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Advanced cycles and replacement working fluids in heat pumps
Applied Thermal Engineering 21 (2001) 237±248
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Advanced cycles and replacement working ¯uids in heat pumps N.J. Hewitt*, J.T. McMullan, P.C. Henderson, B. Mongey Energy Research Centre, University of Ulster, Coleraine, Londonderry, Northern Ireland, BT52 1SA, UK Received 22 January 2000; accepted 28 March 2000
Abstract Heat pumps will continue to make a strong positive contribution to the reduction of carbon dioxide emissions associated with energy production. They are energy ecient under certain conditions and also cost eective (especially when displacing electric heating). It is this problem of cost eectiveness that aects market penetration and limits their use. One method of improving the payback period is by improving the eciency so as to increase the energy savings, and thus the cost savings. This also has, of course, a positive eect on the environment. This paper examines a number of alternative ¯uids and systems in an attempt to improve performance of heat pumps for both space heating and industrial processes. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: HFC zeotropic mixtures; Ammonia±water; Capacity modulation
1. Introduction The approach taken to improve the eciencies of heat pumps for space heating and industrial process heating is the use of higher temperature working ¯uids. While their direct use is obvious for higher temperature applications, their use as components of zeotropic mixtures provides the mixture of working ¯uids with a number of advantages over appropriate azeotropic and pure ¯uids. For example, industrial heating processes in the food and pharmaceutical industries can require large amounts of hot water. To meet their health and * Corresponding author. Tel.: +44-01265-324-895. 1359-4311/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 0 5 3 - 3
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safety standards through the `one-pass' nature of these processes, fresh water, often cold, has to be utilised. Fresh water can be heated in a number of ways but if this can be achieved by a variable heat source that can eectively match the gradient of the water to be heated, irreversibilities in the heat exchanger process can be minimised and overall performance improved. Indeed, some form of composition variation would also modulate heat pump capacity as the process varies. Composition modulation as a form of capacity control is also believed to be a method of improving the performance of heat pumps operating in space heating applications. Building heating pro®les change on an hourly, daily and seasonal basis and greater overall heat pump eciency can be gained by prolonged steady state operation at the correct capacity rather than by cycling on an `on±o' basis. There are a number of ¯uid combinations and cycles capable of meeting these goals. High temperature heat pumps supply a market need for heat in the range of 60±908C, traditionally occupied by systems using the ¯uid R114. The future use of R114 as a CFC is either banned or very limited and ®nding a replacement gives the opportunity to investigate new and improved cycles. One combination of ¯uids is ammonia and water and they being natural ¯uids, their use as refrigerants, will create no signi®cant increase on the direct global warming eect. Their combination in the resorption cycle has an improved performance at higher temperatures but at a cost of increased complexity and limitations in use because of the ammonia content. Alternatively, vapour compression systems using mixtures of HFCs, HFEs and HCs will improve performance, although some of these ¯uids are ¯ammable and thus, care must be taken. The ®rst step in any design procedure is to establish the properties of the chosen ¯uids and their mixtures. PvTx, VLE and transport properties have been either measured or modelled successfully by a range of systems. Heat pumps with mixtures as working ¯uids can operate on the Lorenz cycle and composition change was noted that is dependant on system dynamics and contact with the compressor lubricant. The management of composition change was also successfully carried out with a rectifying column. Energy eciency cannot be achieved by the ¯uid alone, for it also requires the careful selection and sizing of the components of the heat pump. Some of the most important of these components are the heat exchangers. Heat transfer coecients have been established and models of heat transfer and pressure drop through smooth tubes have been successfully developed. Such a far-reaching body of work is beyond most institutions and was funded in part by the European Commission JOULE III Non-Nuclear Energy Programme. The partners were the University of Ulster (UK), Forschungzentrum fur Kaltetechnik und Warmpumpen GmbH (Germany), Forschungzentrum fur Kaltetechnik und Umwelttechnik GmbH (Germany), Consiglio Nationale delle Ricerche, Instituto per la Tecnico del Freddo (Italy), Calor Gas (UK), Ausimont SpA (Italy), Dipartimento di Fisica Tecnica dell`Universita di Padova (Italy), Dipartimento di Energetica, Universita degli Studi di Ancona (Italy), Department of Heat and Power Technology, Chalmers University of Technology (Sweden) and Moy Park (UK).
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2. Use of refrigerant mixtures as working ¯uids in compression cycles Heat pumps are widely recognised as environmentally acceptable in terms of energy use. However, they are expensive to install as compared to other heating devices and also have a limited temperature range. The overriding goal of this (and any other) heat pump research is, therefore, improved eciency to improve environmental performance and reduce capital cost. As a special aspect of this project, improved eciency will be achieved by use of wide boiling refrigerant mixtures to modulate capacity to suit the load, to improve heat exchanger design and to increase operating range. 3. The ammonia±water cycle This section describes the utilities of an ammonia and water mixture which appears to be particularly suitable for high temperature heat pump applications because of the reduced pressure when working at higher temperatures. The advantages as compared to a single component working ¯uid are the possibility of matching the temperature glide of the source/ sink and variation in the circulation composition to enable better matching of the source/sink capacities. To investigate the ammonia±water cycle, several stages were required. Initial testing involved an investigation of the lubrication oil. Miscibility tests indicated that certain PAG and PAO compressor oils were suitable for use with the working ¯uid. Gas loop tests for the two types of oil showed that there was no excessive wear associated with the use of these oils. A large scale application using an absorption cycle with a screw compressor, has been evaluated on a simpli®ed test facility and the eects of water in the compressor lubricant has been noted. In the tests, the discharge pressure was 16 bar and the suction pressure was 4 bar, these pressures represent normal mechanical loading on the compressor. Fig. 1 represents the
Fig. 1. Water and ammonia concentrations in the oil of the oil separator.
