Agrivoltaics on rice fields, not a lost cause

Researchers in Japan have made another attempt to make agrivoltaics on rice fields technically and economically feasible, despite well-known productivity issues when rice is grown below solar modules. They used double-axis tracking, finding potential with careful management of shading and tilt angles.

Maintaining high crop productivity in rice fields hosting solar panels remains a major concern for agrivoltaic projects, as demonstrated by a recent research project conducted by the University of Tokyo in Japan. This six-year experiment showed that the grain yield in the control plot was 8.5 t per hectare, while in the agrivoltaics field, it was 6.5 t per hectare, representing a 23% reduction.

The results of this work confirmed that grain yield in the agrivoltaics system would be limited by the reduced biomass and the reduced panicle number, which are critical traits for rice productivity. Grain quality, and particularly the grain chalkiness and the head rice yield, were also found to deteriorate under partial shading in the agrivoltaics system.

With these issues in mind, another research group from the University of Tokyo has investigated whether agrivoltaic systems based on dual-axis trackers may achieve better results in terms of rice productivity.

They analyzed the performance of an agrivoltaic facility with trackers at an 830 m2 rice paddy in Miyada-mura, Nagano Prefecture, Japan. “The system was designed to adjust panel angles daily and seasonally, prioritizing rice growth during the planting season and maximizing energy production during the off-season,” they explained.

The system utilized 352 PV panels with a power output of 130 W and a size of $\mathrm{1,504}\mathrm{mm}\times 679\mathrm{mm}$. They were grouped into units of 7 to 8 panels and rotated through six torque motors. Galvanized materials were used to withstand strong winds of up to $25m/s$. “The PV panels are remotely controlled and rotated using torque motors around the main east-west axis,” the research group emphasized. “The inclination of PV panels along the north-south axis is regularly adjusted at an angle of between 10 and 30 deg from the horizontal.”

During the April-August period, the modules were positioned at 11 degrees from the horizontal, while in September and October they were placed at 20 degrees. From November to the end of January, they were positioned at 30 degrees and from February to the end of March at 20 degrees.

These experiments showed that, during two growing seasons, rice yields under the panels were 75% and 85% lower compared to benchmark paddies without modules located nearby. However, the scientists found that, while slightly lower in the first year, yield improved significantly in the second year after fine-tuning the amount of sunlight reaching the crops.

“Importantly, the rice also met Japan’s highest grain quality standards,” they further explained. “The average content of protein was 6.34% as compared to 6.47% for the rice produced in the local village region, whereas that of amylose was 18.71% as compared to 18.67%.”

The PV system was found to be able to generate 961.4 kWh/kW, a value that the researchers described as comparable with agrivoltaic systems in Europe.

Their findings are available in the paper “Case study of rice farming in Japan under agriphotovoltaic system,” published in the Journal of Photonics for Energy.

“The study underscores the tradeoffs involved in balancing crop productivity with solar energy output,” the research team concluded. The researchers noted that careful management of shading, including adjusting the panels’ angles throughout the day and season, can help achieve both goals. They also highlighted future directions such as using AI to optimize sunlight sharing in real time and experimenting with high-efficiency or semi-transparent solar panels to further reduce crop shading.