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Theoretical assessments of HFC134a and alternatives to CFC12 as working fluids for heat pumps
Applied Energy 41 (1992) 285-299
Theoretical Assessments of HFC134a and Alternatives to CFC12 as Working Fluids for Heat Pumps S. Devotta & S. Gopichand Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India
ABSTRACT HFC134a has been identified as an alternative to CFC12 for refrigeration applications. Significant progress has been made in the use of HFC134a for refrigeration. This paper analyses the suitability of HFC134a along with those of HCFC22, HCFC124, HFC134, HCFC142b and HFC152a as alternatives to CFC12 for heat-pump applications. Some basic and derived thermodynamic data have been used for a comparative assessment. HFC134a appears to be the closest alternative to CFC12, but HFC152a is likely to be a better alternative from an energy point of view.
NOMENCLATURE H P (PR) Qco s (SCD) T
Rankine coefficient of performance (dimensionless) Enthalpy per unit mass (kJ kg -1) Pressure (bar) Pressure ratio (dimensionless) Specific heating load (kJ kg -~) Entropy per unit mass (kJ kg- ~K- ~) Specific compressor displacement (m 3 MJ -~) Temperature (°C or K)
PEV
Vapour density at TEv (kg m-3)
(COP)RH
Subscripts C
CO EV
Critical Condensing/condenser Evaporating/evaporator 285
Applied Energy 0306-2619/92/$05.00 ~ 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain
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S. Devotta, S. Gopichand
INTRODUCTION Many chlorofluorocarbons (CFCs) and hydrofluorocarbons (HCFCs) are being used extensively as refrigerants in air-conditioning and refrigeration (AC& R) and as working fluids in heat pumps. They possess most of the desirable characteristics for mechanical vapour-compression systems, such as thermal and chemical stability, thermodynamic suitability, non-toxicity, non-flammability and low cost, but unfortunately they have a damaging effect on the stratospheric ozone layer. Therefore the continued use of CFCs will not be permitted. The ozone-layer depletion has become an international issue. An international agreement--the 'Montreal P r o t o c o l ' on the curtailed production and phasing out of CFCs on a global scale has been signed by many countries. Obviously alternatives to CFCs have to be identified and developed. This has attracted the attentions of scientists from many disciplines. CFCs have been used for over five decades in refrigeration systems and for over two decades in heat-pump systems, as they possess some of the best characteristics. Now scientists and technologists are faced with a dilemma to identify a compound which, as long as it is within the system, performs satisfactorily, perhaps even better than the currently used fluids, but if it leaks out should be harmless to human health and benign to the environment. Therefore any alternative has to have a low or preferably zero ozone-depletion potential (ODP). Ideally one should screen for alternatives, not only with zero ODPs but with relatively low values of the global warming potential (GWP), as the 'greenhouse effect' is also becoming a major international issue. These constraints limit the number of alternatives. It is now recognised that in order to satisfy the environmental regulations one has to compromise on other factors such as flammability, energy efficiency and cost.
HEAT PUMPS A N D CFCs For refrigeration there are not many viable and simple competitive processes for mechanical vapour-compression systems, whereas for heating there are many alternative methods. Therefore the economic acceptabilities of heat pumps have often been compared unfavourably with those of conventional processes. The fact that the recycling of energy also leads to a reduction in CO2 emission and consequently less 'greenhouse effect' is likely to give an additional advantage to heat pumps. Some weightage to CO 2 emission reduction along with energy savings should be collectively compared with the conventional process.
Assessments o f alternatives to CFC12 Jbr heat pumps
287
The heat-pump industry derived a great deal from the already existing advances in the AC& R industry. The system developments were based on the availability of components for refrigeration applications. This was particularly so for compressors. Most of the limitations of the readily available compressors were accepted for heat-pump developments. Some metallurgical modifications were made to suit the higher temperature of operation. To begin with, working fluids HCFC22 and CFC12 were used for low-temperature heating. Searches were made to screen the working fluids for a given range of high temperatures. With further developments of the fluids, high-temperature heat pumps were developed using CFC114.
C U R R E N T STATUS Heat pumps are generally used for space heating in residential and commercial sectors (their sizes ranging from a few kW to several MW) and for process-heat recovery and recycling in the industrial sector (from about 100 kW to several MW) using various types of source and delivering heat to various types of sink depending upon the type of application. According to Kuijpers ~, the present worldwide installed capacity of heat pumps is approximately 7500 MW, about 6000 MW for space heating and 1500 MW in the industrial sector. Most of the residential heat pumps use hermetic reciprocating compressors, while the bigger sizes use both semi-hermetic and open-type reciprocating, scroll and screw type compressors. Turbocompressors are also used for larger sizes. Most of the heat pumps in the international market have been developed using HCFC22, CFC12 and CFCI14 having upper limits for their condensing temperatures of about 60, 70 and 120°C, respectively. CFC 114 is used exclusively in industrial applications where high temperatures are essential. The temperature limits were fixed with respect to many factors arising out of the operating characteristics of the available compressors and the thermodynamic characteristics of the working fluid under consideration. Heat-pump designs using these fluids are well established. The continued use of CFC12 and C F C l l 4 will not be permitted as their ozone-depleting potentials are relatively high, namely 1-0 and 1.0, respectively. HCFC22, with an ODP of 0.05, is not currently covered under the Montreal Protocol. However, it has been agreed that HCFCs, including HCFC22, will be phased out by the year 2020, at the latest by 2040. The working-fluid charge in existing heat pumps is usually between 0.5 and 1-5 kg k W - 1, depending upon the type of heat exchangers and compressors used and the application. The quantity is very much lower, as low as 0.1-0"2 kg kW-1, for units with small hermetic compressors. It is
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S. Devotta, S. Gopichand
estimated 1 that the annual global consumption of CFCs is about 800 tonnes, mainly for new units and as recharges in existing units. For an anticipated increase of 10%o for the heat-pump market, this implies an increase of 45% in CFC consumption by 1994 compared with 1986, while in 1998 a reduction of about 45% is expected, with reference to the same year. Therefore it is imperative that ozone-layer compatible alternatives to CFC 12 are identified and the systems using these fluids designed and developed. This is essential if the potential for heat pumps, still at the emerging stage, has to be safeguarded. This will be the major factor determining the future of mechanical vapour-compression heat pumps.
