ionic liquid working fluids for single-effect and compression-assisted absorption refrigeration systems

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Performance analysis of R1234yf/ionic liquid working fluids for single-effect and compression-assisted absorption refrigeration systems Yanjun Sun , Gaolei Di , Jian Wang , Xiaopo Wang , Wei Wu PII: DOI: Reference:

S0140-7007(19)30429-3 https://doi.org/10.1016/j.ijrefrig.2019.10.007 JIJR 4545

To appear in:

International Journal of Refrigeration

Received date: Revised date: Accepted date:

18 August 2019 11 October 2019 13 October 2019

Please cite this article as: Yanjun Sun , Gaolei Di , Jian Wang , Xiaopo Wang , Wei Wu , Performance analysis of R1234yf/ionic liquid working fluids for single-effect and compressionassisted absorption refrigeration systems, International Journal of Refrigeration (2019), doi: https://doi.org/10.1016/j.ijrefrig.2019.10.007

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Highlights 

Several R1234yf/IL working pairs are compared for single-effect and compression-assisted absorption refrigeration cycles.



R1234yf/[hmim][Tf2N] pairs shows the best performance among the studied substances.



IL with higher solubility, lower heat capacity, and smaller molecular weight has better performance.

1

Performance analysis of R1234yf/ionic liquid working fluids for single-effect and compression-assisted absorption refrigeration systems Yanjun Sun1, Gaolei Di1, Jian Wang1, Xiaopo Wang2,*, Wei Wu3 1

Institute of Building Energy & Sustainability Technology, School of Human Settlements and Civil Engineering, Xi'an Jiaotong University, Xi’an, Shaanxi, 710049, China

Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, Xi’an

2

Jiaotong University, Xi’an, Shaanxi, 710049, China 3

School of Energy and Environment, City University of Hong Kong, Hong Kong, China

*Corresponding author, email: [email protected]; Tel:86-29-82668210

Conflict of interest statement The authors declared that they have no conflicts of interest to this work. Xiaopo Wang Xi’an Jiaotong University

Abstract: Due to the disadvantages of conventional working pairs (NH3/H2O and H2O/LiBr) in the absorption-refrigeration cycles, 2,3,3,3-Tetrafluoroprop-1-ene(R1234yf)/ionic liquid (IL), which possessed of remarkable properties, has received more and more attention. In this work, the thermodynamic performance of single-effect and compression-assisted absorption refrigeration cycles were analyzed using R1234yf as refrigerant and ILs (including [emim][BF4], [hmim][BF4], [omim][BF4], [hmim][Tf2N], [hmim][PF6] and [hmim][TfO]) as absorbent, the thermodynamic properties of working pairs was estimated by the NRTL model. The effects of generation, evaporation, condensation and absorption temperature as well as compression ratio on the cooling 2

performance and circulation ratio were studied in various working conditions. Compared to the single-effect cycle, the compression-assisted cycle effectively improves the cooling performance, reduces the circulation ratio and extends the operation range of generation, evaporation and absorption temperatures. At the same working condition, [hmim][Tf2N] performs the best while [emim][BF4] has the lowest COP, the cooling performance of [hmim][BF4], [omim][BF4], [hmim][TfO] and [hmim][PF6] is similar. Keywords: Absorption refrigeration; Single-effect cycle; Compression-assisted cycle; Ionic liquid; Hydrofluoroolefins

Nomenclature B

Second virial coefficient (m3/mol)

Cp

Heat capacity (kJ/(mol·K))

f

Circulation ratio

h

Mass-based specific enthalpy (kJ/kg)

H

Mole-based enthalpy (kJ/mol)

m

Mass flow rate (kg/s)

p

Pressure (Pa)

Q

Heat input/output (kW)

R

Specific gas constant (J/(mol·K))

T

Temperature (K)

V

Mole-based liquid volume (m3/mol)

Vr

Saturated molar liquid volume (m3/mol)

Wc

Energy consumption of the compressor (kW)

Wp

Solution pump work (kW)

