This paper presents a model-based feedback control scheme for energy minimization in an air-conditioning system to maintain a desired level of thermal comfort. The proposed control scheme consists of two layers of control - setpoint optimizing control and dynamical optimizing control. In setpoint optimizing control, the optimum setpoints for dry-bulb temperature, relative humidity and relative velocity of air, evaporator pressure, and condenser pressure are computed on-line under operational and performance constraints of the system. Dynamical optimizing control deals with the design of centralized continuous control based on output feedback to track the optimum setpoints. This control law is designed for the complete dynamical system, i.e., the refrigeration circuit and air circuit simultaneously. The constraints on control inputs were also considered while designing the proposed optimizing control scheme, which has been validated on an air-conditioning unit provided in an Indian Railway passenger coach. The result shows that the proposed control scheme saves substantial energy as compared to the constant-setpoints-based control scheme to maintain the same level of thermal comfort.
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