IL working pairs

Applied Thermal Engineering 172 (2020) 115161

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Gaseous solubility and thermodynamic performance of absorption system using R1234yf/IL working pairs

T

Yanjun Suna, , Gaolei Dia, Jian Wanga, Yusheng Hub,c, , Xiaopo Wangd, Maogang Hed ⁎

⁎

a

Institute of Building Energy & Sustainability Technology, School of Human Settlements and Civil Engineering, Xi'an Jiaotong University, Xi’an 710049, China State Key Laboratory of Air-conditioning Equipment and System Energy Conservation, Zhuhai 519070, China c Gree Electric Appliances Inc. of Zhuhai, Zhuhai 519070, China d Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, Xi'an Jiaotong University, Xi’an 710049, China b

HIGHLIGHTS

were measured for three new R1234yf/IL working pairs from 283.15 to 343.15 K. • Solubilities R1234yf/IL working pairs are analyzed for single-effect and compression-assisted cycles. • Several • [P ][Cl] performs best among the existing R1234yf/IL working pairs. 66614

ARTICLE INFO

ABSTRACT

Keywords: Absorption-refrigeration system Solubility Ionic liquid R1234yf

To overcome the weaknesses of traditional working pairs (H2O/LiBr and NH3/H2O) in the absorption-refrigeration systems, 2,3,3,3-tetrafluoroprop-1-ene (R1234yf)/ionic liquid (IL) as a promising working pair has attracted wide attention. Using an isochoric saturation method, the solubilities were measured for three working pairs (R1234yf/[emim][Ac], R1234yf/[bmim][Ac] and R1234yf/[P66614][Cl]) from 283.15 to 343.15 K and the data was correlated by the NRTL model. Solubilities rise with a decrease in temperature and an increase in pressure. Highest solubility is in [P66614][Cl], followed by [bmim][[Ac] and [emim][Ac]. The cooling performance was studied for the single-effect and compression-assisted system using several R1234yf/IL mixtures. The effects of compression ratio, generation and evaporation temperature on the coefficient performance (COP) and circulation ratio were analyzed. Under the condensation temperature of 303 K, generation temperature of 363 K and absorption temperature of 303 K, the maximum COPs for [emim][Ac], [bmim][Ac] and [P66614][Cl] are respectively 013, 0.23, and 0.46 in the compression-assisted system. [P66614][Cl] has the highest COP and exergy coefficient of performance (ECOP) while [emim][Ac] shows lowest under the same condition.

1. Introduction With the increasing gradually energy consumption and approaching depletion of primary energy, the energy conservation has become a serious issue. The statistics indicate that buildings account for 15–25% of total energy consumption in developing countries and 30–40% in developed countries [1]. The major building-energy consumption is used in the heating, ventilation and air-conditioning (HVAC) system (over 70% in Middle-East, 57% in European Union (EU) and 39% in U.S.) [2]. To deal with the problems, the absorption-cycle technology driven by low-quality thermal energy, such as solar and geothermal energy, or industrial waste heat, has become attractive for heating and cooling in HVAC system [3,4].

⁎

The thermodynamic properties of refrigerant/absorbent are the main limited factors to the cooling performance of an absorption-refrigeration cycle. At present, the commonly used working pairs are the aqueous solution of lithium bromide (H2O/LiBr) and ammonia/water (NH3/H2O). However, the NH3/H2O system suffers from corrosion and toxicity, and requires a complex and costly rectifier for refrigerant purification [5]. H2O/LiBr is mainly applied in room air conditioning owing to the high refrigerant freezing point [6]. Meanwhile, they may crystallize and give undesired pressure below atmosphere [7]. Thus, it is of great significance to explore the alternative working pairs to overcome the weaknesses. Due to the negligible vapor pressure, good thermal and chemical stability, and green environmental protection, ionic liquids (ILs) are

Corresponding authors at: State Key Laboratory of Air-conditioning Equipment and System Energy Conservation, Zhuhai 519070, China (Y. Hu). E-mail addresses: [email protected] (Y. Sun), [email protected] (Y. Hu).

https://doi.org/10.1016/j.applthermaleng.2020.115161 Received 5 December 2019; Received in revised form 1 March 2020; Accepted 3 March 2020 Available online 05 March 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved.

