CFD Analysis of a Thermocline Storage System with Phase Change Material for CSP Applications

In the context of concentrating solar power (CSP) systems, the thermocline thermal storage system is a potentially cost-effective alternative to the more common storage system based on two tanks, one hot and one cold. As part of the European ORC-Plus project, ENEA (Italian institute for new technologies, energy and sustainable development) has proposed an innovative molten salt storage system based on thermocline technology, of which a 1:100 scale prototype has been built at the Solar Collector Test Facility (PCS) at Casaccia. The aim of this thesis is to numerically evaluate the potential to insert phase change material (PCM) into the tank in terms of thermal performance. First, a brief introduction to the general topic of decarbonisation of the energy sector and the growing importance of low-carbon technologies with a focus on CSP technology in current and future power generation developments is given to readers. Afterwards, the research activities previously carried out are provided: the idea behind the design of the prototype, the experimental campaign and the development of a validated CFD model. Based on this solid basis, the previously CFD model was modified to include phase change inserts. To maintain an acceptable computational cost, the domain of the CFD model does not include the tube walls and the PCM, which are replaced by a Robin-type boundary condition governed by a lumped parameter model. To this end, it is also determined in which region of the storage tank it is convenient to insert the PCMs, which phase change material is the most suitable, depending on the melting temperature, and which geometry to adopt for the inserts containing the phase change material. In conclusion, the CFD model coupled with the lumped parameter model is used to simulate a charging transient and the results are compared with those of the storage system without the PCM inserts. The results show that the phase change material allows the temperature of the salts in the upper part of the tank to be stabilised, also leading to a clearer separation between the hot salts and the rest of the thermocline.