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Latent heat thermal energy storage by urea-based eutectic mixtures

18th national competition for scientific and technical research

Renewable energy: materials and processes

Senior Researcher : Álvaro Campos Celador

Research Centre or Institution : Universidad del País Vasco (UPV-EHU).


The research carried out in this fourth year has focused on two areas: increasing the thermal conductivity of storage systems and the stability of urea-sodium nitrate mixtures.

In relation to enhancing of the thermal conductivity, work continued on the expanded graphite and PCM composite developed during the second year. Despite the fact that the composite has very low leakage levels for air, it was considered appropriate to cover it to prevent the carryover of PCM in cases where water is used as a working fluid. For this purpose, different materials that can perform well under a sheet of water, at high temperatures (70º) and in contact with the PCM (paraffin), were collected to be used as coating. With this in mind, the following materials were selected: epoxy resins, silicones, butyl band, dry teflon spray and zinc spray. Prismatic composite samples were coated with each of the selected materials and tested in a climatic chamber in order to have preliminary results. The samples whose coatings did not show degradation after this test: epoxy resins, butyl band and galvanised zinc, were tested on the prototype developed during the second year of the project, without showing any PCM leaks.

In the field of mixture stability, a complete study of degradation at maximum working temperature was carried out (samples degraded for periods of up to 1 year). Additionally, the thermal characteristics of the degraded samples were measured by Differential Scanning Calorimetry, analysis of the composition by Liquid Chromatography, X-ray and Infrared Diffraction. In order to study the phase segregation of the eutectic mixture after being subjected to melting/crystallization cycles in tubes with a PCM mass of approximately 4 grams, characterization tests were carried out: reversibility, behavior at different cooling speeds and vertical or horizontal arrangement. In addition, samples were prepared in thin films under different cooling conditions, and analysed by microscopy (PLM and SEM) to determine the microstructures formed. Finally, a characterisation of the overcooling of the eutectic mixture of urea and sodium nitrate and polyethylene glycol 10000, under different thermal conditions, was carried out. R software was used to process the data and obtain the most relevant variables to describe the phenomenon and create models to predict the phenomenon in the materials studied.


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