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expected to be the most potential absorbents for absorption-refrigeration systems [8]. In 2004, Kim et al. [9] firstly proposed [bmim][Br]/ TFE and [bmim][BF4]/TFE as candidates for working pairs in absorption chillers; [bmim][Br]/TFE is more favorable than [bmim][BF4]/ TFE. Since 2006, Shiflett and Yokozeki [10–15] reported lots of papers about ILs as absorbents, that are impressive researches at this field in recent decade. The solubilities of NH3, H2O, CO2, and HFC in ILs were measured, and some working pairs were proposed as possible alternatives for absorption cycles. In 2006, Sen and Paolucci [16] found that [bmim][PF6] combined with supercritical CO2 can be used in absorption systems. Their studies on the vapor pressure, dissolve ability and thermal stability of working pairs showed that thermophysical properties of IL-based working pairs are still rare. In 2010, Martin and Bermejo [17] demonstrated that CO2/ [omim][NO3] and CO2/[bmpyrr][Tf2N] were more suitable for the cooling absorption by evaluating the energetic efficiency of the process using 25 CO2/IL mixtures. But these IL-based working pairs have a lower COP than traditional NH3/H2O system owing to the necessity of operating with a higher circulation ratio. In 2008, Wang and Zheng [18] studied a double-effect absorption system using [bmim][Br]/TFE as a working pair, suggesting that [bmim][Br]/TFE was a good potential working pair. In 2012, Cheng et al. [19] showed that the COP of CH3OH/[mmim][DMP] was slightly lower than that of H2O/LiBr but higher than that of NH3/H2O. In 2014, Ruiz et al. [20] studied 8 NH3/IL pairs under different operation conditions and found that ILs with high NH3 solubility can enhance the cooling performance. In 2016, Chen and Bai [21] analyzed the cycle efficiency of NH3/[emim][Cu2Cl5] and found it is better than other NH3/IL pairs. Sun et al. [22] evaluated 16 NH3/IL mixtures, indicating that NH3/[bmim][Ac] has the best potential. In 2010, Yokozeki and Shiflett [10] evaluated the performance of 13 H2O/IL working pairs, and the results showed that H2O/IL pairs (when a suitable IL was used) can compete with H2O/LiBr. In 2011, Zhang and Hu [23] analyzed the cooling performance of absorption-refrigeration system using H2O/[emim][DMP], and found that its COP was lower than that of H2O/LiBr but still higher than 0.7. Meanwhile, a lower generation temperature was obtained using H2O/[emim][DMP] compared to H2O/LiBr, indicating that this working pair can be driven by hot water or waster heat with lower temperature. In 2012, Dong et al. [24] showed that the COP of H2O/[dmim][DMP] was slightly lower than that of H2O/LiBr. The advantage is that H2O/[dmim][DMP] can improve the limitation of corrosion and crystallization. They also studied the suitability of several H2O/IL mixtures upon calculating excess Gibbs function, suggesting that IL with halogen and phosphate anion could be as an absorbent [25]. In 2012, Kim et al. [26] evaluated the COP and feasibility of absorption-refrigeration cycles using [emim][Tf2N], [bmim][BF4], [emim][BF4], [bmim][PF6], [hmim][PF6], [hmim][Tf2N] and [hmim] [BF4] paired with R32, R134a, R143a, R125, R152a, R114, R124. The results showed the refrigerant-solubility in ILs largely affected the circulation ratio. R32 combined with [PF6]− and [BF4]− anions based ILs have the lower circulation ratio and higher COP. In 2014, Kim and Kohl [27] compared the cooling performance of R134a/[hmim][PF6] with that of R134a/[hmim][Tf2N], showing that R134a/[hmim][Tf2N] has a maximum COP of about 0.5 while R134a/[hmim][PF6] is about 0.2 under the evaporation, absorption, and condensation temperature of 25 °C, 35 °C, and 50 °C, respectively. The presence of electronegative moieties (F, O and N) in the [Tf2N]− anion leads to higher R134a-solubility, that is favorable to the cooling performance using [hmim] [Tf2N] as an absorbent. Depending on the refrigerant, the novel working pairs are mainly divided into five categories: NH3/IL, H2O/IL, CO2/IL, alcohol/IL and hydrofluorocarbon (HFC)/IL. The above investigations suggest the great potential and significant application prospects of these novel working pairs. For HFC/IL working pairs, HFCs are not harmful to atmospheric ozone but they are greenhouse gases because of their high

global warming potential (GWP). 2,3,3,3-tetrafluoroprop-1-ene (R1234yf) as one of the unsaturated fluorinated hydrofluoroolefin refrigerant has the atmospheric lifetime of only 11 days and a 100-year time horizon GWP of 4 relative to carbon dioxide, and its thermodynamic properties are quite similar to those of R134a [28,29]. Therefore, R1234yf is considered a “fourth generation” refrigerant in the refrigeration industry. However, there is little research on the thermodynamic performance of R1234yf/IL mixtures. Sujatha and Venkatarathnam [30] studied the cooling performance of absorption systems using R32, R152a, R125, R1234ze(E), R1234yf as a refrigerant and [hmim][Tf2N] as an absorbent, showing that the cooling efficiency of R32 and R152a is higher than that of R1234yf at various operating temperatures. Wu et al. [31] compared the COP and ECOP for R1234yf/[hmim][Tf2N] with those for R1234ze(E)/[hmim][Tf2N] in two absorption systems. The COP and ECOP of R1234yf are lower than those of R1234ze(E); both are largely lower than H2O/LiBr and NH3/H2O. Liu et al. [32] analyzed the effects of compressor position on the COP of absorptionrefrigeration systems using R1234yf/[hmim][Tf2N], R1234yf/[hmim] [PF6], and R1234yf/[hmim][TfO]. Among the three working pairs, R1234yf/[hmim][TfO] has the highest COP. Our group [33] studied the cooling performance of single-effect and compression-assisted cycles using [hmim][TfO], [emim][BF4], [hmim][PF6], [hmim][BF4], [hmim] [Tf2N], and [omim][BF4] as an absorbent and R1234yf as a refrigerant, suggesting that [hmim][Tf2N] has the highest COP and ECOP while [emim][BF4] performs worst. The studied ILs used as absorbents are mostly focused on the imidazolium-based ILs. Compared to imidazolium-based ILs, phosphonium-based ILs are always less dense than water and have an extremely low melting temperature. Moreover, they are more stable in basic and nucleophilic conditions owing to the absence of acidic protons in their moieties [34]. Sousa et al. [35,36] measured the solubilities of R41, R11, R23 in [P66614][Cl], [P66614][Tf2N], [P4441][C1SO4], and [P4442] [(C2)2PO4] from 288 to 308 K at atmospheric pressure. Shiflett et al. [37] measured the solubility and diffusivity of R134a in [P66614][TPES] and in [P4441][HFPS] from 283 to 348 K and pressure up to 0.35 MPa. Their results indicate that phosphonium and imidazolium based ILs have similar solubility for several refrigerants. However, there is little information about performance of absorption cycles using phosphonium-based ILs as absorbents. Towards better understanding the cooling performance of absorption cycles using imidazolium and phosphonium based ILs as absorbents and R1234yf as a refrigerant, solubilities of R1234yf in [emim] [Ac], in [bmim][Ac] and in [P66614][Cl] were measured from 283.15 to 343.15 K and pressure up to 0.83 MPa using an isochoric saturation method. The data was correlated by the Non-Random Two-Liquid (NRTL) model. The performance of single-effect and compression-assisted cycle was analyzed using several R1234yf/IL working pairs. Furthermore, the influences of compression ratio, generation temperature, and evaporation temperature on the COP and circulation ratio were investigated. 2. Experimental section 2.1. Materials 2,3,3,3-tetrafluoroprop-1-ene (R1234yf, CAS No. 754-12-1) was supplied by Honeywell Corporation with a declared mass purity better than 99.9%. 1-ethyl-3-methylimidazoliun acetate ([emim][Ac], CAS No. 143314-17-4), 1-butyl-3-methylimidazolium acetate ([bmim][[Ac], CAS No. 284049-75-8), and trihexyltetradecylphosphonium chloride ([P66614][Cl], CAS No. 258864-54-9) were purchased from Sigma -Aldrich with a stated mass purity higher than 95.0%. Before measurements, R1234yf was purified 3–5 times using liquid nitrogen and a high-vacuum system (freeze–pump–thaw) to eliminate non-condensable gases. The ILs were degassed and dried under vacuum while 2

