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Natural working fluids for solar-boosted heat pumps
International Journal of Refrigeration 26 (2003) 637–643 www.elsevier.com/locate/ijrefrig
Natural working fluids for solar-boosted heat pumps C. Chaichanaa, Lu Ayea,*, W.W.S. Chartersb a
International Technologies Centre (IDTC), Department of Civil and Environmental Engineering, The University of Melbourne, Victoria 3010, Australia b Department of Mechanical and Manufacturing Engineering, The University of Melbourne, Victoria 3010, Australia Received 18 March 2001; received in revised form 8 January 2003; accepted 17 March 2003
Abstract The option of using natural working fluids as a substitute of R-22 for solar-boosted heat pumps depends not only upon thermal performance and hazardous rating but also on potential impacts on the environment. This paper presents the comparative assessment of natural working fluids with R-22 in terms of their characteristics and thermophysical properties, and thermal performance. Some justification is given for using natural working fluids in a solar boosted heat pump water heater. The results show that R-744 is not suitable for solar-boosted heat pumps because of its low critical temperature and high operational pressures. On the other hand, R-717 seems to be a more appropriate substitute in terms of operational parameters and overall performance. However, major changes in the heat pumps are required. R-290 and R-1270 are identified as candidates for direct drop-in substitutes for R-22. # 2003 Elsevier Ltd and IIR. All rights reserved. Keywords: Solar energy; Heat pump; Refrigerant; Rice; Replacement; Carbon dioxide; Propane; Propylene
Fluides actifs pour les pompes a` chaleur aide´es par l’e´nergie solaire Mots cle´s : E´nergie solaire ; Pompe a` chaleur ; Frigorige`ne ; Riz ; Substitut ; Dioxyde de carbone ; Propane ; Propyle`ne
1. Introduction In the early days, choices of working fluid for heat pumps were made based on thermal performance and hazardous rating. CFCs were introduced and used worldwide in household and small to medium applications due to their superiority in terms of safety aspects over natural refrigerants. However, since the discovery of the ozone hole and the following introduction of the
* Corresponding author. Tel. +61-3-8344-6879; fax. +61-38344-6868. E-mail address: [email protected] (Lu Aye).
Kyoto Protocol, environmental impacts have become one of the most important aspects. Therefore, the use of CFCs has largely been phased out due to their ozone depletion and global warming effects. Consequently, a search for suitable substitutes is essential. Alternatives for CFCs can be categorised into two groups: short term replacement and long term replacement working fluids. The short-term working fluids are HCFCs and HFCs such as R-22, R-124, and R-134a. These working fluids have very low ozone depletion potential (ODP) but still have high global warming potential (GWP). The longterm working fluids are halogen-free natural working fluids. The working fluids in this category are environmentally benign due to their very low or near zero ODP and GWP.
0140-7007/03/$35.00 # 2003 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/S0140-7007(03)00046-X
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C. Chaichana et al. / International Journal of Refrigeration 26 (2003) 637–643
Attempts have been made in order to prove that natural refrigerants can be used as simple substitutes for R22. Charters et al. [1] compared theoretical performance predictions of three refrigerants, which includes R-12, R-22, and propane, for low and medium ranges of evaporating temperature application. It was found that at low evaporating temperatures ( 20 to 10 C) the three refrigerants studied give similar COPs. At medium evaporating temperatures (0–10 C), the COP of the R-12 was found to be the highest. A further experimental study by Urma [2] showed that it is possible to use propane as a drop-in replacement for R-12 in a small domestic refrigerator. Comparison of R-717 with other refrigerants, R-22, R-123, and R-134a, by Stoecker [3] concluded that R-717 is a viable candidate as a refrigerant for district cooling plant water chillers. A number of comparison works between R-22 and its potential substitutes were also reported in the Gustav Lorentzen Conference [4] and other open literature surveyed [5–9]. A range of worldwide activities on compression systems with natural working fluids can be found in reference [10]. However, none of these reported on effects of solarboosting on natural working fluid heat pumps, especially in water heating applications which have unique working conditions to other applications. Thus, the main objective of this paper is to investigate the possibility of applying natural working fluids to solar-boosted heat pump water heating systems. Natural working fluids that have potential for use as a primary working fluid in solar-boosted heat pump water heater can be grouped as hydrocarbon and non-hydrocarbon base substances. The hydrocarbon group is comprised of propane (R-290), butane (R-600), isobutane (R-600a), and propylene (R-1270). The nonhydrocarbon group is comprised of ammonia (R-717) and carbon dioxide (R-744). Air (R-729) and water (R718) are excluded here because they are not suitable for this application. In this study, comparative assessments of the natural working fluids for a solar-boosted heat pump have been carried out. Physical properties and vapour compression cycle thermal performance of each working fluid are compared with those of R-22. In addition, the focus of this paper includes the possibility of implementing the chosen natural working fluid in solar-boosted heat pumps for domestic water heating applications.
