Expected energy and economic benefits, and ...

Applied Energy 127 (2014) 202–218

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Expected energy and economic benefits, and environmental impacts for liquid-to-air membrane energy exchangers (LAMEEs) in HVAC systems: A review Mohamed R.H. Abdel-Salam ⇑, Melanie Fauchoux, Gaoming Ge, Robert W. Besant, Carey J. Simonson Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK S7N 5A9, Canada

h i g h l i g h t s LAMEEs can be used for air dehumidification and energy recovery in HVAC systems. The economic benefits and environmental impacts of LAMEEs are reviewed. LAMEEs can effectively improve indoor air quality. Several topics are suggested for future research.

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Article history: Received 16 November 2013 Received in revised form 25 March 2014 Accepted 1 April 2014 Available online 4 May 2014 Keywords: Liquid-to-air membrane energy exchanger (LAMEE) Run-around membrane energy exchanger (RAMEE) Liquid desiccant Hybrid air-conditioner Energy savings

a b s t r a c t A review of the literature on liquid desiccant-to-air permeable-membrane energy exchangers will indicate opportunities for future research and application of this new technology in HVAC systems. HVAC systems are responsible for a large fraction of energy consumption and some airborne emissions other than CO2. Liquid-to-air membrane energy exchangers (LAMEEs) can be used in run-around membrane energy exchanger (RAMEE) systems for passive energy recovery and can be used as a dehumidifier/regenerator for active air dehumidification in HVAC systems. LAMEEs have been under research and development for over a decade. This paper reviews the applications of LAMEEs for ventilation energy recovery and air dehumidification in buildings and automotive air-conditioners. Ó 2014 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3. 4.

5.

6.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Direct-contact liquid desiccant energy exchangers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Liquid-to-air membrane energy exchangers (LAMEEs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LAMEE types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Flat-plate LAMEE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Hollow-fiber LAMEE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LAMEE applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Run-around membrane energy exchangers (RAMEEs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. LAMEE-based hybrid liquid desiccant air-conditioner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1. Building hybrid air-conditioners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2. Automotive hybrid air-conditioners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . LAMEE economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. RAMEE systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.1. Applications of RAMEEs in office buildings. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1.2. Applications of RAMEEs in hospitals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Tel.: +1 306 966 5476; fax: +1 306 966 5427. E-mail address: [email protected] (M.R.H. Abdel-Salam). http://dx.doi.org/10.1016/j.apenergy.2014.04.004 0306-2619/Ó 2014 Elsevier Ltd. All rights reserved.

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Nomenclature Acronyms ACH Air Changes Per Hour ANSI American National Standards Institute ASHRAE American Society of Heating, Refrigerating and Air Conditioning Engineers AUD Australian Dollar COP Coefficient of Performance ECOP Electrical Coefficient of Performance HVAC Heating, Ventilation, and Air Conditioning NTU Number of Heat Transfer Units PP Polypropylene PTFE Polytetrafluoroethylene RH Relative Humidity TCOP Thermal Coefficient of Performance tCO2e Tons of Carbon Dioxide Equivalent VAV Variable-Air-Volume Symbol Cr*

Ratio between solution and air heat capacity rates

Chemical CO CO2 CH4 CaCl2 H2S K2CO3 KMnO4 LiBr LiCl LiNO3 MgCl2 NOx NO2 O3 PM SOx SO2 VOCs

symbols Carbon monoxide Carbon dioxide Methane Calcium chloride Hydrogen sulfide Potassium carbonate Potassium permanganate Lithium bromide Lithium chloride Lithium nitrate Magnesium chloride Nitrogen oxides Nitrogen dioxide Ozone Particulates Sulfur oxides Sulfur dioxide Volatile organic compounds

6.2. Building hybrid liquid desiccant air-conditioners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Automotive hybrid liquid desiccant air-conditioners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7. Indoor air quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8. Environmental impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Suggested topics for future research. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Between 2000 and 2011, global energy consumption increased by 32% [1,2] and the building sector is responsible for 40% of this total [3,4]. In recent years, research [5–20] has been conducted to reduce cooling and heating energy consumption in buildings. Heating, ventilation, and air-conditioning (HVAC) systems are responsible for a substantial portion of the total energy consumed in buildings; for instance, space cooling consumes 70% of the building total energy consumption in the Middle East region [21], and space heating represents 60% of the building total energy consumption in the UK [22]. Buildings account for more than 30% of global CO2 emissions [23]. Since the industrial revolution, global warming, outdoor air quality, and climate change have become serious environmental problems, where a large percentage increase in greenhouse gases contained in our planet’s atmosphere (e.g. CO2 and CH4, etc.) prevent low temperature re-radiation of solar gain from easily leaving the atmosphere, causing the global average air temperature to increase. Sims [24] reported the following facts: (1) the atmospheric concentration of CO2 has increased by 31% during the last 200 years, (2) the global mean surface temperature has increased by 0.4–0.8 °C during the past century, (3) due to ice melting, sea levels have increased by an average rate of 1-2 mm/year during the past century, and (4) the number of frost days all over the world has decreased over the past century [24].

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Since people spend more than 90% of their time in buildings [25], it is important to provide good and healthy indoor environment for occupants. Depending on the climate (cold, hot, humid, etc.) and building type (hospital, school, residence, etc.), outdoor ventilation air conditioning can consume up to 30–60% of the total energy consumption in buildings [26]. Wyon [4] found that poor indoor air quality may reduce the performance of the occupants in an office by up to 6–9%. Therefore, if ventilation rates are reduced below acceptable standards for the sake of energy conservation, occupant productivity losses are likely to follow. In conventional air-conditioning systems, dehumidification of the supply air is achieved by cooling the air to a temperature below its dew point temperature to remove moisture, and then reheating the air to a comfortable temperature before it is supplied to the conditioned space. This process consumes large amounts of energy. Energy recovery is an effective method to reduce the energy consumption in building HVAC systems. Different types of energy recovery ventilators (ERV) have been investigated [27–33] and integrated with air dehumidification systems [34–37]. However, they encounter some challenges in practical applications, especially for the retrofitting of existing buildings.