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Y. Sun, et al.

described previously [38]. Here, we give only its main characteristics. As shown in Fig. 2, two stainless-steel cells were put in a thermostatic bath. The volumes of the two cells were calibrated with carbon dioxide which was supplied by Praxair Inc. with declared mass purity of 99.999%. The charged mass of carbon dioxide was weighed using the analytical balance with an uncertainty of 0.002 g (Mettler Toledo ME204, 220 g full scale); the density was calculated by Refprop 9.1 database [39]. One cell is the equilibrium cell with calibrated volume 31.33 cm3; the other is the vapor cell with calibrated volume 73.26 cm3. The temperature was measured by a calibrated Pt100 resistance thermometer (Fluke 5608). The pressure in the cell was measured by a pressure transducer (KELLER 33X) with a maximum range of 3000 kPa. The standard uncertainty in temperature was 0.03 K and in pressure was 2.0 kPa. A well-known amount of dried IL was firstly injected into the equilibrium cell, and then R1234yf was charged into the vapor cell after eliminating the air inside the experimental system by a vacuum pump. The charged mass of R1234yf was determined by the vapor cell volume and density gotten from Refprop 9.1. After that, the valve (V4) between the equilibrium cell and the vapor cell was opened, R1234yf started to be absorbed into IL. The magnetic stirrer was turned off when the pressure approached a constant value. The pressure was recorded. The next step was to change the bath temperature to obtain a new equilibrium pressure. The mole fraction x1 of R1234yf in the IL was determined by x1 = n1/(n1 + n2), where n1 is the number of mole for R1234yf dissolved in the IL; n2 is the number of mole for IL. Because of the extremely-low vapor pressure of IL, the tiny gaseous component of IL can be neglected. As shown by literature [40], n1 was calculated by

Table 1 Samples used in experiment. Material

Source

Mass Fraction Purity

Purification Method

Water Mass Fraction

R1234yf [emim][Ac] [bmim][Ac] [P66614][Cl]

Honeywell Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich

≥0.999 ≥0.95 ≥0.95 ≥0.95

freeze–pump–thaw vacuum-dry-heat vacuum-dry-heat vacuum-dry-heat

None < 100 ppm < 100 ppm < 100 ppm

H

F

F

C

C

C

N

+N

F O

H

F

O

[Emim][Ac] Molecular Weight:170.21

R1234yf Molecular Weight:114.04

N

-

+N P O

+

-

Cl

O

[Bmim][Ac] Molecular Weight:198.26

-

[P66614][C l] Molecular Weight:519.31

Fig. 1. The molecular structures of R1234yf and ILs.

n1 = n10

heated and stirred at about 353 K for at least 48 h. The final mass fraction of water was checked by Karl-Fischer titration (MKC-710), that is no more than 100 ppm for each dried ILs. Table 1 gives pertinent data for all samples used here. Fig. 1 presents the molecular structures of R1234yf and three ILs.

(1)

n11

The initial number of moles of gas in the system (n10 ) is calculated by

n10 =

Vsys (2)

vgas (Tini,pini )

When the refrigerant/IL mixture reaches equilibrium, the number of moles of gaseous refrigerant (n11) is calculated by

2.2. Solubility measurement The solubility was measured by an isochoric saturation method

13 13 10 V2

12

11

8

9

V1

7 V4

V3 5

6 4

1

3

2

1 Gas Bottle 2 Thermostatic Bath 3 Cooling Thermostat Bath 4 Stirrer 5 Gas Cell 6 Equilibrium Cell 7 Thermometer 8 PID Controller 9 Pressure Transducer 10 Vacuum 11 Resistance 12 DC Power 13 Multimeter Fig. 2. Schematic diagram of the solubility measurement. 3

Applied Thermal Engineering 172 (2020) 115161

Y. Sun, et al.

n11 =

Vsys vgas (Tequilib, pequilib )

+

Vcell V2, cell vgas (Tequilib, pequilib )

Table 3 Vapor-liquid equilibrium of R1234yf (1) + [bmim][Ac] (2).

Vabs, gas vgas (Tequilib, pequilib ) (3)

where the subscripts 'equilib' and 'ini' stand for the equilibrium and initial condition; 'sys' and 'cell' refer to the vapor and equilibrium cell; V2,cell is the volume of the IL charged into the equilibrium cell; The vapor molar volume of R1234yf (υgas) is gotten from Refprop 9.1. The volume of gas absorbed in the solvent, Vabs, gas, can be expressed by (4)

Vabs, gas = n1 vabs, gas

Since the measured conditions are lower than the critical parameters of R1234yf, the partial molar volume of dissolved R1234yf in the IL (υabs,gas in Eq. (4)) is assumed to its saturated liquid volume at the equilibrium temperature. Then we can derive the equation of n1

Vsys

n1 =

gas (Tini,

1

Vsys pini )

gas (Tequilib,

pequilib )

+

V2, cell gas (Tequilib,

Vcell pequilib )

abs, gas gas (Tequilib,

pequilib )

(5)

Considering uncertainties of pressure, cell volume, temperature, mass and density of solvent, the standard uncertainty in solubility is better than 3.0%. 3. Experimental data and correlation Tables 2–4 show solubility data (pTx) for R1234yf in [emim][Ac], [bmim][[Ac] and [P66614][Cl] from 283.15 to 343.15 K. For binary solution, thermodynamic equilibrium is given by [40–42] i yi p

= i xi pis

T/K

p/MPa

x1

T/K

p/MPa

x1

283.15 283.15 283.15 283.15 283.15 283.15 283.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15

0.078 0.147 0.201 0.257 0.310 0.357 0.405 0.081 0.156 0.214 0.276 0.334 0.385 0.439 0.495 0.551 0.086 0.166 0.227 0.293 0.355 0.410 0.469 0.530 0.591 0.658 0.090 0.174 0.239 0.309 0.375 0.433 0.496

