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The Magazine

Renewables storage

Electricity storage

As the share of renewables grows, storing electricity is crucial for balancing generation and loads. Decisionmakers have to analyze the economic and technical merits of the various storage options depending on the use case.

In the new world of energy, the supply of electricity from renewable sources fluctuates dramatically. A balance can be achieved by storing the energy to decouple the moment of power generation from the moment of consumption. Energy storage also provides greater stability in transmission and distribution grids, and greater security for the energy system as a whole.
Electricity storage can be used in industrial plants, craft businesses, private homes, and electric and hybrid vehicles. The systems in commercial use today can be broadly categorized as mechanical, electrical, chemical, electrochemical, and thermal. There are technologies suitable for large-scale storage, and others for smaller-scale applications. Siemens is working to develop solutions for many different storage technologies and systems. Obviously, all these forms of storage have their pros and cons.
Decisionmakers must carefully ponder the merits of each alternative in light of the particular use case. We’ve summarized the options and some of their main characteristics.

Batteries: great prospects and very flexible in use

Experts see batteries, and especially lithium-ion (Li-Ion) batteries, as having the greatest potential. They can be activated within seconds, and are capable of storing electricity for several hours. Their scalability makes them suitable for private households, small trade, industrial plants, and stabilizing the grid. They can also help to implement backup power supply. Lithium-ion batteries are used in many mobile devices and in electric cars.

Hydrogen: a mixed picture

Storage via hydrogen pathways is the best way to serve long-term storage needs of several weeks or months. However, its roundtrip efficiency is limited due to losses through electrolysis and power reconversion. A higher benefit is typically achieved if the generated hydrogen is used for mobility purposes. Hydrogen-powered vehicles with fuel cells have a longer range than battery-powered electric cars. They reduce CO2 emissions, and help meet climate protection targets and reduce fuel costs per km.

Pumped storage: a proven technology worldwide

Pumped storage is the form of grid energy storage with the greatest installed capacity worldwide. It is an established mechanical storage technology for large plants that typically store power for several hours. At times of low electrical demand and correspondingly low prices, excess power generation is used to pump water from a lower into a higher reservoir. When demand rises, water is released back into the lower reservoir through a turbine, generating electricity that is sold at higher prices.

Compressed air energy: like giant air pumps

Compressed air energy storage works like a giant air pump, using a compressor for pumping. Upon release of the compressed air, a turbine or engine connected to a generator produces electricity, thereby recovering part of the charged energy. The disadvantages are rather high losses and associated low efficiency levels of 50-70 percent, depending on the existence of heat recuperation. This storage form is mainly used at sites with underground gas caverns.

Thermal storage: lowest-cost storage medium

Thermal storage can take many forms. Low-temperature storage in hot water tanks is state of the art for heating purposes. Nowadays, with increasing volatility, excess electricity can be converted via a heating rod into heat for a district heating system as a means of adding flexibility to CHP plants.

Supercapacitors: performance for vehicles

Supercapacitors offer ultra-short-term performance storage, bridging the gap between conventional capacitors and rechargeable batteries. They are mainly used for mobile applications such as electric cars, buses, and streetcars, where they recover energy from braking, and discharge it for acceleration.

Flywheels: energy by rotation

Similarly, flywheels cannot store large amounts of energy. They work by accelerating a rotor in a vacuum enclosure to a very high speed and maintaining the rotational energy in the system. When energy is extracted, the flywheel’s rotational speed is reduced, whereas adding energy to the system results in a corresponding increase in speed. Flywheels are a fairly mature technology. However, self-discharge rates are rather high, and modularity and scalability are limited.

Ulrich Hottelet, business journalist based in Berlin.
Picture credits: Jochen Stuhrmann