Optimal design of thermal energy storage systems for renewable driven heat and power energy systems

Solar Thermal Brayton Cycle (STBC) is the technology chosen for this analysis. The plant uses a parabolic dish to collect and concentrate the solar beam onto a small area, where is placed a receiver. This thermal energy promotes the heating of the working fluid, which moves the turbine of power block, producing electicity. Furthermore the system operates at low pressure, thanks to the regenerative cycle involved; a recuperator is placed after the compressor in order to pre-heat the incoming air using the exhaust gas from the turbine. STBC technology is very promosing among the CSP, thanks to the major peak efficiency, due to the higher operating temperature reachable and concentration ratio designed. Particularly, the small-scale plant (1-20kW) has been analyzing, thought to provide electricity for the small villages and isolated communities, which aren’t often linked to the main grid as the transmission line can’t cross vast stretches of land. For all this reasons, it has been nalysed in this work. In the first part, a model of the STBC plant has been built, considering the main governing equations valid for all components: recuperator, receiver, dish and turbocharger group. After the validation, it has been envolved for quasi-steady state analysis, simulating the plant function over the time, following an actual irradiance profile. To do that, some control strategies have been proposed, in order to keep the operating temperature near the design value and to avoide the turbine overheating when the overload phase occurs. Among them, the TES introduction has shown great results as control strategy: it allows to keep constant the power generation as well as the turbine inlet temperature and leads to the best exploiting of surplus solar energy without any waste. This means increasing the STBC plant efficiency during the peack hours. In the second part, the integration of the TES in the STBC system has been studided more deeply, underling the advantages in term of working hours, plant efficiency and economical convenience. Since the literature was quite lacking about this investigation, a preliminary study has been conducted, to find the best TES typology among all available possibilities. In this view, a passive TES with sensible material since it is the chepest technology. On the other hand it should be placed on the ground, distant from the other plant component which are supported by the tracking system. This displacement leads a greater heat losses, but is necessary owing to the enourmous weight predicted. Anyway a plant layout has been conceptually designed in order to allow the air flow in two differet direction, during the charging or discharging phase. Particularly a packed-bed configuration has been selected and its more detailed model has been built. At the end the funcioning of a STBC plant with the TES inegration has been simulated, showing and discussioning the results.