Improving solar thermal receiver performance via spectral and directional selectivity

Abstract

Adoption of renewable energy technologies has accelerated rapidly in recent years due to growing energy demand and concerns over climate change. Among renewable energy sources, solar energy conversion systems are particularly promising due to the abundance of solar energy reaching Earth. Despite its abundance, the solar resource is dilute, so solar energy must be collected efficiently in order for it to meet an appreciable portion of demand. The efficiency of solar energy conversion systems can be improved by taking advantage of the spectral and directional properties of sunlight. Spectral properties refer to the distribution of wavelengths associated with solar photons, with most solar energy arriving as photons with wavelengths from 300 - 2500 nm. Spectral selectivity entails absorbing these solar photons while suppressing losses associated with infrared photons at longer wavelengths. Directional properties refer to the incident vector of sunlight, which spans a small solid angle due to the sun's distance from Earth. Directional selectivity entails absorbing radiation from the direction of the sun while suppressing losses to other directions. This thesis explores the theoretical limits of performance enhancement via spectral and directional selectivity, as well as practical devices designed to take advantage of those effects. Limits to spectral selectivity are investigated by applying the Kramers-Kronig relations to spectrally selective absorbers. Limits to directional selectivity are studied via geometrical limits, and are compared to the limits of concentrating sunlight. Two silica aerogel based solar receivers are presented as practical devices utilizing spectral selectivity. A solar thermal aerogel receiver is predicted to achieve similar performance to state of the art vacuum tube receivers, and a hybrid aerogel receiver that collects electricity from photovoltaic cells and heat is shown to potentially achieve higher efficiency than photovoltaics or a thermal receiver alone. A macroscale reflective cavity is demonstrated as a method for achieving directional selectivity in solar absorbers, and can be used to improve the performance of both solar thermal systems and photovoltaic cells.