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Self-sustaining off-grid energy systems may require both short-term and seasonal energy storage for year-around operation, especially in northern climates where the intermittency in both solar irradiation and energy consumption throughout the year is extreme. This paper examines the technical feasibility of an off-grid energy system with short-term battery storage and seasonal hydrogen storage, comprising a water electrolyzer and a fuel cell. The study is based on data from a currently grid-connected residential single-family house in Finland with an existing 21 kWp photovoltaic (PV) installation and a ground source heat pump based heating system. Energy system performance is simulated using real PV power generation data as well as data on grid electricity import and export from the house over a three-year period to find the minimum combination of battery and hydrogen storage system capacities capable of year-round off-grid operation. It is concluded that technically feasible solutions are available for the proposed setup. The most significant factor affecting the system dimensioning is found to be high peak electric power demand during times of low PV power generation, indicating a need for smart power control. Demand for battery storage capacity is found to be significant only to about 20 kWh. Fuel cell and electrolyzer nominal powers of at least 4 kW and 5 kW to 7 kW, respectively, were found to be sufficient for off-grid operation with the studied system. The storage capacity demand for 170 kg to 190 kg of hydrogen annually is, however, impractically large for residential houses. Post-print / Final draft

Solar photovoltaic systems have been growing in popularity in prosumer households as a means of increasing the share of renewable energy and decreasing electricity import. The available self-consumption is, however, limited by a temporal supply–demand imbalance. In this paper, options for improving the self-consumption of a prosumer household are studied by using three-year data sets of electricity import and export data from two distinct, real-life cases from Finland. Two separate approaches are analysed: the use of energy storages, physical or monetary, and changing of the electricity metering method. A switch of the electricity metering method from instant phasewise to hourly net metering was found to increase the self-sufficiency by about 3 to 5 percentage points and have an annual monetary benefit of a few tens of euros when a network storage was used. Considering the energy storage methods under study, the network energy storage was found to be more economically feasible than a physical or a virtual battery energy storage, even though a physical battery storage could increase the self-sufficiency as much as by 30 percentage points with a storage capacity of 20 kWh. The studied virtual battery concept was found to limit the profitable solar photovoltaic plant size if high enough storage capacity was not provided. When a physical battery energy storage is used, switching to hourly net metering does not add value to the system. A significant decrease in the system cost is required for a physical battery energy storage to be economically competitive in northern climate conditions. Publishers version

The capacity of each component in an off-grid water electrolyzer hydrogen production plant integrated with solar photovoltaics and a battery energy storage system represents a significant factor affecting the viability and reliability of the system. This paper describes a novel method that optimizes simultaneously the component capacities and finite-state machine based control of the system to minimize the cost of green hydrogen production. The components and control in the system are referenced to a proton exchange membrane water electrolyzer stack with a fixed nominal power of 4.5 kW. The end results are thus scalable by changing the nominal power of the electrolyzer. Simulations are carried out based on data collected from a residential solar photovoltaic installation with 300 s time resolution. Optimization of the system is performed with particle swarm optimization algorithm. A sensitivity analysis performed over the prices of the different components reveals that the price of the water electrolyzer has the greatest impact on the green hydrogen production cost. It is found that the price of the battery has to be below 0.3 €/Wh to become a feasible solution as overnight energy storage.