Clean solutions to human energy demands are essential to our future. While sunlight is the most abundant source of energy at our disposal, we have yet to learn how to capture, transfer and store solar energy efficiently.
According to University of Toronto chemistry professor Greg Scholes, the answers can be found in the complex systems at work in nature.
"Solar fuel production often starts with the energy from light being absorbed by an assembly of molecules," said Scholes, the D.J. LeRoy Distinguished Professor at U of T. "The energy is stored fleetingly as vibrating electrons and then transferred to a suitable reactor. It is the same in biological systems.
“In photosynthesis, for example, antenna complexes comprised of chlorophyll capture sunlight and direct the energy to special proteins called reaction centres that help make oxygen and sugars. It is like plugging those proteins into a solar power socket."
In an article in Nature Chemistry, Scholes and colleagues from several other universities examine the latest research in various natural antenna complexes. Using lessons learned from these natural phenomena, they provide a framework for how to design light harvesting systems that will route the flow of energy in sophisticated ways and over long distances, providing a microscopic "energy grid" to regulate solar energy conversion.
A key challenge is that the energy from sunlight is captured by coloured molecules called dyes or pigments, but is stored for only a billionth of a second. This leaves little time to route the energy from pigments to molecular machinery that produces fuel or electricity. How can we harvest sunlight and utilize its energy before it is lost?
"This is why natural photosynthesis is so inspiring," said Scholes. "More than 10 million billion photons of light strike a leaf each second. Of these, almost every red-coloured photon is captured by chlorophyll pigments which feed plant growth."
Learning the workings of these natural light-harvesting systems fostered a vision, proposed by Scholes and his co-authors, to design and demonstrate molecular "circuitry" that is 10 times smaller than the thinnest electrical wire in computer processors.
Dr Alexadra Olaya-Castro, co-author of the paper from UCL's department of Physics and Astronomy said: "On a bright sunny day, more than 100 million billion red and blue "coloured" photons strike a leaf each second.
"Under these conditions plants need to be able to both use the energy that is required for growth but also to get rid of excess energy that can be harmful. Transferring energy quickly and in a regulated manner are the two key features of natural light-harvesting systems.
"By assuring that all relevant energy scales involved in the process of energy transfer are more or less similar, natural antennae manage to combine quantum and classical phenomena to guarantee efficient and regulated capture, distribution and storage of the sun's energy."
Fellow researcher Graham Fleming, Vice Chancellor for Research at the University of California (UC) Berkeley, added: “Solar energy is forecasted to provide a significant fraction of the world’s energy needs over the next century, as sunlight is the most abundant source of energy we have at our disposal.
“However, to utilize solar energy harvested from sunlight efficiently we must understand and improve both the effective capture of photons and the transfer of electronic excitation energy.”
“In solar cells made from organic film, this brief timescale constrains the size of the chromophore arrays and how far excitation energy can travel,” Fleming says. “Therefore energy-transfer needs and antenna design can make a significant difference to the efficiency of an artificial photosynthetic system.
“There remains a number of outstanding questions about the mechanistic details of energy transfer, especially concerning how the electronic system interacts with the environment and what are the precise consequences of quantum coherence.
“However, if the right research effort is made, perhaps based on synthetic biology, artificial photosynthetic systems should be able to produce energy on a commercial scale within the next 20 years.”
Last year, Scholes led a team that showed that marine algae, a normally functioning biological system, uses quantum mechanics in order to optimize photosynthesis, a process essential to its survival. These and other insights from the natural world promise to revolutionize our ability to harness the power of the sun.
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