Polar Night Energy's sand battery scales from 8 MWh pilot to the world's largest

Builds sand-based thermal energy storage plants that charge on cheap renewable power and discharge heat into district-heating and industrial networks.

Surplus renewable power gets cheap often enough that storing it as heat in crushed rock beats chemistry — no fire risk, no degradation, media priced by the ton.

Finnish engineers Markku Ylönen and Tommi Eronen started Polar Night Energy around an almost embarrassingly simple idea: make a really big pile of sand and heat it with surplus renewable power to about 500°C, where it stays hot for months. The first commercial system went online in 2022 at utility Vatajankoski in Kankaanpää — a 4×7 m steel container holding hundreds of tonnes of sand, 100 kW of heat power, 8 MWh capacity, feeding the town's district heating and priming the utility's data-server waste heat.

In 2024 the company announced a ten-times-bigger unit for Pornainen: a 13 m tall, 15 m wide silo holding 2,000 tonnes of crushed soapstone — waste chips from a local fireplace maker — rated 1 MW and 100 MWh, about a week of the town's winter heat. It was inaugurated in 2025, and the town's district heating oil was eliminated entirely; the operator reports 90% round-trip efficiency in operation. TechCrunch flagged the economics: the earlier plant stored heat for roughly $25 per kWh against about $115/kWh for lithium-ion packs, charging from Finland's Europe-cheapest power. In November 2025 Polar Night signed with utility Lahti Energia for a 2 MW / 250 MWh system in Vääksy — the largest of its kind — sized to also bid into grid operator Fingrid's reserve and balancing markets.

Heat is the right market to store cheaply: district heating is a large, concentrated, temperature-tolerant demand where lithium batteries are structurally uncompetitive.

The storage medium is industrial waste — crushed soapstone from a fireplace factory — which turns the input cost line into roughly zero.

Geography did the customer acquisition: Finland's cold winters, oil-fired heating towns, and post-2022 import shocks created utilities actively looking for exactly this product.

Grid-services revenue (Fingrid reserves) turns a heat product into a two-sided asset, which is how the second and larger plant got financed.

When the product is heat, not electrons, the cheapest medium is whatever solid is under your feet — and the winning revenue stream may be grid reserve markets, not the one you pitched.

As of late November 2025, the 100 MWh Pornainen plant is in operation and has taken oil out of the town's heating mix, and the 250 MWh Lahti Energia project in Vääksy is being prepared as the largest sand-based thermal storage built. An electricity-production pilot in Valkeakoski is under construction, and the company reports active negotiations for large-scale industrial heat and process steam projects.