Heat RecoverySystems Vol. 6, No. 4, pp. 305-31I, 1986 Printed in Great Britain.
0198-7593/86 $3.00 + .00 Pergamon Journals Ltd
WORKING FLUIDS FOR HIGH TEMPERATURE HEAT PUMPS M. A. R. EISA,* R. BESTand F. A. HOLLAND Department of Chemical and Gas Engineering, University of Salford, Salford M5 4WT, U.K.
(Received 10 January 1986) Abstract--The suitability of working fluids for use in high temperature heat pump systems has been considered on the basis of their volumetric and other thermodynamic properties. The change of thermodynamic properties with temperature has been graphically illustrated. It is shown that water becomes increasingly attractive as a working fluid as the delivery temperature approaches 200°C. A heat pump using water as the fluid is capable of an actual coefficient of performance of five and a gross temperature lift of 70°C for a condensation temperature of 200°C.
(HPV) Pco Per Qco QEv
rc rco r~ rEv rs (VPH) (VR) w
coefficient of performance [dimensionless] coefficient of performance actually obtained from the heat pump [dimensionless] theoretical Rankine coefficient of performance of the heat pump [dimensionless] compression ratio of the heat pump = Pco/Pev [dimensionless] flow rate of working fluid [m~s-1] enthalpy per unit mass of working fluid at state condition X [kJ kg -~] heat pump effectiveness compared to the Rankine coefficient of performance [dimensionless] latent heat of condensation per unit volume of vapour [MJ m -z] vapour pressure of condensing working fluid [bar] vapour pressure of evaporating working fluid [bar] rate of heat delivered by the condensing working fluid [kW ] rate of heat absorbed by the evaporating working fluid [kW] critical temperature of the working fluid [°C or K] temperature of condensing working fluid [°C or K] temperature of the heat delivered by the heat pump [°C or K] temperature of evaporating working fluid [°C or K] temperature of the heat extracted from the heat source [°C or K] volume of vapour per unit latent heat of condensation [m~MJ -~] ratio of volume of vapour to volume of liquid [dimensionless] rate of work delivered to the shaft of the compressor [kW]
Heat pumps are devices for raising the temperature of low grade heat energy to a more useful level using a relatively small amount of high grade energy. They are amplifiers or multipliers of useful heat and the coefficient of performance is a measure of how many times more effective the heat pump is as a supplier of heat than the high grade energy would be if used directly for heating. The most common type of heat pump is the vapour compression heat pump using a mechanical compressor as shown schematically in Fig. 1. It consists of two heat exchangers, a compressor, an expansion valve and a working fluid. In the evaporator heat exchanger, the working fluid evaporates at an absolute temperature Trv while extracting an amount of heat Qtv from the source which may be in the gaseous, liquid or solid state. The working fluid is then compressed and gives up an amount of latent heat Qco at a higher absolute temperature /'co in the condenser heat exchanger. The condensed working fluid is then expanded through the expansion valve and is returned to the evaporator to complete the cycle.
*Mahmoud Abdel Rahman Eisa, Cairo, Egypt. 305
M . A . R . EISA et
High ( grade • Energy input
r !v i.v, Fig. 1. Schematic diagram for a mechanical vapour compression heat pump.
The coefficient of performance of a compressor driven vapour compression heat pump can be written in the form ( C O P ) = Qco _ Qco W Qco - QEv"
The selection of a suitable working fluid is a critical step in the design of a heat pump :system, An ideal working fluid should be: (1) (2) (3) (4)
thermodynamically and physically favourable thermally and chemically stable safe (nonflammable, nontoxic, nonexplosive) readily available and not too expensive
and (5) compatible with the materials of construction and lubricant used in the compressor. The selection of a working fluid for a particular application is always a compromise between the various factors.
