Graphite foams may be the key to capturing and storing thermal energy from solar farms, say researchers from the University of Pretoria.

In the context of climate change and renewable energy, mentions of carbon generally set alarm bells ringing. But a new composite material may change that – especially for concentrated solar power (CSP) systems. CSP is really all about capturing, storing and discharging thermal energy. They work by concentrating the sun’s light onto a single point and using this to heat a storage material. This forms a thermal energy reservoir that can be ‘tapped into’ at night or during bad weather. So-called phase change materials (PCM) are generally used to form this reservoir, storing latent heat energy as they transition from solid to liquid, and releasing it as it cools. But a group of South African researchers have demonstrated that a composite based on graphite foam may be suitable for both collecting and storing thermal energy.

This work, to be published in an upcoming issue of Carbon [DOI: 10.1016/j.carbon.2015.11.071], focused on two areas – 1) the design of a low-cost thermal receiver and storage material and 2) the development of a solar concentrator test platform to characterise these materials. Graphite was chosen for its excellent thermal conductivity and a very low thermal expansion coefficient. Foams of varying composition and density were prepared using a low-cost pitch material. They were then impregnated with a PCM, and directly heated using the solar concentrator to test their thermal transfer properties.

The test platform itself used sunlight, rather than electrical lighting systems, to accurately reflect the operating environment of the materials. Light entered it through a Fresnel lens concentrator, which was shown to produce concentration ratios in excess of 1000 suns. The foam with the lowest density showed the best overall performance – it absorbed the largest volume of phase change material, and demonstrated dramatically improved thermal behaviour compared to the pure PCM.

The geometry of the foam was also found to be a factor – a decrease in thickness resulted in a decreased efficiency in thermal transfer. However, the overall efficiency of the composite foam was lower than that achievable with conventional flat plate collectors. The team believe that this is due to the reflective nature of graphite, and the next stage of this work will be to incorporate carbon black into the structure to improve light absorption.

H. Badenhorst, N. Fox, A. Mutalib “The use of graphite foams for simultaneous collection and storage of concentrated solar energy”, Carbon, 99 (2016) 17 – 25, DOI: 10.1016/j.carbon.2015.11.071.