Performance Evaluation of a Combined Heat and Power Generation System with Borehole Thermal Energy Storage: A Feasibility Study of a Combined Heat Pump and Organic Rankine Cycle System

The current research is focused on the introduction of a heat pump (HP)-assisted organic Rankine cycle (ORC), which runs on the heat extracted from a high-temperature borehole thermal energy storage (BTES). By varying different source temperatures from 40 °C to 60 °C, the HP cycle works to upgrade the heat to run the ORC. Different combinations of environmentally friendly fluids are studied in comparison to match the top and bottom cycles and to make the overall system a combined heat and power (CHP) system. A power sufficiency condition is defined to compare and identify the best working fluid combination for the HP cycle and ORC. Based on the analysis, ammonia for the HP and R1234zee for the ORC emerged to be a suitable combination among all the studied combinations. As an example, for a BTES heat source of 237 kW at the source temperature of 60 °C, the BTES–HP–ORC–district heating system with the ammonia–R1234zee pair has resulted in the HP compressor work input of 21.9 kW with the coefficient of performance (COP) of 10.9 for the HP cycle and the ORC net work output and district heating supply of 10.4 kW and 209 kW, respectively, with the thermal efficiency (η) of 4.3% for the ORC at the evaporation temperature of 65 °C. A study in terms of the greenhouse gas (GHG) emissions reveals the feasibility of the system depending on the regional GHG intensity and emission factor of electricity and natural gas.