In situ measurement methods of air to air heat pump performance

There is no in situ reliable measurement method of air-to-air heat pump heating performances. This paper tests and validates two methods in laboratory and in steady-state conditions. The first method based on refrigerant fluid measurements determines the refrigerant flow rate by using the compressor thermal balance. In particular, the evaporation pressure is measured by a saturation temperature measurement and the compressor ambient heat losses are evaluated from the heat exchanges. The method uses only non-intrusive sensors, except for the condensation pressure sensor installed at the refrigerant charging plug. The second method based on air measurements determines the air flow rate via a multi-point velocity measurement. According to the experimental results, these methods are fully applicable on field with deviations of 4% and 10%, for the refrigerant method and the air method respectively, when compared to the “etalon” measurement.     

Theoretical analyses of a new two-stage absorption-transcritical hybrid refrigeration system

In this context, a two-stage absorption-transcritical hybrid refrigeration system is proposed. R744 is chosen as a refrigerant for the transcritical heat pump subsystem and LiBr-H₂O working pair for the two-stage absorption refrigeration subsystem. Based on the mathematical and physical models, theoretical investigation is carried out on its performance. The main effects are discussed on COP_net (the ratio of cooling capacity powered by low-grade heat to the low-grade heat consumption for the hybrid system) and COP_mt (the ratio of cooling capacity powered by mechanical work to the mechanical work consumption for the hybrid system). Comparing with the normal two-stage absorption refrigeration system, theoretical results show that COP_net could be improved up to about 55% when the refrigeration temperature is 7 °C. In addition, COP_mt are more than 50% higher than that of the conventional transcritical refrigeration system. It is also found that both 45–55 °C low-grade heat and condensing heat could be used as actuating heat of the two-stage absorption refrigeration subsystem.     

CO2 heat pump systems

After the CFCs and the HCFCs were deemed unfit as working fluids in refrigeration, air conditioning, and heat pump applications, there has been a renaissance for carbon dioxide technology. Heat pumps is one of the application areas where theoretical and experimental investigations are now performed by an increasing number of research institutions and manufacturers. This paper gives an overview of some of the current activities in the CO₂ heat pump field. Discussed are the important characteristics of the transcritical CO₂ process applied to heat pumps, and also discussed are theoretical and experimental results from several heat pump applications. Provided that calculations and system designs are performed on the premises of the working fluid, and that test plants are constructed and operated to fully exploit the specific characteristics of both the fluid and the transcritical process, the results show that CO₂ is an attractive alternative to the synthetic fluids. Competitive products may be launched in the near future.     

Effects of refrigerant charge amount on the performance of a transcritical CO2 heat pump

A typical transcritical CO₂ system shows lower performance than conventional air conditioners in cooling mode operation. In addition, the CO₂ system shows a large variation of the performance according to refrigerant charge whereas the conventional systems do not show large variation. In this study, the performance of the CO₂ heat pump was measured and analyzed by varying the refrigerant charge amount at standard cooling condition. In addition, the performance sensitivity of the CO₂ system as a function of refrigerant charge was compared to those for the R22, R410A, and R407C systems. The cooling COP of the CO₂ system was reduced more significantly at undercharged conditions than at overcharged conditions as the deviation from the optimal charge increased. The expansion loss was the dominant factor affecting system performance at undercharged conditions, while the gascooler loss became the major parameter at overcharged conditions. Among the systems investigated and compared in this study, the CO₂ system showed the most reduction in performance at undercharged conditions.     

Investigation of R-744 Voorhees transcritical heat pump system

Using economizer in R-744 heat pump cycle is an effective way to improve the heating capacity in cold climates. In this paper, a modification construction of reciprocating compressor with economizer port, a Voorhees compressor was introduced and the heat pump cycle with Voorhees economizer was compared with the traditional screw or scroll economizer cycles. Both the R-744 transcritical heat pumps with and without Voorhees economizer were tested at the same conditions with different air mass flow rates and different evaporating temperatures. The results show that the heating capacity of the heat pump with Voorhees economizer can be two times higher than the transcritical heat pump without economizer at low evaporating temperature conditions. At the same capacity operation conditions, the efficiency of the heat pump with Voorhees economizer is higher at high refrigerant mass flow rate conditions. The optimum discharge pressure of the heat pump with Voorhees economizer is found to be higher than the heat pump without economizer at the same ambient conditions. For mobile heat pump application, CO₂ transcritical heat pump with Voorhees economizer demonstrates better performance comparing to the conventional transcritical CO₂ heat pump without economizer when the evaporating temperature is lower than −20 °C, or when the mobile is idling with low compressor RPM.     

A study on the performance of multi-stage condensation heat pumpsEtude sur la performance des pompes à chaleur à condensation en plusieurs étages

In this study, computer simulation programs were developed for multi-stage condensation heat pumps and their performance was examined for CFC11, HCFC123, HCFC141b under the same condition. The results showed that the coefficient of performance (COP) of an optimized ‘non-split type’ three-stage condensation heat pump was 25–42% higher than that of a conventional single-stage heat pump. The increase in COP differed among the fluids examined. The improvement in COP was due largely to the decrease in average temperature difference between the refrigerant and water in the condensers, which resulted in a decrease in thermodynamic irreversibility. For the three-stage heat pump, the highest COP was achieved when the total condenser area was evenly distributed to the three condensers. For the two-stage heat pump, however, the optimum distribution of total condenser area varied with working fluids. For the three-stage system, splitting the condenser cooling water for the use of intermediate and high pressure subcoolers helped increase the COP further. When the individual cooling water for the intermediate and high pressure subcoolers was roughly 10% of the total condenser cooling water, the optimum COP was achieved showing an additional 11% increase in COP as compared to that of the ‘non-split type’ for the three-stage heat pump system.     

