Solar Assisted Heat Pump System for High Quality Drying Applications: A Critical Review J. Assadeg 1,2 , Ali H. A. Alwaeli 1 , K. Sopian 1‡ , H. Moria 3 , A. S. A. Hamid 1,4 and A. Fudholi 1 1 Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600, Bangi, Selangor, Malaysia 2 Renewable Energy Engineering Department Collage of Energy and Mining Engineering, Sebha University, Libya 3 Department of Mechanical Engineering Technology, Yanbu Industrial College, Yanbu Al-Sinaiyah 41912, Kingdom of Saudi Arabia 4 Faculty of Science and Natural Resources, Universiti Malaysia Sabah, 88400 Kota Kinabalu, Sabah, Malaysia (jassadeg@gmail.com, ali9alwaeli@gmail.com, ksopian@ukm.edu.my, hazimmoria@gmail.com, pian@ums.edu.my, a.fudholi@ukm.edu.my) ‡ Corresponding Author; K. Sopian, Solar Energy Research Institute, Universiti Kebangsaan Malaysia, 43600, Bangi, Selangor, Malaysia, Tel: +03 89118572, Fax: + 03 89118574, ksopian@ukm.edu.my Received: 15.01.2020 Accepted:28.02.2020 Abstract- Solar assisted heat pump (SAHP) system integrates a solar thermal energy source with a heat pump. This technique is a very fundamental concept, especially for drying applications. By combining a solar thermal energy source such as solar thermal collectors and a heat pump dryer will assist in reducing the operation cost of drying and producing products with high quality. Many review papers in the literature evaluated the R&D aspects of solar-assisted heat pump dryers (SAHPD). This critical review paper studies some of the researches conducted in this field to understand and provides an update on recent developments in SAHPD. Also, a detailed explanation of principles and operation for SAHPD and its applications are presented. The used types of solar thermal collectors, as well as various heat pump dryers, are all discussed in this article. Finally, it is concluded that there is a clear lack of research in the techno-economic and environmental evaluation, while most of SAHPD studies focused on the performance study of the system. Keywords heat pump, collector, two phases, refrigerant, compression index. Nomenclature Volumetric efficiency Mass flow rate Mass refrigerant flow Ts Wetted surface temperature P Pressure Uwi Combined coefficient of heat transfer for tube wall, refrigerant and water film V Velocity k Compression index absolute humidity ratio c Clearance volume of compressor ratio PD Piston displacement Cmin Smaller heat capacity rate Subscripts Cc Heat capacity rate of cold fluid a Air Heat exchanger effectiveness tp Two phases Heat exchanger effectiveness of two phases i Inlet or Inside Outlet temperature of air o Outlet or Outside Intel temperature of air s Saturated condition Intel temperature of working refrigerant r Working refrigerant Temperature of water film h Hot fluid Ua Heat transfer coefficient of the air c Cold fluid A Heat transfer area w properties evaluated at wall temperature or water film Q Heat transfer rate h Heat transfer coefficient v Saturated vapor