2. Characteristics and thermophysical properties Physical properties of refrigerants are the important factors that define both type and range of applications. For instance, critical pressure and temperature indicate just how far the phase change of the refrigerant can occur. In Table 1, characteristics and thermophysical
properties of natural working fluids are presented for comparison. R-744 was reported as a promising refrigerant in heating and cooling applications by taking advantage of heat rejection from temperature glide in the supercritical region [4]. Using the R-744 heat pumps in cold climate areas, their evaporating coils were exposed to ambient air or low temperature sources at about 10 C or below. Therefore there were no reports on evaporating temperature (Te ) at near-critical point. In contrast, a comprehensive study on solar-boosted heat pumps by Dixon [11] showed that on a good sunny day the evaporating temperature of the solar-boosted heat pump studied rose up to 30 C, which is very close to the critical temperature of R-744. Generally, the latent heat of evaporation of a working fluid drops significantly at temperature approaching its critical temperature. In such case, if R-744 is used in the solar-boosted heat pump it is clear that greater working fluid mass flow rate is required in order to maintain a required water heating capacity. For instance, if Te changes from 0 to 25 C, mass flow rate in R-22 heat pump increases only about 10% while it is about 100% in R-744 case. A refrigerant reservoir may be used to provide the extra charge needed. In addition, consequently larger compressor displacement volume and larger tube size are required to satisfy the increased refrigerant mass flow rate. As mentioned earlier, the unique characteristic of solar-boosted heat pumps is their wide range of working conditions. In summer, Te can be higher than 25 C due to high solar radiation while it can be lower than 0 C in winter. On the other hand, the condensing temperature (Tc ) can be in the range of 30–70 C, depending on the water temperature in the storage tank. At each operating condition, the expansion valve used must be able to feed the correct amount of working fluid to the evaporator. In commercial heat pumps, a thermostatic expansion valve (TXV) is generally used. Selection of the correct size TXV is critical. In general, the valve is selected to meet the annual average weather conditions of a specific geographical location. The valve should also be able to cope with the off-design conditions. If severe fluctuation of mass flow rate occurs, it may not be possible to select the thermostatic expansion valve correctly. An electronically controlled valve may be used. However, this would increase the initial cost of the system. There are other issues in cases where there are dramatic changes in mass flow rate. Generally, a heat pump must be designed to accommodate the designed maximum mass flow rate that is required during actual operation. The suction and discharge lines are sized to achieve refrigerant velocities higher than the minimum recommended values to maintain good oil return. However, at much lower mass flow rate, refrigerant velocities in every part of the system decrease considerably. In this
C. Chaichana et al. / International Journal of Refrigeration 26 (2003) 637–643
situation, lubricant oil may not be able to fully return back to the compressor. This oil may deposit on heat exchangers’ tube wall and degrade the heat transfer between the working fluid and the heat source/sink. Poor oil return may lead to overheating and wear of the compressor. On the other hand, very high refrigerant velocity inside tubes may then cause high pressure losses across the equipment. It is estimated that, for the same tube diameter, increase in the R-744 mass flow rate by 50% can increase the pressure losses in the evaporator by 300% (Jung and Radermacher [17] correlation was used in this estimation). Another disadvantage of using R-744 for heat pump water heaters is its high condensing pressure. If the requirement for hot water temperature in a household is 60 C a very high compressor outlet pressure is required in order to meet this specified temperature heat sink. The pressure can be as high as 13 MPa, while it is only 3 MPa for R-22. This level of pressure is inappropriate for household applications since there is too high a risk of potential explosion. Although Table 1 shows that R-717 is toxic and flammable it is the least harmful working fluid to the ozone layer. It only stays in the air for less than 2 weeks with no effects to the ozone layer while R-22 can stay in the atmosphere for up to 12 years. In addition, the latent heat of evaporation of R-717 is 8 times higher than that of R-22, which indicates that a smaller circulating mass flow rate per unit of heat output is required, which also reduces the risks when leakage occurs. From this point, the hydrocarbons and R-717 are more likely to be the suitable choice for substitution in solar-boosted heat pumps. R-744 will hereafter be excluded from comparison due to its mentioned unsuitability.
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3. Thermal performance Single stage vapour compression cycle, which represents regular heat pump working cycle, was chosen in this study. Degrees of superheating and subcooling are 11 and 8 C respectively. The isentropic efficiency of compression process was assumed at 0.8. Working fluid mass flow rate per unit of heat output, compressor discharge temperature, condensing pressure, two-phase pressure losses, compressor displacement volume, and heating coefficient of performance (COP) were then computed given a certain geometry based on a commercially available R-22 solar-boosted heat pump water heater. Properties of each working fluid were estimated using Ref. [14]. For domestic water heating application, the four extreme working conditions, which usually occur at the beginning and the end of heating periods on cloudy and sunny days, are assumed. The condensing temperatures are from 30 to 70 C given initial water temperature of 20 C and final temperature of 60 C. At zero solar radiation, the evaporating temperature is estimated at 0 C while it is 30 C at maximum solar radiation. It should be noted that these estimations were made based on experimental data from tests carried out by Dixon [11] and preliminary tests by the authors. Results of the analysis are presented in Figs. 1–6 in the form of percent deviation from R-22 results. In these figures, the results of these conditions were calculated. The following arguments can be made based on the results shown: 1. Mass flow rate per kW-heating for the R-717 heat pump (Fig. 1) is the lowest compared to the other four in every case. Lower mass flow rate can result in lower pressure losses through equipment.
