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.