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concentration of water and ammonia in the oil of the oil separator and is presented as the function of the discharge temperature. During system operation, the water should be separated from the ammonia before compression, but some water can be carried over, therefore the eects of water in the compressor are important. The values of the water concentrations, found at the compressor discharge for both types of oil, were in the same range (max 2.1%) because both oils are immiscible with water. These results also con®rmed the results of laboratory tests that the absolute content of water in the PAG oil is low [1]. In the second part of this investigation, a small reciprocating ammonia compressor was tested using a resorption cycle. This type of cycle is capable of attaining high temperatures while maintaining conventional refrigeration/heat pump pressures. The cycle can exist in one of the two general forms, namely, as an incomplete evaporation with vapour being compressed and the liquid pumped to the resorber, or as a wet compression. The latter cycle has problems due to the compressor having questionable problems with their ability to tolerate liquid and therefore the ¯ow splitting approach of separating liquid and vapour at the compression stage must be adopted. The cycle is illustrated in Fig. 2. The performance of this unit was found to be satisfactory across a pressure lift of 450±1800 kPa. Evaluation of the circulating composition was achieved through instrumentation and an equation of state [2]. A plot of COPc (Fig. 3) suggests a maximum in this parameter at a circulating composition of approximately 0.7, for a resorber water inlet temperature of 42228C: Modelling of the resorption cycle revealed that the relative heat exchanger area distribution between desorber and resorber had little eect on system COP. Furthermore, when the system was examined as a whole, composition changes did not have a serious eect on COP. However, it was shown that for falling ®lm heat exchangers, the tubes should be made as long as possible in order to maximise COP (Table 1). 4. Wide boiling ¯uids This section covers ¯uids selected as components for wide-boiling mixtures for use in Lorenz cycle machines. Therefore, the properties of the ¯uids and their mixtures will have to be established, models and equations developed and eects of lubricants and chemical and thermal stability noted. Problems with ¯uid availability limited the project to two higher temperature components, Table 1 COP for dierent resorber pressures and lengths
5 m tubes 13 m tubes 20 m tubes
25 bar
30 bar
32 bar
5.79 6.18 6.26
5.95 6.35 6.43
6.00 6.40 6.47
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Fig. 2. Diagram of the ammonia±water test facility.
Fig. 3. COPc for the ammonia±water cycle.
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i.e. R236ea and R236fa. These two ¯uids have no ozone depletion potential and seem to be promising substitutes for components containing chlorine in high temperaure heat pumps. Further investigation revealed that there is very little published information on the properties of these pure ¯uids, or their use as components in mixtures. PVT and VLE data necessary for property calculations were established experimentally for the pure ¯uids and selected mixtures [3±5] that appeared to have the best performance characteristics e.g. R236fa and R134a. The VLE properties of this mixture appear `ideal' in appearance (Fig. 4) and are in agreement with those calculated with the Carnahan±Starling±De Santis equation of state from measured PvTx data. Models of thermodynamic and transport properties of the new ¯uids and their mixtures were also established. These were developed using the Soave±Redlich±Kwong, the Peng±Robinson, the Carnnahan±Starlig±De Santis and the Lee±Kesler equations of state [6]; improved models for the corresponding states approach (Teja and Wilding±Rowley) [7,8] and a general model for density prediction in the liquid and vapour phases. Predictive and semi-predictive models were also developed for viscosity using a three-parameter corresponding states model in which the acentric factor is substituted by a temperature dependant function [9]. However, while the properties of the ¯uids are important, the ¯uids must also be compatible with the materials of the system in which they are to be utilised. Two important features of any refrigerant are its chemical and thermal stability and the eects of lubrication. Stability was measured up to at least 1208C by observing the formation of by-products, corrosion, swelling of elastomers, etc. and plating. VLE and LLE solubility studies on ¯uid-lubricant systems led to modelling of the experimental binary data by the Flory±Huggins ®ve-parameter extended model. Results show that with a polyol ester oil (i.e. Castrol Icematic SW 32), R236ea, R236fa and E134 exhibit good solubility in the range Ð 60±808C. For R236ea and
Fig. 4. Isothermal VLE data for R134a and R236ea (CSD EOS).
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R236fa, no corrosion was noted on copper, steel or aluminium, while in the presence of E134 and polyol ester oils, stainless steel exhibited evidence of corrosion, placing doubts on the future use of E134. Elastomers containing BUNA-N seem to be the most impervious to change. An apparatus was also developed to measure gas-phase pressure and composition and cloud point of working ¯uid oil mixtures. Fig. 5 illustrates the eect of soluble lubricant on vapour pressure of R236fa. After evaluation, it was noted that R236fa and POE oil deviated less from the ideal Raoult's law than R134a and the same oil. P-T-y-x measurements of the ternary system R236fa/R134a/POE oil also revealed deviations from the ideal Raoult's law as expected. Again, modelling of refrigerant and lubricant systems was carried out by the extended version of the Flory±Huggins equation. It calculated accurately the immiscibility conditions and the upper and lower critical solution temperatures. 5. Smart control of mixture composition Having evaluated mixture compositions, the project then sought methods to develop composition modulation. Smart control is ideal for buildings, where the amount of heat required by the building is inversely proportional to the ambient temperature. Thus the source temperature and heat demand changes and the performance must also vary in order to maintain maximum COP and reduce energy related emissions to the environment. One method of maximising COP is to alter the circulating mixture composition so that the optimum composition for that temperature regime is maintained. Preference was initially given to the mixture R407c, which consists of R32, R125 and R134a. Calculations with this mixture were carried out for various compositions and showed that the concept of using zeotropic refrigerant mixtures to change volumetric capacity by separating the components with a rectifying column was feasible. However, this eect was limited due to the limited dierence in boiling points of the individual ¯uids (R32: ÿ51.78C, R125: ÿ488C and R134a: ÿ26.18C). Since R125 and R32 together behave as near azeotropic mixture, R134a was
Fig. 5. R236fa icematic SW 32 (Castrol), isothermal pressure vs. composition.
