Analysis of resorption working pairs for air conditioners of electric vehicles

Applied Energy xxx (2017) xxx–xxx

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Analysis of resorption working pairs for air conditioners of electric vehicles L.W. Wang ⇑, L. Jiang, J. Gao, P. Gao, R.Z. Wang Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China

h i g h l i g h t s
Resorption cycle is proposed for the air conditioners (ACs) of electric vehicles (EVs).
Intermittent working modes of the cycle won’t consume the electricity of on-board batteries.
Resorption working pair of CaCl2-NH4Cl-NH3 has reasonable energy density and high COP.
Energy consumption of resorption AC is reasonable if compared with conventional AC of EVs.

a r t i c l e

i n f o

Article history: Received 5 January 2017 Received in revised form 13 June 2017 Accepted 27 June 2017 Available online xxxx Keywords: Electric vehicle Air conditioner Resorption Halide Energy density COP

a b s t r a c t Conventional compression type air conditioners (ACs) consume a large part of the electricity of batteries on-board of electric vehicles, and that will make the cruising mileage shorter. Sorption and resorption cycles, which are intermittent, may solve this question by the energy storage phases. Both sorption and resorption cycles are analyzed and compared, and both of them have simpler structure if compared with conventional AC for that only two heat exchangers are required. The equilibrium performance analysis shows that resorption working pairs has higher energy density and coefficient of performance (COP) than that of sorption working pairs when the high temperature salt of resorption cycle is same with the halide of sorption cycle. The experimental Clapeyron curves are studied, and CaCl2-NH4Cl-NH3 has best performance. Compared with MnCl2-CaCl2-NH3 and MnCl2-NH4Cl-NH3, the energy density and COP of CaCl2-NH4Cl-NH3 improves by 160% and 35% at least, respectively. The performance of CaCl2-NH4Cl-NH3 is also compared with that of CaCl2-NH3. They have similar smallest energy density, and CaCl2-NH4Cl-NH3 has higher COP if consider the working conditions in the whole year. The energy required for the electric car with a resorption AC is 0.23–0.265 kWh/km, which is acceptable if compared with the results of conventional AC. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Along with the lack of petroleum in the world, serious pollution of atmosphere, and the advance of battery technology, electronic vehicles (EVs) have been recognized as the main way of alteration and development of automobile industry in 21st century [1,2]. But EVs have two shortcomings of short driving distance and high cost owing to the batteries. The reducing energy consumption of onboard batteries in driving process is paramount for the long cruising mileage [3]. Air conditioner (AC) is the important component in the EVs to keep the cabin comfort. Currently the AC in EVs is mainly conventional compression type AC driven by the electricity of batteries ⇑ Corresponding author. E-mail address: [email protected] (L.W. Wang).

onboard. It was reported that the air conditioning system constituted the major energy consumption of a EV, which was up to 65% [4]. More specifically, for EV with passengers the energy consumption due to air conditioning system can reduce the overall driving range by 40–60% under typical standard driving testing conditions [5,6]. But the research nowadays mainly focusses on increasing the comfort level other than reducing the energy consumption of the ACs for EVs. For example, Miranda et al. designed a new climate control system by using a direct energy conversion principle in the commercialization of modern EVs [7]. Chiu et al. investigated the regulation problem of thermal comfortableness and proposed control strategies for cabin environment. Results proved that the performance of the near optimal order-reduced control law was the most suitable method as that of standard LQR (LinearQuadratic Regulator) [4]. Wang et al. established an integrated

http://dx.doi.org/10.1016/j.apenergy.2017.06.077 0306-2619/Ó 2017 Elsevier Ltd. All rights reserved.

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Nomenclature a, b, n, m reaction equilibrium constants Cp specific heat capacity (kJ/(kg K)) C pAmðNH3 Þ;hal specific heat capacity of ammoniate compound with 1 kg sorbed ammonia (kJ/(kgK)) COP coefficient of performance E energy density ENG-TSA expanded nature graphite treated with sulfuric acid HTH high temperature halide LTH low temperature halide mhal mass of halide p pressure (Pa) Q heat quantity or refrigeration quantity (kJ) T temperature (K, °C) x sorption quantity DH evaporating latent heat of ammonia or the reaction heat for desorption process (kJ)

