31%

[52]

R134a

+44.1% +57.7%

R134a

[54]

R134a

– 1.25 2.7 3 3.25 1.4 –

–

[53]

−10 °C −20 °C 10 °C 0 °C −10 °C −20 °C −20 °C

Basic VC-HP AC system with EVI. Relative COP and capacity based on basic VC-HP system. Basic VC-HP AC system with EVI. Relative capacity based on basic VC-HP system. Improved VC-HP system with demisting and EVI Relative capacity based on improved VC-HP system with demisting. Improved VC-HP system with demisting and EVI. Relative COP and capacity based on improved VC-HP system with demisting.

+6% –

+14–44% +28.6%

[55]

R134a

−15 °C

2.65

+10%

–

[56]

R134a

−10 °C

–

+7.5–9.8%

–

[58]

R744

–

[59] [60]

R744 R134a

33 °C 15 °C −20 °C −10 °C

−43 to 65% +22% – –

−40 to 60% +54% – –

[61] [62]

R134a R134a

– –

R134a

3.0 2.0 2.4 –

– –

[63] [67]

R134a

9.3% −4.3% –

31.5% −10.5% –

[64]

R134a

0 °C 35.0 °C 10.0 °C 0 °C −10 °C Cooling Heating 2 °C

[66] [68]

R134a R134a

7 °C –

– 1.3–2.1

10.81% 28.07% 5.2% –

24.16% 46.58% 75.8% –

[94]

R744

35 °C

2.5

+24.3%

–

[95]

R744

−20 °C

1.7

–

–

– –

– –

Improved VC-HP system with three HXs and EVI Relative capacity based on improved VC-HP system with demisting. Improved VC-HP system with three HXs and EVI. Relative capacity based on improved VC-HP system with demisting. Basic VC-HP AC system with EVI. Relative COP based on basic VC-HP system. Radiator-front arrangement for outdoor HX. The stack coolant as the heat source. The waste heat of the batteries, electric motor and power control unit, etc as an additional external heat source. Energy consumption decreased by 15%. Using wasted heat from the coolant of electric equipments. With a waste heat of the electric equipments for heating and an air source for cooling. Dual source heat pump using both air and waste heat. Recovering heat generated inside the motor. Energy consumption decreased by 20% −25%. Dual source heat pump using both ambient air and waste heat of the motor. Dehumidifying heat pump using an additional waste heat source. Equipped with a regenerative HX. Energy consumption decreased by 17% −52%. Basic VC-HP AC system. Relative COP based on R134a system. Basic VC-HP AC system.

the continuous cooling of the cabin. In 2011, Wu et al. [118] developed a set of AC system combined with water-storage heating and ice-storage cooling. It was found that the TES system can save almost 20% for the costing or increase the driving range by 30%. In 2016, Jarzyna et al. [119] developed and built a TES system for public transport vehicles. The water-glycol was used as the material storing thermal energy. It was found that the system efficiency depends largely on the capability to transfer energy in the hydraulic system. The average COP was reported to be about 1.5. The cost of the TES tank was proved to be only 11% of that of electrochemical batteries.

of 26% compared with PTC heating. This option can be further improved if one can use the carried mass of the system to assist with summer time air conditioning. In 2017, Li and Jiang [117] proposed a TES system for EVs as shown in Fig. 19. Compared with the traditional AC system, the cold regenerator (including refrigerated evaporator and EXV2), ice water pump and ice water radiators were added. When the EV is plugged in, the TES system would be turned on to storage energy through the control panel by connecting to the electric grid. The EXV2 and the compressor are opened to realize the cooling of the evaporator in the cold regenerator. The EXV1 is opened to maintain the temperature of the cabin. In the process of driving, cold regenerator was first used to cool the cabin by opening the water pump. When the water temperature in the regenerator is higher than the set value, the water pump would be stopped, and the compressor and the EXV1 are opened to guarantee

4.2. AC systems using thermoelectric (TE) or magnetic effect (ME) In 1997, Chan and Chau [120] represented that the thermoelectric 455


Table 4 Summary of potential candidates for AC systems of EVs. Refrigerant

Advantages

R1234yf

Few variations compared with the conventional R134a system

AC evaporator

Disadvantages

HX

Ref.

EXV2

EXV1

R744

R717

Hydrocarbons

R152a

Energy-efficient in heating Environment friendly

Inexpensive, Environment friendly Higher COP Inexpensive, Environment friendly Higher COP Environment friendly Higher COP

[84–90]

Mild flammability Slightly lower COP Expensive High working pressure Strict sealing requirements Lower COP Toxic and slightly flammable Secondary circuit is needed Flammability Secondary circuit is needed Flammability Secondary circuit is needed

Condonser

[73,93–95]

Pump [97]

Compressor

Specific energy (Wh/kg) Cycle life Overall Efficiency Start-up shock Cooling/Heating response rate Control procedure Energy capacity Cost ($/ kWh)

EES

the surrounding space and inner surfaces of the vehicle. The temperature effect is generated by a combination of conduction to the passenger through the seat and backrest and through convection of conditioned air escaping through the surface of the seat. Furthermore, it is environmental friendliness due to no refrigerants contained. Magnetic cooling and heating technology can potentially decrease the energy consumption of the AC and increase the vehicle autonomy. The European project ICE focused on the development of an efficient Magneto-caloric reversible heat pump AC system for the electric minibus [121]. Payá et al. [122] presented that such systems can almost double the efficiency of conventional AC systems. Torregrosa-Jaime et al. [123–125] developed an overall model of a minibus, including a thermal model of the cabin, hydraulic loops, distribution system and the heat pump. The results showed that a temperature span of 37 K and 40 K has to be overcome in summer and winter, with thermal powers of 1.60 kW and 3.39 kW respectively. The simulation results indicated that the required temperature span can be reached with Gd-Tb alloys with the Curie temperatures in the range of 2–37 °C. In 2016, TorregrosaJaime et al. [126] investigated the characteristics of an active magnetic regenerator refrigerator (AMRR) for its application in AC systems of EVs. The thermal requirements of the EV was obtained and resulted in a cooling demand of 3.03 kW at a temperature range of 29.3 K. A permanent-magnet parallel-plate AMRR made of Gd-like materials was considered. The results indicated that an AMRR made of plates between 30 and 40 µm thick and channels between 20 and 40 µm high could meet the vehicle cooling demand with a COP between 2 and 4 and a total mass between 20 and 50 kg. Compared with vapor compression

[101]

TES

NiMH

Li-ion

Ice-storage cooling

Waterstorage heating

60–120

100–265

90

70

Short < 85% Moderate Slow

Long < 85% Moderate Slow

Infinite > 85% Small Fast

Infinite > 90% Small Fast

Complicated 150–1500

Complicated 500–2500

Simple Litlle

Simple Litlle

Cold storage tank

Fig. 19. AC system with TES proposed by Li and Jiang [117].

