A review of the application of carbon materials in solar thermal energy storage

Graphitic materials can potentially mitigate the issue of low thermal conductivity in phase change materials (PCM) when used in solar thermal energy storage. However, carbon can form an exceedingly wide variety of allotropes which are difficult to distinguish. This study has examined an extensive range of energy storage carbon composites including: synthetic and natural graphite, graphitic fibres, graphitic foams, expanded graphite, graphite nano-platelets, graphene, carbon nanotubes and amorphous carbons. The thermal energy storage media covered include conventional low temperature materials such as paraffins, alcohols, fatty acids and numerous others, as well as high temperature salts. In addition, a wide range of both steady and unsteady state thermal conductivity measurement techniques are represented. Based on the collated results it is evident that particulate additives are limited, potentially due to contact resistance, to improving PCM thermal conductivities by a factor of less than ten. On the other hand, matrix materials like compressed expanded graphite and graphitic foams are capable of achieving enhancements in excess of 10,000%. Compressed expanded graphite composites appear to be the most economically favourable option. Nanomaterials do not perform as anticipated and indications are that costs must drop by more than three orders of magnitude before they become attractive. It has been demonstrated that it may be possible to predict composite thermal conductivity for all carbon materials using a single model, if an additional structural parameter could be measured.