Even back in ancient Rome and Egypt, people used sand as a heat storage medium to keep bathhouses and villas warm. Polar Night Energy, a Finnish startup, has now taken this principle to a whole new level by developing the world’s first commercial sand battery to supply cities and districts in Finland with thermal energy. A sand battery consists of a silo filled with surplus industrial sand and pipes circulating heat inside it, heating up the sand to up to 600°C. The heat can be stored for several days, weeks, or even months. The system is heated using electricity from the grid, solar power, or wind energy.
In an interview with co-founder and CEO Tommi Eronen, we learn more about the technology, its potential, and a pilot project exploring the use of sand batteries for electricity generation.
Before Markku Ylönen, our CTO at Polar Night Energy, and myself founded the company, we studied together at Tampere University of Technology. Living in Finland, a cold country with a high energy demand, especially for heating, we became deeply interested in supporting Finland’s energy self-sufficiency.
However, after finishing our studies, we realized that as the climate crisis progresses, vast amounts of energy storage will be needed to support the rapid growth of clean energy. That is why we came up with the idea of building a sand battery.
We started off by searching for the cheapest and most readily available solid material. At first, we considered soil from industrial sites, but soon realized the odors produced when heating organic materials made it unsuitable. Sand emerged as the second most affordable option, and it turned out to be the best choice. That is how the sand battery was born – and in 2018, we founded Polar Night Energy to turn our idea into reality.
While the efficiency of the sand battery would be slightly higher in a warmer climate, this difference is likely only a few percentage points – except in extreme conditions, like -30°C in winter.
Our system is well-insulated with traditional rock wool, and the sand itself also has insulating properties. If you were to touch the outer surface of our silo in Kankaanpää, it would not feel warmer than the ambient temperature. While our thermal storage does have minor energy losses, our first commercial plant in Kankaanpää operates at around 75 percent efficiency. Larger installations are expected to reach efficiencies of 85 to 90 percent.
The most profitable use cases are industry applications that rely on thermal processes requiring heat above 100°C. This is why we do not compete with heat pumps, which typically provide heat at lower temperatures.
One example: District heating companies in Finland often have the option to use an electrode boiler combined with a large water-based storage tank, which limits the top temperature to around 95°C. However, when the required temperature exceeds 100°C, our system becomes the ideal solution. Storing steam at high temperatures would carry extreme risks associated with pressurized steam tanks. That is why our thermal storage system, which uses sand instead of steam, offers a much safer and more efficient alternative for storing energy above 100°C.
One of the key advantages of our system is its flexibility in charging. Unlike heat pumps, which have a coefficient of performance (COP) above 300 percent, our system operates below 100 percent COP. However, we can be strategic and charge the battery when electricity spot prices are very low or even negative, significantly reducing costs. Additionally, we can sell capacity at specific times to help balance the grid, further enhancing profitability.
Our first commercial unit, located in Kankaanpää, produces 0.2 megawatts (MW) of thermal power, which is a fraction of the local grid’s total demand of approximately 20 MW.
Our next unit, currently being prepared for market in the city of Pornainen, has a thermal output of 1 MW. For a small grid like Pornainen’s, this is significant. In fact, it will cover nearly all of the city’s heating demand for most of the year. Looking ahead, we can certainly scale up and build much larger systems for bigger cities, enabling the sand battery to provide sustainable heat on an even greater scale.
We offer a highly relevant option for project developers looking for thermal energy storage. Our sand battery comes in two sizes: a 2 MW and a 10 MW system. We are currently optimizing the energy storage capacity of the larger unit to offer potential customers a more cost-effective solution.
Compared to electrical batteries, our systems are not yet competitive. However, when choosing between lithium batteries and thermal batteries, project developers should consider a key advantage of thermal storage: While lithium battery prices have decreased significantly, large-scale lithium battery projects – in the 100 MW range – still cost around one million euros per MW of power. Our sand battery offers a significantly lower price point.
Moreover, lithium batteries are typically designed to store energy for one to four hours, meaning they need to be charged and discharged frequently. In contrast, the sand battery allows for much easier energy storage. The key factor is the amount of sand surrounding the pipe for heat transfer, which makes increasing energy capacity straightforward and cost-effective.
While our system has a lower efficiency than lithium batteries, it is specifically designed to produce thermal energy for cities or various industrial processes – large-scale lithium batteries cannot do that. In many ways, our sand battery functions similarly to the combined heat and power plants (CHP) currently used in Finnish cities, but with a sustainable and renewable energy source.
Power-to-Heat-to-Power is currently the pilot phase. Our approach involves converting thermal energy back into electricity using a modified gas turbine – without combustion. Instead of burning fuel, we have adapted the turbine so that the air passes through the compressor, is extracted via a nozzle and redirected through the sand battery, heating it.
In a conventional gas turbine, heat is generated through combustion. In our system, however, the air is heated inside the sand battery before being reintroduced into the turbine, exactly as would be the case with heat generated through combustion.
If we can bring this technology to market at a competitive price, we can deploy large-scale sand batteries across Europe, particularly in locations where conventional gas turbines will need to be replaced. Existing turbines could be modified to integrate with our sand battery. Implementation would be straightforward because the necessary grid connections are already in place – we would simply need to install the sand battery.
For the next few years, we want to focus on European markets, with plans to expand globally in the future. Our primary target industries are those with a high thermal energy demand, such as food and beverage, chemical and manufacturing.
In general, we are particularly relevant to industries that require hot water, steam, or hot air for drying and other thermal processes.