Cool Roof 2: Cool Roof, Cool City

 Los Angeles is in the middle of a record-setting (again!) heat wave. NWS reposts that LAX airport, along the coast, recorded a high of 102 F (old record of 99 F in 2020) while Downtown LA hit 112 F. 

Several people asked about our Cool Roof and Heat Pump. I wrote about replacing our asphalt shingle roof with a composite shingle "Cool Roof" back in 2012. Our roof went from reflecting 30% of solar heat (absorbing 70% of the heat) to reflecting over 70% and absorbing less than 30%. If you look closely at the shingles, they incorporate bits of recycled glass, tumbled to a frosted gray, green or blue. From a distance, it looks like a light-medium gray. You wouldn't even know it was a cool roof unless I brag about it. 

When you install a cool roof, you mitigate the Urban Heat Island (UHI) impact of urbanization. "Georgescu et al. (2012, 2013) reported that summertime statewide warming due to projected urban expansion for Arizona could be reduced by about 50% with the complete integration of highly reflective cool roofs." SoCal would likely be similar. 

So, which should you pick, a cool roof, a green roof, or a photovoltaic (PV) solar roof? The answer turns out to be dependent on local climate and geography. Materials change the radiative impact of buildings. They can reduce the amount of heating during the day, but they can also disrupt the amount of cooling at night. They can heat the air above the city, strengthening the urban heat dome. There are so many variables, it takes a radiative full-physics model and a supercomputer to track them all. 

Take the case of Phoenix and Tucson, studied by Salamanca et al: Citywide Impacts of Cool Roof and Rooftop Solar Photovoltaic Deployment on Near-Surface Air Temperature and Cooling Energy Demand. "In Arizona, cool roofs reduced daily citywide cooling energy demand by 13–14%, while rooftop solar photovoltaic panels reduced them by 8–11% (without considering the additional savings derived from their electricity production)."

But, if you were in London, Brousse et all found that cool roofs would be most effective in reducing extreme heat. Nonintuitively, green roofs wouldn't make much difference overall (but help during the day). Vegetation makes a minimal difference on a city-wide scale. Rooftop solar might have an overall heating effect, as would running AC. But, if the rooftop solar energy was used to run AC, and people could take shelter from the heat indoors, they could be a net benefit. 

In Sydney, Khan et al found that PV solar could disrupt the cooling diurnal land-sea breeze and heat the Sydney area overall. 

Governments can encourage behaviors good for society and suppress behaviors that are detrimental with subsidies/fines. If you were running the government, you would like to find out the cost-effectiveness of different interventions (eg subsidies) and fund what gives you the most bang for the buck. If you had even more money, then fund the slightly less effective but still helpful stuff. 

COOL ROOFS reduce heat experienced inside the buildings and in the urban environment. Require them in new construction. Replace dark roofing materials with cool roofs first. If you have money left over install rooftop solar. 

In 2012, I got a 10% tax credit on the cool shingles we used, but that is only a small amount compared to total cost of installation.  The shingles cost the same as the darker colors in the same product line that absorbed more heat and didn't qualify for the tax credit. I didn't need a nudge of a few hundred bucks to do what I knew would make my house cooler. 

Adding rooftop solar was not on my radar back in 2012 because I figured that the prices were still high and technology was still changing. A few people in our neighborhood installed them, but they were much bigger energy consumers than we are. We hadn't installed a heat pump yet. 

BTW, Lawrence Berkeley Laboratory puts out an annual pricing survey called Tracking the Sun, with useful data on actual installed prices of solar systems. 

DIGRESSION:

When we got our heat pump back in 2020 (or maybe 2019?), people asked why we didn't get solar panels as well. I dragged my feet because the rooftop solar salesmen (and they are always bros) were so sleazy, and I also didn't want to buy anything made with forced labor. I was waiting for the solar supply chain to clean up. I am also waiting until I replace my roof again. The timing just wasn't right. 

Besides, I know that California has an excess of solar power during the day. It used to be worse, but utility-scale batteries have been soaking up the excess solar power so that we need to shut down feed in (curtail) solar farms less than in prior years. For the last month, we have been curtailing solar farms during all the hours rooftop solar would have produced at my location. 

Even during the heat wave this week, we had to curtail cheap solar farm power. 

Yes, even during the hottest day ever (so far) in LA. 

There just wasn't much value-add for society if I add solar panels to our cool roof.