Researchers in Finland have demonstrated the technical feasibility of an off-grid PV system for private households in combination with short-term battery storage and seasonal hydrogen storage. The proposed model is only applicable to the northern climate, as more solar radiation in southern locations would mean less need for seasonal storage. It was tested in an existing family home in Finland with a 21 kW roof field and a 6 kW geothermal heat pump.
December 18, 2020
Scientists at the Technical University of Lappeenranta (LUT) in Finland have found that off-grid PV solutions for private households in northern climates are only technically feasible if they are coupled with short-term battery storage and seasonal hydrogen storage at the same time and the peak consumption of the household is not too high .
In the newspaper The technical feasibility assessment of an off-grid solar PV-based house energy system with battery and hydrogen storage in northern climates, published in Solar Energy, simulated a model for a similar approach in an existing single-family house in Finland with a 21 kW roof system and a 6 kW heat pump for grounding, which is included in the electricity consumption.
Data on hourly average electricity data for PV electricity generation and electricity consumption was collected for three years from January 2017 to December 2019.
The two-story house is designed as a zero-energy building and the east-west PV system is based on a 16 kW inverter. “If the size of the inverter is too small compared to the PV peak power, it is economically preferable for installations in northern locations,” said the scientists. “The capacity of the PV system is such that further improvement in self-sufficiency by increasing the PV peak power is no longer possible,” they further explained, adding that the self-sufficiency of the house was 36.81% in line with the values of all northern European countries. The average annual PV electricity surplus was estimated at around 200%.
In their simulation, the scientists used a battery bank for short-term energy storage and to control peak demand, as well as a hydrogen tank connected to a water electrolyzer and a fuel cell for seasonal storage. Excess PV power is mainly used to charge the battery and only when it is charged is used to power the electrolyzer. On the other hand, overload is always covered first by the battery itself. “The unnecessary sudden switching on and off of the fuel cell is minimized by limiting the output power based on the state of charge of the battery,” they also specified.
Using a sensitivity analysis and a current account analysis, the Finnish group was able to collect data on the unmet electricity demand according to storage phases as well as on the annual hydrogen consumption and the annual hydrogen production. “Based on the simulation results, it is clear that neither a battery nor a hydrogen energy storage system alone is sufficient to maintain year-round, off-grid operation under northern climatic and solar radiation conditions,” concluded the study’s authors. They found that using just one battery would require an “impractically” large system for this type of project, and hydrogen production alone would be wasteful due to its poor round-trip efficiency. However, when combined, the two technologies could make off-grid solar power a viable solution for homes, although limiting high peak power consumption would be critical.
The proposed system could only work with a battery with a minimum storage capacity of 20 kWh and a fuel cell and an electrolyzer with an installed power of at least 4 to 7 kW. “In order to keep the system running during the winter months, a hydrogen storage capacity of around 170 kg to 190 kg is required. So if no additional compressors are used, a relatively large area in a dormitory would be required for the physical storage of hydrogen, ”the scientists confirm.
The validity of these results is limited to the northern climate, since higher solar radiation in more southern locations would mean less need for seasonal storage.
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