Thermodynamic Investigation and Comparison of Absorption Cycles

Article pubs.acs.org/IECR

Thermodynamic Investigation and Comparison of Absorption Cycles Using Hydrofluoroolefins and Ionic Liquid Wei Wu,*,† Haiyang Zhang,‡ Tian You,† and Xianting Li*,† †

Department of Building Science, School of Architecture, Tsinghua University, Beijing 100084, China Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China

‡

ABSTRACT: To overcome the problems of conventional absorption working fluids, the novel hydrofluoroolefins (HFOs) and ionic liquid (IL) were analyzed. The refrigerants were 2,3,3,3-tetrafluoropropylene (R1234yf) and trans-1,3,3,3tetrafluoropropene (R1234ze(E)), and the absorbent was 1hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([hmim][Tf2N]). The property models of HFO/IL and the thermodynamic models of absorption cycles were built with verified accuracy. Analyses show that the coefficient of performance (COP) of R1234yf/[hmim][Tf2N] is up to 0.414, while that of R1234ze(E)/[hmim][Tf2N] is up to 0.498. The better performance of R1234ze(E)/[hmim][Tf2N] is contributed by its higher solubility and lower saturation pressure. The compression-assisted cycle not only extends the operation range but also improves the COP for both HFO/IL pairs. The minimum generation and evaporation temperatures are reduced by 17 and 11 °C under a compression ratio of 1.5. The COPs of HFO/IL are lower than those of H2O/LiBr and NH3/H2O. More HFO/IL mixtures need to be explored for further performance improvement.

1. INTRODUCTION The building sector accounts for a big percentage of the total energy consumption, reaching 30−40% in developed countries and 15−25% in developing countries.1 Space heating, water heating, and space cooling account for 37%, 12%, and 10%, respectively, of the building energy use in the United States.2 Under the background of global energy savings and environment protection, building energy efficiency is playing a more and more significant role.3 Absorption heating and cooling technologies offer great opportunities to reduce energy consumption, recycle waste heat, and utilize renewable energy.4,5 The working fluids (refrigerant/absorbent) have a great influence on the performance of absorption cycles.6 H2O/LiBr and NH3/H2O are presently the major working fluids for absorption cycles.7 Despite the higher efficiency and better safety, H2O/LiBr is limited in many applications because of its high refrigerant freezing point, which makes it impossible to supply refrigeration temperatures lower than 5 °C in the evaporator side.4 In addition, H2O/LiBr systems always suffer from the solution crystallization problem8 and negative pressure problem. Though NH3/H2O is free of those problems, the toxicity and corrosion are always concerns,9 while a rectifier is usually required for refrigerant purification.10 There have been continuous worldwide investigations into alternative working fluids for absorption cycles. The refrigerants mainly include chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC), and hydrofluorocarbon (HFC), while the © 2017 American Chemical Society

absorbents include organic solvents and ionic liquids (ILs).11−13 Because of the concerns of ozone depletion potential (ODP) and global warming potential (GWP), the low-GWP refrigerants are attractive for absorption cycles from the sustainability point of view, much similar to vaporcompression cycles.14 Owing to the negligible vapor pressure, good thermal and chemical stability, and sound solubility with many refrigerants, ILs (composed of one organic cation and one organic or inorganic anion) are regarded as promising absorbents for absorption cycles.12,15 Recently, a number of HFC/IL working fluids have been studied for absorption cooling cycles. Kim et al. investigated the theoretical performance of an absorption refrigeration system using 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) paired with five HFCs (R134a, R32, R125, R143a, R152a).16 An experimental setup using R134a/[bmim][PF6] was built to evaluate the feasibility.17 In addition, Kim and Kohl compared R134/[bmim][PF6] with R134a/[bmim][PF6], indicating that R134/ [bmim][PF6] had much better performance than R134a/ [bmim][PF6].18 They also compared the absorption refrigeration cycle using R134a/[hmim][PF6] and R134a/[hmim][Tf2N] (1-hexyl-3-methylimidazolium bis(trifluoromethylReceived: Revised: Accepted: Published: 9906

June 7, 2017 August 9, 2017 August 14, 2017 August 14, 2017 DOI: 10.1021/acs.iecr.7b02343 Ind. Eng. Chem. Res. 2017, 56, 9906−9916

Article

Industrial & Engineering Chemistry Research

Figure 1. Principle of the single-effect absorption cooling cycle.

