Ceramic particles and supercritical CO2 for more efficient solar thermal energy
The main objective of the COMPASsCO2 project (Components and Materials Performance for Advanced Solar Supercritical CO2 Power Plants) is to integrate two innovative materials into solar thermal power plants: ceramic particles instead of molten salt as heat transfer material and supercritical CO2 (sCO2) instead of water vapour in the power plant cycle. Together with the project participants, DLR is developing a suitable heat exchanger that transfers the thermal energy of up to 1000 degrees Celsius from the particles to the sCO2.
Renewable energies make a significant contribution to climate protection and are a sustainable answer to the challenge of finite raw materials. The largest source of renewable energy is the sun. In solar tower power plants, mirrors concentrate the sunlight and direct it to a heat receiver at the top of the tower. In the commercial tower power plants in operation today, the radiation heats liquid salt to up to 560 degrees Celsius. Pipelines transport the hot salt to the power plant section, where the heat converts water into steam in a heat exchanger. In the so-called Rankine steam cycle, the steam drives a turbine that transfers its rotational energy to a generator that produces CO2-free electricity.
The use of supercritical CO2 in the so-called Brayton power plant cycle opens up new possibilities for solar thermal power plants to reduce costs. At a temperature of 31°C and a pressure of over 73.8 bar, CO2 reaches its supercritical state, where it has the density of a liquid and the viscosity of a gas. These properties enable a higher efficiency of energy conversion and a very compact design of the components compared to the conventional Rankine steam cycle. A solar thermal power plant with a Brayton cycle therefore gets by with a smaller heliostat array and the heat receiver and thermal storage can also be dimensioned smaller than in modern molten salt tower power plants. The lower investment costs have a positive effect on the electricity generation costs and should make this technology more competitive.
However, this requires heat transfer materials, such as ceramic particles, which absorb and store high temperatures of around 1000 degrees Celsius, which the sCO2 needs to be able to exploit its efficiency potential. To adapt these particles even better for use in solar power plants, the participant scientists are developing new compositions and coatings for the ceramic particles. The optimised particles are supposed to be able to absorb and store more heat and be more durable.
The key component for combining the two innovative materials in the cycle of a solar thermal power plant is the heat exchanger. It transfers the heat of the ceramic particles as a heat transfer medium to the supercritical CO2 in the Brayton cycle. Due to the fact that both materials are hotter than those used in conventional power plants, a higher pressure is generated and the particles produce a higher abrasion of the tube coating, the heat exchanger must be much more robust than the molten salt-to-water heat exchanger of commercial power plants. In order to design a suitable heat exchanger, DLR and the project participants simulate the extreme conditions and test various materials in laboratory experiments and test setups. The scientists are researching on new metal alloys and coatings to produce tubes for the heat exchanger that can withstand the conditions in commercial operation for over 20 years.
The project, funded by the European Community under H2020 (Grant Agreement No. 958418), is coordinated by the DLR Institute of Solar Research. The institute also defines the technical boundary conditions and together with the DLR Institute of Technical Thermodynamics tests heat exchangers with the new metal alloys and particles under relevant conditions. The DLR Institute of Materials Research optimises and tests the ceramic particles in terms of their material and functional properties. In total, the project team consists of twelve institutions from seven countries, including companies, research centres and a university.
The virtual kick-off meeting took place on 3 November 2020.
Figure 2: Scientific participants of the COMPASsCO2 project. Source: COMPASsCO2.
Co-funded by the Horizon 2020 Framework Programme of the European Union
Grant Agreement No. 958418