Carbon materials for solar thermal energy

Abstract

Paper presented to the 3rd Southern African Solar Energy Conference, South Africa, 11-13 May, 2015.

Cost-effective capture and storage of solar energy still remains a challenge facing broad-based implementation of this renewable technology. Work at the South African Research Chairs Initiatives (SARChI), Chair in Carbon Materials and Technology at the University of Pretoria is aimed at developing materials from cheap, local carbon sources for solar thermal applications. The focus is split on developing composites for both solar energy capture and thermal energy storage. Preliminary investigations are based on systems designed to provide heating needs like hot water through solar energy. Two different approaches to the capture of solar energy were explored. Firstly the use of carbon black and expanded graphite pressed discs and secondly the production of mesophase based graphitic foams. The carbon black discs demonstrated the ability to reach a peak collector temperature of around 70 °C whilst providing approximately 4 kW.hr per m2 per day. The foam with the best capture had an efficiency calculated at around 95%. This is based on measured incoming radiation converted into sensible heating of the water. Phase change materials present an efficient and innovative solution to the problem of thermal energy storage. These materials suffer from a major drawback due to their low thermal conductivity. Two alternatives were explored for enhancing this property. Firstly the direct incorporation of phase change material into expanded graphite was tested. A variety of loadings were explored. It was found that at low concentrations of 6 mass % the time required for the sample to undergo the phase transition could be reduced by 70% for heating and 85% for cooling. Beyond this point the decrease in phase transition time diminishes. Ongoing work is also being done to explore the use of highly conductive sheets manufactured from roller-pressed expanded graphite. Unfortunately these sheets suffer from structural integrity issues which are being optimized.