Table 1 Characteristics and properties of some refrigerants Working fluid
R-22
R-290
R-600
R-600a
R-1270
R-717
R-744
Chemical formula Molar mass (g mol 1) Critical temperature ( C) Critical pressure (kPa) Critical density (kg m 3) Boiling pointa ( C) Latent heat of evaporationb (kJ kg 1) Flammability Toxicity ODP GWPc
CHClF2 86.46 96.14 4 990 562.0 40.9 187.6 N N 0.05 1 700
C3H8 44.10 96.7 4 247 220.0 42.1 343.9 Y N 0 3
C4H10 58.12 152.0 3 796 227.8 0.5 366.4 Y N 0 3
C4H10 58.12 134.7 3 640 224.4 11.6 335.2 Y N 0 3
C3H6 42.08 92.4 4 665 222.6 47.7 344.2 Y N 0 3
NH3 17.03 131.9 11 333 225.0 33.3 1 186.2 Y Y 0 0
CO2 44.01 31.1 7 384 466.5 78.4 153.7 N N 0 1
a
Boiling point at atmospheric pressure. At 20 C. c Reference to CO2 with base values of 1. Sources: Bitzer International [12], Charters & Lu Aye [13], NIST [14] and ASHRAE [15]. b
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Besides, a smaller amount of working fluid will greatly reduce risks due to leakage during operation and maintenances. However, the sizes of piping remain almost the same as that of the R-22 heat pump due to low density of R-717. The major difference of the R-717 heat pump is that changes in Tc do not affect the mass flow rate required. Thus, the pressure loss due to the momentum term is almost constant. 2. Discharge temperature of the R-717 heat pump in Fig. 2 is the highest among the working fluids compared. It can be raised as high as 200 C due to high entropy at Te =0 C and Tc =70 C. This high temperature increases the temperature difference between water and refrigerant and improves the heat transfer rate at the desuperheating region. Nonetheless, the high discharge temperature can cause a large amount of lubricant oil to be carried away from the compressor with the working fluid since the solubility is increased at high temperature. Another concern is that chemical stability of the lubricant oil can be destroyed, resulting in poor lubricating and consequent wear at the compressor. 3. Both R-600 and R-600a have much lower condensing pressures (50–70%) compared to that of R-22 (Fig. 3). This demonstrates that the
maximum pressure that could occur in the heat pump is also lower. In such case, maximum tensile strength of the materials used in the heat pump can be reduced. In an example of tubes, a smaller wall thickness will slightly upgrade heat transfer and decrease capital costs of material used. 4. Two-phase pressure losses across the evaporator (denoted by Te ) and condenser (denoted by Tc ) for each refrigerant (Fig. 4) were estimated based on Cavallini et al. [16] and Jung and Radermacher [17] correlations. It was assumed that a smooth, 10-m horizontal tube with inside diameter of 10 mm was used in the calculation. An arbitrary mass flow rate was also chosen. It was found that
Fig. 3. Comparison of condensing pressure at certain working conditions.
Fig. 1. Comparison of mass flow rate per kW-heating at certain working conditions.
Fig. 2. Comparison of compressor discharge temperature at certain working conditions.
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the computed pressure losses for every natural working fluid are higher than those of R-22. In average, the computed pressure losses of R-600, R-600a and R-717 are among the highest of the fluids surveyed. 5. R-290, R-1270 and R-717 require similar compressor displacement volume as R-22 does (Fig. 5), given the same heating output. This implies that existing compressors for R-22 can be used with both working fluids if material compatibility is not an issue. However, most of the compressors available for domestic use contain copper and this can not be used with R-717. Moreover, some
Fig. 4. Comparison of two-phase pressure losses at certain working conditions.
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manufacturers recommended not to use R-717 with hermetic compressors due to two main reasons. Firstly, in a sealed condition, motor winding temperature can exceed (at high discharge temperature) the limit of motor insulation and cause damage to the compressor. Secondly, at a certain level of R-717 vapour can become an electrical conductor and cause short-circuiting to motor windings. The other working fluids need a larger displacement volume compressor to deliver the same amount of system heating capacity. 6. Fig. 6 shows the relative COP of heat pumps. The computed COP of the R-22 heat pump is used as reference, and given a value as one. A system with better COP will have its calculated COP higher than one. It is clear that every working fluid investigated here has comparable COP to the R22 heat pump. From this figure, it can be identified that R-600 and R-600a perform better than the other working fluids in every condition. Further investigation shows that only R-290, R-600 and R-600a have increased COP when the superheating increases. This feature is an advantage to the solar-boosted heat pump since high superheat settings are common in these systems in order to cope with the changes in weather conditions and to protect the compressor.
Fig. 5. Comparison of displacement volume at certain working conditions.
Fig. 6. Comparison of relative COP at certain working conditions.
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Among the natural refrigerants compared, it may be concluded that R-717 is the prime candidate for R-22 over the others in this comparison. Refrigerant charge required in R-717 system is only 20% of that in R-22. Thus, we can substantially minimise the effects to global warming and environmental hazard. Its compressor discharge temperature is high which can improve heat transfer rate in desuperheating zone. Computed displacement volume is comparable to that of R-22. The pressure losses at evaporator and condenser are predicted to be lower. Although the R-600 and R-600a show higher COP levels at Tc =35 C the COP level of R-717 at Tc =70 C is comparable to these two working fluids. Note that in water heating applications with regular hot water usage rate, heat pumps work at high condensing temperatures for most of the time.
