How many energy storage systems are there

Energy storage

Lexicon> Letter E> Energy storage

Definition: A system that can absorb energy and then release it again later

More specific terms: short-term storage, long-term storage, seasonal storage, heat storage, storage for electrical energy, chemical storage, flywheel storage

English: energy storage

Categories: Energy Storage, Basic Terms

Author: Dr. Rüdiger Paschotta

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Original creation: 11/24/2012; last change: 15.05.2021


A Energy storage is a system that can absorb energy and release it again later. As a rule, the energy is extracted in the same form in which it was stored. However, it is not necessarily saved in the same form. For example, in a pumped storage power plant, electrical energy is converted into mechanical energy (namely positional energy) for storage and later back into electrical energy.

There are energy storage systems for different forms of energy:

Some storage systems use completely different technologies for injection and withdrawal. For example, a storage device for electrical energy can be realized by combining an electrolyzer (which can generate hydrogen with electrical energy), a hydrogen tank and a fuel cell (for converting it back into electrical energy).

An expanded concept of energy storage can include systems in which the energy is taken in a different form than is stored. For example, surplus electricity could be used to generate RE gas, which is then fed into the gas network and z. B. is used for gas heating. Electric storage heaters convert electrical energy into heat and store it as such. There are also storage facilities whose capacity is only filled by natural resources.

Typical applications of energy storage

There are very different applications of energy storage. Some examples of this:

  • Storage systems for electrical energy can currently store excess generation capacity from power plants and release it again later when consumption is higher or the generation capacity is lower. This is z. B. realized with pumped storage power plants and compressed air storage power plants.
  • If renewable energies are to take over a large part of the electricity generation, one approach would be to use storage facilities on a large scale (e.g. solar energy storage) in order to reconcile the fluctuating feed-in and electricity demand. However, well-suited (low-loss and inexpensive) large memories are not available for the time being.
  • Heat storage tanks (e.g. hot water storage tanks) are often used in order to be able to provide high output for a short period of time. The reloading takes place over a longer period of time, e.g. B. with a low power heat pump.
  • Seasonal storage systems for heat or electrical energy can absorb surpluses in summer and release them in winter.
  • Vehicles need a storage device for the drive energy if a steady supply of energy is not possible while driving (e.g. via an overhead line).

Basic characteristics of energy storage

Energy storage systems have a number of important characteristics that determine their suitability for various purposes:

Energy storage for a certain form of energy can differ in many ways, not just in terms of their capacity.
  • Of central importance is that Storage capacity. It indicates how much energy can be drawn from the storage unit after it has been fully charged. See also the article on the capacity of a battery.
  • Especially in mobile applications (such as electric cars) there is also a high Energy density rather than a high specific (weight or volume-related) storage capacity, because storage of sufficient capacity would otherwise be too large or heavy.
  • There are limits to that power, with which energy can be stored or withdrawn. One speaks of the maximum Loading capacity (or Feed-in power) or. Withdrawal rate (or Withdrawal rate), which can be different. Such boundaries are not necessarily sharply defined; it may be that the efficiency or the service life suffers with higher charging or withdrawal capacities, or that high capacities would lead to overheating in the long term, but are possible in the short term. Here, too, the specific (weight or volume related) performance (the Power density) to be relevant.
  • Some energy storage devices can be charged or discharged practically without delay in accordance with short-term demand, while others require certain lead times.
  • As a rule, energy losses occur, on the one hand during injection and withdrawal (quantified overall by the Cycle efficiency) and on the other hand while holding the load (Self-discharge). The losses during storage tend to be less relevant than those during withdrawal, as they do not affect the storage capacity.
  • The lifespan of a memory can e.g. B. be limited by the number of charge / discharge cycles or to a certain age. It often depends heavily on the operating conditions.
  • Of great practical importance are of course the costs of building and operating a storage facility.
  • In particular, storage systems with a high energy density often cause certain dangers due to accidents in which the stored energy is suddenly released in an undesired way. This applies, for example, to dams that can trigger destructive tidal waves if destroyed in an earthquake or war, or to gasoline tanks and batteries in vehicles in traffic accidents.

