A debate rages in the scientific community over which is more efficient at harvesting solar energy: a plant or a solar cell? An article in the May 13 issue of Science rather provocatively proclaims that the solar cell is the hands-down winner. However, there are scientists who maintain that photosynthesis, an otherwise naturally occurring process, could be altered to help “level the playing field”.
Dr. Robert Blankenship is a scientist who studies photosynthesis at Washington University in St. Louis and serves as a member of the DOE’s Council and Biochemical Sciences. At Blankenship’s suggestion, the DOE held a workshop two years ago in Albuquerque, NM that brought together biologists, chemists, physicists and solid-state scientists. The group congregated to discuss what they knew and “hash everything out.” The paper that was recently published in Science is reportedly a by-product of that workshop.
Dr. David M. Kramer, Professor of Biochemistry and Molecular Biology at Michigan State University and one of the co-authors of the paper says that the their intention was to “resolve the long-standing controversy over the efficience of photosynthesis” because “the efficiency of photosynthesis, and our ability to improve it, is critical to whether the entire biofuels industry is viable.”
The trick, according to the scientists, was to make the competition between plants and solar cells a fair one. Unlike solar cells, plants don’t just capture solar energy-they also store it. For the testing to be fair and yield accurate data, the solar array would need to store the energy it generated, too. So, the scientists connected a solar array to an electrolyzer that used the array’s electricity to split water into hydrogen and oxygen. Apparently, the free energy needed to split water is essentially the same as that needed for a plant to form carbohydrate, so the comparison was deemed even-handed.
Given these terms, the results showed that the annual averaged efficiency of photovoltaic-driven electrolysis was about 10 percent whereas solar energy conversion efficiencies for crop plants was typically about 1 percent.
So the solar cell, for now at least, is the winner. However, Blankenship says the results led them to ask why plants are so inefficient and what can be done to improve their efficiency. He cites an analogy regarding teosinte, a grass native to Central America that is thought to be the ancestor of corn saying, “You couldn’t feed yourself very well off that stuff. The giant ear and juicy kernels are things people imposed on corn to make it suitable for our needs. I think the same sort of thing will go on with bioenergy. We’ve bred plants for food production, not for energy storage, and the two things are not the same.”
At the same time, Blankenship also stresses that efficiency is just one several considerations that need to be taken into account when making decisions about viable energy technologies. He says that life-cycle costs, the capital cost and valuation of the environmental impact of a product from its creation to its destruction are even more important considerations. For now, then, it would seem that the question about whether research toward photosynthesis improvement for the sake of biofuel production is still up in the air.