2. Direct-contact liquid desiccant energy exchangers In the last two decades, direct-contact liquid desiccant air-conditioning systems, where the supply air is dehumidified through

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direct contact with a liquid desiccant solution, have been studied [38–47]. Direct-contact liquid desiccant systems investigated have included designs with two packed beds, where one bed dehumidifies the airflow and the other bed regenerates the desiccant solution. In the dehumidifier, a cooled desiccant solution is sprayed over the packed bed which comes in direct contact with the hot humid supply air. The supply air is cooled and dehumidified in the dehumidifier and the desiccant solution is heated and humidified. After the dehumidifier, the air is supplied to the HVAC unit whereas the diluted desiccant solution is heated and pumped to the regenerator, where it comes in direct contact with the exhaust air from the building, which cools and de-waters the solution. Further cooling of the solution may be required in an auxiliary heat exchanger [38,43–46]. Despite the fact that direct-contact liquid desiccant systems may show significant energy savings when integrated in HVAC systems, there are several problems associated with these systems: (1) a high pressure drop on the airside increases the operating costs of fans and decreases the HVAC system COP, (2) carry-over of liquid desiccant aerosols where some droplets of the desiccant solution are carried downstream by the supply airflow, thus reducing indoor air quality, causing health problems for occupants, and corroding the ducting system. 3. Liquid-to-air membrane energy exchangers (LAMEEs) Liquid-to-air membrane energy exchangers (LAMEEs), where the air and desiccant solution are separated by a micro-porous membrane, have much lower pressure drop on the airside than that in the direct-contact liquid desiccant packed beds, and can eliminate the carry-over problem associated with direct-contact liquid desiccant air-conditioning systems. In addition, for welldesigned exchangers without significant air and liquid flow maldistributions, the effectiveness of each exchanger can be increased to more than 80%. The elimination of the carry-over of desiccant droplets in LAMEEs means not only a reduction in the operating costs, but also the transient response delay times are reduced because smaller masses of liquid desiccant solution are required within each exchanger [48]. LAMEEs can be used as active air dehumidifiers and desiccant solution regenerators in liquid desiccant dehumidification systems. As well, two or more LAMEEs can be combined to constitute a run-around membrane energy exchanger (RAMEE) system for passive energy recovery from the exhaust air to precondition the supply air in HVAC systems. By rerouting the liquid desiccant flow with valves and pumps in the liquid flow loops, several HVAC system configurations can be achieved using the same set of exchangers (i.e. the passive operating condition may be used for one set of inlet air conditions and external auxiliary energy inputs make it an active HVAC system for other inlet air conditions).

Number of publications

6 5

A comprehensive review of the design, performance, and development of LAMEEs was presented in a previous paper by the authors [49]. For the first time, the current paper presents a review of the development, energy savings, economics, and environmental impacts of LAMEE applications in buildings and automotive airconditioners. Fig. 1 shows the number of studies published on LAMEE applications each year. The first study was published in 2002, thereafter few works were published prior to 2008. As industrial interest and support has grown, publications on LAMEE applications significantly increased between 2009 and 2014. 4. LAMEE types 4.1. Flat-plate LAMEE A flat-plate LAMEE has a structure similar to a parallel-plate heat exchanger, only the stiff metal or plastic plates are replaced by flexible micro-porous membranes. Fig. 2 shows a schematic of a flatplate LAMEE. The micro-porous membranes separate the desiccant solution and air streams, and allow simultaneous heat and moisture transfer between the two streams. A flat-plate LAMEE may have a cross-flow, counter-flow, or counter-cross-flow configuration. It can be employed as a dehumidifier or a regenerator in the liquid desiccant air conditioning systems. When a LAMEE is used for air cooling and dehumidifying, the heat and moisture are transferred from the hot humid air to the desiccant solution, whereas the heat and moisture are transferred from the desiccant solution to the air when the LAMEE is employed as a solution regenerator. In the past decade, research [50–64] has been conducted to measure and model the performance of the flat-plate LAMEEs. 4.2. Hollow-fiber LAMEE Micro-porous hollow-fiber membranes have been used for liquid-to-liquid medical, reverse osmosis, distillation, water purification, gas separation, etc. applications for many years [65]; but gas or air-to-liquid exchangers often require very large contact areas. Nonetheless, hollow-fibers, with well-known geometries, are readily available and they can be constructed into a shell-and tube heat exchanger geometry, where the metal/plastic tubes are replaced by hollow-fiber micro-porous membranes, for experiments using liquid desiccants and air. Fig. 3 shows a schematic of a hollow-fiber LAMEE and Fig. 4 shows (a) a photograph of the outside of an exchanger and (b) a photograph of the internal end view of a hollow-fiber LAMEE. Typically, the desiccant solution flows inside the hollow fibers so the liquid pressure can be high, and the air flows outside the hollow fibers at a lower pressure. A hollow-fiber LAMEE is characterized by high packing density (i.e. about 6000 hollow fibers can be included in a 0.0144 m3 exchanger [68]) and good control of the liquid flow rates and distributions within the tubes but poor control of the airflow distribution outside the tubes [68–70]. Over the past decade, several research studies [66–77] have been performed to measure and model the performance of the hollow-fiber LAMEEs.

4

5. LAMEE applications

3

5.1. Run-around membrane energy exchangers (RAMEEs)

2 1 0

2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014

Fig. 1. Publications relating to the applications of liquid-to-air membrane energy exchangers by year.

A RAMEE system is typically composed of two flat-plate LAMEEs coupled within a liquid desiccant solution loop. One of the LAMEEs is located in the supply air stream and is referred to as the supply LAMEE, while the other LAMEE is located in the exhaust air stream and is referred to as the exhaust LAMEE. Fig. 5 shows a schematic diagram of a RAMEE installation in a building. During


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Fig. 2. Schematic diagram of a counter-cross flat-plate LAMEE.

