Vertical Bifacial Solar Panels: A Winning Solution for Agrivoltaics and Farmers

      In 2022, we developed our experimental design and research methods for the full intended agrivoltaic experiment, which was originally planned to begin that year and to be repeated in 2023 and 2024. We would grow specialty crops in one field between two rows of the vertical agrivoltaic array and in the neighboring field in full sun using the same randomized layout. After it became clear that the installation of the vertical array would be delayed beyond the 2022 growing season, we adjusted our plans and instead grew crops only in the full-sun reference field, in a reduced version of the complete experimental layout. The reference data collected from these crops would support our understanding of their typical growth outside of an agrivoltaic system.

      We used three full-sun crop beds in 2022 to grow root vegetables (beets and carrots), climbing vegetables (pole beans and snap peas), mini head lettuce, and saffron. As planned for future years, each crop category was grown in three repeated blocks. Root and climbing vegetable blocks were randomized separately to determine which crop type (beets or carrots, beans or peas) would be on the north or south end of each of the block. We direct-seeded ‘Boro’ beets (Beta vulgaris ‘Boro’) and ‘Negovia’ carrots (Daucus carota ‘Negovia’) (High Mowing Organic Seeds, Wolcott, VT) in all three crop beds on July 8 and harvested a random sample of beets on September 2 and carrots on October 7. We direct-seeded beans and peas at the southmost end of the outer two crop beds on July 26, installed trellises for them to climb, and harvested all mature beans on October 2 and peas on October 2, October 21, and October 31. We planted lettuce seeds in plug trays on July 7, transplanted seedlings to the field in the southmost end of the center crop bed on August 3, and harvested mature lettuce heads on September 30. We planted saffron corms just south of the beets and carrots on August 24 and harvested flowers as they emerged between October 28 and December 8. All crop beds were drip-irrigated on an as-needed basis by UVM farmer trainees.

      After harvests in 2022, we collected yield data as planned for future years, using a random subsample of individual plants of each crop type from each crop bed in each block. For beets, we recorded the total number of beet roots per randomly sampled area and we collected data for individual unwashed beet roots with leaves removed. This included mass, maximum diameter, and a subjective damage rating. For carrots, we recorded the total number of carrots per randomly sampled area and we collected data for individual washed carrots with leaves removed. This included mass, maximum diameter, length, and subjective damage and quality ratings. We counted bean and pea pods and collected data for each pod including mass, maximum width, length, number of beans or peas per pod, and subjective damage and quality ratings. We counted lettuce heads and collected data for each washed head including mass, maximum circumference, length, and a subjective damage rating. For saffron, we observed mass as the total combined dry weight of all stigmas collected from each crop bed in each block over the entire harvest period. These data represent typical full-sun crop growth.

      In 2023, after learning that the installation of the vertical array would again be delayed beyond the growing season, we designed shade fence structures to mimic the conditions expected in three different locations within the planned agrivoltaic system. The UVM vertical solar array plan involved three rows of solar modules running north-to-south with inter-row spacing of 9.14 m (30 ft.) east-to-west. Each row would be ~2.74 m to ~3.05 m (~9 ft to ~10 ft) tall, including ~0.61 m to ~0.91 m (~2 ft to ~3 ft) of above-ground empty space underneath two stacked solar modules. We selected our shade fence parameters based on output from the Dual-Use Shading Analysis Tool from the Massachusetts Department of Energy Resources, which predicts shade on crops in customized agrivoltaic installations. We designed the shade fences so that the shade cast on the two crop rows of interest would match a range of shade levels (by duration and time of day) predicted for different crop beds in the 9.14 m (30 ft.) space between two rows of our planned vertical agrivoltaic system. We developed a 2x4 factorial completely randomized block design with two levels of crop type (beet and carrot, our main crops of interest), four levels of shade (no shade, brief morning shade, brief afternoon shade, and moderate duration afternoon shade), and three blocked replications (south, center, and north). Each block contained each combination of crop type and shade level. We collected data on microenvironments and crop yield in each shade level.

