Heating performance characteristics of stack coolant ... - Springer Link

Journal of Mechanical Science and Technology 26 (7) (2012) 2065~2071 www.springerlink.com/content/1738-494x

DOI 10.1007/s12206-012-0516-2

Heating performance characteristics of stack coolant source heat pump using R744 for fuel cell electric vehicles† Ho-Seong Lee1, Jong-Phil Won1, Choong-Won Cho1, Yong-Chan Kim2 and Moo-Yeon Lee1,* 1

Thermal Management Research Center, Korea Automotive Technology Institute, 74 Yongjung-Ri, Pungse-Myun, Chonan City, 330-912, Korea 2 Department of Mechanical Engineering, Korea University, Seoul, 136-701, Korea (Manuscript Received February 22, 2012; Revised March 16, 2012; Accepted April 10, 2012)

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Abstract The objective of this study is to investigate the performance characteristics of a stack coolant source heat pump using R744 with a stack coolant heat source for fuel cell electric vehicles under cold weather conditions. Electric heaters are currently used in fuel cell electric vehicles, and the high levels of energy consumption involved lead to lower fuel efficiency and a reduction in the vehicle's driving range. In order to improve the efficiency of the fuel cell electric vehicles in this study, a heat pump using R744 as a refrigerant and making use of wasted heat from the stacks is developed to cover the heating capacity. This heat pump is tested and performance optimized for stack coolant heat recovery under the compressor speeds, air temperatures, and flow rates of the interior heat exchanger, as well as the coolant flow rates of the CO2-coolant heat exchanger. In addition, the heating capacity of the tested system was sufficiently attained over 5.0 kW at the coolant flow rate of 5.0 l/min under extremely cold weather conditions of -20°C. Keywords: Heat pump; Fuel cell; Coefficient of performance (COP); R744 ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction The automotive industry is under much pressure to aid in the reduction of fossil fuel consumption due to limited resource levels. As such, zero emission vehicles such as electric vehicles, fuel cell electric vehicles, and hybrid electric vehicles are under development by major automotive companies in various countries. However, electric vehicles and hybrid vehicles using battery packs possess a large drawback due to their comparatively short driving range with the operation of both heating and cooling systems under cold and hot weather conditions. The fuel cell electric vehicle (FCEV) is thus considered a more viable option. Despite the fact that many leading automotive companies have developed and demonstrated FCEVs under lab and road conditions, numerous engineering issues must be address before such vehicles can measure up to the performance of internal combustion engine vehicles. These issues are in areas such as system efficiency, stack performance, thermal management technology, and compatibility. One of the technical challenges for the FCEV is to achieve an acceptable heating capacity in concert with lower levels of power consumption. The present heating system found in *

Corresponding author. Tel.: +82 41 559 3312, Fax.: +82 41 559 3091 E-mail address: [email protected] † This paper was presented at the ICMR2011, Busan, Korea, November 2011. Recommended by Guest Editor Dong-Ho Bae © KSME & Springer 2012

FCEVs is a positive temperature coefficient (PTC) heater. This system results in lower fuel efficiency and a reduction in the vehicle's driving range due to the high levels of energy consumption involved. One possible alternative is a heat pump that utilizes R744 and stack coolant wasted heat in order to heat the FCEV, thus minimizing the power consumption needed to improve the heating capacity. Over the years, much research into the development of a heat pump using R744 has been carried out. This is due to the attractiveness of attaining such a vehicle heating capacity free of environmental regulations as a refrigerant. Neksa [1] describes the various applications of air source heat pumps using R744. Hafner et al. [2] report on the favorable features of an air source mobile heat pump using R744. However, these studies on air source heat pumps using R744 are very difficult to apply to the heating system of vehicles due to the frost that builds up on an exterior heat exchanger, caused by the condensation of water. From this viewpoint, it is necessary to use a stable and an effective heat source in the R744 heat pump for the purposes of stack coolant heat recovery. If wasted heat from the stack coolant is used as the evaporating heat source under cold weather conditions, it is possible for the heat pump using R744 to serve as the FCEV's heating system. Therefore, the objective of this study is to investigate the heating performance characteristics of such a heat pump using

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Table 1. Specifications of tested heat pump system. Components

Specifications

Gas cooler (Interior heat exchanger) CO2-Coolant heat exchanger

Internal heat exchanger Fig. 1. Schematic diagram of the stack coolant source heat pump system using R744 for FCEV.

