We have a liftoff. A crane lifts a massive steel cylinder off a truck in Vatajankoski’s power plant area in Kankaanpää, Western Finland. The seven-meter times four-meter cylinder swings slightly on a wire and looks a bit like a space rocket.
Although Vatajankoski, an energy utility company, is not preparing for a space flight, this is a historic event. The “rocket” is the first commercial sand-based high-temperature heat storage in the world. It utilizes an automatic heat transfer system developed and patented by Polar Night Energy, a Finnish startup from the city of Tampere.
Once the heat storage has safely landed on the earth's surface, about 50 cubic meters of sand is poured into it from an opening on the roof of the cylinder.
Vatajankoski uses the heat stored in the sand to raise the temperature of waste heat generated by their data servers. The temperature of the 60-degree waste heat from the servers needs to be raised to between 75 and 100 degrees Celsius depending on the season, before it can be fed into the Kankaanpää district heating network. The heat storage built in the beginning of 2022 can provide 100 kW of heating power and can store 8 MWhs of thermal energy.

– The starting point of this project was not long-term storage, but could we make seasonal energy storages with the same technology, with more than thousand times higher capacity? ponders Lauri Hölttä, Production Manager at Vatajankoski.

– Such a storage would allow us to utilize the cheap energy of the summer season as much as possible to charge the storage, and then use this stored energy to fulfill the heat demand of the winter season. It would reduce heat production based on combustion.

Energy Storage Is a Challenge

Storing energy is one of the key challenges as we move from combustion technologies to more sustainable forms of energy production. By 2030, the world’s current energy storage capacity is projected to multiply, and annual growth rates are in double digits.

R&D around the topic is active. Polar Night Energy is part of an EU ERA-Net SES project called NewSETS together with LUT University, Flexens and Pumped Hydro Storage. The project studies the utilization of electricity and heat storages as part of a flexible and independent energy network. NewSETS will do a case study about the Åland archipelago.

– Storage is needed when energy production and consumption do not meet. One good way to store energy for long periods of time is heat, says Tommi Eronen, CEO of Polar Night Energy.

Globally, heat production is largely based on combustion and fossil fuels, which are being feverishly sought to be replaced. It means there’s a need to electrify heat production, to recover waste heat, and to find new ways of storing heat.

Markku Ylönen (left), CTO, and Tommi Eronen, CEO of Polar Night Energy.

Sand-based Thermal Energy Storage

A common way to store heat is to heat up water. This includes both domestic water boilers and water-based district heating storages. Water-based heat storages function as short-term and high-power storages. However, storing large amounts of energy in water is expensive and takes up a lot of space.

Many solid materials, such as ordinary sand used by Polar Night Energy, can be heated to temperatures well above the boiling point of water. Sand-based heat storages can store several times the amount of energy that can be stored in a water tank of a similar size; this is thanks to the large temperature range allowed by the sand.

– In addition, a sand-based heat storage can be used in industrial applications that require temperatures above the boiling point of water. In practice, the maximum temperature of a sand-based heat storage is not limited by the properties of the storage medium, but by the heat resistance of the materials used in the construction and automation of the storage. The maximum temperature of our storages built so far is about 600 degrees Celsius, but we are planning to raise it even higher, says Eronen.

From Season to Season

Polar Night Energy's solution can be used for both short-term and seasonal storages, allowing energy to be stored even for months. The sunshine of the summer can be stored for the darkness and cold of the polar night. At the same time, part of the capacity of the storage can be utilized in leveling the shorter-term consumption profile or in the frequency management of the electricity grid.

– The structure of our heat storage is simple. It is an insulated tank made of concrete or steel, filled with sand and pipes. Additionally, equipment outside the storage is required, such as automation components, valves, a fan and a heat exchanger or a steam generator, Eronen explains.

The basic operating principle of the storage is also simple. Electricity is converted to heat by heating air in a closed-loop piping using electrical resistors. When hot air is circulated in a closed loop through the storage, heat moves from the air to the sand.

The storage is unloaded by blowing cool air through the pipes. It heats up as it passes through the storage, and it can be used for example, to convert water into process steam or to heat district heating water in an air-to-water heat exchanger.

In addition to electric heaters, heat can be recovered from hot gas or sufficiently warm waste heat streams. However, the stored waste heat must be above 500 degrees Celsius, as the storage is driven by temperature difference and the storage temperature is also high.

Sector coupling enables the optimization of a large entity

Towards Climate-neutral District Heating

The Eagle has landed. Vatajankoski is on its way to climate-neutral district heating.

– We want to mitigate climate change and offer our customers sustainable energy solutions. Most of our production is already renewable or completely emission-free. A small number of fossil fuels are still used in heat production, but they are planned to be phased out soon, says Lauri Hölttä.

The use of renewable and zero-emission energy has long been slowed down by high prices. According to Hölttä, this has now changed through taxation and emission rights, although some challenges remain.

There is an economic pressure to give up fossil fuels. In the case of renewables, which culminate on biofuels in Finland, the problem will be availability. Using waste heat, the challenge is that production and consumption do not meet, so thermal energy must be stored somewhere. The energy demand for district heating is many times higher in winter than in summer.

According to Hölttä, seasonal storages play a big part in giving up fossil fuels and later biomass.

– Different technologies exist, but for example sand and blast-furnace slag are suitable for longer-term storage. Sand is a simple solution – something you may not even think of.

Sand is indeed an effective solution for reducing climate emissions. According to a report by Mission Innovation, sand-based high-temperature energy storage could save more than 100 Mt CO2e annually after 2030. Saved emissions would be around 3 percent of the current emissions of the whole European Union.

Liisa Naskali, Project Manager and
Miika Peltola, Communications Specialist,
Polar Night Energy