[99,100]

Table 5 Comparison of the performances between EES and TES for EVs. Parameters

Frozen evaporator

Z. Zhang et al.

variable temperature seat is particularly appropriate to the EVs. The heating or cooling is offered by a thermoelectric heat pump and blower embodied in the seat. Higher energy efficiency can be obtained by using energy to heat or cool the passenger directly rather than to heat or cool

Fig. 18. AC system with TES proposed by Wang et al. [115,116]. 456


Z. Zhang et al.

devices, the AMRR worked optimally with fluid flow rates at least 3 times larger for automobile AC systems.

results showed that the solar-assisted heat pump AC system could operate stably in the heating mode as well as in the cooling mode. The third-generation Prius was equipped with the solar ventilation system [134]. The solar ventilation system uses energy provided by a solar panel built into the roof to operate a fan contained within the AC system. This allowed ventilation of the vehicle interior when the vehicle was parked in direct sunlight. The solar panel consisted of 36 poly crystalline silicon cells connected in series. The panel produced a nominal 22 V DC and 3.6 Amps (cell temperature 25 °C), sunlight intensity of 1000 W/m2. In 2017, Li et al. [135] proposed an air cycle heat pump system integrated with a turbocharger, a blower and a regenerated HX for EVs. The results of thermodynamic analysis indicated that the blower installed before the compressor can achieve a higher heating capacity and thus a higher COP. The results also showed that the power consumption of the air cycle with the turbocharger can be reduced about 23% compared with the PTC system under the identical conditions for EVs and its operating temperature range is wider in cold climates than the conventional vapor compression heat pump. But the performance of the system in cooling mode was not performed. The cooling COP of the air cycle systems is usually below 1.0, which is much lower than the vapor compression systems [136]. In 2018, Jiang et al.[137] proposed a sorption AC system for EVs. Fig. 22 presents the layout of the system. Compared with traditional vapor compression AC system, a compressor is substituted for a sorption reactor in the new system, and other components are still remained. Expanded natural graphite is selected in the development of composite sorbent. It was found that the application of sorption technology to the AC system of EV is feasible. Energy density in winter and in summer ranges from 757 kJ/kg to 1980 kJ/kg and 387 kJ/kg to 990 kJ/kg whereas energy efficiency ranges from 0.34 to 0.82 and from 0.19 to 0.42. It was also found that the extra mass of sorption AC system will have limited impact on driving range of EVs, which results in a reduction less than 4.3%. In winter, the highest extended driving range by the new system is estimated to be close to 100 km. Even the lowest extended driving range in summer is still able to attain 21 km, which is about 7.5% of the maximum driving range.

4.3. Waste heat driven (WHD) AC systems In 2012, Javani et al.[127] investigated the performances of ejector and absorption refrigeration cycles for cabin cooling in hybrid vehicles and PEVs by energy and exergy analysis. The ejector cooling cycle (Fig. 20) and the LiBr-water absorption cycle (Fig. 21) using the waste heats from battery packs and the exhaust gas in the ICE mode have been studied. The refrigerating fluid absorbs heat and transfers it to the generator of the absorption system or the boiler of the ejector cooling system. In the hybrid vehicles, researches showed that recovering the waste heat in the ejector cooling cycle can bring about a cooling effect of 7.23 kW, the energetic and exergetic COP being 0.48 and 0.15 respectively. In the absorption cycle, the cooling effect was 7.92 kW, with the energetic and exergetic COP is 0.52 and 0.21 respectively. In the PEVs, as there is no heat from the exhaust gases, the cooling capacity is below 2 kW, which is not enough to offer cooling. Moreover, the LiBrwater absorption cycle has a higher COP and cooling capacity than the ejector cycle. He et al. [128] developed a LiBr-water absorption AC system using waste heat in FCEV in 2007. The thermal management system of the fuel battery was directly connected to the generator of the absorption refrigeration system. It eliminates the energy consumption caused by the secondary heat transfer. As the upper part of the main HX is connected with a bypass branch, when the heat of the fuel battery is greater than it actually want, the extra heat can be discharged by the bypass branch. The results of analysis for a 60-seat passenger car showed that it is applicable to the fuel cell bus. The system has the advantages of saving energy and convenient operation, etc. However, it also has some insufficiency, such as the volume is bigger than the conventional VCR equipment and problems of the sealing and anti-corrosion. As the fuel battery needs to work in a relatively stable temperature environment, the requirement of the thermal management system is higher. It is also feasible that directly use the waste heat of the FCEVs for heating in cold weather. In 2014, Colmenar-Santos et al. [129] designed a heating system by integrating the heat generated by the fuel cell into the heating system of the vehicle. The system designed is based on EVs driven by a 12 kW PEM (proton exchange membrane) fuel cell. It is found that the fuel cell never surpasses its maximum operating temperature and enough heat has been generated by the fuel cell to be exploited in the heating system of the vehicle. The new system was predicted to improve the vehicle driving range up to 21 km in the most optimal case (17%) owing to the fact that the improved cooling system which uses residual heat for heating the cabin prevents the dissipation of heat via the HX.

5. Integrated thermal management (ITM) system combined AC and battery pack Battery temperature influences the availability of discharge power (for start up and acceleration), energy, and charge acceptance during energy recovery from regenerative braking [138]. Temperature also affects the life of the battery. Uneven temperature distribution in a pack lead to electrically unbalanced modules/packs, and reduced pack performance [139]. Currently, large-scale batteries will generate much heat during high current discharge process. In order to prevent

4.4. Other non-VC AC systems

Catalytic converter

Solar-assisted AC systems are also one interesting option of EV climate control systems since solar cells can not only provide a heat insulating layer, but also recharge the battery. This system generally puts batteries full of the roof, which can not only provide part of energy for the AC system but also can effectively prevent the radiant heat of the sun through the roof. The peak value of the needed cooling capacity can reduce about 40% [130]. Ma et al. [131] designed a solar-assisted heat pump system for EVs. The solar cells covered the roof of a compact car, and could generate about 225 W of power. The theoretical analysis showed that the maximum increment of the cooling capacity of this system can be attained by 27%. Zhao [132] declared that two hours of generating electricity capacity by a solar panel could keep the solarassisted system running for half an hour. In [133], a solar controller and an AC controller were designed. The solar controller managed the battery features, such as charging and over-discharging protection, and communicated with the vehicle control system. The experimental

Shell& Tube H.X

Captured heat from exhaust gases (HTF,out)

Mid-Vehicle Muffler

Aft-Muffler

To exhaust pipe end

To the primary heat exchanger (HTF,in)

Boiler Ejector

EXV

Evaporator

Condenser

Fig. 20. Schematic of the steam ejector cooling system [127]. 457


Z. Zhang et al.

Catalytic converter

Mid-Vehicle Muffler

Shell& Tube H.X

Captured heat from exhaust gases (HTF,out)

Aft-Muffler

To exhaust pipe end

To the primary heat exchanger (HTF,in)

Condenser

Generator

Heat Exchanger Pump

EXV1 EXV2 Cold air to cabin

Absorber

Evaporator

Air from cabin

Fig. 21. Schematic of LiBr-water absorption cooling system [127].

