Green Roofs - Living Architecture Monitor - Summer 2010 - (Page 25)

n recent years, there has been a growing interest in urban rooftops from multiple distinct subgroups within the sustainable development community. Some are interested in using rooftops for distributed power generation with wind turbines or photovoltaic (PV) panels, or the tremendous potential for highly reflective roofs to save energy and cool the urban and global environment. Others are interested in developing a large infrastructure of vegetated green roofs to help reduce stormwater runoff while providing a host of other environmental benefits. Without debating the relative merits of any single use of the rooftop resource, a group of researchers at Portland State University (PSU) in Portland, Oregon has set out to explore the potential benefits of integrating multiple technologies. Specifically, through a research grant from the National Science Foundation, PSU is exploring the potential synergistic interactions between PV power generation and green roofing, when installed together on a single roof. The underlying premise of the research is that the green roof will help to cool the PV panels and thus improve their operating efficiency. At the same time, it is expected that the PV panels will improve vegetation health and biodiversity by providing shade during the hot and dry summer months. This project seeks to quantify these effects and explore the potential to optimize the design of such integrated systems. EXPERIMENTAL FACILITY The experimental facility for this research consists of four green roof pans located on the rooftop of Portland State University’s Science Building 2. The facility has relatively unobstructed sky view with the only significant obstruction being a greenhouse wall to the north of the site. Each green roof pan is 15-feet by 12-feet and is planted with a mix of Sedum species (S. acre, S. pachyclados, S. spurium, S. tematum, S. album, S. rupestre, S. divergens, and S. Angelina) in approximately six inches of pumice-based growing media. Four Solar World 170 W monocrystalline silicon panels are mounted above the back side of each pan (see image at right). The facility will be fully instrumented with a weather station, temperature sensors in multiple locations on each panel and within the soil, soil moisture sensors, sensors for measuring vegetation surface temperatures, and data logging power monitoring equipment for each panel. The weather station will record temperature, humidity, wind speed/direction, and precipitation. Flow meters will also be mounted at the drains for each pan. The entire monitoring system is in the final phases of being deployed and will eventually be integrated with an Internet-accessible database tool. GREEN ROOF IMPACTS ON PV PANEL EFFICIENCIES The vast majority of PV materials are based on semiconductors, primarily silicon, for which elevated temperatures lead to an increase in resistivity and a loss of overall efficiency. A general rule of thumb is to expect a loss of 0.5 percent in power output for every degree rise in temperature above standard test conditions (25°C), but more dramatic losses have been documented. The array of four Solar World PV panels installed over the back part of each green roof pan are all monocrystalline silicon panels; these are the most efficient and most common type of silicon PV material, but also the most highly temperature dependent. It is well-documented that green roofs can result in lower, rooftopsurface temperatures and lower near-surface air temperatures (particu- I THE GREEN ROOF PHOTOVOLTAIC TEST FACILITY LOCATED AT PORTLAND STATE UNIVERSITY IN OREGON larly on hot days with calm winds). It stands to reason then that green roofs could result in reduced long-wave radiative heating of the underside of PV panels and also enhance convective cooling of panels, leading to improved panel performance. This effect will be quantified in this project by simultaneously measuring roof temperatures, panel temperatures, incident solar radiation, and panel power output for panels installed above traditional roofing and above vegetated green roofs. IMPACT OF PV ON PLANT HEALTH & DIVERSITY The green roof environment can be a very stressful habitat, experiencing periodic drought and rapid fluctuations in soil moisture and temperature. As a result, green roof plantings have historically focused on shallow-rooting, low maintenance, drought-tolerant species such as Sedum or drought-tolerant grasses. As a plant characteristic, drought tolerance is generally negatively correlated with leaf transpiration rate and photosynthetic carbon gain. Furthermore, most green roof plant species are selected for full-sun exposure, using species from very exposed habitats. The partial shade provided by the PV array may result in a mix of conditions that would support a wider range of plant species, and potentially provide for a more robust and diverse rooftop ecosystem. The impact of PV panels on the vegetation will be evaluated in this project using sampling methods to evaluate plant species diversity, above/below ground plant biomass, and measurements of plant respiration/transpiration. These measurements will be conducted within the shade footprint of the panels and, also, in nonshaded control patches of green roof. LIVING ARCHITECTURE MONITOR SUMMER 2010 25

Table of Contents for the Digital Edition of Green Roofs - Living Architecture Monitor - Summer 2010

Green Roofs - Living Architecture Monitor - Summer 2010
Contents
From the Founder
Strata
On the Roof With...
What is Integrated Design?
How Do We Move the Practice of Integrated Design Forward?
West Coast Integrated Design
Integrated Design for Solar & Green Roofs
Do Green Roofs Improve the Efficiency of PV Panels?
An Invitation to CitiesAlive! in Vancouver this Fall
GHRC Professional Development Calendar
Roof to Plate
Research Showing Early Promises
Advanced Green to Roof Maintenance Course to Launch
New GRPS
New Online Tools
New Corporate Members
Making Government Programs Work Harder

Green Roofs - Living Architecture Monitor - Summer 2010