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The world's first sand battery is already operating in Finland, supplying heat to residential and office buildings in the city of Kankaanpää.
If you've walked along the beach barefoot at the end of a hot day, you'll know that sand retains heat well. For example, a container with one kilogram of sand takes more than five hours to go from 40°C to 20°C. This is due to the low heat transfer coefficient of silicon dioxide, the main component of sand. And that is the principle behind an innovative sand battery recently installed in Finland to store green power throughout the harsh Scandinavian winter.
There are currently numerous renewable energy storage projects, some more fanciful, such as gravity systems, and others already operational, such as this pilot project based on recycled electric batteries from cars installed in a photovoltaic plant in Navarra. Now the sand batteries complement these experiences with a promising alternative approach.
Markku Ylönen and Tommi Eronen are two Finnish researchers who have set out to create what is touted to be the first commercial sand battery. The initial aim was to compensate for the intermittency of renewable energy sources such as wind and solar power by converting surplus electricity production into heat. To this end, they have accumulated 100 tons of sand in a silo with insulating materials installed next to a power plant.
This sand battery uses resistive heating, also known as the Joule effect, whereby electrons flowing through a resistor generate heat, as in a toaster or stove. Subsequently, the hot air generated is circulated through a heat exchanger, which transfers the heat to the sand. The developers claim temperatures above 500°C can be reached, storing up to 8 MWh.
The battery is already in operation and supplies heat to residential buildings, offices, and even a public swimming pool in the Finnish town of Kankaanpää. This battery is capable of storing heat from renewable energy for months. In the case of sand, using the heat directly is more efficient than converting it back into electricity, as a lot of energy is lost in the process. Once the feasibility of the project has been demonstrated, the next step is to create a thousand times larger battery to move towards a more sustainable energy system.
Across the pond, in the USA, NREL (the National Renewable Energy Laboratory) is working on a more ambitious prototype, although based on the same principles as its Finnish counterpart. The innovative battery, dubbed ENDURING, could reach a storage capacity of up to 26,000 MWh. But how exactly does it work?
In this case, gravity is added to the equation. NREL's prototype uses conveyor belts that lift the sand onto heating elements that heat it to 1200º C for storage in silos, like dropping sand through the heating elements of a toaster. When power is needed, the particles are gravity-fed through heat exchangers to power steam turbines that generate electricity fed back into the grid.
In an article about the ENDURING prototype, the U.S. laboratory points out that sand is a stable, low-cost material, with a price ranging from thirty to fifty dollars per ton, and a low ecological impact in its extraction and at the end of its useful life. It also points out that although thermal energy storage has a lower density, its cost can be as low as two dollars per kWh.
As they are used as heat accumulators, sand batteries are not as versatile as other technologies such as lithium, but in return, they offer numerous advantages:
Sand batteries are part of a new wave of large-scale electricity storage systems, such as liquid batteries. You can read more about these types of technologies in this article or subscribe to our newsletter at the bottom of this page to keep up with the latest news.