Modelling, design and analysis of innovative thermal energy storage systems using PCM for industrial processes, heat and power generation

The topic of this PhD thesis is framed on the study and the analysis of thermal energy storage (TES) systems based on phase change materials (PCM) to be used as a back-up for intermediate temperature applications (up to 250 °C). The work is divided in two part: the first part presents the development of numerical models of the latent heat thermal energy storage (LHTES) devices. Different models are developed by means of a 2D and 3D numerical simulation codes specifically implemented in the COMSOL Multiphysics environment. Design of LHTES requires knowledge of the heat transfer process within them, as well as the phase change behaviour of the PCM used. For simulate the PCM, two approaches are used: the first approach takes only into account heat transfer by conduction during the entire process, also when the PCM is in the liquid phase. In the second, the energy equation considering both heat conduction and natural convection is solved to predict the behaviour of the PCM. Different PCM materials, geometries and configurations of the storage device are considered and tested in order to evaluate the effectiveness of the adopted numerical codes under different working conditions. Finally, the models are validated using experimental data obtained from tests carried out on a double tube heat exchanger with fins and with Rubitherm® RT35 paraffin as PCM. The tests are conducted in the laboratories of the University of Lleida (Spain) by the research group GREA Innovació Concurrent. The second part of this work concerns the design and implementation of a test rig, specifically built for experimental investigation of heat storage devices in the laboratory for TES technologies of the University of Cagliari. An accurate study and selection of both the test rig layout and all the needed equipment is carried out to perform experimental analysis. The test bench is composed of an electrical heater, which heats the HTF up to the operating temperature, an air cooler, which simulate the thermal demand during the discharge phase, a HTF circulating pump, two test sections for thermal energy storage systems, and a preliminary TES device which consists in a shell and tube heat exchanger, where the HTF flows in the tubes while the PCM is placed in the shell side. At this stage, the thermal energy storage system, the measuring devices and the data acquisition system are under implementation.