Fig. 23. Thermal management system with air cooling.

et al. [146] found that air forced convection cooling could alleviate temperature growing in the battery, but the non-uniform distribution of temperature on the battery surface is inevitable. Nelson et al. [147] found that it is difficult to cool the battery to below 52 °C through air forced convection cooling if the battery temperature exceeds 66 C. In 2013, Fan et al. [148] numerically indicated that the application of a reciprocating air flow is able to improve temperature uniformity of a lithium-ion battery pack. Zolot et al. [149] used conditioned air from the cabin and parallel airflow scheme for the thermal management system of the battery in Toyota Prius HEV, the schematic is demonstrated in Fig. 23(a). The forced air system is composed of two vents, one for cabin air to return and the other to supply outside air. Ahmad [150] suggested that the air should be precooling before entering into battery pack, and the thermal management system is shown in Fig. 23(b). High air mass flow is indispensable for this option owing to the bad heat transfer at the cell surface and low heat conductivity inside the cell. Also, the dust contained in the air may smudge the battery pack. The dust may be deposited between the cells and, in conjunction with condensed humidity, forms a conductive layer. This layer facilitates the generation of leak currents in the battery. Besides, this option is difficult to heat the battery quickly during cold startup due to its low conductivity when the battery is operated at a very low temperature such as −30 °C in cold winter [144]. Nelson et al. [147] found that just 5 kW could be provided by the battery at −30 °C for a 25 kW delivering power. The battery could not heat itself quickly with I2R heating. In this case, heating the battery became a challenge. Then two methods for quick heating were proposed: with electric heaters within the battery or by heating the battery coolant with heat transferred from the engine coolant.

Fig. 22. Layout of the system of sorption AC system proposed by Jiang et al. [137].

overheating or risk in high temperature ambient and hard conditions, the investigations of battery thermal management have become more important. Based on medium, battery thermal management can be mainly divided into four categories, include air cooling, liquid cooling, the use of heat pipes, and the use of PCMs. These approaches have been reviewed critically by Rao and Wang [140] and Zhang et al. [141]. Here the integrated thermal management systems combined AC and battery pack in the EVs are reviewed critically. 5.1. Air cooling Air cooling approach is dominant among the cooling schemes owing to the virtues of low cost, availability, simplicity and space-saving [142]. The cooling can be achieved by blowing the air serial or parallel through the accessible surfaces of the battery pack. Pesaran et al. [143] found that parallel cooling had better temperature uniformity through 2 dimensional simulated computations. Chen and Evans [144] found that the heat dissipation could not be remarkably alleviated by air natural or forced convection due to the low thermal conductivity of the polymer, specifically in large-scale batteries. Sabbah et al. [145] and Wu et al. [146] found that air natural convection cooling is not an effective means for removing heat from the battery, especially at high discharge rates and high ambient temperatures. Sabbah et al. [145] investigated the air forced convection cooling by simulation and experiment. The results demonstrated that battery temperature variation was increased with air velocity. When ambient temperature is 45 °C and discharge rate is 6.67 °C, battery temperature can not be kept under 55 °C through increasing the air velocity. Wu

5.2. Evaporative cooling A special evaporator inside the battery cell is connected to the AC system in the vehicle, shown as Fig. 24. It is a very compact solution owing to the integration of the battery evaporator with the AC system. The special design of the evaporator and its resulting integration into the battery offer a large contact surface for the heat exchange. But as soon as the battery temperature reaches critical values, the refrigeration cycle has to be switched on even if the vehicle cabin doesn’t need air conditioning. In addition, the battery evaporator cannot heat up the 458


Z. Zhang et al.

optimization, and a 13% exergy efficiency improvement and 5% environmental impact reduction was obtained at the expense of a 27% increase in the total cost through the exergoenvironmental optimization. In 2014, Javani et al. [156] performed an exergy analysis and optimization of a new ITM system for HEVs. In the system, a latent heat thermal energy storage cell consisting of a shell and tube HX filled with Octadecane as the PCM is integrated with a refrigeration cycle. The PCM HX operated in parallel with the chiller, therefore, decreases the heat load of the chiller, leading to the decrease of the compressor work. The results indicated that the maximum exergy efficiency of the system is 34.5% while the minimum total cost rate is 1.38 $/h. In 2011, Yokoyama et al. [157] developed an ITM system called Thermal Link System for EVs. It integrates a motor/inverter cooling system and a heat pump, and it controls the device cooling and AC system simultaneously. Fig. 27 shows the configuration of the system. The cooling and heating capacity were transferred by a secondary loop to the cabin and devices. In the heating mode the EXV in the route to the device cooling HX is in an off state so that the HX is nonfunctional. The system warms the cabin by using the heat from the heat pump and waste heat from the motor/inverter. In the cooling mode, the coolant water travels through the bypass line in the device cooling cycle and does not travel through the interior HX. With the heat pump and waste heat recovery, the heating energy consumption can be decreased to below 580 W for 2.0 kW heating capacity. It implies that the COP was more than 3.3. In 2015, Zhang et al. [141] has established ITM system by liquid circulation, which include HVAC, battery thermal management, electronic devices cooling system (motor, PCU, DC-DC) PCM TES and etc. In 2016, Zou et al. [158] proposed an ITM system combining a heat pipe battery cooling/preheating system with a heat pump AC system to fulfill the comprehensive energy utilization for EVs. A test bench with the ITM system for a regular five-chair EV was set up to study the system performance under different working conditions. The schematic diagram of the system is shown in Fig. 28. The system mainly consists of a variable-frequency scroll compressor, an exterior HX with a fan, a liquid vapor separator, four refrigerant valves (RV), a condenser followed by EXV1 for cabin heating, a refrigerant–air evaporator following with EXV2 for cabin cooling and a refrigerant–water evaporator for battery cooling called battery chiller. The RVs is used to switch the system for cooling or heating. A water–air HX is arranged in front of the vehicle to utilize the natural cooling source and a PTC heater is used to preheat the battery in cold season. A heat pipe HX called battery HX box is installed among the battery group. Under the cooling mode, RV1 and RV4 are open while RV2 and RV3 are closed. The opening of EXV2 and EXV3 are changed repeatedly to get the optimum branch refrigerant flow rate of the cabin evaporator and battery chiller. Under

EXV 2

Fig. 24. ITM system with evaporative cooling.

battery to the ideal temperature when the outdoor temperatures are very low. However, it can be achieved by driving with the internal combustion engine for HEVs. A battery heater must be installed in PEVs so that the vehicle can be started and driven whatever the situation in the winter. Krüger et al. [151] investigated the impact of the battery cooling of the refrigerant cycle and the resulting additional load on the compressor. The evaporative cooling of the cell was selected as the temperature management option. It was found that the energy consumption of the refrigerant cycle was increased by up to 10%. The comfort of the passenger was affected by the battery cooling in hot weather conditions. Zhong [152] investigated the performance of the two evaporator system which provides 2 kW cooling capacity for battery steadily. But heat exhausted from the battery was achieved by the thermostatic heat water during the test. The effects of refrigerant charge, the superheat, the refrigerant leak of the heat EXV on the performance of the system were investigated. The experiments also showed that the electronic EXV in the control of superheat has an advantage than the traditional thermal EXV. Ou [153] designed a reversible heat pump integrated thermal management system. The schematic of the system is shown in Fig. 25. But the performance of the system is needed to be further investigated. 5.3. Secondary loop cooling An additional coolant loop that provides cooling or heating to the battery is connected to the AC system by a battery chiller, shown as Fig. 26. During temperate ambient temperature days, the coolant is cooled by the ambient air flowing through the battery cooler so that the refrigeration cycle doesn’t have to be operated (route A). If the cooling by the battery cooler is not sufficient for high ambient temperatures, the coolant flows through the battery chiller and exchanges heat with the refrigerant, i.e. the electric AC compressor is operated and the refrigerant will be throttled by the thermal EXVs and passes through the chiller (route B). At significantly lower ambient temperatures, the coolant is heated by the heater until the batteries attain a desirable operating temperature (route C). The coolant can be water, glycol, oil, or acetone. This scheme will face the challenges such as big weight, complex structure, better sealing requirements and the addition of external cooling circulatory system. Besides, more power will be required due to the addition of pumps, valves, chillers, and radiators, etc. In 2012, Hamut et al. [154] performed a second law analysis of the secondary loop cooling scheme for high ambient temperatures. The refrigerant is R134a. It was found that the energetic COP and exergetic COP ranges of 1.8–2.4 and 0.26–0.39 respectively. The largest impact on the overall exergetic COP was the ambient temperature for the system. In 2014, they [155] performed an optimization analysis of the system for an HEV using single and multi-objective evolutionary algorithms in order to maximize the exergy efficiency and minimize the cost and environmental impact of the system. A 14% exergy efficiency improvement and 5% cost reduction was obtained at an expense of a 14% increase in the environmental impact through the exergoeconomic