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the component usually separated from the other two ¯uids, to cause the capacity to change. The range of this change in capacity is not sucient for heat pumps. For the central European hydronic heating systems, a much wider range of capacity control has to be achieved. Therefore, a higher boiling component than R134a must be added to other low boiling refrigerants. After screening HFC-¯uids for high temperature replacements, isomers of R236 and R245 as well as the ¯uid R227ea have been identi®ed as suitable replacements for R32 and R125. Initial experiments were carried out with R227ea, a ¯uid with a higher boiling point than R134a, but once again the capacity changes were not sucient. The cycle design presented in Fig. 6 shows the normal heat pump cycle which consists of an indoor and an outdoor unit, and a rectifying column that has been integrated between evaporator and compressor on the low pressure side of the cycle. This rectifying device together with the column consists of one accumulator/separator above and another accumulator below the rectifying column, which is heated by some form of heat source, preferably the compressor discharge gas. This cycle was operated as a normal heat pump by using an electronic expansion valve for achieving a certain amount of superheat at the evaporator outlet. When capacity control is necessary, the expansion valve is opened in such a manner that a ¯ooding of the evaporator occurs, and the accumulator above the rectifying column is ®lled partially with liquid. The liquid refrigerant enters the rectifying column and ¯ows down in counter ¯ow to the evaporated refrigerant from the heated lower accumulator. This causes a change in the composition of the refrigerant ¯owing to the compressor from the evaporator and the
Fig. 6. Diagram of the cycle designed by FKW.
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rectifying column, by enriching the cycle with the lower boiling components and hence increasing the capacity of the heat pump cycle. The ammonia±water cycle is also capable of capacity control through circulating composition management. Results from ammonia water test facility show that composition change varies with liquid level in the low-side receiver. The circulating composition (mXi) is observed to change between 0.55 and 0.75 NH3 (Fig. 7). The lowest circulating concentration approaches the charge concentration, when the liquid receiver is empty. The bulk of the charge is contained in the high-side reservoir and has the same composition as the one circulating throughout the system. The circulating composition is calculated from density and temperature measurements in the liquid line prior to the expansion valve, while a ¯oat assembly housed within the receiver measures the liquid level. To facilitate a change in the circulating composition, a fraction of the system charge is stored in the liquid receiver. As more of the system charge accumulates in the receiver the circulating concentration of NH3 increases. 6. Evaporator and condenser design In addition to the ¯uid analysis, improvements to heat exchangers have been evaluated. For a counter current heat exchanger operating with zeotropic mixtures, the temperature dierence between working ¯uid and heat transfer medium has been reduced and this aects the design of these units. An important aspect of any heat exchanger design is the heat transfer coecient of the ¯uids being used. A test facility examined condensation heat transfer. In addition, a data bank of over 4000 references to condensation heat transfer, evaporating heat transfer and pressure drop was assembled. Furthermore, to design a heat exchanger for wide boiling mixtures, a number of changes to conventional equipment are required. These are primarily aimed at air handling units that are mostly cross-¯ow. To attain the best results while using wide-boiling mixtures, these would necessarily have to become counter-¯ow in nature. To improve the heat transfer, it is necessary to disrupt the growth of thermal boundary layer created around the ®ns and to achieve this aim, several approaches may be taken. Firstly, louvred ®ns within the heat exchanger will
Fig. 7. Circulating composition (NH3 by mass) vs. receiver liquid level.
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adjust the air ¯ow through the system, but may not be ideal for a small scale installation where space is critical. The second method is to introduce vortex generators onto the plates. This involves changes to the outer ®n surfaces in a similar manner to ri¯ing on the inner surface. By using a thick wire mesh, an ideal turbulent ¯ow may be developed, in a similar manner to the vortex generators. Any of these solutions used in conjunction with staggered tubing will create a turbulent air¯ow, thus aiding the boiling eect. To further enhance the heat transfer of a refrigerant mixture, it is possible to create a counter-current ¯ow between the refrigerant and the air. Rearranging the internal tubing and interconnections creates this. The refrigerant ¯ow will be set to ¯ow against the inlet air¯ow, thereby creating a counter-¯ow. This is improved by staggering the refrigerant ¯ow pattern in such a manner that it will pass up and down each row of tubes before passing onto the next section. The more the rows, the closer the ¯ow regime approach to the counter-¯ow. 7. Industrial process integration Heat pump design improvements are only part of the solution. The correct size of heat pump and the assurance that it is performing the correct task will ensure that the design improvements are realised in an actual application. The objective of this work is to demonstrate the possibility of integrating some form of energy recovery system into a modern food processing factory. Table 2 illustrates the measured energy values for the `accessible' heating and cooling streams, given the limitations encountered in the factory, because of the need for cleanliness. The streams have been slightly simpli®ed and some have been added together in order to make a sensible process. The goal is to preheat the large amounts of hot water and steam used in cleaning and cooking processes within the plant. It was therefore decided to use waste heat from the product and from the extensive refrigeration capacity, but not from the cleaning processes, as use of this may contravene hygiene regulations. In the food processing industry, heat is required for hot water and steam. Indeed, the food processing industry is one of the few industries that maintains a high requirment for process steam. Given the requirements for cleanliness etc., the water and steam tends to have a `onepass' use and therefore fresh incoming water is heated from a relatively low temperature. This is an ideal situation for Lorenz cycles, i.e. heat pumps that utilise working ¯uids that boil/ condense across a temperature glide. Glide matching can improve performance by reducing Table 2 Useable process streams Name Air cooling of product Packing and dispatch Fresh chicken Cold store Prechilling Water heating
Temp. IN (8C)
Temp. OUT (8C)
80 40 40 40 40 10
50 20 20 20 20 70
kW 250 309 212 560 670 1500
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2nd Law losses in the heat exchangers. During this research, three alternative cycles were investigated, two conventional vapour compression cycles and an ammonia±water resorption cycle. The two vapour compression systems were a hydrocarbon system utilising R290 and npentane investigated by Mongey et al. [10] and a system utilising R236fa and R32. Both systems had values of COPh in excess of 5.2 and produced only some 400 kg of CO2 per year. This compares favourably with the CO2 emissions of about 1620 kg/year for an oil-®red boiler, but with UK prices, the values of COPh would need to be in excess of 5.6 for an electrically driven heat pump to be economically viable. The Lorenz cycle has some potential in this area, because current laboratory studies on a system delivering some 15 kW at a resorber temperature of 80±908C indicate that a COPh 4:0 is occurring. It is believed that larger systems would have improved performance. However, as has been already indicated, this performance is not sucient for electric driven units, but may be sucient for engine driven units.
