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Energy transition

Hot stones store renewable energy

Environmentally friendly and cost-efficient energy storage solutions will contribute considerably towards ensuring the success of the energy transition. The team led by Siemens development engineer Till Barmeier has achieved a breakthrough in this field by creating a high-temperature heat storage system.

The energy transition is coming, but there are still a few obstacles to overcome. The goal in Germany is to achieve between 55 and 60 percent renewable energy share in terms of consumed energy. Further ahead the goal is to reach 80 percent in 2050. Last year, wind, sun, and other renewable energy sources already contributed 30 percent towards gross electricity generation in Germany. Currently, wind turbines generate 9 percent of Germany’s electricity, with a rapidly rising trend. However, a crucial piece of the puzzle is still missing in the energy transition that was initiated with the Renewable Energy Act (EEG) on April 1, 2000. Researchers, engineers, and developers both in Germany and around the globe are frantically searching for this piece of the puzzle.

When long periods of cloudy weather cast shade on photovoltaic systems, when the turbine rotors stand still for days on end for lack of wind, and the phasing out of fossil fuels means that the power plant boilers and steam turbines remain cold, we run the risk of energy shortages. Does this mean that consumers might have to resort to candles, generators, blankets, and warm sweaters for light and warmth? “Most certainly not. But in order for the energy transition to succeed, we need storage solutions that can balance the natural fluctuations in renewable energy generation and which can also provide electricity in a flexible manner during periods without wind or sun and thus ensure security of supply,” Till Barmeier explains. The Siemens development engineer holds a PhD in mathematics and is in charge of the Energy Storage Program at Siemens Wind Power. For the past three years, he and his team of up to ten specialists have been carrying out basic research in this important field. And it appears that Barmeier’s team has achieved a breakthrough that may finally provide the missing puzzle piece of energy storage that is so important for the energy transition.

A high-temperature heat storage unit: energy-efficient and
environmentally friendly

And this piece of the puzzle is as simple as it is smart: “We draw electricity from the power grid and convert it into heat. Thanks to its density, thermal energy allows a great deal of energy to be stored,” Till Barmeier explains. “You can imagine the electric heater we use to convert the electricity into heat as being like a hairdryer. The hot air that is generated is fed into the heat storage unit via a pipe system.” And the storage unit is based on an equally simple principle: “It consists of a sturdy, insulated shell, which is loosely filled with suitable small pieces of rock. The flow of hot air heats up the pebbles, which must be able to withstand temperatures of over 600 degrees centigrade without melting or cracking.”

Hot stones: Development engineer Till Barmeier and his team make energy storage cost efficient.


But how can this stored energy be converted back into electricity? “This part of the process is also based on an extremely simple principle,” Till Barmeier explains. “Here, we rely on a tried and tested technology that has been providing us with very reliable energy for more than 120 years: the steam power process.” Cold air is blown into the storage unit, heated while it passes through the stones, and used to heat a boiler. This results in high-pressure steam. The steam is transported via pipes to a turbine, where electricity is generated and fed into the grid. Till Barmeier: “During days with high winds or a lot of sun, this procedure allows the excess electricity to be stored – and used during days when the photovoltaic systems and wind turbines cannot generate electricity.” Barmeier’s team believes that the innovative storage unit can keep the energy stored for an entire week. “However, the main goal of the storage solution is to be able to continuously generate electricity for two to three days during energy shortages.” And he adds: “It is important to note that our system is not a seasonal storage solution that allows energy to be collected during autumn storms and conserved for three months for the winter period when renewable energy systems do not generate much electricity.”

During days with high winds or a lot of sun, this procedure allows the excess electricity to be stored.
Till Barmeier, Head of the Energy Storage Program at Siemens


Thermal storage systems are only one technological possibility among many different storage solutions. Pumped hydro, in comparison, which generates energy by channeling water from a reservoir lake at a higher altitude down to a lower reservoir, represents a well-established alternative. It is suitable for balancing day-night cycles of approximately eight hours. However, this technology is very costly and depends on very specific geographical requirements. There is an abundance of such locations, for example, in Norway, but in Germany, almost all the suitable locations have already been used. Till Barmeier points out an additional disadvantage: “If one considers the area requirements for the water reservoirs needed for pumped storage power stations, we can store over 30 times the energy with our thermal solution.”


Electrochemical storage solutions, such as lithium-ion batteries, are also particularly effective. However, these batteries are still very expensive and contain toxic substances. Many other mechanical and chemical storage solutions are currently also limited due to their high costs. This also applies to power-to-gas (P2G) systems, which are based on the principle of water electrolysis. Additional research is required in order to make these different technologies more efficient, more secure, and more environmentally friendly.

A test model for the future
“All these storage possibilities have their place,” Till Barmeier says. “But for us it was clear that we must develop a simple technology that fulfills the requirements in terms of low energy-specific costs, a high potential for scalability, and a low environmental impact.” Already now, it’s clear that the innovative energy storage system does not require an array of toxic or chemical substances. It is based on safe components and processes. “Our solution does not involve any combustion,” Till Barmeier explains. “We produce virtually no emissions and there is no explosion or fire hazard. The rocks we use for our storage solution can be sourced from local quarries in many places around the world.”

The innovative energy storage system is based on reliable and simple principles.


Within the framework of the joint Future Energy Solution (FES) research project, which is sponsored by the Federal Ministry for Economic Affairs and Energy, Siemens, in cooperation with energy provider Hamburg Energie and the Hamburg University of Technology, is constructing a model and testing system for the innovative technology. The system’s thermal storage unit will span an area of approximately 400 square meters. “Over the next two years, we will demonstrate that the system works as we expect it to,” Till Barmeier explains. “And we will find out what potential for additional optimization and scalability it offers.” The construction of a pilot plant with an output of around 30 megawatts could happen as soon as 2019. Also for this plant, Siemens is considering cooperating with an energy supplier or project development specialist. Once the system is market-ready, it will aim to support the energy supply of a medium-sized city. Then the generation output will be 100 megawatts or more.

Till Barmeier is convinced that the FES technology is the missing piece of the puzzle required to complete the energy transition. But the resourceful engineer is already thinking another step ahead. “It is also conceivable for fossil power plants that will be phased out in the future to be converted with our technology,” he explains. “This would only require the power plants to be equipped with a heater and one of our heat storage units.” Thermal power stations would become energy storage plants. In other words: The equipment of the past would be harmoniously combined with the technology of the future.

Ingo Petz is a freelance journalist based in Berlin
Picture credits: Siemens AG