Magnetite thermal energy storage for CSP plants

Abstract

The Department of Mineral Resources and Energy estimates that the industrial sector is the largest consumer of energy in South Africa. Approximately 66% of energy end-use in industry is for heat generation during manufacturing. South African industry has been previously developed in the context of low energy prices for coal and electricity. This has resulted in a wide range of industrial processes that are inefficient and carbon intensive. With rising fuel prices, the prospect of fossil fuel depletion, and the continuous global effort to minimise environmental impact, it is necessary to develop alternate energy sources for heat generation. A significant portion of thermal energy can be generated using solar technology. However, solar energy supply is variable in nature and does not always match demand. It is therefore necessary to integrate thermal energy storage systems into solar plants to ensure availability. Thermal energy can be stored in three main ways namely, sensible, latent and thermochemical heat form. Magnetite is a material that undergoes an antiferromagnetic phase change at ~570 °C. This causes a reversible spike in the heat capacity of the material. This is highly advantageous for thermal energy storage applications and allows it to store more heat than other typical sensible storage media. Magnetite is widely available in South Africa and is often a waste product of other production processes. A lab-scale prototype was developed to analyse the thermal storage characteristics of magnetite in an open (non-pressurised) system with air as the working fluid. The magnetite was heated using a gas burner in a packed bed reactor and discharged using ambient air. Magnetite has the ability to store heat up to 1000 oC which makes it suitable for CSP plants. The experimental results will be used to validate a CFD model to inform future CSP plant designs and for industrial process heating applications.