“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 (Ẇnet) and a higher exergetic plant efficiency (ηex) than the convenient series and parallel configurations for the connection to a state-of-the-art 75/50 DH system.For the considered cases, Ẇnet and ηex 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 Ẇnet and ηex 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 differencesTsupply-Treturn, the “Preheat-parallel” or series configurations might perform better than the parallel configuration for the connection to a high-temperature DH system.