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Thermodynamic performances of non-CFC working fluids in heat-pump cycles
Applied Thermal Engineering Vol. 16, No. 7, pp. 571-578, 1996
Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1359-4311/96 $15.00 + 0.00
Pergamon 1359-4311(95)ooo58-5
THERMODYNAMIC PERFORMANCES OF NON-CFC WORKING FLUIDS IN HEAT-PUMP CYCLES Wen-Lu Department
of Chemical
Weng*
and Bin-Chang
Huang
Engineering, Ming-Hsin Institute of Technology Hsin-Chu, 304 Taiwan, R. 0. China
and Commerce,
Hsin-Feng,
(Received in final form 26 September 1995) Abstract-Thermodynamic performances of a variety of fluids circulating in ideal heat-pump cycles were evaluated by using the Iwai-Margerum-Lu equation of state. The evaluations were implemented at three operating conditions (evaporating temperatures from 323.15 to 343.15 K and condensing temperatures from 363.15 to 413.15 K). The results showed the normal boiling point and the critical pressure of compounds to be the key properties for selecting the thermodynamically proper working fluids. Several potential non-CFC compounds were suggested for the heat pumps with respect to each application. Copyright 0 1996 Elsevier Science Ltd KeywordsNon-CFC
fluids; heat pumps;
thermodynamic
performance.
NOTATION COP H I ic se SCD SH T V W, Z
coefficient of performance molar enthalpy (kJ molt ‘) performance index pressure (MPa) heat rejected in condenser (kJ mol- ‘) heat absorbed in evaporator (kJmol-‘) molar entropy (kJ molt I K- ‘) specific compressor displacement (m’ kJ - ‘) superheat of vapor (K) temperature (K) molar volume (m’ mol - ‘) work requirement for compressor (kJ) compressibility factor
Greek letters 0
acentric
factor
Superscripts and subscripts t performance value of CFC-114 at normal boiling point b at critical state C cond condenser COP coefficient of performance min minimum evaporator evap SCD vapor volume by unit energy superheat of vapor SH therm overall thermodynamic performance
INTRODUCTION CFC-114 was one of the working fluids widely used for heat pumps to recover low-temperature waste heat. Unfortunately, the fully halogenated compounds including CFC-114 will be phased out in 1996. Some proper non-CFC compounds were urgently needed to replace those regulated substances. Thermodynamically, a good working fluid for a heat pump should have the following desirable characteristics: high cycle efficiency, small specific compressor displacement, low *Author
to whom
correspondence
should
be addressed. 571
572
Wen-Lu Weng and Bin-Chang Huang
minimum superheat of vapor, moderate vapor pressures in the heat-exchange units (generally above atmospheric pressure but not too much above 2 MPa). Numerous efforts have been devoted to the search for new refrigerants in the last decade [l-6]. Since the refrigeration cycle is also a heat-pump cycle, the experiences in studying CFC-alternatives for refrigeration systems are also useful for heat-pump energy-recovery systems. The only difference between these two systems is the operating conditions of the cycles. Heat-pump energy-recovery systems generally operate at higher evaporating and condensing temperatures than refrigeration systems. A good refrigerant may no longer be an appropriate working fluid for the heat pumps. As a consequence, it is of interest to find some other potential alternatives from various chemicals for heat-pump energy-recovery systems. Evaluating the thermodynamic performances of fluids by using a predictive thermodynamic model appears to be an efficient method. Pradhan and Larson [7] reported a power cycle analysis with the aid of a corresponding-states model and Bertinat [S] employed the Lee-Kesler equation to simulate high-temperature heat-pump operations. Several promising cubic equations of state have been developed in the past decade. Among these new equations of state, the Iwai-Margerum-Lu (IML) equation of state [9] is one of the reliable models for predicting the thermodynamic properties of fluids [9, lo]. This equation of state was applied recently to study the CFC-alternatives for refrigeration systems [ll] and for the organic Rankine cycle waste-heat recovery systems [12]. The IML equation of state is also adopted in the present study. A diversity of substances were investigated in this work, which include halogenated substances, hydrocarbons, ethers, ketones, esters, alcohols and inorganic compounds. With the assistance of the simulation results, the criteria for finding thermodynamically proper substances are presented and some potential non-CFC compounds for the heat pump systems are suggested.
