Non-linear and adaptive control of a refrigeration system

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© The Institution of Engineering and Technology
Published 14/07/2012

Abstract

In a refrigeration process heat is absorbed in an evaporator by evaporating a flow of liquid refrigerant at low pressure and temperature. Controlling the evaporator inlet valve and the compressor in such a way that a high degree of liquid filling in the evaporator is obtained at all compressor capacities ensures a high energy efficiency. The level of liquid filling is indirectly measured by the superheat. Introduction of variable-speed compressors and electronic expansion valves enables the use of more sophisticated control algorithms, giving a higher degree of performance and just as important are capable of adapting to a variety of systems. This study proposes a novel method for superheat and capacity control of refrigeration systems, namely by controlling the superheat by the compressor speed and capacity by the refrigerant flow. A new low-order non-linear model of the evaporator is developed and used in a backstepping design of a non-linear adaptive controller. The stability of the proposed method is validated theoretically by Lyapunov analysis and experimental results show the performance of the system for a wide range of operating points. The method is compared with a conventional method based on a thermostatic superheat controller.

Inspec keywords: adaptive control; heat transfer; Lyapunov methods; nonlinear control systems; refrigeration; filling; evaporation; compressors; stability; valves; refrigerants

Other keywords: liquid filling; liquid refrigerant flow; variable-speed compressors; electronic expansion valves; evaporator inlet valve control; thermostatic superheat controller; stability; backstepping design; refrigeration system; nonlinear control; adaptive control; Lyapunov analysis

Subjects: Control of heat systems; Engineering materials; Self-adjusting control systems; Heat and thermodynamic processes (mechanical engineering); Stability in control theory; Nonlinear control systems

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