Capturing sunlight in a bottle and using it when needed

Thanks to the initiatives of UNESCO and the United Nations Framework Convention on Climate Change (UNFCCC), most countries across the world have adopted ‘green’ solutions that reduce environmental carbon dioxide. The use of solar panels to capture sunlight and turn it into electricity has been a particularly successful strategy. Many countries across the world have seen a large-scale expansion in the installation of solar panels.

In India, a typical home solar panel installation connected to the grid today costs about Rs. 30,000 per kW after subsidies under the ‘Surya Ghar Mufti Bijili Yojana’. A 2-kW system can cover the basic electricity needs of a typical household. And there are crores of such panels across the country. India’s rooftop solar capacity is now 23 gigawatt (GW). 

In all, India’s land-based solar panels generate about 150 GW of power. Ingenious solar panel installations, such as those over irrigation canals, and floating solar panels in lakes and dam-based water reservoirs are expected to produce an additional 102 GW in the near future. Recently, India has also become ‘greener’ by installing wind turbines and introducing electric trains, trucks, buses, and cars.

MOST molecules as heat stores

An aerial view of Kandulavaripalle panchayat in Chandragiri mandal of Tirupati district, where almost every household has a rooftop solar connection.
| Photo Credit:
File photo

Solar panels convert energy from the sun into electricity when photons excite electrons in silicon crystals, causing them to be freed and to flow from a positively charged layer in the solar cell towards a negatively charged layer, creating an electric current. Inverters are used to convert this flow of electrons into a usable alternating current.

But solar panels work during the daytime when the sun is shining. Home systems that have batteries to store power cost 1.5-times more. If there is a way by which we can more efficiently store captured energy, it would be ideal. This is what a group of researchers at the University of California at Santa Barbara, led by Prof. Grace Han, have done by designing what are referred to as molecular photo-switches (DOI: 10.1126/science.aec6413). These involve molecules that, upon absorbing solar light, convert themselves into a different shape (called isomerisation) and store the energy. This is referred to as photo-switching. Here, the molecule stores solar thermal energy  in a molecular form, also known as molecular solar thermal (or MOST).

The molecule the researchers have chosen, after consultation with a group at University of California at Los Angeles, is an alkaloid called 2-pyrimidone. It switches itself upon receiving photons into an isomer called Dewar pyrimidone. The latter isomeric molecule is highly strained and releases heat as much as 1.65 million joules per kg of MOST molecules. This heat allows this technology to be a practical tool to store heat and deliver it when needed for water heating, cooking, and other needs.

As a member of the research team mentioned, thermal energy from the sun can now be bottled for later use. Silicon-based panels use red and near-infrared regions of the light spectrum (400-1,100 nm). Solar thermal molecules use wavelengths below 500 nm, in the ultraviolet region.

Apart from the pyrimidones, there must be other compounds that too can act as photo-switches and heat storage units. As the solar panels across India capture sunlight during the day, our scientists should be able to synthesise more MOST molecules and distribute them to the people who put up solar panels on their roofs. That would make our country greener.

dbala@lvpei.org