HEAT-PUMP
CYCLE
SIMULATION
Thermodynamic performances of fluids can be evaluated via cycle simulations. The paths of ideal heat-pump cycles are illustrated in Fig. 1. There are two types of substances (type-A and type-B), as shown in the graph, which are categorized by the slopes of the saturated vapor curve in T-S diagrams. The slope of the type-A fluids is positive and that of the type-B fluids is negative. The following assumptions were made for those ideal heat-pump cycles: an isobaric process through each heat-exchanger, an isentropic process through the compression, an isenthalpic process during the expansion, and the outlet of the condenser at saturated state. The cycles are specified by two operation variables, for example, evaporating temperature ( Tevap)and condensing temperature (T,,&. When these two temperature levels were given, state properties of the streams in the cycles can be calculated by a standard procedure with an equation of state. The required input variables for the cycle simulation are the fluid’s critical properties (T,, PC, ZJ, acentric factor (0) and heat capacity at ideal gas states. These properties were taken from Reid et al. [13] and McLinden [14]. On the basis of 1 mole of circulating fluid, the ideal cycle efficiency (coefficient of performance, COP) is given by COP = Q/W,,
(1)
where Qc denotes the heat rejection from the working fluid in the condenser which was calculated from the molar enthalpy difference of the streams across the condenser (i.e. H4 - H5 for the type-A fluids and H3 - H5 for the type-B fluids) and W, is the energy input for the isentropic compression (estimating from H4 - H3 for the type-A fluids and H3 - H2 for the type-B fluids). Meanwhile, the specific compressor displacement (SCD) is calculated from the following equation: SCD = Vcamp.~/Qe, where Qe is the heat duty of the evaporator, i.e. H3 - Hl for the type-A fluids and H2 - Hl for the type-B fluids; Vcomp.,n is the molar volume of the compressor-inlet vapor. The compressor inlet is located at point 3 for the type-A fluids and at point 2 for the type-B fluids. Additionally, the minimum superheat of vapor (SHmi,) for the type-A fluids is calculated from SH,,
= T3 -
TI,
(3)
Performance
of non-CFC
working
573
fluids
where T2 = Tevapand the SHmin is positive. For the type-B fluids, SHti, is given by SH,,,,. = T4 -
T,,
(4)
where T4 = Tcondand the SHmi” is negative. SIMULATION
RESULTS
Three illustrative cases are studied in this work. The operating conditions of the heat pump cycles are specified at (1) Tcond= 363.15 K and Tevap= 323.15 K; (2) Tcond= 383.15 K and Tevap= 343.15 K; (3) zond = 413.15 K and Tcvap= 343.15 K, respectively, and the performances of CFC-114 are taken as the comparison basis. Figure 2 plots the COP against the fluid’s normal boiling point (TL,)
Tspe ----
A fluid Saturation
cuwe
-T cm4 I’
/’ ,’ I’
_T m,
I’
1
1’2 #’
S
Type B fluid ---Saturation
curve I
SE
.
I
eon4
T . ‘mm
2
\ I
S Fig. 1. T vs S diagram
for ideal heat-pump
cycle.
514
Wen-Lu
Weng and Bin-Chang
Huang
R114
=363.15K %;;;=323.15K I
150
I
200
I
250
I
I
300
350
Tb
tK)
Fig. 2. COP vs Tb for heat-pump
system
I
I
400
450
at specified
500
5 i0
conditions.
for the case (1). It shows the cycle efficiency increasing with Tb, regardless of the structure of the substances. The figure also reveals the fact that the COP would be higher than that of CFC-114 if the fluid’s Tb is greater than about 250 K. The variation of SCD with rb is presented in Fig. 3. It appears that the SCD is approximately proportional to Tb. Since a good working fluid should have low SCD, the preferable range of Tb is about Tb < 290 K. Meanwhile, Fig. 4 illustrates the SH,, decreasing with PC and changing from positive to negative. The favorable critical pressure range for this application purpose is about from 3.5 to 4.2 MPa, where the absolute value of 5’Hminis smaller. Figures 5 and 6 show that the saturated pressures in the condenser and in the evaporator are correlated well with Tb. To meet the moderate pressure requirement in the heat exchangers, Tb of potential CFC-alternatives is likely to be located within the range 250-325 K. Considering the thermodynamic performances in all aspects, a fluid whose Tb is located within the range 250-290 K and PC within 3.5-4.2 MPa could be a potential working fluid for the application case (1). Likewise, the favorable ranges of Tb and PC are 27&315 K and 3.74.5 MPa, respectively, for the case (2). The fluids with Tb between 300 and 340 K and PC between 3.8 and 4.7 MPa could be thermodynamically appropriate for the higher temperature application in case (3).
-10
E
??? ? ?-0 Hydrocarbons Ed .*a.* Aromatic8 - ??g=== Ethers - AAAhA Ketones +++++ Esters ~~~~~ Inor anics 0~000 Alto f 01s 10 : 0 00 0 0 Halogenated
10 -’
150
I
200
I
250
I
I
I
I
I
300
350
400
450
500
Tb Fig. 3. SCD vs Tb for heat-pump
tK) system
at specified
conditions.
5 10
Performance
of non-CFC
working fluids
.Y
w-l-l
T,,,,=36315K T,,,=323.15K
00
vu
15
Ml4
0
in 1 ... 4
Q-. b
IC /
-13
*.
Hydrocarbons
b
0
*****Aromatics
-30
mlmmEthers AAAAA Ketones +++++ Esters
-45
omoo Inor anics • ~O~Q Alto %01s
0
??
b 0 b b 0
0
.
0
Halogenated
o
??
I
I
I
I
2
1
Pc4(MPaa) Fig. 4. SH vs PC for heat-pump
POTENTIAL
a
system at specified conditions.
NON-CFC
WORKING
FLUIDS
Similar to Lee et al. [12], four performance indices are employed in this study to represent the fluid’s thermodynamic capability. The individual performance indices for a fluid of interest are defined as relative values to those of CFC-114 as follows: COP/COP*
(5)
Zset, = SCD*/SCD
(6)
I cop =
IsH*tinI/Is&n(.
1S.H =
(7)
The variables with superscript ‘*’ denote the performance value of CFC-114. As defined in equation (7) the values of ZSHare extremely large for a few fluids having small absolute values of SH,,. Thus, an upper limit of ZSHwas set to be ‘three’ according to the normal range of other indices. Another index (Ilh,,) is defined to measure the overall thermodynamic capability of a compound by &,,, =
ICOP
x
&CD
x
(8)
ISH.
Ok_
Rll4
*
1:
Hydrocarbons **** Aromatics ??mlmmEthers z AA... Ketones $10 -I= +++++ Esters : ~~~~~ Inor anics L - ~0000 Alto %01s - 0 0 0 0 0 Halogenated
-
_ _
lo-=
I
??????????
??
150
I
200
I
250
9.
*
.+ + ‘r
I
300 Tb
Fig. 5. Pcond vs Tb for heat-pump
I
350
4t)O
4&J
560
cK) system at specified conditions.
