Fluids for high temperature heat pumps

The theoretical performances of some 250 potential work fluids in vapour compression heat pumps condensing at 150°C and evaporating at 100°C have been predicted, using expression for coefficient of performance (COP) and minimum superheat that involve only easily accessible physical properties. Expected correlations were found between COP and critical temperature, between specific compressor displacement and normal boiling point, T_bp, and between condensing pressure and T_bp. Correlations were also found between minimum superheat and both molecular weight and critical pressure. From these correlations, the desirable basic properties of a high temperature heat pump fluid are deduced. The principle of corresponding states is invoked to explain the connection between minimum superheat and critical pressure, and hence the reason why perfluorinated compounds tend to make poor work fluids.     

Calculation of the performance of vapour compression heat pumps with solution circuits using the mixture R22-DEGDME

In this paper the results of performance calculations of two vapour compression heat pump cycles, one with a single-stage solution circuit and one with a two-stage solution circuit, are presented. As a working fluid combination R22-DEGDME was selected out of a number of candidates in order to employ a safe and non-toxic mixture. It was found that both cycles show a significant increase in coefficient of performance (COP) (up to 50% for the two-stage cycle) compared to R22. The two-stage cycle shows a pressure ratio which is only 45% that for pure R22, resulting in increased mass flow rate but reduced capacity.     

Supercritical heat pump cycles

Supercritical heat-pump cycles suited for high-temperature heat generation and in which heat is delivered in the form of sensible heat of a high-pressure fluid are examined and their energy performance is evaluated. The main variables governing the energy efficiency of the process and the temperatures of the heat produced are recognized to be the fluid critical temperature, the molecular complexity, the top cycle pressure and the amount of internal regeneration of heat. Two cycle configurations are examined: one featuring fluid compression after a regenerative preheating and one that also includes turbine expansion of a fraction of the high-pressure fluid in order to achieve a more effective regeneration. General diagrams giving the operating characteristics of a supercritical heat-pump cycle for any kind of fluid are reported. Some fluids are presented (SF₆, C₃F₈, C₂HF₅, c-C₄F₈), which exhibit a high level of thermal stability and are thermodynamically suitable for supercritical cycles: for each one a detailed performance chart is given. An example application in which a conventional high-temperature cycle is compared with two supercritical solutions is presented. The following conclusions summarize the findings of the thermodynamic analysis. (1) In supercritical cycles high heat-output temperatures are achievable with moderate compressor pressure ratios and with a comparatively simple cycle arrangement, while conventional cycles require a large pressure ratio and a complex cycle organization. Sub-atmospheric pressures, which may be required in conventional cycles, can be avoided. (2) As heat is available in supercritical cycles within a certain temperature range, applications implying the use of heat at variable temperature could benefit from the natural matching between temperature availability and process requirements. (3) The comparatively high pressures at which heat is produced in supercritical cycles could represent a drawback for small-capacity plants but are probably acceptable or even beneficial for large systems. (4) The internal regeneration of a sizeable amount of heat, which is requested in supercritical cycles, represents a definite cost item for this type of heat pump.     

Compression heat pump with solution circuit Part 1: design and experimental results

This Paper presents the design and experimental results of a compression heat pump with solution circuit (CHSC) that has been constructed at the Swiss Federal Institute of Technology. The CHSC offers two major advantages over a single fluid Rankine cycle: 1, the heating capacity is easily varied by a large factor by adjusting the composition of the mixture; and 2, the approximation of the Lorenz process allows for substantially high COP values in cases with gliding temperatures. The test plant heats water from 40 to 70°C and cools water from 40 to 15°C. The heating capacity can be varied from 5 to 15 kW. A COP of 4.3 was measured, representing an energy saving of 23%, compared with a good single fluid Rankine cycle. Measured heat and mass transfer coefficients are presented and discussed.     

Performance analysis of ammonia absorption GAX cycle for combined cooling and hot water supply modes

An ammonia Generator–Absorber heat eXchange (GAX) absorption cycle with combined cooling and hot water supply modes is developed in this study. This paper proposes new multi-modes GAX cycles which function in three different modes (case 1, case 2 and case 3) of cooling and hot water supply with one hardware (ammonia/water GAX absorption heat pump), and finds the best cycle for performance improvement by the parametric analysis. The key parameters are the outlet temperature of hot water and the split ratio of the solution. It is found that the COP_c values for case 1, case 2 and case 3 are 60%, 42% and 87% of COP_c for case 0, respectively, which is the standard cooling mode for the conventional GAX cycle. From the viewpoints of hot water supply, case 1 gives the best performance. However, during the summer season when the cooling mode is the primary purpose rather than the hot water supply, case 3 is the most desirable cycle. The split ratio of the solution should be carefully determined depending on the primary application of the modified GAX cycle; cooling or hot water supply applications. It is also recommended that the optimum design values of UA_SCA and UA_HCA for case 3 should be less than those for case 1.     