Electric driven compressor

R744 with stack heat recovery for FCEVs under cold weather conditions. Kim et al. [3] have previously carried out experimental studies on the feasibility of a heat pump with R744 using hot water under ordinary conditions. In addition, the optimal operating conditions to cope with the heating load in the cabin room are estimated for the tested vehicle.

Expansion valve Accumulator

Type

Multi-flow

Core size

260×250×35 mm

Type

Co-axial

Coolant

176.7 mm2 at Φ15 mm

Ref.

102.6 mm2 at Φ3.3 mm

Type

Co-Axial

Capacity

0.5~2.0 kW at 2 m/s

Length

750~1250 mm

Type

Radial piston/inverter

Displacement

8.6 cc/rev

Control

Manual type

Flow Rate

50~250 kg/h

Pressure

Max.12.5 MPa at 90°C

Volume

500 cc

Table 2. Test conditions of the heat pump.

2. Experimental setup and data

Interior heat exchanger

2.1 Test setup Fig. 1 shows the schematic diagram of the stack coolant source heat pump system using R744 for FCEV considered in this study. The test set up mainly consists of an electric driven external variable displacement compressor for CO2, internal heat exchanger, interior heat exchanger for the interior-side, CO2 and coolant heat exchangers for the exterior-side, an expansion device (electronic expansion valve), and accumulator. The test set up is installed in a psychrometric calorimeter to provide a pre-controlled ambient temperature. The psychrometric calorimeter consists of two different environmental chambers. One is used to simulate the cabin room and is constantly controlled from -30°C to 65°C. The other is used to simulate outdoor conditions. The psychrometric calorimeters, equipped with an air-handling unit including a cooling coil, a heating coil, and a humidifier, are set to -30°C ~ 90°C to an accuracy of ± 0.2°C. The psychrometric calorimeters are controlled via the PID (proportional-integral-derivative) control method. The coolant temperature and flow rate in the CO2 and coolant heat exchangers are controlled by a coolant supply system via a constant water bath (0 ~ 90°C). The water pump's flow rate ranges from 0 to 400 l/min. The compressor frequency is fixed at 60.0 Hz and the current used to drive the compressor is measured by an inverter driver (SV-IG5A), manufactured by LS industrial systems. The work done by the compressor is calculated using power input and current. The expansion valve is manually controlled by a metering valve, which can achieve the desired vapor fraction at the outlet of the CO2 and coolant heat exchangers and adjust the super heat

Air temperature [°C] 0, -10, -20

Exterior heat exchanger

Compressor

Relative humidity [%]

Air Coolant Coolant flow flow temperature rate rate [°C] [m3/min] [l/min]

Rotation speed [rev/min]

-

20, 30, 40, 50, 60

3,000 4,000 5,000 6,000

7

5, 10, 15, 20

condition of the compressor's suction. During the experiments, the major operating and measuring parameters are monitored graphically and numerically in real time. Table 1 shows the specifications of a stack coolant source heat pump system using R744. The core size of the gas cooler is 260 × 250 × 35 mm3. The lengths of the co-axial type internal heat exchangers vary from 750 to 1250 mm. The compressor is electric driven with an inverter driver and radial piston compression. The accumulator has a volume of 500 ml and a maximum pressure of 12.5 MPa at a temperature of 90 oC. A co-axial type coolant and refrigerant heat exchanger is used in this study. Table 2 shows the test conditions used in this study. In order to reflect the various driving conditions in winter season, the air temperature of the interior heat exchanger is set to 0, -10, and -20°C with an air flow rate of 7 m3/min. The coolant temperature of the exterior heat exchanger is set to 20, 30, 40, 50, and 60°C and the coolant flow rate is set to 5, 10, 15, and 20 l/min. The compressor speed is set to 3000, 4000, 5000, and

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Heat capacity of the refrigerant side (kW)