ALTERNATIVES TO CFC12 In spite of concerted efforts to broaden the families of compounds for CFC substitutes, the most likely working-fluid substitutes are emerging only from CFC-related families, either from HCFCs or from HFCs (with no chlorine but with some hydrogen atoms). After an extensive molecular inventory of more than 800 compounds, McLinden and Didion 2 concluded that HFC134a, which has no chlorine in it and hence has a zero ODP, is the most worthy potential substitute candidate for CFC12. The substitution of CFC12 by any alternative would involve substantial changes in various components, such as the heat exchangers, motor, insulation and lubricants. Tests have to be carried out to optimise the system performance and to ensure the reliability and safety of the system. Performance data of refrigeration systems with these three possible alternatives must be generated first, so that eventually it would be possible to substitute CFC12 by more ozone-friendly compounds. There are also some possibilities of using non-azeotropic mixtures (HCFC22/HCFC142b and HCFC22/HFC152a) of compounds with lower ODPs. These mixtures can be screened to match the properties of CFCs being replaced as closely as possible. For mixtures, the thermodynamicproperty package development and property data collection will have to be carried out independently. The property package should be capable of predicting the required properties of mixtures of various combinations. Currently available predictions are somewhat limited by the fact that a few P - V - T measurements of the mixture under consideration should be available to predict accurately the thermodynamic data. One major refrigerant manufacturer has already identified a ternary mixture, HCFC22/HFC152a/HCFC124, as a potential substitute for CFC12 for automobile air-conditioning and refrigeration. Some experimental evaluation of this ternary mixture for refrigeration has been
Assessments o f alternatives to CFCI2 for heat pumps
289
undertaken by Kuijpers e t al. 3 The performance of this blend for automotive air-conditioning was reported to be slightly worse than CFC12. Suitable compatible materials either are available or are being developed. 4 Thus further discussion is limited to pure fluids only. HFC134a is considered to possess the best potential as a long-term substitute for CFC12 in refrigeration processes. Preliminary information on HFC134a indicates that it could be available by 1992, but substantial toxicity and other testing must be completed by producers. From molecular studies and some short-term testing, HFC134a is expected to be non-toxic. The refrigeration performance and its optimisation under actual operating conditions with respect to CFC12 are being studied. The entire process is expected to take 10 years or more. s Initially, HFC134a is expected to be significantly costlier than CFC12 and the other substitutes considered. Oil-refrigerant characteristics of HFC134a are entirely different from those ofCFC12. Spauschus 6 has dealt with this topic elaborately. The higher polarity of HFC134a results in its low solubility in non-polar lubricants. such as the mineral oils and synthetic hydrocarbons currently used as refrigerant compressor lubricants. Mineral oils are immiscible, even at temperatures as high as 65°C, with HFC134a. Similar behaviour has been reported with alkyl benzenes. Certain of the most popular candidate synthetic lubricants are reported to exhibit unconventional solubility behaviour, including decreasing solubility with increasing temperature. This behaviour can lead to serious difficulties in heat-pump systems operating over wide ranges of temperature and pressure. The stability of a working fluid is very important for the satisfactory operation of any heat-pump system. The working fluid and lubricant combination in the presence of impurities found in the system should be chemically compatible at the operating conditions. This depends very much upon the type of oil used. Generally the products of decomposition are noncondensable and affect the performance and life of the system. There is an abundant literature on the thermal and chemical stabilities of conventional working fluids in the presence of oils and metals. However, similar information on the potential alternative fluids remains closely guarded. For CFC12, oils could be obtained independently from lubricant dealers. For CFC substitutes, the oils are also tested by the manufacturers of alternative fluids and specified. The working fluid should be pure, otherwise the impurities present in it (such as moisture, residue, dissolved gases) interfere with the machine operation. According to Spauschus 6 the chemical stability of HFC134a is not likely to be a limiting condition in refrigeration applications. However, one is yet to initiate such an assessment for heat-pump applications using HFC 134a. The high stability of HFC134a results in a relatively low response by
290
S. De~otta, S. Gopichand
electronic halogen detectors. This must be resolved in order to avoid the problems for manufacturers and service engineers. Using the normal boiling-point as a guide for screening alternatives to CFC12, one could also include HFC134 a n d HCFC124. HCFC124 is currently produced on a laboratory scale only. The suitability of HCFC124 for refrigeration is being studied as part of the international effort. 7 The Electricity Research and Development Centre, Capenhurst, England, has noted that HFC134 is superior even to HFC134a and it has not been given adequate attention so far. 8 The developmental activities on HFC 134 are not as well known as for other fluids. Other possible substitute fluids which are already commercially available from the CFC family, but with some compromises, are HCFC142b and HFC152a. 9 While HCFCI42b has an ODP of 0.05, i.e. the same as that of HCFC22, HFC152a with no chlorine has an ODP of zero. However, with more H atoms in the molecule, its flammability is higher. Therefore use of HFC152a and HCFC142b, especially in heat pumps, entails much better safety standards. It is worth noting that HFC152a is widely used as one of the components in an azeotropic mixture (R500) with CFC12 for lowtemperature refrigeration applications. Anticipating the potential applications of some of these compounds in refrigeration, their toxicities are already being studied under international consortia efforts, e.g. HCFC22, HCFC123, HCFC124, HCFC141b, HCFC142b, HFC134a, HFC152a and HFC125 under Alternative Fluorocarbon Environmental Acceptability Studies (AFEAS) and HCFC123, HCFC124, HCFC141b, HFC134a and HFC125 under the Program for Alternative