Plants, algae and some bacteria naturally harvest sunlight to produce energy and raw materials for themselves. Now, scientists have found a way to take the same processes from plants to generate electricity and make chemicals like drugs and nutrients. They are creating a power system that is literally green.
Engineers have attempted to mimic plants using devices like photovoltaic panels for some time, but now they can simply co-opt a plant mechanism directly to do the same things at very low costs.
The process in question uses a molecular complex called photosystem-I (PSI). The complex arranges 96 chlorophyll molecules -- the compound that captures sunlight and gives plants their green color -- matching them to electron donors and acceptors to generate a current.
In plants, this helps them make sugar from sunlight and gives them energy to grow. Developed over 2.7 billion years, this mechanism is highly efficient, moving one electron for every photon of light that strikes the complex, a yield unmatched by any man-made system to date.
At the University of Copenhagen in Denmark, researchers are harnessing PSI's efficiency to make chemicals that are ordinarily difficult and time-consuming to make. Such molecules include expensive treatments for cancers, which require intricate and specific structures, explained Birger Lindberg Møller, a professor in the plant biochemistry laboratory at the University of Copenhagen.
"They have a very, very complex structure," he said. "It's difficult to synthesize those molecules using organic chemistry."
Instead, Møller wants to repurpose PSI to make complicated molecules, bypassing the plant's own processes. "We don't want the intermediate steps with the sugars, we want to feed the electrons directly into the systems we want to operate," said Møller. These systems can then use the energy to make enzymes that assemble proteins, lipids and pharmaceuticals.
This way, manufacturers can avoid the tedious chemical reactions often needed to make drugs in large quantities, driving prices down. "We are really focusing on costly molecules," he said. In addition, by using sunlight as energy, chemical manufacturing plants can reduce their environmental impact.
These techniques are part of a shift toward what Møller called a "bio-based society," where living organisms feeding on light efficiently and sustainably replace many current processes. "The more processes you can run by light, the better," said Møller, whose findings were published in yesterday's edition of Trends in Plant Science.
Low-cost solar roofs out of plant wastes
PSI can also be used to cheaply generate electricity from the sun. By using existing green plants or even agricultural waste as starting material, researchers said they can eventually turn most surfaces into low-cost solar cells. "You take plant matter and isolate the photosynthetic membrane through centrifugation," said Barry Bruce, a professor in the Department of Biochemistry and Cellular and Molecular Biology at the University of Tennessee, Knoxville.
Once isolated, PSI can be stabilized and coated on a surface, like a roof or a wall. Add a transparent cover, a couple of wires and an electrolyte, and you have a solar generator.
Andreas Mershin, who worked with Bruce on this research, said that low-cost ways of producing solar power have existed for some time, but many of these processes were not worth the effort. "Everybody knew 30 or 40 years ago this was doable, but the juice you were getting was so low," said Mershin, a research scientist at the Center for Bits and Atoms at the Massachusetts Institute of Technology. Bruce, Mershin and their colleagues published their findings last month in Scientific Reports, an open-access journal.
Though their device is a substantial improvement, with 10,000 times greater photocurrent density over previous attempts, the PSI solar cell is only 0.1 percent efficient, producing barely enough current to charge a cellphone. However, because the process is so simple, Mershin expects that people all over the world can come up with new ways to make this work better.
"The real experiment is the effect of this paper. We're hoping that a lot of people will jump in," he said. "There are places where if you lower the cost enough, 2 percent [efficiency] is compelling."
A breakthrough for the off-grid world
The idea is to make electricity accessible in areas off of the grid, where standard photovoltaic panels are too fragile, and where companies don't really see an investment potential. "No private company is ever going to fund this because no one is ever going to make money from this," said Mershin.
As a result, he wants to reach out to the tinkerers in garages, basements and backyards along with other research labs to make PSI solar generators even cheaper and more effective.
One method is to increase the surface area for the coating at nanometer scales. (A nanometer is one-billionth of a meter, which measures 39.37 inches.) The researchers used zinc oxide and titanium dioxide to make nanostructures, allowing more PSI complexes to adhere to the surface. Mershin said these textured surfaces can actually be made quite easily, and by experimenting with the recipe, he expects that someone will find a way to improve on their results.
Bruce said in his lab, they are engineering the PSI in bacteria to make the complex easier to repurpose and more robust in capturing sunlight. His team is also working on multilayered devices to capture sunlight at different wavelengths, extracting more usable energy.
Eventually, Mershin said, people around the world could easily produce their own power using a bag of some raw materials and following an instruction sheet that is simpler than an Ikea manual.
Mershin noted that evening lighting helps poor workers gain more productive hours in their day, allowing them to study or learn skills that will help them advance economically. "It would be a disruptive event for humanity in general," he said.
"It's an upside-down way of looking at electricity generation," he added. "People who have never been thought of either as producers or consumers of solar power can use it."
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