The intermittent availability of renewable energy generation is often cited as a key challenge. How do energy consumers keep the lights on or their homes warm when the wind stops blowing? Or if there is an overcast sky?

Permanent and modular lithium-ion battery systems are the most common way of storing surplus renewable energy. In fact, used electric vehicle batteries are increasingly finding a second service life for this exact purpose.

But there are other creative ideas to store this energy. One such instance is happening in the cooler reaches of Europe, where a Finnish town is pioneering a new method for storing unused solar and wind power. This project might just revolutionize the sustainable energy industry by stockpiling the excess energies in a sand battery technology.

Supplementing wind farms and solar panels as they collect nothing but dust on still, chilly nights, this gritty tech promises to store cheap, carbon-neutral power as heat by utilizing what is often underfoot.

It’s a new twist on an old idea

Old storage heaters in twentieth-century homes used a similar concept. The back-breaking appliances, packed with ceramic bricks, stored thermal energy for many hours, releasing it overnight to maintain room temperature when residents couldn't stoke a fire. Sand, the literal building blocks of such bricks, is still an attractive solution to energy problems. It’s stable and inert, non-toxic, inexpensive to mine and is generally found everywhere on Earth.

Sand energy storage is part of a burgeoning group of technologies known as thermal energy storage. In the case of the sand, energy is stored as heat, not chemically. And the tech isn’t limited to sand. Molten salts are in R&D labs, being studied as a potentially viable thermal storage medium. In fact, products of this nature have been around for years. For example, the Solana Generating Station in Arizona is a massive solar farm covering over 1,750 acres. Notably, this facility implemented 12 massive molten salt tanks to store thermal energy overnight, when the plant's 32,000 solar collectors go dark.

Finland is a northern country with some arctic territories. Since the population is widespread, it’s not easy to manage the nation’s electricity grid in a unified system. Oil heating has always long held significance in Finland’s heating energy mix, but reliance on such fossil fuel energy sources has experienced a significant drop since 1990, as per this recent United Nations Climate Change report. Oil also faces challenges in terms of pipeline infrastructure and a long winter that hampers road transit.

Polar Night Energy is implementing a project in Pornainen, Finland, a district just a short 54 km drive northeast of Helsinki, that uses crushed soapstone, not molten salts. The same general principle applies, just on a more sustainable scale.

The sand battery being built in Pornainen is designed to store excess energies generated by local solar panels and wind farms. Previously, this surplus energy was wasted. The idea is to construct a thermally insulated 42 ft tall cylinder, then fill this storage unit with a granular sand medium that features superior thermal energy storage properties, as covered in this article from ACS Omega. The sustainable thermal energy storage system is capable of earning the township a 70% drop in emissions.

Implementation data suggests a resistive electricity conversion circuit will impart the sand battery with 600° C of thermal energy. The heat collects in what’ll amount to several hundred tons of sandy media, as supplied by a Finnish fireplace company. To convert the heat back to electricity, the heat is used to generate steam and spin turbines on demand. The design will hold an estimated 100 MWh of thermal-electric energy, and the estimated conversion efficiency is up to 95%.

The hope is to eliminate 160 tons of carbon dioxide from the atmosphere annually, which is a mighty impressive claim for a cylinder full of sand. The Pornainen sand battery will take around 13 months to complete, going into operation as one of the first of what will be a scalable technology, built to increase in size to encompass several hundred tons of heat-stockpiling sand.

The engineering resources come from municipal heating company Loviisan Lämpö as they partner with the startup Polar Night Energy to build a system that efficiently holds thermal energy in the inert, commonly sourced granules.

The Pornainen project isn’t the first of its kind. The Polar Night Energy team has already completed a prototype in western Finland at the Vatajankoski power plant. The prototype is a 23 ft tall steel construct that uses the thermal energy directly. It supplies heat to about 100 homes through a municipal heating network.

Polar Night Energy believes that they can build sand battery storage systems up to 20 GWh that can insulate sand in temperatures up to 1,000° C. Key seems to be in providing better tank insulation and designing the resistive heating elements that convert the sustainable electricity into thermal, sand-stored energy. Also, there are numerous intensive engineering tasks to complete, focusing on the cubic thermal storage properties of the sand and also on the characteristics of how the energy is transferred into the air circulating systems and heat exchangers.

With no toxic wear to worry about and very little energy lost, a sand battery won’t age anywhere near as quickly as a comparable molten salt storage unit might. Most units are estimated to last 50 years.

Summary

Thermal energy systems were some of the first convenient ways for homeowners to heat their homes through the night. Now, as renewable energy systems become a larger part of the energy mix, engineers must solve for their intermittency issues. It is very likely that large scale, commercial thermal energy storage will become a viable and prominent solution to some of these challenges.