Strategies for Improving Temperature Control and Energy Use in Household Refrigerators

Conventional control of household refrigerators is typically accomplished with simple, single-sensor control systems. Though cost-effective this approach allows temperature dependencies to exist between compartments, producing non-optimal energy consumption. In order to study the performance of the conventional refrigerator a computer model was developed. Analytical results were generated to determine transient performance of the plant while operating under various control approaches. The model consists of refrigerant and airflow subsystems to represent the plant's refrigerant system .thermodynamics and air flow control algorithm. Improved performance can be obtained by adding "dual" control components and using nonconventional control strategies. Specifically, variable temperature bandwidths and uncoupled compressor and evaporator fan algorithms were examined. Also, plant configurations using automatic air flow dampers to regulate the refrigerator fresh food temperature, with separate thermostatic control of the freezer were examined. Two types of air flow dampers were considered: the open/closed (snap-action) type and the proportional (creep) damper. This study indicates that use of automatic dampers in combination with these nonconventional strategies can provide near optimal control at all plant operating points. INTRODUCTION An air flow diagram for a conventional refrigerator is shown in Fig. 1. Typical temperature control uses two basic features: (i) a compressor on/off thermostatic sensor located in the fresh food compartment, and (ii) a manually-adjusted damper (D in Fig. 1) regulating air flow between the freezer and fresh food compartments. The damper controls freezer temperature by regulating the flow of cold air to the fresh food compartment. The air flow is generated by the evaporator fan which cycles simultaneously with the compressor. Typically, the compressor and evaporator fan are turned on and off by a thermostatic sensor with a constant (i.e., flXed) temperature bandwidth. The set point is manually adjustable but the bandwidth remains constant. This single control approach permits coarse control of the freezer but relatively good fresh food temperature control. The advantages of conventional control lie in its simplicity and domestic market-acceptance. Drawbacks include temperature dependency between compartments, temperature shifts between ambients, limited control range, and energy inefficiency. To improve on conventional refrigerator performance, approaches were considered which added degrees of freedom to the control problem using non-traditional control algorithms. Four strategies were studied: constant and variable temperature bandwidths with compressor and evaporator fan coupled and uncoupled. These four strategies were also investigated using two types of automatic air flow dampers: "snap action" and, "creep" type dampers. When "uncoupled" the evaporator fan operates whenever the compressor runs but it also can run without the compressor. "Variable temperature bandwidths" refers to the use of sensor limits which can be set uniquely for each plant control point. THE PLANT MODEL A steady state thermodynamic cycle model developed by Jaster [1] was extended by Bessler [2] to include transients and the air flow analysis. It consists of refrigerant and air flow system submodels which compute the states of both working fluids as they circulate around their respective plant subsystems. The control