theoretical and experimental investigation of an...

Theoretical and Experimental Investigation of an AbsorptionRefrigeration System Using R134/[bmim][PF6] Working FluidSarah Kim and Paul A. Kohl*

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States

ABSTRACT: 1,1,2,2-Tetrafluoroethane (R134) and ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate([bmim][PF6]), were investigated as a new working fluid pair in an absorption refrigeration system. The R134/[bmim][PF6] pairwas compared to previous studies using 1,1,1,2-tetrafluoroethane (R134a) and the same IL. The R134/[bmim][PF6] fluid pairhad up to 92.3% greater cooling-to-total-energy efficiency than the R134a/[bmim][PF6] fluid pair even though R134 and R134aare isomers and have nearly identical physical properties. The coefficient of performance of the R134/[bmim][PF6] fluid pair wasup to 3 times larger than that of the R134a/[bmim][PF6] fluid pair when only waste heat was used at the desorber. A workingrefrigeration system with R134/[bmim][PF6] was constructed, and the measurements of its performance showed that R134/[bmim][PF6] had 1.9 times larger cooling capability than R134a/[bmim][PF6] at a desorber temperature as low as 63 °C.

1. INTRODUCTION

The United States Department of Energy (DOE) hasestablished energy efficiency standards that apply to varioustypes of appliances and equipment in an effort to reduceexponentially growing electricity consumption. Manufacturersof residential central air conditioners and heat pumps have beenrequired to comply with the DOE energy conservationstandards since 1992.1 The mandatory standards drive theneed for more efficient heat pumps. Toward this end, theabsorption refrigeration cycle based on an ionic liquid (IL)working fluid is of interest as a means of utilizing low-qualitywaste heat. Absorption chillers offer an opportunity to recyclelarge amounts of industrial waste heat. Also, renewable energysources which generate heat can be directly used forrefrigeration without the need of conversion to electricalpower.2

However, the use of the absorption refrigeration cycle hasbeen somewhat limited due to technological challenges, healthhazards, and environmental concerns of existing systems.3

Commonly used absorption refrigeration working fluid pairs,ammonia/water and water/LiBr, have drawbacks includingtoxicity (ammonia), negative effects of crystallization (LiBr),incompatibility with metal components (corrosion), and theneed for a rectifier for postdesorption separation of the twofluid streams.4 Further, the pressure and temperature of theevaporator in the ammonia/water and water/LiBr systems issignificantly different from that of Freon-based vapor-compression systems so that many refrigeration applicationsare not within reach.ILs are a liquid salt at ambient temperature and are being

considered as absorbents in a variety of applications because oftheir tunable properties, zero vapor pressure, high thermalstability, and environmental safety.5 In particular, the near-zerovolatility of the IL enables easy separation of the volatileworking fluid by thermal stratification. There have been severaltheoretical studies that have evaluated the performance of ILbased working fluids including 1-ethyl-3-methylimidazoliumethylsulfate ([emim][EtSO4]),6 1,3-dimethylimidazolium di-

methylphosphate ([emim][DMP]),7 and various imidazolium,pyridinium, and pyrrolidinium type ILs.8,9

In a previous report, an experimental demonstration of acooler/heat pump using a freon/IL working fluid pair wasreported for the first time.10 The application was targeted forcooling in high power electronics using waste heat. An IL,[bmim][PF6], was selected as the absorbent because of its highmolar uptake of freons, which is essential to deliveringmaximum cooling. The analysis showed 1,1,1,2-tetrafluoro-ethane (R134a) to be a promising refrigerant when paired with1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]-[PF6]) as the working fluid for a waste heat recyclingabsorption refrigeration system. 1,1,2,2-Tetrafluoroethane(R134), an isomer of R134a (Figure 1), was expected to havesimilar results to that of R134a. However, the shift in fluorinesubstitution (1,1,1,2-tetrafluoroethane to 1,1,2,2-tetrafluoro-ethane) has resulted in a significant change in the absorptionof the fluorocarbon in [bmim][PF6] and the resultingperformance of the absorption refrigeration system. In this

Received: March 27, 2013Revised: July 23, 2013Accepted: August 21, 2013Published: August 22, 2013

Figure 1. Structures of materials used. (a) 1-Butyl-3-methylimidazo-lium hexafluorophosphate ([bmim][PF6]), (b) 1,1,2,2-tetrafluoro-ethane (R134), and (c) 1,1,1,2-tetrafluoroethane (R134a).