0.021 0.040 0.056 0.076 0.095 0.113 0.133 0.019 0.034 0.047 0.060 0.073 0.087 0.101 0.113 0.129 0.015 0.027 0.036 0.047 0.058 0.069 0.080 0.088 0.100 0.113 0.012 0.023 0.030 0.039 0.047 0.056 0.065

313.15 313.15 313.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15

0.561 0.627 0.699 0.095 0.182 0.250 0.324 0.393 0.456 0.522 0.590 0.661 0.738 0.099 0.189 0.261 0.338 0.410 0.477 0.546 0.618 0.693 0.775 0.103 0.197 0.271 0.351 0.427 0.497 0.570 0.646 0.724 0.811

0.071 0.080 0.091 0.010 0.018 0.025 0.031 0.039 0.045 0.052 0.059 0.067 0.074 0.008 0.015 0.021 0.0270 0.033 0.038 0.044 0.050 0.057 0.063 0.007 0.013 0.018 0.023 0.028 0.033 0.038 0.042 0.049 0.054

Table 4 Vapor-liquid equilibrium of R1234yf (1) + [P66614][Cl] (2).

(6)

where

T/K

p/MPa

x1

T/K

p/MPa

x1

Table 2 Vapor-liquid equilibrium of R1234yf (1) + [emim][Ac] (2).

283.15 283.15 283.15 283.15 283.15 283.15 283.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15

0.064 0.130 0.180 0.233 0.281 0.321 0.362 0.068 0.142 0.198 0.258 0.313 0.362 0.413 0.460 0.505 0.073 0.154 0.215 0.280 0.341 0.396 0.454 0.510 0.566 0.628 0.079 0.165 0.230 0.300 0.366 0.426 0.490

0.158 0.299 0.380 0.456 0.516 0.570 0.623 0.146 0.261 0.330 0.397 0.451 0.501 0.550 0.594 0.641 0.126 0.220 0.284 0.344 0.394 0.439 0.483 0.524 0.566 0.608 0.102 0.188 0.244 0.299 0.344 0.383 0.425

313.15 313.15 313.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 323.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15

0.552 0.615 0.686 0.084 0.175 0.244 0.319 0.389 0.453 0.522 0.589 0.659 0.737 0.088 0.184 0.256 0.336 0.410 0.478 0.552 0.624 0.698 0.783 0.092 0.193 0.269 0.352 0.430 0.502 0.580 0.656 0.735 0.825

0.463 0.501 0.538 0.087 0.159 0.209 0.258 0.299 0.336 0.375 0.409 0.442 0.477 0.074 0.137 0.181 0.225 0.262 0.296 0.332 0.364 0.395 0.427 0.061 0.118 0.158 0.198 0.230 0.262 0.296 0.325 0.355 0.384

T/K

p/MPa

x1

T/K

p/MPa

x1

283.15 283.15 283.15 283.15 283.15 283.15 283.15 293.15 293.15 293.15 293.15 293.15 293.15 293.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 313.15 313.15 313.15 313.15 313.15 313.15 313.15

0.073 0.145 0.207 0.266 0.319 0.371 0.420 0.076 0.153 0.219 0.282 0.339 0.395 0.449 0.080 0.161 0.230 0.297 0.358 0.417 0.474 0.084 0.168 0.240 0.311 0.375 0.438 0.500

0.009 0.019 0.027 0.035 0.043 0.051 0.060 0.009 0.016 0.022 0.028 0.034 0.039 0.046 0.007 0.012 0.018 0.022 0.027 0.032 0.037 0.006 0.011 0.015 0.019 0.022 0.026 0.030

323.15 323.15 323.15 323.15 323.15 323.15 323.15 333.15 333.15 333.15 333.15 333.15 333.15 333.15 343.15 343.15 343.15 343.15 343.15 343.15 343.15

0.087 0.175 0.251 0.324 0.392 0.458 0.523 0.091 0.182 0.261 0.338 0.408 0.478 0.545 0.094 0.189 0.271 0.350 0.424 0.496 0.568

0.005 0.009 0.012 0.015 0.019 0.021 0.024 0.004 0.007 0.010 0.013 0.015 0.017 0.020 0.003 0.006 0.009 0.010 0.013 0.015 0.018

4

Applied Thermal Engineering 172 (2020) 115161

Y. Sun, et al. i, vap (T ,

=

i

i, pure [T ,

p, y) s

pi (T )]

ViL (pis

exp

exp

ps

1 RT

p

i, vap (T ,

ViL dp RT

i, pure [T ,

0.7

p, y ) s

pi (T )]

0.6

p)

0.5

The subscript 'i' represents two components in the system: refrigerant (1) and IL (2); xi is the liquid-phase mole fraction of component i; yi is the vapor-phase mole fraction of component i; pis is the saturated vapor pressure of component i; p is the equilibrium pressure; γi is the activity coefficient of component i; ViL refers to the saturated molar liquid volume. ζi is the correction factor. The exponential term in Eq. (7) is the Poynting factor. φi is the fugacity coefficient. The virial EoS is used to estimate the properties of the gas phase. The vapor-phase fugacity coefficient of the solute writes:

0.4

(2B12 B p ln[ 1, vap (T , p , y )] = 11 + RT

B22 ) y22 p

B11

p/MPa

(7)

RT

0.2 0.1 0.0 0.00

In the present case, y2 = 0. 1, vap

B11 p RT

= exp

(T )] = exp

V1L )(p

(B11

1

p1s )

RT

= x 22

21

exp( 21 ) x1 + x2 exp(

2

+

12 exp( [x2 + x1 exp(

0.0 0.00

2 12 )]

(12)

0.8

+

(1) 12 /T

(13)

0.7

21

=

(0) 21

+

(1) 21 /T

(14)

0.6

(0) (1) 12 , 12 ,

(0) 21 ,

and

(1) 21 .

The ob-

p/MPa

where T is in kelvins. Table 5 presents jective function is

x exp | (15)

0.09 x

0.12

0.15

0.18

0.5

283.15 K 293.15 K 303.15 K 343.15 K 313.15 K 323.15 K 333.15 K

0.4

0.1 0.0 0.0

Table 5 The adjustable parameters of NRTL equation.

