Energy storage

Every storage device basically works according to the same principle: it stores energy (charge), it stores energy for a certain time (hold) and it releases energy again (discharge). Different storage units essentially differ in terms of:

• their weight and volume (energy density),
• the amount of energy they can store and
• their performance while they store and release energy.

Overall, storage can be divided into two types: Power-to-Power storage and Power-to-X.

Power-to-Power storage refers only to storage facilities that store electrical energy and withdraw it again in electrical form. They are generally used for short-term storage. Forms of Power-to-Power storage are:

• Electrochemical energy storage (lead-acid, lithium-ion, sodium-sulphur accumulators/high-temperature battery, redox flow batteries)
• Electrical energy storage (double-layer capacitors, also called supercaps)
• Mechanical energy storage (pumped storage, pressure storage, flywheel storage) 

Power-to-Power storage can be used in the following cases: 

• If supply and demand in the power grid are not balanced or fluctuations occur due to renewable energy generation (grid service). If the generation plants do not provide enough electricity, stored electricity can be used.
• In the event of high short-term electricity consumption, especially in businesses and the industrial sector. A large part of the electricity costs for companies is determined by their highest consumption period during a 15 minute window. Therefore, it is important to avoid above-average power consumption or peak loads. This is also called peak shaving and is used to reduce electricity costs. A battery storage system can help to reduce these peak loads internally, so that exceptionally high and expensive power is not drawn from the grid.
• If more electricity is privately produced than can be consumed at a given moment. The excess electricity can then be stored and consumed at another time (self-consumption optimisation). 

If electricity is very cheap at a certain time. Cheap electricity can then be stored and used later as needed or fed back into the grid at a higher price. The profit is the difference between the cheaper purchase price and the higher price when it is used later or fed back into the grid (spread) minus the storage costs and losses. 

Power-to-X refers to storage types that are usually charged using renewable electricity as the energy source but, unlike Power-to-Power storage, store it in thermal form or in the form of chemical energy carriers. 

Electricity stored in chemical energy carriers can be used to generate electricity again. This is why chemical energy carriers are suitable for long-term storage. One disadvantage, however, is that a lot of energy is lost during conversion. Therefore, one must always assess whether it is more financially sound to convert electricity into chemical energy carriers or to use the renewable electricity directly. 

Forms of Power-to-X storage are:

• Thermal energy storage (sensitive heat and cold storage, latent heat storage, thermochemical storage). In the industrial sector, these are heat (including steam) and cold storage systems. In the construction sector, this equates to space heating and hot water supply.
• Chemical energy storage (power-to-gas, electrolysis of hydrogen, synfuels). 

This is also referred to as sector coupling, as energy is used across sectors. The following are possible through conversion into chemical energy carriers:

• Hydrogen can be produced,
• methane and synfuels can be produced for mobility applications,
• long-term storage facilities for reverse power generation can be created and
• fuel for heating and cooling applications can be extracted.

It is important for the storage possibilities in the different sectors to be accessible to each other. Electrical power storage in the form of heat (Power-to-Heat) is also an important element for the energy system of the future.