Artificial photosynthesis for solar power?
Photosynthesis, the natural process used by green plants for millions of years to convert sunlight, carbon dioxide, and water into oxygen and carbohydrates is the basis of an emerging technology that promises to not only provide a renewable source of electrical energy but also to potentially help scrub the atmosphere of excessive carbon dioxide.
A team of researchers from the Department of Energy’s (DOE’s) Lawrence Berkeley National Laboratory, the University of California at Berkeley, and the University of Verona, among other institutions, are looking to create artificial versions of photosynthesis in an effort to find new ways to harness solar power. Their scope of study focuses on how plants are able to harvest and convert sunlight to chemical energy at an energy transfer efficiency rate of approximately 97%. Last month, these scientists announced they had discovered a “dimmer switch”—a molecular mechanism that controls the flow of solar energy as it moves through the system of light-harvesting proteins. The discovery sheds light on a potential pitfall affecting sunlight-harvesting systems and presents an important implication for the design of future artificial photosynthesis systems.
The scientists realized that if the photosynthesis system becomes overloaded with absorbed solar energy, it most likely would suffer some form of damage. Plants solve this problem on a daily basis with a photo-protective mechanism called energy-quenching, the researchers found. Excess energy, detected by changes in pH levels, is safely dissipated from one molecular system to another, where it can then be routed down relatively harmless chemical reaction pathways.
The pigment-binding protein CP29, one of the “minor” light-harvesting proteins in green plants, has been identified as a valve that permits or blocks the critical release of excess solar energy during photosynthesis (Figure 1). It has also been proposed that the opening and closing of this valve—much like a dimmer switch that can be turned on and off—can be controlled by raising or lowering ambient pH levels.
1. Natural dimmer switch. Researchers looking to develop an artificial power-generating version of photosynthesis have identified a botanical “dimmer switch” that controls the flow of solar energy as it moves through a system of light-harvesting proteins. Within this protein supercomplex, energy-quenching takes place in D1 and D2 proteins (light-blue and dark-blue areas). The surrounding proteins, CP29, CP26, and CP24, function as energy flow-controlling valves. Courtesy: Roberto Rossi, Berkeley Lab
“This is really the first detailed picture ever obtained of the molecular mechanism behind the regulation of light harvesting energy,” Graham Fleming, one of the scientists, said. “We believe we will soon be in position to build a complete model of the flow of energy through the photosynthetic light harvesting system that will include how the flow is controlled. This model could then be applied to the engineering of artificial versions of photosynthesis.”
The results of this study were reported in the journal Science (May 9, 2008). The researchers’ next step is to examine the energy-quenching mechanism in the rest of the of the PhotosystemII protein complex to see how it is used to regulate the flow of energy throughout the light-harvesting system.
Poultry litter to fuel 55-MW N.C. plant
A renewable power generating company has found a creative use for poultry litter. Though the litter traditionally is used as fertilizer or cattle-feed, tons go to waste. Rather than waste litter, a new biomass plant planned for construction in Sampson County, N.C., will burn the droppings to generate 55 MW of power.
Fibrowatt LLC, a Pennsylvania-based renewable energy company, announced in April that, as well as providing an alternative, beneficial use for poultry litter, its plant would increase alternative energy supplies to North Carolina, the first state in the southeast U.S. to require minimum levels of renewable energy by 2012.
Fibrowatt was founded in 2000 by a management team that built the world’s first three poultry litter–fueled power plants in the UK in the 1990s. The company’s subsidiary Fibrominn opened the first U.S. poultry litter–fueled power plant in Benson, Minn., in mid-2007. The 77-acre site uses more than 500,000 tons of poultry litter as well as other biomass to produce 55 MW at capacity (Figure 2). The electricity is sold to Xcel Energy.
2. The food-fuel cycle. Tons of excess poultry litter will be burned at a planned 55-MW biomass plant in North Carolina. A crane moves poultry litter within a fuel hall at a similar plant in Minnesota. Courtesy: Fibrowatt LLC
The company is currently conducting site evaluations for two additional poultry litter–fueled plants in North Carolina. The selection of the Sampson County site, which took more than a year, was announced first because of strong support from the poultry industry and locale advantages, which included ample fuel supplies and access to a utility transmission line. To complete development of the Sampson County project, the company must agree to a power purchase agreement and obtain project financing, which is expected to exceed $200 million. Fibrowatt expects to begin construction of the plant in 2009, and it aims to begin operations in Sampson County by 2011.
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