Fig. 3. Schematic diagram of a hollow-fiber LAMEE [66].

summer seasons, heat and moisture are transferred from the hot humid outdoor air into the desiccant solution in the supply LAMEE. The outdoor air is cooled and dehumidified and the desiccant solution becomes diluted. At steady-state conditions for the loop, the exhaust air from the conditioned space passes through the exhaust LAMEE to regenerate the diluted desiccant solution while the exhaust air is discharged to the outside at a higher temperature and humidity content than its inlet conditions. The same process may occur during winter but the direction of heat and mass transfer is reversed; however, when the outside air is very dry in the winter, it is necessary to continuously add water to the loop to prevent the crystallization of the desiccant solution and maintain steady-state operating conditions. RAMEEs can be installed in existing or new buildings because they do not require the supply and exhaust air streams to be adjacent. Moreover, RAMEEs reduce capital costs by downsizing the HVAC heating/cooling equipment

capacities and reduce the operating costs by reducing the heating and cooling energy consumption of the HVAC system [78–85]. Research on RAMEE systems started at the University of Saskatchewan in 2002. Fig. 6 shows a schematic diagram of the RAMEE test facility used in this research. Ge et al. [80] presented a comprehensive review of the research conducted on the RAMEE, so only a brief review is presented here. In 2005, Fan et al. [81] developed a numerical model to simulate the performance of a RAMEE composed of two cross-flow LAMEEs. The steady-state performance of the RAMEE was presented by Fan et al. [82]. This model was then modified by Vali [83] for a RAMEE composed of two counter-cross flow LAMEEs. Hemingson et al. [84] numerically studied the steady-state performance under different outdoor air conditions. Mahmud et al. [85] experimentally investigated the performance of a RAMEE with two counter-cross flow LAMEEs under steady-state AHRI winter and summer [87]

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Fig. 4. Hollow-fiber LAMEE photographs of (a) a shell and tube configuration and (b) an end-view with the end-cap removed showing the distribution of hollow fibers bound in plastic inside the LAMEE [67].

Fig. 5. Schematic diagram of a RAMEE installed in a building during summer operating conditions [78].

conditions. Akbari et al. [88] developed an artificial neural network model to predict the steady-state performance of a RAMEE at different indoor and outdoor air conditions and system design parameters. Ge et al. [89] developed an analytical model for a RAMEE and studied its performance under both balanced and unbalanced supply and exhaust air flow rates. A transient numerical model was developed by Seyed-Ahmadi et al. [90,91], and performance sensitivity studies were done to investigate the effect of several dimensionless parameters on the transient performance of a RAMEE. Erb [92] experimentally investigated the transient performance of a RAMEE and presented a control strategy to reduce undesirable transient step response time delays. Akbari et al. [93] developed an artificial neural network model to predict the transient performance of a RAMEE at different indoor and outdoor air conditions and system design parameters. The TRNSYS software program was used by Rasouli et al. [94] to investigate the impact of installing a RAMEE system on the annual heating and cooling energy consumption of a 10-storey office

building located in four American cities (i.e. Chicago, Helena, Miami, and Phoenix). A sensitivity study was conducted by Rasouli et al. [95] to determine the effect of uncertainties in building and HVAC system parameters on the economics of a RAMEE. The artificial neural network [88] was used in TRNSYS by Rasouli et al. [96] to study the effect of the ventilation rate, desiccant solution flow rate, and the indoor and outdoor air conditions on the performance of a RAMEE. The effect of different membrane properties on the effectiveness of a RAMEE was studied for two different membranes by Larson et al. [97]. Beriault [98] further studied the membrane properties and built a prototype RAMEE with a new membrane with different moisture transfer properties. Afshin [78] studied the effect of different liquid desiccants on the performance of a RAMEE and investigated the effect of the liquid desiccant type, system design parameters, and outdoor air conditions on the occurrence of crystallization in the RAMEE for a small range of operating conditions. The influence of flow maldistributions inside the LAMEEs on the effectiveness of a RAMEE has been studied for one simple case by Hemingson et al. [99]. The transfer of a few typical indoor gaseous contaminants between the exhaust and supply airstreams in a RAMEE was studied by Patel et al. [100]. The applications of a RAMEE system in HVAC systems were also investigated by other researchers. Bergero et al. [101] investigated the energy savings of a RAMEE in an HVAC system (shown in Fig. 7), compared with a system without energy recovery. This energy recovery unit is comprised of two identical cross-flow flat-plate LAMEEs with structures as shown in Fig. 8. Fig. 9 shows that under winter conditions (outdoor air at 0 °C & 80% RH and indoor air at 20 °C & 50% RH), the energy savings increased as the recirculation ratio (RR), which is the ratio between the amount of indoor air recirculated and outdoor fresh air supplied, increased until a specific value (approximately 0.6) where, for the selected application, any further increase in the recirculation ratio caused a decrease in the energy savings. Higher energy savings were achieved as the outdoor air relative humidity increased or the membrane resistance decreased. In addition, the energy savings increased as the LiCl solution mass flow rate increased for recirculation ratios less than


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Fig. 6. Schematic diagram of the run-around liquid-to-air membrane energy exchanger test facility showing the instrument measurement locations at the University of Saskatchewan [79,86].

Fig. 7. Schematic diagram of the energy recovery system studied by Bergero et al. [102].