Compressor

Heating

Cooling

4-way valve

Battery evaporator

d Va lve 3

Battery

Valve 2

Interior HX

e Gas-liquid separator

EXV 1

Battery

Valve 1

Refrigerant circuit

Battery evaporator

Condenser

Evaporator

Vehicl ecabin

Compressor

f

EXV

b

Dryer

Exterior HX

a

Fig. 25. Reversible heat pump ITM system proposed by Ou [153]. 459


Z. Zhang et al. Coolant circuit

A

4-way valve

B

Refrigerant circuit

EXV 1

C

Battery heat exchanger

Battery chiller

Vehicle cabin

Evaporator

Condenser

Battery cooler

Compressor

heating was around 1.34 at −20 °C ambient temperature and 20 °C indoor temperature. There existed a necessary temperature condition for the heat pipe HX to start action. The heat pipe heat transfer performance was around 0.87 W/°C under cooling mode and 1.11 W/°C under preheating mode. The better heat transfer performance of the heat pipe on preheating mode compared with that on cooling mode was ascribed to the gravity.

Coolant Pump

Battery

6. Conclusions

EXV 2 Heater

In this paper, the state of the art for various AC system solutions to EVs was critically reviewed. A comparison of the main solutions to AC systems of EVs is shown in Table 6 including advantages and disadvantages. The investigation of alternative solutions is continuing along many parallel routes, e.g. VCR-DH systems, VC-HP systems, nonvapor compression AC systems, ITM system combined AC and battery pack and so on. Each solution has its individual benefits and limitations that have been extensively discussed in this paper. At present, the solution of the VCR-DH system has been extensively applied owing to the few variations compared with the conventional ICEVs. But it suffers from the defect of large electricity consumption for electric heaters or unpractical issues for fuel-operated heaters. Comparably, VC-HP system is more preferred for EVs due to its high energy efficiency. It seems to be flexibly adapted to different types of EVs with few modifications, which was regarded as a promising option of AC for EVs. However, the application of this solution will be faced with many challenges, such as the attenuation of heating capacity and the energy efficiency in cold weather, defrosting of exterior HX, further improvement of energy efficiency, alternative refrigerant and so on. But these problems are being overcome by the technologies such as EVI cycle, innovative defrosting methods, hybrid source heat pump and heat pump systems using low GWP refrigerants or natural refrigerants. The TES system offers a relative inexpensive way to extend the driving range. It will be a preferential solution if the specific thermal energy density of the TES material is much more than the energy density of batteries. Otherwise it may be better availability to substitute equivalent additional battery mass for the thermal system and extend EVs

Fig. 26. ITM system with secondary loop cooling.

Fig. 27. System configuration of Thermal Link System [157].

the heating mode, RV1 and RV4 are closed while RV2 and RV3 are open. The battery is preheated by the PTC heater. It was found that the additional parallel branch of battery chiller can supply about 20% additional cooling capacity without input power increase as the system is designed to satisfy the basic cabin cooling demand. The COP for cabin

Fig. 28. Schematic of the ITM system proposed by Zou et al. [158]. 460


Z. Zhang et al.

Table 6 Comparisons of the main solutions to AC systems of EVs. AC systems VCR-DH systems

Advantages VCR-EH systems

VCR-FH systems

VC-HP systems

Non-VC systems

TES system AC systems using TE or ME

WHE systems Solar-assisted AC systems

Air cycle heat pump system

ITM systems

Disadvantages

Low noise; Simple; Reliable. Reliability; Simplicity and economy; Little influence on the driving range. High capacity density; Compact system; Normally COP > 1.0; Current AC technologies can be used. Energy-efficient; Continues high performance running. Quiet; Compact system; Easy mode switch; No alternative refrigerant issues Energy-efficient. Energy-efficient; Cooling load reduction; Able to recharge the battery. Environmentally friendly. Air is everywhere available and totally free; Maintenance is simple and cheap.

Air cooling

Low cost; Simplicity; Space-saving.

Evaporative cooling

Compact; Good heat transfer performance.

Secondary loop cooling

Energy-efficient; An ideal solution.

driving range when AC is not needed. The other solutions are likely to be the supplementary method for future EVs AC systems. The ITM system combined AC and battery pack is a final solution and will become the future direction. But it will face the challenges of complex structure, battery descaling and higher requirement on control of the system. The development of AC system for EVs offers hope for the future, but it is far from the satisfaction of the practical application. With the ever improving technology of EVs such as advanced battery energy storage technology, rapid charging technology, improved charging infrastructure and so on, the problems of AC systems should be solvable in the foreseeable future. However, the pursuit of efficient AC system of EVs is an eternal theme. Only with the worldwide continuous efforts can this concept become a reality.

Heating COP < 1; Cause serious range problems; Alternative refrigerant issue. Not a real zero emission vehicle subsystem; Two different fuels are needed. Alternative refrigerant issue. Range loss at low outdoor temperature; Lower COP under extreme conditions; Control strategy; Component re-design; Alternative refrigerant issue. Cumbersome; Lower energy density of TES materials. Less capacity density; Lower COP; Driving range losses. Insufficient waste heat for PEVs. Discontinuous nature of solar energy.

