Dynamic Modeling Of Heat Pump Systems

Heat pump systems include evaporators, condensers, compressors and throttling valves. And accessories include pipes, fans and accumulators. The principle is that to maintain a specified temperature and relative humidity for a space by thermal energy transportation. Heat pump cycles can be divided into cooling mode and heating mode. A typical cooling mode cycle contains four steps: compression, condensation, throttling and evaporation. When refrigerant absorbs heat from the room during evaporation, room temperature will decrease. For heating mode, indoor unit works as condenser. When refrigerant releases heat during condensation room temperature will increase.In order to study factors of system performance, firstly the principle of heat pumps are analyzed. The models of each component and the whole system are build. Evaporator and condenser include convection heat transfer and pressure drop calculation. Throttling is considered as an isenthalpic process.MATLAB/SIMULINK software is used to build models for above components, including evaporator, condenser, compressor, throttling valve, pipes, and fans. Then the whole system is build based on all components.Control loops are designed based on above thermal model. Superheat control and compressor speed control are main factors of system performance. A cascade PID control is used to find the relationship between valve opening and superheat. Meanwhile, compressor speed control also use a cascade PID control to control room temperature. Results show that system performance can be higher by optimizing above parameters.