8. Conclusions The ammonia±water cycle oered a natural approach to higher temperature heat pumps, capable of reaching temperatures of 1208C while maintaining conventional pressures. In addition, the large temperature glide in both the desorber (evaporator) and resorber (condenser) suggested the potential for higher eciencies especially in industrial processes. The programme investigated the eects of water in the compressor, concentrating on compressor lubricant solubility; the precise control of the circulating ¯uid concentration to maximise performance and the potential integration of this type of cycle into an industrial process. Alternatively, mixtures of non-ozone depleting ¯uids oered similar performance enhancements when replacing CFC R114, the ¯uid typically used in higher temperature heat pumps. As there was no single direct replacement for R114, the properties and stability of new ¯uids and their mixtures were established. Of equal importance was the understanding and management of the circulating mixture composition, as aected by the dynamics of the system components and the dierential solubility of the compressor lubricating oil. This last point is of particular importance, when dealing with small systems where the oil to working ¯uid ratio is quite high. As part of the circulating composition management process, the smart control of circulating mixture composition was attempted, again using mixtures with a high glide, but with an alternative goal i.e. the ecient management of heat delivered to buildings. The composition would be controlled to meet the variable (daily and seasonal) demands of building heating requirements. It has been mentioned that a performance enhancement would be associated with a refrigerant that boils or condenses across a range of temperatures. This means that there would be a lower temperature dierence between the working ¯uid and the external ¯uid of interest to be heated or cooled. Again, to maximise eciency, the heat transfer and transport properties of the new ¯uids and their mixtures have been identi®ed and the necessary design enhancements recommended. Finally, process integration was studied in an industrial food
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processing which indicated that performance needs to be considerably higher in order for electric driven heat pumps to be economically viable across a wider operation range. References [1] N.P. Halm, H. Kruse, in: 6th IEA Heat Pump Conference, Berlin, 1999. [2] B. Mongey, J.T. McMullan, N.J. Hewitt, G.A. Molyneaux, An equation of state for NH3±H2O mixtures, in: Natural Working Fluids '98, IIR Conference, Oslo, Norway, 1998, pp. 487±494. [3] G. Di Nicola, G. Giuliani, F. Polonara, R. Stryjek, Saturated pressure and PvT measurements of 1,1,1,3,3,3hexa¯uoropropane (R236fa), Journal of Chemical Engineering Data 44(4) (1999) 696±700. [4] G. Di Nicola, G. Giuliani, F. Polonara, R. Stryjek, Virial coecients from isochoric measurements for R236ea, R236fa, R32+R134a, R32+R236ea, R125+R236ea, R125+R236fa and R134a+R236fa systems, in: 20th International Congress of Refrigeration, Sydney, September, 1999. [5] S. Bobbo, R. Camporese, L. Fedele, M. Scattolini, Vapour-liquid equilibrium measurements and correlation of the binary refrigerants mixtures Di¯uoromethane (HFC-32)+1,1,1,2,3,3-hexa¯uoropropane (HFC-236ea) and Penta¯uoroethane (HFC-125)+1,1,1,2,3,3-Hexa¯uoropropane (HFC-236ea) at 288.6, 303.2 and 318.3K. International Journal of Thermophysics 21(3) (2000) 781±791. [6] G. Scalabrin, L. Garavello, R. Camporese, Prediction of the thermal conductivity of pure refrigerants through an extended corresponding states model, in: Proceedings of the 1996 International Refrigeration Conference, I.I.R. Commissions B1, B2, E1, E2, Purdue University, West Lafayette, IN, USA, 23±26 July, 1996, pp. 415± 422. [7] G. Scalabrin, M. Grigiante, Improved corresponding states models for pure and mixed hydrocarbons with an innovative predictive mixing rule, in: Proceedings of International Refrigeration Conference, I.I.R. Commissions B1, B2, E1, E2, Aarhus, (DK), 3±6 Sept., 1996, pp. 747±757. [8] G. Scalabrin, S. Dal Santo, M. Grigiante, Thermodynamic properties prediction of polar ¯uids in a fourparameter corresponding-states format, in: Proc. 5th Asian Thermophysical Properties Conference, Seoul (Korea) Aug. 30±Sept. 2, 1998. [9] G. Scalabrin, M. Grigiante, Improved corresponding states models for polar ¯uids, in: Proceedings of 16th European Seminar on Applied Thermodynamics, Pont-aÁ-Mousson (F), 19±22 June 1997, pp. 104±111. [10] B. Mongey, J.T. McMullan, M.G. McNerlin, An examination of hydrocarbon mixtures for use in high temperature heat pump applications, in: Applications for Natural Refrigerants, IIR Conference, Aarhus, Denmark, 1996.