Low volumetric refrigerating/heating effect; Poor energy efficiency; Noise; High rotation speed of rotors. Bad heat transfer performance; High air mass flow; Dust deposit; Heating the battery is a challenge in cold winter. Need conflict between AC and battery pack; Battery descaling; Heating the battery is a challenge in cold winter; Not enough experimental results are available. Complex system; Battery descaling; Not enough experimental results are available.

in an automobile. Heat Mass Transf 2005;41:449–58. [6] Chiu C-C, Tsai N-C, Lin C-C. Near-optimal order-reduced control for A/C (airconditioning) system of EVs (electric vehicles). Energy 2014;66:342–53. [7] Daanen HAM, van de Vliert E, Huang X. Driving performance in cold, warm, and thermoneutral environments. Appl Ergon 2003;34:597–602. [8] Norin F, Wyon DP. Driver Vigilance-the effects of compartment temperature. SAE Technical Paper; 1992. [9] Qi Z. Advances on air conditioning and heat pump system in electric vehicles – A review. Renew Sustain Energy Rev 2014;38:754–64. [10] Wyczalek FA. Heating and cooling battery electric vehicles-the final barrier. Aerosp Electron Syst Mag IEEE 1993;8:9–14. [11] Khoury G, Clodic D. Method of test and measurements of fuel consumption due to air conditioning operation on the New Prius II hybrid vehicle. SAE Tech Pap no2005-01-2049; 2005. [12] Lee JT, Kwon S, Lim Y, Chon MS, Kim D. Effect of air-conditioning on driving range of electric vehicle for various driving modes. SAE Technical Paper; 2013. [13] Farrington R, Rugh J. Impact of vehicle air-conditioning on fuel economy, tailpipe emissions, and electric vehicle range. Washington, D.C.: Earth technologies forum; 2000. p. 1–6. [14] Pino FJ, Marcos D, Bordons C, Rosa F. Car air-conditioning considerations on hydrogen consumption in fuel cell and driving limitations. Int J Hydrog Energy 2015. [15] Masaaki K. Recent air-conditioning technologies for environment-friendly vehicles. Denso Tech Rev 2014;19:117–22. [16] Umezu K, Noyama H. Air-conditioning system for electric vehicles (i-MiEV). SAE Automotive Alternate Refrigerant Systems Symposium; 2010. [17] Talley E. Hybrid air conditioning systems overview. The Spring 2011 ICAIA Conference: Department of Automotive Technology at OpenSIUC; 2011. [18] Brodie BR, Takano Y, Gocho M. Evaporator with integrated ejector for automotive cabin cooling. SAE Technical Paper; 2012. [19] Sumeru K, Nasution H, Ani FN. A review on two-phase ejector as an expansion device in vapor compression refrigeration cycle. Renew Sustain Energy Rev 2012;16:4927–37. [20] Sun X-f, Cai W-x, Luo Y-l. Design and matching for heating system of electric vehicles. Automob Sci Technol 2014:21–6. [21] Nemesh M, Martinchick M, Ibri S. Cabin heating and windshield defrosting for extended range electric, pure electric, & plug-in hybrid vehicles. SAE Technical Paper; 2012. [22] Apfelbeck R, Barthel F. Heating comfort and range perfectly combined-heating systems for vehicles with alternative drive system. Prospects and challenges of biofuel-operated water and air heaters. SAE Technical Paper; 2013. [23] Cap C, Hainzlmaier C. Layer heater for electric vehicles. ATZ Worldw 2013;115:16–9.

Acknowledgments The authors appreciate the support of University Scientific Research Project of Hebei Province (No. ZD2017061), Construction Technology Research Project of Hebei Province (No. 2017-131), and Natural Science Foundation of China (No. 51378172). References [1] Tie SF, Tan CW. A review of energy sources and energy management system in electric vehicles. Renew Sustain Energy Rev 2013;20:82–102. [2] Amjad S, Neelakrishnan S, Rudramoorthy R. Review of design considerations and technological challenges for successful development and deployment of plug-in hybrid electric vehicles. Renew Sustain Energy Rev 2010;14:1104–10. [3] Hannan MA, Azidin FA, Mohamed A. Hybrid electric vehicles and their challenges: a review. Renew Sustain Energy Rev 2014;29:135–50. [4] Chan C, Chau K. Modern electric vehicle technology. Oxford University Press on Demand; 2001. [5] Kaynakli O, Pulat E, Kilic M. Thermal comfort during heating and cooling periods

461


Z. Zhang et al.

Refrig 2007;30:1195–206. [59] Lee H-S, Won J-P, Cho C-W, Kim Y-C, Lee M-Y. Heating performance characteristics of stack coolant source heat pump using R744 for fuel cell electric vehicles. J Mech Sci Technol 2012;26:2065–71. [60] Pomme V. Reversible heat pump system for an electrical vehicle. SAE Technical Paper; 1997. [61] Cho C-W, Lee H-S, Won J-P, Lee M-Y. Measurement and evaluation of heating performance of heat pump systems using wasted heat from electric devices for an electric bus. Energies 2012;5:658–69. [62] Lee D-Y, Cho C-W, Won J-P, Park YC, Lee M-Y. Performance characteristics of mobile heat pump for a large passenger electric vehicle. Appl Therm Eng 2013;50:660–9. [63] Ahn JH, Kang H, Lee HS, Jung HW, Baek C, Kim Y. Heating performance characteristics of a dual source heat pump using air and waste heat in electric vehicles. Appl Energy 2014;119:1–9. [64] Qian C, Gu B, Tian Z, Ma L, Yang L. Performance analysis of dual source heat pump in electric vehicles. J Shanghai Jiaotong Univ 2016;50:569–74. [65] Ahn JH, Kang H, Lee HS, Kim Y. Performance characteristics of a dual-evaporator heat pump system for effective dehumidifying and heating of a cabin in electric vehicles. Appl Energy 2015;146:29–37. [66] Ahn JH, Lee JS, Baek C, Kim Y. Performance improvement of a dehumidifying heat pump using an additional waste heat source in electric vehicles with low occupancy. Energy 2016;115:67–75. [67] Suh I-S, Lee M, Kim J, Oh ST, Won J-P. Design and experimental analysis of an efficient HVAC (heating, ventilation, air-conditioning) system on an electric bus with dynamic on-road wireless charging. Energy 2015;81:262–73. [68] Bellocchi S, Leo Guizzi G, Manno M, Salvatori M, Zaccagnini A. Reversible heat pump HVAC system with regenerative heat exchanger for electric vehicles: analysis of its impact on driving range. Appl Therm Eng 2018;129:290–305. [69] Aceves SM, Smith JR. A desiccant dehumidifier for electric vehicle heating. J Energy Resour Technol 1998;120:131–6. [70] Antonijevic D, Heckt R. Heat pump supplemental heating system for motor vehicles. Proc Inst Mech Eng Part D: J Automob Eng 2004;218:1111–5. [71] Li H. Frost and defrost mechanism in heat pump air conditioning for electrical vehicle [Master Thesis]. South China University of Technology; 2015. [72] Dong J, Deng S, Jiang Y, Xia L, Yao Y. An experimental study on defrosting heat supplies and energy consumptions during a reverse cycle defrost operation for an air source heat pump. Appl Therm Eng 2012;37:380–7. [73] Steiner A, Rieberer R. Parametric analysis of the defrosting process of a reversible heat pump system for electric vehicles. Appl Therm Eng 2013;61:393–400. [74] Zhou G, Li H, Liu E, Li B, Yan Y, Chen T, et al. Experimental study on combined defrosting performance of heat pump air conditioning system for pure electric vehicle in low temperature. Appl Therm Eng 2017;116:677–84. [75] Steiner A, Rieberer R. Simulation based identification of the ideal defrost start time for a heat pump system for electric vehicles. Int J Refrig 2015;57:87–93. [76] Huang C, Liu X, Chen E. Design and experiment research of heat pump air conditioning system for electric vehicle. Cryog Supercond 2016;44:55–61. [77] Chen E, Jiang B, Liu X, Wu H, Zhaozong H. Establishment and analysis of numerical model of hot gas bypass refrigeration system. Cryog Supercond 2017:72–7. [78] Lin H, Nan J, Qiao M, Liu L. Experimental study of the characteristic of composite defrosting for heat pump air conditioner of electric vehicle. J Refrig 2017;38:29–33. [79] Yan R, Shi J-y, Qing H, Chen J, Song J. Experimental study on heat exchangers in heat pump system for electric vehicles. SAE Technical Paper; 2014. [80] Peters AR. Interior window fogging-an analysis of the parameters involved. SAE Technical Paper; 1972. [81] The National Archives. Regulation (EC) No 842/2006 of the European Parliament and of the Council on certain fluorinated greenhouse gases; 2006. [82] Akasaka R, Tanaka K, Higashi Y. Thermodynamic property modeling for 2,3,3,3tetrafluoropropene (HFO-1234yf). Int J Refrig 2010;33:52–60. [83] Tanaka K, Higashi Y. Thermodynamic properties of HFO-1234yf (2,3,3,3-tetrafluoropropene). Int J Refrig 2010;33:474–9. [84] Daviran S, Kasaeian A, Golzari S, Mahian O, Nasirivatan S, Wongwises S. A comparative study on the performance of HFO-1234yf and HFC-134a as an alternative in automotive air conditioning systems. Appl Therm Eng 2017;110:1091–100. [85] Zhao Y. Performance of automotive air conditioning systems with R1234yf [Ph.D.]. Shanghai: Shanghai Jiao Tong University; 2012. [86] Lee Y, Jung D. A brief performance comparison of R1234yf and R134a in a bench tester for automobile applications. Appl Therm Eng 2012;35:240–2. [87] Navarro-Esbrí J, Mendoza-Miranda JM, Mota-Babiloni A, Barragán-Cervera A, Belman-Flores JM. Experimental analysis of R1234yf as a drop-in replacement for R134a in a vapor compression system. Int J Refrig 2013;36:870–80. [88] Mota-Babiloni A, Navarro-Esbrí J, Ángel B, Molés F, Peris B. Drop-in energy performance evaluation of R1234yf and R1234ze(E) in a vapor compression system as R134a replacements. Appl Therm Eng 2014;71:259–65. [89] Cho H, Park C. Experimental investigation of performance and exergy analysis of automotive air conditioning systems using refrigerant R1234yf at various compressor speeds. Appl Therm Eng 2016;101:30–7. [90] Li Y, Xiong F, Xuan X, Li Q. Experimental research on R1234yf application in pure electric vehicle air conditioning. Refrig Air Cond 2017;17:96–100. [91] Austin BT, Sumathy K. Transcritical carbon dioxide heat pump systems: a review. Renew Sustain Energy Rev 2011;15:4013–29. [92] Ma Y, Liu Z, Tian H. A review of transcritical carbon dioxide heat pump and refrigeration cycles. Energy 2013;55:156–72. [93] Itoh S. World's frst CO2 air conditioning system. AutoTechnology 2004;4:40–3. [94] Lee M-Y, Lee H-S, Won H-P. Characteristic evaluation on the cooling performance of an electrical air conditioning system using R744 for a fuel cell electric vehicle. Energies 2012;5:1371–83. [95] Wang D, Yu B, Hu J, Chen L, Shi J, Chen J. Heating performance characteristics of