4. Opportunities for natural working fluids in solarboosted heat pumps In the foreseen future, the choice of working fluid for heat pumps will be largely driven by environmental factors. However, there is no universal replacement that can cover the wide application ranges and sizes catered for currently by R-22. In regular unitary heat pumps, 90% of the total equivalent warming impact (TEWI) comes from indirect or energy used effect and only 10% from leakage of working fluid [18]. Please note that 4% annual make-up rate and 5% loss of charge upon equipment decommissioning were assumed. Replacing R-22 by R-717 in heat pump water heaters can take advantage of better thermal efficiency and smaller amount of charge to minimize greenhouse gases emissions and risks associated with charge leakage. Although R-717 is shown to be a better replacement in terms of thermal efficiency and its minimal environmental effects the expanding use of R-717 will depend on a number of factors as described by Larminant [19]. Specially designed components such as compressors and heat exchangers of R-717 heat pumps are required to cope with safety, material compatibility, and degradation of lubricant oil. Actual thermal performance of solar-boosted heat pumps depends mainly on the equipment and control systems used. At the moment, equipment such as compressors and heat exchangers specially designed for R-717 are not widely available for domestic applications. Moreover, they are costly. Another difficulty involved is the material compatibility. Generally, steel or stainless steel pipes are recommended for R-717. Using these materials in domestic heat pump water heater can be reluctantly accepted due to inconveniences in cutting, bending and connecting. Thus, the heat pump should be designed as a maintenance-free unit to avoid this problem.
Other choices of substitute are R-290 and R-1290. They can be used as directly drop-in for most of the existing solar-boosted heat pumps with slightly reduced thermal performance. There are no special material compatibility and lubricant problems. However, some modifications are needed in order to fulfill safety requirements. Researches have indicated that actual thermal performance of a heat pump could be either increased or decreased depending on many other factors such as operating conditions and refrigerant mass flow control system.
5. Conclusions A comparison of selected natural working fluids for solar-boosted heat pumps has been carried out in terms of characteristics, thermal performance, and heat transfer coefficient. Practical assessments of safety, material compatibility and lubrication are also included. The results from this study have shown that CO2 is not an appropriate choice for solar-boosted heat pumps due to its low critical pressure leading to lower heat pump COP at evaporating temperatures approaching the critical temperature. It is, however, found that R-717 has an opportunity to be a prime candidate for a solar-boosted heat pump as it has advantages over the other natural working fluids compared. Apart from being ODP and GWP free, R-717 has demonstrated similar or better system performance to R-22 depending on working conditions. Safety issues including compatibility with existing systems, and lubrication are major weak points for using this working fluid. Drop-in substitutes are also proposed in this study.
Acknowledgements The authors would like to thank the International Technologies Centre (IDTC), the Department of Civil & Environmental Engineering and the Department of Mechanical & Manufacturing Engineering. The financial support of the Australian Research Council (ARC) for this study is acknowledged. Chatchawan Chaichana gratefully acknowledges the scholarship from the Royal Thai Government. References [1] Charters WWS, Megler VR, Urma I, Lu Aye. Propane as a working fluid in domestic heat pumps. In: IIR/IIFMelbourne 1996: Refrigeration, Climate Control and Energy Conservation. 1996 February 11-14; Melbourne, Australia. Melbourne: International Institute of Refrigeration, 1996. [2] Urma I. Propane as a replacement for domestic applications. MEngSc thesis, Department of Mechanical and
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Manufacturing Engineering, The University of Melbourne; 1997. Stoecker WF. Comparison of ammonia with other refrigerants for district cooling plant chillers. ASHRAE Trans 1994;100(2):1126–35. International Institute of Refrigeration (IIR). Proceedings of the Conference of Natural Working Fluids’98 IIR - Gustav Lorentzen Conference, 2–5 June 1998. Oslo, Norway. Bivens DB, Allgood CC, Rizzo JJ, Shiflett MB, Patron DM, Chisolm TEC, et al. HCFC-22 alternative for air conditioners and heat pumps. ASHRAE Trans 1994; 100(2):566–72. Cleland DJ, Adams SD Keedwell RW. Use of hydrocarbons as drop-in replacements for fluorocarbon refrigerants in on-farm milk cooling equipment. 20th International Congress of Refrigeration, International Institute of Refrigeration. 19–24 September 1999. Sydney, Australia. Halozen H. Natural refrigerants- an option for heat pumps? Proceedings of the IIR.IIF Conference: Refrigeration, Climate Control and Energy Conservation, February 11–14 1996, Melbourne, Australia. Hwang Y, Judge JF, Radermacher R. Evaluation of R-22 alternatives for heat pumps. American Society of Mechanical Engineers Advanced Energy Systems Division (Publication) AES 1995;34:11–17. Maclaine-Cross IL, Leonardi E. Why hydrocarbons save energy. AIRAH Journal 1997;51:6 33–37. Stene J. Compression systems with natural working fluidsresults and conclusions from IEA Annex 22 (1995-98). In: Natural working fluids’ 98- Proceedings of the IIR-Gustav
