The Case for Rooftop Agrivoltaics: Innovation at the Food-Energy-Water Nexus
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Jennifer Bousselot giving a tour of the rooftop agrivoltaics plots at Colorado State University (CSU) Spur campus on the Hydro building rooftop. Photo: Kevin Samuelson, CSU Spur.
Introduction
Nearly three years ago our team introduced the concept of rooftop agrivoltaics in the Spring 2022 edition of the Living Architecture Monitor in the article entitled Exploring the Potential of Rooftop Agrivoltaics. Rooftop agrivoltaics involves integrating green roofs for food production under solar panels. Since that time, we have expanded our capacity and explored this topic from many angles at two research locations. Colorado State University (CSU) as an institution, as well as our team specifically, have invested significant resources in the infrastructure for, and investigation of, the new opportunities that rooftop agrivoltaics can provide.
In our earlier article, we reported on the preliminary findings from a small-scale simulated rooftop agrivoltaics system at our CSU Foothills campus in Fort Collins, Colorado. This research is in conjunction with our CSU physics and mechanical engineering laboratories that examines the possibilities of solar energy production and solves problems for the solar manufacturing industry. Two years ago, CSU opened a new rooftop agrivoltaics research facility that is approximately three times larger on our Denver-based campus called CSU Spur. We have 102 two square meter silicon solar panels installed over an 18 inch deep American Hydrotech green roof system. Sempergreen sedum mats surround our crop research plots to add year-round interest and provide separation between food growing plots. Click here to view our rooftop agrivoltaics research fly through two minute video of the facility produced by our communications team.
Rooftop agrivoltaics array on the Colorado State University (CSU) Spur campus rooftop on the Hydro building. Photo: Kevin Samuelson, CSU Spur.
What We Already Know
Crop production on green roofs has already been implemented in multiple cities, including New York City, Montréal, Toronto and Paris. There have been rooftop farms as large as 14,000 m2, providing produce directly to the city (Brooklyn Grange; IGA Supermarket of Quebec; Nature Urbaine, 2024). Rooftop agrivoltaics, which is defined as growing food under solar panels on rooftops, can add a synergistic renewable energy source to rooftop farms. Rooftop agrivoltaics are an innovative system that can supplement our food supply, energy production, and water conservation efforts in cities.
Studies have found that substrate moisture is higher under solar panels than in comparable full sun conditions (Barron-Gafford et al., 2019, Bousselot et al., 2017; Dinesh and Pearce, 2016). Due to the microclimatic conditions that form under the shaded, more moderated temperature solar panel arrays, humidity is higher under the solar panels. Plant water stress can be reduced when growing underneath the solar panels. Coupled with protection from intense direct solar radiation, this reduces overall irrigation needs for agrivoltaic plants, relative to plants growing in full sun.
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Energy production is the other primary ‘harvest’ from rooftop agrivoltaics systems. Plants grown under solar panels evapotranspire moisture, and therefore help cool the solar panels above them. This cooling effect reduces the panel temperature of the solar modules, allowing them to operate at a more optimal temperature and therefore produce energy more efficiently, approximately 5-8% more energy (Köhler et a., 2007; Hui and Chan, 2011).
There are many kinds of solar panels on the market now, most of which are opaque, and silicon based. Cadmium telluride (CdTe) solar panels are a more recent invention that our team has investigated for use in rooftop agrivoltaics research. CdTe has a higher theoretical efficiency than silicon based solar panel systems, and the panels themselves require less materials than silicon panels to produce. This technology results in a thin-film glass panel, which can also be more cost effective to produce.
Due to the characteristics of the thin-film panels, CdTe modules can be fabricated at incremental light transparency levels. These semi-transparent panels can protect the plants below them from harsh environmental conditions like inclement weather, intense solar radiation, and strong winds while still providing light, but less intensely. Semi-transparent panels are similar to greenhouse glass or what humans often use to prevent too bright of light - sunglasses! Currently, there are no grid-ready domestic UL certified semi-transparent CdTe panels, so they are not yet available for commercial rooftop installation.
Eliza Gross, Masters student, transplants her chile pepper research under the rooftop agrivoltaics plots at the Colorado State University (CSU) Spur campus rooftop on the Hydro building. Photo: Kevin Samuelson, CSU Spur.
Our Ongoing Research
Too much shade from opaque solar panels can cause plants to stretch or etiolate. Etiolation is a well-documented shade avoidance response in plants. Etiolation is a process in flowering plants grown in partial or complete absence of light. It is characterized by long, weak stems; smaller leaves due to longer internodes; and a pale yellow color. For some crops, like leafy greens, etiolation is a beneficial side effect. But for flowering and fruiting crops such as chile peppers, plant stretching can lead to taller yet less structurally dense plants that are prone to things like wind damage (Gross et al., 2024).
Another reality of the shaded conditions of rooftop agrivoltaics is a delay in flowering that crops experience as a result of low light conditions. Depending on the crop, most plants that we have investigated to date end up blooming at least two weeks later under solar panels. The delay is only part of the story as overall bloom is reduced under those conditions, too. Of course, it can be argued that the penalty in yield is more than made up for by adding in the financial benefit of clean energy production occurring in the same space.