[24] Shin HY, Ahn KS, Kim CS. Performance characteristics of PTC elements for an electric vehicle heating system. Energies 2016;9:813. [25] Bauml T, Dvorak D, Frohner A, Simic D. Simulation and measurement of an energy efficient infrared radiation heating of a full electric vehicle. Vehicle Power and Propulsion Conference (VPPC) IEEE; 2014. p. 1–6. [26] Müller EA, Onder CH, Guzzella L, Kneifel M. Optimal control of a fuel-fired auxiliary heater for an improved passenger vehicle warm-up. Control Eng Pract 2009;17:664–75. [27] Mimuro T, Takanashi H. Fuel operated heaters applied to electric vehicles. Int J Autom Technol 2014;8:723–32. [28] Kohle U, Pfister W, Apfelbeck R. Bioethanol heater for the passenger compartments of electric cars. ATZ Worldw 2012;114:36–41. [29] Chongpyo C, Gangchul K, Youngdug P, Wookhyun L. The development of an energy-efficient heating system for electric vehicles. 2016 IEEE Transportation Electrification Conference and Expo, Asia-Pacific (ITEC Asia-Pacific); 2016. p. 883–885. [30] Clodic D, Zgheib E, Mortada S. Impacts of heating and cooling on electrified vehicles. In: Proceedings of the 4th European Workshop MAC and Vehicle Thermal Systems; 2011. [31] Zhang Z, Li W, Zhang C, Chen J. Climate control loads prediction of electric vehicles. Appl Therm Eng 2017;110:1183–8. [32] Zhang Z, Liu C, Chen X, Zhang C, Chen J. Annual energy consumption of electric vehicle air conditioning in China. Appl Therm Eng 2017;125:567–74. [33] Peng Q, Du Q. Progress in heat pump air conditioning systems for electric vehicles—A review. Energies 2016;9:240. [34] Li L, Peng F, Zhang H. Design and experiment of a heat pump air-conditioning system for electric vehicles. J Refrig 2013;34:60–3. [35] Peng F, Wei M, Huang H, Zhang H. Effects of different ambient temperatures on performance of electric vehicles' heat pump air conditioning. J Beijing Univ Aeronaut Astron 2014;40:1741–6. [36] Wang Z, Wei M, Peng F, Liu H, Guo C, Tian G. Experimental evaluation of an integrated electric vehicle AC/HP system operating with R134a and R407C. Appl Therm Eng 2016;100:1179–88. [37] Lee H-S, Lee M-Y. Steady state and start-up performance characteristics of air source heat pump for cabin heating in an electric passenger vehicle. Int J Refrig 2016;69:232–42. [38] Zhang Z, Wang D, Zhang C, Chen J. Electric vehicle range extension strategies based on improved AC system in cold climate – a Review. Int J Refrig 2018;88:141–50. [39] Iritani K, Suzuki T. Air conditioning system for electric vehicle. JSAE Rev 1996;17:438. [40] Suzuki T, Ishii K. Air conditioning system for electric vehicle. SAE Technical Paper; 1996. [41] Kondo T, Katayama A, Suetake H, Morishita M. Development of automotive airconditioning systems by heat pump technology. Mitsubishi Heavy Ind Tech Rev 2011;48:27. [42] Qin F, Shao S, Tian C, Yang H. Experimental Investigation on Heating Performance of Heat Pump for Electric Vehicles in Low Ambient Temperature. Energy Procedia 2014;61:726–9. [43] Feng L, Hrnjak P. Experimental study of an air conditioning-heat pump system for electric vehicles. SAE Technical Paper; 2016. [44] Xuan X, Chen F, Rong S. Experimental research on heating performance of heat pump air-conditioning system for electric vehicle. Refrig Air Cond 2017;17:47–50. [45] Hosoz M, Direk M. Performance evaluation of an integrated automotive air conditioning and heat pump system. Energy Convers Manag 2006;47:545–59. [46] Kim KY, Kim SC, Kim MS. Experimental studies on the heating performance of the PTC heater and heat pump combined system in fuel cells and electric vehicles. Int J Automot Technol 2012;13:971–7. [47] Zhang G, Zou H, Qin F, Xue Q, Tian C. Investigation on an improved heat pump AC system with the view of return air utilization and anti-fogging for electric vehicles. Appl Therm Eng 2017;115:726–35. [48] Li H, Li X, Zhou G, Li A, Li Y. Simulation study on ultra - low temperature heat pump air conditioning system in electricity-driven completely vehicles. Cryog Supercond 2014;42:63–7. [49] Li H-J, Zhou G-H, Li A-G, Li X-G, Li Y-N, Chen J. Heat pump air conditioning system for pure electric vehicle at ultra-low temperature. Therm Sci 2014;18:1667–72. [50] Qin F, Xue Q, Albarracin Velez GM, Zhang G, Zou H, Tian C. Experimental investigation on heating performance of heat pump for electric vehicles at −20 °C ambient temperature. Energy Convers Manag 2015;102:39–49. [51] Tang J, Zuo C. Structural analysis of heat pump scroll compressor for electric automobile air conditioning. J Refrig 2014;35:54–8. [52] Zhang Z, Li W, Shi J, Chen J. A study on electric vehicle heat pump systems in cold climates. Energies 2016;9:881. [53] Kwon C, Kim MS, Choi Y, Kim MS. Performance evaluation of a vapor injection heat pump system for electric vehicles. Int J Refrig 2017;74:136–48. [54] Qin F, Zhang G, Xue Q, Zou H, Tian C. Experimental investigation and theoretical analysis of heat pump systems with two different injection portholes compressors for electric vehicles. Appl Energy 2017;185:2085–93. [55] Choi YU, Kim MS, Kim GT, Kim M. Performance analysis of vapor injection heat pump system for electric vehicle in cold startup condition. Int J Refrig 2017;80:24–36. [56] Jung J, Jeon Y, Lee H, Kim Y. Numerical study of the effects of injection-port design on the heating performance of an R134a heat pump with vapor injection used in electric vehicles. Appl Therm Eng 2017;127:800–11. [57] Ogburn M, Nelson DJ, Luttrell W, King B, Postle S, Fahrenkrog R. Systems integration and performance issues in a fuel cell hybrid electric vehicle. SAE Technical Paper; 2000. [58] Kim SC, Kim MS, Hwang IC, Lim TW. Performance evaluation of a CO2 heat pump system for fuel cell vehicles considering the heat exchanger arrangements. Int J