When optimizing energy storage devices for certain applications, compromises often have to be made. For example, optimizing a battery for high performance can compromise storage capacity and service life.

Long-term and short-term storage

Depending on the characteristics described above, a storage unit can often be used more as a long-term storage unit or also as a short-term storage unit:

  • Long-term storage (e.g. seasonal storage) must be able to hold its charge over long periods of time without excessive energy losses, i.e. H. have a low self-discharge. In addition, since relatively few charge / discharge cycles are typically carried out, it must have low costs per amount of stored energy. Otherwise the costs per amount of energy drawn will be very high.
  • A short-term storage device usually experiences more frequent charging / discharging cycles and should therefore lose as little of its service life as possible. The energy losses during injection and withdrawal are relevant, less so are the losses during the (short) storage. It is often also important to be able to store or withdraw a high output for a short time.

Storage facilities provided for certain periods of time are also used e.g. B. as Daily memory or Weekly memory designated.

The usability of storage z. B. as long-term storage depends not only on their theoretical technical suitability, but often also on economic aspects. For example, many pumped storage power plants can be used as frequently used day storage (Circulators) be very economical, as long-term storage (e.g. seasonal storage) by no means. If there are only a few charging / discharging cycles per year, the proportional costs per converted kilowatt hour are far too high. That is why water storage power plants with large natural reservoirs that are only used temporarily with powerful turbines are used as seasonal storage.



Rechargeable batteries are used to store electrical energy. As a rule, they are well suited as short-term storage devices; At least some types can withstand many thousands of charge / discharge cycles and have relatively low energy losses (e.g. around 10%). For long-term storage, however, they are out of the question, even if they show a low level of self-discharge: the costs per storable kilowatt-hour are too high for that.

For example, systems with lithium-ion batteries cost roughly € 800 per storable kilowatt hour (as of 2012). If such a system were operated as a seasonal storage facility, where it is only stored and withdrawn once a year, within a service life of z. B. 10 years only 10 charge / discharge cycles possible. Then there would be horrific costs of 80 € / kWh based on the extracted energy (without operating costs). You can see that even a ten-fold cheaper production or a much longer service life would never make such batteries suitable as seasonal storage.

It looks much better when used in an electric car. A heavily used vehicle could result in 500 charging / discharging cycles per year, i.e. 5000 within a service life of 10 years. Then the proportionate battery costs would be 16 ct / kWh based on the energy drawn. This is still a lot - roughly comparable to the cost of generating electricity. However, it is easy to imagine that further technical advances will soon bring these costs to an acceptable level.

When used in an electric car High performance batteries required that can temporarily store and withdraw high performance. Lithium-ion batteries meet this requirement quite well. If necessary, they could also be supplemented by a supercapacitor, which can handle such high power levels with little loss and wear, but has a low specific storage capacity.

Storage power plants

Pumped storage power plant pumps water into a high-altitude reservoir in order to later (when there is a higher demand for electricity) use it to generate electrical energy again with the help of turbines. Here, the costs per storable kilowatt hour are much lower than for batteries, but they are heavily dependent on the respective topographical conditions. The storage over longer periods of time would be technically possible without any problems, but again the operation is much more economical if the storage is loaded and unloaded frequently; after all, the operator can generally only generate income with this. Nothing stands in the way of this from a technical point of view: the pumps and turbines used can withstand a large number of charging / discharging cycles and the energy losses are relatively low (e.g. 15 to 25% per cycle).

The situation is different for water storage power plants with reservoirs that are only fed by natural tributaries. The limited inflows only allow a limited operating time per year, which of course is then set in times of high electricity demand and correspondingly high prices on the electricity exchange. This often leads to its use as seasonal storage, i.e. H. with electricity production mainly in winter. Of course the term applies Storage limited here in that it is not electrical energy that can be stored, but energy from natural inflows.