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© 2013 American Chemical Society 13459 dx.doi.org/10.1021/ie400985c | Ind. Eng. Chem. Res. 2013, 52, 13459−13465

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study, the performance of the R134/[bmim][PF6] workingfluid pair was modeled using a two-phase pressure dropEquation-of-State (EOS) model.11 The effect of a counter-flowing solution heat exchanger, desorber temperature, andabsorber temperature on the system performance wasevaluated. The fluid was also tested in the benchtop absorptioncooling cycle for an experimental demonstration. Bothcomputational and experimental results of R134/[bmim][PF6]were compared with those of R134a/[bmim][PF6].

2. COMPUTATIONAL MODEL2.1. Thermodynamic System Analysis. The principal

features of the absorption refrigeration cycle are shown inFigure 2. The cycle resembles that of the vapor compression

refrigeration (heat pump) system, except the vapor compressoris replaced with a thermochemical process. Pressurization of thethermochemical process starts in the absorber, where therefrigerant vapor from the evaporator (state point 2) isexothermically absorbed into the strong-IL solution (statepoint 10), resulting in a weak IL solution at state point 5. TheIL solution is pressurized by the liquid pump after the absorber.Then, the solution (regenerative) heat exchanger preheats theweak-IL solution at state point 6, creating state point 7 usingheat from the strong-IL solution flowing back to the absorber(from the desorber). In the desorber, superheated refrigerantvapor is released at high temperature and pressure from the ILvia desorption from the weak-IL solution by the addition ofheat (preferably waste heat). The strong-IL returns to theabsorber through the solution heat exchanger and an expansiondevice. The condensation/absorption process at the absorberand vaporization/desorption process at the desorber both occurin the liquid phase. This allows use of a liquid pump to createthe pressure difference between the condenser and evaporator.System-level simulations have been carried out with

refrigerant/ionic liquid combinations. In all the calculations,the operating temperature of the condenser and evaporatorwere set at 50 and 25 °C, respectively. The cooling capacity atthe evaporator was set at 100 W. The energy and massconservation equations for all components in the system weresimultaneously solved to determine the heat and workloads.The overall energy balance for the system is given in eq 1.

+ + = +Q Q W Q Qd e p c a (1)

where Wp is the liquid pump work and Q is the heat input/output. The subscripts d, e, c, and a correspond to the desorber,evaporator, condenser, and absorber, respectively. All values ofheat are expressed as positive (magnitude) values regardless ofthe direction (in or out) of heat flow.For the calculations, the heat exchanger efficiency, 35%, was

used. The estimated heat transfer is thus given by eq 2.11

= = −Q Q C T T0.35 0.35 ( )actual max min hot,in cold,in (2)

where Cmin is the heat capacity coefficient of the stream thatlimits the amount of heat transfer. This estimate of the heattransfer at the counter-flow heat exchanger can then be used todetermine the temperature and enthalpy of the exit streams ofthe solution heat exchanger.The energy conservation for a subsystem consisting of a

regenerative heat exchanger and a pump is given by eq 3.

− − = − −h h m m h h m W( )( ) ( )9 8 w r 7 5 w p (3)

where h is enthalpy and the subscripts correspond to thelocations shown in the system diagram, Figure 2. Also, mw andmr are the mass flow rates of the weak-IL solution andrefrigerant leaving the desorber, respectively. Energy con-servation for the desorber is given by eq 4.

= − + −Q h m m h m h m( )d 8 w r 3 r 7 w (4)

Similarly, the heat rejected at the absorber is given by eq 5.