A novel defrosting control method based on the degree of refrigerant superheat for air source heat pumps

When an air source heat pump (ASHP) unit operates for space heating at a frosting environment, periodic defrosting is necessary to maintain a high system performance. To defrost efficiently, it is necessary to find an effective defrosting control method. In this paper, an experiment was carried out on an ASHP unit with a capillary tube as a throttle device, under simulated frosting and defrosting conditions using time control defrosting method, and the experimental results are firstly presented. Secondly, a novel defrosting control method based on the degree of refrigerant superheat (DS) is reported. To validate the novel defrosting control method, a further experiment was conducted on another ASHP unit with an electronic expansion valve (EEV) as a throttle device, under simulated frosting and defrosting conditions. The experimental results demonstrated that when applying the novel defrosting control method, defrosting was initiated before the operating performances of ASHP unit rapidly deteriorated, which was more reasonable.     

Modelling the performance of a transcritical CO2 heat pump for high temperature heating

A prototype transcritical CO₂ heat pump was constructed for heating water to temperatures greater than 65°C while providing refrigeration at less than 2°C. The heating capacity was 115 kW at an evaporation temperature of +0.3°C and a hot water temperature of 77.5°C, with a heating coefficient of performance (COP) of 3.4. Performance data is presented for each of the compressor, the gas cooler, and the recuperator as well as for the overall heat pump system. Equipment performance data was incorporated into a computer model to enable parametric investigations of heat pump performance. Model predictions showed that the hot water temperature could be increased from 65 to 120°C with a relatively small reduction in heating capacity and heating COP of 33 and 21%, respectively. Model predictions also highlight the potential for significant capacity improvements by eliminating the recuperator in favour of a larger gas cooler.     

Heating performance enhancement of a CO2 heat pump system recovering stack exhaust thermal energy in fuel cell vehicles

A CO₂ heat pump system using recovered heat from the stack coolant was provided for use in fuel cell vehicles, where the high temperature heat source like in internal combustion engine vehicles is not available. The refrigerant loop consists of an electric drive compressor, a cabin heater, an outdoor evaporator, an internal heat exchanger, an expansion valve and an accumulator. The performance characteristics of the heat pump system were investigated and analyzed by experiments. The results of heating experiments were discussed for the purpose of the development and efficiency improvement of a CO₂ heat pump system, when recovering stack exhaust heat in fuel cell vehicles. A heater core using stack coolant was placed upstream of a cabin heater to preheat incoming air to the cabin heater. The performance of the heat pump system with heater core was compared with that of the conventional heating system with heater core and that of the heat pump system without heater core, and the heat pump system with heater core showed the best performance of the selected heating systems. Furthermore, the coolant to air heat pump system with heater core showed a significantly better performance than the air to air heat pump system with heater core.     

Hybrid absorption heat pump with ammonia/water mixture – Some design guidelines and district heating application

An ammonia/water mixture can be used as an efficient working fluid in industrial-type heat recovery heat pumps and heat transformers. Several configurations of such systems are possible depending on the availability of the waste (thermal) and primary (thermal or electrical) energy sources. This article presents the configurations, the main thermodynamic and hydraulic parameters, and some design guidelines and operating experiences of a medium-temperature, ammonia/water-based compression/re-sorption heat recovery system for district domestic hot water production. In-field experiments have proven the advantages of the concept and its applicability limits in a particular economical environment, while hot water was produced at 55 °C with industrial cooling water at 36 °C as a waste heat source.     

Performance evaluation of a CO2 heat pump system for fuel cell vehicles considering the heat exchanger arrangements

A novel CO₂ heat pump system was provided for use in fuel cell vehicles, when considering the heat exchanger arrangements. This cycle which had an inverter-controlled, electricity-driven compressor was applied to the automotive heat pump system for both cooling and heating. The cooling and heating loops consisted of a semi-hermetic compressor, supercritical pressure microchannel heat exchangers (a gas cooler and a cabin heater), a microchannel evaporator, an internal heat exchanger, an expansion valve and an accumulator. The performance characteristics of the CO₂ heat pump system for fuel cell vehicles were analyzed by experiments. Results for steady and transient state performance were provided for various operating conditions. Furthermore, experiments to examine the arrangements of a radiator and an outdoor heat exchanger were carried out by changing their positions for both cooling and heating conditions. The arrangements of the radiator and the outdoor heat exchanger were tested to quantify cooling/heating effectiveness and mutual interference. The improvement of heating capacity and coefficient of performance (COP) of the CO₂ heat pump system was up to 54% and 22%, respectively, when using preheated air through the radiator instead of cold ambient air. However, the cooling capacity quite decreased by 40–60% and the COP fairly decreased by 43–65%, for the new radiator-front arrangement.