0.8, while the effect of the LiCl solution flow rate became negligible at recirculation ratios higher than 0.8. Bergero and Chiari [102] used a mathematical model for a flatplate LAMEE [54,55,103] to investigate the performance of the energy recovery unit in a selected HVAC system. The ventilation and exhaust air volumetric flow rates were assumed to be equal in all conditions. Under summer operating conditions (outdoor air at 32 °C & 60% RH and indoor air at 26 °C & 55% RH), the effectiveness of the energy recovery unit decreased as the volumetric air flow rate increased, however that reduction was decreased as the exchange surface area or the volumetric solution flow rate was increased. The effectiveness of the energy recovery unit increased as the volumetric solution flow rate increased. Under winter operating conditions (outdoor air at 0 °C & 60% RH and indoor air at 20 °C & 45% RH), the volumetric solution flow rate did not have a significant

influence on the effectiveness, and the reduction in the effectiveness as the volumetric air flow rate increased was less than that during the summer conditions. In conclusion, the energy recovery unit achieved a total effectiveness up to 50%, and compared to an HVAC system without energy recovery, the installation of the energy recovery unit achieved up to 20% of energy savings during summer conditions and 43% during winter conditions. 5.2. LAMEE-based hybrid liquid desiccant air-conditioner A LAMEE-based hybrid liquid desiccant air-conditioner is composed of a conventional air-conditioning system integrated with a LAMEE-based liquid desiccant dehumidification system, where two flat-plate or hollow-fiber LAMEEs are used as an active dehumidifier and regenerator. The novel hybrid air-conditioner

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Fig. 8. The structure of the LAMEE used by Bergero et al. [101].

Fig. 9. Energy savings versus recirculation ratio (RR) under winter conditions [101].

has the ability to eliminate the problem of desiccant solution droplet carry-over and maintains the advantages of conventional direct-contact liquid desiccant dehumidification systems. The hybrid air-conditioner can be utilized either in buildings or automobiles. 5.2.1. Building hybrid air-conditioners In 2006, a LAMEE-based hybrid liquid desiccant air-conditioner was briefly introduced by Bergero et al. [104]. In 2010, Bergero and Chiari [105] studied the hybrid air-conditioner proposed by Bergero et al. [104] and developed a Simulink model to evaluate the performance of that hybrid air-conditioner by comparing it with a traditional direct expansion air-conditioning system. Fig. 10 shows a schematic diagram of this hybrid air-conditioner which is composed of an air dehumidification flow and a solution regeneration loop. Two flat-plate LAMEEs are used as the dehumidifier and the regenerator. During air dehumidification (process 10 –2– 3–10 ), the solution is cooled inside the evaporator and pumped through the dehumidifier to cool and dehumidify the supply air. Afterwards, the diluted desiccant solution leaves the dehumidifier and enters the solution tank, where it is mixed with the regenerated desiccant solution. In the regeneration loop (process 5–6–7– 70 –5), the diluted desiccant solution is heated by the condenser

before entering the regenerator, and a mixture of ambient and exhaust air is used for the regeneration process. Afterwards, the concentrated desiccant solution leaves the regenerator and passes back to the solution tank. An economizer (solution-to-solution heat exchanger) is used to recover heat between the regenerated and diluted desiccant solutions. The performances of the traditional and hybrid air-conditioners have been compared at indoor air design conditions of 25 °C & 50% RH, a sensible heat flux of 5.7 kW, an internal latent load of 5 kg/h, and ambient air conditions of 32 °C & 70% RH. The results showed that the hybrid airconditioner was able to achieve more than 50% energy savings compared to the traditional system. Bergero and Chiari [106] conducted further research to investigate the effect of changing some climatic conditions on the energy savings and system COP of this hybrid air-conditioner by using a theoretical model developed by Bergero and Chiari [105]. The design parameters and design indoor air conditions were the same as in Bergero and Chiari [105]. During a typical summer day, the COPs of the hybrid and traditional air-conditioners were found to be 5.6 and 3.3, respectively. Results revealed that the COP of the hybrid system significantly increased compared to the traditional system and the energy savings increased as the internal latent load increased. The energy savings and the COP decreased as the outdoor air relative humidity increased. This is because the outdoor air was used in the desiccant solution regeneration, and thus the potential for moisture transfer inside the regenerator decreased as the relative humidity gradient decreased. Fig. 11 shows the compressor power, COP, percentage of power savings and the R1 ratio for different indoor relative humidity. The R1 ratio is the ratio between the solution flow rate through the regeneration loop and the solution flow rate through the dehumidification loop. In conclusion, depending on the climatic conditions, the proposed hybrid air-conditioner can accomplish power savings up to 65%. Bergero and Chiari [105,106] presented comprehensive studies for their LAMEE-based hybrid liquid desiccant air-conditioner, and concluded that the hybrid air-conditioner can achieve significant energy savings (i.e. 65%). However, annual energy savings and economic studies are needed to show a complete evaluation of the performance of this hybrid air-conditioner and the energy savings throughout the whole year. Vestrelli [56] proposed a hybrid membrane liquid desiccant airconditioner and numerically investigated its performance. A schematic diagram of this system is shown in Fig. 12, where two flat-


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Fig. 10. Schematic diagram of the hybrid air-conditioner proposed by Bergero and Chiari [105,106].

plate LAMEEs are used as the dehumidifier and the regenerator. Two Simulink–Matlab models were developed; one for a conventional vapor compression system and the other for the hybrid air-conditioner. Results revealed that the mechanical power savings increased as the outdoor air relative humidity increased, or as the membrane thickness decreased. The hybrid air-conditioner achieved up to 40% energy savings in a 100 m3 space at design indoor and outdoor air conditions of 26 °C & 50% RH and 30 °C & 60% RH, respectively. Zhang et al. [107] proposed a heat pump driven membrane based liquid desiccant air-conditioner, shown in Fig. 13. This system has a configuration similar to the system proposed in Bergero and Chiari [105]. The major difference is the dehumidifier and the regenerator are hollow-fiber LAMEEs in this system. The performance of the system studied by Zhang et al. [107] has been numerically and experimentally investigated, where the influences of several operating parameters (i.e. air temperature, air relative humidity, desiccant solution flow rate, and air flow rate) were studied. It was found that the COP of the system increased with the increase of either the air temperature, air relative humidity, or air flow rate; on the other hand, increasing the desiccant solution flow rate was found to decrease the COP. In addition, the dehumidification efficiency was found to increase with increasing either the air relative humidity or the desiccant solution flow rate; while, the dehumidification efficiency decreased with the increase of either the air temperature or flow rate. Abdel-Salam et al. [108] proposed a membrane liquid desiccant air-conditioning system (M-LDAC), shown in Fig. 14. Two counterflow flat-plate LAMEEs were employed as a dehumidifier and a regenerator in the system. The cooled desiccant solution flowed through the dehumidifier to cool and dehumidify the supply air,