= − − −Q h m h m m h m( )a 5 w 10 w r 2 r (5)

Energy conservation for the condenser and the evaporator yieldthe respective heat loads, eqs 6 and 7.

= −Q h h m( )c 4 3 r (6)

= −Q h h m( )e 2 1 r (7)

The cooling-to-total-energy (CE) is defined as the heatremoved at the evaporator divided by the power supplied tothe desorber and the pump, eq 8.

=+

≈Q

Q W

Q

QCE e

d p

e

d (8)

Since waste heat is intended to be used in heating the desorber,the practical coefficient of performance (COP), η, is defined byeq 9.

η =Q

We

p (9)

Equation 9 is the usual figure of merit for absorptionrefrigeration/heat pump systems where waste heat is used.In this analysis, the Redlich−Kwong (RK) type EOS was

used to calculate the thermodynamic properties of the fluids ateach point in the cycle. The RK-EOS was chosen because it hasa temperature dependent attractive term, a(T), and fits the datavery well. Binary interaction parameters (BIPs) wereintroduced to improve the accuracy of the model. Severalassumptions were made for convenience in the calculations: (i)The expansion process is isenthalpic. (ii) The compressionprocess is isentropic. (iii) State 4 is a saturated liquidrefrigerant. (iv) State 2 is a saturated vapor refrigerant. (v)The vapor quality at state 5 is zero.

Figure 2. Schematic diagram of an absorption refrigeration systemusing IL/refrigerant mixture as a working fluid.

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie400985c | Ind. Eng. Chem. Res. 2013, 52, 13459−1346513460

The general RK-EOS can be written in the following form,eqs 10−12.12

=−

−+

PRT

V ba T

V V b( )

( ) (10)

α=a TR T

PT( ) 0.42748 ( )

2c2

c (11)

=bRTP

0.08664 c

c (12)

The subscript c represents the critical properties of thesubstance. P is the pressure. T is the temperature. V is themolar volume. R is the gas constant, and a and b are constants.The temperature dependent function of the α parameter isexpressed by eq 13.

∑α β= − ≡=

≤

T T T T T T( ) (1/ ) , /k

kk

0

3

r r r c(13)

The parameter βk was determined so as to yield the vaporpressure of each pure compound, the refrigerant, and the ionicliquid.11,13 The critical properties along with the β values aresummarized in Table 1.

Three binary interaction parameters (BIPs), τ, l, and k, wereintroduced in the a and b parameters for N componentmixtures.15

∑ α= − ==

a a a f T l x x aR T

PT( )(1 ) , 0.42748 ( )

i j

N

i j ij ij i j ii

i, 1

2c2

c

(14)

τ τ τ τ= + = =f T T( ) 1 / , where and 0ij ij ij ji ii (15)

∑= + − −

=

=b b b l k x x b

RTP

12

( )(1 )(1 ) ,

0.08664

i j

N

i j ij ij i j i

i

i

, 1

c

c (16)

where lij = lji, lii = 0, kij = kji, and kii = 0.A detailed description of finding the BIP can be found

elsewhere.11 The optimum BIPs for R134 and the [bmim]-[PF6] mixture are listed in Table 2.

The EOS was then used to find the enthalpy values at thepoint of interest, eq 17.

∫ ∑= + +=

H H x C T HdT

T

i

N

i piR

1

00

0 (17)

where HR is the residual enthalpy, T0 is the referencetemperature which was set at 273.15 K, and H0 is an arbitraryconstant (an enthalpy at the reference state). The ideal-gas heatcapacity of the ith species, Cpi

0 , is modeled in eq 18.

= + + +C C C T C T C Tpi0

0 1 22

33

(18)

The coefficients in eq 18 are given in Table 3. Using the heatcapacity for each species in the mixture, the enthalpy for the RKEOS is given by eq 19.

