Zhong Lin Wang, a nanotechnologist at the Georgia Institute of Technology, likes van der Beek's proposal. "The shape of the leaf of a tree is the ideal shape to capture the power of the sun," Wang says. "I think the Solar Botanic idea could have tremendous potential." Wang and his Georgia Tech colleagues have demonstrated the potential of piezoelectrical generation on a small scale by developing nanowires that could be woven into fabrics, generating electricity as the wearer moves, to power an iPod, for example.
But other scientists are skeptical. "A lot of people are trying to catch the green energy wave, and sometimes it's half-baked science," says Michael Woodhouse, a solar energy materials scientist at the National Renewable Energy Laboratory (NREL) in Golden, Colo.
For one, could solar and thermoelectrics really work together on a single leaf blade? Joseph Heremans, a professor of mechanical engineering and physics at The Ohio State University in Columbus, says that's "spatially problematic, to say the least." Van der Beek recognizes this as probably the chief remaining engineering hurdle, saying "We do have to find a symbiosis between these materials [photovoltaics and thermoelectrics].... They can't get in each other's way."
Then there's the efficiency of green solar panels: "If Mother Nature wanted photosynthesis to be efficient, she would have made leaves black," Woodhouse says. Black materials absorb all of the sun's visible light, explaining why solar panel makers opt for pitch rather than designer colors. Van der Beek concedes Solar Botanic may have to settle for a dark shade of green, but he believes that ever-improving photovoltaics will make energy generation that sacrifices some reflected green wavelengths economical.
And it is unclear how much the thermoelectric component of the trees will contribute, Heremans says. The temperatures required for real thermoelectric power generation in those environments vastly exceeds the heat that green leaves in the sun normally experience. "I don't see [nanoleaves] working with small temperature gradients," he says. "The second law of thermodynamics tells you that small gradients equal poor efficiency." Van der Beek acknowledges that this third component would contribute the least to the overall energy equation.
Still, he hopes to have a working prototype in about three years. "I know this might still be far off," van der Beek says, "but we have the energy and the will to do this." Including van der Beek, Solar Botanic has four full-time employees, patents pending in several countries (a U.S. filing is in the works), and has secured funding from California-based investors (he won't say how much). Technical assistance for Solar Botanic has come from the nonprofit, England-based Center for Sustainable Engineering as well as the University of Reading–backed Biomimetics Network for Industrial Sustainability (BIONIS), he says.
All told, an energy harvesting tree with a 20-foot-diameter canopy is expected to cost between $12,000 and $20,000, van der Beek notes, and would produce about 120,000 kilowatt hours over a 20-year life span. As such, each kilowatt-hour would cost about 13.5 cents—not particularly competitive with most power sources, which are average around 5 cents.