No. of points

0.06

0.2

where N represents the experimental point. xexp and xcal are the experimental and calculated data. Figs. 3–5 present the p-T-x diagrams of R1234yf/[emim][Ac],

(0) 12 (1) 12 (0) 21 (1) 21

0.03

323.15 K 333.15 K 343.15 K

0.3

x exp

Parameters

283.15 K 293.15 K 303.15 K 313.15 K

0.9

(0) 12

i=1

0.08

Fig. 4. Solubility in mole fraction of R1234yf in[bmim][Ac] (dots: experiment data; lines: calculated results from the NRTL model).

12 )

=

|x cal

0.07

R1234yf/[bmim][Ac]

0.1

12

F=

0.06

0.4

0.2

where α is assumed to be 0.2. τ12 and τ21 are adjustable binary interaction parameters. For a fixed refrigerant -IL system, τ12 and τ21 are functions of temperature

N

0.05

0.5

0.3

(11)

21 )

0.04

0.6

(10)

RT

where B11 refers to the second virial coefficient of solute; V1L refers to the saturated molar liquid volume of solute. Both are obtained from Refprop 9.1. In this work, we chose the NRTL equation to calculate the activity coefficients γ1 [43]:

ln

0.03

0.7

B11 p1s

Thus, ζ1 can be described by 1 = exp

0.02

0.8

p/MPa

1, pure [T ,

0.01

0.9

(9)

Similarly,

p1s

R1234yf/[emim][Ac]

Fig. 3. Solubility in mole fraction of R1234yf in[emim][Ac] (dots: experiment data; lines: calculated results from the NRTL model).

(8)

RT

283.15 K 293.15 K 303.15 K 313.15 K 323.15 K 333.15 K 343.15 K

0.3

R1234yf/[emim] [Ac]

R1234yf/[bmim] [Ac]

R1234yf/[P66614] [Cl]

6.364

4.857

15.456

−3039.4

−2543.0

797.8

3.664

2.996

0.499

319.6

302.5

−354.1

49

66

66

R1234yf/[P66614][Cl] 0.1

0.2

0.3

0.4 x

0.5

0.6

0.7

0.8

Fig. 5. Solubility in mole fraction of R1234yf in[P66614][Cl] (dots: experiment data; lines: calculated results from the NRTL model).

R1234yf/[bmim][[Ac] and R1234yf/ [P66614][Cl]. Solubilities rise with a decrease in temperature and an increase in pressure. The deviations in mole fraction between experiment and calculation are plotted in Fig. 6. The average deviations for R1234yf/[emim][Ac], R1234yf/[bmim] [[Ac] and R1234yf/[P66614][Cl] are respectively 2.58%, 1.76%, and 1.20%; the maximum deviations are respectively 5.00%, 5.21% and 5

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6 4

compression-assisted absorption system is presented in Fig. 8b. Compared to the single-effect cycle, a compressor is added between the absorber and evaporator for compression-assisted cycle.

2

4.2. Thermodynamic model

100(xexp-xcal)/xexp

Y. Sun, et al.

To simplify the calculation, the following assumptions are made: (1) No superheat after evaporation and no sub-cooling after condensation; (2) The solutions at the outlet of absorber and generator are in the vapor-liquid phase equilibrium state; (3) Heat losses and pressure drop are ignored; (4) The throttling process is isenthalpic. To determine the heat and workloads, the energy conservation equations in the absorption system are used. For the generator

0 -2 -4 -6 0.0

R1234yf/[emim][Ac] R1234yf/[bmim][Ac] R1234yf/[P66614][Cl]

0.1

0.2

0.3

0.4 0.5 p/MPa

0.6

0.7

0.8

0.9

Qg + mr fh7 = mr h3 + mr (f

For the absorber Qa + mr fh5 = mr h2 + mr (f Qa + mr fh5 = mr h2 + mr (f (17b) For the condenser

Fig. 6. Deviation between experimental and calculated results.

0.5 0.4

Qc + mr h4 = mr h3

0.3

Qe + mr h1 = mr h2

(16)

1) h8

1) h10 (single-effect system) (17a) 1) h10 (compression-assisted system)

(18)

p/MPa

For the evaporator (19)

For the solution heat exchanger

0.2

Qsxe = mr f (h6 R1234yf/[emim][Ac] R1234yf/[bmim][Ac] R1234yf/[P66614][Cl]

0.1

0.2

0.4

x

0.6

0.8

1)(h8

h 9)

(20)

For the solution pump

Wp = mr f ·(h6

Raoult's law

0.0 0.0

h7) = mr (f

(21)

h5)

where the subscripts a, c, e, g, and sxe refer to the absorber, condenser, evaporator, generator and solution heat exchanger, respectively. Q is the heat input/output. The subscripts 1–9 stand for the state point. h is the mass-based specific enthalpy; mr means the mass flow rate of circulation refrigerant; f stands for the circulation ratio, that represents the ratio of mass flow rate of the weak-IL solution to that of the circulation refrigerant; Wp is the solution pump work determined by the Eq. (21) [27]. The state point temperatures of the solution heat exchanger are calculated by

1.0

Fig. 7. Solubility in mole fraction of R1234yf in three ILs at 283.15 K.