. 5
0
Wen-Lu Weng and Bin-Chang Huang
T,,,,=363.15K T,=323.15K . . . . . Hydrocarbons . . . . . Aromatics . . . . . Ethers . . . . . Ketones +++++ Eaters ImE
c0000 ? ?OODO
fmp
%
* Y
OObOO Halogenated
10 -q
=.
I
150
.+ +
200
I
250
I
I
300
350
Tb
tK)
I
I
400
450
I
500
5 i0
Fig. 6. P,,,, vs Tb for heat-pump system at specified conditions.
As defined in equations (5)-(8), a fluid with the larger index value has the better thermodynamic performance and the indices for the reference fluid (CFC-114) are always unity. Tables l-3 list several potential substances in the sequence of I thmnfor the three application cases, respectively. These tabulated compounds have thermodynamic performances comparable to or even better than those of CFC-114. The saturated pressures of fluids at both condensing and evaporating temperatures are also displayed. The uncertainties of the calculated values are estimated to be within 10%. In the case of Tcond= 363.15 K and Tevap= 323.15 K, HCFC-124 gives the highest value of the &,,, but its cycle efficiency is slightly lower than that of CFC-114, as shown in Table 1. HCFC-142b is another possible alternative for the heat pump. Meanwhile, some members of ethers (vinyl methyl ether, dimethyl ether, methyl ethyl ether) and Cq hydrocarbons (2-butenes, 1-butene, methyl acetylene, isobutylene, butadienes, 2-butyne and cyclobutane) are also superior to CFC-114 at the operating conditions. However, all these potential compounds, except for HCFC-124, are flammable. Consequently, HCFC-124 could be the most favorable working fluid for this application due to its nonflammability, low toxicity, low SH,,,,, and SCD, moderate vapor pressures and comparable cycle efficiency. Table 2 indicates that HCFC-142b, HCFC-14lb, and most of the ethers and the hydrocarbons listed above, have good thermodynamic capabilities for the illustrative case (2). Unfortunately, all
Table Working
fluid
HCFC-124 1,3-Butadiene Vinyl methyl ether 2-Butene-cis Dimethyl ether 1,2-Butadiene Diclamine 1-Butene Cyclopropane HCFC-142b Cyclobutane Methyl acetylene Isobutylene Methyl-ethyl ether 2-Butene-tram Triclamine Isobutane I-Butyne n-Butane *CFC-114:[COP
I. Thermodynamical
performance
indices at operation
conditons*.
T,.d = 363.15 K and Tea, = 323.15 K
ICOP
lsco
ISH
Lm
f’wd (MPa)
Pea, (MPa)
0.930 1.045 1.093 1.068 0.980 1.084 1.109 1.020 0.974 0.995 1.053 1.012 1.079 1.079 1.053 1.051 0.964 1.137 1.027
1.748 1.327 1.140 1.110 2.304 0.999 1.309 1.316 2.522 1.948 0.935 2.47 1 1.323 1.087 1.139 1.098 1.336 0.976 1.128
3.0 3.0 3.0 3.0 1.591 3.0 2.193 2.327 1.289 1.537 3.0 1.608 1.981 2.106 1.970 1.514 I .374 1.551 1.248
4.476 4.160 3.735 3.558 3.518 3.241 3.182 3.122 3.119 2.695 2.872 2.671 2.650 2.469 2.363 I.747 1.704 1.616 1.446
2.276 1.434 1.137 1.151 2.699 1.010 1.276 1.477 3.037 2.242 0.926 2.749 1.506 1.119 1.219 1.175 1.642 0.991 1.257
0.980 0.537 0.939 0.447 1.137 0.364 0.456 0.598 1.346 0.980 0.339 1.143 0.610 0.419 0.48 I 0.456 0.685 0.413 0.499
= 5.962, SCD = 0.408 x lo-’ m’ kJ-I,
SH = 11.237 K, PFond= 1.158 MPa, P.,, = 0.448 MPa].
Performance Table 2. Thermodynamic
Vinyl methyl ether R-142b 1,3-Butadiene 2-Butene-cis 1,2-Butadiene Vinyl acetylene Cyclobutane 1-Butene Isobutylene 2-Butene-tram Methyl ethyl ether Dimethyl ether Cyclopropane Iso-butane HCFC-14lb Tticlamine I-Butyne 2-Butyne Methyl acetylene n-Butane ‘CFC-114:[COP
performance
1.143 0.939 1.064 1.111 1.158 1.170 1.162 1.023 1.009 1.064 1.131 0.907 0.886 0.914 1.200 1.083 1.151 1.217 0.964 1.044
of non-CFC
indices at operation
1.2744 1.5296 1.3995 1.1664 1.1156 1.175 1.025 1.291 1.290 1.165 1.170 2.033 2.154 1.203 I.264 1.140 1.014 0.785 2.279 1.138
working conditons’.