Figure 2. Principle of compression-assisted absorption cooling cycle.

different working fluids. To extend the operation range and improve the cooling performance, the compression-assisted absorption cycle was also studied. Comparisons with the singleeffect cycle were conducted to further evaluate the potential of the novel HFO/IL working fluids used in absorption cycles.

sulfonyl)imide) and found that R134a/[hmim][Tf2N] showed higher system efficiencies in most cases.19 Dong et al. proposed to use the infinite dilution activity coefficients to select novel working fluids. Comparison among 18 HFC/IL pairs revealed that the combinations of R32, R134, and [Tf2N]-based (bis(trifluoromethylsulfonyl)imide) ILs might be two kinds of potential working pairs.20 Compared to HFC, hydrofluoroolefins (HFOs) stand out because of very low GWP.21,22 The commonly investigated 2,3,3,3-tetrafluoropropylene (R1234yf) and trans-1,3,3,3-tetrafluoropropene (R1234ze(E)) have a GWP of only about 6 and 4, respectively.23,24 In this regard, HFO/IL combinations will be very environmentally friendly in absorption cycles. To date, there are few studies on HFO/IL mixtures. Liu et al. studied the solubilities of R1234yf and R1234ze(E) in [hmim][Tf2N].24 However, there is no investigation on the performance of absorption cycles using HFO/IL as working fluids. In this study, the cooling performance of the single-effect absorption cycle using HFO/IL as working fluids were explored. The effects of various working parameters on the system efficiencies were analyzed and compared between

2. METHODOLOGY 2.1. Description of Absorption Cooling Cycles. The basic single-effect absorption cooling cycle is illustrated in Figure 1. The generator is heated by the external heat sources (waste heat, renewable energy, boiler, etc.), so that the vapor HFO (7) is generated from the incoming weak solution (3), which has a diluted IL fraction and weak absorption ability. Then the strong solution (4) with concentrated IL flows through the solution heat exchanger to be cooled (5), goes through the solution expansion valve to be throttled (6), and finally enters the absorber to absorb the incoming vapor HFO (10). Cooled by the external coolant (water or air), the weak solution (1) is pressurized by the solution pump (2) and passes the solution heat exchanger for heat recovery (3) before entering the generator. The generated vapor HFO (7) is 9907

DOI: 10.1021/acs.iecr.7b02343 Ind. Eng. Chem. Res. 2017, 56, 9906−9916

Article


Figure 3. Structures of the HFO refrigerants and IL absorbent.

where Xi and Yi are the liquid and vapor phase mole fraction of the ith species; p is the system pressure; pSi is the saturated vapor pressure of the ith species; γi is the activity coefficient of the ith species; Φi is the correction factor for the ith species, which is calculated by

condensed to liquid refrigerant (8) in the condenser, gets throttled by the refrigerant expansion valve (9), and finally vaporizes in the evaporator (10) to produce cooling or refrigeration effect. As shown in the PTX diagram in Figure 1b, the concentration difference between the weak solution and strong solution has a significant effect on the cycle performance. Too small a concentration difference will worsen the performance or even make it unable to operate. To solve this problem, the compression-assisted absorption cycle25 can be used, as demonstrated in Figure 2. A compressor is configurated between the evaporator and absorber to boost the absorption pressure from pe (10) to pa (11). In this manner, the weak solution becomes much weaker after absorption under a higher pressure, contributing to an increased concentration difference. On the basis of the previous studies, the compression-assisted absorption cycle is especially promising under lower available driving temperatures26 or lower required cooling temperatures.27 2.2. Property Method of HFO/IL. Two HFO/IL working pairs were studied in this work, i.e., R1234yf/[hmim][Tf2N] and R1234ze(E)/[hmim][Tf2N]. The structures of the refrigerants and absorbent are demonstrated in Figure 3. The thermodynamic properties, like pressures and enthalpies under different solution concentrations and temperatures, are quite essential for the performance simulation of absorption cycles. These properties were derived from the experimental vapor−liquid equilibrium (VLE) data, using the nonrandom two liquid (NRTL) activity coefficient property method. The VLE behavior of a multicomponent system can be described by the following γ/Φ formulation:28 Yp i Φi =

XiγipiS

(i = 1, ..., N )

