Rooftop gardens are valuable in many ways to people and buildings. These gardens produce oxygen, reduce building energy costs, capture rainwater and reduce stormwater runoff, create habitats for wildlife, and offer beautiful natural spaces for the enjoyment of building inhabitants, among other benefits. Large rooftop gardens can even promote health and wellness by offering walking paths and other fitness opportunities.
The Environment and Natural Resources 2 Building on the University of Arizona’s main campus is LEED Platinum certified and was originally designed to feature a green roof with solar panels. In 2019, the Office of Sustainability, in partnership with Planning, Design & Construction, the School of Geography & Development, the School of Natural Resources & the Environment, the School of Landscape Architecture & Planning, the Institute of the Environment (now the Arizona Institutes for Resilience) and Tucson Electric Power (TEP), received approval to move forward with a green roof and solar panel installation.
How Does It Work?
This project will couple the solar panels with hardy rooftop garden plots in a system called “agrivoltaics”. The system combines the green roof garden (“agri”-culture) with solar technology (photo-”voltaics”) to create a hybrid that maximizes the efficiency and benefits of each.
Why use agrivoltaics? Typical solar panels perform optimally at 77F, 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 living green roof below the panels allows transpiration from the plants to cool the panels, in some cases by as much as 20F, 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%.
The construction on the green roof component of the project (Phase 1) began in November 2019, with completion scheduled for the end of January 2020. The installation of the solar panels (Phase 2) is expected to begin in May 2020, with completion scheduled for the end of summer 2020. Once fully complete, the green roof will be planted with a variety of native, pollinator-friendly plants and will be maintained in a manner that provides habitat and food resources for our native pollinators.
On January 8, 2020, the CORE construction crew at ENR2 erected a crane and flew bags of soil (weighing 2,500 pounds each!) onto the roof and poured the soil into the planters. The building occupants looked out the north-facing windows to see the crane build itself up and hoist the ton-and-a-half bags of soil six stories into the air.