www.elsevier.com/locate/apthermeng
Advanced cycles and replacement working ¯uids in heat pumps N.J. Hewitt*, J.T. McMullan, P.C. Henderson, B. Mongey Energy Research Centre, University of Ulster, Coleraine, Londonderry, Northern Ireland, BT52 1SA, UK Received 22 January 2000; accepted 28 March 2000
Abstract Heat pumps will continue to make a strong positive contribution to the reduction of carbon dioxide emissions associated with energy production. They are energy ecient under certain conditions and also cost eective (especially when displacing electric heating). It is this problem of cost eectiveness that aects market penetration and limits their use. One method of improving the payback period is by improving the eciency so as to increase the energy savings, and thus the cost savings. This also has, of course, a positive eect on the environment. This paper examines a number of alternative ¯uids and systems in an attempt to improve performance of heat pumps for both space heating and industrial processes. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: HFC zeotropic mixtures; Ammonia±water; Capacity modulation
1. Introduction The approach taken to improve the eciencies of heat pumps for space heating and industrial process heating is the use of higher temperature working ¯uids. While their direct use is obvious for higher temperature applications, their use as components of zeotropic mixtures provides the mixture of working ¯uids with a number of advantages over appropriate azeotropic and pure ¯uids. For example, industrial heating processes in the food and pharmaceutical industries can require large amounts of hot water. To meet their health and * Corresponding author. Tel.: +44-01265-324-895. 1359-4311/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 0 5 3 - 3
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safety standards through the `one-pass' nature of these processes, fresh water, often cold, has to be utilised. Fresh water can be heated in a number of ways but if this can be achieved by a variable heat source that can eectively match the gradient of the water to be heated, irreversibilities in the heat exchanger process can be minimised and overall performance improved. Indeed, some form of composition variation would also modulate heat pump capacity as the process varies. Composition modulation as a form of capacity control is also believed to be a method of improving the performance of heat pumps operating in space heating applications. Building heating pro®les change on an hourly, daily and seasonal basis and greater overall heat pump eciency can be gained by prolonged steady state operation at the correct capacity rather than by cycling on an `on±o' basis. There are a number of ¯uid combinations and cycles capable of meeting these goals. High temperature heat pumps supply a market need for heat in the range of 60±908C, traditionally occupied by systems using the ¯uid R114. The future use of R114 as a CFC is either banned or very limited and ®nding a replacement gives the opportunity to investigate new and improved cycles. One combination of ¯uids is ammonia and water and they being natural ¯uids, their use as refrigerants, will create no signi®cant increase on the direct global warming eect. Their combination in the resorption cycle has an improved performance at higher temperatures but at a cost of increased complexity and limitations in use because of the ammonia content. Alternatively, vapour compression systems using mixtures of HFCs, HFEs and HCs will improve performance, although some of these ¯uids are ¯ammable and thus, care must be taken. The ®rst step in any design procedure is to establish the properties of the chosen ¯uids and their mixtures. PvTx, VLE and transport properties have been either measured or modelled successfully by a range of systems. Heat pumps with mixtures as working ¯uids can operate on the Lorenz cycle and composition change was noted that is dependant on system dynamics and contact with the compressor lubricant. The management of composition change was also successfully carried out with a rectifying column. Energy eciency cannot be achieved by the ¯uid alone, for it also requires the careful selection and sizing of the components of the heat pump. Some of the most important of these components are the heat exchangers. Heat transfer coecients have been established and models of heat transfer and pressure drop through smooth tubes have been successfully developed. Such a far-reaching body of work is beyond most institutions and was funded in part by the European Commission JOULE III Non-Nuclear Energy Programme. The partners were the University of Ulster (UK), Forschungzentrum fur Kaltetechnik und Warmpumpen GmbH (Germany), Forschungzentrum fur Kaltetechnik und Umwelttechnik GmbH (Germany), Consiglio Nationale delle Ricerche, Instituto per la Tecnico del Freddo (Italy), Calor Gas (UK), Ausimont SpA (Italy), Dipartimento di Fisica Tecnica dell`Universita di Padova (Italy), Dipartimento di Energetica, Universita degli Studi di Ancona (Italy), Department of Heat and Power Technology, Chalmers University of Technology (Sweden) and Moy Park (UK).
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2. Use of refrigerant mixtures as working ¯uids in compression cycles Heat pumps are widely recognised as environmentally acceptable in terms of energy use. However, they are expensive to install as compared to other heating devices and also have a limited temperature range. The overriding goal of this (and any other) heat pump research is, therefore, improved eciency to improve environmental performance and reduce capital cost. As a special aspect of this project, improved eciency will be achieved by use of wide boiling refrigerant mixtures to modulate capacity to suit the load, to improve heat exchanger design and to increase operating range. 3. The ammonia±water cycle This section describes the utilities of an ammonia and water mixture which appears to be particularly suitable for high temperature heat pump applications because of the reduced pressure when working at higher temperatures. The advantages as compared to a single component working ¯uid are the possibility of matching the temperature glide of the source/ sink and variation in the circulation composition to enable better matching of the source/sink capacities. To investigate the ammonia±water cycle, several stages were required. Initial testing involved an investigation of the lubrication oil. Miscibility tests indicated that certain PAG and PAO compressor oils were suitable for use with the working ¯uid. Gas loop tests for the two types of oil showed that there was no excessive wear associated with the use of these oils. A large scale application using an absorption cycle with a screw compressor, has been evaluated on a simpli®ed test facility and the eects of water in the compressor lubricant has been noted. In the tests, the discharge pressure was 16 bar and the suction pressure was 4 bar, these pressures represent normal mechanical loading on the compressor. Fig. 1 represents the
Fig. 1. Water and ammonia concentrations in the oil of the oil separator.