x

Mole fraction in liquid phase

y

Mole fraction in gas phase

Greek symbols α

Adjusting parameter

γ

Activity coefficient

ξ

Solution heat exchanger efficiency

ηe

Electricity generation efficiency

η

Isentropic efficiency

φ

Fugacity coefficient

τ

Binary interaction parameter

subscripts 1-9

State points 3

a

Absorber

c

Condenser

e

Evaporator

g

Generator

IL

Ionic liquid

p

Solution pump

r

Refrigerant

ref

Reference value

sxe

Solution heat exchanger

Superscript E

Excess parameter

s

Saturated state

1. Introduction Due to the rapid world population growth, increasing energy consumption and environmental pollution stimulate attention to energy conservation. The statistics showed that approximately 40% of the total energy is consumed in buildings and over 60% of the building-energy consumption allotted to the heating, ventilation and air-conditioning (HVAC) systems (Soltani et al., 2018). Specifically, HVAC systems account for almost 39% of the building-energy consumption in the U.S., 57% in European Union (EU) and more than 70% in Middle-East (Razmi et al., 2019). Nearly 80% of the energy consumption relates to the primary energy yet. For this reason, it is worth exploring the energy efficient HVAC systems independent of the primary energy that are favorable to energy usage. Absorption-refrigeration systems driven by low-grade thermal energy, such as industrial waste heat, geothermal and solar energy, have become attractive in HVAC system (Moreno et al., 2018; Chen et al, 2010). The performance of an absorption cycle depends on its configuration and thermodynamic properties of working pairs composed of a refrigerant and an absorbent (Sun et al., 2012). The commonly used working pairs, NH3/H2O and H2O/LiBr, have some backdraws including toxicity (NH3), crystallization (LiBr), corrosion (NH3), complexity and higher cost of NH3/H2O system (the need of a rectifier for post-desorption separation of fluid streams) (Wang et al., 2011; Fong and Lee, 2014). Therefore, it is extremely vital to explore the alternative working pairs to overcome the shortcomings. 4

Ionic liquids (ILs), unlike a single molecule cation and anion as in LiBr, the cation is usually a large organic molecule paired with either organic or inorganic anion designed to meet the specific requirements. In particular, the near-zero volatility of the ILs can easily separate the volatile working fluid by thermal stratification. A number of researchers analyzed theoretical performance of absorption-refrigeration cycles using IL as an absorbent and H2O, NH3, CO2, hydrofluorocarbons (HFCs), alcohol as a refrigerant (Abumandour et al., 2016; Chen et al., 2014; Chen and Liang, 2016; Kim et al., 2013; Kim and Kohl, 2014; Ruiz et al., 2014; Zhang and Hu, 2012). These investigations present great potential and significant application prospects of these novel working pairs. The Montreal protocol for phase out of ozone depleting refrigerants has recently been extended to HFC refrigerants that have high global warming potential (GWP) values. All high GWP HFC refrigerants will be phased out over a period of time depending on the specific country. 2,3,3,3-tetrafluoroprop-1-ene (R1234yf) refrigerant has the atmospheric lifetime of only 11 days and a 100-year time horizon GWP of 4 relative to carbon dioxide, and is being studied extensively for use in the vapor compression refrigeration systems (McLinden et al., 2014; Sondergaard et al., 2007). Literature survey shows that there is little information about the performance of absorption cycles using R1234yf/IL as the working pair. Sujatha and Venkatarathnam (2018) analyzed the cooling performance of a single-effect absorption cycle for [hmim][Tf2N] as absorbent and R32, R152a, R125, R1234ze(E), R1234yf as refrigerant, their results indicate the COPs of HFCs (R32 and R152a) are better than those of HFOs (R1234yf and R1234ze(E)). Wu et al. (2017) compared the cooling performance of R1234yf/[hmim][Tf2N] and R1234ze(E)/[hmim][Tf2N] in single-effect and compression-assisted cycle, they found that the COP of R1234ze(E) is better than that of R1234yf and the performance is improved by the compression-assisted cycle. Liu et al. (2019) discussed the effect of the compressor position on the performance of absorption cycles using several R1234yf/IL as working pairs, results show that R1234yf/[hmim][TfO] has best performance among their studied pairs. To select suitable-performing R1234yf/IL working pair in the absorption system, in this work, the energy and exergy performance of six R1234y/IL working pairs were analyzed. The molecular structures of six ILs ([emim][BF4], [hmim][BF4], [omim][BF4], [hmim][Tf2N], [hmim][PF6] and 5

[hmim][TfO]) are illustrated in Figure 1. The influence of generation, evaporation, absorption and condensation

temperatures

on

the

performance

for

both

cycles

(single-effect

and

compression-assisted cycle) was explored. To obtain the maximum exergy coefficient of performance, the various compression ratio was compared.

