Enhancing partial load efficiency of air source heat pump heating systems: A comprehensive review of magnitude, influencing factors, and improvement strategies

Air source heat pumps (ASHP) are considered worldwide as an effective alternative to heating with fossil fuels. With the improvement of heating performance under low temperature conditions and the continuous development of advanced defrosting methods, the deterioration of partial load performance in heating applications has become more and more obvious. Many researchers have found that performance in field testing is lower than performance tested in steady-state laboratory operation. This article summarizes the research of the past decades on the aspects of the partial load efficiency loss phenomenon, the magnitude, the influencing factors and improvement strategies, and the efficiency improvement methods for existing ASHP heating systems could be divided into three categories: 1.Improving the thermal inertia of the system, 2.Reducing the deviation between heating capacity and building load, 3.Reducing the water pump performance. It is hoped that with the continuous improvement of the energy efficiency of ASHP units, more attention will be paid to the influencing factors and improvement methods of systems.