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[18] [19]
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Lorentzen Conference. 2–5 June 1998, Oslo, Norway. Paris: International Institute of Refrigeration, 1998, p. 171–79. Dixon CWS. A practical study of a solar boosted heat pump applied to space heating and cooling. MEngSc thesis, Department of Mechanical and Manufacturing Engineering, The University of Melbourne; 1979. Bitzer International. Refrigerant—Report 9. http:// www.bitzer.de/doc/a/a-501-9.pdf. Charters WWS, Lu Aye. Electrical and engine driven heat pumps for effective utilisation of renewable energy resources. In: World Renewable Energy Congress’ 99, 10– 13 February 1999, Perth, Australia. NIST, REFPROP Version 6.02: Thermodynamic Properties of Refrigerants and Refrigerant Mixtures. ASHRAE Chapter 18 Refrigerants. In: 1997 ASHRAE Fundamentals Handbook (SI). Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. 1997. Cavallini A, Del Col D, Doretti L, Longo GA, Rossetto L. Heat transfer and pressure drop during condensation of refrigerants inside horizontal enhanced tubes. International Journal of Refrigeration 2000;23:1 4–25. Jung DS, Radermacher R. Prediction of pressure drop during horizontal annular flow boiling of pure and mixed refrigerant. Int J Heat Mass Transfer 1989;32:12 2435–46. Kruse H. Refrigerant use in Europe. ASHRAE Journal 2000;9:16–24. Larminat Pde. Expanding the use of ammonia. ASHRAE Journal 2000;3:35–40.
Natural working fluids for solar-boosted heat pumps C. Chaichanaa, Lu Ayea,*, W.W.S. Chartersb a
International Technologies Centre (IDTC), Department of Civil and Environmental Engineering, The University of Melbourne, Victoria 3010, Australia b Department of Mechanical and Manufacturing Engineering, The University of Melbourne, Victoria 3010, Australia Received 18 March 2001; received in revised form 8 January 2003; accepted 17 March 2003
Abstract The option of using natural working fluids as a substitute of R-22 for solar-boosted heat pumps depends not only upon thermal performance and hazardous rating but also on potential impacts on the environment. This paper presents the comparative assessment of natural working fluids with R-22 in terms of their characteristics and thermophysical properties, and thermal performance. Some justification is given for using natural working fluids in a solar boosted heat pump water heater. The results show that R-744 is not suitable for solar-boosted heat pumps because of its low critical temperature and high operational pressures. On the other hand, R-717 seems to be a more appropriate substitute in terms of operational parameters and overall performance. However, major changes in the heat pumps are required. R-290 and R-1270 are identified as candidates for direct drop-in substitutes for R-22. # 2003 Elsevier Ltd and IIR. All rights reserved. Keywords: Solar energy; Heat pump; Refrigerant; Rice; Replacement; Carbon dioxide; Propane; Propylene
Fluides actifs pour les pompes a` chaleur aide´es par l’e´nergie solaire Mots cle´s : E´nergie solaire ; Pompe a` chaleur ; Frigorige`ne ; Riz ; Substitut ; Dioxyde de carbone ; Propane ; Propyle`ne
1. Introduction In the early days, choices of working fluid for heat pumps were made based on thermal performance and hazardous rating. CFCs were introduced and used worldwide in household and small to medium applications due to their superiority in terms of safety aspects over natural refrigerants. However, since the discovery of the ozone hole and the following introduction of the
* Corresponding author. Tel. +61-3-8344-6879; fax. +61-38344-6868. E-mail address: [email protected] (Lu Aye).
Kyoto Protocol, environmental impacts have become one of the most important aspects. Therefore, the use of CFCs has largely been phased out due to their ozone depletion and global warming effects. Consequently, a search for suitable substitutes is essential. Alternatives for CFCs can be categorised into two groups: short term replacement and long term replacement working fluids. The short-term working fluids are HCFCs and HFCs such as R-22, R-124, and R-134a. These working fluids have very low ozone depletion potential (ODP) but still have high global warming potential (GWP). The longterm working fluids are halogen-free natural working fluids. The working fluids in this category are environmentally benign due to their very low or near zero ODP and GWP.
