Development and Comparison of Compressor Models for Heat Pump Applications in TRNSYS

Heat pump systems may serve as a replacement for many heating systems using fossil fuels for heating purposes. Heat pumps are widely used in Norway where electric energy is abundant and inexpensive. Research and development of heat pump components are important to further improve component and system performance. The introduction of alternative refrigerants set new demands to heat pump components and the compressor especially. The compressor is the driving component of the heat pump cycle and requires work input. Improving compressor performance is of great interest, as the system performance is highly affected by the compressor power demand. In addition, the proper choice of compressor for the given system and operational conditions are of great importance. The Transient System Simulation Tool (TRNSYS) is used for simulations of the vapor compression cycle in this project. Components for heat pump simulations on component level are not available in the component library included in the software. The source codes of TRNSYS components are available to the user, and components may be changed or created. The available components in TRNSYS are written in the Fortran90 programming language. The aim of this project is to create models of variable speed compressors and other heat pump components for use in TRNSYS simulations. The compressor models are integrated into simple vapor compression refrigeration cycles, which work in heat pump mode. The behavior and properties of the heat pumps implemented with different compressor models are investigated as function of the time dependent heat demand of a simple building located in Bergen, Norway. Two compressor components are created. The compressor models differentiate in the method of calculating the compressor power input. One compressor model is based on a 10-coefficient polynomial for the compressor power input, the other compressor model is based on a polytropic compression process. In addition, an evaporator and condenser component, a throttling device component and a simplified building load component are created. Various simulations are conducted in order to verify that the models and components are consistent with theoretical predictions and published data. The report provides an extensive description of TRNSYS and the process of creating and implementing new components. The components perform as expected in terms of calculating compressor power input and heat transfer rates of the condenser and evaporator. These results are consistent with the energy balance for heat pump systems. The total energy consumption of the polytropic compressor is 21 % lower than the total energy consumption of the polynomial based compressor at standard conditions. The components and models have certain limitations in terms of simulations of the heat pump and compressor performance at varying operational conditions. This is due to the calculations and simplifications of the various components. The components are considered a good starting point for further development, making the components and TRSNYS models better reflect the varying operational conditions of a real heat pump system