then the diluted desiccant solution was pumped to the regenerator to be regenerated. A cooler and heater were used to control the desiccant solution temperatures at the dehumidifier and regenerator inlets. To recover heat between the regenerated and diluted desiccant solutions, a sensible heat exchanger was installed between the dehumidification and regeneration cycles. The heat exchanger played an important role in decreasing the load and the capacity of the cooler and the heater. The performance of the proposed M-LDAC was modeled using TRNSYS [109]. Results revealed that when Cr* (Cr* is the ratio between the solution and air heat capacity rates) increased from 2 to 4, the moisture removal rate (MRR) increased by 48%, the sensible heat ratio (SHR) decreased by 12%, and the cooling capacity (CC) increased by 35%. At Cr* values higher than 4, the change in MRR, SHR, and CC became less sensitive to Cr*; however, the thermal COP (TCOP), electrical COP (ECOP), and total COP increased over the entire range of Cr*. When the number of heat transfer units (NTU) increased from 1 to 10, the MRR increased by 166%, the SHR decreased by 24%, the CC increased by 105%, the COP increased by 58%, the ECOP increased by 70%, and the TCOP increased by 23%. Only a slight enhancement in the system performance occurred at NTU values higher than 10 [108]. Based on the results found by Abdel-Salam et al. [108], the LAMEE is the key component for the optimum design and performance of a hybrid air-conditioner. Consequently, Abdel-Salam and Simonson [110] numerically investigated the effect of various design and operating parameters on the dehumidifier and the regenerator performance (i.e. sensible, latent, and total effectivenesses) installed in the M-LDAC system [108]. The results revealed that the dehumidifier and regenerator effectivenesses substantially

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Fig. 11. (a) The compressor power (b) COP (c) percentage of power savings, and (d) R1 ratio, of the traditional and hybrid air-conditioners versus different indoor air relative humidities [106].

Fig. 13. Schematic diagram of the hybrid air-conditioner proposed by Zhang et al. [107].

Fig. 12. Schematic diagram of the hybrid air-conditioner proposed by Vestrelli [56].

increased when NTU increased in the range between 1 and 7, however the effectiveness became almost constant at NTU values higher than 10. The regenerator effectiveness increased as the Cr

value increased between 2 and 6, whereas the dehumidifier effectiveness increased significantly as Cr increased up to 4, thereafter the effectiveness became almost constant at higher Cr values. The temperatures of the desiccant solution at the dehumidifier and the regenerator inlets did not have significant influence on the dehumidifier or regenerator effectivenesses. Increasing the outdoor air temperature caused a significant decrease in the regenerator effectiveness, whereas the dehumidifier effectiveness decreased only slightly. The relative humidity of the outdoor air affected the performance of the dehumidifier and the regenerator in a trend similar to the outdoor air temperature, however, the influence of the


211

energy consumption of the M-LDAC system was the same as the CAC-ERV system, whereas the M-LDAC-ERV system reduced the annual primary energy consumption, compared to the CAC system, by 32%. Abdel-Salam et al. [113] integrated a solar thermal system with the M-LDAC system. The solar thermal system is used to regenerate the dilute liquid desiccant solution. The main objective of the study was to investigate the energy, economic and environmental performances of the M-LDAC system, when different technologies are used to cover the thermal energy required for the regeneration of dilute desiccant solution. In total, the performance of eight systems with different configurations were investigated. Fig. 16 shows a schematic diagram of the eight systems and Table 1 gives the details of each system. Results revealed that the feasibility of using a solar thermal system mainly depends on thermal load required to be covered (i.e. an energy recovery ventilator (ERV) is used or not) and the type of the used heating system (i.e. natural gas boiler or electrical heat pump).

Fig. 14. Schematic diagram of the M-LDAC system proposed by Abdel-Salam et al. [108].

Fig. 15. Annual primary energy consumption of the four air-conditioning systems [110].

outdoor air relative humidity was more significant than the outdoor air temperature. Abdel-Salam and Simonson [110] evaluated the performance and the annual energy savings of the M-LDAC system by comparing four systems; a conventional air-conditioning system (CAC), a conventional air-conditioning system with an energy recovery ventilator (CAC-ERV), a membrane liquid desiccant air-conditioning system (M-LDAC), and a membrane liquid desiccant air-conditioning system with an ERV (M-LDAC-ERV). The performance and the energy consumption of the four air-conditioning systems were evaluated using TRNSYS [109] for a one-storey office building with an area of 511 m2, an occupant density of 5 persons/100 m2 and a ventilation rate of 10 L/s/person (0.6 ach) based on ANSI/ASHRAE Standard 62.1-2007 [111]. The study was performed in Miami, Florida, USA, which is characterized by a hot and humid climate according to ANSI/ASHRAE/IESNA Standard 90.1-2004 [112]. The results revealed that the M-LDAC system and the M-LDAC-ERV system could reduce the capacity of the air chilling system by 50%, and eliminate the energy consumed in reheating the overcooled supply air stream in the conventional air conditioning system. However, solution chilling systems with capacities of 12 kW and 9 kW, and solution heat pumps with capacities of 18 kW and 14 kW, are required to be added for the M-LDAC and M-LDACERV systems, respectively. Fig. 15 shows that the annual primary