THERMODYNAMIC CONSIDERATIONS The critical temperature of the working fluid provides the upper limit at which a condensing vapour heat pump can deliver heat energy. The workingfluid should be condensed a t a temperature sufficiently below the critical temperature to provide an adequate amount o f latent heat per unit mass. As the condensing temperature approaches the critical temperature of the working fluid, the latent heat of vaporisation decreases rapidly. Consequently, the amount of working fluid required to be circulated increases. This implies an increase in the work of compression per unit of heat transferred so that the coefficient of performance decreases. The condensing pressure of the working fluid should also be significantly below its critical pressure. Moderate condensing pressures allow the use of light weight materials in construction. thereby reducing the size, weight and cost of the equipment. However the evaporating pressure should be above atmospheric to avoid air leaks into the system. Therefore the working fluid should have a reasonably high evaporating pressure PEr and a relatively low condensing pressure Pco implying a low compression ratio (CR) = Pco/Prv. Low compression ratios result in a low power consumption for the compressor and a high volumetric efficiency.
Working fluids for high temperature heat pumps
The latent heat of vaporisation of the working fluid should be as large as possible at the normal operating temperature of the system so that the maximum amount of heat can be transferred for a given circulation rate. There are four important parameters to consider when selecting a working fluid for a vapour compression heat pump system : (l) (2) (3) and (4)
temperature of condensation Tco, the compression ratio (CR)= Pco/PEv, the gross temperature lift (Tco- TEv), the theoretical Rankine coefficient of performance (COP)R.
These parameters are not independent and when two are specified, the other two are determined automatically for a given working fluid. The net temperature lift (To- Ts) is of course less than the gross temperature lift (Tco- TEv) by the sum of the temperature difference driving forces in the condenser and evaporator heat exchangers. The actual coefficient of performance (COP)A is also less than the theoretical Rankine coefficient of performance (COP)R and the difference in both cases is a function of the design of the equipment. A heat pump effectiveness compared to the theoretical Rankine cycle can be defined as
(COP)A (HPE)R = (COP)---~R"
The (HPE)R is effectively the ratio of work required in an ideal Rankine cycle to that in an actual cycle in order to transfer a given amount of heat to the condenser. It is possible to exceed (HPE)R values of 0.8 in a well designed heat pump system. The volumetric thermodynamic properties of a working fluid have a greater significance than the mass based properties in the design and performance of heat pump systems. Of particular importance are the volume per unit latent heat of condensation (VPH) and its inverse the latent heat of condensation per unit volume (HPV) together with the ratio of the volume in the vapour state to the volume in the liquid state (VR) at a given temperature.
COMPARISON OF WORKING FLUIDS The vapour pressure against temperature curve for a particular working fluid determines the condensing and evaporating pressures corresponding to a given pair of condensing and evaporating temperatures. Figure 2 shows the vapour pressure against temperature curves for ten common working fluids. It is seen that the rate of increase of vapour pressure with temperature increases with temperature. The rate of increase is highest for ammonia R717 and lowest for water R718 [I-3]. It is clear that of the ten working fluids represented in Fig. 2, only R11 (trichiorofluoromethane), methanol, ethanol, R I 13 (Trichlorotrifluoroethane) and water can be considered for use in high temperature heat pumps. Working Fluid RII4B2 (dibromotetrafluoroethane) is not shown in Fig. 2, as it has similar thermodynamic properties as R113 in the range of temperatures considered. Figure 3 is a plot of the theoretical Rankine coefficient of performance (COP)R and the compression ratio (CR) for a gross temperature lift (Tco- Tev) = 40°C for R113, RI 1, methanol, ethanol and water, Water is more attractive than the other four working fluids on the basis of theoretical Rankine coefficient of performance and in fact is the only contender at temperatures approaching 200°C. Figure 4 is a plot of the vapour volume per unit latent heat of condensation (VPH) against condensation temperature Tco for RI I, methanol, ethanol, R113 and water. The plot shows that the (VPH) value for water is much too high for it to be considered for use at temperatures little more than 100°C. However the decrease of (VPH) with increasing temperature makes water increasingly attractive at the higher temperatures.
M . A . R . E]SA et al.
Fig. 2, Vapour pressure vs temperature.