Zeolite heat pump — adsorption of methanol in synthetic zeolites 13X, 4A and 5A

The adsorption isotherms for methanol adsorption in synthetic zeolites 13X, 4A and 5A were measured gravimetrically at various temperatures. The model of Langmuir and the potential theory were used for correlating the equilibrium data. The monolayer capacity was calculated using these two models. A small deviation at elevated temperatures was observed. The differential heat of adsorption was measured calorimetrically for all three zeolites. The isosteric heat of adsorption was calculated for zeolite 13X using measured equilibrium data. The log p, 1/T equilibrium diagram for zeolite 13X was computed using the Langmuir equation.     

Development of an ejector cooling system with thermal pumping effect

This paper presents a feasibility study of an ejector cooling system (ECS) that utilizes a multi-function generator (MFG) to eliminate the mechanical pump. The MFG serves as both a pump and a vapor generator. The MFG is designed based on the pressure equilibration between high and low pressures through heating and cooling process. In this design, an ECS that contains no moving components and is entirely powered by heat can be practicable. A prototype using refrigerant R141b as working fluid was constructed and tested in the present study. The experimental results showed that the system coefficient of performance (COP_o) was 0.218 and the cooling capacity was 0.786 kW at generating temperature (T_G) 90 °C, condensing temperature (T_C) 32.4 °C and evaporating temperature (T_E) 8.2 °C. While taking into account the extra heat needed for the MFG operation, the total coefficient of performance (COP_t) is 0.185. It is shown that a continuous operation for the generation of cooling effect in an ECS with MFG can be achieved. This cooling machine can be very reliable since there is no moving part.     

Effects of metal mass on the performance of adsorption heat pumps utilizing zeolite 4A coatings synthesized on heat exchanger tubesEffets de la masse métallique sur la performance des pompes à chaleur à adsorption utilisant des revêtements en zéolite 4A synthétisé sur les tubes de l'échangeur de chaleur

The effects of the wall thickness of stainless steel heat exchanger tubes on the performance of adsorption machines, employing zeolite 4A coatings synthesized on metal heat exchanger tubes, are investigated. A recently developed mathematical model is used to determine the cycle durations when various wall thicknesses of the heat exchanger tubes as well as different zeolite layer thicknesses are utilized. For each case, the power and the COP_cycle values of the system are estimated. In general, very high power and quite low COP_cycle values are obtained when the proposed arrangement is utilized in the adsorption heat pumps. The zeolite layer thicknesses that may result in obtaining high COP_cycle values are generally much higher than the optimum layer thickness value that maximizes the power and the utilization of layers thicker than the optimum value may lead to significant extensions in the cycle durations and hence to a decrease in the power obtained from the system. Decreasing the wall thickness of the heat exchanger tubes increases both the power and the COP_cycle values when the optimum zeolite layer thickness for each wall thickness is taken into account. The possibility of such an enhancement will most probably be limited by the minimum wall thickness value that can actually be obtained by the available technology. The COP values of adsorption heat pumps may also be increased by using regenerative processes. Due to the generally low COP values obtained, the proposed arrangement seems especially suitable to be employed in adsorption machines utilizing energy sources of low economical value, such as waste heat. An optimum compromise between the COP value, which is closely related to the operating costs, and the power of the system should be provided, in case more valuable energy sources are utilized.     

Study of different internal vapour transports for adsorption cycles with heat regeneration

A ‘three-temperature’ model of the adsorption cycles with heat regeneration is used for investigating and analysing the influence of different parameters on the performance of such cycles. The influence of the heat source temperature on the thermodynamic efficiency (COP/COP_Carnot) is investigated. The result is that the thermodynamic efficiency of the cycle is always limited. In order to reduce the internal irreversibilities, different internal vapour transports for pressurising (depressurisation) the adsorber are investigated: first, adiabatic direct pressurisation (depressurisation) with the condenser (evaporator) instead of pressure changes by heat transfer; second, adiabatic internal vapour recovery between the adsorbers (partial pressurisation/depressurisation); third, separation of the adsorber into separate compartments between which vapour cannot be redistributed during pressurisation or depressurisation. Results show that the first process significantly reduces the COP, while the second one enhances the cooling power, and the third one does not change the performance. Analysis gives satisfactory explanation of these results.     

