Excess power from wind turbines pumps water uphill to holding tanks. The water is released when power is needed, powering traditional turbines to create more electricity. Pumped hydro storage is one of the oldest ways to store energy, but Ireland is planning to do so on a massive scale in up to fifty locations. This could turn the country into a net exporter of electricity, and solves a major problem with renewable energy, its variability in producing power. With pumped hydro, power can be generated at night or when the wind isn’t blowing.
Other forms of stored energy include compressed air in underground caverns and solar thermal storing excess heat from the sun in molten salt.