Fluorocarbon Toxicity (PAFT). The American Household Appliances Manufacturers (AHAM) have identified the following single component fluids: HFC134a, HFC152a and HFC134; and the following non-azeotropic mixtures: HFC134a/HFC152a, HCFC22/HCFC124, HFC22/HCFC142b, CFCl14/HCFC22/HFC152a and HCFC124/ HCFC22/HFC152a, as the potential alternatives to CFC12 in domestic refrigeration, l Preisegger l° suggested HCFC227 (CF3-CHF-CF3) as an alternative refrigerant to CFC 12. HCFC227 has not been considered in this analysis as the pertinent thermodynamic data are not readily available. Hogberg e t al. H have considered many alternatives, including the hydrocarbons isobutane, n-butane, dimethyl propane, 2-methyl butane and n-pentane along with HFC143, HCFC142b and HFC152a. Most of the hydrocarbons have not been considered because of their flammabilities and other constraints. The aim of this study is to compare the performances of HFC134a, HCFC22, CFC12, HFC152a, HCFC124 and HCFC142b for condensing
Molecular formula Mol. wt (kg kmo1-1) Tc ( C ) Pc (MPa) N.B.P. ( C ) PEr (MPa) P¢o (M Pa) Qco (kJ kg -1) (SCD) (m 3 M J - 1) (PR) (COP)R H ODP G W P relative to C F C I 2 (NRSC Group/underwriters class)
CHCIF 2 86"5 96.00 4.97 - 40-76 0"91 2.993 146"8 0-176 5 3.32 5-12 0-05 0.288 I/5a
HCFC22 CHzF-CF 3 102'0 101"7 3.78 - 26.2 0"572 2.115 131.7 0.273 3-7 4-94 0.0 0.114 7
HFC134a CC12F2 120-9 112-0 4-113 - 29.79 0"567 1-878 110"3 0.281 1 3.32 5.2 1.0 2.941 1/6
CFC12 CH3-CHF2 66"0 113.5 4.49 - 24.7 0"45 1'726 237-6 0-267 7 3.69 5.44 0"0 0-022 7 --
HFCI52a
--
CHFz-CHF2 102'03 115.6 3.77 - 19.7 0"450 7 1-727 114-24 0-329 3.83 5.285 0.0
HFCI34
CHCIF-CHF2 136"48 122.23 3"615 - 11.9 0"327 7 1.262 154.01 0.426 3 3'85 5.227 0-02 0-294 --
HCFC124
TABLE 1 Comparative Data for HFC134a and Some Working Fluids for Tco = 7 0 C and TEv = 20°C
CH3-CCIF2 100.49 137"0 4.123 - 9-78 0-289 1.124 170.1 0.446 3"88 5.81 0.06 0'323 1/5
HCFC142b
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~
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292
S. Devotta, S. Gopichand
temperatures up to 70°C. A similar analysis for refrigeration has been presented by Devotta and Gopichand} 2 Some basic properties of the working fluids covered in this analysis are indicated in Table 1. DERIVED T H E R M O D Y N A M I C DESIGN DATA The operation of a refrigeration system approximates to that of the Rankine cycle. The ideal Rankine cycle is illustrated in Fig. 1, which is a plot of pressure (P) against enthalpy per unit mass (H) for HFC134a. The cycle consists of an isobaric evaporation (1 -~ 2), an isentropic compression (2 -~ 3), an isobaric condensation (3-*4 ~ 5) and an isenthalpic expansion (5-~ 1). There are four critical parameters which determine the choice of a working fluid ~a for a given set of conditions. They are ---evaporating temperature, TEv -----condensing temperature, Tco --pressure ratio, (PR) --Rankine coefficient of performance, (COP)Rn If any two of these parameters are fixed, for a given working fluid, the values of the other two parameters are automatically fixed. In reality there are other factors to be considered, such as capacity of the available compressors, heat transfer and pressure drop characteristics of heat exchangers and interconnecting piping, thermal and chemical stabilities of the fluid, lubricant and materials of construction combination, miscibility characteristics of the working fluid, and lubricant, toxicity and safety aspects of the 10"0
a.o 1"0
~ 0'1
0-01 I
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1OO 150 200 250 ENTHALPY H~ kJ kg-~
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Fig. 1. Pressure against enthalpy per unit mass for R134a.
Assessments o f alternatives to CFCI2 for heat pumps
293
fluid for the application under study. If the chosen fluid is not capable of meeting the required conditions then another has to be selected. For a hermetic compressor, the choice is further limited by the fixed combination of volumetric throughput and motor power. Therefore it is possible that a compressor will be operating under an inefficient condition while meeting the required heating load. This is extremely important when a hermetic compressor, designed for a particular working fluid, is operated with another working fluid. This has been one of the limitations of hightemperature heat pumps using C F C l l 4 . Depending upon the type of cycle followed, the relative ratings of various fluids may vary. 14 However, the simplest cycle is the ideal Rankine cycle. The design data can be derived readily from basic thermodynamic data as follows: Pressure ratio (PR) -
eco
(1)
eEv
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(2)
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1000
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/43-/42
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+ Tco(S 3 -- 54)
(5)
where S is the entropy per unit mass of working fluid. Because the compression from 2 to 3 is isentropic, S 3 = S 2. Therefore H3 = / / 4 + Tco(S2 -- S,)
(6)
Equations (1}-(6) can be used to calculate any derived thermodynamic design data for a working fluid for a given combination of TEVand Tco using the saturation properties. For the condition $2 less than $4 the vapour at 2 is provided with a required superheat such that, upon isentropic compression, the discharge vapour is just saturated (i.e. state 4).
294
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(COP)a H against Tco for various (PR) and ( T c o - TEv) for R134a.
D E R I V E D T H E R M O D Y N A M I C DATA FOR HFC134a A composite plot between Tco, (Tco-TEv ) and (PR) for HFC134a is presented in Fig. 2. All the basic thermodynamic properties of HFC134a have been taken from McLinden e t a l l s This plot can be used conveniently to check the suitability of HFC134a for a particular application• It can be seen that if the (PR) of an available compressor is limited to 2-5, for a condensing temperature Tco of 70°C, the maximum possible evaporating temperature is about 33°C and the theoretical Rankine coefficient of performance (COP)R H will be about 7"35. Similarly, if the compressor is capable of achieving a (PR) of 6.0, for a Tco of 70°C, the maximum temperature lift will be about 65°C with a (COP)R H of about 3"6. COMPARATIVE ASSESSMENT OF HFC134a From the set of equations presented above, the same design parameters were calculated lbr other fluids• The thermodynamic data for HCFC124 and
Assessments of alternatives to CFCI2 for heat pumps
295
8.°i I 7"O-
R134B R134., R 152(] -
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(PR) against (Tco - TEV) for Tco= 70 C.