Table 3. Uncertainties of the experimental parameters. Items

Accuracy

Thermocouples (T-type)

± 0.1 °C

Pressure gage (Sensors, PI3H) Mass flow meter (Coriolis type) for refrigerant Mass flow meter (Coriolis type) for coolant

± 0.1% (Max 16.0 MPa) ± 0.15%, Max 680 kg/h

Data logger (Gantner)

± 0.1% E. Gate IP (V3) (2.93 W at 12.06 V)

Heating capacity

5.0%

COP

6.8%

9

Heat balance 8

+5.0%

7

-5.0%

6 5 4 3

3

4

5

6

7

8

9

Heat capacity of the air side [kW]

2.2 Data reduction The refrigerant enthalpy method is used to calculate the refrigerant side capacity [6, 7]. The heat capacity for the air side is determined by utilizing both the air flow rate and sensible heat change calculated as shown in Eq. (2): Q& ref = m& ref Δhref

(1)

Q& a = m& aC p ,a (Ta ,in − Ta ,out ) .

(2)

The heat capacity of the air side is validated against the heat capacity of the refrigerant side; both results fall within 5.0% of each other as shown in Fig. 2. As such, the present experimental setup is found to be appropriate. The heating COP of the stack coolant source heat pump using R744 is calculated by Eq. (3). The compressor work is based on an electric compres-

Fig. 2. Heat balance between the air side and the refrigerant side. o

120

3

Interior air inlet condition : -20 C, 7 m /min o Coolant inlet temperature : 60 C, Compressor speed : 4,000 rev/min

100

Pressure [bar]

6000 rev/min. All tested conditions are chosen to mimic the air and coolant conditions of real vehicle driving conditions. Table 3 shows the uncertainties of the measured and calculated parameters used in this study. The temperatures of the refrigerant, air, and coolant are measured by T-type thermocouples with an uncertainty of ± 0.1°C. The refrigerant flow rate is measured by a Coriolis type flow meter with an uncerstainty of ± 0.15% and an upper limit of 680 kg/h. This is installed at the point of compressor discharge to measure the single-phase refrigerant state. The experiment uses a mass flow meter of the stack coolant based on the Coriolis effect with an uncertainty of ± 0.1%. Pressure sensors with an uncertainty of ± 0.1% are installed at the inlet and outlet of each component and measure absolute pressure up to 16.0 MPa. In order to verify the measured data of the heating capacity and the heating COP, an uncertainty analysis is performed in accordance with the 95% confidence level set by the standards of ANSI/ASME [4] and Moffat [5]. Estimates are made of the precision limits and bias limits in all the parameters associated with the heating capacity and the heating COP. The uncertainties of the heating capacity and the COP show that the average uncertainties for the experimental data are 5.0% and 6.8%, respectively.

80 60 Coolant flow rate [l/min] 5 10 15 20

40

20 -300

-200

-100

0

100

Enthalpy [kJ/kg]

Fig. 3. Pressure and enthalpy relation.

sor power input, which is measured by a watt meter and excludes the inverter loss. Fig. 2 shows the heat balance between the air side and the refrigerant side used in this study. Q& COP = & a Wcomp

(3)

3. Results and discussion Performance characteristics of the stack coolant source heat pump using R744 are assessed here. This study focuses on revealing the heating capacity and the heating COP needed to cope with the heating load in the tested vehicle's cabin room under cold weather conditions. These are compared with engine coolant heating systems found in the internal combustion engine. Fig. 3 shows the pressure and enthalpy relation of the coolant source heat pump for a FCEV using wasted heat from the stacks that is designed in this study. By using the refrigerant charge matching method for the heating system as mentioned in Lee et al. [8], the refrigerant charge of the heat pump


for heating is set at 2.0 kg for an air temperature of 20.0°C and an air flow rate of 7 m3/min for the interior heat exchanger. The same setting is used for a coolant temperature of 40°C and a flow rate of 5.0 l/min in the CO2-coolant heat exchanger. In addition, the mass flow rate of the heat pump for acceptable heating is 145.0 kg/h at a frequency of 60.0 Hz and a compressor speed of 4000 rev/min. The R744 heat pump using wasted heat from stacks in the heating mode requires relatively low levels of compressor work. This explains the high value of the heating heat transfer efficiency (heating COP) when compared with other heating systems such as a PTC heater, as mentioned by Cho et al. [9]. This is because working across a small temperature gradient between the heat source and heat sink requires the input of little work to transport heat, due to the adding of stack heat recovery.

