Performance evaluation of a two-stage compression heat pump system for district heating using waste energy

• The present study investigated a two-stage compression heat pump system for district heating utilizing unused energy.
• A two-stage compression heat pump system comprising an intercooler and flash tank was designed.
• Controlling the intermediate pressure resulted in the ability to improve performance by 5.2% under identical heat source conditions.

The present study investigated a two-stage compression heat pump system for district heating utilizing waste energy. An evaporator and condenser exhibit a large difference in temperature when hot water is produced for heating by a heat pump. With single-stage compression, this causes a dramatic drop in the compressor efficiency and lowers the system performance; so, in the present study, a two-stage compression heat pump system comprising an intercooler and flash tank was designed, and the performance characteristics under various operating conditions were tested. When the heat source temperature was raised from 10 °C to 30 °C, the COP (coefficient of performance) was improved by up to 22.6%. As the superheating at the low-stage compressor was increased from 2 °C to 11 °C, the refrigerant flow rate and heating capacity decreased by as much as 7.6% and 2.2%, respectively, but there was no major impact on the temperature of the hot water produced nor on the system performance. Controlling the frequency of the high-stage compressor to control the intermediate pressure resulted in the ability to improve performance by as much as 5.2% under identical heat source conditions.