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concentration of water and ammonia in the oil of the oil separator and is presented as the function of the discharge temperature. During system operation, the water should be separated from the ammonia before compression, but some water can be carried over, therefore the eects of water in the compressor are important. The values of the water concentrations, found at the compressor discharge for both types of oil, were in the same range (max 2.1%) because both oils are immiscible with water. These results also con®rmed the results of laboratory tests that the absolute content of water in the PAG oil is low [1]. In the second part of this investigation, a small reciprocating ammonia compressor was tested using a resorption cycle. This type of cycle is capable of attaining high temperatures while maintaining conventional refrigeration/heat pump pressures. The cycle can exist in one of the two general forms, namely, as an incomplete evaporation with vapour being compressed and the liquid pumped to the resorber, or as a wet compression. The latter cycle has problems due to the compressor having questionable problems with their ability to tolerate liquid and therefore the ¯ow splitting approach of separating liquid and vapour at the compression stage must be adopted. The cycle is illustrated in Fig. 2. The performance of this unit was found to be satisfactory across a pressure lift of 450±1800 kPa. Evaluation of the circulating composition was achieved through instrumentation and an equation of state [2]. A plot of COPc (Fig. 3) suggests a maximum in this parameter at a circulating composition of approximately 0.7, for a resorber water inlet temperature of 42228C: Modelling of the resorption cycle revealed that the relative heat exchanger area distribution between desorber and resorber had little eect on system COP. Furthermore, when the system was examined as a whole, composition changes did not have a serious eect on COP. However, it was shown that for falling ®lm heat exchangers, the tubes should be made as long as possible in order to maximise COP (Table 1). 4. Wide boiling ¯uids This section covers ¯uids selected as components for wide-boiling mixtures for use in Lorenz cycle machines. Therefore, the properties of the ¯uids and their mixtures will have to be established, models and equations developed and eects of lubricants and chemical and thermal stability noted. Problems with ¯uid availability limited the project to two higher temperature components, Table 1 COP for dierent resorber pressures and lengths
5 m tubes 13 m tubes 20 m tubes
25 bar
30 bar
32 bar
5.79 6.18 6.26
5.95 6.35 6.43
6.00 6.40 6.47
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Fig. 2. Diagram of the ammonia±water test facility.
Fig. 3. COPc for the ammonia±water cycle.
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i.e. R236ea and R236fa. These two ¯uids have no ozone depletion potential and seem to be promising substitutes for components containing chlorine in high temperaure heat pumps. Further investigation revealed that there is very little published information on the properties of these pure ¯uids, or their use as components in mixtures. PVT and VLE data necessary for property calculations were established experimentally for the pure ¯uids and selected mixtures [3±5] that appeared to have the best performance characteristics e.g. R236fa and R134a. The VLE properties of this mixture appear `ideal' in appearance (Fig. 4) and are in agreement with those calculated with the Carnahan±Starling±De Santis equation of state from measured PvTx data. Models of thermodynamic and transport properties of the new ¯uids and their mixtures were also established. These were developed using the Soave±Redlich±Kwong, the Peng±Robinson, the Carnnahan±Starlig±De Santis and the Lee±Kesler equations of state [6]; improved models for the corresponding states approach (Teja and Wilding±Rowley) [7,8] and a general model for density prediction in the liquid and vapour phases. Predictive and semi-predictive models were also developed for viscosity using a three-parameter corresponding states model in which the acentric factor is substituted by a temperature dependant function [9]. However, while the properties of the ¯uids are important, the ¯uids must also be compatible with the materials of the system in which they are to be utilised. Two important features of any refrigerant are its chemical and thermal stability and the eects of lubrication. Stability was measured up to at least 1208C by observing the formation of by-products, corrosion, swelling of elastomers, etc. and plating. VLE and LLE solubility studies on ¯uid-lubricant systems led to modelling of the experimental binary data by the Flory±Huggins ®ve-parameter extended model. Results show that with a polyol ester oil (i.e. Castrol Icematic SW 32), R236ea, R236fa and E134 exhibit good solubility in the range Ð 60±808C. For R236ea and
Fig. 4. Isothermal VLE data for R134a and R236ea (CSD EOS).
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R236fa, no corrosion was noted on copper, steel or aluminium, while in the presence of E134 and polyol ester oils, stainless steel exhibited evidence of corrosion, placing doubts on the future use of E134. Elastomers containing BUNA-N seem to be the most impervious to change. An apparatus was also developed to measure gas-phase pressure and composition and cloud point of working ¯uid oil mixtures. Fig. 5 illustrates the eect of soluble lubricant on vapour pressure of R236fa. After evaluation, it was noted that R236fa and POE oil deviated less from the ideal Raoult's law than R134a and the same oil. P-T-y-x measurements of the ternary system R236fa/R134a/POE oil also revealed deviations from the ideal Raoult's law as expected. Again, modelling of refrigerant and lubricant systems was carried out by the extended version of the Flory±Huggins equation. It calculated accurately the immiscibility conditions and the upper and lower critical solution temperatures. 5. Smart control of mixture composition Having evaluated mixture compositions, the project then sought methods to develop composition modulation. Smart control is ideal for buildings, where the amount of heat required by the building is inversely proportional to the ambient temperature. Thus the source temperature and heat demand changes and the performance must also vary in order to maintain maximum COP and reduce energy related emissions to the environment. One method of maximising COP is to alter the circulating mixture composition so that the optimum composition for that temperature regime is maintained. Preference was initially given to the mixture R407c, which consists of R32, R125 and R134a. Calculations with this mixture were carried out for various compositions and showed that the concept of using zeotropic refrigerant mixtures to change volumetric capacity by separating the components with a rectifying column was feasible. However, this eect was limited due to the limited dierence in boiling points of the individual ¯uids (R32: ÿ51.78C, R125: ÿ488C and R134a: ÿ26.18C). Since R125 and R32 together behave as near azeotropic mixture, R134a was
Fig. 5. R236fa icematic SW 32 (Castrol), isothermal pressure vs. composition.
