2nd law analysis of ON/OFF vs. frequency modulation control of a refrigerator

The cooling capacity of any refrigerator or freezer should be somewhat oversized to assure that temperature can be retained over a range of conditions such as variable outside temperatures, opening of the doors, uncertainties of insulation and also for an acceptable temperature response to the introduction of new contents. Thus, a temperature control technique is required. The easiest control to implement is ON/OFF control via a thermostat. However, a more efficient control is continuous operation with temperature controlled by adjusting the compressor speed. The speed control advantage results from a lower temperature difference across the heat exchangers with continuous operation relative to ON/OFF control. This paper presents a 2nd law analysis of a freezer that has a 12000 (Btu/hr) capacity, but the load only requires 6000 (Btu/hr). It is shown that speed control results in 41% electric energy savings relative to ON/OFF control.