About a decade ago, a certain midwestern college was planning a new science and math building. Several faculty members in the engineering department lobbied for a rooftop photovoltaic array, but the powers-that-be decided it was too costly. (They did, however, go for a geothermal heating and cooling system, which paid for itself in energy savings after six years.) To appease the sustainable energy proponents (but without consulting faculty), the architects added a smattering of thin-film PV cells in a checkerboard pattern on the sunniest corner of the building - the same corner that houses the biology department's greenhouse. So now the building has a PV array that generates a trickle of electricity, blocks half of the light reaching the greenhouse, and irritates faculty in two disciplines.
As it turns out, photovoltaics vs photosynthesis does not have to be a zero-sum game.
Solar with a Side of Veggies
As engineers in Europe are demonstrating, agriculture and solar power can peacefully coexist in order to maximize land-use efficiency. And in the US, researchers at the University of California at Santa Cruz are exploiting the fact that PV cells and plants use different components of the solar spectrum. They're testing greenhouses with a magenta tint, coupled with wavelength-selective photovoltaics (WSPV), and finding that they can generate low-cost electricity with no detrimental effects on the crops. In fact, some of the plants actually grow better in a pink greenhouse, with tomato plants producing the same yield while using 5% less water.
Luminescent Solar Concentrators
Although the exterior looks like a simple sheet of pink plastic, it's actually made of high-tech luminescent solar concentrators (LSCs), materials that absorb solar radiation, internally reflect it toward the edge, and shine the accumulated sunlight on a narrow photovoltaic panel. This allows light over a large area to be concentrated onto a smaller cell, reducing the cost of the photovoltaics.
The LSCs are tuned to absorb the blue-green wavelengths of light that plants don't use, direct those colors to the PV cells, and allow the remaining photons to pass through and trigger photosynthesis. Effectively, the LSC acts as a sorting mechanism, allowing the appropriate wavelengths of light to reach the components that need them.
The Numbers
The power that's generated by the magenta greenhouse isn't going to the grid; it's used to power the greenhouse fans, sensors, and data logging equipment, making the greenhouse an off-grid building. Experimental data showed that each square meter of WSPV on the greenhouse generates an average of 3.5 kWh of electricity per month. A 3m x 7m greenhouse covered in WSPV could produce upwards of 70 kWh per month.
At the time of this writing, a standard PV panel costs about $1 per watt (retail) and generates 170 watts per square meter. The WSPV material costs $45 per square meter (one-fourth the cost of standard PV) and generates one-third the power. In other words, 100 square meters of traditional PV panels can generate 17kW at a cost of $17k ($1/watt), while the same area covered in WSPV could produce 5.7kW at a cost of $4500 - just $0.80/watt. That's 20% lower cost for the panels.
These figures don't suggest that we should ditch traditional PV altogether, just that in some applications - those where power density isn't the highest priority - WSPV can be added at a low cost. In fact, the price of PV continues to fall, so unless the cost of the WSPV material declines at the same rate, I suspect that the cost difference will become insignificant over time. Even if that happens, however, WSPV will continue to make sense for applications where it's desirable to let some light pass through while the remaining photons produce power. And remember, the wavelength-specific PV can be tuned to other colors - it doesn't have to be pink. Picture atriums, sunrooms, and offices with tinted glass that produces electricity.
When Physics Met Botany
The magenta greenhouse research is led by Dr. Michael Loik, UCSC Professor of Environmental Studies, but it's a truly interdisciplinary project, as it uses wavelength-selective photovoltaic systems developed by Professors Sue Carter and Glenn Alers of UCSC's Department of Physics. Sustainability is a team effort - we're all in this together.
Here is Professor Loik talking about the research:
Video courtesy of UC Santa Cruz
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