Black coloured rooftops have become the norm of many of the cities’ landscape with increasing number of houses switching over to sustainable, efficient and clean energy source – solar energy. Solar-thermal power systems that convert solar energy to heat or electricity are becoming ubiquitous. These systems typically consist of a flat plate collector that utilizes solar absorber coatings to get maximum conversion efficiency from incident solar radiation to heat. These collectors are coated black to enhance the absorptance- the effectiveness of absorbing radiant energy. Now, a group of researchers, led by Prof. Bikramjit Basu from the Material Research Centre at the Indian Institute of Science, Bangalore, and Dr. Harish C Barshilia from CSIR-National Aerospace Laboratories, has developed a new, colourful coating for flat plate collectors, thereby increasing its absorptance without compromising the aesthetic appearance of the roofs where they are installed.
This research is a part of a gigantic Indo-US solar energy research project called SERIIUS, launched to discover the revolutionary power of solar energy by photovoltaic (PV) and concentrating solar power (CSP) where IISc has tied up with the National Renewable Energy Laboratory (NREL), USA. “As a part of the SERIIUS project, our research group has been actively working on developing solar selective absorber coating for last few years. Prof. Kamanio Chattopadhyay from Materials Engineering and Interdisciplinary Centre for Energy Research, IISc is instrumental in successfully accomplishing this work”, says Atasi Dan, a PhD scholar at Materials Research Centre, IISc and has been working on the project since March, 2014.
“To meet the ever increasing energy demands of mankind, it is necessary to harness the renewable source of solar energy. One of the most efficient ways to harness solar energy to generate solar thermal power is through the photo-thermal route, wherein suitable concentrated collectors are coated with spectrally selective coating with desired properties”, says Dr. Barshilia, explaining the motivation behind the study. An ideal solar absorber needs to absorb much of the incident solar radiation and emit very less thermal infrared radiation. These properties depend largely on the material and the coating of the absorbers. Most of the spectrally selective coloured paints used as absorber coatings have the limitation of high emittance.
The multilayer absorber coating developed by the team is fabricated using tungsten, aluminium, Nitrogen and Oxygen. The team had to address the challenge of ensuring maximum absorptance while minimizing heat loss in the absorber, and also improve the environmental and thermal stability of the coating that can be affected by humidity, short-term thermal shock, etc. This coating exhibited coloured appearance, a long-term thermal stability of up to 500°C in air for 150 hours, and a durability of 25 years. The aluminium oxide layer provided thermal and chemical stability for the coating while tungsten prevented diffusion of iron and chromium atoms from the stainless steel substrate towards the coating.
The researchers have developed an improved solar absorber coating that has four layers - a metallic reflector, an absorber with high metallic content, a semi-transparent layer with low metallic content and an anti-reflection layer. “We have performed more than 100 experiments by varying a number of parameters to achieve desired performance of the multilayer stack. A slight change of these experimental parameters may result in drastic change in the selective performance of the coating”, explains Ms. Dan. “Our aim was also to find out the physics behind achieving such an outstanding selectivity and the colour appearance of the coating”, she adds. “The colour of the coating had to be carefully chosen since it is based on the part of the solar spectrum that is being reflected - attractive colours result in high reflectance and low absorption. We have modulated the number of layers in the coating to get a blue colour”, Prof. Basu said.
Concentrating Solar Power (CSP) systems – the technology that uses mirrors and lenses to concentrate sunlight onto a small area to produce heat and then subsequently electricity - are carbon neutral unlike fossil fuels, have lower operational cost and supply energy during high demands since they store energy. “The newly developed solar selective absorbers will improve the efficiency of these CSP systems. Further research to develop new solar selective functional materials will certainly address the global energy-related challenges”, concludes Prof. Basu, talking about how this innovation may help reduce the dependency on imported oil.