Tuesday, June 16, 2009

More on Reflective Roof Coatings

As Climate Progress reports, Energy Secretary Stephen Chu talked up the value of reflective roof coatings last week in Europe. So why is Chu, who used to run the Lawrence Berkeley National Laboratory and has a Nobel Prize, talking about something so simple as roof paint? It turns out that the lab has measured the value of reflective roof (and road) coatings:
Most existing flat roofs are dark and reflect only 10 to 20% of sunlight. Resurfacing the roof with a white material that has a long-term solar reflectance of 0.60 or more increases its solar reflectance by at least 0.40. Akbari et al. estimate that so retrofitting 100 m2 (1000 ft2) of roof offsets 10 tonnes of CO2 emission. (For comparison purposes, we point out that a typical US house emits about 10 tonnes of CO2 per year.) Emitted CO2 is currently traded in Europe at about $25/tonne, making this 10-tonne offset worth $250.
This is not a new insight. Researchers in the 1990s identified light (and heat) absorbing surfaces as the most significant cause of the urban heat island effect.
On a summer afternoon, central Los Angeles registers temperatures typically 5°F higher than the surrounding suburban and rural areas. Hot roofs and pavements, baked by the sun, warm the air blowing over them. The resulting urban "heat island" causes discomfort, hikes air-conditioning bills, and accelerates the formation of smog.
This accumulation of heat is not due solely to urban density:
Contrary to popular opinion, heat islands do not arise mainly from heat leaking out of cars, buildings, and factories. In summertime, such anthropogenic heat gain accounts for a mere 1 percent of the heat island's excess temperature.
There are other low tech ways to reduce the heat island effect in cities. I noted several years ago that Lawrence Berkeley National Laboratory has used its supercomputers to calculate the energy savings of street trees planted in front of buildings.


Post a Comment

<< Home