Electric-heater defrosting performance of frost-free refrigerator-freezer and its improvement by step-reduction power

•Defrost energy and freezer-temperature rise fluctuated as heater power increases.•Two-way melting of frost found with warm air accumulation in evaporator chamber.•Mismatch between frost mass and defrost heat with time led to low defrost efficiency.•Step-reduction power relieved the mismatch and improved overall defrost performance. The electric-heater defrosting is widely used for frost-free refrigerator-freezers, but suffers from high energy consumption and cabinet-temperature rise. In this article, the electric-heater defrosting performance and its improvement were studied numerically and experimentally for a side-by-side refrigerator-freezer. First, the defrost cycle was tested under different constant power, and the optimal 200 W was obtained with the lowest 118.8 W·h energy consumption and mild 9.9 K freezer-temperature rise in the allowed 36.0 min duration. Then, a numerical model was built to simulate the defrost process with the constant 200 W heater power. Low defrost efficiency was found with only 12.2 % heat absorbed by frost, attributed mainly to the mismatch between reducing frost mass and constant heater power during defrosting. Accordingly, step-reduction power was proposed to mitigate this mismatch and optimized with 4 min initial interval, 6 min steady interval, 20 W power step and 80 W minimum power, which finally led to 27.1 % reduction in energy consumption with similar duration and freezer-temperature rise for the defrost cycle. The results could provide guidelines for the improvement of defrost performance in the refrigerator-freezer industry.