AC/HP (Heat Pump) system and applied it in a compact EV for improving the heating performance [8]. Qin et al. studied the traditional air source heat pump for EVs, and developed a refrigerant injection air source heat pump system in the cold region, which improved the heating capacity by 28.6% compared with the traditional system [9]. The ACs studied in all the references above have one common feature on the electricity input, which is provided by the on-board battery [10] because the cooling power output and electricity input processes proceed simultaneously. If the electricity consumption and cooling power output phases of AC are able to be fulfilled by other alternative technologies separately, the energy consumption of AC can be provided by the mains electricity other than the batteries of EVs, and that means the cooling power energy can be stored in the system when a EV is charged by the mains electricity. Then the drawback of short driving distance caused by the AC will be overcome and the potential of the EVs will be further expected. The sorption refrigeration technology provides the possibility for that. The sorption technology driven by the heat is recognized as one of the most prospective energy conversion technologies, which manifest various functions of refrigeration [11], energy storage [12,13], and electricity generation [14]. The intermittent working modes for heating and cooling of a sorption cycle [15,16] make it possible for serving as a type of energy storage AC [17,18]. It takes the advantages of high energy density, little heat loss, time and space discrepancy adjustment, which happens to overcome the problems for conventional EVs’ AC [19]. High energy density accords with the concept of lightweight while time and space discrepancy adjustment with little heat loss provides more flexibilities. The research nowadays mainly focuses on the thermal energy storage with different sorption working pairs. Such as that Lourdudoss et al. [20] evaluated the three phase sorption cycle for thermal energy storage, which indicated that three phase sorption processes enjoyed a higher heat supply when compared with the sensible and latent heat energy storage. But the study didn’t analyze the cooling power supply, which is required by the AC of EVs. Yu et al. [21] investigated on LiCl2-water sorption thermal energy storage system for combined heat and cold output. Results showed that the novel composite adsorbent could reach a cold and heat storage density of 108 kWh/m3 and 163.6 kWh/m3, respectively. Such a working pair cannot be used for the EVs as well because the volume of the system is quite big for the low density of the sorbent. Later, an innovative dual-mode ammonia thermochemical energy storage cycle was proposed for seasonal storage

Subscripts con condensation de, des desorption desL desorption state of LTH env environmental ev evaporation hal halide HP heat pump ref refrigeration res resorption sor sorption sorL sorption state of LTH sum summer win winter

and heat supply with little heat loss. It was indicated that coefficient of performance (COP) for heat storage could reach 0.6, and energy density was higher than 1000 kJ/kg under different working conditions [22]. Nonetheless, there will be one possible deficiency of such technology because ammonia is used as working fluid. The safety problem caused by liquid ammonia will be difficult to be dealt with for the bumps and vibrations of the EVs in the driving process. Similar as sorption technology, resorption technology also could be applied for refrigeration and thermal energy storage. The basic resorption cycle is composed of two reactors with different halides, and evaporator/condenser is replaced by a reactor. Compared with sorption systems, resorption system has no ammonia liquid in the system [23]. Bao et al. [24] established one resorption system for cold storage and long distance air conditioning transportation. Results pointed out that COP under different conditions ranged between 0.20 and 0.31. Li et al. [25] analyzed MnCl2-CaCl2-NH3 resorption cycle, and indicated its potential for high energy storage density. Moreover, our previous work have investigated the characteristics of resorption system for both direct heat supply [26] and combined heating and cooling modes [27]. Experimental results indicated that the largest energy storage density could reach 1706 kJ/kg. The maximum average cooling power achieved 1.07 kW during cold releasing phase. But there is no research work for adopting such type of cycles for AC of EVs. In order to verify the feasibility of such types of ACs for EVs, based on our previous research on resorption system, different sorption and resorption working pairs are analyzed. The Clapeyron curves and sorption/desorption properties of sorbents are both tested under non-equilibrium conditions. Based on the results, the performance of resorption energy storage AC for EVs is analyzed.

2. Working principle of the cycles and development of sorbents The conventional compression type AC includes the compressor, evaporator, condenser, liquid receiver, and expansion valve. The refrigerant vapor needs to be compressed by the compressor, and such a process consumes the electricity of batteries when the EVs are in motion, and will make the cruising mileage shorter. Comparably, the sorption and resorption technology will separate the heat input (that is provided by the electricity) and cooling power output into intermittent phases, then the heat will be pro-

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vided by the mains electricity other than electricity of batteries of EVs. Both sorption and resorption cycles for AC of EVs are shown in Figs. 1 and 2. Figs. 1a and 2a shows that both sorption and resorption cycles are simpler than conventional compression AC for that only two heat exchangers are required in a cycle, while two heat exchangers and one compressor are involved in one conventional AC. The structure of sorption cycle (Fig. 1a) is similar to that of resorption cycle (Fig. 2a). The equilibrium Clapeyron diagrams for both sorption and resorption cycles are shown in Figs. 1b and 2b. The equilibrium lines are referred from the Ref. [28]. The reaction processes shown in Figs. 1b and 2b are monovariant. The universal reaction formula for the complex reaction between halide and NH3 is as follows [29]:

Ma Xb ðNH3 Þn þ ðm  nÞNH3 () Ma Xb ðNH3 Þm

ð1Þ

where M represents metal elements, X represents Cl, and a, b, n, m are the reaction equilibrium constants. The whole working processes include the synthesis and decomposition processes. For example, in Fig. 1b at point d MnCl2 (NH3)2 is cooled by the environmental heat transfer fluid and reacts with NH3, and the product is MnCl2(NH3)6. At point c MnCl2 (NH3)6 is heated by the electricity, and is decomposed into MnCl2(NH3)2 and NH3. 2.1. Working processes of sorption cycle and choice of working pairs The sorption cycle shown in Fig. 1a shows that only one reactor and one condenser/evaporator are involved. In summer time the environmental temperature is always higher than 30 °C. The cooling and condensing temperature is taken as 35 °C. For the heating process reactor is heated by the mains electricity when the EVs are charging the electricity, and the temperature can be as high as around 200 °C. In regard of the safety, the highest desorption temperature is taken as lower than 180 °C. Taking the line 5 in Fig. 1b as the example, in the summer time the working processes are as follows: (1) In the night the reactor of halide is heated by the mains electricity, and as Fig. 1a shows that the condenser is cooled by the cooling air through the pipeline connected with V3, V4 and Fan 1. The sorbed refrigerant is desorbed in the reactor