0140-7007/03/$35.00 # 2003 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/S0140-7007(03)00046-X
638
C. Chaichana et al. / International Journal of Refrigeration 26 (2003) 637–643
Attempts have been made in order to prove that natural refrigerants can be used as simple substitutes for R22. Charters et al. [1] compared theoretical performance predictions of three refrigerants, which includes R-12, R-22, and propane, for low and medium ranges of evaporating temperature application. It was found that at low evaporating temperatures ( 20 to 10 C) the three refrigerants studied give similar COPs. At medium evaporating temperatures (0–10 C), the COP of the R-12 was found to be the highest. A further experimental study by Urma [2] showed that it is possible to use propane as a drop-in replacement for R-12 in a small domestic refrigerator. Comparison of R-717 with other refrigerants, R-22, R-123, and R-134a, by Stoecker [3] concluded that R-717 is a viable candidate as a refrigerant for district cooling plant water chillers. A number of comparison works between R-22 and its potential substitutes were also reported in the Gustav Lorentzen Conference [4] and other open literature surveyed [5–9]. A range of worldwide activities on compression systems with natural working fluids can be found in reference [10]. However, none of these reported on effects of solarboosting on natural working fluid heat pumps, especially in water heating applications which have unique working conditions to other applications. Thus, the main objective of this paper is to investigate the possibility of applying natural working fluids to solar-boosted heat pump water heating systems. Natural working fluids that have potential for use as a primary working fluid in solar-boosted heat pump water heater can be grouped as hydrocarbon and non-hydrocarbon base substances. The hydrocarbon group is comprised of propane (R-290), butane (R-600), isobutane (R-600a), and propylene (R-1270). The nonhydrocarbon group is comprised of ammonia (R-717) and carbon dioxide (R-744). Air (R-729) and water (R718) are excluded here because they are not suitable for this application. In this study, comparative assessments of the natural working fluids for a solar-boosted heat pump have been carried out. Physical properties and vapour compression cycle thermal performance of each working fluid are compared with those of R-22. In addition, the focus of this paper includes the possibility of implementing the chosen natural working fluid in solar-boosted heat pumps for domestic water heating applications.
2. Characteristics and thermophysical properties Physical properties of refrigerants are the important factors that define both type and range of applications. For instance, critical pressure and temperature indicate just how far the phase change of the refrigerant can occur. In Table 1, characteristics and thermophysical
properties of natural working fluids are presented for comparison. R-744 was reported as a promising refrigerant in heating and cooling applications by taking advantage of heat rejection from temperature glide in the supercritical region [4]. Using the R-744 heat pumps in cold climate areas, their evaporating coils were exposed to ambient air or low temperature sources at about 10 C or below. Therefore there were no reports on evaporating temperature (Te ) at near-critical point. In contrast, a comprehensive study on solar-boosted heat pumps by Dixon [11] showed that on a good sunny day the evaporating temperature of the solar-boosted heat pump studied rose up to 30 C, which is very close to the critical temperature of R-744. Generally, the latent heat of evaporation of a working fluid drops significantly at temperature approaching its critical temperature. In such case, if R-744 is used in the solar-boosted heat pump it is clear that greater working fluid mass flow rate is required in order to maintain a required water heating capacity. For instance, if Te changes from 0 to 25 C, mass flow rate in R-22 heat pump increases only about 10% while it is about 100% in R-744 case. A refrigerant reservoir may be used to provide the extra charge needed. In addition, consequently larger compressor displacement volume and larger tube size are required to satisfy the increased refrigerant mass flow rate. As mentioned earlier, the unique characteristic of solar-boosted heat pumps is their wide range of working conditions. In summer, Te can be higher than 25 C due to high solar radiation while it can be lower than 0 C in winter. On the other hand, the condensing temperature (Tc ) can be in the range of 30–70 C, depending on the water temperature in the storage tank. At each operating condition, the expansion valve used must be able to feed the correct amount of working fluid to the evaporator. In commercial heat pumps, a thermostatic expansion valve (TXV) is generally used. Selection of the correct size TXV is critical. In general, the valve is selected to meet the annual average weather conditions of a specific geographical location. The valve should also be able to cope with the off-design conditions. If severe fluctuation of mass flow rate occurs, it may not be possible to select the thermostatic expansion valve correctly. An electronically controlled valve may be used. However, this would increase the initial cost of the system. There are other issues in cases where there are dramatic changes in mass flow rate. Generally, a heat pump must be designed to accommodate the designed maximum mass flow rate that is required during actual operation. The suction and discharge lines are sized to achieve refrigerant velocities higher than the minimum recommended values to maintain good oil return. However, at much lower mass flow rate, refrigerant velocities in every part of the system decrease considerably. In this
C. Chaichana et al. / International Journal of Refrigeration 26 (2003) 637–643
situation, lubricant oil may not be able to fully return back to the compressor. This oil may deposit on heat exchangers’ tube wall and degrade the heat transfer between the working fluid and the heat source/sink. Poor oil return may lead to overheating and wear of the compressor. On the other hand, very high refrigerant velocity inside tubes may then cause high pressure losses across the equipment. It is estimated that, for the same tube diameter, increase in the R-744 mass flow rate by 50% can increase the pressure losses in the evaporator by 300% (Jung and Radermacher [17] correlation was used in this estimation). Another disadvantage of using R-744 for heat pump water heaters is its high condensing pressure. If the requirement for hot water temperature in a household is 60 C a very high compressor outlet pressure is required in order to meet this specified temperature heat sink. The pressure can be as high as 13 MPa, while it is only 3 MPa for R-22. This level of pressure is inappropriate for household applications since there is too high a risk of potential explosion. Although Table 1 shows that R-717 is toxic and flammable it is the least harmful working fluid to the ozone layer. It only stays in the air for less than 2 weeks with no effects to the ozone layer while R-22 can stay in the atmosphere for up to 12 years. In addition, the latent heat of evaporation of R-717 is 8 times higher than that of R-22, which indicates that a smaller circulating mass flow rate per unit of heat output is required, which also reduces the risks when leakage occurs. From this point, the hydrocarbons and R-717 are more likely to be the suitable choice for substitution in solar-boosted heat pumps. R-744 will hereafter be excluded from comparison due to its mentioned unsuitability.
