Heat pump drying — A review

Drying is one of mankind's oldest preservation techniques for different products such as furs, wools, textiles, clay, timber, grains, fruits and vegetables. Statistics show that developed as well as developing countries use over one third of their primary energy for industrial operations including drying processes. In the past few years, it has been discovered that heat pump drying is an efficient method in drying industries. It delivers more heat than the work input to the compressor. Currently, South Africa has experienced an increase of drying processes for various industrial, commercial and residential applications. As a result it increased energy consumption. It is important to develop the market of heat pump drying systems in South Africa because this technology is still in its infancy stages. Apparently, most of the research in this area has been conducted in European and Asian countries. Therefore, the results are based on their specific environmental and climatic conditions which cannot be directly applied to a different region with different conditions like South Africa. The present study deals with reviewing different studies on heat pump drying. The historical development of heat pump drying systems (HPDSs), process efficiency, drying mechanism of different products, optimization and economic analysis of heat pump drying is reported.