Solar Thermal Characterisation of Micro-Patterned High Temperature Selective Surfaces applied to a Thermionic Device

Abstract

Solar thermal energy converters have been widely studied for many years and are suitable for both small and large-scale deployment. Solar thermionic energy converters are a much more direct and potentially more efficient way of generating electricity from solar energy. An ideal absorber aims to maximise solar absorption while minimising infra-red emissivity in order to achieve the highest possible temperature. Preventing conduction and convection heat loss of the solar absorber is also a key to obtain a higher temperature. In this thesis, molybdenum samples were fabricated using laser micromachining. Finite Difference Time Domain modelling of the microstructure has shown the absorptivity increased over the visible and near-infrared region. The microstructures were characterised using an integrating sphere and an infrared microscope. In-air solar simulator-heated temperature results for the molybdenum samples with different microstructures showed good agreement with the absorptivity measurements. Two vacuum devices were developed to reduce the convection heat loss with a mount designed to minimise conduction loss. The molybdenum samples were heated in the vacuum devices with a solar simulator and focused sunlight and >400℃ has been achieved. COMSOL Multiphysics has been used to model the heat transfer between the molybdenum sample and the holder. Based on the simulation results, a prototype of fully sealed thermionic emission convertor was proposed which has a new design of the sample and the holder. Overall, this work shows that low-cost laser micro-patterned molybdenum samples have good potential to obtain high temperature for thermionic emission applications.

Date of Award	29 Sept 2020
Original language	English
Awarding Institution	University of Bristol
Supervisor	Martin J Cryan (Supervisor)