The global energy transition process is confronting humanity with its greatest challenge in the years ahead, but without it, climate change cannot be curbed. The energy sector is currently causing roughly two thirds of all climate-damaging greenhouse gases by burning fossil fuels. Going forward, a major share of the energy is planned to be produced from renewable instead of from fossil fuels in order to significantly reduce use of the latter. However, this reorientation calls for greater flexibility in energy management due to fluctuations in wind and solar power production, which is dependent on the time of day and weather conditions. Consequently, it’s hard to control steady and even feeding of electric power into grids. Energy storage systems that store surplus energy and feed it back into the grid on demand can resolve this predicament by temporally separating energy production and use, so enabling system and supply reliability.

In the context of energy transition, it becomes clear, again and again, that there’s no such thing as THE single technology that solves all problems

Dr. Bernhard Ernst, Deputy Head of Energy Storage at Fraunhofer Institute for Energy Economics and Energy System Technology

Basically, there are two applications for storage technology: short-term storage systems can absorb and release energy several times a day, while long-term storage systems are intended to store energy across periods of days or weeks for on-demand release. In terms of technology, these storage systems are based on highly diverse designs. “In the context of energy transition, it becomes clear, again and again, that there’s no such thing as THE single technology that solves all problems but that a mix of many different systems has to work together in ways that make sense,” says Dr. Bernhard Ernst, Deputy Head of Energy Storage at Fraunhofer Institute for Energy Economics and Energy System Technology IEE. Battery storage systems such as lithium-ion batteries are already part of everyday life today. Electricity can also be converted into other forms of energy, such as hydrogen and synthetic fuels, by so-called power-to-x systems for longer-term storage. However, mechanical energy storage systems that keep achieving new breakthroughs play an important role as well.

Proven and innovative technologies

Pumped hydro storage plants are arguably the oldest, most mature, highest-capacity plus an extremely efficient way of mechanically storing energy. Such a power station that was used by a weaving mill began to exist in Switzerland as far back as in 1863. In Germany and the United States, the first larger pumped hydro storage plants were established in the nineteen-twenties and nineteen-thirties. These power stations consist of two water reservoirs on two different elevations that are connected to each other by pipes. In the event of an electricity surplus water is pumped from the lower reservoir into the upper one and when additional power is needed the potential energy in the upper reservoir is used. The water is discharged to drive current generators with an efficiency of up to 80 percent. The greater the difference in elevation between the two reservoirs, the more energy can be stored. That’s why undulating or mountainous landscapes are ideal sites.

“Pumped hydro storage plants are a technology that has been established for decades and offers many advantages without which electric power supply would be facing much greater challenges,” says Ernst. 99 percent of the worldwide capacities for electric power storage are covered by pumped hydro storage plants, according to the German Energy Agency (dena). The largest pumped hydro storage plant is the Bath County Pumped Storage Station in the United States with a capacity of 24,000 MWh that could supply a big city with electric power for one day.

However, storing energy for entire regions and longer periods of time solely in this way is hard to imagine, if for no other than space and geological reasons. A potential alternative is the utilization of former mining tunnels or residual cavities from open-pit mining as water reservoirs, although the opportunities are equally limited in this case.

The StEnSea (Stored Energy in the Sea) project of Fraunhofer Institute introduces another option: so-called hollow-sphere power stations for offshore use. For this purpose, hollow concrete spheres with a diameter of 35 meters (115 feet) have to be dropped in the sea. There’s a vacuum inside these hollow spheres and when water flows into a sphere due to the pressure difference it drives a turbine that generates electric power. When there’s a surplus of energy the water is pumped back out of the sphere, so creating a vacuum again. The deeper the spheres are deposited on the seabed the higher the pressure differences and resulting energy yield. In that case, however, the walls of the spheres would have to become increasingly thicker. Consequently, the StEnSea engineers have identified levels between 700 and 800 meters (2,297 to 2,624 feet) below sea level as optimal depths. The prevailing pressure there is at least 70 bar (1,015 psi). In these conditions, the storage potential of the hollow-sphere power stations is supposed to amount to roughly 1,000 times that of the pumped storage capacity currently used worldwide – at comparably low costs: “Using today’s standardized and available technology, and with a storage capacity of 20 MWh per sphere, we envision a total worldwide electric storage capacity of 893,000 MWh. In future sites combining a large number of such systems comparably low cycle costs of estimated 2.0 euro-cents per kWh would be incurred. That would enable an important low-cost grid balancing contribution to offset the fluctuations in wind and solar power generation,” says Fraunhofer Department Head Jochen Bard. An initial test phase using a 1-10-scale model in Lake Constance has already been successfully completed. Energy storage expert Dr. Bernhard Ernst considers the concept of his Fraunhofer colleagues to be a promising alternative as well. “StEnSea is comparable to traditional pumped storage in terms of application and costs,” he says. “Its activation period of less than one minute is even faster. In addition, approvals are easier and faster to obtain because the environmental impact is much lower than that of conventional pumped storage stations.”

Inspired by pumped storage stations, engineers of Swiss startup EnergyVault were looking for a solution that does not rely on water, though, but can be operated anywhere. Their idea was to use mobile blocks weighing up to 35 metric tons (39 short tons) that can be stacked by cranes to create towers. The higher the stacking of these blocks the more energy they can store. In times of power scarcity, the blocks are lowered again and the moved mass serves to produce electricity by means of generators.

