MEET THE TEAM: Discover work package 3 with CIEMAT-PSA


Discover the people and work done behind POLYPHEM in our blog articles series!

For this third episode, meet Esther Rojas Bravo from CIEMAT-PSA (Spain) who presents work package 3 dealing with Research and Development on Thermal Energy Storage System.

Main researcher

Esther Rojas Bravo coordinates most of Thermal Storage activities at Plataforma Solar de Almería (PSA). With a Ph.D. in Physics, her activity, during more than 25 years, has been focused on heat transfer enhancement and efficiency optimization, both theoretical and experimental, of active solar thermal systems, mainly thermal energy storage and parabolic trough collectors.

POLYPHEM: Thermocline storage systems with concrete

POLYPHEM project explores the extensive use of concrete for the thermocline storage system to both enlarge tank sizes and reduce their cost.

Currently, commercial thermal storage systems use solar salt as a storage medium in a two-tank configuration. The potential advantages to replacing this configuration for a single storage tank where both cold and hot medium live together separated by a temperature gradient (thermocline) have been theoretically highlighted for a long time. Therefore, the POLYPHEM project will verify such approach by erecting by the beginning of 2022 a thermocline storage tank of more than 2 MWh capacity, with a cost target of 28€/kWh.

This cost target is expected to be achieved by using concrete as the main material in the tank walls, its foundations, and filler. Metallic walls are state-of-the-art tank walls which implies a certain limitation in the size of the tank which will be overcome with the concrete walls. The current problems found for a metallic storage tank at high temperatures are expected to be solved with the combination of concrete both at the tank walls and foundations.

Another innovation of the POLYPHEM thermocline storage tank is the use of structured filler made of concrete. Fillers reduce the cost of the storage systems, whenever their cost is lower than the heat transfer and storage fluid they replace. Here a cheap filler as concrete bricks will be used. The problems of thermal ratcheting associated with unstructured packed beds are expected to be outstripped with POLYPHEM bricks. Using a standard procedure in manufacturing concrete bricks, a filler able to stand by itself is achieved.

The research process through POLYPHEM

The use of concrete in storage systems requires solving certain challenges which have been addressed in the POLYPHEM project.

  • The first one is to have a concrete mixture with good thermophysical characteristics as storage media while keeping good structural properties. This mixture has to be compatible with the heat transfer used, in this case, thermal oil, in terms of negligible filtration, contamination and maintenance of their thermo-structural properties.
  • The thermal inertia the tank walls may provide has to be considered to foresee the thermal behaviour of the whole system and therefore in the design phase.
  • In the same sense, the heat transfer between the oil and the structure filler has to be understood and simulated.
  • A good predictive model is required not only for designing the storage system for the POLYPHEM plant, but also for extending the use of such approach for other size plants, and even other configurations and applications.
  • Erecting a concrete storage tank with a structured filler walk next to several tasks to optimize construction such as deciding between pouring the concrete mixture between metallic slabs or going for ensembling precast pieces, disposal of bricks inside the tank to maximize heat transfer, etc.

Main findings and key innovations

Between several new concrete mixtures developed by Arraela Company within the POLYPHEM project, two have been chosen based on their thermophysical and structural properties and considering the role these materials play in the POLYPHEM storage system.

Therefore HEATEK RV has been selected for the tank walls and filler and HEATEK-RC for insulating the foundations. Studies for the compatibility of the first one with the thermal oil used in the POLYPHEM plant assure the good behaviour of such a mixture in contact with it.

Between the available geometries of already commercialized concrete bricks, that one with, according to thermal studies, has the better thermal performance has been chosen and bricks have been manufactured with the specific mixture of HEATEK-RV.

MicroSol facility, at CNRS-PROMES premises, has given the opportunity to test this type of filler in real conditions, but at a small scale, giving the necessary clues on both the heat transfer between oil and bricks and the appropriate commissioning procedures to follow for the allocation of the filler. Erecting and running the 2 MWh storage prototype at the POLYPHEM plant will give the required information to launch this technology approach in a thermal storage system for large capacity needs.

In a conclusion, the extensive use of concrete in thermal storage systems of large capacity is much closer thanks to the POLYPHEM project. The knowledge of the thermal behaviour of structured fillers in thermocline system is much well understood after this project, opening the window to an approach where thermal ratcheting is not an issue.

The achievements of this project in the storage system will not be that without the invaluable contributions of Arraela, a company for special concrete mixtures outside the building sector, Fraunhofer-ISE, CEA, and CIEMAT-PSA, research centers with a proven wide experience in modeling, design, and testing of innovative storage systems.