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3. Thermal performance Single stage vapour compression cycle, which represents regular heat pump working cycle, was chosen in this study. Degrees of superheating and subcooling are 11 and 8 C respectively. The isentropic efficiency of compression process was assumed at 0.8. Working fluid mass flow rate per unit of heat output, compressor discharge temperature, condensing pressure, two-phase pressure losses, compressor displacement volume, and heating coefficient of performance (COP) were then computed given a certain geometry based on a commercially available R-22 solar-boosted heat pump water heater. Properties of each working fluid were estimated using Ref. [14]. For domestic water heating application, the four extreme working conditions, which usually occur at the beginning and the end of heating periods on cloudy and sunny days, are assumed. The condensing temperatures are from 30 to 70 C given initial water temperature of 20 C and final temperature of 60 C. At zero solar radiation, the evaporating temperature is estimated at 0 C while it is 30 C at maximum solar radiation. It should be noted that these estimations were made based on experimental data from tests carried out by Dixon [11] and preliminary tests by the authors. Results of the analysis are presented in Figs. 1–6 in the form of percent deviation from R-22 results. In these figures, the results of these conditions were calculated. The following arguments can be made based on the results shown: 1. Mass flow rate per kW-heating for the R-717 heat pump (Fig. 1) is the lowest compared to the other four in every case. Lower mass flow rate can result in lower pressure losses through equipment.
Table 1 Characteristics and properties of some refrigerants Working fluid
R-22
R-290
R-600
R-600a
R-1270
R-717
R-744
Chemical formula Molar mass (g mol 1) Critical temperature ( C) Critical pressure (kPa) Critical density (kg m 3) Boiling pointa ( C) Latent heat of evaporationb (kJ kg 1) Flammability Toxicity ODP GWPc
CHClF2 86.46 96.14 4 990 562.0 40.9 187.6 N N 0.05 1 700
C3H8 44.10 96.7 4 247 220.0 42.1 343.9 Y N 0 3
C4H10 58.12 152.0 3 796 227.8 0.5 366.4 Y N 0 3
C4H10 58.12 134.7 3 640 224.4 11.6 335.2 Y N 0 3
C3H6 42.08 92.4 4 665 222.6 47.7 344.2 Y N 0 3
NH3 17.03 131.9 11 333 225.0 33.3 1 186.2 Y Y 0 0
CO2 44.01 31.1 7 384 466.5 78.4 153.7 N N 0 1
a
Boiling point at atmospheric pressure. At 20 C. c Reference to CO2 with base values of 1. Sources: Bitzer International [12], Charters & Lu Aye [13], NIST [14] and ASHRAE [15]. b
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Besides, a smaller amount of working fluid will greatly reduce risks due to leakage during operation and maintenances. However, the sizes of piping remain almost the same as that of the R-22 heat pump due to low density of R-717. The major difference of the R-717 heat pump is that changes in Tc do not affect the mass flow rate required. Thus, the pressure loss due to the momentum term is almost constant. 2. Discharge temperature of the R-717 heat pump in Fig. 2 is the highest among the working fluids compared. It can be raised as high as 200 C due to high entropy at Te =0 C and Tc =70 C. This high temperature increases the temperature difference between water and refrigerant and improves the heat transfer rate at the desuperheating region. Nonetheless, the high discharge temperature can cause a large amount of lubricant oil to be carried away from the compressor with the working fluid since the solubility is increased at high temperature. Another concern is that chemical stability of the lubricant oil can be destroyed, resulting in poor lubricating and consequent wear at the compressor. 3. Both R-600 and R-600a have much lower condensing pressures (50–70%) compared to that of R-22 (Fig. 3). This demonstrates that the
maximum pressure that could occur in the heat pump is also lower. In such case, maximum tensile strength of the materials used in the heat pump can be reduced. In an example of tubes, a smaller wall thickness will slightly upgrade heat transfer and decrease capital costs of material used. 4. Two-phase pressure losses across the evaporator (denoted by Te ) and condenser (denoted by Tc ) for each refrigerant (Fig. 4) were estimated based on Cavallini et al. [16] and Jung and Radermacher [17] correlations. It was assumed that a smooth, 10-m horizontal tube with inside diameter of 10 mm was used in the calculation. An arbitrary mass flow rate was also chosen. It was found that
Fig. 3. Comparison of condensing pressure at certain working conditions.
Fig. 1. Comparison of mass flow rate per kW-heating at certain working conditions.
Fig. 2. Comparison of compressor discharge temperature at certain working conditions.
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the computed pressure losses for every natural working fluid are higher than those of R-22. In average, the computed pressure losses of R-600, R-600a and R-717 are among the highest of the fluids surveyed. 5. R-290, R-1270 and R-717 require similar compressor displacement volume as R-22 does (Fig. 5), given the same heating output. This implies that existing compressors for R-22 can be used with both working fluids if material compatibility is not an issue. However, most of the compressors available for domestic use contain copper and this can not be used with R-717. Moreover, some
Fig. 4. Comparison of two-phase pressure losses at certain working conditions.