o

Interior heat exchanger capacity [kW]

2068

6.0

4.0

2.0 3,000

4,000

5,000

6,000

(a) Heating capacity

3.2 Effect of air inlet temperatures for the interior heat exchanger Fig. 5(a) shows the effects of compressor speed on the heating capacity and the heating COP with variations of the inlet air temperature of the interior heat exchanger. The heating capacity is seen to increase with the decrease of the inlet air temperature of the interior heat exchanger from 0 to -20°C, due to higher temperature differences between a refrigerant inlet and an air inlet. This is because the decrease of the inlet air temperature of the interior heat exchanger causes a reduction in the refrigerant mass flow rate and thus decreases the compressor's work. Consequently, the heating COP increases when decreasing the inlet air temperature of the interior heat exchanger. However, as shown in Fig. 5(b), the heating COP

Coefficient of performance [COP]

8.0

3

Interior air inlet conditions : -20 C, 7m /min Coolant flow rate : 5 l/min

Coolant temperature o 20 C o 30 C o 40 C 6.0 o 50 C o 60 C

4.0

2.0 3,000

4,000

5,000

6,000

7,000

Compressor speed [rev/min]

(b) COP o

5.0

Compressor work [kW]

Compressor speed is an important factor affecting the heating performances of the tested heat pump. Fig. 4 shows the effects of compressor speed on the heating capacity and heating COP with variations of the stack coolant temperatures. Conditions depicted include a coolant flow rate of 5.0 l/min and the air temperature of -20.0°C as well as an air flow rate of 7.0 m3/min of the interior heat exchanger. The heating capacity of the interior heat exchanger increases with the rise in compressor speeds, but the heating COP decreases due to the rapid increase of the compressor work as shown transfer efficiency of the heat exchanger decreases with the increase of the required heating load at given conditions. The heating capacity at the coolant temperature of 20.0°C increases by an average of 13.9% with the rise of the compressor speed from 4000 to 6000 rev/min. However, the compressor work increases by an average of 61.9% and the heating COP decreases dramatically with the same rise in compressor speed, as shown in Fig. 5(c). In addition, the heating COP decreases with the rise of the coolant temperatures but converges when the compressor speed reaches 5000 rev/min.

7,000


o

3.1 Effect of compressor speed

3

Interior air inlet conditions : -20 C, 7m /min Coolant flow rate : 5 l/min 10.0 Coolant temperature o 20 C o 30 C 8.0 o 40 C o 50 C o 60 C

3

Interior air inlet conditions : -20 C, 7m /min Coolant flow rate : 5 l/min

Coolant temperature o 20 C 4.0 o 30 C o 40 C o 50 C 3.0 o 60 C

2.0 1.0 0.0 3,000

4,000

5,000

6,000

7,000


(c) Compressor work Fig. 4. Heating performances of the stack coolant source heat pump with the variations of the compressor speeds.

decreases rapidly with a rise of the compressor speed. This is due to the increased compressor work at given conditions, as shown in Fig. 5(c). The heating capacity at the inlet air temperature of the interior heat exchanger of -20.0°C increases by



3

8.0

3.3 Effect of coolant flow rates for a CO2-coolant heat exchanger

Interior air inlet condition : 7m /min o Coolant inlet conditions : 40 C, 5 l/min

Interior air inlet temperature o 0C o -10 C o -20 C 6.0

4.0

2.0 3,000

4,000

5,000

6,000

7,000



Coefficient of performance [COP]

3

Interior air inlet condition : 7m /min o 10.0 Coolant inlet conditions : 40 C, 5 l/min Interior air inlet temperature o 0C o 8.0 -10 C o -20 C

6.0 4.0 2.0 0.0 3,000

4,000

5,000

6,000

7,000


(b) COP 3


5.0 4.0

Interior air inlet condition : 7m /min o Coolant inlet conditions : 40 C, 5 l/min Interior air inlet temperature o 0C o -10 C o -20 C

3.0 2.0 1.0 0.0 3,000

4,000

5,000

6,000

2069

7,000


(c) Compressor work Fig. 5. Heating performances of the stack coolant source heat pump with the variations of the interior air temperatures.

