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How Should We Use Our Roofs?

Covering roofs with solar panels will block cool roofs.

Have you heard of cool roofs? The idea is that if we paint a lot of roofs white, or generally make them more reflective, they won’t absorb as much of the sun’s energy. I’ve always thought of them as a kind of a gimmick – maybe they’ll shave a couple bucks off a building’s air conditioning bill, but they’re not something that will do much to address climate change.

Boy, was I wrong! I was recently at a talk by a physicist who helped me realize that the air conditioning savings are only part of the story. And, given the California Energy Commission’s recent decision to require that new homes install solar panels, I’m worried that we’re going to be misusing our roof space.

Here’s my non-technical understanding of how cool roofs work. In general, we’re worried about greenhouse gasses because they trap the heat that comes into the Earth’s atmosphere every day from the sun. But, what if the sun didn’t create as much warmth? Think about a parking lot in the hot sun – most of us know that if we’re walking barefoot, we should stick to the white lines and avoid the black pavement.

Not only do white roofs stay cool themselves, but they also transfer less heat to the atmosphere. The arrows in the figure below illustrate what’s happening. The yellow arrows show the energy from the sun coming in and hitting the white roof on the right or the black roof on the left. The red, white and orange arrows depict the heat that’s transferred back to the atmosphere and the lengths are proportional to heat flows. So, the black roofs send a lot more heat back to the atmosphere compared to white roofs. The small red arrows below the roofs show that black roofs heat up the building a lot more, too.

Source: Heat Island Group, Lawrence Berkeley National Laboratory

From an economics perspective, cool roofs are a great example of a positive externality – they provide benefits both to the people who own them as well as to others. Markets don’t provide enough positive externalities. People are underinvesting in cool roofs because they’re only thinking about the lower air conditioning bill and not accounting for the benefits to the rest of us in terms of less heat in the atmosphere. Externalities provide a really good justification for some kind of government intervention. It could make sense, for example, to have building codes that require light-colored roofs.

Cool roofs are potentially a pretty big deal, too. Several articles (summarized here) have shown that making most urban roofs a bit more reflective would be the equivalent of removing 300 million vehicles, more than are currently registered in the United States.

This is great news. Cool roofs will reverse some of the heat-related impacts of climate change. It’s analogous to sucking carbon out of the atmosphere for as long as the roof lasts. There aren’t many technologies that actually accomplish this goal.

Researchers at the Lawrence Berkeley National Laboratory are looking for ways to make cool roofs more appealing. For example, Paul Berdahl and his colleagues are investigating materials that appear red and an Egyptian blue to the naked eye, but are actually very reflective. So, if you prefer the aesthetics of a dark-colored roof, you’re in luck. Another Berkeley Lab scientist, Art Rosenfeld, dubbed the “godfather of energy efficiency,” and his colleague Hashem Akbari were early pioneers on cool roofs.

Unfortunately, the California Energy Commission is trying to cover our roofs with something much less useful – solar panels. And, a roof with PV panels is nearly as bad as an all-black roof. Physicists measure the albedo of different materials on a scale from 1 (white) to 0 (black). According to this article, PV panels are 0.05, so very close to black. I spoke to some physicists who said the albedo of some PV panels could be as high as 0.25, but still not close to 1.

Both Jim and Severin have highlighted the flawed logic behind the Commission’s rooftop solar mandate. I’m worried about something more specific – the lost opportunity to use our roofs more constructively. Misguided policies aren’t just bad themselves. They can also crowd out good policies.

In fact, the California Energy Commission’s current building standards require cool roofs on homes. This seems super redundant. We’ll be required to have cool roofs and then required to cover them up with PV panels?

Ah, you may be thinking, we could have both cool roofs and rooftop solar if we just leave solar panels on people’s roofs and create fields of white panels in the middle of the dessert. But, Lucas’ post convinced me that the benefits of locating electricity close to consumption are low, and grid-scale solar is a lot cheaper than rooftop. Plus, there are economic reasons to locate solar in particular places, near existing transmission, for example.

Finally, installing solar panels on roofs involves installing inverters, wiring, and lots of other balance of system work. Making roofs lighter color is much easier and cheaper. I’m not necessarily advocating tearing the existing roofs off all our buildings and replacing them with something lighter colored. But, when existing roofs are being replaced anyway and when we’re putting up new buildings, it seems sensible to require something reflective.

So, I hope policy makers elsewhere will support cool roofs. And, to combine heat-related metaphors, I hope cool roofs become hot.

Keep up with Energy Institute blogs, research, and events on Twitter @energyathaas

 

Catherine Wolfram View All

Catherine Wolfram is the Cora Jane Flood Professor of Business Administration at the Haas School of Business, University of California, Berkeley. ​During Academic year 2018-19, she will serve as the Acting Associate Dean for Academic Affairs at Berkeley Haas. She is the Program Director of the National Bureau of Economic Research's Environment and Energy Economics Program, Faculty Director of The E2e Project, a research organization focused on energy efficiency and a research affiliate at the Energy Institute at Haas. She is also an affiliated faculty member of in the Agriculture and Resource Economics department and the Energy and Resources Group at Berkeley.

Wolfram has published extensively on the economics of energy markets. Her work has analyzed rural electrification programs in the developing world, energy efficiency programs in the US, the effects of environmental regulation on energy markets and the impact of privatization and restructuring in the US and UK. She is currently implementing several randomized controlled trials to evaluate energy programs in the U.S., Ghana, and Kenya.

