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.
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.
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Catherine Wolfram View All
Catherine Wolfram is the William F. Pounds Professor of Energy Economics at the MIT Sloan School of Management. She previously served as the Cora Jane Flood Professor of Business Administration at the Haas School of Business at UC Berkeley. From March 2021 to October 2022, she served as the Deputy Assistant Secretary for Climate and Energy Economics at the U.S. Treasury, while on leave from UC Berkeley. Before leaving for government service, she was the Program Director of the National Bureau of Economic Research’s Environment and Energy Economics Program, Faculty Affiliate of the Energy Institute at Haas from 2000 to 2023, as well as Faculty Director of the Energy Institute from 2009 to 2018. Before joining the faculty at UC Berkeley, she was an Assistant Professor of Economics at Harvard. 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 working on several projects at the intersection of climate and trade. She received a PhD in Economics from MIT in 1996 and an AB from Harvard in 1989.
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This is an interesting point – I don’t know what the CEC envisions in their solar roof requirement in terms of how the solar panels are placed on the roof – are they integrated as part of the roof itself or are they mounted on top of the roof – as is currently the case for most residential PV installations. Even in those cases where the panels are mounted parallel to the roof plane, there is a several inch air gap that should serve to help decouple heat transmission into the building. Typically the entire roof isn’t covered with solar panels, so there is still some advantage to having a ‘cool’ roof.
The albedo effect is another matter – and as has been pointed out – likely underappreciated. My understanding is that most PV cells use light in the ~380 to ~ 750 nm wavelength range – which happens to be visible light. It also happens that low-e windows have coatings that pass visible light but reflect IR and UV wavelength light (not with 100% transmission or reflectance). I have no idea whether this is a topic of current research – but given the importance of the albedo effect, perhaps it should be.
Interesting post, Catherine. And great discussion! It shows how much we still have to learn about how to design truly efficient roofs and rooftop PV panels. And how much room and need for for innovation there is in this space!
A bit fuller consideration is needed. The influence of the white roof on solar radiation lasts only as long as the white roof is there (optimistically, a couple of decades). Depending on where the energy for air conditioning in the home is coming from, there may also be reduced CO2 emissions, and what would really be the critical issue in the long-term is whether CO2 emissions are reduced more by the white roof or more by having PV on the roof, for the CO2, once emitted to the atmosphere has a very long life-time, and so its influence on the net radiative forcing over time is really the key issue. I’d also note that, yes, there are all sorts of caveats to be calculated our when one considers issues like the energy and its source to make and install the solar cells or white roof, etc. I’m just noting that the analysis needs to be integrated over time and not just be based on the instantaneous difference.
Catherine: besides other comments about solar capturing energy and studies that show how it helps decrease heat in homes (personally experienced in my home), solar panels rarely cover a whole roof. SW facing roofs in CA are most important, many of these can be garage roofs. So using white roofing in combination with solar on roofs absolutely makes sense for the entire structure.
I may be showing my ignorance, but isn’t most of the PV power coming from shorter wavelength solar rays? It that is true, it may be possible that panels can be designed to reflect infrared (the heat from the sun) and absorb the shorter wavelength light that produces the electricity. Selective coating have been used for years space vehicles as a way of controlling their internal temperature. Selecting the wavelengths that are reflected may be one that is worth looking at. Catherine’s point about need the converters as part of the roof solar panels is a good one. The need for converters as part of the panels does affect their cost. Large arrays of panels can have converters that are separate from the panels, which should be a cost saving. Jim’s point about solar panels saving on air conditioning is good for the building being air conditioned is an interesting one, but the solar panels with low albedo for infrared still contributes to the heat island effect of large cities.
Interesting – I never put much thought into “cool roofs.” Do cool roofs make sense in a cold climate where much of the energy use is for heating, rather than cooling? It’s not obvious I would reduce energy use by reflecting heat here in New England. I could see it going the other way!
I think your analysis that compares the absorption and emissivity of a solar panel to a black roof is a bit simplistic… you’re forgetting that solar panels convert photons into electricity. That means much more of the sun’s energy is not radiated locally because is transported elsewhere as electrons (which may end up as heat when later converted and used, but that offsets other energy sources).
Solar panels capture energy that would otherwise either be absorbed by or reflected by the roof.
One study from San Diego showed that the presence of solar panels reduced air conditioning loads in the building by about 0.3 kWh for each kWh of electricity produced by the solar panel. So, in essence, a 30% bonus on the output of the solar panel.
This has been little studied, but when the Mauna Lani Hotel on the Big Island of Hawaii installed 1 MW of solar, more than 15 years ago, they found that they had overshot the mid-day consumption of the building, because they had not factored in the reduced AC requirements.
It will be interesting to see some serious science comparing the cool roof vs. solar panel impact of building cooling requirements.
Please consider the environmental costs of covering green fields and natural habitats with PV panels before discounting the idea of PV on urbanized roofs and parking lots. For your argument to be convincing, you also need to show that (1) the building energy saved from cool roofs exceeds the power generation from an equivalent-sized PV array; and (2) it’s not possible to design low-albedo PV panels.