Performance Characteristics of Heat Pump-Boiler System : Part 1-Performance Analysis of the System

Central heat supply plants for district heating and cooling systems or industrial use are appreciated for their capability to use diversified energy sources, especially oil alternative fuels like coal and wastes, and for their energy saving ability by waste heat recovery, or by simultaneous electricity and heat supply. But, the simultaneous electricity and heat supply system needs to be on a large scale due to load leveling. So it is not popular in Japan. Individually equipped heat supply boilers can cut down the piping heat loss of the central heat supply system. But, they cannot fully utilize the available energy of the high temperature heat in the boiler due to the reduction in energy quality accompanied by the temperature reduction from high temperature to relatively low supply temperature. To overcome these defects, a new system, which pumps up heat from a low temperature heat source by a heat pump, was proposed. This system is directly driven by a Rankine cycle steam expander and supplies the pumped up heat with the Rankine cycle condenser waste heat. Former studies analytically and experimentally investigated the characteristics. The lack of exergie analysis makes it difficult to investigate the efficiency improvement. This report analyzed the efficiency and the exergie balance of the "Heat pump-Boiler System" mentioned above by considering practical losses. To increase the efficiency, power recovery is examined in the expansion process of the heat pump cycle. The results are as followed: 1) A thermal efficiency of 120% can be realized by arranging the steam and heat pump cycle heat exchangers in series and by adopting power recovery in the heat pump cycle expansion process. This showed the energy saving ability of the Heat pump-Boiler System compared to the individual boiler system (thermal efficiency 90%) by the considering piping heat loss accompanying central heat supply systems. 2) Without power recovery, the thermal efficiency differs with the type of the heat pump cycle compression process. The two stage compression type showed maximum efficiency. 3) Exergie analysis showed that power recovery in the heat pump cycle is effective, and also that with power recovery the thermal efficiency is not affected by the types of the heat pump cycle compression process. So, the compression type can be chosen by supply water temperature level or from an economical point of view. 4) Increasing the temperature differences in the heat exchangers decreases the system thermal efficiency. The exergie analysis showed that temperature differances reduction, especially in the steam condenser and heat pump evaporator, effectively increases thermal efficiency.