5.73%. For the NRTL model, small deviations (mostly < 4%) are randomly distributed as a function of pressure. Toward better understand of R1234yf-solubility in ILs, the solubilities are compared for R1234yf in three ILs at 283.15 K (Fig. 7). [P66614][Cl] has the highest solubility while [emim][[Ac] is the lowest. R1234yf/[bmim][Ac] and R1234yf/ [emim][Ac] systems show a high positive deviation from ideality, suggesting that R1234yf is less soluble in the two ILs.

sxe

=

T8 T8

T9 T6

(22)

where ξsxe (set to 0.8 in this work) is the solution heat exchanger efficiency. The outlet parameters of compression are calculated by

4. Thermodynamic system analysis

=

4.1. Absorption-refrigeration system description

h2 , ideal h2 h2 h2

(23)

where h2′,ideal refers to the ideal outlet refrigerant enthalpy in an isentropic compression; η (set to 0.7) is the isentropic efficiency of compressor. The COP of absorption-refrigeration cycle is

Fig. 8a illustrates the principle of the single-effect absorption cooling system. The refrigerant vapor from the evaporator (state point 2) is absorbed into the strong solution (state point 10) that leads to a weak solution (state point 5) in the absorber. Then, the weak solution is carried to the solution heat exchanger by the solution pump (state point from 6 to 7). The refrigerant vapor (stat point 3) is released from the weak solution by the external heat (renewable energy, waste heat, boiler, etc.) in the generator. The strong solution (state point 8) flows through the solution heat exchanger (the state point 9) that could preheat the weak solution (state point from 6 to 7). After the solution heat exchanger, the strong solution is throttled by the expansion valve and comes back to the absorber. The gaseous refrigerant released from the generator condenses into the liquid refrigerant in the condenser (state point 4), then gets throttled by the expansion valve. Finally, the liquid refrigerant vaporizes in the evaporator to produce cooling. The

COP =

Qe Qg + Wp + Wc

e

(24)

where ηe (set to 0.38) is the electricity generation efficiency. Wc (energy consumption of compressor) is equal to 0 for the single-effect system and Wc = mr (h2′ - h2) for the compression-assisted system. According to the second law of thermodynamics, the exergy coefficient of performance (ECOP) of absorption cycle is determined by [27]

ECOP =

Qe |1 Q g (1

Tref Te |

Tref Tg ) + Wp + Wc

where Tref is set to 298.15 K. 6

(25)

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condenser

Qc 4

3

evaporator

9

6

10

5

3

expansion valve

solution heat exchanger expansion valve

Qg generator

heat rejection

7

8

condenser

4

generator

heat rejection expansion valve

Qc

Qg

8

7

9

6

10

5

solution heat exchanger expansion valve

pump

evaporator

absorber

absorber 2

1

2

1

Qc

heat rejection

pump

2

Qa

heat rejection

Qc

(a) Single-effect cycle

Qa

(b) Compression-assisted cycle

Fig. 8. The principle of absorption cooling systems.

The molar enthalpy of the refrigerant/IL mixture is obtained by

H = x r Hr + xIL HIL +

Table 7 State point parameters of R1234yf/IL absorption cycle (single-effect).

(26)

HE

State point

where Hr is the molar enthalpy of refrigerant gotten from Refrop 9.1 (at the reference temperature is 298.15 K); xr refers to the refrigerant-solubility in the IL; HE is the excess enthalpy [44,45]; HIL is the molar enthalpy of IL, which is calculated by

HIL =

T T0

Cp, IL dT + Href

1 2 3 4 5 6 7 8 9 10

(27)

where Href is the specific enthalpy at the reference state (298.15 K). The molar heat capacity, Cp,IL, is calculated by (28)

Cp, IL = A + DT + ET 2

The coefficients (A, D and E) are regressed by the experimental data [46–51]. The calculated results are presented in Table 6. For the refrigerant/IL mixture, the excess enthalpy is determined by

HE =

RT 2 x r

ln T

r

+ xIL p, x

ln IL T

State point

where γ is the activity coefficient calculated from the NRTL model

1 2 3 4 5 6 7 8 9 10 2´

5. Results and discussions Energy and exergy performance are analyzed for single-effect and compression-assisted system using three new working pairs under different working conditions including the air-conditioning, sub-zero, and high-temperature cooling conditions, that is to meet different actual applications. At the condensation temperature of 303.15 K, generation temperature of 363.15 K and evaporation temperature of 278.15 K, the sate point parameters of new working pairs are presented in Table 7 (single-effect cycle) and Table 8 (compression-assisted cycle). From

A

D

E

[emim][Ac] [bmim][Ac] [P66614][Cl]

249.3215 381.9632 623.8823

0.1333 −0.5647 0.1087

3.88 × 10−4 1.885 × 10−3 1.499 × 10−3

[emim][Ac]

[bmim][Ac]

[P66614][Cl]

100 100 100 100 1.96 1.96 1.96 1.05 1.05 1.05

100 100. 100 100 3.70 3.70 3.70 2.02 2.02 2.02

100 100 100 100 13.9 13.9 13.9 8.17 8.17 8.17

Pressure (kPa)

372.9 372.9 783.5 783.5 559.4 783.5 783.5 783.5 783.5 559.4 559.4

Refrigerant concentration (%) [emim][Ac]

[bmim][Ac]

[P66614][Cl]

100 100 100 100 2.88 2.88 2.88 1.05 1.05 1.05 100

100 100 100 100 5.74 5.74 5.74 2.02 2.02 2.02 100

100 100 100 100 21.69 21.69 21.69 8.17 8.17 8.17 100

Table 7 and Table 8, it can be seen that the refrigerant concentrations of weak and strong solution, and concentration difference between weak and strong solution for both cycles are in the same order: [P66614] [Cl] > [bmim][[Ac] > [emim][Ac]. Remarkably, [P66614][Cl] is much higher than the other studied ILs. Comparing the single-effect cycle, the absorption pressure rises from 372.9 to 559.4 kPa in the compression-assisted cycle. This increasing pressure will lead to higher refrigerant concentration of the weak solution and higher concentration difference. Therefore, it is beneficial to the absorption process by

Table 6 Coefficients of the heat capacity of ILs. ILs

372.9 372.9 783.5 783.5 372.9 783.5 783.5 783.5 783.5 372.9

Refrigerant concentration (%)

Table 8 State point parameters of R1234yf/IL absorption cycle (compression-assisted).

(29)

p, x

Pressure (kPa)

7

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0.5

Single-effect cycle [emim][Ac] [bmim][Ac] [P66614][Cl]

0.4

Compression-assisted cycle [emim][Ac] [bmim][Ac] [P66614][Cl]

COP

0.3

0.2

0.1

0.0 315

320

325

330

335

340

345

350

355

360

365

370

Generation temperature /K Fig. 9. Effect of generation temperature on COP of two absorption systems.

integrating the pressure-boosting compressor when the refrigerant-solubility is lower.