Members of the Bousselot research team discuss layout of research on the Colorado State University (CSU) Spur campus green roof on the Hydro building. Photo: Kevin Samuelson, CSU Spur.
Non-crop plants such as pollinator plants show a similar response in rooftop agrivoltaics systems. We characterized rooftop agrivoltaics environmental growing conditions in Uchanski et al. (2023) and then reported the larger plant size, yet delayed flowering of pollinator plants in Hickey et al. (2024). There is evidence that ornamental plants will overwinter at higher rates when protection is provided by the solar panels which produce a more favorable environment for plant survival. (Bousselot et al., 2017; Hickey et al., 2024).
Our recent studies on leafy greens have shown that they produce higher yields under 40 percent semi-transparent solar panels compared to full sun and opaque solar panels (Villa-Ignacio 2024). The ideal level of transparency is still under investigation for optimizing both plant growth and energy production. We are currently evaluating 20 percent semi-transparent panels in an upcoming raspberry production research project. These panels filter out light evenly across the spectrum.
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Another possibility for expanding the water conservation opportunities of rooftop agrivoltaic systems is to incorporate the use of an atmospheric water generator (AWG). At CSU Spur, we will be collaborating next year with a company that manufactures AWG systems to evaluate the practicalities of incorporating these systems into rooftop agrivoltaics.
Saffron flower in bloom on November 26, 2024, under the rooftop agrivoltaics array at Colorado State University (CSU) Spur campus on the Hydro building rooftop. Photo: Jennifer Bousselot.
Conclusion
Now that we have begun the process of characterizing the conditions and typical plant response to the rooftop agrivoltaics environment, we can start predicting and evaluating the iterative responses that certain crops and ornamental plants will produce. An example of both an expected and an unexpected response is from the crocus plant that produces culinary saffron. As we expected, flowering was delayed under the solar panel conditions. However, we didn’t expect that the moderated temperature and moisture conditions would allow this fall harvested crop to produce weeks, and even months, later in the year. Watch for our team to continue to evaluate and report on these high value, shade-tolerant crops for use in rooftop agrivoltaics systems.
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Jennifer Bousselot, PhD, GRP, is an Assistant Professor of Horticulture at Colorado State University. She and her research team investigate crop production, pollinator plant production, and stormwater management capacity of green roofs, green infrastructure, and rooftop agrivoltaics systems.
Armando Villa-Ignacio, GRP, is a PhD student in the Bousselot lab at Colorado State University studying raspberry production in rooftop agrivoltaics systems. Armando completed his Masters of Science in Horticulture in June, also at CSU studying leafy green production in rooftop agrivoltaics systems.
References
Barron-Gafford, G., M. Pavao-Zuckerman, R. Minor, L. Sutter, I. Barnett-Moreno, D. Blackett, M. Thompson, Y. Dimond, A. Gerlak, G. Nabhan, and J. Macknick. 2019. Agrivoltaics provide mutual benefits across the food–energy–water nexus in drylands. Nature Sustainability, 2: 1-8. https://doi.org/10.1038/s41893-019-0364-5.
Bousselot, J.M., T. Slabe, J.E. Klett, and R.D. Koski. 2017. Photovoltaic array influences the growth of green roof plants. Journal of Living Architecture, 4(3): 9-18. https://doi.org/10.46534/jliv.2017.04.03.009.
Brooklyn Grange. Green Roof System and Eco-friendly landscaping in Brooklyn. https://www.brooklyngrangefarm.com/.
Dinesh, H. and J.M. Pearce. 2016. The potential of agrivoltaic systems. Renewable and Sustainable Energy Reviews, 54: 299-308. https://doi.org/10.1016/j.rser.2015.10.024.
Gross, E., M. Chavez and J.M. Bousselot. 2024. Chile pepper production in a Colorado rooftop agrivoltaic system. In Proceedings of CitiesAlive! 2024. Toronto, Canada. https://citiesalive.org/s/Gross-Chavez-and-Bousselot-CitiesAlive-2024-Research-Paper.pdf
Hickey, T., M. Chavez and J.M. Bousselot. 2024. A rooftop agrivoltaic system: Pollinator plant establishment. Journal of Living Architecture, 11(1): 41–55. https://doi.org/10.46534/jliv.2024.11.01.041.
Hui, S.C. and M.S. Chan. 2011. Integration of green roof and solar photovoltaic systems. In Paper submitted to Joint Symposium.
IGA Supermarket of Quebec. Canada’s Largest Organic Garden on a supermarket rooftop. https://www.iga.net/en/in_the_community/environment/frais_du_toit.
Köhler, M., W. Wiartalla and R. Feige. 2007. Interaction between PV-systems and extensive green roofs. In Proceedings of the 5th Greening Rooftops for Sustainable Communities Conference.
Nature Urbaine. Notre ferme. 2024. https://www.nu-paris.com/notre-ferme/.
Uchanski, M., T. Hickey, J.M. Bousselot and K.L. Barth. 2023. Characterization of agrivoltaic crop environment conditions using opaque and thin-film semi-transparent modules. Energies, 16(7): 3012. https://doi.org/10.3390/en16073012.
Villa-Ignacio, A. 2024. Evaluating leafy green production in a Colorado rooftop agrivoltaic system. Evaluating Leafy Green Production in a Colorado Rooftop Agrivoltaic System - ProQuest.