2. Thermodynamic system analysis 2.1 Absorption refrigeration system description Figure 2a shows the single-effect absorption cooling cycle which consists of an evaporator, a condenser, a generator, an absorber and a solution heat exchanger. The refrigerant vapor is generated from the weak solution (state point 7) by the addition of heat (renewable energy, waste heat, boiler, etc.) in the generator. The strong solution (state point 8) flows back to the absorber through the solution heat exchanger and expansion device. The refrigerant vapor from the evaporator (state point 2) is exothermically absorbed into the strong solution (state point 10) that results in a weak solution at state point 5 in the absorber. Then, the weak solution is pressurized by the liquid pump and passes the solution heat exchanger for heat recovery before entering the generator. The refrigerant vapor from the generator condenses into liquid refrigerant (state point 4) in the condenser, then gets throttled by the refrigerant expansion valve. Finally, the liquid refrigerant vaporizes in the evaporator to produce cooling or refrigeration effect. The principle of compression-assisted absorption cycle is illustrated in Figure 2b. The significant feature of the cycle is that a compressor is configured between the evaporator and absorber to improve the absorption pressure. The increasing pressure in the absorber can result in higher solubility of the refrigerant in the solution, that means the weak solution becomes much weaker after absorption under a higher pressure. Investigations (Wu et al., 2017; Liu et al., 2019)) show that the compression-assisted absorption cycle is potentially promising under the lower available driving temperatures or lower required cooling temperatures. 2.2 Thermodynamic model The following assumptions are made to simulate the thermodynamic performance: (1) The degree of superheat of the refrigerant leaving the evaporator is zero and the degree of sub-cooling of the liquid leaving the condenser is zero; (2) The solutions of refrigerant and IL in the generator

6

and absorber are in the vapor-liquid phase equilibrium state; (3) Pressure drop in all components is zero; (4) The solution pumping work is ignored; (5) The throttling process is isenthalpic. The energy and mass conservation equations in the absorption cycle were simultaneously solved to determine the heat and workloads. For the generator

Qg  mr fh7  mr h3  mr  f  1 h8

(1)

For the absorber

Qa  mr fh5  mr h2  mr  f  1 h10

(single-effect cycle)

(2a)

Qa  mr fh5  mr h2  mr  f  1 h10

(compression-assisted cycle)

(2b)

For the condenser

Qc  mr h4  mr h3

(3)

Qe  mr h1  mr h2

(4)

Qsxe  mr f  h6  h7   mr  f  1 h8  h9 

(5)

For the evaporator

For the solution heat exchanger

For the solution pump

h6  h5 

Wp mr f

(6)

where Q is the heat input/output. The subscripts g, a, c, e, and sxe stand for the generator, absorber, condenser, evaporator and solution heat exchanger, respectively. mr is the mass flow rate of circulation refrigerant; h is the mass-based specific enthalpy; f is the circulation ratio defined as the ratio of mass flow rate of the weak solution to that of the circulation refrigerant. Compared with the cooling capacity, the solution pump work (Wp) is too small that is ignored in this work (Herold et al., 2016). The state point temperatures of the solution heat exchanger are determined by

 sxe 

T8  T9 T8  T6 7

(7)

where ξx is the solution heat exchanger efficiency that is set as 0.8 in this work. The outlet parameters of compression are calculated by



h2,ideal  h2 h2  h2

(8)

where η is the isentropic efficiency of compressor that is set to 0.7; h2',ideal is the ideal outlet refrigerant enthalpy in an isentropic compression. The COP of the absorption-refrigeration system is defined as

COP 

Qe Qg  Wc e

(9)

where ηe is the electricity generation efficiency used to convert a fuel to electricity, that is set to 0.38 in this work. Wc is the energy consumption of the compressor. For the single-effect cycle, Wc = 0; For the compression-assisted cycle, Wc = mr(h2' - h2). For a second law thermodynamic analysis, the exergy coefficient of performance (ECOP) of absorption-refrigeration cycle is calculated by (Kim et al., 2013; Kim and Kohl, 2014)

ECOP 

Qe 1  Tref Te

Qg 1  Tref Tg   Wc

(10)

where Tref is the reference temperature that is set as 298.15 K. The mole-based enthalpy of the binary mixture is calculated by

H  xr H r  xIL H IL  H E

(11)

where xr is the refrigerant-solubility in the IL; Hr is the mole-based enthalpy of the refrigerant obtained from Refrop 9.1 (Lemmon et al., 2013); HIL is the mole-based enthalpy of the IL; HE is the excess enthalpy. The mole-based enthalpy of the IL is calculated by T

H IL   C p , IL dT  H ref T0

(12)

where Href is the reference specific enthalpy of IL at reference state; Tref is the reference temperature that is set to 273.15 K. The heat capacity of the IL, Cp,IL, is obtained by

C p, IL  A  DT  ET 2 8

(13)

A, D and E are the coefficients of eq. (13), that are regressed by the experimental data of the heat capacity of IL gotten from references (Waliszewski et al., 2005, 2008; Paulechka et al., 2015; Blokhin et al., 2006; Diedrichs and Gmehling, 2006; Li et al., 2011). The calculated results are shown in Table 1. For the refrigerant (1)/IL (2) mixture, the excess enthalpy is calculated by

   ln  r    ln  IL   H E   RT 2  xr    xIL     T  p , x    T  p , x

(1)

where γ is the activity coefficient that is calculated by the NRTL model 2     G21  12G12  ln  r  x  21    2   xr  xILG21   xIL  xr G12  

(15)

2     G12  21G21  ln  IL  x  12    2   xIL  xr G12   xr  xILG21  

(16)

