Analysis and design of devices for medium temperature solar thermal energy conversion
Creators: name: Francesconi, Marco
Publisher: name: Università di Pisa
Year: 2017
Subjects: solar energy
energy conversion

description: The increase of energy consumption as well as the growth of the prices of fossil fuels and the need to reduce pollutant emissions strongly stimulate the development of renewable sources and waste heat recovery. Large scale power generation from renewable energy sources or from waste heat recovery inherently relies on a large number of small size distributed energy systems. At the moment, the components which can be employed in those applications still need a large effort to increase their performances, their efficiencies and to reduce their costs. The technical aspects, involved in the design of some energy components, were considered in this thesis. As a matter of fact, the main aim of this research was the analysis of the effects that geometric parameters or construction characteristics as well as operating conditions introduce in the performances of a number of devices used in solar thermodynamic plants based on ORC technology. Typical components of this application such as pumps, condensers and heat exchangers have already been developed and much of work has been already done, while for other devices such as solar collectors and volumetric expanders it is still necessary to further study their behaviour and to improve their performance. For these reasons two essential components of a micro generation solar plant were analysed; a compound parabolic collector (CPC) with evacuated pipe, to investigate the device for the collection of solar energy, and a steam expander, obtained from a modified Wankel engine, for power production. The research was based on numerical techniques and CFD simulations to analyze the physical phenomena that occur during working conditions of these energy devices. The solar collector with evacuated pipe was first analyzed with a numerical model to investigate its transient behaviour, and then, a CFD model was developed to understand the heat transfer mechanism inside the enclosure when the evacuated pipe is not present. In this context, several CFD studies were focused to investigate the thermal influence due to the housing of several concentrators in a single panel, considering also the presence of an external barrier and the truncation of the concentrators. In the end, a series of low cost technical solutions were shown with the aim to increase the efficiency of the collectors. The research on the collector was also completed with an experimental apparatus, to evaluate the performances of a CPC concentrator with an evacuated pipe and validate the numerical simulations. The studies about the expander were done to assess the influence of the valves on the performances, combining a CFD analysis with experimental tests performed using compressed air as working fluid. The research continued with the development of a numeric model of the expander, that was calibrated with the results of the experimental tests, with the aim to evaluate friction losses as well as other ancillary losses. Those components can also be used separately in other applications; as a matter of fact the CPC could be employed to heat a fluid for industrial or domestic purposes or it could be used for desalination or for water chemical treatment, while the Wankel expander can be employed in other ORC systems using other renewable sources, such as biomass and geothermal fluids, or waste heat recovery.