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[en] Highlights: • Frost retarding and defrosting studies published in 2000–2017 are reviewed. • Two types of 12 frost retarding measures are classified and analyzed. • 5 defrosting methods and 6 improvement methods are summarized. • Initiation and termination control strategies of defrosting operation are presented. • The existing gaps in the research works are identified and classified as 5 aspects. - Abstract: Air source heat pump (ASHP) units have found worldwide applications due to their advantages of high energy efficient and environmental friendly. Frost deposition and accumulation on the surface of the outdoor coil in an ASHP unit is inevitable and always play significant negative effects. To accurately predict and control a frosting-defrosting cycle, the interrelated heat, mass, and momentum transport phenomena within frost, melted frost and at the air-frost interface, a moving boundary condition, should be clearly understood. This review paper focuses on the developments in frost retarding and defrosting investigations for ASHP units from 2000 to 2017. 12 frost retarding measures and 5 defrosting methods are firstly introduced, followed by 6 typical system optimization methods during reverse cycle defrosting. Alternative control strategies to start and end a defrosting operation are thereby presented. Basing on previous analysis, the existing gaps in the research works on frost retarding and defrosting are identified, and recommendations are finally offered as per the viewpoint of the present authors. This comprehensive and systematic review around an entire frosting-defrosting cycle might provide an overview of the analytical tools for scholars, researchers, product developers, and policy makers, and shed new light on the designing and performance optimization of ASHP units.
[en] Highlights: • Heat performances of RIASHP system for EVs is designed and tested at −20 °C out-car. • Two scroll compressors are used to analyze the effect of injection portholes shapes. • Heating capacity of RIASHP system is raised up to 28.6% than traditional system. • The refrigerant cycle and injection process and the influence factors are discussed. • Summary the analysis: bigger area of injection porthole make better heat capacity. - Abstract: The traditional Air Source Heat Pump (ASHP) for Electric Vehicles (EVs) has many limits in cold region because of poor heating performance and operating safety in low ambient temperature, which can be solved by the ASHP with refrigerant injection. A test bench that can be switched between a traditional ASHP system and a refrigerant injection ASHP system for EVs in the cold region is developed in this study. Compressors applied in the test bench are refitted with different injection portholes from the regular electric scroll compressor to analyze the influence of porthole shape on the system performance. The experimental results show that the heating capacity of refrigerant injection ASHP system is raised up by 28.6% compared with the traditional system. The larger injection porthole helps to increase the heating capacity when in-car inlet air temperature is higher, and the effect of injection porthole shapes to injection process and refrigerant cycle is also analyzed. The research will contribute to the application of refrigerant injection technology in EVs.
[en] The dynamic performance characteristics of the air source heat pump (ASHP) with refrigerants R22 and R407C during frosting and defrosting are studied. The results show that both refrigerant systems have similar performance characteristics, except that the performance of the R407C system deteriorated faster than that of the R22 system under frosting, and the performance of the R407C system attains its steady state faster than that of the R22 system after defrosting. R407C refrigerant can be used in either existing systems or in new systems that were originally designed for R22
[en] In this study, the performance of an air source heat pump system installed in a commercial building is analyzed using the developed heat pump performance model and building load simulation data of several regions in Korea. The performance test of an air source heat pump system with a variable speed compressor is tested to develop model that considers changes in the performance characteristics of the heat pump system under various operating conditions. The heat pump system is installed in an environmental chamber, and the experimental equipment is set up according to the manufacturer' specifications as well as the AHRI 1230 test specifications. The performance test conditions of the heat pump system are selected using a central composite design method, in which 29 points for each cooling and heating mode are selected. The developed performance model based on experimental data predicts experimental values with an error of ±5 %. Building cooling and heating loads in three regions in Korea are analyzed using TRNSYS software, which includes standard building and weather data from Seoul, Daejeon and Busan in Korea. The effects of outdoor air temperature and part load ratio on the performance and regional monthly average power consumption of the heat pump system are analyzed
[en] Rotary compressor is most widely used in small capacity refrigeration and heat pump systems. For the air source heat pump, the heating capacity and the COP will be obviously degraded when it is utilized in low temperature ambient. Gas injection is an effective method to enhance its performance under those situations, which has been well applied in air source heat pump with scroll compressor. However, the development of the gas injection technology in rotary compressor is relatively slow due to limited performance improvement. In this research, the essential reason constraining the improvement of the gas injection on the rotary compressor and its heat pump system is identified. Two new injection structures for rotary compressors has been put forward to overcome the drawback of traditional injection structures. Based on a verified numerical model, the thermodynamic performance of air source heat pumps with the new gas-injected rotary compressor are investigated. The results indicate that, compared to the air source heat pump with the traditional gas injected rotary compressor, the new injection structures both can enhance heating capacity and COP of the air source heat pump obviously. (paper)