(point c in Fig. 1b) and flows to the condenser through valve of refrigerant, and then condenses there (point b in Fig. 1b). There are five types chlorides with the highest desorption temperature lower than 180 °C, i.e. chlorides 1–5 (Fig. 1b). (2) In the daytime of summer when the refrigeration effect is required, the chilling air exchanges the heat with evaporator through the pipeline with Fan 2, V7, and V8, while the reactor of halide is cooled by the air through the pipeline of Fan 1, V1, and V2 (Fig. 1a). The reactor sorbs (point d in Fig. 1b) the refrigerant in the evaporator, and the evaporating (point a in Fig. 1b) effect provides the cooling power. Such a process doesn’t need the energy input anymore. As mentioned before in sorption phase the sorption temperature should higher than 35 °C, and Fig. 1b shows that Four halides (2– 5) can satisfy the refrigerating requirement. For the winter time, the environmental temperature is around 10 °C, which is condensing temperature as well as the evaporation temperature for the AC. The working process in the night is similar with process (1) for summer time. In the daytime the heat pump effect will be provided by the sorption heat of the reactor. The sorbent inside the reactor sorbs the refrigerant from the evaporator. As Fig. 1a shows, the evaporator absorbs the heat from environmental air by the air flowing process through Fan 1, V3, and V4. The air in the cabin flows through the reactor by the pipeline of Fan 2, V6, and V5, and absorbs the reaction heat, which is released inside the cabin for providing the heating effect. Fig. 1b shows that the sorption and desorption happens as the equilibrium sorption/ desorption points in winter (such as point g for sorption/desorption and point e for condensation/evaporation) show. The highest desorption temperature is 180 °C, and the lowest sorption temperature should be higher than 25 °C, which is the temperature controlled in the cabin of the car. Fig. 1b shows that chlorides 1–6 can satisfy the conditions.

2.2. Working processes of resorption cycle and choice of working pairs Resorption cycle shown in Fig. 2a consists of two reactors, and the working processes are explained by the working pair of MnCl2-NH4Cl-NH3 shown in Fig. 2b. In the summer two phases are as follows:

(a)

(b)

0:NH3, 1:NH4Cl(3-0 NH3), 2: Ba(8-0 NH3), 3: Ca(8-4 NH3), 4: Ca(4-2 NH3), 5: Mn(6-2 NH3), 6: Fe(6-2 NH3), 7: Mg(6-2 NH3), 8: Ni(6-2 NH3) Fig. 1. Sorption cycle for EVs (a) working principle; (b) Clapeyron diagram.

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Fig. 2. resorption cycle for EVs (a) working principle; (b) Clapeyron diagram.

(1) In the night the reactor of high temperature halide (HTH) (MnCl2)is heated by the electric heater, and the reactor of low temperature halide (LTH) (NH4Cl) is cooled by the cooling air through the pipeline connected with V3, V4 and Fan 1. The sorbed refrigerant is desorbed by the HTH (point c1) and flows to the reactor of LTH through valve of refrigerant, and then is sorbed there (point b1). For the resorption working pair the sorption/desorption rate is larger when the temperature difference of two types of halides is bigger (Fig. 2b), considering of this feature only four HTHs can be chosen for NH4Cl, which are 4–7. (2) In the daytime of summer when the refrigeration effect is required, the chilling air exchanges the heat with LTH through the pipeline of Fan 2, V7, and V8 (Fig. 2a), while the reactor of HTH is cooled by the air through the pipeline of Fan 1, V1 and V2. HTH of MnCl2 sorbs (point d1 in Fig. 2a) the refrigerant in the reactor of LTH, and the desorption effect of LTH (point a1 in Fig. 2b) provides the cooling power. The refrigeration is generated by the desorption heat of LTH. Halides of 4–7 (Fig. 2b) can satisfy the condition of sorption temperature higher than 35 °C. It should be paid attention for that the conditions will change if different LTH is chosen. For example, if the CaCl2 is chosen as the LTH, in the night the sorption point of LTH is b2 and the desorption point of HTH (taking MnCl2 as the example) is c2. In the daytime the desorption refrigeration point of LTH is a2 and the sorption phase of HTH is d2. The available HTHs are 5–8. For the winter time, the working phase in night is similar with the process (1) for summer time. In the daytime the environmental temperature is the desorption/sorption temperature of LTH, and the heat pumping effect will be provided by the sorption heat of HTH. As Fig. 2a shows, the reactor of LTH exchanges the heat with environmental air by Fan 1, V3, and V4, and the air in the cabin of the EVs exchanges the heat with reactor of HTH by the pipeline of Fan 2, V6, and V5. HTH sorbs the refrigerant from the reactor of LTH, and the sorption heat of HTH provides the air conditioning effect. The sorption and desorption of HTH happens as the ‘‘equilibrium sorption/desorption points of HTH in winter” (such as point g1) shows in the Fig. 2b. The highest desorption temperature of HTH is 180 °C, and the lowest sorption temperature of HTH should be higher than 25 °C. Fig. 2b shows that halides 4–7 can satisfy the