R134 were generated by using a computer program from the Allied Signal Corporation IncJ 6 The thermodynamic data for other fluids were taken from the A S H R A E H a n d b o o k 1988 Fundamentals. ~7 Comparative performance data are presented in Table 1. It can be seen from the table that, for a given condition of Tco = 70°C, the condenser pressure will be highest with HCFC22 (= 2.993 MPa), which is well above the usual design upper limit for a compressor. While the behaviour of H F C I 3 4 a follows very closely that of CFC12, though on the higher side, HFC152a is much closer on the lower side. The variation of pressure ratio (PR) with ( T c o - TEv) for a Tco of 70°C is shown in Fig. 3. The pressure ratios for HFC134a and HFC152a are almost the same, and (PR) values for HCFC124, HCFC142b and HFC134 are much higher than for HFC134a. Typically, for Tco=70°C and TEv=30°C, H C F C 142b requires the highest (PR) of 2.885 while H FC 134a and H F C 152a require an intermediate value of 2.74. This means the volumetric efficiency of the compressor operating with HCFC142b (or with HCFC124) will be the
S. Devotta, S. Gopichand
296
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-
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Fig. 4.
(COP)R. against
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LIFT ( T c o - T E v ) ~'C
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least, while with HCFC22 it is likely to be the same as that with CFC12. Higher pressure-ratios lead to various unwanted conditions like compressor cylinder overheating due to high discharge temperatures and valve leakage. With the available components for the usual pressure-ratio range only HFC152a and HFC134a appear to be alternatives to CFC12. The variation of the Rankine coefficient of performance for heating (COP)RH with ( T c o - TEV) is shown in Fig. 4. Of all the fluids considered, HCFC142b offers the best (COP)RH value. For the condition of Tco = 70°C and TEV= 30°C, HCFC142b has the highest value of 7.5 while HFC134a has the least value of 6"49. Obviously using HFC134a means some energy penalty with respect to CFC12. However, the use of HCFC142b entails more stringent safety design to mitigate the flammability of HCFC142b. The specific compressor displacements for HFC134a and HFC152a, ~as shown in Fig. 5, are closer to CFC12 while HCFC124 and FICFC142b require bigger compressors for the same heat duty. Though from an energy
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s. Devotta, S. Gopichand
point of view HCFC142b appears to be extremely attractive, it has the disadvantage of requiring a bigger compressor. With respect to the use of standard CFC 12 compressors, HFC134a appears to be the ideal choice. If a redesign is considered, HFC152a is an even better choice because of its highe r (COP)~H, zero ODP and relatively low GWP. However, safety aspects have to be mitigated for HFC152a. It is generally believed that this is possible I with the current level of compressor technology. Figure 6 is a plot of condenser load Qco against (Tco-TEv) for a condensing temperature of 70°C. For the identical conditions the condenser load for HFC152a is the highest. The values for CFC12 and HCFC124 are almost the same within the range of this study. For Tco= 70°C and T~v = 20°C, HFC 152a has the highest value of 237.6 kJ kg-1 while CFC 12 has the lowest value of 110.3 kJ kg-~; HFC134a has an intermediate value of 131.7 kJ kg- 1.
CONCLUSIONS This comparative assessment suggests the choice of HFC 134a as the logical alternative to CFC12 as the working fluid for heat pumps. Nevertheless, there is significant scope for the use of HFC152a for heat pumps from an energy efficiency point of view. HFC152a is likely to be as stable a fluid as HFC134a, but with more H atoms will pose a miscibility problem similar to that with HFC134a. Also the flammability aspect has to be considered. It is already commercially available and it is cheaper than HFC134a. The compressor discharge temperature is also likely to be relatively high. This will require some metallurgical considerations for the discharge ports and better insulating material for the motor windings, if used for hermetic compressors. China is and US-EPA, Washington DC, are working on H FC 152a as an alternative to CFC12 in refrigeration applications. There is an immediate need for similar research and development work for heatpump applications.
REFERENCES 1. Kuijpers, L. J. M., Technology progress on protecting the ozone layer: refrigeration, air-conditioning and heat pumps--technical options report. UNEP, Nairobi, 1989. 2. McLinden, M. O. & Didion, D. A., The search for alternative refrigerants--a molecular approach. Proc. LLF.-LLR.--Commissions B1, B2, El, E2, Purdue, USA, 1988, pp. 91-9. 3. Kuijpers, U J.M., De Wit, J.A., Bencschop, A.A.J. & Janssen, M.J.P.,
Assessments of alternatives to CFC12.Ibr heat pumps
4.
5. 6. 7.
8. 9. 10. 11. 12. 13. 14. 15.
16. 17. 18.
