Storage facilities are playing an increasingly important role in the energy system of the future. They address a bottleneck in the energy transition by better aligning generation with consumption.
In the spring of 2024, the Steag Iqony Group began construction of a heat storage facility adjacent to the Gelsenkirchen-Schalke heating plant.
During a ceremonial inauguration on July 8, 2026, the recent commissioning was celebrated together with Mona Neubaur, Minister for Economic Affairs, Industry, Climate Protection, and Energy of the State of North Rhine-Westphalia, and Andrea Henze, Mayor of Gelsenkirchen.
“Our heat storage facility makes the Ruhr district heating system more flexible, reliable, and sustainable. With this giant thermos, we can use climate-friendly heat independently of the time of generation: We capture more of it as it is produced and make it available when demand is high and generation is low. This milestone was a true team effort. My sincere thanks go to everyone involved in the project, our partner Bilfinger, the State of North Rhine-Westphalia as the funding provider, and the City of Gelsenkirchen as a strong supporter,” said Gundolf Schweppe, CEO of the Steag Iqony Group, at the grand opening.
Support from the State of North Rhine-Westphalia and the City of Gelsenkirchen
Minister Mona Neubaur stated: “57 meters tall, 31 million liters of water—and yet, at its core, quite simple: a thermos for the heating transition. What the Steag Iqony Group has built here in Gelsenkirchen demonstrates how district heating in metropolitan areas can be made future-proof. We specifically fund such storage facilities because they make the difference between heat that’s simply there and heat that’s available when people need it.”
The project, with an investment volume of approximately 30 million euros, is funded by the state of North Rhine-Westphalia with funds from the “Program for Rational Energy Use, Renewable Energy, and Energy Conservation”—progres.nrw for short—with funding totaling approximately 7.1 million euros.
Mayor Andrea Henze adds: “Our draft municipal heat plan is currently open for public comment. The draft shows that the district heating network will be further expanded and that further growth is possible. This makes the project all the more important, as it is a milestone for decarbonization and a key contribution to the success of the local heating transition. Potentially, the new storage facility can supply the local district heating network with heat for two full days, thereby increasing the flexibility and reliability of this form of heat supply.”
How a Thermos Works
The heat storage tank works like a giant thermos. Standing 57 meters tall and 28 meters in diameter, it can store up to 31 million liters of water—enough to fill about 200,000 bathtubs.
What makes it unique is that the heat storage tank is completely filled with water at all times. The upper section contains water at around 90 degrees, which exerts pressure on the separated lower section (the so-called “working section”). This allows the water there to be heated to up to 115 degrees without beginning to evaporate. In the working section, the water is stored in two layers. The hot water is located at the top in the supply line, while the cooler water—at a temperature of around 60 degrees—is located at the bottom in the return line.
When the storage tank is being charged, the colder water is pumped out from the bottom via a piping system, heated to 115 degrees using heat exchangers, and fed back into the storage tank at the top. If the storage tank is to supply energy to the grid, the hot water is pumped from the top of the storage tank into the grid, and the colder water returning from customers is replenished at the bottom.