conditions. Again if the halide of CaCl2 is taken, the conditions will change (such as e2 and g2 shows), and HTHs of 5–8 will be available. In order to verify the performance of halides with different reaction temperatures, three types of chlorides are chosen for the analysis, and they are the low temperature salt of NH4Cl, middle temperature salt of CaCl2, and high temperature salt of MnCl2. 2.3. Theoretical performance analysis The development procedures of the composite sorbents with expanded natural graphite treated with sulfuric acid (ENG-TSA) are as follows: (1) The halides are mixed with the water. After that the ENGTSA is mixed in the mixture of water and halides. (2) The composite is dried in the oven at the temperature over than 120 °C for about 4 h, and then the composite is consolidated by compression. In the analysis the energy density per kilogram sorbent (E) and COP are considered. For the energy storage type AC, the desorption time can be as long as that of the electricity charging process of EVs, and the sorption time is also long enough which is same to the time for the continue voyage course, and the whole cycle time is as long as about 6 h. For such a long cycle time the energy storage quantity of kJ is more important than kW, so for the E the unit of kJ/kg is used. The bulk density of halide will be different when the density of composite sorbent (including the halide and ENG-TSA) is different. For the composite sorbent the ENG-TSA is the matrix and doesn’t have sorption ability, so the sorption performance depends only on the properties of halides. Moreover the mass of ENG-TSA for different hylides is commonly same, thus the bulk density of halide (kg halide/m3 composite sorbent) is generally taken for evaluating the sorption performance of the system. Considering the reasons above the mass of halide is taken to calculate the energy density instead of the mass of composite sorbent. 2.3.1. Models for the sorption cycle In summer time the cooling effect is provided by the evaporating heat in the evaporator. The sensible heat of the refrigerant

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liquid from the condenser to evaporator will consume a part of the refrigeration power. The desorption heat is provided by the mains electricity. The sensible heat of the sorbent and ammoniate in the reactor can be neglected in the calculation. The equations for refrigeration in summer are calculated by 1 kg halide, and the equations are as follows:

Q ref xcycle  mhal  ½DHNH3  C pNH3  ðT env  T ref Þ ¼ mhal mhal ¼ xcycle  ½DHNH3  C pNH3  ðT env  T ref Þ ð2Þ

Esor;summ ¼

COPres;summ

COP sor;summ ¼ ¼

Q ref ¼ ½xcycle;LTH  DHdes;LTH  ðxsor;LTH  C pAmðNH3 Þ;LTH þ 1=4 mHTH   xcycle;HTH ð6Þ  C pENG-TSA Þ  ðT env  T ref Þ  xcycle;LTH

Eres;summ ¼

For resorption cycle in winter time the heat is provided by the sorption heat of HTS. The Eres,win and COPres,win are calculated by the equations:

 C pENG-TSA Þ  ðT sor  T env Þ

xcycle  ½DHNH3  C pNH3  ðT env  T ref Þ xcycle  DHdes þ ½xsor;hal  C pAmðNH3 Þ;hal þ 1=4  C pENG-TSA   ðT de  T env Þ

where Qref is the refrigerating quantity, mhal is the mass of halide, xcycle is the cycle sorption quantity, i.e. kg ammonia per kg halide. DH is the evaporating latent heat of ammonia or the reaction heat for desorption process, Cp is the specific heat capacity, Tenv is the environmental temperature, Tref is the refrigeration temperature, C pAmðNH3 Þ;hal is the heat capacity of ammoniate compound with 1 kg sorbed ammonia, and subscript of sor, hal, and summ are for sorption, halide, and summer respectively. In winter time the heat is provided by the sorption heat of sorbent reactor. The sensible heat of the ENG-TSA in the sorption reactor will consume a part of the sorption heat. The Esor,win and COPsor,win are calculated by the equations:

Q HP ¼ xcycle  DHsor  ½xsor;hal  C pAmðNH3 Þ;hal þ 1=4 mhal  C pENG-TSA   ðT sor  T env Þ

ð7Þ

Eres;win ¼ xcycle;HTH  DHsor;HTH  ðxsor;HTH  C pAmðNH3 Þ;HTH þ 1=4

ð3Þ

Esor;win ¼

In order to make a comparison with sorption cycle, 1kg HTH is taken as the reference for the calculation of the performance. The equations are as follows:

  xcycle;HTH Q ref ½xcycle;LTH  DHdes;LTH  ðxsor;LTH  C pAmðNH3 Þ;LTH þ 1=4  C pENG-TSA Þ  ðT env  T ref Þ  xcycle;LTH ¼ ¼ xcycle;HTH  DHdes;HTH þ ðxsor;HTH  C pAmðNH3 Þ;HTH þ 1=4  C pENG-TSA Þ  ðT de  T env Þ Q de