299
Experimental investigations into the ternary blend HCFC-22/124/152a as a substitute in domestic refrigeration. Proc. 1990 ASHRAE-Purdue CFC Conference, pp. 315-33. Batemen, D.J., Bivens, D.B., Gorski, R.A., Wells, W.D., Lindstorm, R.A., Morse, R.L. & Shinon, R.L., Refrigerant blends for the automative airconditioning after market. SAE Technical Paper Series no. 900216, International Congress and Exposition, Detroit, Michigan, 26 February-2 March 1990. Likes, E W., Impact of CFC regulations on commercial refrigeration equipment manufacturers. Int. J. Refrigeration, I1 (1988) 222-3. Spauschus, H. O., HFC134a as a substitute refrigerant for CFCI2. LLF.-LLR.-Commissions B1, B2, El, E2, Purdue, USA, 1988, pp. 397 400. Hanhoff-Stemping, I., Newmeier, G., Stix, E. & Garber, W. D., The ecological effects of ozone--relevant halogenated hydrocarbons and their substitutes. In Re,~ponsibility Means Doing Without--How to Rescue the Ozone Layer. Federal Environment Agency, Berlin, 1989. Kuijpers, L. J. M. & Miner, S. M., The CFC-issue and the CFC-forum at the 1988 Purdue IIR meeting. Proc. LLF.-LLR.--Commissions BI, B2, El. E2, Purdue, USA, 1988, pp. 291-303. Kruse, H. & Hesse, U., Possible substitutes for fully halogenated chlorofluorocarbons using fluids already marketed. Int. J. Refrigeration, 11 (1988) 276-83. Preisegger, D. E., HFC134a and HFC227--future fluids for heat pumps? Proc. Workshop on 'High-Temperature Heat-Pumps, Hanover, pp. 59-66. Hogberg, M., Vamling, L. & Berntsson, T., Possible working fluids in hightemperature heat-pumps. Proc. Workshop on 'High-Temperature Heat-Pumps', Hanover, pp. 67-79. Devotta, S. & Gopichand, S., Comparative assessment of HFCI34a and some refrigerants as alternatives to CFCI2. Int. J, Refrigeration (accepted). Holland, F. A., Watson, F. A. & Devotta, S., Thermodynamic Design Datalor Heat-Pump Systems. Pergamon Press, Oxford, 1982. Domanski, P. A. & McLinden, M. O., A simplified cycle simulation model for the performance rating of refrigerants and refrigerant mixtures. Proc. 1990 ASHRAE-Purdue CFC Cotfference, pp. 466-75. McLinden, M.O., Gallagher, J.S., Weber, L.A., Morrison, G., Ward, D., Goodwin, A. R. H., Moldover, M. R., Schmidt, J. W., Chae, H. B., Bruno, T. J., Ely, J. F. & Huber, M. L., Measurement and formulation of the thermodynamic properties of refrigerants 134a(l,l,l,2-tetrafluoroethane) and 123(l,l-dichloro2,2,2-trifluoroethane). A S H R A E Trans., 92(2)(1989) 263-83. Allied Signal Inc., Genie Program--Saturation Temp--Press Table & Thermodynamic Properties Table. Genetron Products, New York, 1989. ASHRAE Handbook 1988 Fundamentals. American Society for Heating, Refrigeration and Air-Conditioning Engineers, New York, 1988. Tan, L. C. & Ge, Y. T., Thermodynamic performance analysis and test of refrigerator using HFCl52a as refrigerant. Proc. 1990 ASHRAE-Purdue Conference, pp. 442-58.
Theoretical Assessments of HFC134a and Alternatives to CFC12 as Working Fluids for Heat Pumps S. Devotta & S. Gopichand Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India
ABSTRACT HFC134a has been identified as an alternative to CFC12 for refrigeration applications. Significant progress has been made in the use of HFC134a for refrigeration. This paper analyses the suitability of HFC134a along with those of HCFC22, HCFC124, HFC134, HCFC142b and HFC152a as alternatives to CFC12 for heat-pump applications. Some basic and derived thermodynamic data have been used for a comparative assessment. HFC134a appears to be the closest alternative to CFC12, but HFC152a is likely to be a better alternative from an energy point of view.
NOMENCLATURE H P (PR) Qco s (SCD) T
Rankine coefficient of performance (dimensionless) Enthalpy per unit mass (kJ kg -1) Pressure (bar) Pressure ratio (dimensionless) Specific heating load (kJ kg -~) Entropy per unit mass (kJ kg- ~K- ~) Specific compressor displacement (m 3 MJ -~) Temperature (°C or K)
PEV
Vapour density at TEv (kg m-3)
(COP)RH
Subscripts C
CO EV
Critical Condensing/condenser Evaporating/evaporator 285
Applied Energy 0306-2619/92/$05.00 ~ 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain
286
S. Devotta, S. Gopichand
INTRODUCTION Many chlorofluorocarbons (CFCs) and hydrofluorocarbons (HCFCs) are being used extensively as refrigerants in air-conditioning and refrigeration (AC& R) and as working fluids in heat pumps. They possess most of the desirable characteristics for mechanical vapour-compression systems, such as thermal and chemical stability, thermodynamic suitability, non-toxicity, non-flammability and low cost, but unfortunately they have a damaging effect on the stratospheric ozone layer. Therefore the continued use of CFCs will not be permitted. The ozone-layer depletion has become an international issue. An international agreement--the 'Montreal P r o t o c o l ' on the curtailed production and phasing out of CFCs on a global scale has been signed by many countries. Obviously alternatives to CFCs have to be identified and developed. This has attracted the attentions of scientists from many disciplines. CFCs have been used for over five decades in refrigeration systems and for over two decades in heat-pump systems, as they possess some of the best characteristics. Now scientists and technologists are faced with a dilemma to identify a compound which, as long as it is within the system, performs satisfactorily, perhaps even better than the currently used fluids, but if it leaks out should be harmless to human health and benign to the environment. Therefore any alternative has to have a low or preferably zero ozone-depletion potential (ODP). Ideally one should screen for alternatives, not only with zero ODPs but with relatively low values of the global warming potential (GWP), as the 'greenhouse effect' is also becoming a major international issue. These constraints limit the number of alternatives. It is now recognised that in order to satisfy the environmental regulations one has to compromise on other factors such as flammability, energy efficiency and cost.
HEAT PUMPS A N D CFCs For refrigeration there are not many viable and simple competitive processes for mechanical vapour-compression systems, whereas for heating there are many alternative methods. Therefore the economic acceptabilities of heat pumps have often been compared unfavourably with those of conventional processes. The fact that the recycling of energy also leads to a reduction in CO2 emission and consequently less 'greenhouse effect' is likely to give an additional advantage to heat pumps. Some weightage to CO 2 emission reduction along with energy savings should be collectively compared with the conventional process.
Assessments o f alternatives to CFC12 Jbr heat pumps
287
The heat-pump industry derived a great deal from the already existing advances in the AC& R industry. The system developments were based on the availability of components for refrigeration applications. This was particularly so for compressors. Most of the limitations of the readily available compressors were accepted for heat-pump developments. Some metallurgical modifications were made to suit the higher temperature of operation. To begin with, working fluids HCFC22 and CFC12 were used for low-temperature heating. Searches were made to screen the working fluids for a given range of high temperatures. With further developments of the fluids, high-temperature heat pumps were developed using CFC114.
