Optimal Configuration for Low-T Geothermal CHP Plants

In this paper, we investigate four combined heat-and-power (CHP) plant configurations which are fueled by low-temperature geothermal energy and connected to a district heating system. The four CHP configurations are compared based on an exergy analysis and the cycle parameters are optimized towards maximal electrical power output, while satisfying the heat demand of the district heating system. We consider reference geothermal brine conditions of 130°C and 150kg/s, according to the expected conditions in Flanders (Belgium). A reference heat demand of 10MW is assumed. Two types of district heating systems are considered: a low-temperature district heating system with supply and return temperatures of 65°C and 40°C, respectively, and a high-temperature district heating system with supply and return temperatures of 90°C and 60°C. The electricity is produced via an Organic Rankine Cycle (ORC). We conclude that the so-called HB4 CHP is the optimal configuration for a wide range of brine conditions and for a wide range of heat demands. For the connection to the 65/40 district heating system and for the reference conditions, the electrical power output is as high as for a pure electrical power plant (4.8MW) and the exergetic plant efficiency is 49.1%, which is 9.5%-pts higher than that for a pure electrical power plant. For the 90/60 district heating system and reference brine conditions, the electrical power output is 4.3% lower and the exergetic plant efficiency is 52.1%, which is 12.5%-pts higher than that for a pure electrical power plant. The exergetic plant efficiency is higher than that for the connection to a 65/40 district heating system due to the higher exergy content of heat at a higher temperature. Furthermore, for all brine conditions and values of the heat demands considered, the optimal CHP configuration has always higher exergetic plant efficiency than the pure electrical power plant, which means that the energy utilization of the low-temperature geothermal source is better in a (optimal) CHP plant.

ISBN:0934412227

Publication year:2017