[Cl] is more than 5 times that in [bmim][Ac], and more than 10 times that in [emim][Ac]. Higher solubility leads to the larger concentration difference between the strong and weak solutions, that will decrease the circulation ratio and improve the COP. [P66614][Cl] with highest solubility performs the best. But the maximum COP of [P66614][Cl] is about only 2.8 times that of [bmim][Ac] and about 5 times that of [emim] [Ac], indicating that the COP depends on not only solubility behavior but also some other properties of ILs. The molecular weights of [emim] [Ac], [bmim][Ac], and [P66614][Cl] are respectively 170.21, 198.26, and 519.34 g/mol. A larger molecular weight means a lower mole concentration of weak solution at the same mass flow rate, that will result in a higher circulation ratio. A higher circulation ratio means more heat input needed at generator, that will lead to a lower COP. Thus, the COP of [P66614][Cl] is not so higher due to its higher molecular weight.

5.1. Influence of generation temperature Under the condensation temperature of 303.15 K, absorption temperature of 303.15 K and evaporation temperature of 278.15 K, the effects of generation temperature on cooling performance for three working pairs in single-effect and compression-assisted system (the compression ratio is 1.5) were shown in Fig. 9. The COP increases first and slightly decreases later with an increase of generation temperature for both systems. An explanation for this behavior can be given by the refrigerant solubility. The higher generation temperature, showing a lower refrigerant-solubility (in the generator), results in the lower refrigerant concentration of the strong solution that will increase the concentration difference between weak and strong solution. This is favorable to COP. However, too high generation temperature may result in increasing irreversibility that is detrimental to the cooling performance. For the single-effect system, the lowest generation temperature of the studied working pairs is around 333 K. [P66614][Cl] shows the highest COPs of 0.03–0.25 while [emim][Ac] has the lowest COPs of 0.004–0.05. When the generation temperature is approximately 365 K, the COP reaches the maximum. The minimum generation temperature is reduced in the compression-assisted system that is approximately 318 K. Therefore, the refrigeration cycle with a pressure-boosting compressor is favorable to utilize the lower temperature heat in the generator. Meanwhile, the COPs of working pairs are improved in the compression-assisted system. The COP of [P66614][Cl] is still highest and the maximum increases to 0.42. For [bmim][Ac] and [emim][Ac], the maximum increases to 0.17 and 0.09, respectively. The COP order is [emim][Ac] < [bmim][Ac] < [P66614][Cl] under the same condition. Fig. 10 shows the trend of circulation ratio as generation temperature. In the single-effect cycle, the circulation ratios of [emim][Ac], [bmim][Ac] and [P66614][Cl] (at the generation temperature of 363 K) are approximately 107, 58 and 16. In the compression-assisted cycle, the circulation ratios are obviously reduced to 54, 26, and 6.8 at the same condition. [P66614][Cl] has the lowest circulation ratio (7.7–6.7 for the single-effect system; 23–15 for the compression-assisted system). The lower circulation ratio means the larger concentration difference between strong and weak solutions, and it is required smaller flow rate of the weak solution to obtain the same amount of refrigerant. Thus, a lower circulation ratio results in a higher COP. From Figs. 3-5, it can be seen that the R1234yf-solubility in [P66614]

5.2. Influence of evaporation temperature The evaporation temperature actually depends on the cooling or refrigeration temperature. For example, the industrial refrigeration usually requires evaporation temperatures below 273 K; the space cooling with dehumidification needs evaporation temperatures below 278 K; the evaporation temperatures of electronic chip cooling could be above 293 K. One of the advantages of R1234yf/IL working pairs is that they can operate in the sub-zero condition, thus the evaporation temperature reaches as low as 250 K in this work. Figs. 11 and 12 present the effects of evaporation temperature on the COP and circulation ratio at the absorption temperature of 303.15 K, generation temperature of 363.15 K and condensation temperatures of 303.15 K. As increasing evaporation temperature, the COP increases and the circulation ratio decreases. Because higher evaporation temperature yields higher saturated vapor pressure in the absorber, which makes the absorption process much stronger. Thus, the concentration difference becomes higher, leading to decreased circulation ratio and increased cooling performance. As shown in Fig. 11, the minimum evaporation temperature is reduced from about 262 to 252 K by adding a compressor in the singleeffect system. At the evaporation temperature of 288 K, the COPs of [emim][Ac], [bmim][Ac] and [P66614][Cl] are respectively 0.08, 0.15 and 0.39 in the single-effect cycle. The COPs increase to 0.13, 0.23 and 0.46 in the compression-assisted cycle. Fig. 12 shows that the circulation ratio of compression-assisted cycle dramatically decreases compared to single-effect cycle. At the evaporation temperature of 288 K, the circulation ratios of [emim][Ac], [bmim][Ac] and [P66614][Cl] are 8

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200

Single-effect cycle [emim][Ac] [bmim][Ac] [P66614][Cl]

175

Circulation ratio

150

Compression assisted cycle [emim][Ac] [bmim][Ac] [P66614][Cl]

125 100 75 50 25 0 315

320

325

330

335

340

345

350

355

360

365

370

Generation temperature /K Fig. 10. Effect of generation temperature on circulation ratio of two absorption systems.

respectively 61.3, 30.9 and 8.1 in the single-effect cycle. The circulation ratios are significantly reduced to 34.5, 16.5 and 3.7 in the compression-assisted system. From the above analysis, we found that the high evaporation temperature and pressure-boosting compressor are beneficial to COP and circulation ratio.

5.4. Comparisons with existing R1234yf/IL working pairs According to the literature and our studies, the existing R1234yf/IL working pairs were compared with traditional pairs (NH3/H2O and H2O/LiBr). Under the condensation temperature of 303.15 K, generation temperature of 363.15 K and absorption temperature of 303.15 K, the COP and ECOP with a cooling temperature (evaporation temperature of 273.15 K or 278.15 K) are shown in Table 9. Among the R1234yf/IL working pairs, R1234yf/[P66614][Cl] has the highest performance whose COP and ECOP are 0.37 and 0.13 at 278.15 K. But the COPs of R1234yf/IL working pairs are lower than that of conventional working pairs (typically 0.6–0.8). Compared to H2O/LiBr, R1234yf/IL have more suitable operation pressures (for example 3.73 bar and 7.84 bar for R1234yf/IL, while 0.009 bar and 0.042 bar for H2O/LiBr at the above conditions). On the other hand, the equipment size of R1234yf/IL working pair will be much smaller than that of H2O/LiBr since the specific volume of H2O is about two orders higher than that of R1234yf, thus the capital cost will be reduced when R1234yf is used as a refrigerant instead of H2O.