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the component usually separated from the other two ¯uids, to cause the capacity to change. The range of this change in capacity is not sucient for heat pumps. For the central European hydronic heating systems, a much wider range of capacity control has to be achieved. Therefore, a higher boiling component than R134a must be added to other low boiling refrigerants. After screening HFC-¯uids for high temperature replacements, isomers of R236 and R245 as well as the ¯uid R227ea have been identi®ed as suitable replacements for R32 and R125. Initial experiments were carried out with R227ea, a ¯uid with a higher boiling point than R134a, but once again the capacity changes were not sucient. The cycle design presented in Fig. 6 shows the normal heat pump cycle which consists of an indoor and an outdoor unit, and a rectifying column that has been integrated between evaporator and compressor on the low pressure side of the cycle. This rectifying device together with the column consists of one accumulator/separator above and another accumulator below the rectifying column, which is heated by some form of heat source, preferably the compressor discharge gas. This cycle was operated as a normal heat pump by using an electronic expansion valve for achieving a certain amount of superheat at the evaporator outlet. When capacity control is necessary, the expansion valve is opened in such a manner that a ¯ooding of the evaporator occurs, and the accumulator above the rectifying column is ®lled partially with liquid. The liquid refrigerant enters the rectifying column and ¯ows down in counter ¯ow to the evaporated refrigerant from the heated lower accumulator. This causes a change in the composition of the refrigerant ¯owing to the compressor from the evaporator and the
Fig. 6. Diagram of the cycle designed by FKW.
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rectifying column, by enriching the cycle with the lower boiling components and hence increasing the capacity of the heat pump cycle. The ammonia±water cycle is also capable of capacity control through circulating composition management. Results from ammonia water test facility show that composition change varies with liquid level in the low-side receiver. The circulating composition (mXi) is observed to change between 0.55 and 0.75 NH3 (Fig. 7). The lowest circulating concentration approaches the charge concentration, when the liquid receiver is empty. The bulk of the charge is contained in the high-side reservoir and has the same composition as the one circulating throughout the system. The circulating composition is calculated from density and temperature measurements in the liquid line prior to the expansion valve, while a ¯oat assembly housed within the receiver measures the liquid level. To facilitate a change in the circulating composition, a fraction of the system charge is stored in the liquid receiver. As more of the system charge accumulates in the receiver the circulating concentration of NH3 increases. 6. Evaporator and condenser design In addition to the ¯uid analysis, improvements to heat exchangers have been evaluated. For a counter current heat exchanger operating with zeotropic mixtures, the temperature dierence between working ¯uid and heat transfer medium has been reduced and this aects the design of these units. An important aspect of any heat exchanger design is the heat transfer coecient of the ¯uids being used. A test facility examined condensation heat transfer. In addition, a data bank of over 4000 references to condensation heat transfer, evaporating heat transfer and pressure drop was assembled. Furthermore, to design a heat exchanger for wide boiling mixtures, a number of changes to conventional equipment are required. These are primarily aimed at air handling units that are mostly cross-¯ow. To attain the best results while using wide-boiling mixtures, these would necessarily have to become counter-¯ow in nature. To improve the heat transfer, it is necessary to disrupt the growth of thermal boundary layer created around the ®ns and to achieve this aim, several approaches may be taken. Firstly, louvred ®ns within the heat exchanger will
Fig. 7. Circulating composition (NH3 by mass) vs. receiver liquid level.
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adjust the air ¯ow through the system, but may not be ideal for a small scale installation where space is critical. The second method is to introduce vortex generators onto the plates. This involves changes to the outer ®n surfaces in a similar manner to ri¯ing on the inner surface. By using a thick wire mesh, an ideal turbulent ¯ow may be developed, in a similar manner to the vortex generators. Any of these solutions used in conjunction with staggered tubing will create a turbulent air¯ow, thus aiding the boiling eect. To further enhance the heat transfer of a refrigerant mixture, it is possible to create a counter-current ¯ow between the refrigerant and the air. Rearranging the internal tubing and interconnections creates this. The refrigerant ¯ow will be set to ¯ow against the inlet air¯ow, thereby creating a counter-¯ow. This is improved by staggering the refrigerant ¯ow pattern in such a manner that it will pass up and down each row of tubes before passing onto the next section. The more the rows, the closer the ¯ow regime approach to the counter-¯ow. 7. Industrial process integration Heat pump design improvements are only part of the solution. The correct size of heat pump and the assurance that it is performing the correct task will ensure that the design improvements are realised in an actual application. The objective of this work is to demonstrate the possibility of integrating some form of energy recovery system into a modern food processing factory. Table 2 illustrates the measured energy values for the `accessible' heating and cooling streams, given the limitations encountered in the factory, because of the need for cleanliness. The streams have been slightly simpli®ed and some have been added together in order to make a sensible process. The goal is to preheat the large amounts of hot water and steam used in cleaning and cooking processes within the plant. It was therefore decided to use waste heat from the product and from the extensive refrigeration capacity, but not from the cleaning processes, as use of this may contravene hygiene regulations. In the food processing industry, heat is required for hot water and steam. Indeed, the food processing industry is one of the few industries that maintains a high requirment for process steam. Given the requirements for cleanliness etc., the water and steam tends to have a `onepass' use and therefore fresh incoming water is heated from a relatively low temperature. This is an ideal situation for Lorenz cycles, i.e. heat pumps that utilise working ¯uids that boil/ condense across a temperature glide. Glide matching can improve performance by reducing Table 2 Useable process streams Name Air cooling of product Packing and dispatch Fresh chicken Cold store Prechilling Water heating
Temp. IN (8C)
Temp. OUT (8C)
80 40 40 40 40 10
50 20 20 20 20 70
kW 250 309 212 560 670 1500
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2nd Law losses in the heat exchangers. During this research, three alternative cycles were investigated, two conventional vapour compression cycles and an ammonia±water resorption cycle. The two vapour compression systems were a hydrocarbon system utilising R290 and npentane investigated by Mongey et al. [10] and a system utilising R236fa and R32. Both systems had values of COPh in excess of 5.2 and produced only some 400 kg of CO2 per year. This compares favourably with the CO2 emissions of about 1620 kg/year for an oil-®red boiler, but with UK prices, the values of COPh would need to be in excess of 5.6 for an electrically driven heat pump to be economically viable. The Lorenz cycle has some potential in this area, because current laboratory studies on a system delivering some 15 kW at a resorber temperature of 80±908C indicate that a COPh 4:0 is occurring. It is believed that larger systems would have improved performance. However, as has been already indicated, this performance is not sucient for electric driven units, but may be sucient for engine driven units.
