Matt is an environment correspondent.

sand battery
Image caption, The sand battery has been installed and is functioning well according to the power company

The world's first fully functioning sand battery can store green power for months at a time.

A major issue for green energy is year-round supply.

The device is charged up using cheap electricity from the sun or wind.

Warm homes can be found in winter when the heat is stored in the sand at 500C.

The war in Ukranian has drawn attention to the issue of green power.

It has the longest Russian border in the EU and Moscow has stopped gas and electricity supplies in the wake of the decision of the Finns to join NATO.

Politicians and citizens alike are concerned about sources of heat and light, especially with the long, cold winter on the way.

There is a new piece of technology in a corner of a small power plant that has the potential to ease some of the concerns.

This device has a key element. There is a large amount of sand in a silo.

These rough and ready grains can be used to store power when needed most.

Climate change and the rising price of fossil fuels have led to a surge in investment in renewable energy production.

New solar panels and wind turbine sources can be quickly added to national grids.

How do you keep the lights on when the sun is out and the wind is blowing?

Adding more renewable sources to the grid means you need to increase other sources of energy to balance the network, as too much or too little power can cause it to collapse.

Large scale batteries can store and balance energy as the grid becomes cleaner.

Image source, Tony Jolliffe
Image caption, A bucket of sand used in the battery pilot plant

Most batteries are made with lithium and are expensive with a large footprint and can only handle a limited amount of power.

In the town of Kankaanp, a group of young Finns have completed the first commercial installation of a battery made from sand that they believe can solve the storage problem in a low-cost, low impact way.

Markku Ylnen, one of the two founders of Polar Night Energy, said they wanted to be able to get the green electricity into the storage quickly.

The Vatajankoski power plant has a district heating system.

The same process that makes electric fires work warms the sand by using low-cost electricity.

A heat exchanger is used to transfer hot air from the ground to the air.

Sand is a good medium to store heat. The developers say that their device can keep sand at 500C for a long time.

When energy prices go up, the battery discharges hot air which warms water for the district heating system which is pumped around homes, offices and even the swimming pool.

Image caption, Finnish researchers Markku Ylönen and Tommi Eronen, who came up with the sand battery idea

The sand battery idea was developed at a former pulp mill in the city of Tampere, with the council donating the work space and funding to get it off the ground.

Elina Seppnen is an energy and climate specialist for the city.

If we have a solution that gives flexibility for the use and storage of heat, that would help a lot in terms of expense.

Image source, Tony Jolliffe
Image caption, The sand battery helps to warm homes, offices and the local swimming pool

One of the biggest challenges is whether the technology can be scaled up to make a difference, and whether the developers can use it to get electricity out as well as heat.

Sand is used to return power to the grid.

Most of the process heat that's used in food and drink, textiles or pharmaceuticals comes from the burning of fossil fuels, so storing green energy as heat is a huge opportunity for industry.

Sand is being looked at as a viable form of battery for green power.

The man who invested in the system says that the Finns are the first with a working system.

The managing director of the Vatajankoski power plant likes the idea of being the first in the world to do something like this.

If you want, it's crazy, but I think it will be a success.

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