By design, greenhouses are solar structures that collect the sun’s rays to create a warm environment in which plants can flourish. A passive solar design goes beyond the conventional greenhouse by utilizing glaze on southern-facing windows to maximize light and heat, ventilating the interior for cooling, insulating northern walls to reduce nightly heat loss, and including an enclosed water mass (such as a water-filled steel drum) to absorb heat during the day and release it at night.
As the rooftop photovoltaic (PV) market started taking off, farmers, horticulturalists and herbalists began realizing they could use the sun’s energy to power their air-conditioning, irrigation, and ventilation systems. However, installing PV panels on the roof of the greenhouse changes its interior conditions—the temperature, humidity, and brightness. These internal changes can threaten the livelihood of crops. “Solar greenhouses must be designed to ensure the right balance between an adequate internal radiation in the winter and adequate protection during the summer,” states the report, Performance Analysis of Greenhouses with Integrated Photovoltaic Modules.
Researchers at the University of California, Santa Cruz are taking a new approach to sustaining crops while creating electricity with a new PV system that uses pigments to absorb sunlight. These Wavelength Selective Photovoltaic Systems (WSPVs) use a pigment called LR305 that catches only green and blue wavelengths. Since plants reflect green light instead of absorbing it, these wavelengths can be converted into electricity while allowing the plants below the panels to flourish.
The WSPVs have yet to be perfected, as not all plant species respond similarly to the altered light quality. The research team found no changes among lettuce and spinach, while tomatoes produced more fruit (yet smaller in size), but strawberries fared poorly. “We can’t solve every puzzle,” USCS physics professor Sue Carter tells Santa Cruz Weekly. “But we can get a good idea on how to proceed forward and how species interact in different environments. This will help make better greenhouses for those plants.”
Another innovative approach to the solar PV greenhouse was developed last year in Spain by ULMA Agrícola and Tecnalia. The system utilizes the sun’s changing trajectory throughout the year with a lens-based optical feature, allowing sunlight in during the winter months and diverting the intense summer rays to PV cells, reports EarthTechling. The electricity generated can then power the greenhouse operations.
ULMA Agrícola reveals that testing of tomatoes and peppers—which require a significant amount of sunlight—was positive and electric production was 15 percent higher (annually) compared to that of a standard PV system. Besides allowing the co-existence of photovoltaics and crops, the new PV installation “acts to even out seasonal variations in sun radiation and temperature inside the greenhouse, [plus] the possible economic benefits to the farmer with the sale and/or consumption of the energy generated,” ULMA Agrícola states.