Configuring agrivoltaic systems around plant physiology can substantially raise energy yields without compromising the light supply to crops. That is the finding of a simulation carried out by researchers at Macquarie University in Australia, with results applicable to other regions as well. The precondition is that agrivoltaic planning targets both electricity and crop yields, since the study is based on the light spectrum that plants can actually use, known as photosynthetically active radiation (PAR).
Existing shading models fall short
Australian researcher Adnin Tazrih Natasha examined the growth behaviour of a medicinal plant in Malaysia as a case study. Her work rests on the principle that agrivoltaics is added to agriculture, not the other way round. “We cannot maximise energy yield through PV system design alone. Agricultural output contributes at least 50 percent to the overall efficiency of the combination,” she says.
In a pilot project, she set out to identify the PAR threshold below which the light spectrum becomes insufficient for plant growth. “Previous modelling has only considered total light, not the light spectrum,” Natasha explains. “But the success of an agrivoltaic project depends precisely on this spectrum and on PAR. Conventional shading models ignore the spectral composition and quality of light and rely on fixed light assumptions instead.”
Photons in the right spectrum is decisive
To address this, Natasha related PAR to energy efficiency in order to design a system that benefits both crops and solar output, allowing agrivoltaic systems to be optimised accordingly. The light intensity that plants require for optimal development, expressed as photosynthetic photon flux density (PPFD), ranges from 200 to 1,000 micromoles per square metre per second, from seedling stage through to flowering. This describes the quantity of photons striking the plants each second that they can actually use, and is the decisive factor in determining PAR.
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To ensure sufficient light in the critical spectrum between 400 and 700 nanometres reaches the plants, system design must be adjusted accordingly, with module density and tilt angle configured to match the required illumination.
Measuring PAR
Natasha tested how this might work using fixed-tilt south-facing and east-west solar arrays installed above various shade-tolerant crops and medicinal plants. Rather than juggling multiple parameters, she deliberately chose a reduced approach: by defining a target PAR percentage in advance, typically 40 to 60 percent of the spectrum, module density, tilt and orientation can be set so that a crop rotation over six months or a full year is reliably supplied with sufficient light.
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To do this, Natasha measured global horizontal irradiance (GHI) and converted the readings into PAR and UV values, from which she could estimate both crop and energy yields and derive an optimal system configuration. Her study compared a reference area without solar installations against the two array configurations. As expected, the reference area delivered the highest PAR values, though these exceeded what the plants actually required, so Natasha set target PAR values of 500 and 700 micromoles per square metre per second.
Designing systems around PAR values
The trial showed that at full and half module density, simulated energy yield rose well above the previous reference value, while PAR supply to the plants remained within the biologically necessary range. Because Natasha focused on PAR values between 500 and 700 micromoles per square metre per second, the results also apply to vegetables and herbs that require somewhat more light, such as tomatoes, lettuce and basil. As a next step, she plans to examine how crop and energy yields can be further optimised using tracking systems and bifacial modules. (su)