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manufacturers recommended not to use R-717 with hermetic compressors due to two main reasons. Firstly, in a sealed condition, motor winding temperature can exceed (at high discharge temperature) the limit of motor insulation and cause damage to the compressor. Secondly, at a certain level of R-717 vapour can become an electrical conductor and cause short-circuiting to motor windings. The other working fluids need a larger displacement volume compressor to deliver the same amount of system heating capacity. 6. Fig. 6 shows the relative COP of heat pumps. The computed COP of the R-22 heat pump is used as reference, and given a value as one. A system with better COP will have its calculated COP higher than one. It is clear that every working fluid investigated here has comparable COP to the R22 heat pump. From this figure, it can be identified that R-600 and R-600a perform better than the other working fluids in every condition. Further investigation shows that only R-290, R-600 and R-600a have increased COP when the superheating increases. This feature is an advantage to the solar-boosted heat pump since high superheat settings are common in these systems in order to cope with the changes in weather conditions and to protect the compressor.
Fig. 5. Comparison of displacement volume at certain working conditions.
Fig. 6. Comparison of relative COP at certain working conditions.
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Among the natural refrigerants compared, it may be concluded that R-717 is the prime candidate for R-22 over the others in this comparison. Refrigerant charge required in R-717 system is only 20% of that in R-22. Thus, we can substantially minimise the effects to global warming and environmental hazard. Its compressor discharge temperature is high which can improve heat transfer rate in desuperheating zone. Computed displacement volume is comparable to that of R-22. The pressure losses at evaporator and condenser are predicted to be lower. Although the R-600 and R-600a show higher COP levels at Tc =35 C the COP level of R-717 at Tc =70 C is comparable to these two working fluids. Note that in water heating applications with regular hot water usage rate, heat pumps work at high condensing temperatures for most of the time.
4. Opportunities for natural working fluids in solarboosted heat pumps In the foreseen future, the choice of working fluid for heat pumps will be largely driven by environmental factors. However, there is no universal replacement that can cover the wide application ranges and sizes catered for currently by R-22. In regular unitary heat pumps, 90% of the total equivalent warming impact (TEWI) comes from indirect or energy used effect and only 10% from leakage of working fluid [18]. Please note that 4% annual make-up rate and 5% loss of charge upon equipment decommissioning were assumed. Replacing R-22 by R-717 in heat pump water heaters can take advantage of better thermal efficiency and smaller amount of charge to minimize greenhouse gases emissions and risks associated with charge leakage. Although R-717 is shown to be a better replacement in terms of thermal efficiency and its minimal environmental effects the expanding use of R-717 will depend on a number of factors as described by Larminant [19]. Specially designed components such as compressors and heat exchangers of R-717 heat pumps are required to cope with safety, material compatibility, and degradation of lubricant oil. Actual thermal performance of solar-boosted heat pumps depends mainly on the equipment and control systems used. At the moment, equipment such as compressors and heat exchangers specially designed for R-717 are not widely available for domestic applications. Moreover, they are costly. Another difficulty involved is the material compatibility. Generally, steel or stainless steel pipes are recommended for R-717. Using these materials in domestic heat pump water heater can be reluctantly accepted due to inconveniences in cutting, bending and connecting. Thus, the heat pump should be designed as a maintenance-free unit to avoid this problem.
Other choices of substitute are R-290 and R-1290. They can be used as directly drop-in for most of the existing solar-boosted heat pumps with slightly reduced thermal performance. There are no special material compatibility and lubricant problems. However, some modifications are needed in order to fulfill safety requirements. Researches have indicated that actual thermal performance of a heat pump could be either increased or decreased depending on many other factors such as operating conditions and refrigerant mass flow control system.
5. Conclusions A comparison of selected natural working fluids for solar-boosted heat pumps has been carried out in terms of characteristics, thermal performance, and heat transfer coefficient. Practical assessments of safety, material compatibility and lubrication are also included. The results from this study have shown that CO2 is not an appropriate choice for solar-boosted heat pumps due to its low critical pressure leading to lower heat pump COP at evaporating temperatures approaching the critical temperature. It is, however, found that R-717 has an opportunity to be a prime candidate for a solar-boosted heat pump as it has advantages over the other natural working fluids compared. Apart from being ODP and GWP free, R-717 has demonstrated similar or better system performance to R-22 depending on working conditions. Safety issues including compatibility with existing systems, and lubrication are major weak points for using this working fluid. Drop-in substitutes are also proposed in this study.
Acknowledgements The authors would like to thank the International Technologies Centre (IDTC), the Department of Civil & Environmental Engineering and the Department of Mechanical & Manufacturing Engineering. The financial support of the Australian Research Council (ARC) for this study is acknowledged. Chatchawan Chaichana gratefully acknowledges the scholarship from the Royal Thai Government. References [1] Charters WWS, Megler VR, Urma I, Lu Aye. Propane as a working fluid in domestic heat pumps. In: IIR/IIFMelbourne 1996: Refrigeration, Climate Control and Energy Conservation. 1996 February 11-14; Melbourne, Australia. Melbourne: International Institute of Refrigeration, 1996. [2] Urma I. Propane as a replacement for domestic applications. MEngSc thesis, Department of Mechanical and
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