Finnish researchers have demonstrated the technical feasibility of off-grid residential photovoltaic systems that combine short-term storage with seasonal hydrogen storage. The proposed model is only suitable for northern climates, as higher solar radiation in southern regions means less seasonal storage needs. The power generation system was tested in an existing single-family house in Finland with a 21 kW rooftop PHOTOVOLTAIC array and a 6 kW ground source heat pump.
Scientists at Finland's Laplanta University of Technology (LUT) have found that off-grid residential PV solutions are technically feasible only if they combine short-term storage with seasonal hydrogen storage and peak household power consumption is not too high.
In "Evaluating the Technical Feasibility of a Solar Photovoltaic based Off-grid home Energy System combining battery and hydrogen Storage in northern Climates," published in the journal Solar Energy, the team modeled a similar approach for an existing single-family house in Finland, The house uses a 21 kW rooftop photovoltaic array integrated into the electricity system and a 6 kW ground source heat pump.
They collected data on the average hourly power of photovoltaic generation and consumption over a three-year period from January 2017 to December 2019.
Designed as a zero-energy building, the two-storey home relies on a 16-kilowatt inverter for its east-west photovoltaic system. "Reducing the load of inverters compared to pv peak power capacity is economically preferable for power generation facilities in northern Regions," the scientists said. "The capacity of the pv system has reached a point where increasing pv peak power cannot further increase self-sufficiency," they further explained. It added that the house is 36.81 percent self-sufficient, which is in line with all Nordic countries. The annual surplus pv power is estimated at about 200%.
In their simulation, the researchers used battery packs for short-term storage and control for peak demand, and hydrogen storage tanks linked to water electrolyzers and fuel cells for seasonal storage. The surplus photovoltaic power is mainly used to charge the battery, and only when the battery is fully charged is used to power the electrolytic cell. Excess demand, on the other hand, has always been met first and foremost by batteries. "By limiting the output power of the battery according to its charging status, unnecessary sudden startup and shutdown of the fuel cell can be minimized," they added.
Using sensitivity analysis and power balance analysis, the Finnish team successfully collected data on the unmet power demand after the storage phase, as well as the total annual hydrogen consumption and production. "Based on the simulation results, it is clear that neither battery nor hydrogen storage systems alone are sufficient to sustain off-grid operation throughout the year in northern climates and sunshine conditions," the authors conclude. Their findings suggest that such a project would require a large system that would be "impractical" if only batteries were used, and that hydrogen production from a single unit would be driven by low energy
Waste in exchange for efficiency. However, combining the two technologies makes off-grid residential solar a viable solution, although limiting peak efficiency is still critical.
The proposed system can operate only with batteries with a minimum storage capacity of 20 KWH and fuel cells and electrolytic cells with a minimum installed capacity of 4 to 7 kW. "In winter, in order to maintain the system, hydrogen storage capacity needs to be about 170 to 190 kilograms. Therefore, unless additional compressors are used, a relatively large physical hydrogen storage area needs to be left in the house." Scientists are sure.
The team goes on to say that the validity of these results is limited to northern climates, as higher solar radiation in southern regions means less seasonal storage needs.