Heat storage

A hot water buffer tank with z. B. 400 liters volume, depending on the temperature stroke z. B. store 20 kilowatt hours of heat - enough z. B. for a family's daily need for hot water. What would be desirable, however, would be much larger seasonal storage facilities that could store excess heat from solar collectors in summer and make it available for heating buildings in winter. For a well-insulated single-family house, this involved a storage capacity of the order of magnitude of 10,000 kWh. Realization as a hot water storage tank with hundreds of cubic meters is difficult, however, as very good thermal insulation would be required in order to keep storage losses sufficiently low.

However, the implementation is easier if even larger hot water tanks are built, which then z. B. supply entire residential areas. This is not only due to the lower specific costs, but also because the ratio of surface area and volume becomes more favorable: If, for example, all dimensions are made twice as large, the surface that loses heat (and needs to be insulated) increases by a factor of 4, the storage volume but by a factor of 8. With very large heat storage systems, long-term storage is much easier.

There is also the possibility of underground heat storage in aquifers without any thermal insulation. The relative heat losses remain low on the one hand because of the favorable ratio of surface and volume and on the other hand because of the increased ambient temperature at a sufficiently great depth.

It is often particularly cost-effective to mobilize masses that are already present, for example in buildings, for heat storage; this concept is known as thermal component activation.

Chemical energy storage

How relevant are memory leaks?

How harmful the energy losses that occur in energy storage systems are depends heavily on the respective application, the objective, the boundary conditions and the chosen economic perspective:

  • If z. If, for example, the batteries of an electric car have losses, this increases the need for electrical energy when charging, which increases costs and reduces the climate protection effect. The range of the vehicle can also be affected. So such losses are very undesirable.
  • If a storage system only occasionally absorbs excess energy that would otherwise not be usable in any other way, considerable losses during storage can often be accepted - in the sense that there is no better solution. Losses when discharging are more damaging, as this reduces the usable storage capacity.
  • If storage on the power grid on a large scale z. B. should absorb excess wind energy, high losses during storage would be economically acceptable if the excess electricity can be purchased at very low prices. From an economic point of view, however, this market situation is very undesirable because electricity generated at considerable cost would lose much of its value. So one would like storage facilities that could significantly reduce the fluctuations in the exchange price for electricity. However, this presupposes low-loss storage, since lossy storage can only be operated economically when the electricity price fluctuates strongly. Therefore z. For example, the power to gas approach only incompletely solves the problem of fluctuations in wind energy over time because the cycle efficiency is too low.

Possible alternatives to energy storage

The basic function of energy storage systems is to compensate for temporary deviations between energy supply and consumption. The need for energy storage is of course higher, the stronger and more frequent such deviations occur. It can therefore be reduced by either making the energy supply more controllable or by adapting the consumption better to the supply. In the field of electrical energy, for example, this can be done in such a way that, on the one hand, power plants that are as flexible as possible (i.e. their output can be controlled quickly) are used and, on the other hand, the demand for electricity is shifted to times when there is good supply with the help of load management. This is also associated with costs - for example because flexible gas-fired power plants have higher operating costs than inflexible coal-fired power plants; this may have to be compared with the cost of additional energy storage.

Another approach to reducing the need for energy storage is improved transport options - in the electricity sector through high-performance high-voltage lines. In this way, generation and consumption can be balanced out by shifting space rather than time. This approach is often much more cost-effective than the use of energy storage devices and also typically entails significantly lower energy losses.

The article on storage for electrical energy contains further details.

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See also: energy, seasonal energy storage, heat storage, storage for electrical energy, solar power storage, battery, capacity of a battery, pumped storage power plant, flywheel storage, thermal component activation, chemical energy storage, electric storage heating, energy density, power, power to gas, RP-Energie-Blog 2015- 05-19, RP Energy Blog 2015-01-19
as well as other articles in the categories of energy storage, basic concepts