8 megawatt-hours of power and heated the sand up to 1000 °F: the world's first sand battery Vatajankowski went into operation in Finland

Wind and solar power are intermittent, so they produce energy when it is available, not when it is required, necessitating a large amount of energy storage. This may take many forms, ranging from traditional lithium-based "big batteries" to flow batteries, silicon phase-change batteries, molten salt batteries, iron-air batteries, gravity batteries, carbon dioxide expansion batteries, and other more unusual technologies like buoyancy cells.

In terms of energy efficiency, output power, location, installation expenses, operating costs, input and output power ratings, longevity, and how long it can store electricity for, each has its own strengths and drawbacks. That's excellent because different solutions will meet distinct demands; some adding to the grid during sudden demand spikes while others smoothing out fluctuating daily curves between demand and renewable supply. Others still assist with seasonal supply shortages like when solar production drops off throughout the winter.

Another from the pile, originating in Finland. The debut of the first commercial sand battery has just taken place at "new energy" firm Vatajankoski, about four hours northwest of Helsinki, according to Polar Night Energy.

This is a thermal energy storage system that uses an enormous, insulated steel tank - around 4 meters (13.1 feet) wide and 7 meters (23 feet) tall – filled with ordinary sand as its heat reservoir. This equipment is capable of storing 8 megawatt-hours of power at a nominal power rating of 100 kW while the sand is heated to somewhere around 500-600 degrees Celsius (932-1112 °F).

When the need arises, this energy is again extracted as heat in the same manner. Vatajankowski is tapping into its accumulated heat, in addition to extra heat generated by its data servers, to power the local district heating system, which draws on pipes for distribution of heat. It can then be used to warm structures or swimming pools, industrial processes, or any other application that demands heat.

The sand, according to the firm, just needs to be dry and free of combustible trash. Indeed, the business thinks it's a great way to store energy. It's so easy and inexpensive that Polar Night Energy claims the installation costs are less than €10 per kilowatt-hour, and it runs itself in a fully automated mode using no consumables at all.

The company promises that it'll scale up as well, with hundreds of megawatts of nominal power generated by installations with a capacity of 20 gigawatt-hours of energy storage and the sand heated to 1000 °C (1,832 °F) in certain designs. If they're the correct form, disused mine shafts may be used to construct big underground storage facilities. There are no high-pressure vessels required; rather, the most expensive component is usually the piping.

The business name "Polar Night" is a play on the fact that certain areas of Northern Finland experience no sunlight during the winter, when they are above the latitude where there is no direct sun for weeks on end in the depths of winter. This sand battery, according to the firm, will have its greatest impact during situations like this, when its long-term storage allows buildings to be heated more cheaply and effectively through the freezing Finnish winter.

The advantage of this solid sand storage container is that it enables many "zones" of energy storage within the sand. It's possible to create a system for longer-term heat storage in the center of the sand cylinder, but shorter-term repeated use cycles closer to the top surface or outside are more practical. Because the liquids would constantly be mixing and moving, this would be impossible in a liquid medium like water or molten salt.

This system will only see widespread usage in regions with district-level heating. However, there is a surprising number of district heating going on. Over half of all Scandinavian homes have it, as do many other countries, including Northern China and the United States.

Because of this, the Mission Innovation climate solutions framework has predicted that deploying all of Polar Night's energy storage technology to its maximum potential could eliminate enough carbon-burning heat sources to lower annual greenhouse emissions by 57 to 283 megatons CO2 equivalent per year by 2030. That would be a significant contribution in and of itself.