5.2.2. Automotive hybrid air-conditioners Automotive air-conditioners increase the fuel consumption of mid-size vehicles by 12-17% [114], and up to 38% under specific circumstances (i.e. extreme hot climates) [115]. Automotive airconditioners are quite attractive for LAMEE applications due to the high density of the latent load inside the conditioned space (the cabin), and the availability of waste heat from the radiator or the engine bay for the desiccant solution regeneration. Vestrelli [56] proposed an innovative automotive hybrid airconditioner, shown in Fig. 17, and developed a Simulink model to investigate its performance. This hybrid air-conditioner was comprised of a dehumidification cycle, a regeneration cycle, and a traditional refrigeration cycle. The dehumidifier and the regenerator were flat-plate LAMEEs with identical structure and specifications. Fig. 18 shows the configuration of the proposed automotive hybrid air-conditioner in a car; the regenerator might be installed behind the radiator or under the floor of the cabin. Initially, the desiccant solution was cooled when it flowed through the traditional refrigeration cycle. Thereafter, it flowed through the dehumidifier (LAMEE) to cool and dehumidify the supply air for the cabin. Finally, it returned to the regeneration cycle to be regenerated, either by waste heat from the radiator or the engine bay. The design indoor air conditions were 23 °C and 50% RH, whereas the outdoor air conditions were chosen according to the EUROCLIM standard (30 °C & 60% RH, and solar radiation of 900 W/m2). The engine coolant was used to heat the liquid desiccant inside a counter-flow liquid-to-liquid heat exchanger. The effect of the air temperature at the regenerator inlet, on the regeneration airflow rate was also investigated. It was found that as the air temperature at the regeneration inlet increased, the flow rate of the regeneration air needed for the regeneration process decreased. Therefore, an effective regeneration process can be obtained with reasonable airflow rates by increasing the air temperature at the regenerator inlet. Vestrelli [56] evaluated the energy savings of the proposed automotive hybrid air-conditioner by comparing it with a traditional automotive air-conditioner. Fig. 19 shows schematics of the traditional and hybrid automotive air-conditioners. Design outdoor air conditions were chosen according to the EUROCLIM standard. For both systems, CO2 was employed as the refrigerant, the cabin was 3 m3, and the air was completely replaced with fresh air. Results showed that the proposed system accomplished up to 20% mechanical power savings (MPS) at a typical occupancy (i.e. five passengers [115 g h1 m3]), and the MPS increased as the indoor vapor production increased. Consequently, the hybrid airconditioner can achieve higher energy savings in bigger vehicles (e.g. buses, trains).

212


Fig. 16. Schematic diagram of the eight hybrid membrane liquid desiccant air-conditioning systems investigated by Abdel-Salam et al. [113].

Table 1 Configurations of the eight systems investigated by Abdel-Salam et al. [113]. System

M-LDAC-NG M-LDAC-ERV-NG S-M-LDAC-NG S-M-LDAC-ERV-NG M-LDAC-HP M-LDAC-ERV-HP S-M-LDAC-HP S-M-LDAC-ERV-HP

Heating system

Air system

NG boiler

Electrical HP

STS

ERV

Without ERV

Yes Yes Yes Yes – – – –

– – – – Yes Yes Yes Yes

– – Yes Yes – – Yes Yes

– Yes – Yes – Yes – Yes

Yes – Yes – Yes – Yes –

6. LAMEE economics Economics is an important concern for the application and promotion of any new technology. However, only a few studies on the economics of LAMEE systems have been reported in the literature. 6.1. RAMEE systems 6.1.1. Applications of RAMEEs in office buildings Rasouli et al. [96] presented a life cycle cost (LCC) analysis (over 15 years) for a RAMEE system in a 10-storey office building located

in four North-American cities (Saskatoon [cold-dry climate], Chicago [cold-humid climate], Miami [hot-humid climate], and Phoenix [hot-dry climate]). The total floor area was 28,800 m2. The cooling system was a variable-air-volume (VAV) HVAC system, and the heating system was composed of radiators operating with hot water (natural convection). Despite the fact that the RAMEE reduced the annual cooling and heating energy consumption, the pressure drop across the RAMEE increased the fan power consumption. Therefore, designers should keep the pressure drop as small as possible to maximize the energy savings. Table 2 gives the capital and annual operating costs of the HVAC system with and without a RAMEE. Installing a RAMEE system caused a maximum increase in the capital costs of 8.5% in Phoenix, and a maximum reduction in the annual operating costs of 11.6% in Chicago. The payback periods were 1.8, 2, 4, and 4.8 years in Saskatoon, Chicago, Phoenix, and Miami, respectively. 6.1.2. Applications of RAMEEs in hospitals Rasouli [116] presented a LCC analysis (over 15 years) for a RAMEE system in a 3-storey hospital located in four North-American cities (Saskatoon, Chicago, Miami, and Phoenix). The total floor area was 3150 m2. Energy savings due to the installation of a RAMEE with a VAV HVAC system were investigated. Fig. 20 shows that installing the RAMEE system caused a maximum reduction of 27% in the total costs in Saskatoon. An immediate payback period


213

Fig. 17. Schematic diagram of the proposed automotive air-conditioner by Vestrelli [56].

6.2. Building hybrid liquid desiccant air-conditioners

Fig. 18. Schematic diagram of the installation of the proposed air-conditioner in a car [56].

occurred in Saskatoon and Chicago due to the downsizing of the HVAC equipment, whereas the payback periods were 1–2 and 2– 3 years in Phoenix, and Miami, respectively.

Vestrelli [56] compared the economics of the hybrid air-conditioner presented in Fig. 12 to a medium-sized conventional airconditioner (i.e. 650 MW h/year) [117]. Vestrelli [56] found that the hybrid air-conditioner accomplished 20% power savings at outdoor air conditions of 30 °C and 60% RH, and the hybrid air-conditioner only consumed 520 MW h/year. The payback period of this hybrid air-conditioner was 3 years. Abdel-Salam and Simonson [110] investigated the economics of the M-LDAC system in a one-storey office building with a floor area of 511 m2, located in Miami, Florida, USA. The economic performance of the M-LDAC system was evaluated by comparing four systems; a conventional air-conditioning system (CAC), a CAC system with an energy recovery ventilator (CAC-ERV), an M-LDAC system, and an M-LDAC system with an energy recovery ventilator (M-LDAC-ERV). A LCC analysis was performed for a lifetime of 15 years. Fig. 21 shows the capital, annual operating, and total life cycle costs of the four systems. It was found that the M-LDAC-ERV system achieved the lowest life cycle costs, and reduced the total costs of the CAC system by 21%. Zhang et al. [107] reported that the COP of the membrane liquid desiccant air-conditioner increased with the increase of either the temperature or the relative humidity of the outdoor air. Therefore, higher energy savings can be achieved in hot and humid climates.

