Can we calculate the efficiency of a natural photosynthesis process?

Engineers have a lot to learn — from plants…

By Sarah Jensen

Great engineering ideas can come right out of your back yard, or from the weeds growing in the sidewalk cracks in front of your house. All you have to do is look closely enough. “Understanding how well plants convert light to energy can have a real impact on photovoltaic systems,” says Ardemis Boghossian, a graduate researcher in MIT’s Department of Chemical Engineering.

Determining the efficiency of photovoltaic cells is relatively easy since the process produces immediate electric power. But calculating the efficiency of plants is complicated by the fact that photosynthesis involves not one but a series of reactions. Each step of photosynthesis takes place in tiny chloroplasts, which are tiny subunits inside plant cells where sunlight and carbon dioxide are converted to oxygen and sugar.

In the lab, Boghossian and her team calculate the efficiency of that process by injecting a dye into the chloroplast and measuring its color change as electrons are produced during photosynthesis. Alternatively, they measure the amount of sugar or oxygen produced relative to the amount of carbon dioxide taken up by the plant. Once the amount of energy lost in performing the chemical conversions is taken into account, they find, the efficiency rate of converting light to energy is approximately 6%, compared to 10% in photovoltaics.

Where plants outpace PV cells, however, is in the amount of light they absorb. Both photosynthesis and photovoltaic systems absorb very high-energy light, but plants are nearly 100% efficient at absorbing light from the visible spectrum — the range of colors from red to blue.   PV cells absorb light over a large range of the spectrum, too, but not as well as collard greens, kale, or goosegrass. And depending on the conditions under which they evolved, some plants are better at this than others. ”Dark green, leafy plants have very active chloroplasts and they’re very good at converting light to energy,” says Boghossian. (Popeye was clearly on the right track.) As researchers learn more about photosynthesis and understand the mechanisms that affect its efficiency, they’re able to combine botany and technology in the creation of more effective PV systems. Currently under study is the design of molecules that replicate those in chloroplasts and leaves. Once installed in PV panels, the “artificial leaves” may absorb light as efficiently as plants.

Boghossian and her team are also investigating ways to create photovoltaics from actual plant proteins. Engineered to use light from all areas of the spectrum, they would combine the best of both natural and synthetic efficiencies. “It’s a fascinating system to study,” she says. “The biological perspective gives so much insight into ways we can improve photovoltaics in the future.”

Thanks to 24-year-old Pawan Singh from Lucknow, India, for this question.

Posted: March 13, 2012