5- E W M h a ! e r ~ n a t n ~
~ R - I I 3
( Tco- TEv)= 40:C
Condensmqternl~ure ( Too,*C ) Fig. 3. Theoretical Rankine coefficient of performance and compression ratio vs condensation temperature.
Working fluids for high temperature heat pumps
fB E 0.8 > 0.7 g
~o I I00
I 150 Condensing temperature ( Tco,*C )
Fig. 4. Volume of vapour per unit latent heat of condensation vs condensation temperature.
The size of the compressor is based on the volumetric flow rate of working fluid. The vapour volume per unit latent heat of condensation (VPH) and the compression ratio (CR) together determine the power of the motor required to drive the compressor. The volumetric flow rate of the working fluid into the compressor can be determined from the equation
F = QEv(VPH).
Figure 4 indicates that the higher the condensation temperature, the smaller the compressor for a given heat load and gross temperature lift. Figure 5 is a plot of the ratio of the volume of vapour to the volume of liquid (VR) against temperature for R113, R11, methanol, ethanol and water. The higher the value of (VR) the greater the inefficiency in the compressor. Figure 6 is a plot of gross temperature lift (Tco- TEv) and latent heat of condensation per unit volume of vapour (HPV) for RI 1 and water for a theoretical Rankine coefficient (COP)R = 6.25. From equation (2) this would be equivalent to an actual coefficient of performance (COP),~ = 5.0 for a heat pump effectiveness (HPE)R = 0.8. Insufficient reliable data at temperatures higher than 150°C exist to justify the inclusion of methanol and ethanol in Fig. 6. RI13 is omitted from Fig. 6 since it is less thermodynamically attractive than R11. Thermodynamically R11 is a fairly attractive working fluid at temperatures up to about 150°C. However it has a tendency to break down in the presence of lubricants . CONCLUSIONS The volumetric thermodynamic properties of a working fluid have a greater significance than the mass based properties in the design and performance of heat pump systems. R114 (dichlorotetrafluoroethane) is an excellent working fluid for use in the heat pumps designed to deliver heat at temperatures up to about 120°C. However there is a lack of suitable working fluids for use in heat pumps required to deliver heat in the temperature range 120 to about 180°C
150 Temperoture (°C)
Fig. 5. Ratio of volume of vapour to volume o f liquid vs temperature.
150 200 Condensing temperoture ( Too *C )
Fig, 6. Gross temperature lift and latent heat of condensation per unit volume vs condensing temperature for a theoretical Rankine coefficient of performance (COP)R = 6.25.
Working fluids for high temperature heat pumps
. At temperatures a p p r o a c h i n g 200°C water becomes increasingly attractive as a working fluid. High temperature heat p u m p s using water as the working fluid are capable o f both high temperature lifts and high coefficients o f performance. A n actual coefficient o f performance o f five can be obtained with a gross temperature lift o f a b o u t 70°C with a condensation temperature o f 200°C. Research workers at Electricit6 de France have delivered heat at 246°C using water as the working fluid with a commercially available Atlas C o p c o twin screw compressor . REFERENCES 1. F. A. Holland, F. A. Watson and S. Devotta, Thermodynamic Design Data for Heat Pump Systems, Pergamon Press, Oxford (1982). 2. ASHRAE Handbook--Fundamentals, American Society of Heating Ventilating and Air-Conditioning Engineers, New York. (1977). 3. N. B. Vargaftik, Tables on Thermophysical Properties of Liquids and Gases, 2nd edn. Hemisphere, Washington (1975). 4. P. Srinivasan, S. Devotta and F. A. Watson, Thermal stability of RI 1, RI2B1, R113 and R114 and compatibility with some lubricating oils. Chem. Engng Res. Des. 63, 230-234 (1985). 5. F. Moser and H. Schnitzer, Heat Pumps in Industry, Elsevier, Amsterdam (1985). 6. B. Dcgueurce, F. Banquet, J. P. Denisart and D. Favarat, Use of twin screw compressors for steam compression. Paper presented at 2nd BHRA Internationl Symposium on the Large Scale Applications of Heat Pumps, York, England, pp 189-196 (1984).