13.5% with the rise of the compressor speed from 4000 to 6000 rev/min; however, the heating COP decreases by an average of 54.2%. This is because the compressor work increases by 166.7% with such a rise in compressor speed.

Fig. 6 shows the effects of the stack coolant flow rate on the heating capacity and heating COP with the variations of the inlet air temperature of the interior heat exchanger. The heating capacity and the heating COP vary with the stack coolant flow rate of the CO2-coolant heat exchanger for given conditions. The heating capacity is seen to increase with the decrease of the inlet air temperature of the interior heat exchanger, due to the increased temperature difference. Considering the rise of the coolant flow rates from 5.0 to 20 l/min, the heating capacity increases by an average of 29.7% with the decrease of the inlet air temperatures from 0 to -20.0°C of the interior heat exchangers, due to the increased heat transfer rate. The heating capacity at the inlet air temperature of the interior heat exchanger of -20.0°C increases by 33.3% with the rise of the coolant flow rate from 5.0 to 20 l/min, but the heating COP decreases by 5.8%. This is because the compressor work increases by 43.8% with the rise of the coolant flow rates from 5.0 to 20 l/min. In addition, under extremely cold weather conditions of -20°C, the heating capacity of the tested system is sufficiently attained over 5.0 kW at a coolant flow rate of 5.0 l/min. As a result, the observed heating performances of the stack coolant source heat pump using R744 may be considered sufficient for heating loads under cold weather conditions. Fig. 6(d) shows the heat recovery capacity of the wasted heat from the stacks of the FCEV. For the purpose of the heat recovery, the wasted heat from the stacks may be used to reach the required heating capacity in the FCEV's heat pump. The heat recovery capacity of wasted heat from the stacks is seen to be an average of 62.1% when compared with the results of Fig. 5(a); coolant flow rates and interior air temperatures are tested in both figures. In particular, at an interior air temperature of -20.0°C for the interior heat exchanger, the heat recovery capacity charged an average of 65.6% of the total heating capacity with the rise of the coolant flow rate from 5.0 l/min to 20.0 l/min, compared with the results of Fig. 6(a). The observed characteristics of the heating COP and the heating capacity for various operating conditions suggest that the coolant source heat pump using wasted heat from the stacks designed in this study is appropriate to use as a cabin heater for a FCEV under cold weather conditions. Additionally, it will also be helpful in increasing the driving range of the FCEV.

4. Conclusion This study investigates the heating performance characteristics of the stack coolant source heat pump using R744 for FCEVs. The experimental investigation in this study includes varying the compressor speeds, air temperatures, and flow rates of the interior heat exchanger, as well as the coolant flow rates of the CO2-coolant heat exchanger. Experimental results show that the heating capacity and heating COP of the stack

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H.-S. Lee et al. / Journal of Mechanical Science and Technology 26 (7) (2012) 2065~2071 3

8.0

Interior air inlet temperature o 0C o -10 C 7.0 o -20 C

6.0 5.0 4.0 3.0

0

5

10

15

20

Interior air flow rate : 7m /min o Coolant inlet temperature : 40 C, Compressor speed : 4,000 rev/min

7.0 Coefficient of performance [COP]


3



5.0 4.0 3.0 2.0

25

0

5

Coolant flow rate [l/min]

10 15 Coolant flow rate [l/min]


3

1.5

1.0

0

5


20

25

(c) Compressor work

Exterior heat exchanger capacity [kW]



Interior air inlet temperature o 0C o -10 C o 2.0 -20 C

0.5

25

(b) COP

3

2.5

20

6.0



4.0 3.0 2.0 1.0

0

5


20

25

(d) Heat recovery capacity of the wasted heat from the stacks

Fig. 6. Heating performances of the stack coolant source heat pump with the variations of the coolant flow rate.