⎡ (B − V L)(p − pS ) ⎤ i i i ⎥ Φi = exp⎢ ⎢⎣ ⎥⎦ RT

(2)

where Bi is the second virial coefficient and VLi is the saturated molar liquid volume of the ith species. pSi , Bi, and VLi can be obtained from the NIST Refprop software.29 For the HFO (1)/ IL (2) binary system, the concentration of IL in the vapor phase can be neglected; thus Y1, is regarded as 1. The activity coefficient of NRTL model for each component is calculated by eqs 3−6:28 2 ⎤ ⎡ τ21G21 τ12G12 ⎥ ln γ1 = X 22⎢ + (X 2 + X1G12)2 ⎦ ⎣ (X1 + X 2G21)2

(3)

2 ⎤ ⎡ τ12G12 τ21G21 ⎥ ln γ2 = X12⎢ + 2 2 (X1 + X 2G21) ⎦ ⎣ (X 2 + X1G12)

(4)

where G12 and G21 are defined as G12 = exp( −ατ12), G21 = exp( −ατ21)

(5)

τ12 and τ21 are defined as τ12 = τ120 +

τ012,

1 τ12 τ1 0 , τ21 = τ21 + 21 T T

9908

τ021,

(6)

where α, and are adjustable parameters, which were regressed from the experimental VLE data with minor deviations. These parameters for R1234yf/[hmim][Tf2N] and

(1)

τ112,

τ121


Article

Industrial & Engineering Chemistry Research Table 1. Adjustable Parameters of the NRTL Model for HFO/IL Pairs24 HFO/IL pair

α

τ012

τ112

τ021

τ121

R1234yf/[hmim][Tf2N] R1234ze(E)/[hmim][Tf2N]

0.2096 −12.83

20.17 0.3359

−1923 −76.07

36.49 0.2888

−1654 80.25

Figure 4. VLE diagrams of HFO/IL working fluids. Lines stand for results calculated from NRTL model; dots stand for data from measurements.

where R is the univeral gas constant, which is 8.314472 J mol−1 K−1. 2.3. Modeling of Absorption Cooling Cycles. To build the absorption cycle models, some reasonable assumptions should be made for simplification: the system is in steady state; the refrigerants leaving the evaporator and condenser are both saturated; the solutions at the outlet of the generator and absorber are in phase equilibrium; the flow resistance, pressure losses, and heat losses are ignored; the throttling process is isenthalpic.31,32 The absorption cycles using HFO/IL working fluids were modeled based on the following mass and energy balances in each component.10,33,34 For the generator, the heat balance is

R1234ze(E)/[hmim][Tf2N] were obtained from ref 24 and are listed in Table 1. To verify the accuracy of the property model and adjustable parameters, the calculated VLE data and measured VLE data are compared in Figure 4. It is seen that the NRTL model can well predict the VLE behaviors of these HFO/IL working fluids. The enthalpy of the binary HFO (1)/IL (2) mixtures was calculated by12 H = X1H1 + X 2H2 + H E

(7)

where H1 is the enthalpy of liquid HFO, which was calculated using Refprop software,29 H2 the enthalpy of IL, and HE the excess enthalpy. The enthalpy of the IL can be calculated by H2 =

∫T

Q g + m0fh3 = m0h7 + m0(f − 1)h4

where Qg is the generator heat exchange rate; m0 is the refrigerant mass flow rate; h is the specific enthalpy on a massbase; f is the circulation ratio defined as the mass flow rate ratio of weak solution to refrigerant, which can be determined by the mass balance of refrigerant:

T

Cp,IL dT + H0

(8)

0

where T0 is the reference temperature, which was set as 273.15 K; H0 is the reference enthalpy at the reference state, which was set as 200 kJ/kg when converted to mass-based enthalpy; Cp,IL is the liquid heat capacity of IL, which was calculated by Cp,IL = C0 + C1T

m0fx w = m0 + m0(f − 1)xs

(12)

where xw and xs are the mass fractions of HFO in the weak and strong solution, respectively. For the absorber, the heat balance is

(9)

The experimental data for the IL heat capacity were obtained from ref 30, and we regressed the coefficients of eq 9 in Table 2. For the HFO (1)/IL (2) binary system, the excess enthalpy can be calculated by ⎡ ⎛ ⎛ ∂lnγ2 ⎞ ⎤ ∂lnγ1 ⎞ E 2⎢ H = −RT X1⎜ ⎟ + X 2⎜ ⎟ ⎥ ⎢ ⎝ ∂T ⎠ ⎝ ∂ ⎠ p , X ⎥⎦ T ⎣ p,X