3.0 3.0 3.0 3.0 3.0 2.680 3.0 2.519 2.230 2.128 1.716 1.083 0.995 1.615 1.055 I.285 1.233 2.055 0.819 1.150
= 7.954, SCD = 0.576 x lo-’ m3 kJ-‘,
SH =
7.81
I K,
fluids
577
Tcond= 383.15 K and T,,.. = 343.15 K
4.374 4.311 4.276 3.887 3.876 3.685 3.573 3.325 2.903 2.638 2.270 1.809 1.675 1.621 1.600 1.587 1.438 1.438 1.418 1.366
1.771 2.726 2.116 1.722 1.539 1.630 1.394 2.166 2.210 1.833 1.687 3.912 4.322 2.383 1.627 1.748 1.437 0.966 4.007 1.856
0.692 1.197 0.932 0.739 0.63 1 0.737 0.572 0.965 0.985 0.788 0.791 I.794 2.069 1.088 0.674 0.754 0.652 0.395 1.822 0.814
P,“d = 1.729 MPa, P,,., = 0.741 MPa]
these substances are flammable. Such working fluids should be used with care. The potential compounds for the high-temperature application (Tcond= 413.15 K and r,,,, = 343.15 K) are given in Table 3. It is apparent that HFCs, HCFCs and ethers are not thermodynamically comparable to CFC-114 in this case. In addition to the hydrocarbons, as mentioned in the previous two cases, acetone, methyl acetate and 2-propanol exhibit good thermodynamic capabilities for the application case (3). Flammability of those substances should also be noted. CONCLUSIONS
Thermodynamic performances of various working fluids in ideal heat-pump cycles were evaluated with the aid of the Iwai-Margerum-Lu equation of state. The results showed that the normal boiling point and the critical pressure of fluids are the key properties to select thermodynamically proper working fluids for the heat-pump systems. We also found that HCFC-124 is the most favorable non-CFC fluid for the heat pump to recover low-temperature waste heat. HCFC-142b, HCFC-14lb, acetone, methyl acetate, 2-propanol and some Cq hydrocarbons and ethers appear to be thermodynamically better than CFC-114 for higher-temperature applications. However, those flammable substances should be used with care. Acknowledgements-The authors wish to acknowledge also grateful for the valuable suggestions from Professor Institute of Technology.
Table 3. Thermodynamic Working
fluid
1,2-Butadiene 2-Butene-cis Acetone 2-Butene-tram Vinyl acetylene 1,3-Butadiene Methyl acetate 1-Butene Isobutylene Isopropyl alcohol 2-Butyne n-Octane I-Butyne Trimethyl borate Vinyl acetate 1-Propanol ‘CFC-114:[COP
the Ministry of Education, R.O.C., for financial support. M. J. Lee, Department of Chemical Engineering, National
performance
indices at operation
1x0
ISH
ICOP 1.915 1.716 2.441 1.500 2.035 2.053 2.423 1.137 1.025 2.489 2.281 1.341 2.050 2.382 2.465 2.638 = 7.565. SCD = 0.402 x 10-l
2.905 2.695 1.356 2.276 3.0 2.938 1.316 1.643 1.357 0.794 2.399 1.897 2.919 0.929 0.865 0.498
3.0 3.0 3.0 2.773 1.488 1.218 1.832 3.0 3.0 2.785 0.768 0.827 0.530 0.793 0.819 1.564
conditons’.
We are Taiwan
T,,.a = 413.15 K and T’,,, = 343.15 K
hb”” 16.69 13.87 9.726 9.467 9.079 7.347 5.839 5.603 4.170 3.835 3.266 1.741 1.681 1.291 1.239 1.023
F’,,.d (MPa)
P,,., (MPa)
2.705 2.932 0.935 3.110 2.695 2.419 0.934 3.623 3.703 0.640 1.671 3.129 2.340 0.725 0.626 0.402
0.63 1 0.739 0.162 0.788 0.737 0.572 0.157 0.965 0.985 0.066 0.395 0.814 0.652 0.107 0.095 0.039
m’ kJ- I, SH = 4.462 K, P,,.d = 2.960 MPa, P,., = 0.741 MPa]
578
Wen-Lu Weng and Bin-Chang Huang REFERENCES
1. H. Kruse and U. Hesse, Possible substitutes for fully halogenated chloroflourocarbons using fluids already marked. Inr. J. Rejiig. 11, 276-283 (1988). 2. L. Kuijpers and S. M. Miner, The CFC issue and the CFC forum at the 1988 Purdue IIR. Inc. J. Refrig. 12, 118-124 (1989). 13, 122-130 (1990). 3. H. Kurse, CFC research programmes in western Europe. Int. J. R&g. 4. K. Watanabe, Current thermophysical properties research on refrigerant mixtures in Japan. Inr. J. Thermophys. 11, 433-453 (1990). 5. H. 0. Spauschus, Compatibility requirements for CFC alternatives, Znf. J. Refrig. 13, 73-78 (1990). 6. M. J. Lee and H. C. Sun, Thermodynamic performance prediction for non-CFC mixtures. C/rem. Engng Comm. 126, 205220 (1993). 7. A. V. Pradhan and V. H. Larson, Power cycles analyses by generalized thermodynamic properties. Proc. 15th IECEC, pp. 68&685 (1980). 8. M. P. Bertinat, Fluids for high temperature heat pumps. Inf. .I. Refrig. 9, 43-50 (1986). 9. Y. Iwai, M. R. Margerum and B. C.-Y. Lu, A new three-parameter cubic equation of state for polar fluids and fluid mixtures. Fluid Phase Equilibria 42, 21-41 (1988). 10. M. J. Lee and Y. L. Chao, Correlation of thermophysical properties of halogenated refrigerants. Fluid Phase Equilibria 67, 11l-125 (1991). 11. M. J. Lee and Y. L. Chao, Thermophysical performance of CFC-alternatives in refrigeration systems. J. Chinese Inst. Chem. Engng 23, 143-151 (1992). 12. M. J. Lee, D. L. Tein and C. T. Shao, Thermophysical capability of ozone-safe working fluids for an organic Rankine cycle system. Heat Recouery Systems & CHP 13, 409418 (1993). 13. R. C. Reid, J. M. Prausnitz and B. E. Poling, The Properties of Gases and Liquids, 4th Edn. McGraw-Hill, New York,
USA (1987). 14. M. 0. McLinden, Thermodynamic properties of CFC alternatives: a survey of the available data. Int. J. Refrig. 13, 149-162 (1990).
