Long ago in Ancient Greece, philosophers and scholars were already contemplating the principles of the four elements as known throughout history: earth, water, air and fire. According to the teachings of Empedocles, the four indestructible and unchangeable natural forces are the basic components of human life, indeed of all that exists. Without them, we would be nothing.

Although our knowledge of the world has since matured, these four elements will continue to be important in the future. The sustainable four elements will advance decarbonization in the world and will to a great extent ensure that we have electricity, light and warmth. But since the wind doesn’t always blow and the sun doesn’t always shine, we also need storage systems to be able to secure energy supplies over longer and shorter time periods. Research on various systems is ongoing, with some more successful than others. “In the future will we need several different systems,” says Myriam Gil Bardají, manager of the Joint Project Energy Storage for the European Energy Research Alliance at the Karlsruhe Institute of Technology. “Right now there isn’t any storage technology that can by itself meet the challenges of storing regenerative energies.”

The following is an overview of some of the promising approaches. Each development has its own unique approach—and each is based on one of the four elements.


Photo: Max Bögl Wind AG

Water – Clever Combination

New ways of thinking are always a good idea when it comes to the future of energy storage. The concept behind the water battery in Gaildorf, Germany, is therefore an innovative step forward. The facility, inspected by TÜV SÜD, combines renewable wind-generated energy with a pumped-storage power plant to form a powerful energy storage facility. The water battery works over the short term to absorb energy, hold it and then feed it back into the electricity grid when it’s needed.  

Up to 70 megawatt-hours, the equivalent of about four hours of wind energy, can be fed into the flexible short-term storage facility as needed, contributing to grid stability. The wind turbines are already turning, and the water battery is scheduled to become operational in summer 2019. The foundations of the wind turbines are used as water reservoirs, connected by pipes to a hydroelectric plant and its basin located down the valley, 200 meters lower in elevation. Extra wind energy will be used to pump the water up to the tanks. When electricity is needed later, perhaps on a windless day, the water is sent back downhill to produce hydroelectric power.“Developing new technologies is essential to master the challenges of the future,” Gil Bardají says. “That’s why such pilot programs for network integration are so important and commendable.” 

This pilot project is just the start: the modular design of this water battery can be deployed as a storage solution for all types of renewable energies. Completely standardized, the water battery can become a ready-made storage power plant. Interest in this technology is already being expressed in Germany, Austria, the United States and Indonesia.


Photo: shutterstock/mmoktp 

Fire – H₂igh H₂opes

Water vapor or droplets—these are the only “waste products” created when hydrogen gas is burned. When hydrogen gas combusts in the presence of oxygen, a large amount of energy is released and the two combine to form water. A cleaner process can scarcely be found. No wonder the chemical processing of hydrogen gas, known as electrolysis, plays a central role in the long-term electricity storage of the future. 

“Hydrogen is versatile,” says Gil Bardají. “It can be transformed into energy for stationary applications such as electricity and heat generation or in turbines, but also for mobile applications such as transportation.” However, the problem at the moment continues to be that hydrogen production through electrolysis uses a large amount of energy with a relatively low efficiency. 

In water electrolysis, water is split into oxygen gas and hydrogen gas. The process requires electricity, best obtained from renewable sources. If the wind isn’t blowing or the heavens are cloudy, the gases can be removed from their storage tanks and for instance combusted in a gas and steam turbine, which in turn drives a generator to produce electricity.

The world’s largest hydrogen plant, currently under construction in Linz, Austria, will demonstrate that hydrogen can achieve a high degree of efficiency. The pilot plant is expected to achieve efficiencies of 80 percent. The stated goal is to find out whether hydrogen can replace coal and coke on a large scale over the longer term. MITNETZ GAS, a natural gas supplier in central Germany, has started building infrastructure for hydrogen testing at the Chemical Park Bitterfeld-Wolfen, part of the Hydrogen Power Storage & Solutions East Germany (HYPOS) project. The project’s more than one hundred members are striving to build a regional model for hydrogen in eastern Germany. The project is currently scheduled to last for two years, with TÜV.


Photo: ALACAES 

Earth – From The Depths of a Massif

The Gotthard Base Tunnel is an engineering masterpiece. It runs almost sixty kilometers through the central Swiss Alps, dramatically reducing travel times. And if that weren’t enough, a three-kilometer long tunnel near the village of Bianca, which was used as a conveyor system to remove excavated materials during tunnel construction, was converted from 2015 to 2017 into a testing plant for a modern compressed air reservoir. 

The principle of such a reservoir is easy to explain: surplus solar and wind energy is used to drive compressors to condense air. This compressed air is stored in a reservoir, known as a cavern, and then released when needed to spin a turbine that drives a power generator. Inhale, hold your breath, exhale—it’s as if a compressed air reservoir fills up with energy on every inhale and then releases it on the exhale. 

However, compressed air reservoirs are few and far between as of yet due to their disadvantages. “They’re not very efficient and require very specific geological conditions,” Gil Bardají explains. The total energy outlays are very high, with an efficiency rate of just 40 percent. Pumped-storage power plants, the most-used storage systems today, achieve rates of around 80 percent.

In contrast to other compressed air reservoirs, however, the Gotthard project has a relatively high degree of efficiency due to heat recycling in particular. The air is compressed in two steps and can reach temperatures of 550 degrees Celsius. This thermal energy is no longer just released unused, but instead is collected in an additional reservoir and re-used during the conversion of compressed air into electric energy, leading to an efficiency of up to 75 percent.

At the end of the decade—the plans are still vague—the pilot project is to be followed by a commercial plant, but many issues need to be resolved before this happens. Experts warn that the technology is not yet mature, particularly with regard to thermal storage. A mountainous country like Switzerland, with many airtight tunnels, could thus play an important role for the future of compressed air reservoirs.


Photo: Frauenhofer ICT 

Air – Giants of the Future

Researchers consider redox-flow batteries to be promising solutions for large-scale electricity storage. What’s special about them: while conventional batteries—whether in cars or smartphones—are closed units with a fixed storage capacity, the capacity of redox-flow batteries depends only on the amount of liquid they have. Whether one or a hundred thousand liters, the larger the tanks, the more energy the battery can store, making this technology interesting for industrial applications, for instance. 

Redox-flow batteries—“red” for reduction = electron uptake, “ox” for oxidation = electron liberation—use fluid electrolytes that flow in two circuits that are independent from one another. An electrolyte is a solution that contains free-moving ions that can conduct electricity. When a charge is placed on the battery, the ions in one of the fluids take up the electrons released from the ions in the other fluid. When the battery supplies electricity, the reaction runs in reverse. 

A pilot project in Pfinztal, Germany, near Karlsruhe, is demonstrating the technology’s potential. A 2-megawatt wind turbine supplies the energy for the Fraunhofer Institute’s campus there. If there isn’t enough wind, the giant battery, which is located in a specially built hangar, supplies enough power for all the connected buildings—as much as a small town with four thousand residents would need.

Its scalability is what makes the redox-flow battery attractive for use in remote areas that aren’t connected to a reliable electricity grid. Research in Pfinztal is currently looking at whether it is possible to dependably supply villages, small towns or companies with electricity with the help of redox-flow batteries. “A plus for the system is that it is usable particularly for medium-term and long-term storage,” Gil Bardají says. “It still needs a lot of further development, but the batteries have already achieved a high efficiency of more than 75 percent for the overall system.”