(11)

Q a + m0fh1 = m0h10 + m0(f − 1)h6

(13)

For the condenser, the heat balance is Q c + m 0 h8 = m 0 h 7

(14)

For the evaporator, the heat balance is Q e + m0h9 = m0h10

(10)

(15)

For the solution heat exchanger, the heat balance is Table 2. Coefficients of the IL Heat Capacity

Q x = m0f (h3 − h2) = m0(f − 1)(h4 − h5)

IL

C0

C1

[hmim][Tf2N]

434.385

0.654

(16)

where Qa, Qc, Qe, and Qx are the heat exchange rates of the absorber, condenser, evaporator, and solution heat exchanger, 9909


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Figure 5. Model verification of absorption cooling cycles using ILs.

The energy consumption of the compressor was calculated as

respectively. The state point parameters of the solution heat exchanger can be determined by

T − T5 ηx = 4 T4 − T1

Wc = (17)

Q a + m0fh1 = m0h11 + m0(f − 1)h6

(18)

Wp m0f

COP =

(19)

where Wp is the solution pump work, which is usually ignored because of its small percentage compared to the cooling capacity.10 The coefficient of performance (COP) of the absorption cycle was defined as the ratio of the cooling capacity produced in the evaporator to the energy consumption in the generator: COP =

(20)

(21)

h11, i − h10 h11 − h10

g

+

Wc ηe

)

(25)

Qe 1 − ⎛ Q g ⎜1 − ⎝

T0 ⎞ ⎟ Tg ⎠

T0 Te

+ Wc

(26)

where T0 is the reference temperature for exergy calculation, taken as 298.15 K. 2.4. Model Verification. To verify the accuracy of the ILbased absorption cycle models, the calculated results were compared with those for R134a/[hmim][Tf2N] and H2O/ [dmim][DMP] mixtures, because of a lack of studies for HFO/ IL mixtures. Figure 5a shows the comparison for a R134a/ [hmim][Tf2N] absorption cycle under different generation temperatures,19 with condensation, absorption, and evaporation temperatures of 50, 35, and 25 °C, respectively. Figure 5b presents the comparison for a H2O/[dmim][DMP] absorption cycle,36 with condensation, absorption, and evaporation temperatures of 40, 30, and 10 °C, respectively. We can observe that good agreements are reached for both working fluids, in terms of the cooling COP, operation range, and

where pa and pe are absorption pressure and evaporation pressure; CR is the compression ratio provided by the compressor. To determine the outlet parameters after compression, the compressor isentropic efficiency was used:34 ηi =

(Q

ECOP =

In addition to the above models for the single-effect cycle, some supplemental models were built for the compressionassisted cycle. The absorption pressure after compression was calculated by

pa = pe CR

Qe

where ηe is the electricity generation efficiency used to convert electricity to heat, which was set as 0.38 in this study.34,35 For a second law thermodynamic analysis, the exergy coefficient of performance (ECOP) of the compression-assisted absorption cycle was defined as

Qe Qg

(24)

The COP of the compression-assisted absorption cycle was defined as

For the solution pump, the heat balance is

h2 = h1 +

(23)

where ηm is the mechanical efficiency of the compressor, which was set as 1.0 for simplification.31,35 Because the refrigerant parameter at absorber inlet has been changed after compression, the heat balance for the absorber is

where ηx is the solution heat exchanger efficiency, which is set as 0.8 in this study.19 For the refrigerant and solution expansion valves, the isenthalpic processes were assumed:10 h9 = h8 , h6 = h5

m0(h11 − h10) ηm

(22)

where ηi is the isentropic efficiency, which was set as 0.7 in this study;31,34 h11,i is the ideal outlet refrigerant enthalpy in an isentropic compression. 9910


