Article - Energy Storage

Storage Technology


Heat storage,  symbolic of storage technology; Source: DLR (CC-By-3.0)

© DLR (CC-By-3.0)

In the course of the energy reforms in Germany, it is planned for 55 to 60 per cent of electricity to come from renewable sources by 2035, rising to 80 per cent by 2050. The main sources are wind energy and photovoltaics, both of which fluctuate in the amount of energy they produce.

Because of the growing proportion of intermittent renewable energy, the energy system must become more flexible so that system stability and a reliable supply can continue to be assured. Energy storage is one of the options to increase the flexibility of the power supply. For example, energy storage systems can balance the generated power with the actual power consumption. In periods with plenty of wind they could absorb electricity, and then feed it into the grid when there is little wind and the sky is cloudy. This would make it possible to generate electricity which is then only used at a later time. Energy storage systems can also help to provide balancing energy and thus to enhance the stability of the grid frequency.

Overview of energy storage systems

Energy storage technology is sub-divided into short-term and long-term storage systems, depending on their use. Short-term storage systems can store energy and give it off again many times during the day. They generally only offer a limited storage capacity. Long-term storage systems, on the other hand, must be able to store electricity for several days or weeks, e.g. to compensate for periods with little wind in which the sun hardly ever shines.

1. Short-term storage

Power reserve storage (seconds or minutes)

  • High energy-to-capacity ratio
  • For short-term fluctuations
  • Especially for grid services / balancing energy
  • Can be used several times per day
  • Especially batteries, condensers, flywheel storage facilities

Displacement storage (minutes or hours)

  • Especially to balance supply and demand over the course of a day (e. g. photovoltaic energy for private consumption)
  • One to two cycles per day
  • Especially batteries, compressed air storage systems, pumped storage hydroelectricity

2. Long-term storage (days or weeks)

  • For long-term periods with low renewable energy output / seasonal storage / back-up
  • Few cycles per year
  • Chemical storage (hydrogen / methane) and large pumped storage hydroelectricity plants (e. g. in Norway)

Battery storage

Batteries are electro-chemical storage devices. This is basically a tried and tested technology. Conventionally, lead-acid batteries have mainly been used. Now, the lithium-ion batteries which we know from mobile phones and laptops are increasingly being used, partly because they have a high overall efficiency rate of approx. 85%. Batteries are still relatively expensive for large-scale technological systems, but significant cost reductions are expected in the next few years.

Pumped storage systems

In pumped storage plants, the energy is stored as potential energy in the form of water which is pumped to a more elevated reservoir, then transformed into energy again when it flows down through a turbine connected to a generator. Pumped storage hydroelectricity plants are technically sophisticated, and in Germany they are currently the only storage technology with a significant capacity.

Compressed air storage systems

Here, excess electricity is used to pump air into underground salt domes or former gas caverns (compression). When the electricity is needed, the compressed air flows out through a turbine and generates electricity again. To improve the efficiency of the system, the heat which results from the compression can also be used (adiabatic compressed air storage).


In power-to-gas systems, energy (electricity) is used to convert water to hydrogen, and sometimes to convert it further to methane. The advantage of this system is that the hydrogen (to some extent) and the methane (without limitation) can be fed into the existing natural gas grid and stored there. The gas fed into the grid can then be reconverted to electricity, or it can be used for other purposes (e.g. heating, gas-powered vehicles). At present, the technology is still expensive and not very efficient.

Research and development

Energy storage is an important topic in an energy supply system which is mainly based on renewable energy. Energy storage systems are necessary for the future, but today they are usually expensive and to some extent still under development. Therefore, the main focus in energy storage is currently on research and development, especially on achieving the necessary cost reduction potential. For this purpose the Federal Government is carrying out the "Energy Storage Research Initiative".

More about the subject

The growing proportion of renewable energy is posing great challenges for the electricity system. The electricity system must be able to adapt flexibly enough to the changing volumes of wind and solar energy. Pumped storage hydroelectricity plants can play a significant role. In April 2012, Germany, Austria and Switzerland (the "D-A-CH" countries) agreed to enhance their exchange and cooperation in relation to pumped storage hydroelectricity plants. Subsequently, a detailed three-part investigation of the legal and economic conditions of pumped storage hydroelectricity plants and their contributions to the power supply was initiated, which gave special attention to the trilateral prospects.

The legal situation for pumped storage systems in the three countries was assessed by the law firm Görg Partnerschaft von Rechtsanwälten mbB on behalf of the Federal Ministry for Economic Affairs and Energy (BMWi). The economic prospects were assessed in an expert report by the Workgroup for Infrastructure Policy (WIP) at the Technical University of Berlin on behalf of the Swiss Federal Office of Energy (SFOE), and the contributions of pumped storage hydroelectricity plants to the power supply are the subject of a study by the Institute of Power Systems and Power Economics (IAEW) of RWTH Aachen University on commission for the Austrian Federal Ministry of Science, Research and Economy (BMWFW). A report by the Energy Science Centre of the ETH Zürich on behalf of the SFOE presents an overview of the individual studies.

On 8 October 2014 Rainer Baake, the Secretary of State in the Federal Ministry for Economic Affairs and Energy, convened a conference on "Storage in the Context of the Energy Transition". At this conference, about 250 experts from business, science, industry and politics discussed the current scientific insights into the demand for storage and the use of storage systems in electricity grids and the electricity market.


The conference aimed for a discerning analysis of the application of energy storage systems in electricity systems (grids, markets, long-term storage) and possible hindrances for a sensible use of such systems. The content of the conference was sub-divided into four themes:

  • Storage systems: demand, potential and technology
  • Use of storage systems in electricity grids
  • Use of storage systems in the electricity market
  • Long-term storage for the electricity system

The investigations and discussions show that the continued expansion of renewable energy requires a greater overall flexibility in the electricity system to enable it to adapt quickly to rapid fluctuations in power generation and demand. Alongside grid expansion and greater flexibility on- the demand for electricity, energy storage systems are one other option to balance the generation of power and the consumption of power. Different storage technologies can fulfil very different functions in the electricity system, so the various storage systems should be used for their individual purpose in the power supply system.