Seasonal Energy Storage in Aluminium for 100 Percent Solar Heat and Electricity Supply

In order to reduce anthropogenic global warming, governments around the world have decided to reduce CO₂ emissions from fossil fuels dramatically within the next decades. In moderate and cold climates, large amounts of fossil fuels are used for space heating and domestic hot water production in winter. Although on an annual base solar energy is available in large quantities in these regions, least of the solar resource is available in winter, when most of the energy is needed. Therefore, solutions are needed to store and transfer renewable energy from summer to winter. In this paper, a seasonal energy storage based on the aluminium redox cycle (Al3+→Al→ Al3+) is proposed. For charging, electricity from solar or other renewable sources is used to convert aluminium oxide or aluminium hydroxide to elementary aluminium (Al3+→Al). In the discharging process, aluminium is oxidized (Al→Al3+), releasing hydrogen, heat, and aluminium hydroxide or aluminium oxide as a by-product. Hydrogen is used in a fuel cell to produce electricity. Heat produced from the aluminium oxidation process and by the fuel cell is used for domestic hot water production and space heating. The chemical reactions and energy balances are presented, and simulation results are shown for a system that covers the entire energy demand for electricity, space heating and domestic hot water of a new multi-family building with rooftop photovoltaic energy in combination with the seasonal Al energy storage cycle. It shows that 7–11 kWp of photovoltaic installations and 350–530 kg Al would be needed per apartment for different Swiss climates. Environmental life cycle data shows that the global warming potential and non-renewable primary energy consumption can be reduced significantly compared to today's common practice of heating with natural gas and using electricity from the ENTSO-E network. The presumptive cost were estimated and indicate a possible cost-competitiveness for this system in the near future.