Polar Night Energy, the Finnish brainchild behind the sand battery, introduces an innovative approach to high-temperature thermal energy storage. These sand-based marvels utilise sand or similar materials as a medium to store energy in the form of heat. Their primary objective is to serve as high-capacity reservoirs for surplus wind and solar energy. By storing this surplus as heat, the potential applications are diverse, including heating homes, generating steam, and fueling high-temperature processes in industries traditionally reliant on fossil fuels.
The founders, Tommi Eronen and Markku Ylönen, aspire to propel the transition toward renewable energy. Presently, the intermittent nature of renewable energy sources presents challenges for energy networks, and conventional storage methods are both costly and imperfect. Sand batteries offer a promising solution, allowing renewable energy production to scale up effectively, ensuring a consistent supply of clean energy.
The world's first commercial sand battery is situated in the Finnish town of Kankaanpää, where it connects to the district heating network, providing warmth to residential and commercial buildings and even the municipal swimming pool. The term "sand battery" gained wide recognition through a BBC news story in July 2022. As of now, sand batteries are not yet available for individual homes.
How Sand Batteries Operate
A sand battery comprises an insulated steel silo housing, containing sand and heat transfer pipes. External components, including automation systems, valves, a fan, and a heat exchanger or steam generator, facilitate the process. These batteries are charged using electricity from the grid or local sources, often stemming from variable sources such as wind or solar power. The electrical energy is directed to the heat storage through a closed-loop air-pipe system, where electrical resistance heating elements heat the circulating air within the heat transfer pipes.
In Kankaanpää, the sand can attain temperatures of up to 600 degrees Celsius, with the potential to achieve even higher temperatures depending on the specific requirements. The maximum temperature is primarily constrained by the heat resistance of the construction and control materials rather than the properties of the sand as a storage medium.
To release the stored heat, cool air is blown through the pipes, gradually heating as it passes through the sand storage. The resulting hot air can be employed to convert water into process steam or to heat district heating water using an air-to-water heat exchanger.
The Advantages of Sand
Sand is particularly well-suited for this application because it can withstand temperatures far exceeding the boiling point of water. It can store several times more energy than a water tank of similar size due to its broad temperature range. In addition to its energy storage capabilities, sand offers the advantage of space efficiency and versatility in industrial applications.
The grain size of the sand is not a critical factor, as heat storage is not sensitive to grain size. Polar Night Energy opts for high-density, low-cost sand that does not deplete scarce sources. They emphasize the use of materials that are unsuitable for the construction industry.
Another compelling feature of sand is its ability to retain heat for extended periods. In the Kankaanpää installation, it undergoes charging cycles of approximately two weeks. The company reports that the heat storage functions optimally when charged and discharged between 20 and 200 times per year.
A Milestone for Renewable Energy
Although the concept of heating sand for energy storage is not entirely novel, Polar Night Energy's innovative model allows for large-scale heat storage applications. This innovation paves the way for the commercialisation of sand batteries, with clients spanning various sectors, including energy utilities, residential and commercial property operators, food and beverage companies, chemical and pharmaceutical industries, metal production, and others.
Even in the frigid Finnish winters, the sand battery operates efficiently, maintaining an internal temperature of 600 degrees Celsius due to superior insulation. When fully charged, the battery can store up to 8 MWh of thermal energy, capable of discharging around 200 kW of power through heat-exchange pipes. This output is sufficient to provide hot water and heating for approximately 100 homes and a swimming pool. In Kankaanpää, the sand battery supplements power from the grid, with charging typically taking place during off-peak hours when electricity prices are lower.
Sand batteries address a crucial gap in renewable energy by providing a low-maintenance solution and utilising lower-quality sand that would otherwise contribute to the global shortage of higher-grade river sand. Moreover, the components of the sand battery experience minimal wear and tear, with the fan being the only moving part, easily replaceable. Alongside its efficiency, sand boasts a remarkably long lifespan, offering a sustainable storage solution for renewable energy.