The material converts over 87% of light falling on it into usable heat energy.

Heat or thermal energy is a fundamental form of energy essential for life. Humans have been harnessing its power for everything from heating our homes and cooking food to powering our industries and generating electricity. Most of the heat we use comes from the sun. Plants and trees absorb and store this energy, and over a million years later turn into coal. For most of human history, we have been burning these fossilised plants, for all our thermal energy requirements. This in turn has had adverse effects on our environment and biodiversity, leading to pollution and increasing greenhouse gas emissions. More recently, however, with the advent of renewable energy sources, humans have learned to harness this light directly from the sun. Yet, we have been able to capture only a miniscule percentage of the total heat energy the sun radiates.

Now, a new material designed at the Indian Institute of Bombay (IIT Bombay) has made a breakthrough in the amount of heat that can be absorbed and stored. Called nanostructured hard- carbon florets or NCF in short, the material has shown an unprecedented solar-thermal conversion efficiency of over 87%. It absorbs more than 97% of the ultraviolet, visible and infrared components of sunlight and converts this efficiently into thermal energy. The heat thus produced can be effectively transferred to either air or water for practical applications. The study demonstrated that NCFs heat the air from room temperature to 60 degree Celsius and can thereby provide smoke-free space-heating solutions. “This is particularly relevant for heating spaces located in cold climatic conditions that receive abundant sunshine such as Leh and Ladakh,” says Prof. C. Subramaniam from Department of Chemistry, IIT Bombay and an author of this study.

Solar thermal converters, like those present in solar water heaters, are already in use in many places around the world. According to the Ministry of New and Renewable Energy, India an estimated 40 million or 2.5% of households in India already use solar water heaters. But 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 (Cr) or nickel (Ni) films. While anodised chromium is a heavy metal and toxic to the environment, both Cr and Ni-films exhibit solar-thermal conversion efficiencies ranging anywhere between 60-70%. In fact, the best commercial ones in the market operate at 70% solar-thermal conversion efficiencies,” remarks Dr. Ananya Sah, the lead author of the study that developed the NCF. NCFs on the other hand, made primarily of carbon, are inexpensive to produce, environment-friendly and easy to use. Vacuum jackets, which are required for other solar heat absorbers to work and are challenging to maintain, are also not required with NCF coatings.

For efficient conversion of solar thermal energy into usable heat, a material needs to have two important but contrasting characteristics. Firstly, the ability to successfully convert a large portion of the incoming packets of light or photons into heat – a process called photon thermalization. And secondly, the ability to retain that heat without loss due to thermal conductivity and radiation. When incoming photons strike a material they cause the atoms of the material to oscillate. These oscillations, called phonons, then travel through the material spreading the heat throughout the material. Materials with higher phonon thermal conductivity spread the heat faster, eventually losing a majority of the heat falling on the material. In short, then, a good heat absorber must have high photon thermalization and low phonon thermal conductivity. NCFs tick both these boxes.

The structure of the nanoparticles of NCF resembles marigolds in appearance, composed of interconnected tiny cones of carbon. The structure is unique in that it allows for both strong phonon activation when photons strike it 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 Prof. Subramaniam.

Apart from its remarkable efficiency in converting sunlight into heat, another advantage of NCFs lies in their processability. Using a technique called chemical vapour deposition, carbon is deposited onto a substrate of amorphous dendritic fibrous nanosilica (DFNS) to form the NCF. The materials are readily available and the technique is easily scalable, making large-scale manufacturing commercially inexpensive. Once manufactured, NCFs can be spray-painted onto almost any surface, similar to powder coating a surface, reducing the cost of application and maintenance as well. “We have shown that the coating is possible over surfaces such as paper, elastomer, metal and terracotta clay,” remarks Prof. Subramaniam.

Beyond solar-thermal conversion, NCFs offer other potential applications, such as space-heating to heat rooms and other smaller spaces. Remarkably, the researchers showed that hollow copper tubes coated with NCFs can heat air flown through them to over 72 degrees Celcius. They have also demonstrated its ability to convert water into vapour with an efficiency of 186%, the highest ever recorded. Solar vapour conversion is a method used to convert water into vapour for purification. 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. Subramaniam.

The groundbreaking study is supported by the Swarna Jayanti Fellowship of the Department of Science and Technology, India and demonstrates a green solution to the energy crises gripping the world and can aid in the global transition to sustainable energy sources. 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. These efforts have been receiving excellent support and mentoring from the IIT Bombay alumni group through their Translational Research Accelerator (TRA) program. The team has also won several awards at the carbon-zero challenge held at IIT Madras and the Vishwakarma Prize for Engineering, 2021 at IIT Gandhinagar, for their invention. “I strongly believe that NCF has the potential to change the solar-thermal energy market in India and lead the way in decarbonization,” concludes Prof. Subramaniam.