energy storage To store energy for a rainy day is one of the great challenges of a renewable energy system. But many technologies for energy storage already exist.

Using a high proportion of solar and wind energy increases the risk of temporary imbalances in the electrical system. Therefore, we need flexible production such as hydro power or gas and ways of controlling demand. In addition, various types of energy storage will also be needed.

Batteries are ideal for frequency regulation, as they can alternate from charging to supplying power to the grid as quick as a flash when needed to balance the system.

“Many people are sceptical of the idea of using batteries in the electrical system, as they represent an expensive way of storing energy. However the main factor here is not energy storage, but output regulation. Although the energy capacity of batteries is measured in kilowatt seconds rather than kilowatt hours, their output is large and makes a difference. Many of them together can produce hundreds of megawatts, like a large hydro plant,” says Mikael Nordlander, Head of the Research & Development Portfolio Future of the Energy System at Vattenfall R&D.

District heating can become another key component, simply by complementing boilers with immersion heaters and accumulator tanks. This technology is already of great interest in Germany and Denmark because these countries already have many hours during the year with large electricity surpluses. The potential is enormous – a review shows that the existing district heating system could in theory, balance all generated Swedish wind power for many hours, should it become necessary.

“We are now also looking into the possibility of high-temperature storage of electricity in a medium that is heated up to several hundred degrees. This would allow us to convert electricity to heat and then back again to electricity when there is a shortfall,” says Mikael Nordlander.

See more examples of energy storage technologies:

Mega batteries
How: Currently, sodium-sulphur batteries are most common, but lithium-ion ones are on their way. Storage time: Up to several hours. Comments: These batteries store a small quantity of energy but can handle large outputs, which is valuable for regulating frequency. Sometimes mega-batteries are used in close proximity to a single wind farm in order to balance the output.
Electricity to heat
How: Surplus electricity heats water for district heating. Storage time: From several hours to several days. Comments: This is good for handling power production peaks. It is cheap to build as the district heat infrastructure is already in place. It cannot normally be converted to electricity again, but reduces the need for other fuels. However, studies are being conducted to find out if high-temperature storage could be a feasible way to convert electricity to heat and back, for example by producing steam to run a turbine.
Compressed air
How: Air is compressed into rock caverns or similar and is then released via a turbine. Storage time: Usually hours. Comments: Plants exist but are not widely used. They have poorer efficiency than a pumped storage plant, which makes it hard to run them profitably.
Home batteries
How: Batteries that store solar power in private homes. Storage time: Several seconds to several hours. Comments: These are already starting to pick up, especially in Germany. They can be used both to store one’s own solar electricity until it is needed and to help regulate the frequency of the system. Batteries in electric cars could be used in a similar way.
Electricity to fuel
How: Electricity splits water to produce hydrogen gas, which can be further developed into other burnable gases such as methane and ammonia, or liquid fuels such as ethanol. Storage time: Theoretically unlimited. Comments: The technology is proven and some 30-40 pilot plants are currently in place in Europe. The challenge today is getting paid enough for fuel produced in this way.
Pumped storage plant
How: Water is pumped back up into the reservoir at times of low demand. Storage time: Usually several hours. Comments: Good efficiency – up to about 80%. A mature and proven technology. Limited expansion potential. Requires large reservoirs or a high head.

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