∫ ∑

= −+

+ −

+ +=

⎜ ⎟⎛⎝

⎞⎠H

ab

Tb

dadT

VV b

RT Z

x C T H

ln ( 1)

dT

T

i

N

i pi1

00

0 (19)

2.2. Pressure Drop. The viscosity of ILs is often relativelyhigh. For example, [bmim][PF6] at 294 K and atmosphericpressure has a viscosity of 376 mPa·s.16 Thus, the IL flow in themicrochannels used at the absorber and desorber may create asignificant pressure drop, which can affect the systemperformance. The average pressure drop through the micro-fluidic channel heat exchangers was evaluated using a two-phasepressure drop equation, eq 20.17

ρφ

ερ ε ρ

− =−

+

+ −−

⎜ ⎟⎛⎝

⎞⎠

⎡⎣⎢⎢

⎤⎦⎥⎥

⎡⎣⎢⎢

⎤⎦⎥⎥

dPdz

f G x

dG

ddz

x x

2 (1 )

( ) (1 )(1 )

mm

l2 v

h ll2 2

v 2

v

v 2

l (20)

where dh is the hydraulic diameter of the channel and f, G, xv, ρ,

and ε are the liquid-phase fanning friction factor, mass flux,vapor quality, density, and void fraction, respectively. z is theaxial direction coordinate along the channel length. Subscripts“l” and “v” stand for liquid and vapor phases, respectively. Inthe two-phase multiplier correlation of Lockhart andMartinelli,18 φl is incorporated with the C value proposed byLee and Mudawar,19 eqs 21−23.

φ = + +CX X

11

l2

2 (21)

= −C Re We2.16 (laminar liquid laminar vapor)lo0.047

lo0.6

(22)

= −C Re We1.45 (laminar liquid turbulent vapor)lo0.25

lo0.23

(23)

where Relo and Welo are liquid-only Reynolds and Webernumbers, respectively. The Martinelli parameter, X, and thesingle phase empirical correlation of the fanning friction factorfor laminar flow in a rectangular channel by Shah and London20

are expressed by eqs 24 and 25.

μμ

ρρ

= −⎛⎝⎜⎜

⎞⎠⎟⎟

⎛⎝⎜

⎞⎠⎟

⎛⎝⎜⎜

⎞⎠⎟⎟X

xx

1l

v

0.5 v

v

0.5v

l

0.5

(24)

Table 1. EOS Constants of [bmim][PF6] and HFCRefrigerants

purecompound Tc (K)

Pc(kPa) β0 β1 β2 β3

[bmim][PF6]

14860.5 2645 1.0 0.62627 0 0

R13413 391.97 4641 1.0012 0.48291 −0.05070 0

Table 2. Binary Interaction Parameters for R134 and[bmim][PF6]

fluid pair l12 = l21 k12 τ12 (K) ΔP (MPa)a

R134/[bmim][PF6] 0.0418 −0.0451 24.4896 0.002

aStandard deviations of nonlinear least-squares in pressures.



β β β

β β

= − + −

+ −

fRe 24(1 1.3553 1.9467 1.7012

0.9564 0.2537 )

2 3

4 5(25)

where μ is the viscosity and β is the aspect ratio of the channel.Also, the void fraction model of Zivi21 was adopted in thisstudy. Microchannel structures are used in the absorber and thedesorber in this study due to their high heat and mass transferrates. However, the magnitude of the negative effects of the ILhigh viscosity on the pressure drop within the microchannelheat exchangers and, in turn, the system performance areassessed in this study. The dimensions (length × width) of theevaporator and condenser are 2 × 2 cm and 3 × 3 cm,respectively. The dimensions of the absorber and the desorberare 8 × 8 cm. The microfluidic channel cross-sectional area forthe heat exchangers is 1 × 1 mm.