462


Z. Zhang et al.

[96]

[97]

[98]

[99]

[100]

[101] [102]

[103] [104] [105]

[106] [107] [108]

[109]

[110]

[111]

[112]

[113] [114]

[115] [116]

[117] [118]

[119] [120] [121]

[122]

[123] [124] [125]

CO2 heat pump system for electrical vehicle in a cold climate. Int J Refrig 2017;85:27–41. Hundy GF, Trott AR, Welch TC. Chapter 3 - refrigerants. refrigeration, air conditioning and heat pumps (Fifth Edition). Butterworth-Heinemann; 2016. p. 41–58. Tan W. Study on ammonia working medium automobile air conditioning system and its control strategy of electric vehicle [Master's Thesis]. Chongqing University; 2016. Harby K. Hydrocarbons and their mixtures as alternatives to environmental unfriendly halogenated refrigerants: an updated overview. Renew Sustain Energy Rev 2017;73:1247–64. Wongwises S, Kamboon A, Orachon B. Experimental investigation of hydrocarbon mixtures to replace HFC-134a in an automotive air conditioning system. Energy Convers Manag 2006;47:1644–59. Santoso B, Tjahjana DDDP. Performance analysis of the electric vehicle air conditioner by replacing hydrocarbon refrigerant. Int Conf Eng, Sci Nanotechnol 2017:030015. Ghodbane M. An investigation of R152a and hydrocarbon refrigerants in mobile air conditioning. SAE Trans 1999. Lohse-Busch H, Duoba M, Rask E, Stutenberg K, Gowri V, Slezak L. et al. Ambient temperature (20°F, 72°F and 95°F) impact on fuel and energy consumption for several conventional vehicles, hybrid and plug-in hybrid electric vehicles and battery electric vehicle. SAE International; 2013. Samadani E, Fraser R, Fowler M. Evaluation of air conditioning impact on the electric vehicle range and li-ion battery life. SAE International; 2014. Jeffers MA, Chaney L, Rugh JP. Climate control load reduction strategies for electric drive vehicles in warm weather. SAE International; 2015. Jeffers MA, Chaney L, Rugh JP. Climate control load reduction strategies for electric drive vehicles in cold weather. SAE Int J Passeng Cars - Mech Syst 2016;9:75–82. Leighton D. Combined fluid loop thermal management for electric drive vehicle range improvement. SAE Int J Passeng Cars - Mech Syst 2015;8:711–20. Zhang Z, Li W, Zhang C, Shi J, Chen J. A study on heat load character of EV in cold climate. J Refrig 2016;37:39–44. Llopis R, Sanz-Kock C, Cabello R, Sánchez D, Torrella E. Experimental evaluation of an internal heat exchanger in a CO2 subcritical refrigeration cycle with gascooler. Appl Therm Eng 2015;80:31–41. Ituna-Yudonago JF, Belman-Flores JM, Elizalde-Blancas F, García-Valladares O. Numerical investigation of CO2 behavior in the internal heat exchanger under variable boundary conditions of the transcritical refrigeration system. Appl Therm Eng 2017;115:1063–78. Pitarch M, Navarro-Peris E, Gonzalvez J, Corberan JM. Analysis and optimisation of different two-stage transcritical carbon dioxide cycles for heating applications. Int J Refrig 2016;70:235–42. Zhang Z, Wang H, Tian L, Huang C. Thermodynamic analysis of double-compression flash intercooling transcritical CO2 refrigeration cycle. J Supercrit Fluids 2016;109:100–8. Dai B, Liu S, Zhu K, Sun Z, Ma Y. Thermodynamic performance evaluation of transcritical carbon dioxide refrigeration cycle integrated with thermoelectric subcooler and expander. Energy 2017;122:787–800. Besagni G, Mereu R, Inzoli F. Ejector refrigeration: a comprehensive review. Renew Sustain Energy Rev 2016;53:373–407. Shahidinejad S, Bibeau E, Filizadeh S. Design and simulation of a thermal management system for plug-in electric vehicles in cold climates. SAE Technical Paper; 2012. Wang M, Wolfe E, Craig T, Laclair TJ, Abdelaziz O, Gao Z. Design and testing of a thermal storage system for electric vehicle cabin heating. SAE International; 2016. Wang M, Craig T, Wolfe E, LaClair TJ, Gao Z, Levin M, et al. Integration and validation of a thermal energy storage system for electric vehicle cabin heating. SAE International; 2017. Li Z, Jiang Y. The schemes of air conditioning system and cooling system in electric vehicle. Shanghai Auto 2017:2–6. Wu W, Lin Y, Gao R, Song W, Feng Z. Design and economic analysis on electrical vehicles air conditioner with energy storage system combined with cold and heat. Refrig Air-Cond 2011;11:25–9. Jarzyna W, Zielinski D, Aftyka M, Fatyga K. Cold storage-supported air conditioning system in urban transport vehicles. J Ecol Eng 2016;17:120–7. Chan C, Chau K. An overview of power electronics in electric vehicles. Ind Electron IEEE Trans 1997;44:3–13. Torregrosa-Jaime B, Payá J, Corberan J, Malvicino C, Di Sciullo F. ICE project: mobile air-conditioning system based on magnetic refrigeration. SAE Technical Paper; 2013. Payá J, Corberán JM, Torregrosa-Jaime B, Vasile C. Innovative air-conditioning systems for conventional and electric vehicles. 13th European Automotive Congress. Valencia, Spain; 2011. Torregrosa-Jaime B, Payá J, Corberan J. Design of efficient air-conditioning systems for electric vehicles. SAE Technical Paper; 2013. Torregrosa-Jaime B, Vasile C, Risser M, Muller C, Corberan J, Payá J. Application of magnetocaloric heat pumps in mobile air-conditioning. SAE Technical Paper. Torregrosa-Jaime B, Corberán JM, Vasile C, Muller C, Risser M, Payá J. Sizing of a reversible magnetic heat pump for the automotive industry. Int J Refrig