G12  exp  12  , G21  exp   21 

(17)

1 2 2 12  120  121 T  122 T 2 ,  21   210   21 T   21 T

(18)

r yr p   r xr prs

(19)

  Br  Vr   p  prs    r  exp  RT  

(20)

2 IL

2 r

where x and y are the molar fraction in liquid phase and gas phase, respectively; φ is the fugacity coefficient; p and T is the equilibrium pressure and equilibrium temperature, respectively; prs , Br and Vr are the saturated pressure, the second virial coefficient and the saturated mole-based liquid volume of refrigerant, respectively. The values of these three parameters are obtained from Refprop 9.1. The subscripts r and IL stand for the refrigerant and IL, respectively. R is the gas 0 1 0 1 2 constant; α,  12 ,  12 ,  122 ,  21 ,  21 and  21 are adjustable parameters that are regressed by

the experimental VLE data (Liu et al., 2019; Sun et al., 2017). Table 2 lists these adjustable parameters for the studied working pairs. Using the adjustable parameters, the calculated results were shown in the Supplementary Material. 9

2.3 Model verification To verify the accuracy of the above thermodynamic model, the COP and circulation ratio of both cycles (single-effect and compression-assisted cycle) working with R1234ze(E)/[hmim][BF4] are calculated. Figure 3 shows the comparisons between our calculated results and those from Wu et al. (2018) at the evaporation temperature of 278.15 K and condensation temperature of 303.15 K. The results show good agreement with the reported results with a deviation of 0.94 % in COP and 2.44 % deviation in circulation ratio for the single-effect cycle, and a deviation of 2.38 % in COP and 6.05 % deviation in circulation ratio for compression-assisted cycle.

3. Results and discussions The energy and exergy performance of the single-effect and compression-assisted cycle using R1234yf/IL working pairs (R1234yf combined with [emim][BF4], [hmim][BF4], [omim][BF4], [hmim][Tf2N], [hmim][PF6] and [hmim][TfO]) are analyzed in the high-temperature cooling, air-conditioning and sub-zero conditions. 3.1 Effect of generation temperature The generation temperature practically depends on the available heat source (industrial waste heat, solar energy, geothermal energy, etc.), and lower generation temperature is favorable to utilize the low-grade heat. Figure 4 show the COP variation with the generation temperature for the six working pairs in both cycles at the condensation temperature of 303.15 K, absorption temperature of 303.15 K and evaporation temperature of 278.15 K. Each separate figure is shown in the Supplementary Material. In both cycles, the COP firstly increases and then slightly decrease with rising generation temperature for all working pairs. A higher generation temperature brings the lower refrigerant concentration after the generation process that can increase the concentration difference between weak and strong solution, and then it can improve the cooling performance. However, too high generation temperatures lead to an increase of irreversibility that may reduce the performance. For the single-effect cycle, the minimum generation temperature is about 335 ~ 343 K for the studied working pairs. [emim][BF4] has the lowest COPs of 0.01 ~ 0.04. The COPs of [hmim][BF4], [omim][BF4], [hmim][TfO] and [hmim][PF6] are similar, and the range of COP is 0.01 ~ 0.13. [hmim][Tf2N] yields the highest COPs of 0.01 ~ 0.18 where the maximum appears at 10

the generation temperature of 365 K. For the compression-assisted cycle (compression ratio of 1.5), the minimum generation temperature is reduced to about 317 ~ 322 K that is more beneficial to use the low-temperature heat. The COP of [emim][BF4] is still the lowest, but the maximum is increased to 0.08. On the other hand, the maximum COP is increased to 0.20, 0.21, 0.29 and 0.22 for [hmim][TfO], [hmim][PF6], [omim][BF4] and [hmim][BF4], respectively. [hmim][Tf2N] performs the best in the studied working pairs that yields maximum COP of 0.35 at about 336 K. The better performance of R1234yf/[hmim][Tf2N] working pair can be explained that [hmim][Tf2N] has a higher solubility, a lower heat capacity and a smaller molecular weight than other ILs, which leads to a higher refrigerant concentration of the weak solution and enthalpy during the absorption process. A higher circulation ratio always leads to a lower COP, and the absorption-refrigeration cycle cannot operate normally when the circulation ratio is too high. Figure 5 presents the variation of circulation ratio with generation temperature for the six working pairs in both cycles. Each separate figure is shown in the Supplementary Material. For both cycles, [emim][BF4] has the highest circulation ratio that indicate the flow rate of the weak solution has to be higher than other studied ILs to obtain the same amount of refrigerant. Similar circulation ratio is obtained from [hmim][BF4], [omim][BF4], [hmim][PF6] and [hmim][TfO], that is about 60 for the single-effect cycle and is about 25 for the compression-assisted cycle at the generation temperature above 360 K. The minimum generation temperature is reduced to about 333 K in the compression-assisted cycle when the circulation ration of [hmim][Tf2N] is about 20, that is the lowest circulation ratio in the studied ILs. Compared to the single-effect cycle, the circulation ratio obviously decreases for each working fluid in the compression-assisted cycle. The high circulation ratio results from the small concentration difference between the strong and weak solutions. The concentration difference mainly depends on the refrigerant solubility and the pressure. To reduce the circulation ratio, higher solubility ILs should be found to increase the concentration difference. 3.2 Effect of evaporation temperature To explore the performance of R1234yf/IL in a larger evaporation-temperature region, the evaporation temperatures from 250 to 290 K have been studied in this work. Figures 6-7 illustrate the COP and circulation ratio of both cycles variations with the evaporation temperature for the 11