Fig. 19. Schematic diagram of an automotive (a) conventional air-conditioner and (b) hybrid air-conditioner [56].

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Table 2 Capital and operating costs of the HVAC system with and without a RAMEE in an office building located in Saskatoon, Chicago, Miami, and Phoenix [96]. Component

Saskatoon

Capital costs ($US) Heating Sys. Cooling Sys. Fans RAMEE Sys. Total Change ($US/m2)

Chicago

Phoenix

HVAC

RAMEE

HVAC

RAMEE

HVAC

RAMEE

HVAC

RAMEE

192,200 199,200 76,000 0 467,400 +3.6% 16.2

148,100 188,400 76,000 72,000 484,500

133,000 293,400 80,400 0 506,800 +7.5% 17.6

99,200 293,400 80,400 72,000 545,000

30,700 470,400 81,600 0 582,700 +3.7% 20.2

19,100 432,000 81,600 72,000 604,700

54,600 331,200 108,000 0 493,800 +8.5% 17.1

38,900 316,800 108,000 72,000 535,700

39,772 154,522 194,294 11.6%

23,568 148,155 171,723

16.8

Operating costs ($US/year) Natural gas 21,432 Electricity 73,869 Total 95,301 Change 11%

14,880 69,947 84,827

18.9

21

640 103,263 103,903 5%

477 98,189 98,666

18.6

6,778 109,005 115,783 8.4%

2,819 103,220 106,039

Table 3 Capital and operating costs of the conventional and hybrid air-conditioners for a 5passenger vehicle [56].

1,200,000 1,000,000

Capital costs

800,000

$ US

Miami

600,000

Conventional system

Hybrid system

Component

Component

Cost (€)

Refrigeration cycle 800 [119] Heat pump 2 membrane contactors Tank and pipes Recovery and mixing unit Total 800 Total

400,000 200,000 0 Saskatoon

Chicago Without RAMEE

Miami

Phoenix

Cost (€) 400 300 50 150 900

Operating costs

With RAMEE

Item Fig. 20. Total life cycle costs of an HVAC system, with and without RAMEE, in a hospital in Saskatoon, Chicago, Miami, and Phoenix [116]. Overconsumption due to airconditioner (L/100 km) Mileage (km/year) Average fuel cost (€/L) Maintenance (annual) Total (€/year)

80,000 70,000

Cost (€) Conventional system

Hybrid system

2 [118]

1.6

10,000 1.2 20 260

10,000 1.2 30 222

60,000

$ US

50,000 40,000 30,000 20,000 10,000 0 CAC

CAC-ERV Capital Costs

M-LDAC

Annual Operating Costs

M-LDAC-ERV Total Costs

Fig. 21. Capital, annual operating, and total life cycle costs of the conventional and hybrid air-conditioning systems proposed by Abdel-Salam et al. [110].

Overall, it can be concluded that the economics of LAMEE systems (i.e. RAMEEs or membrane liquid desiccant air-conditioners) installed in buildings, strongly depend on the outdoor air conditions. Thus, the economic performance of a LAMEE system will vary from one city to another. 6.3. Automotive hybrid liquid desiccant air-conditioners Investigating the economics of automotive air-conditioners is quite complicated as it depend on many factors (e.g. outdoor air conditions, fuel price, mileages, driving style, car model, number of passengers, etc.). Generally, a conventional air-conditioner increases the fuel consumption by 1–3 L/100 km [118]. Vestrelli [56] compared the economics of a conventional automotive air-

conditioner with the proposed automotive hybrid liquid desiccant air-conditioner presented in Section 5.2.2. The economic study was conducted on a 5-passenger vehicle with an annual mileage of 30,000 km including 10,000 km during the summer, at outdoor air conditions of 30 °C & 60% RH, and a fuel overconsumption of 2 L/100 km due to the conventional air-conditioner [118]. Table 3 shows the capital and operating costs of the conventional and hybrid air-conditioners. Results showed that the hybrid air-conditioner consumed less fuel than the conventional air-conditioner by 20%, with a payback period of 3 years. When the number of passengers increased to 8, the hybrid air-conditioner fuel savings increased to 30% with a payback period of 2 years. 7. Indoor air quality Indoor air quality has become a crucial issue in recent years. One of the main objectives of an HVAC system is to keep the indoor air quality at acceptable levels. Poor indoor air quality, due to the presence of some gaseous pollutants such as CO, CO2, SO2, NOx, O3, radon, and VOCs, has several adverse impacts on occupants’ health and performance. For instance, VOCs may cause headaches, dry coughs, dizziness, nausea, tiredness, and eye, nose, and throat irritations [120]. Table 4 gives detailed information about sources and adverse impacts of some typical pollutants in modern buildings.

215


Pollutant

Sources

Health impacts

CO2

Metabolic activities, combustion, humans, and pets Particle board, carpet, insulation, plywood, ceiling tile, tobacco, and smoke Aerosol sprays and cleaning products

Acidosis and irregular breathing

Formaldehyde

1,1,1-Trichloroethane

Toluene

20

(a)

18 16 14

Ton CO2

Table 4 Sources of different pollutants and their impacts on human health [121,122].