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0.191, while that of R1234ze(E)/[hmim][Tf2N] is always higher, reaching 0.081−0.260. The better performance of R1234ze(E)/[hmim][Tf2N] is comprehensively contributed by two main factors. On the one hand, R1234ze(E) has a higher solubility in [hmim][Tf2N], leading to a higher refrigerant concentration of the weak solution after the absorption process, as shown in Figure 6b. On the other hand, R1234ze(E) has a lower saturation pressure under the same conditions, as compared in Tables 3 and 4, resulting in a lower refrigerant concentration of the strong solution after the generation process. Consequently, the concentration difference of R1234ze(E)/[hmim][Tf2N] is much larger, which finally improves the cooling performance of the absorption cycle. 3.2. Effect of Condensation Temperature. The condensation temperature is practically determined by the external cooling medium, such as underground water, surface water, evaporative cooling water, and ambient air, and it is in the range of 20−40 °C in this work. With a fixed generation temperature of 90 °C and an evaporation temperature of 5 °C, the effects of the condensation temperature on the cooling COP and solution concentration of HFO/IL working fluids are compared in Figure 7. As the condensation temperature increases, the COP decreases dramatically for both R1234yf/[hmim][Tf2N] and R1234ze(E)/[hmim][Tf2N]. A higher condensation temperature yields a higher saturation pressure in the condenser and generator, leading to a higher refrigerant concentration of the strong solution after the generation process. In addition, a higher absorption temperature will weaken the absorption ability, resulting in a lower refrigerant concentration of the weak solution after the absorption process. Therefore, the concentration difference becomes much smaller as the condensation temperature increases, which finally deteriorates the cooling performance of the absorption cycle. The COP of R1234yf/[hmim][Tf 2 N] is 0.323−0.054, while that of R1234ze(E)/[hmim][Tf2N] is 0.400−0.058. Similarly, the better performance of R1234ze(E)/[hmim][Tf2N] is comprehensively attributed to the higher solubility of R1234ze(E) in [hmim][Tf 2 N] and the lower saturation pressure of R1234ze(E). 3.3. Effect of Evaporation Temperature. The evaporation temperature is practically determined by the required cooling or refrigeration temperature, and it ranges from −5 to 20 °C in this work. With a fixed generation temperature of 90 °C and a condensation temperature of 30 °C, the effects of the evaporation temperature on the cooling COP and solution concentration are illustrated in Figure 8. For both R1234yf/[hmim][Tf2N] and R1234ze(E)/[hmim][Tf2N], as the evaporation temperature increases, the COP keeps increasing. A higher evaporation temperature produces a higher saturation pressure in the evaporator and absorber, which makes the absorption process much smoother. As a consequence, the refrigerant concentration of the weak solution becomes higher. Besides, the refrigerant concentration of the strong solution is little affected by the evaporation temperature (Figure 8b). Therefore, the concentration difference becomes much larger and the cooling performance of the absorption cycle is improved substantially. The COP of R1234yf/ [hmim][Tf2N] ranges between 0.036 and 0.414, while that of R1234ze(E)/[hmim][Tf2N] ranges between 0.046 and 0.498.

performance trend. With verified model accuracy, we can use the established models to investigate the performance of absorption cycles using different HFO/IL working fluids.

3. PERFORMANCE OF SINGLE-EFFECT ABSORPTION CYCLE The performance of the single-effect absorption cycle using HFO/IL working fluids was analyzed under different generation temperatures, condensation (absorption) temperatures, and evaporation temperatures. For typical conditions with generation, condensation, and evaporation temperatures of 90, 30, and 5 °C, respectively, the state point parameters of R1234yf/[hmim][Tf2N] and R1234ze(E)/[hmim][Tf2N] are listed in Tables 3 and 4, respectively. Table 3. State Point Parameters of R1234yf/[hmim][Tf2N] Single-Effect Cycle state point

temperature (°C)

pressure (kPa)

vapor fraction

mass flow rate (kg s−1)

refrigerant concentration (%)

1 2 3 4 5 6 7 8 9 10

30.0 30.0 77.1 90.0 42.0 42.0 76.4 30.0 5.0 5.0

372.92 783.51 783.51 783.51 783.51 372.92 783.51 783.51 372.92 372.92

0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.213 1.000

1.00 1.00 1.00 0.97 0.97 0.97 0.03 0.03 0.03 0.03

6.98 6.98 6.98 3.75 3.75 3.75 100.00 100.00 100.00 100.00

Table 4. State Point Parameters of R1234ze(E)/ [hmim][Tf2N] Single-Effect Cycle state point

temperature (°C)

pressure (kPa)