[en] Highlights: • An ASHP system with refrigerant injection for EVs is designed, for cold regions. • The heat performances of the system are tested at −20 °C ambient temperature. • The system cycle process with refrigerant injection are analyzed on lgP-H diagrams. • The effects of refrigerant injection, dryness, and in-car inlet state are discussed. • The new system can improve heating and own better application prospect. - Abstract: Since the performance of conventional air source heat pump (ASHP) for electric vehicles (EVs) is apt to decline sharply in low ambient temperature, it will consume more electricity of the cell, and affect driving mileage in cold regions. Aiming at developing high efficiency heating system for EVs in cold regions, an ASHP system applying refrigerant injection for EVs is designed, as well as the test bench is built to investigate its performance. According to the operation condition of EVs, heating performances are tested on different in-car inlet air temperature and various fresh air ratios under −20 °C ambient temperature. The system cycle process with refrigerant injection, as well as the influences of refrigerant injection and dryness are also analyzed and discussed. The results show that the heating capacity of the ASHP with refrigerant injection can be increased up to 31%, and in comparison with the conventional heat pump system its heating performance is better when in-car inlet temperature is above −10 °C. Therefore, ASHP with refrigerant injection has great potentiality to be applied for the EVs in cold regions
[en] The double whammy dealt by the economic crisis and housing slump has stifled expansion of the ground-source heat pump market in many European countries. The European Union market contracted for the second year running (by 2.9% between 2009 and 2010), and this despite the fact that more than 100,000 units were sold over the twelve-month period, taking the number of installed units past the one million mark.
[fr]La crise economique ainsi que la crise immobiliere qui touchent de nombreux pays europeens ne facilitent pas l'essor du marche de la pompe a chaleur geothermique. Pour la deuxieme annee consecutive, le marche de l'union europeenne est en baisse (-2,9 % entre 2009 et 2010). il parvient tout de meme a se maintenir au-dessus des 100 000 unites vendues par an, ce qui lui permet de depasser pour la premiere fois le cap du million d'unites installees.
[en] The effects of air flow maldistribution on the performance of an air source heat pump chiller under frosting conditions were investigated experimentally. The results indicated that air flow maldistribution was the dominant factor leading to hunting of the thermostatic expansion valve for medium and/or large size finned tube evaporators. With air flow maldistribution degree (AMD) increasing, frost occurred earlier, and the frost layer grew faster. The operating characteristics became lower when AMD was increased. We found such phenomenon seemed to be related to both the difference of refrigerant outlet superheat and the frosting velocity. In the hunting stage, the frost block effect became the main factor degrading the refrigeration system performance. With AMD increasing, the heat pump system pertinent performance data (suction pressure, evaporation temperature, discharge pressure, refrigerant outlet temperature, etc.) were degraded more dramatically
[en] It is difficult for an air source heat pump (ASHP) system to provide enough heat for heating buildings with high working efficiency when it runs at low ambient temperature. An advanced ASHP system that, in the paper, composes a main refrigerant circle and a bypass circle would supply a lot of heat for the buildings in Beijing at same working condition. In the system, the bypass refrigerant can be used to increase the density of refrigerant at the inlet of the hermetic compressor employed in refrigeration cycle, and so, the advanced ASHP could appear relative high efficiency even when it runs at low ambient temperature. The assistant throttle apparatus in the bypass circle is a part of capillary that was enlaced tightly around to the compressor. The capillary can be used to reuse the heat from surface of the compressor. Compared with a traditional system, the advanced ASHP system could provide three times heat capacity and more than 35% of efficiency at the working condition -10 oC.
[en] Highlights: • A special experimental rig was built and its details are reported. • The negative effects of downwards flowing of the melted frost were shown. • Defrosting duration was shortened after installing water collecting trays. • Temperature of melted frost decreased after installing trays. - Abstract: When the surface temperature of the outdoor coil in an air source heat pump (ASHP) unit is lower than both freezing point of water and the air dew point, frost can be formed and accumulated over outdoor coil surface. Frosting affects the energy efficiency, and periodic defrosting therefore is necessary. Reverse cycle defrosting is currently the most widely used defrosting method. A previous related study has indicated that during reverse cycle defrosting, downwards flow of the melted frost over a multi-circuit outdoor coil could affect the defrosting performance, without however giving detailed quantitative analysis of the effects. Therefore an experimental study on the effects has been carried out and a quantitative analysis conducted using the experimental data. In this paper, the detailed description of an experimental ASHP unit which was specifically built up is firstly reported. This is followed by presenting experimental results. Result analysis and conclusions are finally given