Copyright 0 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1359-4311/96 $15.00 + 0.00
Pergamon 1359-4311(95)ooo58-5
THERMODYNAMIC PERFORMANCES OF NON-CFC WORKING FLUIDS IN HEAT-PUMP CYCLES Wen-Lu Department
of Chemical
Weng*
and Bin-Chang
Huang
Engineering, Ming-Hsin Institute of Technology Hsin-Chu, 304 Taiwan, R. 0. China
and Commerce,
Hsin-Feng,
(Received in final form 26 September 1995) Abstract-Thermodynamic performances of a variety of fluids circulating in ideal heat-pump cycles were evaluated by using the Iwai-Margerum-Lu equation of state. The evaluations were implemented at three operating conditions (evaporating temperatures from 323.15 to 343.15 K and condensing temperatures from 363.15 to 413.15 K). The results showed the normal boiling point and the critical pressure of compounds to be the key properties for selecting the thermodynamically proper working fluids. Several potential non-CFC compounds were suggested for the heat pumps with respect to each application. Copyright 0 1996 Elsevier Science Ltd KeywordsNon-CFC
fluids; heat pumps;
thermodynamic
performance.
NOTATION COP H I ic se SCD SH T V W, Z
coefficient of performance molar enthalpy (kJ molt ‘) performance index pressure (MPa) heat rejected in condenser (kJ mol- ‘) heat absorbed in evaporator (kJmol-‘) molar entropy (kJ molt I K- ‘) specific compressor displacement (m’ kJ - ‘) superheat of vapor (K) temperature (K) molar volume (m’ mol - ‘) work requirement for compressor (kJ) compressibility factor
Greek letters 0
acentric
factor
Superscripts and subscripts t performance value of CFC-114 at normal boiling point b at critical state C cond condenser COP coefficient of performance min minimum evaporator evap SCD vapor volume by unit energy superheat of vapor SH therm overall thermodynamic performance
INTRODUCTION CFC-114 was one of the working fluids widely used for heat pumps to recover low-temperature waste heat. Unfortunately, the fully halogenated compounds including CFC-114 will be phased out in 1996. Some proper non-CFC compounds were urgently needed to replace those regulated substances. Thermodynamically, a good working fluid for a heat pump should have the following desirable characteristics: high cycle efficiency, small specific compressor displacement, low *Author
to whom
correspondence
should
be addressed. 571
572
Wen-Lu Weng and Bin-Chang Huang
minimum superheat of vapor, moderate vapor pressures in the heat-exchange units (generally above atmospheric pressure but not too much above 2 MPa). Numerous efforts have been devoted to the search for new refrigerants in the last decade [l-6]. Since the refrigeration cycle is also a heat-pump cycle, the experiences in studying CFC-alternatives for refrigeration systems are also useful for heat-pump energy-recovery systems. The only difference between these two systems is the operating conditions of the cycles. Heat-pump energy-recovery systems generally operate at higher evaporating and condensing temperatures than refrigeration systems. A good refrigerant may no longer be an appropriate working fluid for the heat pumps. As a consequence, it is of interest to find some other potential alternatives from various chemicals for heat-pump energy-recovery systems. Evaluating the thermodynamic performances of fluids by using a predictive thermodynamic model appears to be an efficient method. Pradhan and Larson [7] reported a power cycle analysis with the aid of a corresponding-states model and Bertinat [S] employed the Lee-Kesler equation to simulate high-temperature heat-pump operations. Several promising cubic equations of state have been developed in the past decade. Among these new equations of state, the Iwai-Margerum-Lu (IML) equation of state [9] is one of the reliable models for predicting the thermodynamic properties of fluids [9, lo]. This equation of state was applied recently to study the CFC-alternatives for refrigeration systems [ll] and for the organic Rankine cycle waste-heat recovery systems [12]. The IML equation of state is also adopted in the present study. A diversity of substances were investigated in this work, which include halogenated substances, hydrocarbons, ethers, ketones, esters, alcohols and inorganic compounds. With the assistance of the simulation results, the criteria for finding thermodynamically proper substances are presented and some potential non-CFC compounds for the heat pump systems are suggested.