C U R R E N T STATUS Heat pumps are generally used for space heating in residential and commercial sectors (their sizes ranging from a few kW to several MW) and for process-heat recovery and recycling in the industrial sector (from about 100 kW to several MW) using various types of source and delivering heat to various types of sink depending upon the type of application. According to Kuijpers ~, the present worldwide installed capacity of heat pumps is approximately 7500 MW, about 6000 MW for space heating and 1500 MW in the industrial sector. Most of the residential heat pumps use hermetic reciprocating compressors, while the bigger sizes use both semi-hermetic and open-type reciprocating, scroll and screw type compressors. Turbocompressors are also used for larger sizes. Most of the heat pumps in the international market have been developed using HCFC22, CFC12 and CFCI14 having upper limits for their condensing temperatures of about 60, 70 and 120°C, respectively. CFC 114 is used exclusively in industrial applications where high temperatures are essential. The temperature limits were fixed with respect to many factors arising out of the operating characteristics of the available compressors and the thermodynamic characteristics of the working fluid under consideration. Heat-pump designs using these fluids are well established. The continued use of CFC12 and C F C l l 4 will not be permitted as their ozone-depleting potentials are relatively high, namely 1-0 and 1.0, respectively. HCFC22, with an ODP of 0.05, is not currently covered under the Montreal Protocol. However, it has been agreed that HCFCs, including HCFC22, will be phased out by the year 2020, at the latest by 2040. The working-fluid charge in existing heat pumps is usually between 0.5 and 1-5 kg k W - 1, depending upon the type of heat exchangers and compressors used and the application. The quantity is very much lower, as low as 0.1-0"2 kg kW-1, for units with small hermetic compressors. It is
288
S. Devotta, S. Gopichand
estimated 1 that the annual global consumption of CFCs is about 800 tonnes, mainly for new units and as recharges in existing units. For an anticipated increase of 10%o for the heat-pump market, this implies an increase of 45% in CFC consumption by 1994 compared with 1986, while in 1998 a reduction of about 45% is expected, with reference to the same year. Therefore it is imperative that ozone-layer compatible alternatives to CFC 12 are identified and the systems using these fluids designed and developed. This is essential if the potential for heat pumps, still at the emerging stage, has to be safeguarded. This will be the major factor determining the future of mechanical vapour-compression heat pumps.
ALTERNATIVES TO CFC12 In spite of concerted efforts to broaden the families of compounds for CFC substitutes, the most likely working-fluid substitutes are emerging only from CFC-related families, either from HCFCs or from HFCs (with no chlorine but with some hydrogen atoms). After an extensive molecular inventory of more than 800 compounds, McLinden and Didion 2 concluded that HFC134a, which has no chlorine in it and hence has a zero ODP, is the most worthy potential substitute candidate for CFC12. The substitution of CFC12 by any alternative would involve substantial changes in various components, such as the heat exchangers, motor, insulation and lubricants. Tests have to be carried out to optimise the system performance and to ensure the reliability and safety of the system. Performance data of refrigeration systems with these three possible alternatives must be generated first, so that eventually it would be possible to substitute CFC12 by more ozone-friendly compounds. There are also some possibilities of using non-azeotropic mixtures (HCFC22/HCFC142b and HCFC22/HFC152a) of compounds with lower ODPs. These mixtures can be screened to match the properties of CFCs being replaced as closely as possible. For mixtures, the thermodynamicproperty package development and property data collection will have to be carried out independently. The property package should be capable of predicting the required properties of mixtures of various combinations. Currently available predictions are somewhat limited by the fact that a few P - V - T measurements of the mixture under consideration should be available to predict accurately the thermodynamic data. One major refrigerant manufacturer has already identified a ternary mixture, HCFC22/HFC152a/HCFC124, as a potential substitute for CFC12 for automobile air-conditioning and refrigeration. Some experimental evaluation of this ternary mixture for refrigeration has been
Assessments o f alternatives to CFCI2 for heat pumps
289
undertaken by Kuijpers e t al. 3 The performance of this blend for automotive air-conditioning was reported to be slightly worse than CFC12. Suitable compatible materials either are available or are being developed. 4 Thus further discussion is limited to pure fluids only. HFC134a is considered to possess the best potential as a long-term substitute for CFC12 in refrigeration processes. Preliminary information on HFC134a indicates that it could be available by 1992, but substantial toxicity and other testing must be completed by producers. From molecular studies and some short-term testing, HFC134a is expected to be non-toxic. The refrigeration performance and its optimisation under actual operating conditions with respect to CFC12 are being studied. The entire process is expected to take 10 years or more. s Initially, HFC134a is expected to be significantly costlier than CFC12 and the other substitutes considered. Oil-refrigerant characteristics of HFC134a are entirely different from those ofCFC12. Spauschus 6 has dealt with this topic elaborately. The higher polarity of HFC134a results in its low solubility in non-polar lubricants. such as the mineral oils and synthetic hydrocarbons currently used as refrigerant compressor lubricants. Mineral oils are immiscible, even at temperatures as high as 65°C, with HFC134a. Similar behaviour has been reported with alkyl benzenes. Certain of the most popular candidate synthetic lubricants are reported to exhibit unconventional solubility behaviour, including decreasing solubility with increasing temperature. This behaviour can lead to serious difficulties in heat-pump systems operating over wide ranges of temperature and pressure. The stability of a working fluid is very important for the satisfactory operation of any heat-pump system. The working fluid and lubricant combination in the presence of impurities found in the system should be chemically compatible