3. EXPERIMENTAL METHODSThe ionic liquid [bmim][PF6] (iolitec, 99%), R134 (SynQuestLaboratory, Inc., 98%), and R134a (Airgas, 99.9%) were used asreceived. An experimental setup for a laboratory scaleabsorption refrigeration system using the IL based workingfluid was built and operated, the details of which are describedelsewhere.10 To investigate the effect of desorber power (wasteheat) input on the evaporator cooling capacity, the desorberpower was increased up to ∼100 W to keep the evaporatortemperature constant at 42 °C with the condenser and absorbercoolant (secondary fluid) inlet temperatures of 22 °C. Whenthe refrigerant was changed from R134a to R134, the R134awas discharged from the benchtop system using a refrigerantrecovery unit. A port on the refrigerant loop prevented drainageof the ionic liquid. The presence of a check valve and verticalcylinder between the solution loop and refrigerant loop alsoworked as a barrier for ionic liquid overflow. The [bmim][PF6]in the system remained under vacuum conditions for severaldays until there was no further change in the pressure. R134was then loaded into the system to the same level as R134a asdetermined by the saturated pressure. The measuredparameters were temperature, absolute pressure, and electricalpower input. The uncertainty in the temperature reading was0.1 K for the calibrated thermocouples relative to each other.The uncertainty in absolute pressure measurement was 0.25%of the maximum value of 2068.43 kPa (300 psi). Theuncertainty in the output value of the electrical powertransducer was 0.14% of the measured value.

4. RESULTS AND DISCUSSION4.1. Computational Results. It was previously shown that

the R134a/[bmim][PF6] working fluid pair could be used in awaste-heat recycling absorption refrigeration system.10 Thechange in fluorine position in tetrafluoroethane from the firstcarbon to the second carbon (R134a is 1,1,1,2-tetrafluoro-ethane and R134 is 1,1,2,2-tetrafluoroethane) has little effect onits physical properties, such as density and boiling point.However, this difference in chemical structure has a significanteffect on its interaction with [bmim][PF6], which affects theperformance of the absorption refrigeration cycle. The

solubility of R134 in the IL absorbent was predicted usingthe two phase pressure drop EOS model, Figure 3. The

calculated values are in good agreement with the experimentallymeasured data reported by Shiflett and Yokozeki.22 In Figure 3,the experimentally determined values are the data points, andthe solid lines correspond to the fitted values. Surprisingly,R134 showed significantly higher solubility at the sametemperature and pressure in comparison to R134a at thesame temperatures, Figure 4. This is most likely due to thehigher probability for hydrogen bonding between the sym-metrical R134 and [bmim][PF6].

24

The operating pressure of the system is determined by thesaturated pressure of the refrigerant. Therefore, the solubilitydifference between the absorber and desorber is important indetermining the overall solubility of the refrigerant. The vaporpressure of R134 and R134a at the condenser and evaporatortemperatures of 50 and 25 °C, respectively, are listed in Table4.

Table 3. Coefficients for Ideal Gas Heat Capacity of Pure Compounds [J·mol−1]

pure compound C0 (J·mol−1) C1 (J·mol−1K−1) C2 (J·mol−1K−2) C3 (J·mol−1K−3)

[bmim][PF6]5 −2.214 0.57685 −3.854 × 10−4 9.785 × 10−8

R13413 15.58 0.28694 −2.028 × 10−4 5.39633 × 10−8

Figure 3. R134 solubility in [bmim][PF6] as a function of temperature(K) and pressure (MPa). Symbols, experimental solubility data;22

lines, computed EOS model.

Figure 4. R134a solubility in [bmim][PF6] as a function oftemperature (K) and pressure (MPa).10 Symbols, experimentalsolubility data;23 lines, computed EOS model.



