‘‘Preheat-parallel” configuration for low-temperature geothermally-fed CHP plants

A novel CHP configuration is presented, which is fueled by low-temperature geothermal energy and delivers heat to a district heating (DH) system. This so-called ‘‘Preheat-parallel” configuration has a higher net electrical power output and a higher exergetic plant efficiency than the convenient series and parallel configurations for the connection to a state-of-the-art 75/50 DH system. For the considered cases, the net electrical power output and the exergetic plant efficiency are 1.3–6.4% and 0.4–1.9%-pts higher than for the parallel configuration, respectively. The highest values correspond to the highest heat demand. With respect to the series configuration the net electrical power output and the exergetic plant efficiency are 2.1–9.9% and 0.7–3.0%-pts higher, respectively, where the highest values correspond to the lowest heat demand. Furthermore, the optimal CHP configuration - series, parallel or ‘‘Preheat-parallel”- is discussed. The optimal configuration depends on the DH system requirements. Supply and return temperatures in the range of Tsupply = 40—110°C and Treturn = 30—70°C are considered. We conclude that the series and parallel configurations have the best performance for the connection to low-temperature and high-temperature DH systems, respectively. However, for a wide range of Tsupply and Treturn, the ‘‘Preheat-parallel” configuration is the most appropriate. The preheating-effect is the main feature of the ‘‘Preheat-parallel” configuration, and is more useful for a large temperature difference Tsupply - Treturn and for low values of Treturn. Furthermore, we found that for high heat demands and small temperature differences Tsupply - Treturn, the ‘‘Preheat-parallel” or series configurations might perform better than the parallel configuration for the connection to a high-temperature DH system.status: publishe