Energy Storage Technologies as Options to a Secure Energy Supply

The current energy system is subject to a profound change: A system, designed to cater to energy needs by supplying fossil fuels is now expected to shift to integrate ever larger amounts of renewable energies to achieve overall a more sustainable energy supply. The challenges arising from this paradigm change are currently most obvious in the area of electric power supply. However, it affects the entire energy system, albeit with different effects. Within the energy system, various independent grids fulfill the function to transport and distribute energy or energy carriers in order to address spatially different energy supply and demand situations. Temporal variations are currently addressed by just-in-time production of the required energy form. 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 also represent an option to compensate for a temporal difference in energy supply and demand. Energy storage systems have the ability for a controlled take-up of a certain amount of energy, storing this energy within a storage media on a relevant timescale and a controlled redispatch of the energy after a certain time delay. Energy storage systems can also be constructed as process chains by combinations of unit operations, each covering different aspects of those functions. Large-scale mechanical storage options for electrical power are currently almost exclusively pumped hydro storage. These systems might be complemented in the future by compressed-air storage and maybe liquid-air facilities. There are several electrochemical storage technologies currently under investigation for their suitability as large scale electrical energy storage in various stages of research, development, and demonstration. Thermal energy storage technologies are based on a large variety of storage principles: Sensible heat, latent heat (based on phase transitions), adsorption/desorption processes or on chemical reactions. The latter can be a route to permanent and loss-free storage of heat. Chemical energy storage systems are based on the energy contained within the chemical bonds of the respective storage molecules. These storage molecules can act as energy carriers. Equally well, these compounds can enter various industrial value chains in energy-intensive industrial sectors and are therefore in direct economic competition with established (fossil) supply routes for these compounds. Water electrolysis, producing hydrogen and oxygen, is and will be the key technology for the foreseeable future. Hydrogen can be transformed by various processes to other energy carriers of interest. These transformations make the stored energy accessible by 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, connect and interweave the various energy streams and sectors. While chemical energy storage offers a route for a stronger integration of renewable energy outside the 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.