A NOVEL APPROACH TO DISTRICT HEATING: USING A TWOPHASE EXPANDER IN REVERSIBLE HEAT PUMP-ORGANIC RANKINE CYCLE SYSTEM

Abstract

A district heating system involves a central generation and distribution of heat to residential buildings through a network of insulated pipes. Heat can be generated, for example, from the combustion of fossil fuels and biomass. Since there are many environmental restrictions and the growth of heat and energy demand, several industrial waste heat and renewable energy sources, such as geothermal or solar thermal energy may be promising to be utilized for district heating purposes. However, in some districts, various low-temperature heat sources might be available; as a result, a heat pump is installed to increase the temperature to accommodate demand peak loads. The heat pump is operated only for a certain period and sometimes not used during summer. To optimize the utilization of the heat source, the district heating system may be transformed into a power generation system that can generate, for example, electricity instead of heat during the summer. This goal may be achieved by applying a reversible system. Therefore, this article discusses the novel reversible heat pump-organic Rankine cycle (RHPORC) system using a two-phase expansion system. Some working fluids were selected based on thermal properties, as well as safety and environmental issues. Some selected two-phase volumetric expanders were introduced in the novel system. In the end, the performances of the system based on the selected working fluid were compared. According to the results of a study, using RHPORC in district heating systems might save annual energy consumption by up to 50%.