While impressive, the last few decades of human achievement in photovoltaics pale in comparison to nature’s equivalent technology: photosynthesis. Just look at the numbers—every year photosynthesis produces about 3,000 exajoules (EJ) of chemical energy, or 7 x 1017 kilocalories, which equates to about half the total energy stored in the world’s petroleum reserves (approximately the average daily caloric intake of eating champ Joey Chestnut). Compare this to the 0.1 EJ of electrical energy produced annually by man-made photovoltaics. Closing this gap is the key to a sustainable energy future, and unlike nature we don’t have the luxury of waiting billions of years to get there.
Researchers are increasingly trying to peek inside nature’s bag of tricks and develop a new generation of biologically-inspired photovoltaics. Two recent discoveries represent significant progress toward this goal. The first of these papers was from a group of UC Berkeley researchers, led by chemistry professors Graham Fleming and Birgitta Whaley. They demonstrated that chloroplasts make use of a quantum physical effect known as entanglement to transport solar energy from light harvesting pigments to chemical reaction centers with extraordinary efficiency. Entanglement causes pairs of electrons that are spatially separated to behave like a single particle, meaning any change to one electron instantaneously affects the other. In plants, this effect allows solar energy to be stored in a high-energy electron configuration for a long enough period of time to be transferred to the chemical reaction centers before any of the energy has a chance to leak away.
Although their result lies in the realm of basic science, it may lead to the reality of utilizing quantum entanglement in man-made devices such as solar cells. It had previously been thought that the chaotic nature of high temperature systems at the molecular level would prohibit electrons from remaining entangled over a useful period of time. Now we know that you have to go no further than your windowsill to disprove this hypothesis; this is certain to change some minds and may lead to significant improvement to solar cell efficiency in the coming years.
The second recent innovation, made by a group led by Professor Michael Strano at MIT, is an artificial light-harvesting structure that has the ability to reassemble after its molecules have been broken apart by light. This mimics the mechanism used by plants to combat gradual reductions in conversion efficiency over time. In plants, proteins in the light harvesting regions typically break apart and reassemble every 45 minutes, a process that maintains the health of the system year after year. Similarly, damaged structures in the MIT group’s concoction reassemble whenever a surfactant is added to and subsequently removed from the solution.
Thought to be the most complex man-made self-assembling system ever developed, their structure consist of seven different compounds, including carbon nanotubes, proteins, and phospholipids. Although their device isn’t quite ready yet to compete with silicon-based solar cells, their work represents the first step towards developing long-lasting, low-cost solar cell materials using nature’s own self-repairing approach.
It’ll be interesting to see what comes next from this line of work. Self-installing solar arrays? Grid-connected rainforests? Photovoltaic jellyfish? Actually, my money is on an artificial Venus fly trap… it offers guaranteed savings on both your electric bill and exterminator bill.
Sarovar, M., Ishizaki, A., Fleming, G., & Whaley, K. (2010). Quantum entanglement in photosynthetic light-harvesting complexes Nature Physics, 6 (6), 462-467 DOI: 10.1038/nphys1652
Ham, M.-H., Choi, J. H., Boghossian, A. A., Jeng, E. S., Graff, R. A., Heller, D. A., Chang, A. C., Mattis, A., Bayburt, T. H., Grinkova, Y. V., Zeiger, A. S., Van Vliet, K. J., Hobbie, E. K., Sligar, S. G., Wraight, C. A., & Strano, M. S. (2010). Photoelectrochemical complexes for solar energy conversion that chemically and autonomously regenerate Nature Chemistry : 10.1038/nchem.822