      We installed the shade fences in 2023 in two crop beds ~1.22 m (~4 ft) wide. The beds were spaced ~4.88 m (~16 ft) apart to limit shading of crops in one bed by fences in the other bed. Due to the short height of the shade fences, which was required for structural stability, we designed their sizes and installation locations so that the intended shade treatments would affect two of the three rows of crops in each crop bed. Each “morning shade” fence was offset to the east of its two main crop rows, with the third row to its east. Each “afternoon shade” fence was offset to the west of its two main crop rows, with the third row to its west. We made the shade fence panels using 90% UV light-blocking woven black plastic (HDPE) shade cloth (Coolaroo USA, GALE Pacific USA, Inc., Charlotte, NC). Each panel was ~3.20 m (~10.5 ft) long with a 1.22 m (4 ft) fiberglass stake in a long sleeve at each end for structural support. Fence panels were centered between two 2.13 m (7 ft) steel t-posts buried ~0.91 m (~3 ft) deep and spaced ~3.66 m (~12 ft) apart. They were attached to the end posts with polyester paracord using adjustable knots, and were also tied to a 1.83 m (6 ft) t-post at the center of each panel, buried ~0.61 m (~2 ft) deep. For “brief shade” treatments (fewer hours of the day in shade), fence panels were 0.61 m (2 ft) tall, installed with ~0.61 m (~2 ft) of empty space below the panels. For “moderate” duration shade treatments, panels were 0.91 m (3 ft) tall, installed with ~0.15 m (~0.5 ft) of empty space below the panels. To reduce wind damage to the shade fences, we cut semicircular wind vent flaps in the shade fabric, which were 42.55 cm (16.75 in) in diameter. “Brief shade” panels had four wind vent flaps in one row across the panel, while “moderate” duration panels had two rows of four flaps across the panel. We added larger secondary flaps on the panels to limit sunlight leakage in low-wind conditions. These were only installed on panels next to carrots due to time constraints.

      For the 2023 shade fence experiment, we direct-seeded ‘Boro’ beets and ‘Negovia’ carrots on June 15. We installed shade fences from June 22 to July 3. We redistributed some beets to fill empty areas and thin dense areas on July 20. Starting on August 11, we conducted microenvironmental monitoring using sensors that automatically logged data every five minutes. We measured sunlight as PAR (photosynthetically active radiation), soil moisture, soil temperature, ambient temperature, and relative humidity for each replication of each shade treatment for carrots. On August 14 and 15, we thinned carrots and beets in the field. We harvested beets on August 23, August 29, and September 6 (south, center, and north blocks, respectively) and carrots on September 12, September 21, and October 10 (south, center, and north blocks, respectively). From the two rows with intended shade treatments applied (or all three rows for full-sun controls), we harvested all beets and carrots in the northmost portion of each 3.66 m (12 ft) experimental section, not counting a 0.30 m (1 ft) buffer on the north end. For beets, the length of this sampling section was 1.83 m (6 ft) south of the buffer, and for carrots it was 1.22 m (4 ft). For each shade level in each replication, we measured the total (unwashed) fresh weight of leaves removed from beets and carrots. We then collected yield data from all harvested beets and carrots after washing, except for the full-sun control carrots, from which we measured a randomly selected two-thirds of the roots. We used yield measures similar to those used in 2022 for the full-sun reference crops.

      In 2024, we repeated the shade fence experiment in two fields using the experimental design from 2023, with crop and shade locations randomized separately for each field. All shade panels had secondary flaps covering the wind vents. For the first field experiment, we installed complete shade fences by June 26. We direct-seeded ‘Boro’ beets and ‘Negovia’ carrots on June 28. Starting on July 7, we conducted microenvironmental monitoring for all carrot sections. On July 16 and 17, we reseeded bare spots in the field. From July 29 to August 1, we thinned beets and carrots in the field. We harvested beets on September 23, September 27, and October 9 (south, center, and north blocks, respectively) and carrots on October 16, October 28, and November 7 (south, center, and north blocks, respectively). For the second field experiment, we installed shade fences without secondary flaps covering all wind vents by July 19 and direct-seeded ‘Boro’ beets and ‘Negovia’ carrots on that date. We finished adding secondary flaps over wind vents on July 26. We did not conduct regular microenvironmental monitoring in this field. On August 2 for carrots and August 6 for beets, we reseeded bare spots in the field. On August 16, 23, and 27, we thinned beets and carrots in the field. We harvested beets on November 20 (south block) and December 11 (center and north blocks) and carrots on December 3, 4, and 5 (south, center, and north blocks, respectively). For both fields, we harvested from all three rows of beets and carrots in each shade section and collected data separately for each row. As in 2023, we harvested only the northmost 1.83 m (6 ft) just south of a 0.30m (1 ft) buffer for each experimental section of beets. We did the same for carrots in 2024. Again, we measured the total (unwashed) fresh weight of leaves removed from beets and carrots for each experimental section and collected post-washing yield data similar to 2023.