Study and optimisation of ultra-high temperature thermal insulation structures with application to thermal energy storage

Abstract

Today, high capacity energy storage remains one of the major obstacles to an economical full-scale development of renewable energy technologies, without which hundred-percent decarbonised electricity grid, power and heating integrated networks will not be achievable. Unfortunately, it would not be economically viable to rely on current grid-connected storage technologies to provide enough capacity to compensate for the inherent intermittency of renewables, since most suffer from high upfront costs, low energy densities, low e ciencies, long-term degradation or inconvenient deployment location requirements. Ultra-high temperatures (>1600°C) would unlock greater energy densities and allow heat engine extraction cycles to operate at higher efficiencies, consequently improving the overall round-trip energy effciency - see, for example, an Ultra-High Temperature Thermal Storage (UHTS) [1, 2]. However with higher operating temperatures come additional energy losses which prove increasingly di cult to prevent as radiative transfers become predominant, and efficient ultrahigh temperature thermal insulators are needed.

Recently, high-temperature thermal insulation has been the focus of numerous studies which have led to the development of promising technologies including aerogels, nano fibres and ceramic fibre blankets. Of all these, ceramic brief and nano fibres have the highest maximum operating temperature of up to 1400°C with low-pressure thermal conductivities of order 0.1 W/m/K at such temperatures. However, while very efficient, such technologies would restrict the operating temperatures and storage duration of UHTS type thermal stores, and do not have sufficient structural strength. To remedy this, this doctoral project investigates the potential of insulating structures based on evacuated honeycomb geometries to both improve their thermal performance at ultra-high temperatures and raise their maximum operating temperatures to UHTS requirements whilst providing the required structural support.

First, a theory based on fundamental energy transfer equations and its numerical implementation are built to model combined conductive-radiative energy transfers in three-dimensional complex rectangular multi-media structures with obstacles and internal boundaries of any kind. Two or more media may be present, with multiple interfaces between them, and may be opaque or semi-transparent to thermal radiation so that radiative and conductive transfers may happen through participating (potentially rare ed) gaseous and solid media alike. Experimental results are then presented to validate the model where a vacuum chamber is built, a metal honeycomb heated up to 600°C, and the measured energy flows compared to numerical predictions. In a second stage, the model is applied to the design and optimisation of uniform honeycomb based ultra-high temperature thermal insulation. First, a thermal optimisation is presented during which a parametric analysis of thermal transfers in uniform honeycombs is conducted which correlates optimal geometries and thermal conductivities to a new dimensionless parameter Nrc which is shown to be an appropriate tool for the thermal characterisation of such structures. All correlations are then validated by numerical results, following which numerical optimisation experiments show that honeycomb-based insulators can retain thermal conductivities as low as 0.01 W/m/K at temperatures of 1400°C, thus largely outperforming current technologies. The case of non-uniform honeycombs is subsequently addressed, for which numerical results are analysed, and an analytical condition for optimal performance is derived and numerically validated. Mechanical loads are then introduced into the model and their effects on thermal performance are analysed, which is necessary in a UHTS context where insulators must bear a load whilst preserving optimal thermal performance. Finally, the results of the thermo-mechanical optimisation are applied to the thermal transient modelling of a UHTS type thermal store which relies on optimal honeycomb-based insulators. Its thermal storage efficiency is characterised for short and long term (seasonal) storage operation, highlighting its operating exibility and making it an essential tool for achieving fully decarbonised electricity, heat and gas networks.