Dusseldorf It is the right time for ideas like this: so-called thermal storage in old coal-fired power plants. According to the federal government, 80 percent of German electricity should come from renewable energies by 2030. More and more coal-fired power plants are going offline.
This is intended to reduce CO2 emissions from electricity generation. But at the same time there is growing concern that Germany is becoming more and more dependent on wind and solar power – which is only available on windy or sunny days. So what if there is no wind and it is dark or cloudy?
Thermal storage should help to solve this problem. Cleantech analyst Antoine Koen from the think tank Future Cleantech Architects says: “The closer we get to the goal of producing 100 percent of our electricity from renewable energies, the more important storage becomes.” Many initially think of batteries to store renewable energy. Thermal storage is sometimes significantly cheaper than rechargeable batteries.
How thermal storage works
The basic idea is to store electricity when the sun is shining and the wind is blowing – and thus more electricity is generated than people consume. The stored electricity can then be used at a later point in time.
Thermal electricity storage can, for example, run as part of so-called heat storage power plants. If there is a lot of green electricity in the grid, the power plant uses it to heat a liquid or an object with the help of a type of immersion heater. This could be liquid salt, for example.
For example, it was planned in a pilot project that RWE, the German Aerospace Center (DLR) and the Aachen University of Applied Sciences ran jointly until 2021. This involved retrofitting an existing lignite-fired power plant to create a thermal storage facility.
In it, liquid salt should be heated up to 560 degrees in times of excess electricity and stored in a tank. If additional electricity is required, the hot molten salt should be used to heat water from the coal-fired power plant’s circuit so that it evaporates. Steam, in turn, can drive a turbine and thus generate electricity – in the same way as steam from burning coal in a coal-fired power plant.
Advantages over batteries
Such thermal storage devices are designed to store electricity over a period of several hours. For example, excess electricity from midday when the sun is high can be stored and used again later in the evening when it is dark.
In Germany there are dozens of coal-fired power plants that have already been shut down or are scheduled to be shut down in the next few years. According to the cleantech expert Antoine Koen, 50 thermal storage power plants with a total storage capacity of 500 gigawatt hours are conceivable in Germany.
For comparison: a battery in a VW ID.3 has a storage capacity of 58 kilowatt hours. The 500 gigawatt hours correspond to around eight and a half million such electric car batteries.
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According to the think tank Future Cleantech Architects, it would be too expensive to solve the electricity storage problem with batteries alone. The experts say: Batteries are cost-efficient when it comes to providing high performance at short notice. However, batteries are ineffective for storing large amounts of energy.
The difference between performance and energy can be imagined using a water or petrol tank. The power would correspond to the pressure – i.e. the speed at which water or petrol can be pumped in or out of the tank. The quantity, on the other hand, describes the number of liters that fit into the tank.
Future Cleantech Architects say thermal storage is significantly more cost-effective when storing large amounts of energy, which is key to integrating solar and wind power.
High degrees of efficiency only when using heat
One disadvantage of thermal storage compared to batteries is that they are less efficient. According to the expert Gerrit Koll from DLR, it is 40 to 45 percent for storing electricity – so only 40 to 45 percent of the stored electricity is later fed back into the grid as electricity. With batteries, the efficiency can be as high as 75 percent. Then only 25 percent of the electricity is lost.
However, Koll points out that in thermal power plants not only electricity but also heat can be generated from the stored energy. This means that the molten salt heated with the help of the electricity not only generates steam to make electricity again, but also heat that can be used. For example, it can be routed as district heating in pipes to households that heat with it.
If the stored energy is also used as heat, according to Koll, thermal storage systems can use almost 100 percent of the electricity used from renewable energy sources.
Synergy effects through thermal storage in old coal-fired power plants
Thermal storage does not necessarily have to be set up in old coal-fired power plants. They can also be built from scratch. But the use of old coal-fired power plants should result in synergy effects.
According to the International Energy Agency (IEA), building a thermal storage power plant from scratch takes four years on average. According to energy expert Koen, converting an old coal-fired power plant into a thermal storage facility is much faster.
In addition, converting a coal-fired power plant into a storage facility instead of demolishing it could also save jobs. Many parts of the coal-fired power plant, such as the turbine, could continue to operate. The previous employees could therefore keep their previous tasks. The US company Malta, for example, which builds electrothermal energy storage systems, promises that this will work.
Practical implementation so far rare
So far, however, the cheap thermal storage is still more theory than practice, at least in this country. A test facility for an electrothermal storage system went into operation in Hamburg in 2019. Inside, a kind of huge Föhn heated up volcanic rock, which later used its heat to generate steam for a turbine. However, the demonstration operation was stopped in May 2022 because the commercial market was still missing.
The project by RWE, DLR and FH Aachen has now also been discontinued. In 2021, feasibility studies had shown that an economic perspective at RWE’s power plant sites in the Rheinisches Revier was not foreseeable. However, that was not a rejection of the technology, but only a statement with reference to the Rhenish Revier, emphasizes an RWE spokesman when asked.
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The Rhenish mining area is a relevant factor when it comes to coal-fired power plants – so the statement that thermal storage would not be worthwhile there initially seems devastating.
However, the expert Gerrit Koll from DLR says: “The market conditions have changed considerably in the meantime.” He refers to the sharp rise in energy and raw material prices due to the lack of gas from Russia, to strongly fluctuating electricity prices and to the current inflation.
Koll also says: “The system planned by RWE did not have any significant extraction of heat for industrial or district heating supply.” As a result, the electricity could not be used as efficiently as it might be necessary for economical operation.
Thermal storage in real operation
Some thermal storage projects are currently being planned. According to Koll, for example, there is a project by a large German energy supply company. A solid-state storage tank in a previous coal-fired power plant is to be heated in order to later use its heat to generate electricity again. If there is no surplus green electricity available, a gas turbine with hydrogen should also keep the power plant running.
According to Koll, there is also a project by an energy supply company in Chile. Based on a feasibility study by DLR and the German Society for International Cooperation (GIZ), it wants to supplement an existing coal-fired power plant with an output of 250 megawatts with large molten salt storage facilities and thus expand it into thermal storage power plants.
Antoine Koen is convinced: “The fact that thermal storage systems have not yet caught on to such an extent has more to do with the markets than with the technology.” The technology is there. “In the past, you just didn’t need storage like this, so it didn’t pay off,” says Koen. “But now they are becoming more and more important.”
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