Solar panels keep greenhouses cooler in summer, let the sun through in winter

Spain's ULMA Agrícola consortium and Tecnalia research center have developed a new type of photovoltaic solar panel for greenhouses that can generate electricity without an adverse effect on the crops grown within, while additionally providing cooling in summer. The system is designed to exploit the annual oscillation - the variation in the height of the sun's path across the sky over the course of the year. In theory, no solar radiation is compromised over winter, but surplus radiation can be diverted to electricity-generation during summer.

The photovoltaics use optical lenses rather than a mechanical means of solar-tracking, which may generate 15 percent more electricity over a standard photovoltaic system according to the first phase of testing during summer. A second phase of winter testing ending in March is currently underway, and UMLA Agrícola is projecting that similar results will be seen.

Though we have not seen precise details of how the optical system works, it appears to exploit surprisingly simple technology operating on a very simple geometric principle. Judging by the photographs, photovoltaic strips run across the greenhouse roof, with wider gaps of ordinary glazing between. Rows of lenses of similar width to, but slightly offset from, the photovoltaic strips are installed in an elevated position above the roof.

My interpretation is that, at the lower solar altitudes of winter, these lenses intercept the solar radiation headed for the solar panels and redirect it through the glazing. In summer, the lenses do not interfere, and some of the sunlight is allowed to fall directly on the solar panels. Though not explicit, it seems plain that the mooted beneficial cooling during summer is as a direct result of reduced solar penetration due to the solar panels, and not through air conditioning powered by the photovoltaics.

The tests are taking place at a 400 square meter (478 sq.yd.) UMLA Agrícola greenhouse owned by Neiker Tecnalia at Derio in the Basque Country. Under these tests, temperature, humidity and total radiation of the interior were monitored, as well as photosynthetically active radiation, which is the band of solar radiation useful to plants for photosynthesis. This band, of wavelengths 400 to 700 nm, falls entirely, though only just, within the band defined as light (380 to 740 nm), to which the human eye is sensitive.

Tomatoes and peppers were chosen for the tests, requiring as they do copious amounts of light in order to flourish - presumably making any adverse effects to growing conditions easier to identify. Additionally, the crops are a widely-grown cash cow for the agricultural industry. If the technology could not be made to work with such profitable goods, the game would very likely be up. The first phase of testing reportedly showed positive results in respect of crop quality and yield.

Source: Basque Research

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