Concentrated Solar Plants (CSPs) utilise sunlight (or
solar thermal energy) to generate electricity. One part of the
generation process involves the use of molten salt mixtures for the
transport and storage of heat. Research carried out at ISIS is looking
to better understand the thermodynamic characteristicsr of these molten
salt mixtures and maximise their transport efficiency.
Traditional molten salts comprise of alkaline and earth-alkaline nitrate/nitrite mixtures. On the face of it such salts offer clear advantages. For instance, not only are they low cost, but they also have a low toxicity and a good heat capacity which is vital for transporting and storing heat. However, they also have limitations, the most significant being a high freezing point.
The freezing point of a molten salt mixture is critical for efficient use in solar plants. Collaboration between ISIS scientists, Dr Mark Telling and Dr Ron Smith, the Department of Chemical Science and Technologies of University of Rome Tor Vergata, and the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA- DTE-STT-ITES, Casaccia, Rome, Italy) hopes to develop a predictive model that can be used to forecast freezing points for different molten salt mixtures. At present, when choosing a new molten salt combination, a phase diagram must be produced experimentally in order to understand the behaviour of the mixtures at different temperatures. Development of a predictive model, however, would lessen the need for such potentially costly and time consuming experimentation and have huge implications for industry
Dr Salvatore Sau from the ENEA says, “Molten salts at the moment have too high a freezing point. Inside a solar plant it is necessary for technological reasons that the fluids remain in liquid phase. We are trying to understand the structure of the substances, especially at high temperatures.”
Dr Anna Tizzoni, from the Department of Chemical Science and Technologies, University of Rome Tor Vergata explains, “ISIS allows us to run experiments using temperatures above the melting point of the salts. We are trying to understand what is happening to the molecules in the salt, to see if there is some behaviour that changes their composition. If there is, we can apply this logic to other mixtures.” Results from such experiments will enable the researchers to develop their predictive model.
Preliminary experiments have been carried out on the POLARIS instrument where the use of high resolution neutron diffraction affords the possibility of accurate structural modelling. Salvatore says, “The idea is to complete our data about the structure of these materials. In Italy we have got an X-ray diffraction instrument, but we cannot perform experiments above room temperature. We are using neutron scattering in order to complete the data sets we have already obtained from X-ray diffraction.“
The future for energy harvesting is certainly a hot topic. The outcome of this research could have real impact on the future of CSPs. Anna says, “Some mixtures might not be used at the moment because they have never been tested as they are too expensive to test. This predictive model will allow testing of such mixtures. With the model you could propose a certain mixture that could be used, or at least exclude some compositions which would save time and money.”
Research date: September 2015
For more information, please contact Dr Anna Tizzoni.