Experimental results for a hydraulic refrigeration system using n-butane

The hydraulic refrigeration system (HRS) is a vapor-compression system that accomplishes the compression and condensation of the refrigerant in a unique manner, by entraining refrigerant vapor in a down-flowing stream of water and utilizing the pressure head of the water to compress and condense the refrigerant. A multi-stage HRS was designed, fabricated, and tested using n-butane as the refrigerant. In general, both the refrigeration rate and the coefficient of performance (COP) increased with a corresponding decrease in the compression fluid temperature of the third and final stage. The refrigeration rate and COP were also found to increase with a corresponding increase in evaporator temperature. The predictions of an enhanced model incorporating two-phase hydraulic losses show excellent agreement with the experimental data with a maximum error of ±20%. The results of the experimental investigation indicate that the HRS offers an attractive and feasible alternative to conventional vapor-compression systems, especially in applications where direct-contact heat exchange in the evaporator is desirable.     

Review of advanced absorption cycles: performance improvement and temperature lift enhancement

The objective of this study is to propose and evaluate advanced absorption cycles for the coefficient of performance (COP) improvement and temperature lift enhancement applications. The characteristics of each cycle are assessed from the viewpoints of the ideal cycle COP and its applications. The advanced cycles for the COP improvement are categorized according to their heat recovery method: condensation heat recovery, absorption heat recovery, and condensation/absorption heat recovery. In H₂O–LiBr systems, the number of effects and the number of stages can be improved by adding a third or a fourth component to the solution pairs. The performance of NH₃–H₂O systems can be improved by internal heat recovery due to their thermal characteristics such as temperature gliding. NH₃–H₂O cycles can be combined with adsorption cycles and power generation cycles for waste heat utilization, performance improvement, panel heating and low temperature applications. The H₂O–LiBr cycle is better from the high COP viewpoints for the evaporation temperature over 0°C while the NH₃–H₂O cycle is better from the viewpoint of low temperature applications. This study suggests that the cycle performance would be significantly improved by combining the advanced H₂O–LiBr and NH₃–H₂O cycles.     

Amélioration de la performance des machines frigorifiques á absorption par l'utilisation de cycles à absorption et désorption étagés

This study deals with staged absorption and desorption cooling systems which increase the performance of absorption cycles that are driven by only low-grade energy, particularly when the working fluids are NH₃H₂0. Instead of working with only one absorber, these systems use a cascade of absorbers composed by one operating at the evaporator pressure, followed by a series of absorbers operating at staged pressures P_j, between P_ev and P_c In the same way, a cascade of generators is used for desorption. For the same operating parameters for other equipment and the same COP, the systems that we propose permit the generators to run at temperatures below those of all other systems offered up to now and using the same working fluids. When T_ev = −10°C, T_a = T_c = 30°C, the temperature of the generators can be as low as 65°C while the COP of the system is 0.258 and the COP_ex 0.317. By increasing the temperature of generators to 85°C while maintaining the other parameters at the same values, COP becomes 0.374 and the COP,, 0.336. These results improve the performance of absorption systems using only low-grade energy (T < 100°C). Particularly, they are better than the performance of two-stage absorption systems which consist of two single-stage absorption cycles coupled with each other through the evaporator of the first cycle and the absorber of the second cycle. With the same operating parameters indicated above for our system at the evaporator, the condenser, and the absorber, these coupled cycles need temperatures at generators of 80 and 100°C, whereas they give a COP of only 0.270     

Review article from ASHRAE symposium, San Francisco, January 1986: Transient performance and frosting of unitary heat pumps

Three papers relating to the transient performance and frosting of unitary air-source heat pumps were presented at this symposium. The titles and respective authors of the papers are:

1. 1. Effect of short cycling and fan delay on the efficiency of a modified residential heat pump by W.J. Mulroy, National Bureau of Standards.

2. 2. Air-to-air heat pumps operating under frosting conditions on the outside coil by C. Tantakitti and R. H. Howell, University of Missouri, Rolla.

3. 3. Design and preliminary analysis for measuring transient mass rate of flow in unitary heat pumps by M. I. Belth and D. R. Tree, Purdue University, West Lafayette.

A short review of each of these is given in this Paper.     