HEAT-PUMP
CYCLE
SIMULATION
Thermodynamic performances of fluids can be evaluated via cycle simulations. The paths of ideal heat-pump cycles are illustrated in Fig. 1. There are two types of substances (type-A and type-B), as shown in the graph, which are categorized by the slopes of the saturated vapor curve in T-S diagrams. The slope of the type-A fluids is positive and that of the type-B fluids is negative. The following assumptions were made for those ideal heat-pump cycles: an isobaric process through each heat-exchanger, an isentropic process through the compression, an isenthalpic process during the expansion, and the outlet of the condenser at saturated state. The cycles are specified by two operation variables, for example, evaporating temperature ( Tevap)and condensing temperature (T,,&. When these two temperature levels were given, state properties of the streams in the cycles can be calculated by a standard procedure with an equation of state. The required input variables for the cycle simulation are the fluid’s critical properties (T,, PC, ZJ, acentric factor (0) and heat capacity at ideal gas states. These properties were taken from Reid et al. [13] and McLinden [14]. On the basis of 1 mole of circulating fluid, the ideal cycle efficiency (coefficient of performance, COP) is given by COP = Q/W,,
(1)
where Qc denotes the heat rejection from the working fluid in the condenser which was calculated from the molar enthalpy difference of the streams across the condenser (i.e. H4 - H5 for the type-A fluids and H3 - H5 for the type-B fluids) and W, is the energy input for the isentropic compression (estimating from H4 - H3 for the type-A fluids and H3 - H2 for the type-B fluids). Meanwhile, the specific compressor displacement (SCD) is calculated from the following equation: SCD = Vcamp.~/Qe, where Qe is the heat duty of the evaporator, i.e. H3 - Hl for the type-A fluids and H2 - Hl for the type-B fluids; Vcomp.,n is the molar volume of the compressor-inlet vapor. The compressor inlet is located at point 3 for the type-A fluids and at point 2 for the type-B fluids. Additionally, the minimum superheat of vapor (SHmi,) for the type-A fluids is calculated from SH,,
= T3 -
TI,
(3)
Performance
of non-CFC
working
573
fluids
where T2 = Tevapand the SHmin is positive. For the type-B fluids, SHti, is given by SH,,,,. = T4 -
T,,
(4)
where T4 = Tcondand the SHmi” is negative. SIMULATION
RESULTS
Three illustrative cases are studied in this work. The operating conditions of the heat pump cycles are specified at (1) Tcond= 363.15 K and Tevap= 323.15 K; (2) Tcond= 383.15 K and Tevap= 343.15 K; (3) zond = 413.15 K and Tcvap= 343.15 K, respectively, and the performances of CFC-114 are taken as the comparison basis. Figure 2 plots the COP against the fluid’s normal boiling point (TL,)
Tspe ----
A fluid Saturation
cuwe
-T cm4 I’
/’ ,’ I’
_T m,
I’
1
1’2 #’
S
Type B fluid ---Saturation
curve I
SE
.
I
eon4
T . ‘mm
2
\ I
S Fig. 1. T vs S diagram
for ideal heat-pump
cycle.
514
Wen-Lu
Weng and Bin-Chang
Huang
R114
=363.15K %;;;=323.15K I
150
I
200
I
250
I
I
300
350
Tb
tK)
Fig. 2. COP vs Tb for heat-pump
system
I
I
400
450
at specified
500
5 i0
conditions.
for the case (1). It shows the cycle efficiency increasing with Tb, regardless of the structure of the substances. The figure also reveals the fact that the COP would be higher than that of CFC-114 if the fluid’s Tb is greater than about 250 K. The variation of SCD with rb is presented in Fig. 3. It appears that the SCD is approximately proportional to Tb. Since a good working fluid should have low SCD, the preferable range of Tb is about Tb < 290 K. Meanwhile, Fig. 4 illustrates the SH,, decreasing with PC and changing from positive to negative. The favorable critical pressure range for this application purpose is about from 3.5 to 4.2 MPa, where the absolute value of 5’Hminis smaller. Figures 5 and 6 show that the saturated pressures in the condenser and in the evaporator are correlated well with Tb. To meet the moderate pressure requirement in the heat exchangers, Tb of potential CFC-alternatives is likely to be located within the range 250-325 K. Considering the thermodynamic performances in all aspects, a fluid whose Tb is located within the range 250-290 K and PC within 3.5-4.2 MPa could be a potential working fluid for the application case (1). Likewise, the favorable ranges of Tb and PC are 27&315 K and 3.74.5 MPa, respectively, for the case (2). The fluids with Tb between 300 and 340 K and PC between 3.8 and 4.7 MPa could be thermodynamically appropriate for the higher temperature application in case (3).
-10
E
??? ? ?-0 Hydrocarbons Ed .*a.* Aromatic8 - ??g=== Ethers - AAAhA Ketones +++++ Esters ~~~~~ Inor anics 0~000 Alto f 01s 10 : 0 00 0 0 Halogenated
10 -’
150
I
200
I
250
I
I
I
I
I
300
350
400
450
500
Tb Fig. 3. SCD vs Tb for heat-pump
tK) system
at specified
conditions.
5 10
Performance
of non-CFC
working fluids
.Y
w-l-l
T,,,,=36315K T,,,=323.15K
00
vu
15
Ml4
0
in 1 ... 4
Q-. b
IC /
-13
*.
Hydrocarbons
b
0
*****Aromatics
-30
mlmmEthers AAAAA Ketones +++++ Esters
-45
omoo Inor anics • ~O~Q Alto %01s
0
??
b 0 b b 0
0
.
0
Halogenated
o
??
I
I
I
I
2
1
Pc4(MPaa) Fig. 4. SH vs PC for heat-pump
POTENTIAL
a
system at specified conditions.
NON-CFC
WORKING
FLUIDS
Similar to Lee et al. [12], four performance indices are employed in this study to represent the fluid’s thermodynamic capability. The individual performance indices for a fluid of interest are defined as relative values to those of CFC-114 as follows: COP/COP*
(5)
Zset, = SCD*/SCD
(6)
I cop =
IsH*tinI/Is&n(.
1S.H =
(7)
The variables with superscript ‘*’ denote the performance value of CFC-114. As defined in equation (7) the values of ZSHare extremely large for a few fluids having small absolute values of SH,,. Thus, an upper limit of ZSHwas set to be ‘three’ according to the normal range of other indices. Another index (Ilh,,) is defined to measure the overall thermodynamic capability of a compound by &,,, =
ICOP
x
&CD
x
(8)
ISH.
Ok_
Rll4
*
1:
Hydrocarbons **** Aromatics ??mlmmEthers z AA... Ketones $10 -I= +++++ Esters : ~~~~~ Inor anics L - ~0000 Alto %01s - 0 0 0 0 0 Halogenated
-
_ _
lo-=
I
??????????
??
150
I
200
I
250
9.
*
.+ + ‘r
I
300 Tb
Fig. 5. Pcond vs Tb for heat-pump
I
350
4t)O
4&J
560
cK) system at specified conditions.