six working pairs at generation temperature of 363.15 K, absorption temperature of 303.15 K and condensation temperatures of 303.15 K. Each separate figure is shown in the Supplementary Material. For the both cycles, the COP increases with rising evaporation temperature while circulation ratio increases exponentially with a decrease in the evaporator temperature. A higher evaporation temperature results in a higher saturated pressure in the evaporator and absorber, and then the solution is much stronger in the absorption process. In this case, the concentration difference of solution increases what contributes to lower circulation ratio and higher cooling performance. For the single-effect cycle, the minimum evaporation temperature is about 262 ~ 268 K for the studied working pairs. [emim][BF4] has the highest circulation ration in the studied ILs, its COP is 0.01 ~ 0.09. Similar COP and circulation ratio for [hmim][TfO], [hmim][PF6], [omim][BF4] and [hmim][BF4] are obtained, the range of their maximum COPs is 0.15 ~ 0.30. [hmim][Tf2N] has the lowest circulation ratio that yields the highest COPs of 0.01 ~ 0.35. For the compression-assisted cycle, the minimum evaporation temperature is decreased to about 250 ~ 257 K. The maximum COP is increased to 0.30, 0.28, 0.31, and 0.29 for [hmim][TfO], [hmim][PF6], [omim][BF4] and [hmim][BF4], respectively. [hmim][Tf2N] performs the best in the studied working pairs that yields maximum COP of 0.47 at 288 K. From the above analysis, R1234yf/IL working pairs have more potential for the refrigeration application with higher evaporation temperatures. 3.3 Effect of condensation temperature At a generation temperature of 363.15 K, an evaporation temperature of 278.15 K and the absorption temperature of 303.15 K, the effects of the condensation temperature on the COP and circulation ratio for both cycles are shown in Figures 8-9. Each separate figure is shown in the Supplementary Material. For both cycles, the COP decreases as the condensation temperature increases. A higher condensation temperature produces a higher saturated pressure in the condenser and generator, that leads to a higher refrigerant concentration of the strong solution after generation process and an increase of circulation ratio. The concentration difference is smaller with rising the condensation temperature which finally decreases the COP. Compared to the single-effect cycle, the circulation ratio obviously decreases and COP dramatically increases in 12

the compression-assisted cycle. [hmim][Tf2N] presents the highest COP in the whole condensation temperature, and its maximum COP is 0.36 at about 293 K in the compression-assisted cycle while is 0.25 in the single-effect cycle. [hmim][TfO], [hmim][PF6], [omim][BF4] and [hmim][BF4] have the similar COP and circulation ratio; their maximum COPs of the compression-assisted cycle are 0.23, 0.24, 0.25 and 0.30, respectively. 3.4 Effect of absorption temperature Figures 10-11 present the variations of COP and circulation ratio with absorption temperature for different R1234yf/IL working pairs in both cycles at the generation temperature of 363.15 K, evaporation temperature of 278.15 K, and condensation temperature of 303.15 K. Each separate figure is shown in the Supplementary Material. The COP decreases as the absorption temperature increases in both cycles. A higher absorption temperature results in a decrease of solubility of refrigerant in the absorbent that has to increase the solution flow rate to meet the required refrigeration load. The increasing solution flow rate needs more heat input to the generator for the same cooling capacity of the evaporator, which leads to a decrease of COP. As shown in Figures 11 and 12, the drop of COP with an increase of absorption temperature in the single-effect cycle is higher than that in the compression-assisted cycle. A similar situation has been found in the circulation ratio. The effect of absorption temperature on the performance in the single-effect cycle is more than that in the compression-assisted cycle. From the above results we can see that the operation range can be improved by the compression-assisted technology. [hmim][Tf2N] has highest COP and lowest circulation ratio; the maximum of COP is about 0.40 at the absorption temperature of 293 K in the compression-assisted cycle. 3.5 Effect of compression ratio Based on a compression ratio of 1.5, the cooling performance of compression-assisted cycle have been analyzed in above sections. To further explore the effect of compression ratio on the cooling performance, the variations of COP and ECOP with compression ratio (between 1.0 and 2.4) for six R1234yf/IL working pairs were presented in Figures 12-13. The compression ratio of 1 means no compressor, i.e., the single-effect cycle. In the calculation process, the generation, evaporation, condensation and absorption temperatures are taken as 363.15 K, 273.15 K, 303.15 K and 303.15 K, respectively. From Figures 12-13, it can be observed that the COP and ECOP 13

