The analysis of a sustained oscillation in the heat pump system with buffer tank

Abstract

The most of domestic scale heat pumps with fixed speed compressors are thermostatic controlled. Control units with a relay characteristic are used to control these pumps. Such pumps operate in the mode of sustainable oscillations. To overcome short cycling, i.e. switching on and off of heat pumps, as this reduce energy efficiency and pump life cycle, buffer tanks are installed with the task of absorbing the excess heat delivered by the pump. The paper analyzes how the properties of a heat accumulator and a heat pump affect the oscillations parameters. The buffer tank is described by the FOPDTmodel while the pump is approximated by relay nonlinearity with symetric hysteresis. A relay feedback technique with a describing function was used for the analysis. The results were confirmed by simulation in Simulink / Matlab environment.