at the operating conditions. This depends very much upon the type of oil used. Generally the products of decomposition are noncondensable and affect the performance and life of the system. There is an abundant literature on the thermal and chemical stabilities of conventional working fluids in the presence of oils and metals. However, similar information on the potential alternative fluids remains closely guarded. For CFC12, oils could be obtained independently from lubricant dealers. For CFC substitutes, the oils are also tested by the manufacturers of alternative fluids and specified. The working fluid should be pure, otherwise the impurities present in it (such as moisture, residue, dissolved gases) interfere with the machine operation. According to Spauschus 6 the chemical stability of HFC134a is not likely to be a limiting condition in refrigeration applications. However, one is yet to initiate such an assessment for heat-pump applications using HFC 134a. The high stability of HFC134a results in a relatively low response by
290
S. De~otta, S. Gopichand
electronic halogen detectors. This must be resolved in order to avoid the problems for manufacturers and service engineers. Using the normal boiling-point as a guide for screening alternatives to CFC12, one could also include HFC134 a n d HCFC124. HCFC124 is currently produced on a laboratory scale only. The suitability of HCFC124 for refrigeration is being studied as part of the international effort. 7 The Electricity Research and Development Centre, Capenhurst, England, has noted that HFC134 is superior even to HFC134a and it has not been given adequate attention so far. 8 The developmental activities on HFC 134 are not as well known as for other fluids. Other possible substitute fluids which are already commercially available from the CFC family, but with some compromises, are HCFC142b and HFC152a. 9 While HCFCI42b has an ODP of 0.05, i.e. the same as that of HCFC22, HFC152a with no chlorine has an ODP of zero. However, with more H atoms in the molecule, its flammability is higher. Therefore use of HFC152a and HCFC142b, especially in heat pumps, entails much better safety standards. It is worth noting that HFC152a is widely used as one of the components in an azeotropic mixture (R500) with CFC12 for lowtemperature refrigeration applications. Anticipating the potential applications of some of these compounds in refrigeration, their toxicities are already being studied under international consortia efforts, e.g. HCFC22, HCFC123, HCFC124, HCFC141b, HCFC142b, HFC134a, HFC152a and HFC125 under Alternative Fluorocarbon Environmental Acceptability Studies (AFEAS) and HCFC123, HCFC124, HCFC141b, HFC134a and HFC125 under the Program for Alternative Fluorocarbon Toxicity (PAFT). The American Household Appliances Manufacturers (AHAM) have identified the following single component fluids: HFC134a, HFC152a and HFC134; and the following non-azeotropic mixtures: HFC134a/HFC152a, HCFC22/HCFC124, HFC22/HCFC142b, CFCl14/HCFC22/HFC152a and HCFC124/ HCFC22/HFC152a, as the potential alternatives to CFC12 in domestic refrigeration, l Preisegger l° suggested HCFC227 (CF3-CHF-CF3) as an alternative refrigerant to CFC 12. HCFC227 has not been considered in this analysis as the pertinent thermodynamic data are not readily available. Hogberg e t al. H have considered many alternatives, including the hydrocarbons isobutane, n-butane, dimethyl propane, 2-methyl butane and n-pentane along with HFC143, HCFC142b and HFC152a. Most of the hydrocarbons have not been considered because of their flammabilities and other constraints. The aim of this study is to compare the performances of HFC134a, HCFC22, CFC12, HFC152a, HCFC124 and HCFC142b for condensing
Molecular formula Mol. wt (kg kmo1-1) Tc ( C ) Pc (MPa) N.B.P. ( C ) PEr (MPa) P¢o (M Pa) Qco (kJ kg -1) (SCD) (m 3 M J - 1) (PR) (COP)R H ODP G W P relative to C F C I 2 (NRSC Group/underwriters class)
CHCIF 2 86"5 96.00 4.97 - 40-76 0"91 2.993 146"8 0-176 5 3.32 5-12 0-05 0.288 I/5a
HCFC22 CHzF-CF 3 102'0 101"7 3.78 - 26.2 0"572 2.115 131.7 0.273 3-7 4-94 0.0 0.114 7
HFC134a CC12F2 120-9 112-0 4-113 - 29.79 0"567 1-878 110"3 0.281 1 3.32 5.2 1.0 2.941 1/6
CFC12 CH3-CHF2 66"0 113.5 4.49 - 24.7 0"45 1'726 237-6 0-267 7 3.69 5.44 0"0 0-022 7 --
HFCI52a
--
CHFz-CHF2 102'03 115.6 3.77 - 19.7 0"450 7 1-727 114-24 0-329 3.83 5.285 0.0
HFCI34
CHCIF-CHF2 136"48 122.23 3"615 - 11.9 0"327 7 1.262 154.01 0.426 3 3'85 5.227 0-02 0-294 --
HCFC124
TABLE 1 Comparative Data for HFC134a and Some Working Fluids for Tco = 7 0 C and TEv = 20°C
CH3-CCIF2 100.49 137"0 4.123 - 9-78 0-289 1.124 170.1 0.446 3"88 5.81 0.06 0'323 1/5
HCFC142b
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292
S. Devotta, S. Gopichand
temperatures up to 70°C. A similar analysis for refrigeration has been presented by Devotta and Gopichand} 2 Some basic properties of the working fluids covered in this analysis are indicated in Table 1. DERIVED T H E R M O D Y N A M I C DESIGN DATA The operation of a refrigeration system approximates to that of the Rankine cycle. The ideal Rankine cycle is illustrated in Fig. 1, which is a plot of pressure (P) against enthalpy per unit mass (H) for HFC134a. The cycle consists of an isobaric evaporation (1 -~ 2), an isentropic compression (2 -~ 3), an isobaric condensation (3-*4 ~ 5) and an isenthalpic expansion (5-~ 1). There are four critical parameters which determine the choice of a working fluid ~a for a given set of conditions. They are ---evaporating temperature, TEv -----condensing temperature, Tco --pressure ratio, (PR) --Rankine coefficient of performance, (COP)Rn If any two of these parameters are fixed, for a given working fluid, the values of the other two parameters are automatically fixed. In reality there are other factors to be considered, such as capacity of the available compressors, heat transfer and pressure drop characteristics of heat exchangers and interconnecting piping, thermal and chemical stabilities of the fluid, lubricant and materials of construction combination, miscibility characteristics of the working fluid, and lubricant, toxicity and safety aspects of the 10"0
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Assessments o f alternatives to CFCI2 for heat pumps
293
