Although the energy consumption of individual household refrigerators is small, their large number represents an appreciable potential for energy savings. Improvements in insulation, compressor efficiency and optimisation of the refrigerant charge have reduced energy consumption significantly in recent years. Still more power can be saved by converting the capacity control strategy from compressor on/off cycling to a variable speed running mode. This control strategy leads to a reduction in the average speed of the compressor and fewer stops, which are then only needed for the periodic defrosting of the evaporator. Numerical simulation showed that this kind of operation can lead to energy savings of up to 30%. The reasons for the savings are: Smaller friction losses in the compressor, higher evaporation temperature, lower condensing temperature and a reduction in losses associated with the pressure equalisation on compressor stops. In the experimental investigation an off-the-shelf hermetically sealed compressor was modified for use with a frequency converter and installed in a household refrigerator. In a temperature-controlled chamber, energy consumption was measured according to DIN EN 153 at different compressor speeds. In nearly continuous running mode, the energy consumption was reduced by approximately 20% compared to the original on/off mode. Furthermore we investigated experimentally and by simulation the difference between fully variable speed control and the operation with just two fixed speeds, i. e. switching between a low and a high speed according to the heat load. It turned out, that the difference in power consumption between these two control philosophies is marginal, in the maximum 3 %. Because the two-speed control can probably be realised at lower cost, it seems more advantageous to implement this control strategy in household refrigerators. REQUIRED COOLING CAPACITY Although the energy consumption of individual household refrigerators is small, their large number represents an appreciable potential for energy savings. Improvements in insulation, compressor efficiency and optimisation of the refrigerant charge have reduced energy consumption significantly in recent years. The purpose of this investigation was to find out, how much power could be saved by optimising the capacity control. The control system has to make sure that the refrigeration provided by the refrigeration cycle matches the required cooling capacity. The required cooling capacity of a household refrigerator is the heat flow, which has to be removed through the evaporator of the refrigeration cycle to keep the average temperature inside the cooling cabinet at a level of e. g. 5°C. This required cooling capacity depends on • the heat conducted through the insulation, which in itself depends on the ambient temperature • the door opening frequency and duration • the cooling-down of warm goods • the water removal from moist goods Since some of these parameters are varying with time, also the required cooling capacity will vary with time. The refrigeration, which has to match the required cooling capacity, is provided by the refrigeration cycle. Since the system has some thermal inertia, the matching has not to be instantaneous, but it must fit to a certain timeaverage of -sayabout 20 minutes. The refrigeration cycle consumes electric power. The amount of consumed power does depend on the (time-averaged) required cooling capacity, the ambient temperature and the efficiency of the refrigeration cycle. Required Cooling Capacity Po w er C on su m pt io n 35°C 25°C 15°C Maximum capacity