coolant source heat pump using R744 are sufficient to cover the heating load of the FCEV under cold weather conditions. In addition, the heating capacity of the tested system is sufficiently attained over 5.0 kW at a coolant flow rate of 5.0 l/min under extremely cold weather conditions of -20°C. (1) The refrigerant charge of the heat pump for heating is set at 2000 g for air temperatures of -20°C and an air flow rate of 7 m3/min for the interior heat exchanger. This setting is also used for the CO2-coolant heat exchanger with a coolant temperature of 40°C and a flow rate of 5.0 l/min. (2) The heating capacity at a coolant temperature of 20.0°C increased by an average of 13.9% with the rise of the compressor speed from 4000 to 6000 rev/min. However, the compressor work increases by an average of 61.9%. The heating COP decreases dramatically with the same rise in compressor speed. (3) The heating capacity of the interior heat exchanger at an inlet air temperature of -20.0°C increases by 13.5% with the rise of the compressor speed from 4000 to 6000 rev/min, but the heating COP decreased by an average of 54.2%.

(4) The heating capacity at an inlet air temperature of the interior heat exchanger of -20.0°C increases by 33.3% with the rise of the coolant flow rate from 5.0 to 20 l/min. However, the heating COP at the same inlet air temperature of the interior heat exchanger decreases by an average of 5.8%. (5) Under extremely cold weather conditions of -20°C, the heating capacity of the tested system is sufficiently attained over 5.0 kW at a coolant flow rate of 5.0 l/min.

Acknowledgment This work was supported by the grant "Development of carbon dioxide mobile A/C system (10005246)" from the Korea Institute of Energy Technology Evaluation and Planning.

Nomenclature-----------------------------------------------------------------------COP : Coefficient of performance GWP : Global warming potential


h m& Q&

Rh T

: Enthalpy (kJ/kg) : Mass flow rate (kg/h) : Heat transfer rate (W) : Relative humidity (%) : Temperature (°C)

Subscripts a comp in out p ref

: Air : Compressor : Inlet : Outlet : Pressure : Refrigerant

References [1] P. Neksa, CO2 heat pump systems, Int. J. Refrigerant, 25 (2002) 421-427. [2] A. Hafner, J. Pettersen, G. Skaugen and P. Neksa, An automobile HVAC system with CO2 as the refrigerant. In : Natural working fluids, 1998 IIR-Gustav Lorentzen conference, Oslo, Norway (1998). [3] S. C. Kim, M. S. Kim, I. C. Hwang and T. W. Lim, Heating performance enhancement of a CO2 heat pump system recovering stack exhaust thermal energy in fuel cell vehicles, Int. J. Refrigerant 30 (2007) 1215-1226. [4] ANSI/ASME, Measurement uncertainty, ANSI/ASME PTC 19-1-1985, Part 1 (1985). [5] R. J. Moffat, Uncertainty analysis in the planning of an experiment, J. Fluids Engineering, 107 (1985) 173-178. [6] ANSI/AMCA 210, Laboratory methods of testing fans for

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rating, ANSI, Arlington (VA) (1985). [7] ASHRAE Standard 116, Methods of testing for seasonal efficiency of unitary air-conditioners and heat pumps, ASHRAE, Atlanta (GA) (1983). [8] M. Y. Lee, C. W. Cho, H. S. Lee, D. Y. Lee, Y. C. Park and J. P. Won, Performance characteristics of a simultaneous hybrid heat pump using coolant and air sources for an electric bus, Int. J. of Energy (2011) in Review. [9] C. W. Cho, M. Y. Lee, H. S. Lee, D. Y. Lee, S. T. Oh and J. P. Won, Heating performance characteristics of a coolant source heat pump using wasted heat of electric devices for an electric bus, JMST (2011) in Review.

Hoseong Lee received his Master’s degree in 2006 from Korea University, Korea. He then worked as a researcher at Hyundai Motors until August 2007. Since then, he has been working in Thermal management system research center, KATECH, Korea.

Mooyeon Lee received his Ph.D degree in 2010 from Korea University, Korea. He then worked as a research professor at Department of Mechanical Engineering, Korea University, Korea, until March January 2011. Currently, he is working in Thermal management system research center, KATECH, Korea.