Effect of Absorber/Desorber Outlet Temperature on CE.The effect of changing several key operating conditions wasevaluated using the model developed in this work. The effect oflowering the absorber temperature from 309.65 K to 300.65 Kwas evaluated. Figure 5 shows the value of CE for the absorber

at 309.6 K, and Figure 6 shows the resulting value of CE for theabsorber at 300.65 K. The CE value was generally observed toincrease when the absorber temperature was lowered from309.65 K to 300.65 K due to higher refrigerant solubility in theabsorber. R134 had a higher CE value than R134a at the sametemperature, with an average improvement of 32.3% and 92.3%for the absorber operated at 300.65 and 309.65 K, respectively.This larger difference in CE values between the refrigerants at309.65 K is due to the higher solubility of R134 compared toR134a. The effect of solubility on the system efficiencybecomes increasingly important for cases where the solubilitydifference between the absorber and desorber is small (i.e., highabsorber temperature).Effect of Solution Heat Exchanger on CE. The introduction

of a solution heat exchanger between the inlet and outlet of thedesorber improves the performance of the cycle. The solutionheat exchanger transfers heat from the strong IL solution atsystem point 8 to the weak IL solution at system point 6, Figure2. The heat exchanger improves the system efficiency because it

lowers the amount of waste heat needed to increase thetemperature of the solution mixture to the desorber temper-ature, and less heat is discharged at the absorber. A 35%efficient heat exchanger improved the CE values by 16.4% and32.6% for R134 and R134a (see Figure 7), respectively,

compared to the case without the heat exchanger, as shown inFigure 5. The system performance was enhanced to a greaterextent for the refrigerant with lower CE values, R134a.

Effect of Absorber/Desorber Outlet Temperature on η. Thecoefficient of performance η is plotted with respect to desorberoutlet temperature in Figures 8 and 9 for absorber temperatures

of 309.65 K and 300.65 K, respectively. At higher desorberoutlet temperatures, the pumping work was reduced due to anincrease in the refrigerant-to-absorbent circulation ratio. Thismeans that the IL absorbent can carry more refrigerant on eachpass, which leads to an efficiency increase. The liquid pumpingwork is the product of the liquid volumetric flow rate andpressure difference between the absorber and desorber: Wp = V̇l× ΔP. Therefore, when free waste heat is used at the desorber

Table 4. Vapor Pressure of R13422 and R134a [kPa]25

temperature [K] R134 R134a

298.15 526.0 664.9323.15 1062.9 1318.6

Figure 5. Effect of desorber outlet temperature on CE (withoutsolution heat exchanger). Ta = 309.65 K.

Figure 6. Effect of desorber outlet temperature on CE (withoutsolution heat exchanger). Ta = 300.65 K.

Figure 7. Effect of desorber outlet temperature on CE (with solutionheat exchanger ε = 0.35). Ta = 309.65 K.

Figure 8. Effect of desorber outlet temperature on η (without solutionheat exchanger). Ta = 309.65 K.

Figure 9. Effect of desorber outlet temperature on η (without solutionheat exchanger). Ta = 300.65 K.



to drive the system, the operating pressure range and solubilitydifference between the absorber and desorber are bothimportant because the goal is to minimize the pumping workby reducing the amount of IL pumped. Both refrigerants hadhigher COP values at lower absorber temperatures due to thehigher refrigerant-to-absorbent ratio, resulting in a lower liquidvolumetric flow rate. The R134/IL pair had 2 (Figure 9) or 3times (Figure 8) the efficiency of the R134a/IL pair dependingon the absorber temperature. The improvement factor byreplacing R134a/IL with R134/IL is greater for the COP valuesthan it is for CE because the pumping work, Wp, whichdominates COP is 2 orders of magnitude smaller than the heatrequired at the desorber (Qd) and absorber (Qa).Wp is the onlyterm in the denominator for computing COP. The R134/ILpair had less pumping work than the R134a/IL pair due toseveral thermophysical properties including (i) a larger molarenthalpy of vaporization (Table 5), (ii) a smaller operating

pressure range (Table 4), (iii) a larger solubility and solubilitydifference in [bmim][PF6] (smaller liquid volumetric flow), and(iv) a larger liquid density (Table 5).4.2. Experimental Results. The evaporator junction