vapor fraction



1 2 3 4 5 6 7 8 9 10

30.0 30.0 76.4 90.0 42.0 42.0 75.4 30.0 5.0 5.0

259.34 578.33 578.33 578.33 578.33 259.34 578.33 578.33 259.34 259.34

0.000 0.000 0.000 0.000 0.000 0.000 1.000 0.000 0.189 1.000

1.00 1.00 1.00 0.95 0.95 0.95 0.05 0.05 0.05 0.05

7.50 7.50 7.50 3.06 3.06 3.06 100.00 100.00 100.00 100.00

3.1. Effect of Generation Temperature. The generation temperature is practically determined by the driving sources, like natural gas, waste heat, and renewable energy, and it ranges from 60 to 95 °C in this work. With a fixed evaporation temperature of 5 °C and a condensation temperature of 30 °C, the effects of the generation temperature on the cooling COP and solution concentration of HFO/IL working fluids are presented in Figure 6. As the generation temperature increases, the COP increases first and stabilizes or slightly decreases later. A higher generation temperature causes the better generation process and higher performance, but the increased irreversibility may reduce the performance when the temperature is too high.10 The COP of R1234yf/[hmim][Tf2N] is in the range of 0.067− 9911


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Figure 6. Effect of generation temperature on the performance of single-effect cycle.

Figure 7. Effect of condensation temperature on the performance of single-effect cycle.

Figure 8. Effect of evaporation temperature on the performance of single-effect cycle.

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4. PERFORMANCE OF COMPRESSION-ASSISTED ABSORPTION CYCLE The cooling performance greatly deteriorates under lower generation temperatures and lower evaporation temperatures for the single-effect absorption cycle. Therefore, the compression-assisted absorption cycle is used to extend the operation range and improve the cooling performance of the novel HFO/ IL working fluids. For a typical condition with generation, condensation, and evaporation temperatures of 70, 30, and 5 °C, respectively, and a compression ratio of 1.5, the parameters of R1234yf/ [hmim][Tf2N] and R1234ze(E)/[hmim][Tf2N] of each state point are listed in Tables 5 and 6, respectively. Compared with Table 5. State Point Parameters of R1234yf/[hmim][Tf2N] Compression-Assisted Cycle state point

temperature (°C)

pressure (kPa)

vapor fraction



1 2 3 4 5 6 7 8 9 10 11

30.0 30.0 60.2 70.0 38.0 38.0 57.0 30.0 5.0 5.0 20.3

559.4 783.5 783.5 783.5 783.5 559.4 783.5 783.5 372.9 372.9 559.4

0.00 0.00 0.00 0.00 0.00 0.00 1.00 0.00 0.21 1.00 1.00

1.000 1.000 1.000 0.935 0.935 0.935 0.065 0.065 0.065 0.065 0.065

11.96 11.96 11.96 5.83 5.83 5.83 100.00 100.00 100.00 100.00 100.00

Figure 9. Performance improvement of compression-assisted cycle under lower generation temperatures.

It is observed that the compression-assisted cycle not only extends the minimum generation temperature but also improves the cooling performance for both R1234yf/[hmim][Tf2N] and R1234ze(E)/[hmim][Tf2N]. Compared with the single-effect cycle, the minimum generation temperature is reduced from 63 to 46 °C. Meanwhile, the cooling COP is maintained in a range of 0.181−0.366 for R1234yf/[hmim][Tf2N] and 0.187−0.430 for R1234ze(E)/[hmim][Tf2N]. It should be noted that the COP difference between these two HFO/IL working fluids is narrowed by the compressionassisted cycle, indicating that the performance improvement has more potential for the worse-performing R1234yf/[hmim][Tf2N]. 4.2. Performance under Lower Evaporation Temperature. Lowering the evaporation temperature is favorable for providing lower-temperature cooling services. The evaporation temperature is chosen to be between −20 and 20 °C for the compression-assisted cycle with a compression ratio of 1.5. Under a fixed generation temperature of 90 °C and a condensation temperature of 30 °C, the cooling COPs of different absorption cycles using different HFO/IL pairs are compared in Figure 10.

