Unlike the massive fixed-tilt installations that require large steel canopies to hold photovoltaic panels above the ground, as their name suggests, VBF panels have an upright orientation and are installed closer to the surface. These walls of panels allow for easier passage of machinery, animals, and people without the need to worry about clearance height, effectively increasing both the land’s usage and the owner’s return on investment.
But because of their geometry, VBF installations do yield slightly less energy—about 5 to 10 percent less—than traditional monofacial fixed-tilt arrays. Power generation peaks for VBF arrays are different as well, given their vertical orientation, in the mid-morning and mid-afternoon, instead of solar noon for fixed-tilt installations. A team of researchers at Rutgers University has used modeling simulations to explore how changes in design can improve the energy output of VBF installations to make them competitive with conventional photovoltaic installations.
This research is detailed in “Design Considerations for Vertical Bifacial Agrivoltaic Installations,” published in the ASME Journal of Solar Energy Engineering in December.
Improvements unlocked
When laboratories shut down during the pandemic, the Rutgers team had to pivot toward a new research area that was doable within the new normal, recalled W. Ross Rucker, a post-doctoral researcher in the Department of Materials Science and Engineering at Rutgers, who completed part of this work for his doctorate.
“Modeling agrivoltaics fit neatly into that since a lot of the work could be done in our homes or outside in the fresh air,” he said. “We had originally started somewhat niche, looking at integrating solar panels into greenhouses, before joining up with the Rutgers Agrivoltaic Program [RAP] and researching more general applications.”
When considering how array design attributes affect energy output, the Rutgers team included such criteria as module height, inter-row spacing, inverter connection where rows are either wired together or separately, and inclusion of bypass diodes.
Another consideration was cell density—either single-high modules or double-high modules, where two landscape-oriented modules are stacked vertically. “Single-high modules built with bypass diodes and installed in New Brunswick, N.J., offered a small increase in power output, up to roughly 1.16 percent when rows were spaced 6.1 meters apart, with diminishing gains as the rows were spaced further apart,” the paper stated.
However, going to a double-high VBF installation resulted in a nearly doubled annual electrical output per acre, the researchers found. When increasing row spacing from 6.1 meters to 12.2 meters, VBF installations that used double-high racking and bypass diodes saw an output increase of more than 80 percent, when compared to single-high installations with row spacing at 6.1 meters. This increased to 95 percent when the double-high installations had row spacing of 24.4 meters.
“Row spacing is a vital consideration when you are planning out a VBF array, especially if you decide to stack up multiple rows of panels, and even more if you’re going to be using the space to grow crops too,” Rucker said. “A lot of the light they collect is going to be coming from a sideways angle, and if they’re too close together, the shadows cast by the panels are going to be blocking each other in a major way.”
But another major consideration for stacked rows is that the bottom modules be wired independently from the top modules—otherwise, the inter-row shading effects will reduce system electrical output, added Dunbar P. Birnie, III, a professor and Corning Saint-Gobain Malcolm G. McLaren Chair in the Department of Materials Science and Engineering at Rutgers and co-author on the study.
“We do expect somewhat lower energy production than other standard array types, but the VBF arrays seem to have the most ideal ‘ease of farming’ because they don’t impede the motion of farming equipment,” he said.
The Rutgers Agrivoltaic Program installation is one-of-a-kind, featuring vertical bifacial solar voltaic arrays in varying row spacing and module heights. With more variations, the Rutgers team will be able to learn more in the future about solar generation and its connection to farming.
Photo: A. J. Both/Rutgers University
In-field testing
Although this research has so far been based just on modeling, the RAP has built several experimental arrays at sites in New Jersey—including a VBF installation at the Rutgers Animal Farm in New Brunswick that is mostly complete.
Birnie and Rucker’s research supported both planning and design of the new arrays for the one-of-a-kind RAP installation, which features VBF solar voltaic arrays with varying row spacing and module heights to provide researchers the opportunity to test and learn more about agrivoltaics.
“During the design process for those arrays, we decided to include a vertical bifacial array. This gave extra meaning to our modeling and highlighted differences in electrical output that might be expected for different wiring patterns,” Birnie said.
RAP is funded by the U.S. Department of Energy through a grant under the Foundational Agrivoltaic Research for Megawatt Scale (FARMS) funding program, which aims to develop replicable models for agrivoltaics.
VBF will be a better choice over traditional arrays farther away from the equator, and as research continues to show more about unexpected sources of improved output in VBF arrays, Rucker believes it may only be a few more years before VBF arrays become more universal.
“Our new VBF array is coming online soon and will allow us to evaluate differences in row spacing—one of the variables tested in our paper,” Birnie said. “As with new technologies in general, there is expected to be cost reduction with time as more installations are built. We hope to collaborate with VBF array installations at different latitudes to test the latitude and climate differences.”
Initial modeling also didn’t simulate albedo light, or light that is reflected from the ground back onto the panels, “because we thought it would be a lot of processing for a very small effect,” Rucker said. “At the urging of the reviewers, we went back to add those calculations to the model and it made a pretty significant difference.”
Although albedo typically has a relatively small effect for fixed-tilt and single-axis tracker arrays, Birnie added that he was also surprised by the quantity of light that could be available from upward scattered albedo effects.
“Comparison with real output production data is critical and will help us fine tune the albedo, glass surface reflection, and thermal effects in our modeling,” Birnie said.
The study’s data indicates that VBF arrays can produce competitive amounts of electricity per acre when designed properly, further noting that “Rows of double-high modules stacked in landscape orientation and placed 6.1 meters apart produce more than 159 MWh/year/acre and still leave plenty of space for farming operations.”
For now, field studies on electrical production, crop studies, and animal studies are all underway at RAP testing installations. Birnie and Rucker are also looking into non-ideal situations and how they can affect the overall productivity of the arrays, such as rotating the array away from perfect cardinal directions and certain farmer-friendly adaptations that trade off some photovoltaic efficiency for day-to-day operational conveniences.
“Modeling the vertical panels has been really fascinating. They’re so different from traditional arrays yet so simple,” Rucker said. “The applications they have in agrivoltaics are going to be game-changing, I believe.”
Louise Poirier is senior editor at Mechanical Engineering magazine.