temperature, which reflects the operating temperature of anelectronic device or other component requiring cooling, wasmaintained to be at 42 °C by controlling the heater power atthe evaporator. Data were collected at different desorbertemperatures by adjusting the heater power attached to thedesorber. The desorber outlet temperature was measured priorto the separation of the refrigerant vapor produced. That is, it isan IL-vapor mixture combining states 3 and 8 in Figure 2. Theevaporator junction temperature, evaporator outlet fluidtemperature, separator outlet fluid temperature (state 8 inFigure 2), and desorber outlet fluid temperature at specificdesorber power values are shown in Figure 10. Thetemperature of the solution leaving the separator and desorberincreased, while the evaporator outlet temperature decreased

with respect to the desorber heater power. This shows thatmore vapor refrigerant was generated at the desorber as a resultof the higher desorber fluid temperature. The evaporator outlettemperature tends to converge to a specific temperature at highdesorber temperature (higher desorber power). The CE had aparabolic shape, Figure 11, as predicted by the computational

analysis due to the trade-off between the desorber heater powerand the cooling occurring at the evaporator. The experimentalR134/[bmim][PF6] efficiency values were smaller than thetheoretical values, as shown in Figure 5, due to experimentallosses in heat at various locations. The experimental benchtopsystem was insulated but not optimized. The temperature andpressure sensors and excess piping added to accommodate theirinstallation made the system larger and less insulated than anoptimized system. In addition, the performance can beimproved by adding a solution heat exchanger and improvingthe design of the microchannel absorber and desorber.Nevertheless, the experimental system results proved that theperformance trends predicted in the model were correct andthat a fluorocarbon/IL refrigerant pair can be made into aworking system.The cooling capacities of the R134/IL and R134a/IL

working pairs were compared, as shown in Figure 12. TheR134/IL pair reached nearly the maximum cooling capacity at arelatively low separator temperature, 63.26 °C. This is 1.9 timeslarger than that of the R134a/IL working pair. Therefore, theR134/IL pair forms a highly effective refrigerant pair forrecycling low grade waste heat. The results are in goodagreement with the EOS model when the measured temper-ature and pressure values are given as input parameters. TheEOS model calculations show that R134/IL is predicted to have1.82 times larger cooling capacity, which is very close to themeasured values.

5. CONCLUSIONThe R134/[bmim][PF6] fluid pair was evaluated in an energyrecycling absorption refrigeration system. The EOS modelshowed that R134 had a higher solubility difference in[bmim][PF6] than R134a in the same IL under the coolingcycle operating conditions, which resulted in higher CE values.The larger molar enthalpy of vaporization, smaller operatingpressure range, and larger liquid density of R134 compared toR134a also contributed to the improvement in COP values.Experimental results confirmed the larger cooling capacity ofthe R134/[bmim][PF6] working fluid pair than the previously

Table 5. Enthalpy of Vaporization (ΔHvap) and SaturatedLiquid Density (ρ) Values of R134 and R134a at 298.15K

ΔHvap [kJ/mol] ρ25,26 [g/cm3]

R134 16.33 1.290R134a 14.64 1.207

Figure 10. Experimentally measured desorber outlet fluid temperature,separator outlet fluid temperature, evaporator junction temperature,and evaporator outlet temperature with respect to desorber powerinput for R134/[bmim][PF6] working fluid.

Figure 11. Experimentally measured CE for R134/[bmim][PF6]working fluid with respect to desorber outlet fluid temperature.



reported R134a/[bmim][PF6] pair. The maximum coolingcapacity of the R134/[bmim][PF6] working fluid pair wasreached at a relatively low separator fluid temperature (63.26°C), which would allow more effective waste heat utilization.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe financial support of the Interconnect Focus Center, one ofsix focus centers of the Semiconductor Research Corporation,is gratefully acknowledged. The authors would also like tothank Mark Evans and Nishith Patel for assistance in operatingthe benchtop absorption system and modeling of new fluids.

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Figure 12. Comparison of cooling capacity between R134/[bmim]-[PF6] and R134a/[bmim][PF6] working fluid pairs.



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