For many years, scientists around the world have been looking for means to efficiently produce and store energy in a climate-friendly manner. Among the most promising materials for this task is sulphur, as it can not only fuel steam or gas turbines, but also be used for energy storage in solar thermal power plants.
When employed in a thermal power cycle, it can absorb, transport and store solar energy as high-temperature heat, since it has an energy density several times higher than that of molten salt. Also, it can easily be integrated into processes which involve the recycling of materials, such as renewable energy processes.
Solar station with an salt energy storage system
Now (2021), scientists at the German Aerospace Center (DLR) have developed a process that can generate electricity in a climate-neutral way using sulphur and solar energy. This research was part of an already completed EU project called PEGASUS project. The process uses a pilot-scale plant and a chemical cycle where sulphur is burnt in special turbines and the exhaust gases converted into sulphuric acid. With the help of solar energy, the sulphuric acid is then converted back into pure sulphur without emitting CO2.
The temperature required to decompose sulfuric acid is provided by a solar thermal plant that is hot enough to break down the acid into sulphur dioxide (SO2) and water (H2O).
The temperatures required to decompose sulphuric acid is provided by a solar thermal plant, which are sufficiently hot to split the acid into sulphur dioxide (SO2) and water (H2O). SO2 and H2O then undergo a process called disproportionation to produce pure sulphur again. The sulphur can then either be stored or burnt in a gas turbine to generate electricity. The gas produced in the turbine is sulphur dioxide (SO2) which can be used to again generate sulphuric acid and heat. In addition, the heat produced in this process might be used to power a steam turbine and thus generate even more electricity. Finally, the cycle starts again with the decomposition of sulphuric acid. If there is lots of sunshine, more sulphur than is needed can be produced during daytime operations, thus enabling the plant to operate continuously.
The research was aimed at testing sub-processes of sulphuric acid decomposition with the help of solar energy and using the resulting sulphur as a fuel in gas turbine power plants. In order to produce the high temperatures required for the decomposition, the researchers combined a newly developed reactor to split sulphuric acid with a solar radiation receiver previously developed at DLR which used ceramic particles as the heat transfer and storage medium. A particle receiver incorporating small ceramic particles absorbed and transported the incoming thermal energy to produce electricity and industrial process heat. This enabled achieving operating temperatures of over 900 degrees in power plants, which increased efficiency and lowered production costs. The scientists at the German Aerospace Center installed a variant of the CentRec particle receiver developed for the Sunlight artificial Sun in Jülich. At the same time, they researched the sub-process of integrating sulphuric acid decomposition into the sulphur cycle in their laboratory.
The Jülich solar towers: A test facility for commercial solar thermal power plants
Source: DLR (CC-BY 3.0)
The Pegasus Project was a transnational joint research endeavour which started in 2016 until 2020 and was funded by INEA – Innovation and Networks Executive Agency (European Commission) as well as the Horizon 2020 Framework Programme of the European Union. Internationally renowned research institutions and companies participated in it, including DLR – Deutsches Zentrum für Luft- und Raumfahrt (Germany), APTL/CERTH – Aerosol and Particle Technology Laboratory, Chemical Process and Energy Resources Institute, Centre for Research and Technology Hellas (Greece), KIT – Karlsruhe Institute of Technology (Germany), BCR – Baltic Ceramics (Poland), PI – Processi Innovativi (Italy), and BRS – BrightSource (Israel). The Greek research institute APTL/CERTH, for example, provided the catalyst materials for the sulphuric acid decomposition. The Italian company NextChem was in charge of the process simulation and the technical and economic study. The Karlsruhe Institute of Technology (KIT) designed and built the sulphur burner for the high pressures required. The Israeli company BrightSource designed the solar field and will be the future end user of the circular process. The PEGASUS project was awarded 4.7 million Euros of funding by the European Commission's Horizon 2020 programme.
Using sulphur instead of molten salts has several advantages: sulphur has a much higher energy density than molten salt which is currently used in solar thermal power plants. Therefore, higher operating temperatures can be achieved. Sulphur can also be easily stored in different forms, which is why it can be transported over long distances. Solar thermal plants with sulphur production operate efficiently, particularly if they are installed in sunny regions. The sulphur produced there can then be transported to less sunny regions.
Effective and economic long-term storage of solar energy is an important step in order to fully replace fossil power plants with renewable sources. The European project PEGASUS with its novel power cycle for renewable electricity production providing 24 hours baseload operation might be an important means to achieve this.