Q ref Q de

5

ð4Þ

Q HP Q de xcycle  DHsor  ½xsor;hal  C pAmðNH3 Þ;hal þ 1=4  C pENG-TSA   ðT sor  T env Þ ¼ xcycle  DHdes þ ½xsor;hal  C pAmðNH3 Þ;hal þ 1=4  C pENG-TSA   ðT de  T env Þ

COP sor;win ¼

ð5Þ

2.3.2. Models of resorption cycle In resorption cycle the cooling effect is provided by the desorption heat of LTH. The sensible heat of the ENG-TSA, which decreases from environmental temperature to refrigeration temperature will consume a part of the refrigeration power. The desorption heat is provided by the mains electricity, and the sensible heat of the ENG-TSA in the reactor needs to be considered in the calculation. In order to make a comparison with the sorption cycle which only has one sorbent reactor, in the resorption cycle only one reactor is considered in the calculation of the energy density, which is the HTH reactor that consumes the electricity for heating phase.

COP sor;win ¼

ð8Þ

xcycle;HTH  DHsor;HTH ðxsor;HTH C pAmðNH3 Þ;HTH þ1=4C pENG-TSA ÞðT sor T env Þ xcycle;HTH  DHdes;HTH þ½xsor;HTH C pAmðNH3 Þ;HTH þ1=4C pENG-TSA ðT de T env Þ

ð9Þ

2.3.3. Theoretical performance comparison of sorption and resorption cycle As mentioned previously the desorption temperature is taken as 180 °C. In summer time the cooling and condensing temperature is 35 °C, and refrigeration temperature is 5 °C. For the winter time the environmental temperature is taken as 10 °C and the air conditioning temperature (sorption temperature) is taken as 25 °C. Under these conditions the cycle sorption quantity of NH4Cl, CaCl2, and MnCl2 are 3, 6, and 4 mol NH3/mol halide, respectively. The theoretical energy density and COP are calculated for different sorption and resorption working pairs, and the results are shown in Fig. 3. Fig. 3 shows that the resorption cycle has higher COP and energy density than that of sorption cycle in summer time when the HTH of resorption cycle is same as the halide of sorption cycle. For example, when MnCl2 is chosen as the HTH, the COP and energy density of MnCl2-CaCl2-NH3 are 0.59 and 1231 kJ/kg respectively, whereas for the MnCl2-NH4Cl-NH3 are 0.38 and 803 kJ/kg separately. If compared with the data of MnCl2-NH3, which are 0.34 and 707 kJ/kg, the data is improved by resorption cycle significantly. Similarly, when CaCl2 is taken as the HTH, i.e. for CaCl2-NH4Cl-NH3 in summer time the COP and energy density are 0.44 and 1354 kJ/kg, which are improved by about 13% compared the data of CaCl2-NH3, which are 0.39 and 1192 kJ/kg. In winter time the energy density and COP of resorption cycles that utilize HTH as same as halide of sorption cycles, such as MnCl2-CaCl2-NH3, MnCl2-NH4Cl-NH3, and MnCl2-NH3, have same energy density and COP (1445 kJ/kg and 0.69). The resorption cycle of CaCl2-NH4Cl-NH3 also has the same data with sorption cycle of CaCl2-NH3 (2178 kJ/kg and 0.7). Compared with other working pairs NH4Cl-NH3 has highest COP in summer time for its smaller reaction heat in desorption phase, but it has smallest COP in winter time again for the relatively low reaction heat, which provides the air conditioning effect in the cabin of the EVs.

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Fig. 3. Theoretical energy density and COP of different sorption and resorption working pairs.

Considering both summer time and winter time, resorption cycles have better performance in summer time and same performance in winter time. Meanwhile resorption cycle has no refrigerant liquid and also has much lower working pressure than that of sorption cycle (as shown in Fig. 2b), which is a safety feature especially for the desorption phase in summer time that has the highest pressure in the whole year. Thus the resorption cycles are analyzed for the energy storage type ACs in the EVs. The optimal sorption working pair, which is CaCl2-NH3 with the best performance in winter and reasonable performance in summer, is also analyzed for the comparison with resorption working pairs. 3. Sorption performance of different resorption working pairs

tor 1 desorbs to the condenser and condenses there, and the data is collected by the data collector. For a fixed condensing temperature, the desorption process of sorbent completes when the level in the refrigerant vessel reaches a predetermined value. (2) After the first step the valve between the condenser and sorption reactor is closed. The temperature of the sorbent reactor is cooled by the cryostat from the highest desorption temperature to the environmental temperature, and for each test point the temperature and pressure of the reactor are collected when the data doesn’t change for five minutes. More information on experimental rig, collecting experimental data and calibration of test data is shown in Ref. [30].

