Scientists at the Indian Institute of Technology, Bombay, (IIT Bombay) have developed a new material that can convert sunlight into heat energy with unprecedented efficiency.

The material, called nanostructured hard-carbon florets (NCFs), is made by depositing carbon onto a substrate of amorphous dendritic fibrous nanosilica (DFNS), using a technique called chemical vapour deposition. The resulting material absorbs more than 97 per cent of the ultraviolet, visible and infrared light, and converts this them into thermal energy. The heat produced can be transferred to either air or water for practical applications.

The researchers have demonstrated that hollow copper tubes coated with NCFs can heat the air flown through them to over 346K. They have also demonstrated its ability to convert water into vapour with an efficiency of 186 per cent, the highest ever recorded. NCFs have outperformed all other competition when it comes to efficient conversion of the sun’s energy. “One metre square of NCF coatings converts 5 litres of water in an hour, that is at least 5 times better than commercial solar stills,” says Prof C Subramaniam.

What’s unique?

To efficiently harness solar thermal energy and convert it to usable heat, a material should possess two essential yet somewhat conflicting qualities. First, it should be able to effectively transform a significant portion of incoming light particles, known as photons, into heat in a process called photon thermalisation. Second, it must retain this heat without losing it through thermal conductivity and radiation.

When incoming photons strike a material, they set the material’s atoms into motion, creating oscillations called phonons, which then propagate through the material, spreading the heat. Materials with high phonon thermal conductivity transmit heat quickly but also tend to lose a significant portion of the absorbed heat. An ideal heat absorber should exhibit both high photon thermalisation and low phonon thermal conductivity. This is precisely what the NCFs offer.

The nanoparticle structure of NCFs resemble marigold flowers consisting of interconnected small carbon cones. This unique structure enables two crucial features: strong phonon activation when photons hit the material and low phonon thermal conductivity.

“NCF has ordered structure in short-range (shorter lengths) and disordered structure in long-range (longer distances). So, when light energy is absorbed by NCF, this short-range ordering causes strong phonon activation (oscillations in the ordered lattice). Anything that has strong phonons should also help in conducting the energy away. However, in NCF, the long-range disorder acts to scatter these phonon-waves. Therefore, the phonon thermal conductivity is low,” explains Subramaniam.

The NCF is highly efficient in converting sunlight into heat energy. The materials used to make NCFs are readily available, and the manufacturing technique easily scalable, making large-scale manufacturing of these florets commercially inexpensive. Once manufactured, NCFs can be spray-painted onto almost any surface, reducing the cost of application and maintenance as well.

Besting the best

According to the Ministry of New and Renewable Energy, an estimated 40 million or 2.5 per cent of households in India already use solar water heaters. However, commonly available solar heat absorbers are often expensive, bulky and potentially harmful to the environment. “Conventional coatings and materials for solar-thermal conversion are based on chromium or nickel films. While anodised chromium is a heavy metal and toxic to the environment, both Cr and Ni-films exhibit solar-thermal conversion efficiencies ranging between 60-70 per cent. In fact, the best commercial ones in the market operate at 70 per cent solar-thermal conversion efficiencies,” says Dr Ananya Sah, the lead author of the study that developed the NCF. The NCFs on the other hand, made primarily of carbon, are inexpensive to produce, environment-friendly and easy to use.

The team has already begun the process of commercialising the product by setting up a company at the Society for Innovation and Entrepreneurship (SINE) at IIT Bombay. The company will focus on scaling up the manufacturing of NCFs and developing NCF-based devices required for water heating and space-heating.