She received a PhD in Economics from MIT in 1996 and an AB from Harvard in 1989. Before joining the faculty at UC Berkeley, she was an Assistant Professor of Economics at Harvard.

20 thoughts on “How Should We Use Our Roofs? Leave a comment

  1. It would be useful to see some serious analysis of this question. The solar panels obviously shade the roof from infrared and other radiation. While solar PV panels work better if they are not overheated the back radiation of heat from the panels is a design question. A light colored roof will still tend to reflect radiant heat. Also, the air gap between panels and the roofing material on a sloped roof will create some air motion, cooling that air gap.
    Ignoring the environmental cost of using land for utility scale installations is absurd. There is a reason people like me want to limit the use of acres of desert land for solar installations. We love that space and the creatures who live there. If solar PV is installed on open land, optimize with solar tracking racks and raise them up so sheep can graze underneath them. (Goats may nibble on the wiring).
    Those black-top parking lots that absorb and re-emit so much solar heat, cover them with more parking or housing floors and top it off with solar panels.
    We should definitely find ways to make our roof-tops useful.

  2. Research has been done on this topic. See e.g.
    “Citywide Impacts of Cool Roof and Rooftop Solar Photovoltaic Deployment on Near-Surface Air Temperature and Cooling Energy Demand”
    https://link.springer.com/article/10.1007/s10546-016-0160-y
    which says in part “… cool roofs reduced daily citywide cooling energy demand by 13–14 %, while rooftop solar photovoltaic panels by 8–11 % (without considering the additional savings derived from their electricity production).”

  3. Jeez. “I saw a picture and now I hate rooftop solar too!” The post fails to account for two obvious benefits from solar panels on roofs, as mentioned by other comments. One, solar shades roofs, reducing heat transfer into the building. Two, solar panels do not accumulate heat like a black parking lot surface. The energy absorbed is converted into electricity. You can put your hand on a solar panel, or walk barefoot on one (I don’t recommend it). No, I haven’t done the math to compare v. cool roofs (which are great), but neither did Dr. Wolfram.

  4. Interesting blog. There are some really fascinating developments in this space involve the use of photonic surfaces that re-transmit solar thermal energy through the “atmospheric window” (8-13 micron wavelength) back into space. These may have the potential to dramatically reduce air conditioning requirements and lower the urban heat island effect.

    Most recent development from Columbia University is here, but earlier work has also been done at Stanford and U. of Colorado.
    https://www.sciencedaily.com/releases/2018/09/180927145555.htm

  5. I think that this is a false dichotomy. There is no apparent reason that one couldn’t have solar at home and also substantially gain the benefits of a cool roof. This merits a much more precise analysis than I will offer here, but please consider this math. Google tells me that the average two-bedroom home has approximately 1500 square feet of roof space. The Energy Commission tells me that the average system size required by the new mandate is 2.8 KV. Google says that a 3KV photovoltaic system will require approximately 198 square feet of roof space. That means that even if every new home complied with the mandate by having rooftop solar, it would on average darken 198/1500, or 13% of the available roof space. But the mandate does not require rooftop solar on every home. Ground mounts could qualify, as well as membership in an offsite community solar facility. I would suggest that concerns about the lost cooling opportunity collect and analyze data a lot more dependable than the numbers I found with my quick-and-dirty Google search before writing off the new solar mandate for this reason.

  6. The idea behind putting panels on the roof is to harvest energy from the sun instead of burning fossil fuels to meet growing energy demands. The root cause of the problem with global warming is emissions from burning fossil fuels. Why not just tackle the root cause?!

    The idea of cool roofs would make more sense in countries like India, where roofs are also a terrace and people usually don’t like to fill them with solar panels.
    It would be cool if researchers at Lawrence Berkeley National Laboratory worked on tiles that are white colour is summer but changed to a darker colour in winter.

  7. Here’s the study that Jim Lazar references: https://phys.org/news/2011-07-solar-panels-cool.html. The question is what is the baseline for that study.

    I’ll also note that the assertions made in the three blogs by other UCEI researchers referenced here have not gone unchallenged on those blog commentaries. For example, it was revealed that the marginal cost for a new transmission line from the California desert cost as much as 8 cents per kilowatt-hour. That would make distant new solar much more expensive than rooftop. There are other unexamined implicit assumptions identified in those comments that undermine the logic of those conclusions.

    • Thanks, McubedEcon. That is a nice piece of work.

      The paper I was referring to in my email is actually this one:
      http://maeresearch.ucsd.edu/kleissl/pubs/DominguezetalSE2011.pdf

      There is another nice paper on the shading value of solar at:
      https://peer.asee.org/impact-of-shading-on-cooling-and-heating-load.pdf

      The point is that solar shading is valuable. Having more air circulation UNDER the panels improves the shading effect. There are additional benefits that should be considered beyond the electricity production. And, to be fair, there are additional costs that should be considered, such as the higher cost of re-roofing the property if that is required before the end of the solar system lifetime.

      In the Roadmap2050 project, we found that remote solar would not be sufficient to reach the 80% reduction by 2050 goal, and rooftop solar was necessary to get there (as was elimination of fossil fuels for domestic space and water heating).
      http://www.roadmap2050.eu/

      Before criticizing any specific solution, be sure you have an alternative that is better, cheaper, more politically plausible, or easier to get us to 80%. Picking one at a time at solutions will not solve the challenge we face. There is no Planet B.

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