Advanced surface and volumetric receivers to convert concentrated solar radiation

This thesis is the results of the work conducted during the three years of Ph.D. at the Department of Industrial Engineering of the University of Padova. The conversion of solar energy into heat in the medium-temperature range (between 80 °C and 250 °C) has recently encountered a renewed interest in heating and cooling applications of industrial, commercial, residential and service sectors. Concentrating solar thermal collectors at medium temperature are suitable for many commercial and industrial applications, such as industrial process heat, solar cooling and desalination of the seawater. It is expected that in the future, a significant technological development can be achieved for these collectors, provided that the conversion of solar energy becomes more efficient and cost-effective. The proper design of the receiver, which is considered the heart of any concentrating collector, is essential to the future improvement in the conversion efficiency of this technology. In this context, the present thesis investigates the application of two innovative concepts of receivers in a prototype of an asymmetrical parabolic trough concentrator installed in the Solar Energy Conversion Lab of the Industrial Engineering Department, at the University of Padova. In Chapter 1, a study on different estimation procedures for the assessment of the direct normal irradiance, which is the solar resource utilized by solar concentrators, is presented. The study includes an indirect evaluation from measurements of global and diffuse horizontal irradiances and the use of semi-physical/empirical models. A detailed analysis of the instrumentation and of the measuring technique as well as the expression of the experimental uncertainty is provided. In Chapter 2, the optical performance of the asymmetrical parabolic trough is experimentally characterized. As a result, a statistical ray-tracing model of the concentrator for optical performance analysis in different working conditions is validated and used to optimize the design of the proposed receivers. In Chapter 3, an innovative flat aluminium absorber manufactured with the bar-and-plate technology, including an internal turbulator, is tested in the asymmetrical parabolic trough collector under single-phase and two-phase flow regimes. A numerical model to predict its performance has been developed and validated against the experimental data. In Chapter 4, this model is used to evaluate the performance of a small solar-powered ORC system by coupling the aforementioned concentrating solar system with direct vaporization of a low-GWP halogenated fluid or by using an intermediate solar circuit to heat pressurized water and evaporate the same organic working fluid in a separate heat exchanger. Finally, in Chapter 5 a new direct absorption receiver is proposed to investigate the capability of a suspension of single-wall carbon nanohorns in distilled water to absorb concentrated sunlight. The volumetric receiver has been designed through the development of a three-dimensional computational fluid dynamics model for its installation in the focus region of the asymmetrical parabolic trough. The capability of the nanofluid in collecting solar radiation when exposed to concentrated and non-concentrated solar flux are experimentally investigated thanks to the cooperation with National Council of the Research (CNR), that provided the aqueous solution. The nanofluid was tested in several conditions, with and without circulation, to investigate its stability with time