3.1. Testing procedures 3.2. Performance of different resorption working pairs The sorption performances of different sorbents are tested by the test unit shown in Fig. 4a and b. The test unit has two sorption reactors, one refrigerant vessel, three cryostats, a pressure transmitter and valves, etc. The refrigerant vessel acts as condenser or evaporator, depending on the operation conditions. Temperature of sorption reactor and the refrigerant vessel are controlled by three cryostat baths. The Clapeyron curves are tested by the following procedures: (1) The temperature of refrigerant vessel (serves as condenser) is controlled by one cryostat at the condensing temperature. The sorbent is filled in the sorption reactor 1, for which the temperature is controlled by one cryostat at the desorption temperature. The refrigerant sorbed by the sorbent in reac-

The experimental Clapeyron curves of resorption working pairs are shown in Figs. 5–7. 3.2.1. Performance of working pair of MnCl2-NH4Cl-NH3 The properties and working processes can be demonstrated by the working pair of MnCl2-NH4Cl-NH3, for which the experimental data is shown in Fig. 5. The experimental Clapeyron curves are bivariant (Fig. 5) other than monovariant as shown in Fig. 2. Fig. 5a is for the summer time. The performance is as follows: (1) For the night: The working processes start from 1–2 and end at 3–4. The ammoniate HTH (MnCl2) is heated and desorbed. The start point of desorption is at point 1. The ammoniate

Fig. 4. Test unit of sorption performance.

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Fig. 5. Performance of MnCl2-NH4Cl-NH3 (a) summer time, (b) winter time.

7

Fig. 7. Performance of CaCl2-NH4Cl-NH3 (a) summer time, (b) winter time.

In the resorption process the sorption quantity of NH4Cl increases from point 2 to point 4 by an isothermal process, and the cycle sorption quantity is 0.63 kg/kg. Meanwhile the sorption quantity of MnCl2 decreases from point 1 to point 3 in the heating process (temperature increases). (2) For the daytime: The temperature of ammoniate NH4Cl is controlled at the refrigeration temperature of 5 °C, and the ammoniate MnCl2 is cooled by the outside cooling source with the lowest cooling temperature of 35 °C. The largest sorption quantity of MnCl2 is controlled by the cooling temperature, i.e. at point 5, which is roughly 0.48 kg NH3/kg MnCl2. The smallest sorption quantity of MnCl2 is at point 3 and 7, which is 0.27 kg/kg, and the cycle sorption quantity is 0.21 kg/kg. In winter time the environmental temperature is 10 °C, and the heating temperature inside the cabin of EVs is controlled at 25 °C. The working processes are shown in Fig. 5b as follows:

Fig. 6. Performance of MnCl2-CaCl2-NH3 (a) summer time, (b) winter time.

LTH (NH4Cl) is cooled by the cooling source with the temperature of 35 °C, and the initial point, which is point 2, has the smallest sorption quantity of 0.32 kg NH3/kg NH4Cl.

(1) For the night: Desorption of MnCl2 and sorption of NH4Cl start at the point of 9 and 10, respectively. The sorption quantity inside NH4Cl increases from point 10 to point 11, and the desorption quantity of MnCl2 decreases from point 9 to point 12. When the resorption is completed the cycle sorption quantity of NH4Cl (between lines of 0.32 and 0.95 kg/kg) is 0.63 kg/kg. For MnCl2 the cycle sorption quantity increases slightly if compared with Fig. 5a for that the cooling temperature decreases from 35 °C to 25 °C, and it is roughly 0.23 kg/kg (between the lines of 0.5 and 0.27 kg/kg). (2) For the daytime: The ammoniate NH4Cl desorbs (from 11 to 10) and ammoniate MnCl2 sorbs (from 12 to 9). The sorption heat of MnCl2 provides heating power.

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3.2.2. Performance of working pair of MnCl2-CaCl2-NH3 and CaCl2NH4Cl-NH3 The performance of MnCl2-CaCl2-NH3 is shown in Fig. 6a. By the similar analyzing method for MnCl2-NH4Cl-NH3, the refrigeration and heat pumping performances are as follows: (1) Refrigeration in summer: The ammoniate CaCl2 has the smallest sorption quantity of around 1 kg NH3/kg CaCl2 (point 2). In the resorption process the sorption quantity of CaCl2 increases from point 2 to point 4 with the highest sorption quantity of 1.23 kg/kg, and the cycle sorption quantity is 0.23 kg/kg. Meanwhile the ammoniate MnCl2 desorbs from point 1 to point 3. Under the condition of daytime the ammoniate CaCl2 is controlled at the refrigeration temperature of 5 °C. The largest sorption quantity of MnCl2 at point 5 is roughly 0.48 kg NH3/kg MnCl2. The smallest sorption quantity of MnCl2 is at point 3 and 7 with the data of 0.27 kg/kg, and the cycle sorption quantity is 0.21 kg/kg. (2) Heat pump in winter: Fig. 6b shows that in the night for desorption of MnCl2 and sorption of CaCl2 the starting points are 9 and 10. After that the resorption completed the cycle sorption quantity of CaCl2 (between lines of roughly 0.99 and 1.23 kg/kg) is 0.24 kg/kg. For MnCl2 the cycle sorption quantity increases slightly for the lower cooling temperature in winter, and it is around 0.23 kg/kg (between the lines of 0.5 and 0.27 kg/kg). In the daytime the working process is the reversed process of that in the night. The performance of CaCl2-NH4Cl-NH3 is shown in Fig. 7. (1) Refrigeration in summer: Fig. 7a shows that for the night the largest cycle sorption quantity of NH4Cl is 0.63 kg/kg (between lines of 0.32 and 0.95 kg/kg). In daytime the ammoniate CaCl2 exchanges the heat with the environment, and the smallest sorption quantity of ammoniate CaCl2 is at point 7 and point 3, which has much lower desorption temperature (around 100 °C) than that of MnCl2. The largest sorption quantity is at point 5 and point 1. The cycle sorption quantity of CaCl2 is around 0.63 (between lines of 0.3 and 0.93). (2) Heat pump in winter: In the night of winter time, the desorption of ammoniate CaCl2 starts at point 9 in Fig. 7b, and the corresponding state of NH4Cl is at point 10. In daytime the desorption of NH4Cl happens at point 11, and the sorption quantity decreases from 11 to 10 by an isothermal process. The temperature of ammoniate NH4Cl is controlled at environmental temperature. The sorption of CaCl2 happens at point 12. The sorption quantity of CaCl2 increases from point 12 to point 9. The lowest temperature is 25 °C, which

