Battery alternatives: Energy in reserve

Sometimes Germany produces more electricity than it consumes, sometimes too little. Volker Quaschning, Professor of Renewable Energy Systems at HTW Berlin, points to two situations: “In the sun-rich summer, we need storage for day–night balancing – about twelve hours. In winter, solar output is low but winds are stronger. However, there are also periods of calm lasting up to two weeks.”

To bridge shortages on the one hand and avoid wasting energy on the other, powerful energy storage systems are indispensable. Batteries are considered the preferred option for short-term peaks, while gas and hydrogen are used for longer periods. In between lies a broad range of additional technologies – from mechanical and thermal to electrostatic storage systems – that vary in effectiveness depending on the application.

Pumped Storage: Water up, water down

Pumped-storage power plants (PSPs) lead in storage capacity and installed output – both in Germany and worldwide. Their efficiency ranges between 75 and 85 percent.

The principle is simple: two reservoirs at different elevations are connected by pressure pipelines. When there is surplus electricity, water is pumped uphill and stored in the upper basin. When energy is needed, it flows back down to the lower reservoir, driving generators in the process. PSPs can reach full output within minutes and stabilize the power grid for several hours. About 30 plants are currently in operation in Germany, many of them for decades. They were originally built to handle peak demand. In Forbach (Baden-Württemberg), the first new facility since 2003 is scheduled for completion by 2027. However, significant additional expansion is unlikely.

Volker Quaschning is skeptical: “In theory, there are suitable sites. In practice, we need the storage within five years. In Germany, projects like this typically take closer to 20 years to implement.” A major drawback is the extensive environmental impact, which slows permitting processes – and is one reason why the domestic potential is considered largely exhausted.

Spherical storage: Pumped storage goes underwater

A project by the Fraunhofer Institute for Energy Economics and Energy System Technology (IEE) transfers the pumped-storage principle to the ocean floor. Heavy concrete spheres filled with water serve as the lower reservoir. When there is surplus electricity, the spheres are pumped empty – the storage system is “charged.” When energy is needed, a valve opens: water rushes back into the spheres under high ambient pressure, driving a turbine and generator. By moving the system to deep water, “space constraints can be avoided and ecological impacts reduced,” explains Dr. Bernhard Ernst, project lead at Fraunhofer IEE for “Stored Energy in the Sea.” Entire “clusters of spheres” could be anchored near offshore wind farms – directly where excess electricity is generated.

The concept has been tested with spheres three meters in diameter in Lake Constance. Next, trials are planned off California at depths of around 600 meters. According to project calculations, the optimal sphere diameter is about 30 meters. In principle, larger spheres deliver higher efficiency – but such dimensions must first be manufactured and deployed. “We consider 30 meters still feasible,” says Ernst. However, further research is required for scaling up. He expects the first commercial applications no earlier than the early 2030s.

Gravity storage: Weights release electricity

Gravity storage systems operate like a giant elevator for heavy masses. When excess electricity is available, weights are lifted. When power is needed, the descending masses drive generators, releasing energy back into the grid. The concept is simple – and that simplicity is part of its appeal.

Such systems are suitable for balancing energy over several hours and potentially for medium- to long-term storage cycles, with efficiencies expected to be comparable to pumped storage. The Scottish company Gravitricity is testing this approach in a decommissioned mine shaft in Finland. Switzerland’s Energy Vault has developed a crane-based system and exported the concept to China. In Rudong, a roughly 120-meter-high “Tetris-like” tower entered operation in spring 2025, with a storage capacity of 100 megawatt-hours.

Volker Quaschning questions the economic scalability: “To store large amounts of energy, I need height and volume. If I compare this with pumped-storage plants in Austria with a head of 1200 meters, the energy that can be stored in such systems is relatively small.” Energy Vault counters with plans for nine additional facilities in China.

Flywheel storage: Energy in motion

Flywheel storage uses a long-known physical principle enhanced by modern materials and control technology. An electric motor accelerates a rotor to very high speeds, storing energy as rotational motion within the system. When energy is needed, the same motor operates as a generator, slowing the rotor and feeding electricity back into the grid.

