Velocity based defrost of evaporator coil of heat pumps

Abstract

Minimization of frost formation on the outdoor coils of residential heat pumps and subsequent defrost cycles to remove the frost in an energy efficient manner remains an active area of research and development in the HVAC industry. Inverting the cycle to reject heat from the outdoor coils is the most common method to run defrost cycles of the residential heat pumps. However, these defrost cycles are energy intensive. This research proposes a novel method that can substantially reduce the energy consumed in such defrost cycles. The method involves controlled use of reverse air flow on the outdoor coil surface during the defrost cycle, resulting in fewer defrost cycles needed for a given duration of heat pump operation. The method further provides better cleaning of the coil surface by improving the draining of the melted frost. It also allows part of the frost to be removed without the need to melt it. In this research, the proposed methods are evaluated experimentally by simulating the frost formation, and defrost cycle in a controlled environment. Defrost cycles were run with two of the most popular control strategies that are in use, time-based defrost and control-based defrost. Experimental results demonstrate that with the use of the proposed method, energy savings of 56% and 31% are possible for the above- and below-freezing environments respectively, as compared to a baseline that represents the ASHRAE recommended operating conditions.