is controlled by the temperature inside the cabin of EVs. The cycle sorption quantity of NH4Cl and CaCl2 are 0.63 and 0.64 (between lines of 0.94 and 0.30), respectively. 4. Performance analysis of energy storage type resorption AC 4.1. Energy density and COP of resorption cycles The energy density and COP for both winter and summer time are calculated by the experimental maximum sorption quantity and cycle sorption quantity shown in Figs. 5–7 and Eqs. (6)–(9). The results are shown in Fig. 8. Fig. 8a shows that the energy density of MnCl2-NH4Cl-NH3 is smilar to that of MnCl2-CaCl2-NH3. In summer time the energy density of MnCl2-CaCl2-NH3 (364 kJ/kg) is slightly higher than that of MnCl2-NH4Cl-NH3 (315 kJ/kg), while in winter time the data for both working pairs are same, which is 581 kJ/kg. The best results are obtained from the working pair of CaCl2-NH4Cl-NH3, and the data is 945 and 1504 kJ/kg, respectively for summer and winter time. COP shown in Fig. 8b has the similar trends as that of energy density in Fig. 8a. The data in summer time is smaller than that in winter time. MnCl2-NH4Cl-NH3 and MnCl2-CaCl2-NH3 have similar performance, and the COP of MnCl2-CaCl2-NH3 is slightly higher, which is 0.31 while the data of MnCl2-NH4Cl-NH3 is 0.27 in summer time. In winter time both of two working pairs have the same data of 0.47. The highest performance is again gotten from CaCl2-NH4Cl -NH3, and the data is 0.43 and 0.62, respectively, for summer and winter time. 4.2. Comparison of resorption and sorption cycles In order to have a better comparison between sorption and resorption technology for energy storage type AC of EVs, the experimental results of CaCl2-NH3 is analyzed as shown in Fig. 9. In summer time the cycle is 1–2–3–4–5–6. In daytime The CaCl2 reactor is cooled by the environmental cooling source, and the sorption starts at point 1 with the evaporation happening at point 3. With the decrease of the sorption temperature, the sorption quantity increases along the 1–2 by an isobaric process. The evaporation effect of NH3 at point 3 provides the cooling effect. In the night the sorption reactor is heated and the evaporator serves as the condenser this time. The desorption happens initially at point 5, where is the largest sorption quantity of 1.07 kg/kg. With the temperature increasing the sorption quantity decreases from point 5 to point 6, and the desorbed NH3 is condensed at point 4. Point 2 and 5 are for the largest sorption quantity while the point 1 and 6 are for the smallest sorption quantity, and the cycle sorption quantity is 0.77 kg/kg.

Fig. 8. Energy density and COP of different working pairs (a) energy density, (b) COP.

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time. Then for the 25 kWh battery, the energy required for the car with the resorption AC is about 0.265 kWh/km in summer time and 0.23 kWh/km in winter time. Comparing the performance of the compression type AC, the features of resorption AC are as follows:

Fig. 9. Sorption performance of CaCl2-NH3.