A theoretical study of an innovative ejector powered absorption–recompression cycle refrigerator

This paper describes a novel cycle which uses a steam ejector to enhance the concentration process by compressing the vapour from the lithium bromide solution to a state that it can be used to re-heat the solution from which it came. The energy efficiency and the performance characteristics of the novel cycle are theoretically investigated in this paper. The theoretical results show that the coefficient of performance (COP) of the novel cycle is better than the conventional single-effect absorption cycle. The characteristics of the cycle performance show its promise in using high temperature heat source at low cost.     

Selective water sorbent for solid sorption chiller: experimental results and modelling

In this paper the experimental results of a lab-scale chilling module working with the composite sorbent SWS-1L (mesoporous silica gel impregnated with CaCl₂) are presented. The interesting sorption properties of this material yield a high COP=0.6 that gives a promising alternative to the common zeolite or silica gel for application in solid sorption units driven by low temperature heat (T 100 °C). The measured low specific power of the device is a result of not optimised geometry of the adsorber and of the pelletised shape of the adsorbent. Heat transfer optimisation is currently under progress to increase the specific power. The experimental results are compared with those of a mathematic model able to describe the dynamic behaviour of the system. The model is used to study the influence of the main operating parameters on the system performance.     

Analysis of advantages and limitations of absorber-generator heat exchange

In the last few years increased interest has been shown in absorption heat pumps utilizing advanced cycles. One particular advanced cycle which works with very large solution volumes is the so-called absorber-generator heat exchange cycle. We will show in this Paper that the theoretical advantages of this cycle can be severely diminished by limitations which are due to the characteristics of the solution field of the working pair used. These limitations are also important if absorption processes are to be used to realize the Lorenz process.     

Carbon dioxide as a working fluid in drying heat pumpsLe dioxyde de carbone comme fluide actif dans des séchoirs à pompe à chaleur

Carbon dioxide as a working fluid in refrigeration and heat pump systems is increasingly important in view of the CFC substitution problem. It is both under ecological and economical aspects an attractive alternative to the HFC working fluids at present in practical use. The thermophysical properties and characteristics of carbon dioxide are quite different from those of refrigerants used in conventional vapour compression cycles. Its application in conventional vapour compression refrigerating systems is limited by its critical parameters (t_c=31.1°C and p_c=73.8 bar). The possibility to use carbon dioxide also beyond these limits in high temperature processes, e.g. heat pumps, is given by the application of a trans-critical process. The design and construction of a commercial drying heat pump system (batch type cabinet dryer with 12 kW heating capacity and closed air circuit) using the natural working fluid carbon dioxide is shown and experimental results of investigations carried out are presented. Energy savings are given compared to manufacturer's data of energy consumption.     

An elemental NTU- model for vapour-compression liquid chillers

This paper presents a steady-state model for predicting the performance of vapour-compression liquid chillers over a wide range of operating conditions. The model overcomes the idealisations of previous models with regard to modelling the heat exchangers. In particular, it employs an elemental NTU- methodology to model both the shell-and-tube condenser and evaporator. The approach allows the change in heat transfer coefficients throughout the heat exchangers to be accounted for, thereby improving both physical realism and the accuracy of the simulation model. The model requires only those inputs that are readily available to the user (e.g. condenser inlet water temperature and evaporator water outlet temperature). The outputs of the model include system performance variables such as the compressor electrical work input and the coefficient of performance (COP) as well as states of the refrigerant throughout the refrigeration cycle. The methodology employed within the model also allows the performance of chillers using refrigerant mixtures to be modelled. The model is validated with data from one single screw chiller and one twin-screw chiller where the agreement is found to be within ±10%.     

Feasibility study of a two-stage vapour compression heat pump with ammonia-water solution circuits: experimental results

A breadboard heat pump was designed and built to test the performance of a vapour compression heat pump with two-stage ammonia-water solution circuits. A major improvement in performance was obtained by incorporating a bleed line to attain water balance between the high and the low temperature solution circuits. The new scheme was first investigated by computer simulation and then incorporated in the experimental setup. The advantages of this scheme are reduced first cost (by eliminating the need for a rectifier), a simplified system and its control mechanisms, a 20–30% improvement in cooling coefficient of performance and a 10–15% increase in cooling capacity as compared to the system with a rectifier. Coefficients of performance in the range of 0.69 to 1.04 were obtained experimentally for a temperature lift of 100 K and cooling capacities in the range of 2.02 to 4.22 kW. The pressure ratios encountered were in the range of 6.9 to 11, which are 35 to 50% of the pressure ratio expected for a conventional single-stage heat pump.