2014;37:156–64. [126] Torregrosa-Jaime B, Payá J, Corberán JM. Application of magnetic cooling in electric vehicles. Sci Technol Built Environ 2016;22:544–55. [127] Javani N, Dincer I, Naterer G. Thermodynamic analysis of waste heat recovery for cooling systems in hybrid and electric vehicles. Energy 2012;46:109–16. [128] He Q, Yang J, Zhu T. Feasibility analysis on absorption air conditioning systems using waste heat in fuel cell buses. Automot Eng 2007;29:1006–8. [129] Colmenar-Santos A, Alberdi-Jiménez L, Nasarre-Cortés L, Mora-Larramona J. Residual heat use generated by a 12 kW fuel cell in an electric vehicle heating system. Energy 2014;68:182–90. [130] Ingersoll J. Integration of solar cells in automobiles as a means to reduce the air conditioner capacity and improve comfort. In: Proceedings of the Ninth EC Photovoltaic Solar Energy Conference, Kluwer Academic Publishers: dordrecht, The Netherlands; 1989. [131] Ma G, Shi B, Chen G, Wu L. Study on solar-assisted heat pump system for electric vehicle air conditioning. Acta Energ Sol Sin 2001;22:176–80. [132] Zhao C. Soar-assisted system study of automobile air conditioner based on parallel technology [Master's Thesis]. Hefei: Hefei University of Technology; 2012. [133] Sun H. Research of automatic control system of the solar-assisted air-conditioning for pure electric vehicle [Master's Thesis]. Nanchang: Nanchang University; 2012. [134] Sardar A, Mubashir S. Prospects of solar energy for electric mobility. Auto Tech Rev 2012;1:18–23. [135] Li S, Wang S, Ma Z, Jiang S, Zhang T. Using an air cycle heat pump system with a turbocharger to supply heating for full electric vehicles. Int J Refrig 2017;77:11–9. [136] Zhang Z, Liu S, Tian L. Thermodynamic analysis of air cycle refrigeration system for Chinese train air conditioning. Syst Eng Procedia 2011;1:16–22. [137] Jiang L, Wang RZ, Li JB, Wang LW, Roskilly AP. Performance analysis on a novel sorption air conditioner for electric vehicles. Energy Convers Manag 2018;156:515–24. [138] Pesaran AA. Battery thermal management in EV and HEVs: issues and solutions. Battery Man 2001;43:34–49. [139] Pesaran AA, Vlahinos A, Burch SD. Thermal performance of EV and HEV battery modules and packs. National Renewable Energy Laboratory; 1997. [140] Rao Z, Wang S. A review of power battery thermal energy management. Renew Sustain Energy Rev 2011;15:4554–71. [141] Zhang T, Gao C, Gao Q, Wang G, Liu M, Guo Y, et al. Status and development of electric vehicle integrated thermal management from BTM to HVAC. Appl Therm Eng 2015;88:398–409. [142] Jung DY, Lee BH, Kim SW. Development of battery management system for nickel–metal hydride batteries in electric vehicle applications. J Power Sources 2002;109:1–10. [143] Pesaran AA, Burch S, Keyser M. An approach for designing thermal management systems for electric and hybrid vehicle battery packs. In: Proceedings of the 4th Vehicle Thermal Management Systems: London, UK; 1999. p. 24–27. [144] Chen Y, Evans JW. Heat transfer phenomena in lithium/polymer‐electrolyte batteries for electric vehicle application. J Electrochem Soc 1993;140:1833–8. [145] Sabbah R, Kizilel R, Selman JR, Al-Hallaj S. Active (air-cooled) vs. passive (phase change material) thermal management of high power lithium-ion packs: limitation of temperature rise and uniformity of temperature distribution. J Power Sources 2008;182:630–8. [146] Wu M-S, Liu KH, Wang Y-Y, Wan C-C. Heat dissipation design for lithium-ion batteries. J Power Sources 2002;109:160–6. [147] Nelson P, Dees D, Amine K, Henriksen G. Modeling thermal management of lithium-ion PNGV batteries. J Power Sources 2002;110:349–56. [148] Fan L, Khodadadi JM, Pesaran AA. A parametric study on thermal management of an air-cooled lithium-ion battery module for plug-in hybrid electric vehicles. J Power Sources 2013;238:301–12. [149] Zolot M, Pesaran AA, Mihalic M. Thermal evaluation of Toyota Prius battery pack. SAE Technical Paper; 2002. [150] Ahmad A. Pesaran. Battery thermal management in EVs and HEVs: Issues and solutions,. Advanced automotive battery conference. Las Vegas, Nevada; 2001. [151] Krüger I, Limperich D, Schmitz G. Energy consumption of battery cooling in hybrid electric vehicles. international refrigeration and air conditioning conference at purdue. Paper 1266; 2012. [152] Zhong h. Simulation and experimental study of electric vehicle air-conditioning system [Master's Thesis]. Hangzhou: Zhejiang University; 2012. [153] Ou Y. Integrated thermal management combined heat pump air conditioner with electric vehicle's battery pack [Master's Thesis]. Guangzhou: South China University of Technology; 2013. [154] Hamut H, Dincer I, Naterer GF. Exergy analysis of a TMS (thermal management system) for range-extended EVs(electric vehicles). Energy 2012;46:117–25. [155] Hamut HS, Dincer I, Naterer GF. Analysis and optimization of hybrid electric vehicle thermal management systems. J Power Sources 2014;247:643–54. [156] Javani N, Dincer I, Naterer GF, Yilbas BS. Exergy analysis and optimization of a thermal management system with phase change material for hybrid electric vehicles. Appl Therm Eng 2014;64:471–82. [157] Yokoyama A, Osaka T, Imanishi Y. Thermal management system for electric vehicles. SAE Int J Mater Manuf 2011;4:1277–85. [158] Zou H, Wang W, Zhang G, Qin F, Tian C, Yan Y. Experimental investigation on an integrated thermal management system with heat pipe heat exchanger for electric vehicle. Energy Convers Manag 2016;118:88–95.

463