To prevent localized grid blackouts or harmful voltage spikes, utilities are increasingly pairing renewables with battery storage and/or rapid-response natural gas plants to smooth out fluctuating supply. Finnish technology startup Polar Night Energy has constructed the world's largest commercial sand battery, a revolutionary high-temperature thermal energy storage design capable of storing 100 MWh of renewable energy. The Pornainen sand battery isn’t the company’s first. Finland’s sand battery is one of the first commercial-scale technologies to demonstrate what long-duration thermal energy storage could look like. As utilities look beyond lithium-ion for longer-duration storage, technologies such as thermal storage are beginning to attract greater commercial interest.