8. Conclusions The ammonia±water cycle oered a natural approach to higher temperature heat pumps, capable of reaching temperatures of 1208C while maintaining conventional pressures. In addition, the large temperature glide in both the desorber (evaporator) and resorber (condenser) suggested the potential for higher eciencies especially in industrial processes. The programme investigated the eects of water in the compressor, concentrating on compressor lubricant solubility; the precise control of the circulating ¯uid concentration to maximise performance and the potential integration of this type of cycle into an industrial process. Alternatively, mixtures of non-ozone depleting ¯uids oered similar performance enhancements when replacing CFC R114, the ¯uid typically used in higher temperature heat pumps. As there was no single direct replacement for R114, the properties and stability of new ¯uids and their mixtures were established. Of equal importance was the understanding and management of the circulating mixture composition, as aected by the dynamics of the system components and the dierential solubility of the compressor lubricating oil. This last point is of particular importance, when dealing with small systems where the oil to working ¯uid ratio is quite high. As part of the circulating composition management process, the smart control of circulating mixture composition was attempted, again using mixtures with a high glide, but with an alternative goal i.e. the ecient management of heat delivered to buildings. The composition would be controlled to meet the variable (daily and seasonal) demands of building heating requirements. It has been mentioned that a performance enhancement would be associated with a refrigerant that boils or condenses across a range of temperatures. This means that there would be a lower temperature dierence between the working ¯uid and the external ¯uid of interest to be heated or cooled. Again, to maximise eciency, the heat transfer and transport properties of the new ¯uids and their mixtures have been identi®ed and the necessary design enhancements recommended. Finally, process integration was studied in an industrial food
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processing which indicated that performance needs to be considerably higher in order for electric driven heat pumps to be economically viable across a wider operation range. References [1] N.P. Halm, H. Kruse, in: 6th IEA Heat Pump Conference, Berlin, 1999. [2] B. Mongey, J.T. McMullan, N.J. Hewitt, G.A. Molyneaux, An equation of state for NH3±H2O mixtures, in: Natural Working Fluids '98, IIR Conference, Oslo, Norway, 1998, pp. 487±494. [3] G. Di Nicola, G. Giuliani, F. Polonara, R. Stryjek, Saturated pressure and PvT measurements of 1,1,1,3,3,3hexa¯uoropropane (R236fa), Journal of Chemical Engineering Data 44(4) (1999) 696±700. [4] G. Di Nicola, G. Giuliani, F. Polonara, R. Stryjek, Virial coecients from isochoric measurements for R236ea, R236fa, R32+R134a, R32+R236ea, R125+R236ea, R125+R236fa and R134a+R236fa systems, in: 20th International Congress of Refrigeration, Sydney, September, 1999. [5] S. Bobbo, R. Camporese, L. Fedele, M. Scattolini, Vapour-liquid equilibrium measurements and correlation of the binary refrigerants mixtures Di¯uoromethane (HFC-32)+1,1,1,2,3,3-hexa¯uoropropane (HFC-236ea) and Penta¯uoroethane (HFC-125)+1,1,1,2,3,3-Hexa¯uoropropane (HFC-236ea) at 288.6, 303.2 and 318.3K. International Journal of Thermophysics 21(3) (2000) 781±791. [6] G. Scalabrin, L. Garavello, R. Camporese, Prediction of the thermal conductivity of pure refrigerants through an extended corresponding states model, in: Proceedings of the 1996 International Refrigeration Conference, I.I.R. Commissions B1, B2, E1, E2, Purdue University, West Lafayette, IN, USA, 23±26 July, 1996, pp. 415± 422. [7] G. Scalabrin, M. Grigiante, Improved corresponding states models for pure and mixed hydrocarbons with an innovative predictive mixing rule, in: Proceedings of International Refrigeration Conference, I.I.R. Commissions B1, B2, E1, E2, Aarhus, (DK), 3±6 Sept., 1996, pp. 747±757. [8] G. Scalabrin, S. Dal Santo, M. Grigiante, Thermodynamic properties prediction of polar ¯uids in a fourparameter corresponding-states format, in: Proc. 5th Asian Thermophysical Properties Conference, Seoul (Korea) Aug. 30±Sept. 2, 1998. [9] G. Scalabrin, M. Grigiante, Improved corresponding states models for polar ¯uids, in: Proceedings of 16th European Seminar on Applied Thermodynamics, Pont-aÁ-Mousson (F), 19±22 June 1997, pp. 104±111. [10] B. Mongey, J.T. McMullan, M.G. McNerlin, An examination of hydrocarbon mixtures for use in high temperature heat pump applications, in: Applications for Natural Refrigerants, IIR Conference, Aarhus, Denmark, 1996.