increase as the compression ratio increases. A higher compression ratio yields a higher absorption pressure that leads to the weak solution becoming much weaker. In the above given temperatures, the maximum COPs of [emim][BF4], [hmim][BF4], [omim][BF4], [hmim][Tf2N], [hmim][PF6] and [hmim][TfO] are 0.11, 0.25, 0.27, 0.38, 0.24 and 0.26, respectively. Due to the saturated vapor pressure of R1234yf, the maximum compression ratio only can be calculated to 2.4. Under the above working condition, the maximum ECOP appears at the compression ratio is 2.4 for each ILs. 3.6 Comparisons with the conventional working fluids From the above investigations, R1234yf/[hmim][Tf2N] is the most potential working pair in the studied R1234yf and IL. Under a generation temperature of 363.15 K, condensation temperature of 303.15 K, and absorption temperature of 303.15 K, the effect of evaporation temperature on COP and ECOP of R1234yf/[hmim][Tf2N], H2O/LiBr and NH3/H2O in the compression-assisted cycle is compared in Figure 14. The COP of R1234yf/[hmim][Tf2N] is lower than that of H2O/LiBr and NH3/H2O. The maximum COPs of R1234yf/[hmim][Tf2N], H2O/LiBr and NH3/H2O are 0.41, 0.85 and 0.80, respectively. Compared to H2O/LiBr working pair, more suitable operating pressures are obtained using the refrigerant/IL working pair. (4.38 bar and 10.2 bar for R1234yf/IL; 0.012 bar and 0.073 bar for H2O/LiBr) at the evaporation and condensation temperatures of 293 K and 313 K, respectively. Since the specific volume of R1234yf is approximately two orders lower than that of H2O, the size of the refrigeration equipment is much smaller leading to smaller capital cost when R1234yf is used as a refrigerant instead of H2O.

4. Conclusions Absorption-refrigeration technology has a great advantage for utilizing the low-grade thermal energy. The potential applications of six R1234yf/IL working pairs in the absorption-refrigeration system was analyzed. The solubilities of refrigerant in different ILs were correlated using the NRTL model. Based on the calculated results from NRTL, energy and mass conservation equations, the effect of generation, evaporation, condensation and absorption temperature, as well as compression ratio, on the cooling performance and circulation ratio has been studied in different working conditions. The main conclusions are as follows:

14

(1) At the evaporation temperature of 278 K, absorption temperature of 303 K, and condensation temperature of 303 K, the minimum generation temperature is about 335 ~ 343 K (It is beneficial to use the lower temperature heat) for the six R1234yf/IL working pairs in the compression-assisted cycle. The maximum COP is 0.20, 0.21, 0.29 and 0.22 for [hmim][TfO], [hmim][PF6], [omim][BF4] and [hmim][BF4], respectively. [bmim][BF4] has the lowest cooling performance and highest circulation ratio, while [hmim][Tf2N] performs the best that yields maximum COP of 0.35 at the generation temperature of 336 K. (2) At the generation temperature of 363 K, condensation temperature of 303 K and absorption temperature of 303 K, the minimum evaporation temperature in the compression-assisted cycle is about 253 ~ 263 K. The maximum COP is 0.13, 0.29, 0.31, 0.47, 0.28 and 0.30 for [emim][BF4], [hmim][BF4], [omim][BF4], [hmim][Tf2N], [hmim][PF6] and [hmim][TfO] at temperature of about 288 K, respectively. R1234yf/IL working pairs present greater advantages in the higher evaporation temperature. (3) The compression-assisted cycle not only improves the COP but also extends the operating range for the R1234yf/IL working pairs. The COP increases as the compression ratio increases, but a higher compression ratio needs more electricity input that results in a decrease of ECOP when the compression ratio is too high. Based on the saturated vapor pressure of R1234yf, the compression ratio was discussed from 1.0 to 2.4. (4) The COPs of the studied R1234yf/IL are lower than those of NH3/H2O and H2O/LiBr. Because the specific volume of R1234yf is much lower than that of H2O, the refrigeration-equipment volume is much smaller leading to smaller capital cost when R1234yf/IL is used as a working pair instead of H2O/LiBr. To obtain cooling-performance improvement, more R1234yf/IL working pairs need to be explored in the future. R1234yf solubility in IL, molecular weight, and heat capacity of IL should be taken into account when selecting R1234yf/IL working pairs.

