Saturday, November 19, 2005
Green Roofs vs Roof-Mounted PV
Green Roof Blocks are 4 sq ft blocks that enable a building to have a green roof added quickly and cheaply. The blocks have seeds for drought-resistant plants and growing medium, and can be laid directly on a flat roof and secured. As with all green roofs, these reduce energy consumption and costs, decrease storm water runoff, and preserve the life of roof materials. This product has been introduced into the St. Louis and Bejing markets.(Thanks to Evangelical Ecologist for the link.)
Worldchanging also has a post on green roofs.
Now, as I've pointed out before, using roof space for PV would put a huge dent in our electricity needs, would be a necessary part of a distributed grid and would also reduce air conditioning loads.
So, which is better? On what basis should an owner determine which to install on his roof? Which, if either, should get more LEED credits?
iGreenBuild has opposing viewpoints on this issue. One, by Jerry Yudelson, says that PV should not be used on roofs due to its high cost and lost opportunity cost for conservation measures elsewhere. He state a case study where a green roof and PV shades were both installed. The other, by Bill McNeese, says that there is a place for both. He notes that PV will help with our grid infrastructure and that advances in PV would improve its costs. He also states that PV will more than likely have a larger impact when installed on windows and cleristories using thin films.
Overall, I have to agree with both: there is a place for both (although Yudelson didn't say that overtly). In retrofitting buildings in higher density areas, both should be used. Shaded areas would be common and can be used for green roofs. Green roofs would prove to be more of a benefit here because of storm water management, energy conservation and potentially even lower AC requirements than a building being shaded by roof-mounted PV. PV would be used where there is clear solar access, such as on the taller buildings.
In retrofitting buildings in lower density areas, the choice can vary depending on the circumstances. On big box stores, PV can be installed over parking lots and green roofs on the buildings themselves. Any glazing used for daylighting in the midst of a green roof can also be used as a substrate for PV. Many buildings in lower-density areas have steeper roof lines than the flat ones on skyscrapers in high density areas. Depending on how steep, these roofs might not be a good location for green roofs, but would be ideal for PV.
As implied in both opinions, in new buildings, the best approach would be a hybrid of both. Since there is more leeway in choices in a new building, it would be easy relative to this approach on an existing building. Green roofs would be used for the roof surface, while PV is installed as BIPV: on awnings, on windows, on cleristories, on shades and on skylights.
Also stated in both opinions is that energy efficiency should be a priority in design.