A Year Ago on Zero Resource – October 2010

Cool Pavements – Melvin Pomerantz

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In Berkeley, we are fortunate to have such events as Science at the Theater, where Lawrence Berkeley National Lab researchers give talks on their work at the Berkeley Repertory Theatre. The lectures are free and get a pretty sizeable audience.

On Monday, October 11, I was in the audience as researchers from Lawrence Berkeley National Laboratory (and the beloved Art Rosenfeld) gave a presentation titled “Cool Roofs, Cool Cities.” The post below consists of Part 3 of my record of the presentation – Melvin Pomerantz gives an overview of cool pavements. All portions are included in chronological order.

An ellipsis (…) indicates that I was not able to capture the words or thoughts skipped. The presentation is transcribed as accurately as possible – punctuation choices are mine. I also added any images.

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Now we’re talking about cool pavements … because they are a significant fraction of a city … including the streets, parking lots, driveways, and sidewalks.

We need to understands that they are a composite material … they are aggregate, an array rocks of different sizes … We then have to hold them together. So we have to glue the rocks and sand together by coating each rock with a  binder of some sort. For “asphalt”, the binder is asphalt, which is a petroleum product …  We can also bind the rocks together with cement, which is a mineral product, and we call that concrete … The key thing is that because you’re coating the aggregate, you mostly see the color of the binding … Our target is the binder.

Of the third of the city that is covered with pavements of various kinds, 50% is streets, 40% are exposed parking, and about 10% are sidewalks … Because it covers about 90% of the paved city streets, our target is the asphalt concrete.

Image credit: Wikimedia Commons

Start with fresh asphalt concrete … which is very black and has a solar reflectance of about 5%. As it ages, sunlight breaks it downs … Typically, its solar reflectance goes up to about 15% … The question is can we do any better … If you use a light-colored aggregate, it will show as the binder wears off … can use seashells, or porcelain … For old pavement, which required resurfacing periodically … we can put on a layer of asphalt emulsion, and put light-colored aggregate on top. That is called a “chip seal” … One issue is that aggregate is heavy, so it’s expensive to ship. So we want to use rock that’s nearby, and it may not be white … If the road stay cool, it doesn’t deform as much …

An example is from San Jose … They happen to have a quarry nearby that has white rock, and they’ve used a chip seal …

The other type of road, a little less common, is cement concrete … Fresh cement has a solar reflectance of about 35% … as it ages, it gets darker and reflectance drops to about 20% … The fine aggregate tends to float to the top … If you have light-colored fine aggregate, you can get an initial solar reflectance of about 40% …

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Part 1 is posted here. Part 2 is posted here. Part 4 is posted here.

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Cool Roofs – Melvin Pomerantz

5

In Berkeley, we are fortunate to have such events as Science at the Theater, where Lawrence Berkeley National Lab researchers give talks on their work at the Berkeley Repertory Theatre. The lectures are free and get a pretty sizeable audience.

On Monday, October 11, I was in the audience as researchers from Lawrence Berkeley National Laboratory (and the beloved Art Rosenfeld) gave a presentation titled “Cool Roofs, Cool Cities.” The post below consists of Part 1 of my record of the presentation – Melvin Pomerantz gives an introduction to the heat island effect and cool roofs. All portions are included in chronological order.

An ellipsis (…) indicates that I was not able to capture the words or thoughts skipped. The presentation is transcribed as accurately as possible – punctuation choices are mine. I also added any images.

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What I’m going to talk about is a very familiar experience to a lot of you – when you go into the center of a city, it’s a lot warmer … That effect, namely the temperature … tends to be anywhere from 5-7 deg F warmer in the city than outside the city …

Image Credit: Wikimedia Commons

The heat has not only an effect of causing discomfort, but has economic effects, too … Looking at some SMUD data … went up to about 107 degrees that day … As the day goes on, it gets hotter and hotter, and the demand for electricity gets higher and higher … finally it cools down and people turn off their air conditioners a little bit. … To get this power, you need about 5.5 power plants, but the power used in January is much less, so the power company has to have in reserve all these power plants. So there is capital involved in having these in reserve … And the ones in reserve are generally the oldest, most polluting plants … so it’s an unfortunate effect …

Another effect that goes on is that when things are very hot, there are actually deaths … Chicago in 1995, there were 739 deaths attributed to the heat wave – almost all occurred on the top floor of buildings with black roofs …

So what can we do about it? … How is the air heated in the first place? The sun does not heat the air directly … Sunlight travels very well through the atmosphere … There are opaque surfaces – the light comes in from the sun and strikes a surface … some stays in the surface and heats the surface, then the air comes along and touches the surface and heats up … The surface is acting as a converter, so if we can modify the surface, we can get a handle on it … If you have a building underneath a roof, that heat travels into the building, then you have to run on the air conditioning. …  Also pavements suffer if it gets too hot … the pavement needs to handle the deterioration that the heat causes … …

One way to look at this is to look at the solar reflectance … If you have no light coming out, it’s black … if it all comes out, it’s very bright. If the light is not all caught by the material, it has a higher solar reflectance, and it’s cooler … If you decrease the solar reflectance, the temperature can rise 80-90 degrees F over the ambient air – and you don’t want that … …

There’s another feature, which is that once the surface is warm, it radiates … there are gases in the air which absorb this thermal radiation, and it’s like a blanket. It’s blocked by the gases in the atmosphere… this is the atmospheric greenhouse effect. If we can affect the light from sticking in the surfaces, we reduces the greenhouse effect, too, and keep the planet a bit cooler …

Image credit: Wikimedia Commons

What can we make cooler? … Looking again at Sacramento … about 39% of what we could see from the sky was pavements … … … If we reflect sunlight, it mostly passes back out of the atmosphere without heating the air…

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Part 2 is posted here. Part 3 is posted here. Part 4 is posted here.

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