Heat Pump with Two Heat Sources on Different Temperature Levels

Aim of this project is the development of a new heat pump system with economizing that is able to improve the heating performance using two or more heat sources at different temperature levels. These heat sources are incorporated into the system with minimal loss of exergy by adding the heat at different pressure levels. Practical applications for such systems are for example buildings with a heat pump and a solar thermal collector or buildings with waste heat recovery. While solar thermal systems can be used for heating and domestic hot water in summer, they fail to produce sufficient temperatures for direct use in fall, spring and winter. By connecting the solar collectors to the here described heat pump system, they are able to improve the heat pump efficiency significantly. Furthermore, the lower temperatures of the collectors help to improve their effectiveness and raise the usable solar gains. While existing systems using two heat sources are either very costly, inefficient or need large amounts of waste heat, the proposed cycle can use varying amounts of waste heat and increases the heat pump efficiency by up to 30% over a wide range of operating conditions. The paper presents an introduction to the topic and various heat pump system designs using two heat sources. All of them incorporate the OptiRef. Simulation results and experimental analysis show that depending on the amount of waste heat and the temperature level of the heat pump cycle, efficiency and heating capacity improvements of 20-30% could be shown in a laboratory prototype. Oil management and control of the system are the main challenges when implementing it in the laboratory or field. Altogether, the functionality of the system could be proven, but there is still room for improvement with respect to the control concept and in order to reduce cost.