Similar to the cycle for summer, the sorption cycle for winter is 9–8–7–8–9, i.e. from point 9 to 8 is the cooling and sorption phase of the sorption reactor, and 7 is evaporation point. From 8 to 9 is the heating and desorption phase, and this time point 7 is for condensation. Again the cycle sorption quantity is 0.77 kg/kg. For sorption cycle of CaCl2-NH3 the energy density and COP for both winter and summer time are calculated by the experimental maximum sorption quantity and cycle sorption quantity shown in Fig. 9 and Eqs. (2)–(5). The energy density for summer and winter are 1008 and 1827 kJ/kg, respectively, while the COP are 0.37 and 0.66, separately. The energy density of CaCl2-NH3 in winter time is higher than that of CaCl2-NH4Cl-NH3, and it is increased by 22%. But for the system design the volume of reactor is mainly limited by the smallest energy density, i.e. energy density in summer, which is similar for both cycles of CaCl2-NH3 and CaCl2-NH4Cl-NH3. COP of CaCl2-NH3 in summer time is about 16% lower than that of CaCl2-NH4Cl-NH3 although it is about 6% higher than resorption cycle in winter time. Thus the CaCl2-NH4Cl-NH3 is a better choice for air conditioners of EVs because it has higher COP in the whole year and the similar smallest energy density. 4.3. Analysis of application of resorption systems for EVs The electric car generally needs about 16 kWh/100 km. For the small type electric car with 25 kWh cell the cruising mileage is about 160 km, and the driving time is around 2–3 h. The AC is taking as 2.5 kW, then the total cooling energy storage density for 3 h should be around 7.5 kWh. When the refrigerating temperature is 5 °C the energy storage density of resorption cycle of CaCl2-NH4Cl-NH3 in summer time is 945 kJ/kg. The HTH required for the car is 28.6 kg. A high density of the sorbent can be taken inside the reactor because the cycle time is long enough, which is much longer than general sorption refrigeration cycle that is mostly shorter than 1 h. When the density of the HTH in composite sorbent is 600 kg/m3 the volume of the sorbent in the sorption reactor should be around 0.048 m3. Considering the metal wall and the heat transfer channels inside the sorption reactor, the volume of the sorption reactor is about 30% larger than the sum volume of the compressor (less than 0.002 m3) and condenser (less than 0.035 m3). The volume of LTH reactor is similar with the HTH reactor because they have similar cycle sorption quantity. This volume is a little big for the limited space under the car bonnet. But because the resorption system doesn’t need electricity when the car is running, the position under the car or near the car trunk can be used for the sorption reactors. For example, the HTH reactor can be installed at the position near to the batteries of EVs, which is below the seat. The LTH can be installed at the same position for the conventional compression The energy for desorbing the sorption reactor is 17.4 kWh in summer time and 12.1 kWh in winter

(1) Considering the conventional AC consumes around 40–60% energy of the passenger EV [4], and around 30–50% energy of electric car, the car with the compression AC has the data of 0.22–0.31 kWh/km. The energy consumed by the resorption AC is about 0.265 kWh/km in summer time and 0.23 kWh/km in winter time, which is acceptable. (2) The resorption AC only needs the mains electricity for desorption phase of HTH. For refrigeration or heat pump phases in daytime it doesn’t need the energy input, which means it won’t consume the electricity of batteries of the EVs. Thus the cruising mileage of EVs will be prolonged significantly, especially for the winter time, and the data is higher than 30%. (3) Resorption AC has bigger volume. Thus for the passenger EVs the resorption system will be more prospective for it has spacious space for the installation of the resorption system. 5. Conclusion The resorption cycles, which has lower working pressure, reasonable sorption performance, no liquid refrigerant inside, and intermittent working phases for energy storage, are analyzed for serving as the ACs for EVs. Such ACs won’t consume the electricity of the batteries on-board and thus won’t influence the cruising mileage of EVs. The conclusions are as follows: (1) Under the condition of that the HTH in resorption cycle is same with halide in sorption cycle, the theoretical analysis of equilibrium working conditions shows that resorption has higher COP and energy density in summer time, while it has same COP and energy density in winter time if compared with that of sorption cycle. Taking CaCl2-NH4Cl-NH3 as the example, in summer time both COP and energy density are improved by about by 13% compared with the data of CaCl2-NH3. In winter time the energy density and COP are same with that of CaCl2-NH3. (2) For different resorption cycles the CaCl2-NH4Cl-NH3 has the best sorption performance for the highest cycle sorption quantity both in summer and winter. HTH and LTH in CaCl2-NH4Cl-NH3 working pair both have the cycle sorption quantity higher than 0.6 kg/kg. Compared with the MnCl2-CaCl2-NH3 and MnCl2-NH4Cl-NH3 working pairs, the energy density and COP of CaCl2-NH4Cl-NH3 improve by 160% and 35% at least. (3) The performance of CaCl2-NH4Cl-NH3 is compared with CaCl2-NH3. The smallest energy density, which is for the summer time and is the limitation for the volume design of the sorption reactor, are similar for both working pairs. The COP of CaCl2-NH3 in summer time is about 16% lower than that of CaCl2-NH4Cl-NH3, and it is 6% higher in winter time. The CaCl2-NH4Cl-NH3 is a better choice for the design of AC of EVs for the higher COP considering the working conditions in the whole year. (4) For the 25 kWh battery the energy required by the resorption AC of a EV is about 0.265 kWh/km in summer time and 0.23 kWh in winter time and it is regenerated when the EV charges the electricity, while for the EV with the conventional AC the data is 0.22–0.31 kWh/km, and the electricity of battery on-board is consumed by the AC. The energy consumed by the resorption AC is acceptable.

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For conventional ACs the cruising mileage is shorten by around 40–60% for passenger EV and 30–50% for electric cars. The resorption air conditioner doesn’t need the electricity of batteries on-board and won’t influence the cruising mileage.

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