EP50 — Unicorn Files — Polar Night Energy: The Sand Battery Storing Renewable Heat

In this Unicorn Files episode we tell the story of Polar Night Energy, a Finnish startup that chose one of the most abundant materials on Earth — sand — to tackle one of the most complex challenges in the energy transition: how to store heat in a way that is cheap, safe, and scalable.

Founded in 2018 in Tampere by Tommi Eronen and Markku Ylönen, Polar Night Energy starts from a precise engineering thesis: industrial heat and district heating represent one of the largest and hardest-to-decarbonize sectors in the economy. Lithium batteries are not built for this. Natural gas still dominates. Something different is needed.

The answer is the Sand Battery: a system that heats ordinary sand to temperatures of up to 500°C using electricity from renewable sources, and keeps it hot for weeks thanks to the material's excellent thermal insulation properties. When heat is needed, a heat exchange system releases it into the district heating network.

The journey follows the classic deeptech arc: years of development, proof of concept, first commercial customer. The world's first commercial Sand Battery goes into operation in Kankaanpää in 2022, at Vatajankoski. Then comes Pornainen — 1 MW of power and 100 MWh of capacity — currently the world's largest sand-based thermal storage plant, designed to help decarbonize local district heating.

In 2024 and 2025 the portfolio grows: the Power-to-Heat-to-Power pilot in Valkeakoski, a partnership with Lahti Energia for a 250 MWh system, and expansion into the Baltic markets with Solsiden and Soldag. In 2026 Polar Night Energy wins the Breakthrough Award from TEK, Finland's most prestigious engineering association.

What makes this story stand out isn't just the technology. It's the strategic consistency: a startup that didn't try to do everything, that identified a specific market — industrial heat and district heating — and built progressive proof points to earn trust in a sector where customers don't buy hype.

They buy reliability. They buy risk reduction. They buy operational fit.

An episode about deeptech, engineering patience, and one lesson for founders: sometimes the most powerful innovation is the one that looks the simplest.

• Il mercato più grande non è sempre il più visibile: il calore industriale è una delle grandi sfide della decarbonizzazione.
• Nel deeptech, la fiducia si costruisce con proof point progressivi: pilot, primo cliente, scala industriale, internazionalizzazione.
• Una tecnologia complessa può avere una narrativa semplice: una batteria fatta di sabbia.
• I clienti B2B industriali non comprano hype, comprano affidabilità, riduzione del rischio e integrazione operativa.
• La sostenibilità scala quando diventa anche un business case: meno combustione, meno volatilità, più controllo energetico.