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  • Electricity made from salt

    STEAG has a future solution up its sleeve that turns coal-fired power plants into energy storage plants

    The energy transition towards resource-saving electricity supply with renewable energies is striding onwards. Wind power and photovoltaic plants are being rapidly developed, yet one important element of the new energy market is receiving comparatively little attention: storage technology. But it is precisely this key technology that is needed when the wind stops blowing, the sun is no longer shining – and conventional power plants are no longer available as a standby option. With six dynamic large batteries, STEAG is already Germany's largest storage operator on the rule energy market. But the energy company from Essen goes even further: the research and development department has, amongst other things, been working with thermal storage power plants that can be integrated into existing coal-fired power plants.

    According to a joint monitoring report by the German Federal Cartel Office and Federal Grid Agency, in October of this year the installed capacity of the solar, wind and other ecopower plants currently constructed in Germany was greater than that of the installed capacity of conventional power plants for the first time: renewable energies had a generation capacity of 112.5 gigawatts (GW) – and therefore more than coal-fired power plants, nuclear power plants and other plants, which together added up to 105.1 GW. In comparison: in 2016 the GW comparison was still 104.5 GW (renewable energies) to 107.5 GW (conventional energy generation).

    However, currently it is precisely these conventional plants, including the coal-fired power plants of STEAG, that balance out sometimes significant deviations in the grid and therefore contribute towards there always being a sufficient supply of electricity for industry, private households and commerce. "Various storage technologies do exist in Germany, including pump storage power plants as a leading stationary storage option. But even these are not currently economically viable," explains Prof. Dr.-Ing. Wolfgang Benesch, Head of the STEAG Research and Development department.

    As early as the end of 2016 the studied mechanical engineer and his team began to get an overview of the storage market. "We want to technically and economically highlight the opportunities of storage plants. But there were as good as no comparable details, such as output, capacity and costs." So STEAG began a so-called orientation enquiry at various manufacturers of storage technologies and therefore soon had a starting point for specific deliberations.

    "When constructing pump storage power plants, alongside the classic version, there are also several interesting models that are currently being discussed in the Rhine and Ruhr regions," explains the Research Leader. "For example, the flooding of former mining tunnels. But this is very laborious and has many approvals linked to it. Such a pump storage power plant is ultimately a renewed interference with nature, requires enormous construction time and causes high costs." Wolfgang Benesch and his colleagues looked for a pragmatic alternative – and found it in salt storage technology. This was no coincidence, as STEAG has many years of experience in this area: the energy company from Essen has been operating a solar thermal power plant with liquid salt storage in Arenales, Spain since 2013, which has an output of 50 megawatts (MW) and a storage capacity of around 1,000 megawatt hours (MWh).

    The STEAG engineers combined this knowledge with considerations of the further use of coal-fired power plants. "Converting an existing coal-fired power plant seemed to us like an interesting alternative," says Wolfgang Benesch. "The entire coal process is decommissioned, but the steam turbine and the generator continue to be used." The boiler, in which coal had previously been burnt, then replaces an electrically heated salt storage tank. These are tanks in which the salt in a liquid state is heated to temperatures of around 500 to 550 degrees Celsius. "In principle, the salt tanks are heated with immersion heaters, which are operated with electricity, possibly even excess electricity from renewable energies. The hot, liquid salt then passes on its heat to the steam cycle in the power plant as required."

    The disadvantage of this solution: the efficiency of the current conversion is determined by the efficiency of the previous steam cycle and averages at 45 percent. However, a clear advantage is the conversion of an existing power plant could be realised in two to three years – and the new plant would not emit any more CO2.

    Alongside the limited market for quick energy storage – such as these salt storage power plants with a capacity of around 1,500 MWh – above all mass storage is needed for energy security. However, these energy storage plants have not previously been clearly classified in terms of energy efficiency. Therefore, as the law stands, they are considered both as final consumers of energy during the storage process as well as consumers of energy during withdrawal.

    As a result, the operators of storage plants must, in principle, pay all taxes and levies for the emitted electricity that a conventional electricity customer would also pay: the EEC levy of 6.354 cent per kilowatt hour (kWh), electricity tax of 2.05 cent/kWh, cogeneration levy of 0.445 cent/kWh, §19 levy of 0.378 cent/kWh, concession fee of 0.11 to 2.39 cent/kWh and, finally, the offshore liability levy of 0.04 cent/kWh.

    "As a result, storage technologies cannot currently be efficiently operated," says Wolfgang Benesch. "This already applies to pump storage power plants and, consequently, also for our concept of a salt storage power plant. This is a question for legislators: if, after the final withdrawal from coal and gas generation, there are no longer any conventional power plants left available in Germany, we will need energy storage plants and their operation must then be economically feasible. If necessary, by a public subsidy."

    These ideas are not utopian. For as recently as at the start of 2018 Union and SPD enshrined in the coalition contract that they would check "to what extent power plant sites no longer required can be used for large thermal storage power plants." Should policy implement this initiative, STEAG would already have a functional solution ready to go thanks to Wolfgang Benesch and his employees in the Research and Development department.

    Profile

    Prof. Dr.-Ing. Wolfgang Benesch (65), has worked for STEAG since 1984. Benesch was responsible for the waste treatment, plant construction and plant and process technology departments at the energy company from Essen, before becoming Head of the Energy Technologies Division of STEAG Energy Services GmbH in 2002 and Head of the R&D department of STEAG Group in 2007. Born in Lünen, he is also a member of the board of STEAG Energy Services India and has lectured as the chair for energy plants and energy process technology at the Ruhr University Bochum. He was named honorary professor there in the spring of 2016. Wolfgang Benesch retired at the end of last year. But those who experience him do not get the impression of a pensioner. As recently as August he was awarded the Guilleaume commemorative coin of VGB PowerTech e.V., the international association of companies from the electricity and thermal energy supply sector. Wolfgang Benesch received the highest distinction of the VGB for especially outstanding achievements in the collaboration of power plant operators. And even after retiring, he continues to make his expertise available to STEAG – as a "retired, not tired" Consultant.