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)
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.
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".