. 5
0
Wen-Lu Weng and Bin-Chang Huang
T,,,,=363.15K T,=323.15K . . . . . Hydrocarbons . . . . . Aromatics . . . . . Ethers . . . . . Ketones +++++ Eaters ImE
c0000 ? ?OODO
fmp
%
* Y
OObOO Halogenated
10 -q
=.
I
150
.+ +
200
I
250
I
I
300
350
Tb
tK)
I
I
400
450
I
500
5 i0
Fig. 6. P,,,, vs Tb for heat-pump system at specified conditions.
As defined in equations (5)-(8), a fluid with the larger index value has the better thermodynamic performance and the indices for the reference fluid (CFC-114) are always unity. Tables l-3 list several potential substances in the sequence of I thmnfor the three application cases, respectively. These tabulated compounds have thermodynamic performances comparable to or even better than those of CFC-114. The saturated pressures of fluids at both condensing and evaporating temperatures are also displayed. The uncertainties of the calculated values are estimated to be within 10%. In the case of Tcond= 363.15 K and Tevap= 323.15 K, HCFC-124 gives the highest value of the &,,, but its cycle efficiency is slightly lower than that of CFC-114, as shown in Table 1. HCFC-142b is another possible alternative for the heat pump. Meanwhile, some members of ethers (vinyl methyl ether, dimethyl ether, methyl ethyl ether) and Cq hydrocarbons (2-butenes, 1-butene, methyl acetylene, isobutylene, butadienes, 2-butyne and cyclobutane) are also superior to CFC-114 at the operating conditions. However, all these potential compounds, except for HCFC-124, are flammable. Consequently, HCFC-124 could be the most favorable working fluid for this application due to its nonflammability, low toxicity, low SH,,,,, and SCD, moderate vapor pressures and comparable cycle efficiency. Table 2 indicates that HCFC-142b, HCFC-14lb, and most of the ethers and the hydrocarbons listed above, have good thermodynamic capabilities for the illustrative case (2). Unfortunately, all
Table Working
fluid
HCFC-124 1,3-Butadiene Vinyl methyl ether 2-Butene-cis Dimethyl ether 1,2-Butadiene Diclamine 1-Butene Cyclopropane HCFC-142b Cyclobutane Methyl acetylene Isobutylene Methyl-ethyl ether 2-Butene-tram Triclamine Isobutane I-Butyne n-Butane *CFC-114:[COP
I. Thermodynamical
performance
indices at operation
conditons*.
T,.d = 363.15 K and Tea, = 323.15 K
ICOP
lsco
ISH
Lm
f’wd (MPa)
Pea, (MPa)
0.930 1.045 1.093 1.068 0.980 1.084 1.109 1.020 0.974 0.995 1.053 1.012 1.079 1.079 1.053 1.051 0.964 1.137 1.027
1.748 1.327 1.140 1.110 2.304 0.999 1.309 1.316 2.522 1.948 0.935 2.47 1 1.323 1.087 1.139 1.098 1.336 0.976 1.128
3.0 3.0 3.0 3.0 1.591 3.0 2.193 2.327 1.289 1.537 3.0 1.608 1.981 2.106 1.970 1.514 I .374 1.551 1.248
4.476 4.160 3.735 3.558 3.518 3.241 3.182 3.122 3.119 2.695 2.872 2.671 2.650 2.469 2.363 I.747 1.704 1.616 1.446
2.276 1.434 1.137 1.151 2.699 1.010 1.276 1.477 3.037 2.242 0.926 2.749 1.506 1.119 1.219 1.175 1.642 0.991 1.257
0.980 0.537 0.939 0.447 1.137 0.364 0.456 0.598 1.346 0.980 0.339 1.143 0.610 0.419 0.48 I 0.456 0.685 0.413 0.499
= 5.962, SCD = 0.408 x lo-’ m’ kJ-I,
SH = 11.237 K, PFond= 1.158 MPa, P.,, = 0.448 MPa].
Performance Table 2. Thermodynamic
Vinyl methyl ether R-142b 1,3-Butadiene 2-Butene-cis 1,2-Butadiene Vinyl acetylene Cyclobutane 1-Butene Isobutylene 2-Butene-tram Methyl ethyl ether Dimethyl ether Cyclopropane Iso-butane HCFC-14lb Tticlamine I-Butyne 2-Butyne Methyl acetylene n-Butane ‘CFC-114:[COP
performance
1.143 0.939 1.064 1.111 1.158 1.170 1.162 1.023 1.009 1.064 1.131 0.907 0.886 0.914 1.200 1.083 1.151 1.217 0.964 1.044
of non-CFC
indices at operation
1.2744 1.5296 1.3995 1.1664 1.1156 1.175 1.025 1.291 1.290 1.165 1.170 2.033 2.154 1.203 I.264 1.140 1.014 0.785 2.279 1.138
working conditons’.
