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The potential of water for thermal energy storage underground

  • The potential of water for thermal energy storage underground

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The city of Västerås in central Sweden used a network of underground caverns to store oil reserves during the Cold War. Now energy company Mälarenergi is planning to decontaminate the caverns and use them to store hot water, to be later used for district heating, reports the BBC.

District heating supplies hot water or steam from a central location to consumers through a network of pipes. The heat is used for space heating and cooling, domestic water heating, and industrial process heat. Globally, it is a growing market with China, Russia and Europe as the major producers of global district heat, according to the International Energy Agency (IEA).

In Västerås, home to about 130,000 people, district heating supplies 98% of the households. Thermal storage below ground makes sense as the ground itself works as insulating material. The caverns will be filled with hot water up to 95 degrees Celsius, and warmth will be sent to a district heating network using heat exchangers. The storage capacity available is about 120 Olympic-sized swimming pools (300,000 cubic metres), which is 11 times larger than the largest above-ground hot water tank the company has nearby. The caverns will be filled with hot water before the year ends, with heat coming from a nearby power plant that burns waste or biomass. The company is considering installing carbon capture technology to reduce emissions from the plant.

Thermal energy storage entails energy conservation, not energy production: excess heat is stored for later use, thereby contributing towards more efficient and environmentally friendly energy use in heating and cooling. Reusing existing infrastructure such as caverns is a good strategy for cities, and underground thermal energy storage for district heating is not new; one of Sweden’s first examples was built in the 1980s in Avesta and is still in use, while other examples are operational or being planned in Sweden and Finland.

Other options suggested for thermal energy storage underground are abandoned coal mines, often flooded; in the UK, 25% of the population lives above them. Heat exchangers can be used to ensure any potential contaminants from mine water would not come in contact with a closed water loop for home supply. Another alternative is aquifer thermal energy storage (ATES), using aquifers for seasonal storage of cold and/or warm groundwater, a standard construction option in The Netherlands where thousands of ATES systems are in operation.

The impact of district heating systems on the environment depends on the type of energy used to heat the water. Whereas district heating networks have potential for efficient, cost-effective integration of low-carbon energy sources, the IEA says their decarbonisation potential is still largely untapped, and fossil fuels account for about 90% of global production, especially in the largest markets, China and Russia. There is potential for improving their efficiency of networks through smart technologies, and switch them to renewable energy sources such as geothermal and bioenergy, and integrate recycled heat, such as excess heat from data centres.

Within Europe, the market penetration of district heating and cooling systems varies widely, with more than 50% of citizens served in some northern countries like Denmark, Sweden, Estonia and Lithuania, and less than 5% is much of southern and eastern Europe. The EU-funded project WEDISTRICT aims to demonstrate innovative 100% fossil free district heating and cooling systems across different climate zones and building types in Europe. This is a sector with great potential to move closer to climate neutrality, if we consider that heating and cooling of buildings accounts for 50% of the total EU energy consumption, a large part of which (70%) is still generated from fossil fuels.

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