scope:

INTRODUCTION

Refrigerator designs today are very different from the past. It has received attention in the U.S. because it consumes about 7% (DOE 2008) of the total primary energy in residences, and it is becoming a primary target in new designs to cut household electricity and also to make refrigerators more green than before (Simpson 2007). Their energy efficiency has improved rapidly in recent years. Vineyard et al. (1997) mentioned that energy consumption of a 1.86 m³ (20 ft³) refrigerator, has dropped from 1726 kWh/y in 1972 down to around 460 kWh/y in 2001 with improving gaskets, insulation, compressors, and other technologies to maximize efficiency. They showed the feasibility to reduce the power consumption of the aforementioned refrigerator down to 1 kWh/day. Inverter controlled variable speed compressors have also been laboratory tested with impressive results (Chang 2006; Chang et al. 2009). However, there has been little research work in improving refrigerator defrosting efficiency.

This paper discusses the design and initial testing of a novel reverse cycle defrosting scheme. Reverse cycle defrosting schemes are not new-this approach is commonly used to defrost the outdoor heat exchangers of air-to-air heat pumps. Condensers for the majority of the refrigerators are located at the bottom of the refrigerator, along with a condenser fan. Applying a heat pump type reverse cycle defrost scheme for this kind of design is relatively simple. A 4-way valve is needed. During the defrosting period, the refrigeration cycle is reversed, heating the evaporator and melting the frost. One challenge for this approach, however, is to reduce the hot refrigerant temperature (superheat) before it enters the evaporator located in the freezer compartment to avoid possible damage to the coil due to thermal shock. For typical Asian refrigerator designs the condenser coils are located on one or both refrigerator side panels as shown in Figure 1. This design is aimed to save space and cost, though it is not very energy efficient. There are at least two reasons why the Asia type refrigerators are less efficient: (1) The condenser coils are attached to the refrigerator side panels. The side panels are warm whenever the compressor is running, and part of the heat is transferred into the refrigerator fresh food compartment. (2) The insulation materials are thinner than that of U.S. refrigerators. A heat pump type reverse cycle defrost scheme is not good for this design. As heat is absorbed from the refrigerator side panels during defrost operation, the temperature of the panels will drop, possibly leading to moisture condensation on the panel and water puddling on the floor. A new reverse cycle defrosting concept has been developed to avoid this moisture condensation problem and to reduce thermal shock to the evaporator.