Acknowledgments The authors are grateful for financial support from the National Natural Science Foundation of China (No. 51606148).

References 15

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17

Table 1 Coefficients of the heat capacity of ILs.

[emim][BF4] [hmim][BF4] [omim][BF 4] [hmim][TfO] [hmim][PF6] [hmim][Tf2N]

A

D

E

282.2 276.0 512.8 298.7 545.7 -119.1

-0.15 0.52 -0.67 0.84 -1.31 3.62

8.07×10-4 0 2.07×10-3 -4.6×10-4 3.034×10-3 -4.13×10-3

Table 2 The adjustable parameters of NRTL equation. Working pair

α

 120

 121

 122

 210

1  21

 212

R1234yf/[emim][BF4] R1234yf/[hmim][BF4] R1234yf/[omim][BF4] R1234yf/[hmim][TfO] R1234yf/[hmim][PF6] R1234yf/[hmim][Tf2N]

-0.1 -0.1 -0.2 -0.1 -0.1 -0.1

4613.2 -6.84 7.24 -1.46 -11.6 -8.27

-26.01 0.089 -0.023 0.048 0.118 0.039

0.038 -1.6×10-4 3.24×10-5 -8.2×10-5 -2.0×10-4 -4.3×10-5

7.12 31.5 -9.84 14.86 34.7 1.24

-0.026 -0.28 0.020 -0.15 -0.3 0.004

4.19×10-5 4.9×10-4 -1.6×10-5 2.64×10-4 5.24×10-4 -1.46×10-5

18

N

N N

F

F

N

F

B F

N

F

F

F

F

B F

N

F B

F

F

[ e m im ][ B F 4 ]

[ h m im ] [ B F 4 ]

[ om im ][ B F 4]

M o le c u l a r W e i g h t:1 9 7 . 9 7

M o le c u l a r W e i g h t : 2 5 4 . 0 8

M o l e c u la r W e i g h t: 2 8 2 .1 3

N

O O

F

F

S O

N

N

F F

F

N

F P

F

N

F F

F

F

[h m im ][T F O ]

[ h m im ] [ P F 6 ]

M o le c u la r W e ig h t:3 1 6 .3 4

M o l e c u l a r W e i g h t :3 1 2 .2 4

Figure 1 The molecular structures of ILs

19

O

O N

F

O

N

F

S O

F F

[ h m i m ][ T F 2 N ] M o l ec u la r W e ig h t:4 4 7 .4 2

Qc

condenser

Qg

4

3

generator

heat rejection expansion valve

8

7

9

6

10

5

solution heat exchanger expansion valve

evaporator

pump

absorber 2

1

Qc

Qa

heat rejection

Figure 2a The principle of single-effect absorption cooling cycles Qc

condenser

Qg

4

3

generator

heat rejection expansion valve

8

7

9

6

10

5

solution heat exchanger expansion valve

evaporator

pump

absorber 1

2’

2

heat rejection

Qc

Qa

Figure 2b The principle of compression-assisted absorption cooling cycles

20

180

0.40

160

0.35

circulation ratio

0.30 0.25 0.20

Reference single-effect cycle compression-assisted cycle

0.15

0.00 310

320

330

340

350

360

370

This work single-effect cycle compression-assisted cycle

100 80

40

This work single-effect cycle compression-assisted cycle

0.05

120

60

0.10

20 0 310

380

320

330

generation temperature /K

340

350

360

370

380

generation temperature /K

Figure 3 Comparison of COP and circulation ratio between our results and reference

0.40

single-effect cycle

[emim][BF4]

0.35

[hmim][BF4]

COP

COP

Reference single-effect cycle compression-assisted cycle

140

0.30

[omim][BF4]

0.25

[hmim][TfO] [hmim][PF6] [hmim][Tf2N]

0.20

compression-assisted cycle

[emim][BF4]

0.15

[hmim][BF4] 0.10

[omim][BF4]

0.05

[hmim][TfO] [hmim][PF6]

0.00 315

[hmim][Tf2N] 320

325

330

335

340

345

350

355

360

365

generation temperature /K

Figure 4 Effect of generation temperature on COP of two absorption systems

21

300 270

single-effect cycle [emim][BF4]

240

[emim][BF4] [omim][BF4]

Circulation ratio

210

[hmim][TfO] [hmim][PF6]

180 150

[hmim][Tf2N] compression-assisted cycle

120

[emim][BF4]

90

[emim][BF4] [omim][BF4]

60

[hmim][TfO] [hmim][PF6]

30 0 315

[hmim][Tf2N] 320

325

330

335

340

345

350

355

360

365

370

generation temperature /K

Figure 5 Effect of generation temperature on circulation ratio of two absorption systems.