Flywheel systems respond within milliseconds, making them suitable for rapid frequency stabilization or as backup power sources. Their efficiency can reach up to 90 percent, and they can withstand hundreds of thousands of charge cycles. Munich’s municipal utilities tested a facility with 28 coupled flywheels several years ago. At the Technical University of Dresden, a flywheel was combined with a wind turbine to buffer electricity directly on site. In Ireland, a massive flywheel helps keep the power grid stable through its inertia.

Despite these advantages, widespread market adoption has yet to occur. The reason is largely physical: flywheels lose energy through friction – typically between 3 and 20 percent per hour, depending on the system. “An exciting technology, but a niche product. Flywheels are unsuitable for seasonal or long-term storage,” says Quaschning.

Compressed air storage: High pressure underground

The oldest plant of this type is located in Huntorf near Oldenburg, Germany. Since 1978, air has been compressed to about 70 bar and stored in deep underground caverns. During compression, heat is generated but traditionally not utilized. When electricity is produced again, the air must be reheated – typically with natural gas – to prevent the turbine from icing. With an efficiency of only about 42 percent, this design has found few imitators, apart from a similar plant in the United States.

Adiabatic compressed air energy storage (A-CAES) is considered a potential “game changer.” In this approach, the heat generated during compression is captured and stored in a thermal reservoir, eliminating the need for fossil fuels. Efficiencies of up to about 70 percent are considered achievable. In Germany, a planned prototype in Staßfurt was never built due to a lack of market prospects. China, however, brought a large-scale facility online in 2024 with an output of 300 megawatts and a storage capacity of 1500 megawatt-hours.

“The technology continues to evolve. But if it were the major breakthrough, it would already have prevailed,” says Volker Quaschning. A “revival” is conceivable, since compressed-air storage fundamentally offers long-term potential. Whether it can truly compete with hydrogen remains to be seen: “Hydrogen is expensive, but far more versatile as an energy carrier. Compressed air is ‘only’ air.”

Liquid air storage: Cold as power in reserve

Liquid air energy storage converts electricity into cold. Using the Linde process, air is cooled to about −195 °C and liquefied. In this compressed liquid state, it can be stored for long periods in insulated tanks. When energy is needed, the liquid air is reheated, expands, and drives a turbine – similar to steam in a conventional power plant.

The approach is appealing because it relies on established industrial components and requires no rare materials. Storage tanks can also be installed wherever sufficient space is available. In practice, however, the technology has rarely progressed beyond pilot projects.

“The effort required to liquefy air is enormous,” says Volker Quaschning. Without heat and cold recovery, efficiency is about 25 percent; with thermal recovery, around 50 percent. If industrial waste heat is utilized, theoretical efficiencies of up to 70 percent are possible, though not yet confirmed in practice. “If compressed-air storage already struggles economically, that applies even more to liquid air,” Quaschning concludes.

Thermal storage: Electricity to heat – and back?

The field of thermal energy storage is vast. It ranges from household hot-water tanks and buffer storage systems to complex industrial heat-recovery systems and thermochemical approaches. Their common goal is to store energy in the form of heat for hours, days, or even months. “Electricity can be converted into heat without losses,” explains Volker Quaschning, “but converting it back into electricity is technically complex and involves significant losses.”

Researchers are working on concepts that combine both directions. These include high-temperature storage systems developed by the Solar Institute Jülich and the Carnot battery concept from the German Aerospace Center (DLR). Such systems heat solid materials like ceramics or salts to temperatures of up to 1000 °C. Part of the stored energy can later be converted back into electricity, typically with efficiencies below 50 percent. Overall efficiency improves when the heat itself is also used directly. The real potential therefore lies not in reconversion alone but in coupling the electricity and heat sectors. Already today, a substantial share of electricity is used for heating. Thermal storage that can flexibly absorb surplus power provides noticeable relief for the grid.