3.0 3.0 3.0 3.0 3.0 2.680 3.0 2.519 2.230 2.128 1.716 1.083 0.995 1.615 1.055 I.285 1.233 2.055 0.819 1.150
= 7.954, SCD = 0.576 x lo-’ m3 kJ-‘,
SH =
7.81
I K,
fluids
577
Tcond= 383.15 K and T,,.. = 343.15 K
4.374 4.311 4.276 3.887 3.876 3.685 3.573 3.325 2.903 2.638 2.270 1.809 1.675 1.621 1.600 1.587 1.438 1.438 1.418 1.366
1.771 2.726 2.116 1.722 1.539 1.630 1.394 2.166 2.210 1.833 1.687 3.912 4.322 2.383 1.627 1.748 1.437 0.966 4.007 1.856
0.692 1.197 0.932 0.739 0.63 1 0.737 0.572 0.965 0.985 0.788 0.791 I.794 2.069 1.088 0.674 0.754 0.652 0.395 1.822 0.814
P,“d = 1.729 MPa, P,,., = 0.741 MPa]
these substances are flammable. Such working fluids should be used with care. The potential compounds for the high-temperature application (Tcond= 413.15 K and r,,,, = 343.15 K) are given in Table 3. It is apparent that HFCs, HCFCs and ethers are not thermodynamically comparable to CFC-114 in this case. In addition to the hydrocarbons, as mentioned in the previous two cases, acetone, methyl acetate and 2-propanol exhibit good thermodynamic capabilities for the application case (3). Flammability of those substances should also be noted. CONCLUSIONS
Thermodynamic performances of various working fluids in ideal heat-pump cycles were evaluated with the aid of the Iwai-Margerum-Lu equation of state. The results showed that the normal boiling point and the critical pressure of fluids are the key properties to select thermodynamically proper working fluids for the heat-pump systems. We also found that HCFC-124 is the most favorable non-CFC fluid for the heat pump to recover low-temperature waste heat. HCFC-142b, HCFC-14lb, acetone, methyl acetate, 2-propanol and some Cq hydrocarbons and ethers appear to be thermodynamically better than CFC-114 for higher-temperature applications. However, those flammable substances should be used with care. Acknowledgements-The authors wish to acknowledge also grateful for the valuable suggestions from Professor Institute of Technology.
Table 3. Thermodynamic Working
fluid
1,2-Butadiene 2-Butene-cis Acetone 2-Butene-tram Vinyl acetylene 1,3-Butadiene Methyl acetate 1-Butene Isobutylene Isopropyl alcohol 2-Butyne n-Octane I-Butyne Trimethyl borate Vinyl acetate 1-Propanol ‘CFC-114:[COP
the Ministry of Education, R.O.C., for financial support. M. J. Lee, Department of Chemical Engineering, National
performance
indices at operation
1x0
ISH
ICOP 1.915 1.716 2.441 1.500 2.035 2.053 2.423 1.137 1.025 2.489 2.281 1.341 2.050 2.382 2.465 2.638 = 7.565. SCD = 0.402 x 10-l
2.905 2.695 1.356 2.276 3.0 2.938 1.316 1.643 1.357 0.794 2.399 1.897 2.919 0.929 0.865 0.498
3.0 3.0 3.0 2.773 1.488 1.218 1.832 3.0 3.0 2.785 0.768 0.827 0.530 0.793 0.819 1.564
conditons’.
We are Taiwan
T,,.a = 413.15 K and T’,,, = 343.15 K
hb”” 16.69 13.87 9.726 9.467 9.079 7.347 5.839 5.603 4.170 3.835 3.266 1.741 1.681 1.291 1.239 1.023
F’,,.d (MPa)
P,,., (MPa)
2.705 2.932 0.935 3.110 2.695 2.419 0.934 3.623 3.703 0.640 1.671 3.129 2.340 0.725 0.626 0.402
0.63 1 0.739 0.162 0.788 0.737 0.572 0.157 0.965 0.985 0.066 0.395 0.814 0.652 0.107 0.095 0.039
m’ kJ- I, SH = 4.462 K, P,,.d = 2.960 MPa, P,., = 0.741 MPa]
578
Wen-Lu Weng and Bin-Chang Huang REFERENCES
1. H. Kruse and U. Hesse, Possible substitutes for fully halogenated chloroflourocarbons using fluids already marked. Inr. J. Rejiig. 11, 276-283 (1988). 2. L. Kuijpers and S. M. Miner, The CFC issue and the CFC forum at the 1988 Purdue IIR. Inc. J. Refrig. 12, 118-124 (1989). 13, 122-130 (1990). 3. H. Kurse, CFC research programmes in western Europe. Int. J. R&g. 4. K. Watanabe, Current thermophysical properties research on refrigerant mixtures in Japan. Inr. J. Thermophys. 11, 433-453 (1990). 5. H. 0. Spauschus, Compatibility requirements for CFC alternatives, Znf. J. Refrig. 13, 73-78 (1990). 6. M. J. Lee and H. C. Sun, Thermodynamic performance prediction for non-CFC mixtures. C/rem. Engng Comm. 126, 205220 (1993). 7. A. V. Pradhan and V. H. Larson, Power cycles analyses by generalized thermodynamic properties. Proc. 15th IECEC, pp. 68&685 (1980). 8. M. P. Bertinat, Fluids for high temperature heat pumps. Inf. .I. Refrig. 9, 43-50 (1986). 9. Y. Iwai, M. R. Margerum and B. C.-Y. Lu, A new three-parameter cubic equation of state for polar fluids and fluid mixtures. Fluid Phase Equilibria 42, 21-41 (1988). 10. M. J. Lee and Y. L. Chao, Correlation of thermophysical properties of halogenated refrigerants. Fluid Phase Equilibria 67, 11l-125 (1991). 11. M. J. Lee and Y. L. Chao, Thermophysical performance of CFC-alternatives in refrigeration systems. J. Chinese Inst. Chem. Engng 23, 143-151 (1992). 12. M. J. Lee, D. L. Tein and C. T. Shao, Thermophysical capability of ozone-safe working fluids for an organic Rankine cycle system. Heat Recouery Systems & CHP 13, 409418 (1993). 13. R. C. Reid, J. M. Prausnitz and B. E. Poling, The Properties of Gases and Liquids, 4th Edn. McGraw-Hill, New York,
USA (1987). 14. M. 0. McLinden, Thermodynamic properties of CFC alternatives: a survey of the available data. Int. J. Refrig. 13, 149-162 (1990).