A new technology is keeping a Finland town warm with renewable energy
The battery has a four metres wide and seven metres high steel container with 100 tonnes of recycled sand. Photo: Polar Night Energy A new battery technology by a Finland-based company that uses sand to store thermal energy may aid clean energy solutions. The innovation comes at a time when Europe is facing an unprecedented energy crisis.
Russia — the supplier of 40 per cent of the European Union’s natural gas supply — has shut off its pipelines to a large extent. Countries in the Northern Hemisphere rely on a central heating system in winters, with natural gas as the most common heating fuel. This is unlike developing countries with a tropical climate.
The sale of heat pumps, considered a renewable source of internal heating, rose by 35 per cent in the EU. A rise in the sale of other controversial alternatives, such as wood pellets, accompanied this simultaneously. The world is increasingly looking at renewable internal heating sources.
Experts generally believe thermal storage will reduce our reliance on fossil fuels, provide storage for intermittent renewable energy and help balance the grid. Thermal energy storage is yet to develop as a field in this sense globally fully.
One such early example is a battery that uses sand as a medium to store thermal energy. It was invented by Polar Night Energy, a company that describes itself as an “innovator of seasonal heat storage”.
The battery has a four metres wide and seven metres high steel container with 100 tonnes of recycled sand. The ‘silo’ supplies warm water to a district heating network connected to both resident and commercial buildings.
This battery is located in a town called Kankaanpää, Western Finland, with just over 11,000 people. A district heating system circulates hot water to keep buildings warm, industrial processes and public water systems.
This patented technology is useful to a country like Finland, one of the countries closest to the North Pole, where the sun sets at around 3 pm in winter months with temperatures as low as minus 30 degrees Celsius.
The latest battery model can store up to 8 megawatt-hours of energy as heat. The reservoir is so well-insulated from the outer environment that it can retain temperatures up to 600 degrees Celsius and prevent heat losses over time.
The sand at the core is very far from the boundary, so the heat stored in the core does not easily get lost, even if we wait for days or weeks, Polar Night Energy’s lead scientist Ville Kivioja told Down To Earth.
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“If we look at the sand in the steel silo, then some of this sand is quite close to the boundary structure, say, a distance less than 0.5 metres. This portion of the sand will conduct the heat quite effectively to the boundary structure, and thus this heat gets partly lost over time,” Kivioja said.
But sand has a low heat conductivity and is far enough from the boundary, so the heat is not lost, he explained. The sand-based heat storage could store heat for months, but this is yet to be scaled to this level.
“The sand battery delivers to a small geography currently, so the heat is discharged at the rate of 0.1 MW to the grid from a storage capacity of 8 MWh. This would technically mean providing this power for 80 hours straight when one starts with full storage,” Kivioja said.
The company has inked a new deal with a vendor for discharging heat at the rate of 2 MW from 200 MWh storage capacity, which would similarly translate to discharging time of 100 hours, the scientist added, keeping the vendor’s name confidential.
The battery receives electricity from the grid through excess solar and wind power, which is converted to heat and transferred to the sand. “Simply put, we heat the air inside using resistors and circulate the hot air using a fan through a series of pipes, which releases the heat to the sand where it is preserved,” Kivioja further said.
The sand battery has three major interconnected components; a steel silo containing 100 tonnes of sand where the heat is stored, an electric air heater with resistors used in regular ovens and an air-to-water heat exchanger. The exchanger has a mechanical pipe and water.
Hot air in the pipe heats up the water, which is connected to the district heating system.
“Air is blown via a fan through the curricular pipe system inside the silo. The cold air then enters the electric air heater, where it becomes hot with the help of a resistor located inside,” Kivioja said.
The hot air circulates inside the air-to-water heat exchanger through a metal structure without contacting the water.
“The hot water is discharged into the district heating system. Water circulates in a closed loop in the district heating system from the heat exchanger to the customer and back. The excess water returns and the process continues,” the scientist explained further.
Air circulates in a separate closed loop, from the heat exchanger to the storage and back. The closed loop of air circulation can both charge and discharge the storage via the district heating system, he added.
Water-based storage systems are a renewable alternative to produce building heat, according to Xiaoshu Lu, associate professor at the School of Technology and Innovations, Energy Technology, at the University of Vaasa in Finland.
“It can store heat up to only 100 degrees Celsius, whereas the sand battery can store heat up to 600 degrees Celsius, thus having a greater advantage,” she told DTE.
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The makers of the sand battery have yet to demonstrate seasonal storage capacity, but so far the technology looks promising, Lu said. Russia cutting off the natural gas supply to the EU had little impact on Finland because it consists of less than 10 per cent of Finland’s energy mix, including nuclear and Biomass, she added.
“Europe currently leads renewables integration in district heating, with about 25 per cent of its district heat supplies produced from renewable sources. Particularly high rates are observed in countries such as Sweden, Denmark, Austria, Estonia, Lithuania, Latvia and Iceland where renewables fuel more than 50 per cent of district heat,” a report by the IEA said last year.
“While bioenergy and municipal waste account for a large majority of renewable district network supplies, large-scale solar thermal systems and heat pumps are seeing growing interest. At the end of 2021, almost 300 solar district heating systems representing 1.6 GWh were in operation worldwide,” the report added.
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