fluid for the application under study. If the chosen fluid is not capable of meeting the required conditions then another has to be selected. For a hermetic compressor, the choice is further limited by the fixed combination of volumetric throughput and motor power. Therefore it is possible that a compressor will be operating under an inefficient condition while meeting the required heating load. This is extremely important when a hermetic compressor, designed for a particular working fluid, is operated with another working fluid. This has been one of the limitations of hightemperature heat pumps using C F C l l 4 . Depending upon the type of cycle followed, the relative ratings of various fluids may vary. 14 However, the simplest cycle is the ideal Rankine cycle. The design data can be derived readily from basic thermodynamic data as follows: Pressure ratio (PR) -
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(1)
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+ Tco(S 3 -- 54)
(5)
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(6)
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294
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D E R I V E D T H E R M O D Y N A M I C DATA FOR HFC134a A composite plot between Tco, (Tco-TEv ) and (PR) for HFC134a is presented in Fig. 2. All the basic thermodynamic properties of HFC134a have been taken from McLinden e t a l l s This plot can be used conveniently to check the suitability of HFC134a for a particular application• It can be seen that if the (PR) of an available compressor is limited to 2-5, for a condensing temperature Tco of 70°C, the maximum possible evaporating temperature is about 33°C and the theoretical Rankine coefficient of performance (COP)R H will be about 7"35. Similarly, if the compressor is capable of achieving a (PR) of 6.0, for a Tco of 70°C, the maximum temperature lift will be about 65°C with a (COP)R H of about 3"6. COMPARATIVE ASSESSMENT OF HFC134a From the set of equations presented above, the same design parameters were calculated lbr other fluids• The thermodynamic data for HCFC124 and
Assessments of alternatives to CFCI2 for heat pumps
295
8.°i I 7"O-
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R134 were generated by using a computer program from the Allied Signal Corporation IncJ 6 The thermodynamic data for other fluids were taken from the A S H R A E H a n d b o o k 1988 Fundamentals. ~7 Comparative performance data are presented in Table 1. It can be seen from the table that, for a given condition of Tco = 70°C, the condenser pressure will be highest with HCFC22 (= 2.993 MPa), which is well above the usual design upper limit for a compressor. While the behaviour of H F C I 3 4 a follows very closely that of CFC12, though on the higher side, HFC152a is much closer on the lower side. The variation of pressure ratio (PR) with ( T c o - TEv) for a Tco of 70°C is shown in Fig. 3. The pressure ratios for HFC134a and HFC152a are almost the same, and (PR) values for HCFC124, HCFC142b and HFC134 are much higher than for HFC134a. Typically, for Tco=70°C and TEv=30°C, H C F C 142b requires the highest (PR) of 2.885 while H FC 134a and H F C 152a require an intermediate value of 2.74. This means the volumetric efficiency of the compressor operating with HCFC142b (or with HCFC124) will be the
S. Devotta, S. Gopichand
296
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least, while with HCFC22 it is likely to be the same as that with CFC12. Higher pressure-ratios lead to various unwanted conditions like compressor cylinder overheating due to high discharge temperatures and valve leakage. With the available components for the usual pressure-ratio range only HFC152a and HFC134a appear to be alternatives to CFC12. The variation of the Rankine coefficient of performance for heating (COP)RH with ( T c o - TEV) is shown in Fig. 4. Of all the fluids considered, HCFC142b offers the best (COP)RH value. For the condition of Tco = 70°C and TEV= 30°C, HCFC142b has the highest value of 7.5 while HFC134a has the least value of 6"49. Obviously using HFC134a means some energy penalty with respect to CFC12. However, the use of HCFC142b entails more stringent safety design to mitigate the flammability of HCFC142b. The specific compressor displacements for HFC134a and HFC152a, ~as shown in Fig. 5, are closer to CFC12 while HCFC124 and FICFC142b require bigger compressors for the same heat duty. Though from an energy
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point of view HCFC142b appears to be extremely attractive, it has the disadvantage of requiring a bigger compressor. With respect to the use of standard CFC 12 compressors, HFC134a appears to be the ideal choice. If a redesign is considered, HFC152a is an even better choice because of its highe r (COP)~H, zero ODP and relatively low GWP. However, safety aspects have to be mitigated for HFC152a. It is generally believed that this is possible I with the current level of compressor technology. Figure 6 is a plot of condenser load Qco against (Tco-TEv) for a condensing temperature of 70°C. For the identical conditions the condenser load for HFC152a is the highest. The values for CFC12 and HCFC124 are almost the same within the range of this study. For Tco= 70°C and T~v = 20°C, HFC 152a has the highest value of 237.6 kJ kg-1 while CFC 12 has the lowest value of 110.3 kJ kg-~; HFC134a has an intermediate value of 131.7 kJ kg- 1.
CONCLUSIONS This comparative assessment suggests the choice of HFC 134a as the logical alternative to CFC12 as the working fluid for heat pumps. Nevertheless, there is significant scope for the use of HFC152a for heat pumps from an energy efficiency point of view. HFC152a is likely to be as stable a fluid as HFC134a, but with more H atoms will pose a miscibility problem similar to that with HFC134a. Also the flammability aspect has to be considered. It is already commercially available and it is cheaper than HFC134a. The compressor discharge temperature is also likely to be relatively high. This will require some metallurgical considerations for the discharge ports and better insulating material for the motor windings, if used for hermetic compressors. China is and US-EPA, Washington DC, are working on H FC 152a as an alternative to CFC12 in refrigeration applications. There is an immediate need for similar research and development work for heatpump applications.
REFERENCES 1. Kuijpers, L. J. M., Technology progress on protecting the ozone layer: refrigeration, air-conditioning and heat pumps--technical options report. UNEP, Nairobi, 1989. 2. McLinden, M. O. & Didion, D. A., The search for alternative refrigerants--a molecular approach. Proc. LLF.-LLR.--Commissions B1, B2, El, E2, Purdue, USA, 1988, pp. 91-9. 3. Kuijpers, U J.M., De Wit, J.A., Bencschop, A.A.J. & Janssen, M.J.P.,
Assessments of alternatives to CFC12.Ibr heat pumps
4.
5. 6. 7.
8. 9. 10. 11. 12. 13. 14. 15.
16. 17. 18.
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