“For energy storage to pay off in this way, at least 20 stories of stacked height must be possible,” says Robert Piconi, CEO and co-founder of EnergyVault. It takes up to 16 hours of stacking work to store 80 MWh of energy in this way. Advanced computer systems coordinate the charge and discharge cycles. Efficiency is said to amount to 90 percent. Currently EnergyVault is building the first commercial gravity energy storage center in China. The EVx plant is scheduled to go into operation in the second half of 2023.

Potential energy can also be stored and retrieved by using vehicles in motion. For instance, by causing trains with heavy cargo to run uphill (energy storage) and downhill again (energy discharge by recuperation, see video below). The International Institute for Applied Systems Analysis in Austria is pursuing a similar idea: In this case, electric trucks are intended to haul water from an upper course of a river to a lower one. Because the trucks running downhill are filled, recuperating electric power in the process, and back uphill in empty condition they even produce a small electricity surplus during their uphill and downhill runs, which benefits the efficiency of this system (additional info).

Gravitational forces are the secret of gravity storage as well. In this case, surplus electricity is used to hydraulically pump a mass of rock or concrete with a size of several hundred meters to a higher elevation. When electricity is needed, this piston is lowered again and pushes the water through turbines driving generators just like in a hydro power station. The water flows back into a reservoir. The system has a minimum efficiency of 80 percent – which makes it particularly attractive economically. Its storage capacity is huge and would amount to several gigawatt hours, so equaling or surpassing the capacities of pumped hydro storage plants. A study by the Imperial College of London also highlights the great potential of this technology. It states: „Gravity Storage systems are the most cost-effective bulk electricity storage technology for storage capacities larger than 1 GWh.“ But before such a system is ready for use, a long to-do list has to be worked through. Because just as huge as the capacity are the technical challenges – such as pressures acting on turbines, pipes and seals.

Flywheel energy storage (FES) is another method of mechanically storing energy. The advantages of these systems are short response times, relatively high site independence, plus high environmental compatibility across their entire lifecycle. In an FES, an electric motor uses surplus electricity to accelerate a flywheel rotating inside a vacuum chamber with low friction. When electric power is needed, the rotational input energy drives a generator. Efficiency is supposed to amount to 85 to 90 percent. At the end of 2021, TU Dresden presented the so far largest flywheel energy storage system in the DEMIKS project. With a capacity of 500 kilowatt hours, the 42-metric-ton (46-short-ton) prototype surpasses previously used FES systems five-fold. Installed in close proximity to windfarms, the fast-response FES systems are intended to help balance fluctuations in wind power generation.

Another way to “park” electricity is compressed air storage. In this case, excess electricity is used to compress gas for storage in underground caverns, for example. To retrieve the energy, the expanding gas drives a turbine. The capacity of such a system can be additionally increased with hydrostatic pressure from a water reservoir. The first compressed air power station was put into operation in 1978. The utilization of so-called adiabatic systems has enabled the efficiency of this storage method to be raised from roughly 40 to more than 70 percent. Adiabatic means that the heat generated in the compression process does not escape but is stored and made available for the expansion process. As California advances towards its objective of 100 percent carbon-free electricity by 2045, Central Coast Community Energy (3CE), one of California’s largest community choice aggregators, is relying on this type of energy storage. The compressed air energy storage facilities of the Willow Rock Energy Storage Center are to provide 1,600 MWh of energy over the next 25 years. The goal: an annual saving of up to 28 million metric tons (31 million short tons) of carbon dioxide – the equivalent of the emissions of more than 120,000 passenger cars.

General conditions must fit the purpose

Time will tell whether or not all of these new technologies will actually prove their viability in field use and offer additional opportunities for storing energy. Numerous factors such as technical feasibility, safety aspects, environmental regulations and funding or subsidies play a role in this context. Among mechanical storage systems expert Bernhard Ernst sees the greatest potential particularly in two technologies: “In pumped storage and in compressed air storage because that’s where the largest storage capacities can be achieved. Actually, in a large synchronous grid like the one that Europe operates very fast storage systems such as flywheel storage aren’t necessary because adequate momentary reserve ensures grid stability within the range of seconds.”

Moreover, for projects to evolve into sustainable business models, the general conditions have to fit the purpose. Bernhard Ernst: “This is a prerequisite for sustainable business models and ranges from fast and legally valid approvals for large-scale projects to regulatory conditions (such as grid charges) to long-term agreements (such as capacity markets). In addition, research and development of new technologies such as StEnSea has to be supported because the potential of approvable pumped storage stations is finite.”

For expert Bernhard Ernst, yet another development is essential to supporting the energy transition process: an extension of the power grid that will then be regulated digitally and centrally. “Intensive integration of electric power supply, for instance in Europe, reduces the requirement for storage because both in the short-term range and for seasonal fluctuations there will be balancing effects in a large area that will make storage systems superfluous to some extent. Added to this is the fact that large-scale storage in Norway or in the Alps can be linked to production and consumption more effectively when adequate transportation capacities are available in the grid. At the moment, grid extension is clearly less costly than storage for achieving the same effect. Consequently, the question to what extent and up to what distances grid extension is approvable and feasible tends to be more relevant and will subsequently result in the required storage.”