0.50

single-effect cycle

0.45

[emim][BF4]

0.40

[hmim][BF4] [omim][BF4]

0.35

[hmim][TfO] [hmim][PF6]

COP

0.30

[hmim][Tf2N]

0.25 0.20

compression-assisted cycle [emim][BF4]

0.15

[hmim][BF4] [omim][BF4]

0.10

[hmim][TfO] [hmim][PF6]

0.05 0.00 250

[hmim][Tf2N] 255

260

265

270

275

280

285

290

evaporation temperature

Figure 6 Effect of evaporation temperature on COP of two absorption refrigeration systems

22

200 180

single-effect cycle [emim][BF4]

160

[hmim][BF4] [omim][BF4]

Circulation ratio

140

[hmim][TfO] [hmim][PF6]

120

[hmim][Tf2N] 100 80

compression-assisted cycle [emim][BF4]

60

[hmim][BF4] [omim][BF4]

40

[hmim][TfO] [hmim][PF6]

20 0 250

[hmim][Tf2N] 255

260

265

270

275

280

285

290

295

evaporation temperature /K

Figure 7 Effect of evaporation temperature on circulation ratio of two absorption systems

0.40

single-effect cycle [emim][BF4]

0.35

COP

[hmim][BF4] 0.30

[omim][BF4]

0.25

[hmim][TfO] [hmim][PF6] [hmim][Tf2N]

0.20

compression-assisted cycle [emim][BF4]

0.15

[hmim][BF4] 0.10

[omim][BF4]

0.05

[hmim][TfO] [hmim][PF6]

0.00 292

[hmim][Tf2N] 294

296

298

300

302

304

306

308

310

312

314

316

condensation temperature /K

Figure 8 Effect of condensation temperature on COP of two absorption refrigeration systems.

23

350

single-effect cycle [emim][BF4]

300

[hmim][BF4] [omim][BF4]

Circulation ratio

250

[hmim][TfO] [hmim][PF6]

200

[hmim][Tf2N]

150

compression-assisted cycle [emim][BF4]

100

[hmim][BF4] [omim][BF4] [hmim][TfO] [hmim][PF6]

50

0 290

295

300

305

310

315

[hmim][Tf2N]

320

condensation temperature /K

Figure 9 Effect of condensation temperature on circulation ratio of two absorption systems

0.45

single-effect cycle [emim][BF4]

0.40

[hmim][BF4]

0.35

[omim][BF4] [hmim][TfO] [hmim][PF6]

0.30

COP

0.25

[hmim][Tf2N] compression-assisted cycle

0.20

[emim][BF4] 0.15

[hmim][BF4]

0.10

[omim][BF4]

0.05

[hmim][TfO] [hmim][PF6]

0.00 292

[hmim][Tf2N] 294

296

298

300

302

304

306

308

310

312

314

316

absorption temperature /K

Figure 10 Effect of absorption temperature on COP of two absorption systems

24

300 270

single-effect cycle [emim][BF4]

240

[hmim][BF4] [omim][BF4]

Circulation ratio

210

[hmim][TfO] [hmim][PF6]

180

[hmim][Tf2N]

150

compression-assisted cycle

120

[emim][BF4]

90

[hmim][BF4] [omim][BF4]

60

[hmim][TfO] [hmim][PF6]

30 0 292

[hmim][Tf2N] 294

296

298

300

302

304

306

308

310

312

314

316

absorption temperature /K

Figure 11 Effect of absorption temperature on circulation ratio of two absorption systems

0.50 0.45 0.40 0.35

COP

0.30

[emim][BF4] [hmim][BF4] [omim][BF4] [hmim][TfO] [hmim][PF6] [hmim][Tf2N]

0.25 0.20 0.15 0.10 0.05 0.00 1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

compression ratio

Figure 12 Effect of compression ratio on COP of absorption refrigeration systems.

25

0.20

[emim][BF4]

0.18

[hmim][BF4]

0.16

[omim][BF4]

0.14

[hmim][TfO] [hmim][PF6]

ECOP

0.12

[hmim][Tf2N]

0.10 0.08 0.06 0.04 0.02 0.00 1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

compression ratio

Figure 13 Effect of compression ratio on ECOP of absorption refrigeration systems.

1.0 0.9

0.45

H2O/LiBr NH3/H2O

0.40

0.7

0.35

0.6

0.30

ECOP

COP

0.8

0.50

R1234yf/[hmim][Tf2N]

0.5 0.4

0.20

0.3

0.15

0.2

0.10

0.1

0.05

0.0 250

255

260

265

270

275

280

285

290

0.00 250

295

R1234yf/[hmim] [Tf2N]

0.25

evaporation temperature /K

H2O/LiBr NH3/H2O

255

260

265

270

275

280

285

290

295

evaporation temperature /K

Figure 14 Comparisons of COP and ECOP for R1234yf/[hmim][Tf2N] and the conventional working pairs.

26