With fossil reserves fast depleting, new technologies promising more efficient use of renewable energy sources, especially the sun, are flooding the market. Researchers at Sri Venkateswara College of Engineering and Technology in Chittoor, Andhra Pradesh, have devised beads that can store solar energy and be used to heat water. R Meenakshi Reddy, the lead author, speaks to Ankur Paliwal about his study published in the International Journal of Renewable Energy Technology on December 2. Edited excerpts:
Describe your work
We have developed beads containing cheap phase change materials. These Phase Change Materials (PCMs) store energy and release it by changing from solid to liquid and vice-versa. A large number of organic and inorganic PCMs are available in the market. The advantage of the beads is that they have high heat storage capacity in small size.
What are these beads made of?
To make the beads paraffin wax and stearic acid are filled in separate spherical capsules made of high density polyethylene. They are safe, reliable, cheap and non-corrosive. They show little volume changes on melting. The same materials have been used by other researchers in the past and the size of the beads was between 100mm to 25mm. But in our research we have optimised the size to 38 mm because it is difficult to fill paraffin wax and stearic acid in beads with size of 25 mm.
How does the technology work?
The beads are uniformly packed in layers in the form of a bed in an insulated thermal energy storage tank full of water. During daytime, the water gets heated, and it transfers heat to the beads through convection. The beads start melting. The melting point of beads is between 57-61°C and can store heat up to 1,000KJ. When the already available hot water has been utilised, fresh water gets filled in the tank. Because of the temperature difference, the beads begin to solidify and heat water.
How are the beads better than conventional methods?
The limitation with the conventional system is cost and storage of heat only in sensible form. The heat that causes change in temperature of an object is called sensible heat. So, if the capacity of the tank is 50 litres only that much hot water can be utilised. In our set up we store heat in sensible as well as latent form. Heat that causes change in the phase of a substance is called latent heat. So almost double the amount of hot water can be utilised. Once charged the beads can hold heat for 48 hours. The system is cheaper as stearic acid and paraffin wax are available at Rs 50 and Rs 75 per kg respectively. The beads can be used up to 10 years.
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