Energy Storage as Part of a Secure Energy Supply

The current energy system is subject to a fundamental transformation:
A system that is oriented towards a constant
energy supply by means of fossil fuels is now expected to
integrate increasing amounts of renewable energy to achieve
overall a more sustainable energy supply. The challenges
arising from this paradigm shift are currently most obvious
in the area of electric power supply. However, it affects all
areas of the energy system, albeit with different results. Within
the energy system, various independent grids fulfill the
function of transporting and spatially distributing energy or
energy carriers, and the demand-oriented supply ensures
that energy demands are met at all times. However, renewable
energy sources generally supply their energy independently
from any specific energy demand. Their contribution
to the overall energy system is expected to increase significantly.
Energy storage technologies are one option for temporal
matching of energy supply and demand. Energy storage
systems have the ability to take up a certain amount of
energy, store it in a storage medium for a suitable period of
time, and release it in a controlled manner after a certain
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time delay. Energy storage systems can also be constructed as
process chains by combining unit operations, each of which
cover different aspects of these functions. Large-scale
mechanical storage of electric power is currently almost
exclusively achieved by pumped-storage hydroelectric power
stations. These systems may be supplemented in the future
by compressed-air energy storage and possibly air separation
plants. In the area of electrochemical storage, various technologies
are currently in various stages of research, development,
and demonstration of their suitability for large-scale
electrical energy storage. Thermal energy storage technologies
are based on the storage of sensible heat, exploitation of
phase transitions, adsorption/desorption processes, and
chemical reactions. The latter offer the possibility of permanent
and loss-free storage of heat. The storage of energy in
chemical bonds involves compounds that can act as energy
carriers or as chemical feedstocks. Thus, they are in direct
economic competition with established (fossil fuel) supply
routes. The key technology here – now and for the foreseeable
future – is the electrolysis of water to produce hydrogen
and oxygen. Hydrogen can be transformed by various processes
into other energy carriers, which can be exploited in
different sectors of the energy system and/or as raw materials
for energy-intensive industrial processes. Some functions of
energy storage systems can be taken over by industrial processes.
Within the overall energy system, chemical energy
storage technologies open up opportunities to link and interweave
the various energy streams and sectors. Chemical
energy storage not only offers means for greater integration
of renewable energy outside the electric power sector, it also
creates new opportunities for increased flexibility, novel
synergies, and additional optimization. Several examples of
specific energy utilization are discussed and evaluated with
respect to energy storage applications.
The article describes various technologies for energy storage
and their potential applications in the context of Germany’s
Energiewende, i.e. the transition towards a more sustainable
energy system. Therefore, the existing legal framework
defines some of the discussions and findings within the
article, specifically the compensation for renewable electricity
providers defined by the German Renewable Energy
Sources Act, which is under constant reformation. While the
article is written from a German perspective, the authors
hope this article will be of general interest for anyone
working in the areas of energy systems or energy technology.