An Italian research team has designed a 1 kW-sized solid oxide fuel cell (SOFC) cogenerator for applications in residential buildings.
The researchers explained that, compared to fuel cell systems based on proton exchange membrane (PEM) technology, SOFC systems offer greater operating temperatures, making them more suitable for full-electric buildings, such as nearly zero-energy buildings (NZEBs), which are residential and commercial buildings that have very low primary energy requirements that can be met to a significant extent by renewable sources.
Other advantages are high combined heat and power efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost.
The group conducted a case study on a system deployed in a NZEB single-floor villa with a size of 80 m2 located at the University of Sannio, in Benevento, southern Italy. It uses an air-to-air heat pump providing heating, cooling, mechanical ventilation and domestic hot water (DHW). It also relies on a solar thermal collector of 2.2 m2 area and horizontal geothermal boreholes, as well as a 5.3 kW rooftop PV system linked to an 11.6 kWh lithium battery.
Image: University of Sannio, International Journal of Hydrogen Energy, CC BY 4.0
The SOFC system measures 1.2 m × 0.5 m x 0.8 m and is equipped with an inverter that is connected to the electric home grid. It also utilizes an internal heat exchanger to recover heat resulting from the electrochemical reaction. Its electrical efficiency reportedly reaches up to 49% and an overall efficiency of up to 90% at 30 C of return temperature.
“The FC is purely hydrogen-fueled, with a gas flow of about 12 L/min at 15-25 millibar considering a nominal operation. The supply of fuel first takes place via a storage station of 64 Sm3 at 200 bar. Eight cylinders and three depressurization stages are placed outside,” the researchers explained. “Currently green hydrogen is purchased. The future development of the system is to install an electrolyzer for the on-site production of the necessary hydrogen.”
The whole cogeneration system includes closed water circuits that provide heat to a 300 L tank, used for pre-heating the DHW generated by the heat pump. It also comprises a hybrid inverter for the management of the battery, the PV system, the FC, the electric home loads, and the grid connection.
Through a series of experiments, the research group could ascertain that, in particular conditions of photovoltaic production, the system is not only able to meet the home electric loads, but also to inject surplus energy into the network. It also found the system can achieve an electrical efficiency of up to 0.48 and a maximum overall efficiency of 0.93.
“Considering all monitored conditions, the whole useful and spent energy over the analyzed period, the electrical efficiency is equal to 0.45,” it further explained. “The overall efficiency decreases as the return water temperature value in the heat recovery circuit increases.”
The researchers also warned that the system may require highly skilled labor for its deployment and managaement, and said another limitation is represented by its high start-up time of the machine, estimated at approximately 24 h. “In addition, it is advisable to operate the FC at nominal conditions and without frequently varying the power levels,” they stated.
The system was described in “Experimental characterization of solid oxide fuel cell hydrogen fueled in a residential small villa,” published in the International Journal of Hydrogen Energy. The research team included academics from Italy's SFOC specialist SolydEra S.p.A., green-hydrogen startup Stress S.c.a.r.l., the University of Sannio, and the University of Molise.
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