Table 6. State Point Parameters of R1234ze(E)/ [hmim][Tf2N] Compression-Assisted Cycle state point

temperature (°C)

pressure (kPa)

vapor fraction



1 2 3 4 5 6 7 8 9 10 11

30.0 30.0 59.6 70.0 38.0 38.0 57.3 30.0 5.0 5.0 20.0

389.0 578.3 578.3 578.3 578.3 389.0 578.3 578.3 259.3 259.3 389.0

0.00 0.00 0.00 0.00 0.00 0.00 1.00 0.00 0.19 1.00 1.00

1.000 1.000 1.000 0.919 0.919 0.919 0.081 0.081 0.081 0.081 0.081

13.24 13.24 13.24 5.58 5.58 5.58 100.00 100.00 100.00 100.00 100.00

the results of the single-effect absorption cycle in Tables 3 and 4, it is found that the absorption pressures are increased from 372.9/259.3 kPa to 559.4/389.0 kPa, contributing to higher concentration differences though the generation temperature is reduced from 90 to 70 °C. 4.1. Performance under Lower Generation Temperature. Lowering the generation temperature is of great significance for utilizing the lower-grade waste heat or renewable energy. The generation temperature is set as 45− 80 °C for the compression-assisted cycle with a compression ratio of 1.5. Under a fixed evaporation temperature of 5 °C and a condensation temperature of 30 °C, the cooling COPs of the compression-assisted and single-effect absorption cycles using HFO/IL working fluids are compared in Figure 9.

Figure 10. Performance improvement of compression-assisted cycle under lower evaporation temperatures. 9913


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Figure 11. Effect of compression ratio on the performance of compression-assisted cycle (R1234ze(E)).

Compared with the single-effect cycle, the minimum evaporation temperature is reduced from −7 to −18 °C for R1234yf/[hmim][Tf2N] and from −10 to −20 °C for R1234ze(E)/[hmim][Tf2N]. The cooling COP is in the range of 0.031−0.563 for R1234yf/[hmim][Tf 2N] and 0.039−0.606 for R1234ze(E)/[hmim][Tf2N]. Also, the COP difference between these two HFO/IL working fluids is narrowed by the compression-assisted cycle, indicating that the performance improvement has more potential for the worse-performing R1234yf/[hmim][Tf2N]. 4.3. Performance under Various Compression Ratios. The above analyses are based on a compression ratio of 1.5, and the effect of the compression ratio on the cycle performance is investigated in this section. Under generation temperatures of 75 and 80 °C, evaporation temperatures of −5 and 0 °C, and a condensation temperature of 30 °C, the cooling COPs and ECOPs versus the compression ratio (varying between 1.0 and 3.0) are depicted in Figure 11. A compression ratio of 1.0 means no compression, i.e., the single-effect cycle. As the compression ratio increases from 1.0 to 3.0, the cooling COP increases because of a higher absorption pressure. The cooling ECOP increases first and decreases later, because a higher compression ratio requires more high-quality electricity input to the generator, leading to a decreased ECOP when the compression is too high. There is an optimal compression ratio at which the ECOP reaches the maximum. Figure 11a shows that the COP can reach 0.546 or 0.538 at generation temperatures of 75 or 80 °C, respectively. To achieve the maximum ECOPs of 0.196 and 0.182, the optimal compression ratios are 2.0 and 2.1, respectively. Figure 11b indicates that the COP can reach 0.450 and 0.496 at evaporation temperatures of −5 and 0 °C, respectively. To achieve the maximum ECOPs of 0.208 and 0.187, the optimal compression ratios are 3.6 and 3.0, respectively. Though a lower evaporation temperature yields a lower COP, it produces a higher-quality cooling effect, contributing to a higher ECOP under the some conditions. In addition, a lower evaporation temperature requires a higher optimal compression ratio. 4.4. Comparisons with Existing Fluids and Systems. The novel HFO/IL working fluids are compared with the conventional H2O/LiBr and NH3/H2O. With a generation temperature of 70 °C and a condensation temperature of 30 °C, the COPs under a cooling condition (te = 5 °C) and a refrigeration condition (te = −5 °C) are presented in Table 7.

