CFD analysis of a thermocline molten salt energy storage for CSP plants hybridized with an electric microwave heating system

The aim of this work is to study, through computational fluid dynamics (CFD) models, the charge transient of a thermocline sensible thermal storage for concentrated solar power plants (CSP). A thermocline storage is characterized by the stratification of the temperature inside the tank, in which the upper part is at a higher temperature than the lower part. This solution allows to avoid the classic double tank configuration used for CSP applications, in which the two tanks contain the fluid at two different constant temperatures. The thermocline storage allows, therefore, to supply the cold fluid to the solar field and the hot fluid to the power plant using a single tank. The choice to study this type of thermal storage derives from the continuation of several studies carried out in previous years by ENEA. The model developed by ENEA consists of a tank with a central channel, at the ends of which there are two heat exchangers, used for charging and discharging the storage. Inside the heat exchangers pipes the heat transfer fluids flow. In this work, it was decided to develop a model in which the charging phase is carried out thanks to an additional device, which uses microwaves (MW) to irradiate and heat up the storage medium (molten salt). Microwaves represent a very efficient method for heating a fluid starting from electrical energy. This aspect allows for a hybridization between CSP plants and photovoltaic (PV) plants. In fact, the idea is precisely to recover the surplus energy (in the central hours of the day) of the PV plants and store it to be used later when the demand increases (generally, during the evening). In this work, after having provided the reader with the theoretical basis regarding CSP plants, thermal storage and microwave heating, the various steps in the construction of the model are explained in detail. First of all, the model developed by ENEA is briefly explained, which represents the basis for the subsequent models. Then, the geometry, the physics of the simulation, the boundary conditions and the mesh generation are presented. The models were built using the CAD Solidworks software, while the fluid dynamic simulations were performed using the Simcenter STAR-CCM+ software. The results are divided into three parts. In the first part, simulations of the charge transient were performed for two different configurations, A and B, which differ in the way in which the microwaves are discharged into the fluid. These simulations led to inadequate results regarding the temperature stratification. Since the poor stratification was seen to be caused by the excessive flow rate circulating in the channel, it was decided to make some changes to improve the performance. The first is the introduction of a “fan interface” that simulates the operation of an impeller inside the channel, to regulate the flow rate. This solution also proved to be inefficient. The second solution undertaken is the introduction of microchannels in the central channel, in order to create concentrated pressure drops, hindering the passage of the fluid. This solution has led to satisfactory results, thanks to a better stratification of the temperature and an adequate temperature difference between the upper and lower part of the tank.