A cheap material that can store heat energy collected from the sun during the day that can be released slowly over night has been developed by Meenakshi Reddy, Sri Venkateswara College of Engineering and Technology, Chittoor, India. The material based on paraffin wax and stearic acid could help keep homes warm in sunny parts of the world that get very cold at night without burning wood or fossil fuels.

Phase change materials (PCM) can store a large amount of heat in the form of latent heat in a small volume. Heated in the sun, the spherical capsules, 38 mm in diameter, containing of a blend of paraffin wax (mp = 37 °C) and stearic acid, become entirely liquid. As they solidify they slowly release the stored heat.

Stearic acid is a lot cheaper on the Indian market than paraffin and more readily available. The team found out that without reducing the overall heating efficiency of the capsules the proportion of paraffin wax can be lowered.

• Solar energy based thermal energy storage system using phase change materials,
  R. Meenakshi Reddy, N. Nallusamy, T. Hariprasad, K. Hemachandra Reddy, G. Ramachandra Reddy,
  Int. J. Renew. Energy Technol. 2012, 3 (1). 11–23.
  DOI: 10.1504/IJRET.2012.043905