Table 7. COP Comparisons of Different Working Fluids working fluid

te = 5 °C

te = −5 °C

R1234yf/[hmim][Tf2N] R1234ze(E)/[hmim][Tf2N] H2O/LiBr NH3/H2O

0.363 0.426 0.805 0.737

0.151 0.206 − 0.634

The COPs of HFO/IL are lower than those of H2O/LiBr and NH3/H2O. Considering the possibilities to overcome the problems of the conventional working fluids (toxicity, crystallization, negative operation, etc.), more ILs need to be explored to improve the performance of the novel HFO/IL working fluids. In addition, the compression-assisted absorption cycle is also compared with the vapor-compression cycle. With the same cooling duty of 100 kW, the electricity and heat consumption for different cycles are presented in Table 8. The absorption cycles require lower electricity but much more heat than the compression cycles. Table 8. Electricity and Heat Consumption of Absorption Cycle and Compression Cycle (kW) te = 5 °C

te = −5 °C

cycle

electricity

heat

electricity

heat

absorption (R1234yf/[hmim] [Tf2N]) absorption (R1234ze(E)/[hmim] [Tf2N]) compression (R1234yf) compression (R1234ze(E))

8.27

254.9

8.67

639.6

7.39

234.3

7.65

381.7

15.08 14.66

0 0

23.10 22.33

0 0

5. CONCLUSIONS Absorption technologies offer great opportunities for waste heat recovery as well as renewable energy utilization. The potential of using novel low-GWP HFO/IL working fluids in absorption cycles was analyzed. The NRTL property models were used, and the thermodynamic models of absorption cycles were built with verified accuracy. The cooling performances of different HFO/IL working fluids were comparatively investigated under various working parameters. In addition, the performance was improved by the compression-assisted 9914


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Industrial & Engineering Chemistry Research Y = vapor mole fraction α,τ = adjustable parameters η = efficiency γ = activity coefficient Φ = correction factor

absorption cycle, and the compression ratio was optimized. The main conclusions are as follows: (1)The cooling COP of R1234yf/[hmim][Tf2N] is up to 0.414, while that of R1234ze(E)/[hmim][Tf2N] is up to 0.498 under a wide range of working parameters. The better performance of R1234ze(E)/[hmim][Tf2N] is comprehensively contributed by its higher solubility and lower saturation pressure. (2)The compression-assisted cycle not only extends the operation range but also improves the COP for both R1234yf/ [hmim][Tf2N] and R1234ze(E)/[hmim][Tf2N]. The minimum generation and evaporation temperatures are reduced by 17 and 11 °C under a compression ratio of 1.5. The performance improvement has more potential for the worseperforming R1234yf/[hmim][Tf2N]. (3) There is an optimal compression ratio at which the ECOP reaches the maximum. A lower evaporation temperature requires a higher optimal compression ratio. (4) The COPs of HFO/IL are lower than those of H2O/LiBr and NH3/H2O. With the same cooling duty, the absorption cycles require lower electricity but much more heat than the compression cycles. More HFO/IL mixtures need to be explored for further performance improvement. This study provides more choices and useful suggestions for novel absorption working fluids, thus promoting better developments for absorption technologies.

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Abbreviations

CFC = chlorofluorocarbon COP = coefficient of performance GWP = global warming potential HCFC = hydrochlorofluorocarbon HFC = hydrofluorocarbon HFO = hydrofluoroolefin IL = ionic liquids NRTL = nonrandom two liquid ODP = ozone depletion potential VLE = vapor−liquid equilibrium Subscripts and Superscripts

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86-10-62785860. Fax: +86-10-62773461. *E-mail: [email protected]. Tel: +86-10-62785860. Fax: +86-10-62773461.

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a = absorber c = condenser, compressor, critical e = evaporator, electricity E = excess i = species index, isentropic, ideal L = liquid m = mechanical g = generator p = pump s = saturation, strong w = weak x = heat exchanger

REFERENCES

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ORCID

Wei Wu: 0000-0002-9657-6682 Haiyang Zhang: 0000-0002-3303-8975 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support of Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51521005) and National Natural Science Foundation of China (Grant No. 51638010).

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NOMENCLATURE B = second virial coefficient, cm3 mol−1 C = heat capacity, J mol−1 K−1 CR = compression ratio f = circulation ratio h = mass-based enthalpy, kJ kg−1 H = mole-based enthalpy, J mol−1 m = mass flow rate, kg s−1 p = pressure, kPa Q = heat exchange rate, kW R = universal gas constant, 8.314472 J mol−1K−1 t = temperature, °C T = temperature, K V = molar volume, cm3 mol−1 W = power rate, kW x = liquid mass fraction X = liquid mole fraction 9915


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Thermodynamic Investigation and Comparison of Absorption Cycles

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