Photosynthesis dynamics’ effects on yield in standard agrivoltaics and even lighting configurations

Crop yields in agrophotovoltaics (APV) suffer from heterogeneity and general low level of
irradiance reaching crops, forcing farmers or project developers to increase spacing between modules. Even-lighting (EL) consists in placing diffusers between modules to distribute light more evenly, to avoid stark sun/shade alternance and mitigate the effect of APV on yield [1].
To explore the theoretical benefits of EL, we conducted simulations of photosynthesis
response to irradiance profiles measured on a sunny day (August 8th, 2023, Belgium,
unpublished data) under an APV structure (four 1.4 m solar modules, 0.6 m spacing) and an EL prototype (identical, but with diffusers between solar modules), designed to diffuse light laterally with a 35° angle and mitigate the alternance of shade and high irradiance on the canopy. Irradiance profiles are presented in Figure 1 (a). Local peaks are observed both at 10:45 AM and 7:30 PM in APV and not in EL which is likely attributable mostly to the diffusers material which, at this particular sun elevation, resulted in total reflection of sun irradiance. Irradiance levels under EL in shade periods (grey background) are always at least equal to or higher than under APV, and much lower in the main peak from 2 to 4 PM.
The photosynthesis simulations take into account (i) the photoprotection developed by plants as acclimation to high irradiance and (ii) this process’ reversibility delay [2]. This approach allows us to analyze transitory response of photosynthesis to shade-to-sun and sun-to-shade dynamics in APV and EL. Simulations were run for horizontal leaf unit surface with parameters representative of soybean [2]. We compare CO2 assimilation rates (Figure 1 b) and cumulated assimilations (Figure 1 c). Figure 1 (b) displays the benefits of even lighting in terms of assimilation rates, especially around solar noon where irradiance is distributed over a longer period of time by means of the diffusers, giving a clear advantage to assimilation rates under EL from 2 to 4 PM EL also performs better in shade periods around solar noon (i.e. 11 AM to 2 PM and 4 PM to 7 PM) due to lower photoprotection levels. Integrated results (Figure 1 c) show that plants in the EL configuration received 86% of the APV configuration total solar energy. However, the plants under EL assimilate 17% more CO2 than under APV. This is due, in part, to photosynthesis maintaining a better efficiency overall in the EL setup than in the APV where high irradiance between 2 PM and 4 PM means higher photoprotection and thus reduced photosynthesis efficiency, as illustrated in Figure 1 (b). Improvements lie in exploring more diverse irradiance profiles since these results are highly dependent on the prototype design and materials used for irradiance trials on that specific date, testing more complete dynamic photosynthesis models (i.e. considering Rubisco activation delays), other species relevant to APV, and finally modelling carbon assimilation and plant growth within a functional structural plant model simulated canopy to analyze not only CO2 assimilation rates on horizontal leaves but resulting crop yield.