Eye, nose, and throat irritation, rashes

The type and amount of harmful emissions associated with energy consumption mainly depend on the type of fuel (e.g. coal, oil, natural gas, clean sources) used for energy generation. The environmental impacts of LAMEEs can play a crucial role on

8

4 2 0

CO2 CAC

(b)

8. Environmental impacts

10

6

Dizziness, cancer, eye irritation, and irregular breathing Cancer, fatigue, eye Wall paper, wall covering, paint, printer, floor covering, irritation, and headache and chipboard

CAC-ERV

M-LDAC

M-LDAC-ERV

100 90 80

Emissions (kg)

Chung et al. [121] reported that because some inorganic gases are soluble in water, there is a potential for some pollutants to be absorbed by the desiccant solution during the cooling and dehumidification process. Welty [123] mentioned that liquid desiccant solutions can trap germs and kill them. It has been reported in [124] that liquid desiccant air-conditioners can eliminate up to 91% of the airborne microorganisms and 80% of particles larger than 5 lm, which results in higher levels of indoor air quality. Chung et al. [125] used a direct-contact liquid desiccant system to simultaneously dehumidify the supply air and remove some pollutants (i.e. toluene, 1,1,1-trichloroethane, CO2, and formaldehyde) from it. The system was composed of a packed bed as a dehumidifier and triethylene glycol with a concentration of 95% was employed as the desiccant solution. The concentrations of the formaldehyde, toluene, 1,1,1-trichloroethane, and CO2, in the supply air were 0.02 ppm, 3 ppm, 24 ppm, and 1000 ppm, respectively. Results revealed that the system removed 100% of the toluene and 1,1,1-trichloroethane, 56% of CO2, and 30% of formaldehyde. Moreover, varying the relative humidity of the air did not have any impact on the pollutants removal rate. Vestrelli [56] reported that Isetti et al. [126] used a semi-permeable membrane, fabricated from a layer of PTFE and supported by a layer of PP fibers, with K2CO3 solution for NO2 absorption, and with a mixture of LiNO3 and KMnO4 solution for simultaneous absorption of NO2 and H2S from the ambient air. Results revealed that there was a promising potential for using semi-permeable membranes with hygroscopic solutions to improve the indoor air quality. Moreover, Zhang et al. [127] reported that the material and properties of semi-permeable membranes have great influences on reducing the transfer of some pollutants (i.e. acetic acid, formaldehyde, acetaldehyde, toluene, and ethane) to the supply airstream. They found that the PVA-1, PVP, and PAM membranes are the best choices for use in air-to-air energy exchangers, because of their high water vapor permeability and high selectivity. In conclusion, liquid desiccant systems not only decrease the energy consumption of the HVAC systems, but may also improve the indoor air quality by removing some pollutants from the supply air. Further research is needed to determine the capability of the LAMEE systems with typical desiccant solutions (i.e. LiCl and MgCl2 solutions) to remove pollutants from the supply air.

12

70 60 50 40 30 20 10 0

CO CAC

NOx CAC-ERV

SOx PM M-LDAC M-LDAC-ERV

Fig. 22. Simulated (a) CO2 emissions and (b) CO, NOx, SOx, and particulate (PM) emissions of four air-conditioning systems in an office building in Miami, Florida, USA [110].

promoting its applications, as a number of countries have implemented a carbon tax where users are charged for CO2 emissions. For example, in Australia, a carbon tax of 23 AUD/tCO2e was implemented in 2012 for industrial emitters [128]. Rasouli [116] conducted an environmental impact assessment of installing a RAMEE with a VAV HVAC system in a 3-storey hospital (with a total floor area of 3150 m2) located in four NorthAmerican cities (i.e. Saskatoon, Chicago, Miami, and Phoenix). They presented some interesting facts to get a better understanding of the environmental impacts of installing the RAMEE system. A mature tree absorbs around 21.6 kg CO2/year [129] and a new mid-size car emits 165 g CO2/km [130]. The environmental impacts of installing the RAMEE in the 3-storey hospital are equivalent to planting of 5450, 3440, 1850, and 1490 trees or removing 36, 23, 12, and 10 cars that travel 20,000 km/year, from the roads in Saskatoon, Chicago, Miami, and Phoenix, respectively. Abdel-Salam and Simonson [110] presented an environmental impact assessment of the four simulated air-conditioning systems (i.e. CAC, CAC-ERV, M-LDAC, and M-LDAC-ERV) mentioned in Section 5.2.1. Fig. 22(a) and (b) shows that the M-LDAC-ERV system produced the lowest emissions compared to the other three systems.

9. Conclusions The current paper presents a review of the applications, energy savings, economics, and environmental impacts of LAMEEs for energy recovery and air dehumidification in buildings and automotive air-conditioners. The LAMEE applications discussed show a great potential to reduce the capital and operating costs of HVAC systems. The following are the most important conclusions drawn from this literature review:

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1. A RAMEE system, comprised of supply and exhaust LAMEEs in a run-around system, can significantly reduce the capital and operating costs of an HVAC system, by downsizing the HVAC equipment, and reducing the annual heating and cooling energy consumptions, respectively. 2. RAMEEs can significantly reduce the amount of cross contamination between exhaust and supply air flows during energy recovery. 3. A LAMEE-based hybrid liquid desiccant air-conditioner is able to save a significant amount of energy consumption in buildings, compared to a conventional air-conditioner. 4. An automotive hybrid air-conditioner shows promising potential for reducing the fuel consumption of air-conditioners in automobiles. 5. LAMEEs can significantly reduce the amount of harmful emissions associated with energy consumption in buildings. 6. Liquid desiccants are able to absorb some pollutants from the supply air, which implies that LAMEE applications not only reduce the cooling and heating energy consumption, but may also improve the indoor air quality compared to other systems. 10. Suggested topics for future research Based on the current literature review of the research and development of LAMEEs, some recommendations are proposed for future research as follows. 1. For specific HVAC application, determine the optimum design of the supply and exhaust LAMEEs in a RAMEE system. 2. For specific HVAC application, investigate the performance of a RAMEE composed of two hollow-fiber LAMEEs. 3. Investigate the transient performance of LAMEE-based hybrid air-conditioners. 4. Study a stand-alone air dehumidification system with a LAMEE for air dehumidification and a solar thermal system for desiccant solution regeneration. 5. Investigate the ability of LAMEEs with typical liquid desiccant solutions to absorb pollutants (e.g. CO, CO2, Toluene, Formaldehyde, SOx, NOx, VOCs), from the supply air during the cooling and dehumidifying process.

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