5.3. Performance under various compression ratio The above studies show that the compression ratio importantly affects the cooling performance of absorption cycles. To gain a deeper comprehension of the compression ratio influence on the COP, various compression ratio (from 1 to 2.4) is discussed in this section. At the evaporation temperature of 273.15 K, generation temperature of 363.15 K, condensation temperature of 303.15 K and absorption temperature of 303.15 K, the effects of compression ratio (from 1.0 to 2.4) on COP and ECOP were illustrated in Figs. 13 and 14. Compression ratio of 1 refers to the single-effect system. As the compression ratio increases from 1.0 to 2.4, the COP and ECOP increase first (A higher compression ratio results in the weak solution becoming much weaker due to the higher pressure in the absorber) and become less steep later (too high compression ratio needs to consume more power in a compressor). 0.5

Single-effect cycle [emim][Ac] [bmim][Ac] [P66614][Cl]

0.4

Compression-assisted cycle [emim][Ac] [bmim][Ac] [P66614][Cl]

COP

0.3

0.2

0.1

0.0 250

255

260

265

270

275

280

285

290

Evaporation temperature /K Fig. 11. Effect of evaporation temperature on COP of two absorption systems. 9

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200

Single-effect cycle [emim][Ac] [bmim][Ac] [P66614][Cl]

175

Circulation ratio

150

Compression-assisted cycle [emim][Ac] [bmim][Ac] [P66614][Cl]

125 100 75 50 25 0 250

255

260

265

270

275

280

285

290

Evaporation temperature /K Fig. 12. Effect of evaporation temperature on circulation ratio of two absorption systems.

0.5

Table 9 The COP and ECOP of absorption cycle using different working pairs.

[emim][Ac] [bmim][Ac] [P66614][Cl]

0.4

Working pairs

COP

0.3

H2O/LiBr NH3/H2O R1234yf/[emim][Ac] R1234yf/[bmim][Ac] R1234yf/[P66614][Cl] R1234yf/[emim][BF4] R1234yf/[hmim][BF4] R1234yf/[Omim][BF4] R1234yf/[Hmim][TfO] R1234yf/[Hmim][PF6] R1234yf/[Hmim][Tf2N]

0.2 0.1 0.0 1.0

1.2

1.4

1.6 1.8 Compression ratio

2.0

2.2

2.4

0.20

ECOP

[emim][Ac] [bmim][Ac] [P66614][Cl]

0.05

1.2

1.4

1.6 1.8 Compression ratio

2.0

2.2

COP

ECOP

COP

ECOP

– 0.69 0.07 0.12 0.31 0.06 0.16 0.17 0.14 0.15 0.28

– 0.31 0.03 0.06 0.14 0.03 0.07 0.07 0.06 0.09 0.11

0.80 0.73 0.09 0.15 0.37 0.08 0.20 0.21 0.19 0.19 0.34

0.39 0.26 0.03 0.06 0.13 0.03 0.07 0.07 0.06 0.08 0.10

(1) Solubilities were measured for R1234yf in [emim][Ac], [bmim] [Ac], and [P66614][Cl] from 283.15 to 343.15 K. For each R1234yf/ IL, solubility rises with increasing pressure and decreasing temperature. The rising solubility order is: [emim][Ac] < [bmim] [Ac] < [P66614][Cl]. The new data was correlated by the NRTL model. From the calculated results, it can be seen that the NRTL model provides good agreement with the experimental data. (2) At the absorption temperature of 303 K, evaporation temperature of 278 K and condensation temperature of 303 K, the minimum generation temperature is around 318 K for the compression-assisted system and about 333 K for single-effect system using the three new R1234yf/IL working pairs. [emim][Ac] has the lowest COP and highest circulation ratio while [P66614][Cl] shows the highest COP and lowest circulation ratio. In the single-effect system, the maximum COPs for [emim][Ac], [bmim][Ac] and [P66614][Cl] are 0.05, 0.08 and 0.25, respectively. In the compression-assisted system, the maximum COPs increase to 0.09, 0.17 and 0.42. (3) At the condensation temperature of 303 K, generation temperature of 363 K and absorption temperature of 303 K, the minimum

0.10

0.00 1.0

Te = 278.15 K

of low-GWP R1234yf/IL working pairs was investigated. Solubilities of R1234yf in three new ILs were measured by an isochoric saturation method and the data was correlated by the NRTL model. According to the solubility, heat capacity and energy conservation law, the effects of compression ratio, generation and evaporation temperature on the cooling performance were analyzed in various working conditions. The main conclusions are as follows:

Fig. 13. Effect of compression ratio on COP of absorption system.

0.15

Te = 273.15 K

2.4

Fig. 14. Effect of compression ratio on ECOP of absorption system.

6. Conclusions Absorption technologies are promising for waste heat recovery and renewable energy utilization. To overcome the disadvantages of traditional NH3/H2O, H2O/LiBr and HFC-based working pairs, the potential 10

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evaporation temperature decreases from 262 to 252 K by the addition of a compressor in the single-effect system. For [emim][Ac], [bmim][Ac] and [P66614][Cl], the maximum COPs increase to 0.13, 0.22 and 0.46, respectively. R1234yf/[P66614][Cl] has more potential in high evaporation temperature. (4) As increasing compression ratio, the COP increases and circulation ratio decreases. But too high compression ratio is detrimental to ECOP because more electricity input is needed. Compared to NH3/ H2O and H2O/LiBr, R1234yf/IL working pairs have a lower COP. However, there are some great advantages for R1234yf/IL working pairs, such as green environmental protection, suitable operation pressure, lower capital cost, etc. More ILs need to be explored to improve the performance of R1234yf/IL working pairs in the future, especially phosphonium-based ILs. This study indicates that refrigerant solubility, heat capacity and molecular weight of absorbent should be considered when selecting working pairs.

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