08-18 plant comparison agrivoltaics

[Image above] Representative examples of basil leaves from plants grown under (H) clear glass panels and (I) tinted semitransparent solar panels. The white horizontal bar represents 50 mm. Credit: Thompson et al., Advanced Energy Materials (CC BY 4.0)

Last August, I looked at a concept gaining increased attention among agriculturists—the concept of agrivoltaics.

Agrivoltaics refers to the practice of co-locating photovoltaic infrastructure and agriculture by planting crops under photovoltaic panels. The technique was originally conceived in 1981, and it became more attractive in recent years as photovoltaic prices dropped, interest in renewable energy rose, and financial pressures on small farmers grew.

Supporters of agrivoltaics note numerous benefits to both photovoltaics and crops, including creation of a favorable microclimate under the solar canopy for plants (e.g., protected from wind, less variation in temperature) and reduced heat stress on the solar panels due to localized cooling from water evaporation.

There is a downside to agrivoltaics, though—lower crop yield of certain plants.

Because solar panels absorb light, they naturally reduce the amount of light reaching underlying plants. And while some plants grow well in the partial shade cast by solar panels, other plants experience detrimental effects.

“For example, for lettuce, the total biomass yield under agrivoltaic installation in Montpellier (France) was 15–30% less than the control conditions (i.e., full-sun conditions). When growth of tomato was tested in Japan, the yield in an agrivoltaic regime was about 10% lower than for conventional agriculture,” researchers write in a recent open-access paper.

The researchers come from several universities in the United Kingdom and Italy and are led by postdoctoral researcher Paolo Bombelli from the University of Cambridge. And in their recent paper, they investigate a possible way to circumvent this limitation.

The key, they propose, is not simply to let more light through—the key is customizing which wavelengths of light get through.

In conventional agrivoltaics, opaque and neutral semitransparent solar panels absorb electromagnetic radiation uniformly across the entire visible spectrum. However, visible wavelengths are not absorbed uniformly by the underlying plants.

Chlorophylls, the main photosynthetic pigments in plants, absorb wavelengths mostly in the red (≈600–700 nm) and blue (≈400–500 nm) regions of the electromagnetic spectrum; wavelengths in the green region (≈500–600 nm) are rarely absorbed (giving chlorophylls, and plants, a green appearance).

At the same time, other pigments such as carotenoids and anthocyanins absorb wavelengths in the blue and green regions, respectively, with the purpose of dissipating excess/harmful solar energy to protect the photosynthetic apparatus.

Taken together, “part of the solar energy absorbed in the blue and green portions of the electromagnetic spectrum is dissipated without contributing to photosynthesis,” the researchers write.

Based on this knowledge, a solar panel that absorbs blue and green wavelengths while allowing red wavelengths to pass through may mitigate the detriments of shade because the wavelength most important to photosynthesis is still reaching the underlying plants.

The technology to customize solar panels to absorb certain wavelengths exists. Yet surprisingly, no experimental data on the effects of customized solar panels on plant growth have been published, according to the researchers—so they decided to conduct a study.

In their study, the researchers created tinted semitransparent solar panels that absorb preferentially blue and green wavelengths, leaving most of the red wavelengths for photosynthesis. They installed the solar panels over beds of basil and spinach for testing.

A) Solar radiation spectrum in the visible range at the ground level. B) Absorption spectrum for the tinted semitransparent solar PV panel used in this study. C) Absorption spectrum for a basil plant leaf. D) Schematic representation of the input (solar energy) and the two contextual outputs of agrivoltaics (i.e., electricity and biomass). Credit: Thompson et al., Advanced Energy Materials (CC BY 4.0)

Compared to plants grown under panels of clear glass, plants grown under the tinted semitransparent solar panels demonstrated more efficient photosynthetic use of light (up to 68% for spinach) and a preferential redirection of metabolic energy toward tissues above ground (up to 63% for basil). In addition, the amount of protein extracted from both plants was increased in leaf (basil: +14.1%; spinach: +53.1%), stem (basil: +37.6%; spinach: +67.9%), and root (basil: +9.6%; spinach: +15.5%).

There was a loss in the yield of marketable biomass for both basil (-15%) and spinach (-26%). However, when electricity generation is taken into consideration, “our experimental data has shown that agrivoltaics could give a substantial overall financial gain calculated to be +2.5% for basil and +35% for spinach compared with classical agriculture,” they write.

In the conclusion, the researchers note a few limitations of their study and areas for more exploration. For example, the study does not allow conclusions to be drawn on the effect of agrivoltaics on plants where underground tissues might have different functions, such as storage in tubers.

“Further experimental trials using semi-transparent solar panels with specific, targeted optical properties might permit the development of novel methods for tailoring the content of specific nutrients in crops,” they write.

The open-access paper, published in Advanced Energy Materials, is “Tinted semi‐transparent solar panels allow concurrent production of crops and electricity on the same cropland” (DOI: 10.1002/aenm.202001189).