With the greening of commercial roofs or, as called in Asia, “Skyrise
Gardens,” it’s only natural that collecting sunlight through
photovoltaic arrays would be the next step. Plant cover impedes
rainwater departure to the ground, streets, storm sewers, streams,
rivers, lakes and oceans. This captured moisture then helps cool the
building, reducing cooling energy costs—a win-win scenario.
What effect does a green roof have on solar collection panels and the production of electricity?
“The greened roof is notably cooler than conventional bituminous roofs: while lower temperatures lead to higher voltages at silicon-based photovoltaic panels, the electricity generation of PV on green roof is higher than on conventional roofs. We are in the process of quantifying that fact. Due to construction activity accompanying the installation of the array it was difficult to evaluate the vegetation during the first year, however, from the second year on investigation of the vegetation under the panel indicates significantly improved growth of the species relative to plant height and foliage density. There also appears to be a change in species from small plants (e.g. sedum) toward larger ones such as artemisia. Soil on the roof retains rainwater, which is in turn evaporated by the plants of the roof garden, removing heat from the building through enthalpy.” 1
Not only do the plants do better, but the building is cooled from rainwater activity. What would happen if a high-efficiency irrigation system controlled with soil moisture sensing devices was designed to maintain a constant moisture level conducive to plant growth and building cooling? Would this not also help to offset energy costs, adding to the sought after level of efficiency that is seldom achieved?
“To be more specific, high rooftop temperatures increase the conductivity of a crystalline silicon panel’s semiconductor, which in turn inhibits charge separation and lowers the voltage of the solar cell. The higher the temperature, the less efficient the panel. The absence of shading and moisture, and the presence of heat-absorbent surfaces on a rooftop can create hot, desert-like conditions. While this open exposure to sunlight makes it an ideal location for solar panels, this hot zone can decrease photovoltaic panel productivity by up to 25 per cent. Installing a green roof, however, can improve solar panel efficiency by reducing rooftop temperatures.
Traditional black roofs can reach temperatures of 158 F, and they have an enormous effect on temperatures in the building and at ground level. In comparison, the rooftop temperature of an extensive green roof rarely exceeds 77 F.” 2
At the John H Daniels Faculty of Architecture, Landscape & Design, University of Toronto located at 230 College St., Toronto, a study has been proposed that takes the current research one step further. This step includes water. GRIT (green roof irrigation test) is an ongoing study on the roof of this building to test plants, soil media and irrigation controlled through a variety of systems (view it live at http://roof-cam.daniels.utoronto.ca, user: View; password: View).
Irrigation water from a variety of green sources can be utilized…rainwater collected at the source on the rooftop collected in above roof cylinders is just one example. As stated, cooling the roof creates more energy and cooling the soil increases the building’s capacity to lose undesired heat. Green roofs once established may be able to survive with the occasional rainwater event during hot summer days.
However, loss of building cooling and lower energy production should indicate that irrigation when supplied could even out the dry-wet cycle and increase cooling and energy, thus beneficially reducing costs while being green.
GRIT 2 is a proposed four-year study about comparing solar PV arrays on top of white and green roof of approximate 75 square metres. The green roof will be installed directly on the roof surface and will be one continuous surface. Dripline irrigation with 12-inch triangular spacing with soil moisture activation control will be utilized to maintain optimum plant growth moisture levels. The moisture will also help energy production through a cooler surface plus building cooling through heat evaporation.
The proposal is for the design, construction, monitoring and analysis of the environmental performance associated with the integration of PV arrays and white and green roofs (with the design potential for integration with irrigation). The initial installation will be at the Daniels Faculty on the eastern part of the roof (21 panels), followed by an installation at the Tremco Facilities (50,000 square feet).
Solar, power, heat loss and irrigation go hand in hand. Provide water that integrates with the other elements and the system works.
Without water, heat islands reduce energy production by photovoltaic solar arrays. Remove moisture from the soil and the buildings cooling costs rise. Water seems to be an inexpensive way to have everything working in harmony. A professionally designed high-efficiency irrigation system with soil moisture control seems to be the answer to providing water in the right place at the right time.
Lorne Haveruk, CID, CWCM-L, CIC, CGIA, CLIA, WCP, Principal, DH Water Management, a water resource management consultancy firm focused on innovative rain, ground, storm, grey and city water supplied indoor and outdoor watering solutions. Visit http://www.dhwatermgmt.com.or email lorne@dhwatermgmt.com for infomation. This material is for information purposes and is not intended to provide legal advice.
References 1,2
• Photovoltaic Panels on Greened Roofs,
http://commons.bcit.ca/greenroof/publications/photovoltaic_kohler.pdf
• Systems Integration
http://www.eco-structure.com/vegetated-roof/systems-integration.aspx
• University of Toronto
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