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ABSTRACT
When air source heat pump (ASHP) units operate in heating mode at low temperatures in high humidity environments, frost forms and accumulates on the surface of its outdoor coils. This frost layer adversely degrades the operating efficiency of ASHP units rapidly, and can even result in sudden shutdown. Due to this operational problem, many researchers have conducted studies on frost-retardation measures. Since the frost that is present on the ASHP unit after shutdown has to be removed, defrosting becomes necessary. Among several reported defrosting methods, such as compressor shutdown defrosting, electric heating defrosting, hot water spray defrosting and hot gas bypass defrosting, the most popular method is reverse cycle defrosting. In addition, defrosting initiation and termination control play an important role in the frosting and defrosting cycle. Due to a scarcity of reviews of the literature on improving the frosting and defrosting performance of ASHP units, this paper provides a comprehensive review of experimental and modelling studies which explore this topic. This review can be used for guiding the future design and control strategy optimisation for ASHP units.
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Notes on contributors
Mengjie Song
Dr Mengjie Song, Ph.D., is an Associate Professor in the Department of Energy Engineering at the Guangdong University of Technology (GDUT), as well as being involved with the “One-Hundred Young Talents” project. He graduated from the Hunan University, the Harbin Institute of Technology (HIT), and The Hong Kong Polytechnic University, respectively. His research interests include the frosting and defrosting performance of ASHP units, indoor thermal comfort, the thermal characteristic stability of PCMs and fin surface treatment for heat transfer enhancement. On the topic of the frosting and defrosting performance of ASHP units, he has nearly eight years’ research experience, during which he has outlined a series of phenomena and terms such as uneven frosting, even frosting, the frosting evenness value and the defrosting evenness value.
Jiankai Dong
Dr Jiankai Dong, Ph.D., is a Lecturer in the Department of Building Thermal Energy Engineering at the HIT. He obtained his doctorate from the HIT in 2012. His research interests include mechanism of frosting/defrosting and icing/de-icing, ASHP technology and building energy efficiency etc. He has published more than 40 papers, including 11 in SCI journals and 14 in EI journals. He has applied for more than 20 invention patents in the People’s Republic of China, 11 of which have been authorised.
Chili Wu
Dr Chili Wu, Ph.D., is a Research Assistant Professor at The Hong Kong University of Science and Technology (HKUST), in which he graduated from. In addition, he managed a research team at the Building Energy Research Centre of the HKUST Fok Ying Tung Research Institute in Guangzhou. He has undertaken a lot of works in the area of energy system studies and used his research in practical applications. He has applied for more than 20 patents in Hong Kong and has been involved in a number of research projects with financing amounting to HK$3.6 million.
Yiqiang Jiang
Prof Yiqiang Jiang, Ph.D., received his bachelor’s, master’s and doctoral degrees from the HIT and is currently a Professor in its Department of Building Thermal Energy Engineering. As an international expert in the frosting and defrosting performance of ASHP units, he has published many academic journal articles and conference papers on this topic. He has a wide range of research interests, including thermal comfort, the cooling and heating of buildings, LNGs and alternative energy systems.
Minglu Qu
Dr Minglu Qu, Ph.D., is an Associate Professor in the School of Environment and Architecture at the University of Shanghai for Science & Technology. She received her Ph.D. degree from The Hong Kong Polytechnic University. Her current research interests include heat pump systems, thermal comfort, energy application systems, and the heat and mass transfer processes in heating, ventilation and air-conditioning (HVAC) equipment. She has published more than 10 international academic papers on heating and cooling systems.