In Fall 2017, Students for Sustainability (SFS), the Office of Sustainability, the Community & School Garden Program, the Schools of Landscape Architecture, Natural Resources & the Environment, and Geography, Development, & the Environment, as well as numerous staff from across campus began to lay the foundation to place an “agrivoltaic” or PV+ system on the roof of the Environment & Natural Resources 2 Building (ENR2).
The term “agrivoltaic” refers to the development of the same area for both photovoltaic energy and agricultural production. In simpler terms, it is the cultivation of partial-shade tolerant plants, "agri"-culture, beneath an array of solar panels, or photo- "voltaics." Similar to agrivoltaic systems, "PV+" systems combine photovoltaic panels and the growth of non-agricultural plants, often pollinator-friendly species, to provide additional ecosystem services.
Why use agrivoltaic or PV+ systems? Typical solar panels perform optimally at 77°F (25°C), but temperatures in the Tucson area regularly exceed that threshold throughout the year, and direct solar exposure further exceeds this optimal performance temperature. Installing a green roof below the solar panels allows transpiration from the plants to cool the panels, in some cases by as much as 20°F (11.2°C), increasing the panels’ efficiency and their electrical output. In turn, shade from the panels both cools the plants and helps prevent water loss in extreme heat, increasing plant productivity by as much as 300%.
In Fall 2018, the Office of Sustainability secured $500,000 from the Division of Business Affairs and $10,000 from the Institute of the Environment (now the Arizona Institutes for Resilience: Solutions for the Environment and Society) to support the installation of the green roof. The installation of the solar panels, was supported by accumulated energy rebates from Tucson Electric Power and by the Large Scale Renewable Energy Agreement reached by the University and TEP in 2019.
Phase 1 - the installation of the green roof - began in November 2019 and was completed in early February 2020. More information on the green roof installation can be seen in this video:
Phase 2 - the installation of the solar panels - began in November 2020, and was completed in January 2021. Due to the COVID-19 pandemic, the green roof planting will be delayed until summer 2021 with the onset of the summer monsoons.
The roof will be seeded with a variety of native, pollinator-friendly seeds that are locally and sustainably sourced. The planting areas will then be maintained in a manner that provides habitat and food resources for our local pollinators. We will also install a number of sensors and other instruments that will allow us to gather data over the next year or longer.
Agrivoltaic and PV+ systems, especially rooftop systems, are an emerging area of research, particularly in arid and semi-arid environments and in areas with limited space and competing land-use needs. Agrivoltaic and PV+ systems could eventually improve sustainability on a large scale, resulting in a 60% increase in land use efficiency through the simultaneous production of electricity and food/ecosystem services.
Our approach is to designate 20 study plots, with half of the plots having a 5” planting medium depth, and half having an 8” planting medium depth. For each depth, we will designate five replicate plots beneath the solar canopy, and five with open sky exposure, so that the plots can be directly compared. From this setup, we expect to obtain quantitative data and insight into the establishment differences between plants grown under solar panels and plants grown under standard open sky conditions, as well as solar panel temperature and performance data correlated to the establishment of the plants. This knowledge will be essential to demonstrate the benefits of agrivoltaic and PV+ systems to both the green roof and solar panel professional communities, encouraging the more efficient integration of green spaces with solar energy installations.
Want to know more about the Rooftop PV+ Project? Want to get involved with the research or start conducting your own research? Fill out our SmartSheet form.
The rooftop is publically accessible during normal building hours, 8am-5pm Monday-Friday. You can access the rooftop through the stairwell on the fifth floor on the north side of the building. Please be respectful of the plants and ongoing research by staying off of the green roof!
For tours of the green roof, contact Trevor Ledbetter, Director for the Office of Sustainability, at firstname.lastname@example.org. For tours of the ENR2 Building, contact Maggie Heard, ENR2 Building Manager, at email@example.com.