Evaluation of a low temperature fuel cell system for residential CHP

摘要

CHP (combined heat and power) is a technology that allows to provide electrical and thermal energy. CHP is normally used in systems that produce wasted heat at high temperature to recover energy and increase overall system efficiency. The aim of this work is to investigate the possibility to recover heat produced by a 5 kW PEFC system for residential applications (hot water and building heating). As known, PEFCs work at low temperature (60-90 ℃) and the experiments have been carried out in order to improve the overall system efficiency by reusing heat that is normally wasted. The work was developed during an Italian National project PNR-FISR "Polymeric and Ceramic Fuel Cell" coordinated by CNR-ITAE. A 5 kW PEFC system, developed with NUVERA Fuel Cells in the framework of the project, was tested in cogeneration configuration recovering wasted heat with a heat exchanger directly connected to cathode out. Tests on PEFC system were carried out in the range 2.5-5 kW, maintaining the working stack temperature at 71 ℃. Heat, produced at different power levels, was removed from the system by using a regulated water flow in the heat exchanger. A peculiar feature of the system is the so-called "direct water injection" at the cathode, that allows simultaneous cooling and humidification of the stack. This characteristic permitted the recovery of most of the waste heat produced by the fuel cell. The performance of the PEFC unit was analyzed in terms of electrical, thermal and total efficiency. Tests showed that it is possible to obtain water at about 68 ℃ under different power levels. Moreover, experimental data showed that heat recovered was maximum when heat exchanger worked at nominal power and, under these conditions, the overall system efficiency increased up to 85%.