According to the current state of the art, electrical energy can only be stored to a very limited extent and for a short period of time, which is why electrical energy storage is almost always referred to as a storage system that converts electrical energy into a form of energy that is easier to store and later converts it back into electricity. Bringing all electrical storage systems into one definition therefore poses challenges for legislators.
For this reason, the definition of energy storage systems was only amended in June 2022 as part of the Easter package and set out in the Energy Industry Act.
Energy Industry Act (EnWG) §3 15d (since June 22)
Energy storage facility:
"Installation in an electricity grid with which the final use of electrical energy is postponed to a point in time later than that of its generation or with which the conversion of electrical energy into a storable form of energy, the storage of such energy and its subsequent reconversion into electrical energy or use as another energy carrier takes place".
In short, an energy storage system is a system that takes energy in the form of electricity from the power grid, converts it into another form of energy if necessary, and then feeds it back into the power grid in the form of electrical energy with a time delay.
Consequently, an energy storage system is not an (end) consumer, since the electricity consumed is not consumed by the user, but it is also not a producer, since the energy was not produced by the storage system itself.
An energy storage system is therefore merely an intermediate storage facility for the temporary storage of energy.
The continuous shutdown of conventional large-scale power plants such as coal, gas or nuclear power plants and the development of several million renewable generation plants immensely increases both the complexity and the volatility of the German power grid.
Already today, there are more and more days on which the electricity price falls into negative territory, especially when a significant surplus is produced by PV & wind during the day and prices rise massively again in the evening and expensive conventional power plants have to be ramped up to cover demand at nightfall.
More details on this can be found in the article "Large-scale battery storage as a key technology for the energy transition".
In addition to these short-term fluctuations within a few hours, there is the weather dependency and the seasonal dependency of renewable sources. Even with a week of overcast and windless weather or a significantly lower production output of all PV plants in winter, the German power grid must be able to combine power generation and power consumption and thus ensure security of supply.
It is therefore not possible to achieve an energy transition solely through the expansion of renewable energies and it is imperative that this be supplemented by an equally rapid expansion of both short-term and long-term energy storage systems, which will make the power grid more flexible and stable and thus prepare it for the high number of volatile plants. This is shown, among other things, by the Fraunhofer study "Paths to a Climate-Neutral Energy System" of 12.11.21, which expects an enormous increase in the number of electrical energy storage systems. (Increase in battery storage from 0.45 GWh in 2021 to 83 GWh in 2030).
The various types of energy storage systems differ fundamentally in the form of energy in which the electricity is stored. In general, a distinction is made between chemical, electro-chemical, thermal, mechanical and electrical energy storage systems.
Depending on the type of storage and its properties, very different applications result for energy storage systems. Some storage systems are better suited for short-term energy storage due to their high efficiencies and fast response times, while others are significantly more effective for long-term storage.
Since both short-term and long-term energy storage will be needed to compensate for the volatility of renewable sources, the following section compares the three energy storage systems whose potential capacity and technical maturity make them the current frontrunners in electrical energy storage.
Large-scale battery storage systems are currently best suited to balancing short-term volatility (e.g. day-night volatility). In addition to the highest efficiency, the scaling of battery storage is significantly easier than that of pumped storage power plants. Pumped storage power plants have high geographical requirements, as they have to be built at a location with a large difference in altitude. In addition, the possible locations for pumped storage often pose risks for nature conservation, which can make the planning and implementation of such projects considerably more difficult. Large-scale battery storage facilities, on the other hand, are geographically independent and can be built anywhere. Due to their high energy density, land consumption is low and there are no significant environmental risks.
In addition, the construction of a pumped storage power plant can take 5-10 years, while a large-scale battery storage facility can be connected to the grid within 1-2 years. As the expansion of energy storage systems must be massively accelerated over the next few years in order to achieve the target of 80% electricity from renewable sources, the realization time of storage systems is a critical point.
As far as long-term storage of electricity is concerned, hydrogen is currently a technology with particularly high potential. Although the efficiency of electrolysis and fuel cells is significantly lower than that of large-scale battery storage or pumped storage, hydrogen can be stored for a long time in suitable storage media (e.g. LOHC technology or in the natural gas grid). Thus, the seasonal fluctuations of renewable sources can also be compensated.
Experts agree that both short-term and long-term storage will be needed on a very large scale to meet key challenges of the energy transition - to compensate for short-term fluctuations in day-night volatility and to hedge against seasonal fluctuations. So the question is not "large-scale battery storage, pumped storage, or hydrogen?" Rather, the energy system of the future requires a "both/and" approach.
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