Countering climate change with cool pavements
news.mit.edu
news.mit.edu
this is one of the reasons why i'm an advocate for street trees, and urban foliage in general (cuts down both incident and reflective radiation), and why i'm maddened when i see urban trees cut down or cut back (as been happening quite often in my urban neighborhood in LA). the tree trimmers i see doing this have no idea what they're doing, being low-paid laborers paid only to run a chainsaw/trimmer, not to contemplate consequences or consider other options. trees take decades to become effective shade, and minutes to butcher irreparably.
that's not to put down cool pavements, which are really cool (haha), but that trees can provide all the same benefits and more, which makes it the better first option, with cool pavements being reserved for where tree shading is difficult or impossible (like roadways and parking lots).
I'm not saying we shouldn't do this. I really like seeing plant life in cities and I think it really makes a psychological difference (I _love_ living in a city with a lot of green spaces and trees everywhere). But it is nowhere near as simple as placing a tree in the ground and cleaning the leaves.
Dude, we have rats running around, pigeons nesting and pooping, what biological contaminants?
We need to have less takeaway left on the street and garbage bins that are cleaned up on time.
And I have never heard of trees contribution negatively to "biological contaminants", whatever that means. Rather, trees contribute to a cleaner and healthier environment.
on our street there are countless trees that were likely planted decades ago, but a lack of care by the residents makes them mere shadows of their potential.
and for anyone interested, LA's a great example of how trees provide a return on investment (actual dollar return outside the tremendous ecological benefits). go onto google maps and look at the satellite view. people who have never been here can point out the expensive neighborhoods because they are the greener ones.
as for planting the trees without the permission of the resident responsible for that parkway, like I said in that previous comment, it doesn't do much good if the folks don't take care of them. we've also learned the hard way that doing things that seem additive to the hood are not always taken that way by neighbors and can cause conflict. been on the end of that a couple of times already. that said, maybe I'll stop talking about doing this and actually try it and see what folks say.
That's all I got after 5 minutes on Google maps.
This is normal in many cities.
Trees can mean high status because
(a) you need excess land to have trees.
(b) old wealthy suburbs often have old trees (a massive old oak tree implies something).
(c) single dwelling houses that are near the city often have very old trees due to the age of the location, and can have high value because the location has now become central (due to city outskirt growth).
(d) maintaining trees costs time and money, as mentioned, so that also implies wealth (and status).
Aside: lawns are another status symbol - that is why so many people care about them so much! Think of a huge old English estate with manicured lawns, and why that is a strong symbol of wealth.
Edit: and I should say I love trees and I think they are really worthwhile, independent of any associated status!
Yep, check out a satellite view of Atherton, CA vs Redwood City next door. A huge contrast.
A big one regarding more trees is more maintenance, you've already pointed out how terrible maintenance can be, and lawns alone are an incredible example of just how inefficient we handle plants in the name of some arbitrary aesthetic, tons of water wasted, many hours spent trimming things to some ideal size that turns out to be terrible for the plant itself, etc.
Of course my paranoia doesn't ignore cool pavements either; the reflectivity does bring some mild worries: some skyscrapers are known to cause large amounts of concentrated reflective heat, but those are due to the more mirror-like reflective properties of glass rather than the diffusive matte grey/whites of these pavements. I also worry about visibility, could we end up having a "snow blindness" effect? Would driving be affected as the harsh unnatural dark contrast of a typical road is very easy to spot compared to...everything else. I'm ofcourse aware not all roads are this black but you can't deny bright yellows and whites of road markers on a deep black road would be easier to pick up on than on a greyish road.
This of course applies mostly to areas that are hot for a good portion of the year.
And I have the same concerns as you about the reflective pavement.
That is, constructed shade structures that provides shade without being unpredictable living things.
There are probably real problems with that approach too. What are they?
Tree trimming and eventually removal can cost 500-1000+$ for older trees which you eventually need to do to avoid having limbs and eventually trees fall on nearby cars. Roots damage local pavement again if you let the trees grow large enough. They also start taking up significant space requiring much wider sidewalks.
Of course you can remove them and add new trees regularly, but that’s another cost and tends to upset locals. Etc. granted his is usually in comparison to an empty sidewalk as it’s unusual to have other forms of public shade.
Note that buildings are also shade structures but they contribute to the heat island effect.
We have a lot of old trees around and the pavement around them is bulged, cracked and deformed.
https://www.npr.org/templates/story/story.php?storyId=561579... (2006)
If that type of tree is known to have extensive roots, perhaps you could use smaller tiles on the pavement nearby and let the roots find their way.
The eventual result might look unsettling to some, though[0] (I know it does to me).
I guess it does make part of the pavement inaccessible for wheelchair users… But then I believe that must be a massive old tree there.
We aren't separate from nature, killing it is killing ourself.
My guess would be evapotranspiration, not photosynthesis.
IMO solving overheated cities is not just more trees but less population density. Less buildings, less concrete, less asphalt. Less generation of heat and pollution in the first place. Less is more essentially.
Smaller, denser cities are more walkable and have more room for parks.
Small, single-family buildings are incredibly energy-inefficient. Large 5-story apartment blocks are way more efficient.
I moved from Perth, Australia to Berlin, Germany - the difference is astounding. Living in Perth without a car was painful. Living in Berlin without a car is normal. The hot water heating is communal, even in some areas provided for entire city blocks, while every house in Perth has its own reverse-cycle air conditioning setup. High density living is just more efficient per person.
Or maybe they do know what they are doing. Trees need trimming.
> not to contemplate consequences or consider other options.
Like?
Often urban trees are struggling, but I've seen workers still prune them heavily.
Not only is cooler, it looks much better than cement… It can be expensive as it uses a lot of hand labour.
Bit of an overly broad generalization. Some European cities, sure. But many are desolate and devoid of trees. Many old cities have very narrow streets, which aren't great for trees. A few shrubs, maybe. Wider strees in Belgian cities are generally devoid of trees, because parking spaces are apparently more important -- and we wouldn't want to impede the view of shops and billboards! Whenever urban trees are cut down, there's a bit of protest, but nevertheless it just keeps happening :-(
It only requires political will.
Edit: A POV drive through the same place https://www.youtube.com/watch?v=qgbgu7Pv60A
Several here have made the much more obvious observation: just use plants.
[1] https://en.m.wikipedia.org/wiki/Thermoelectric_generator
Not pavement but still
1. Pavement is a very poor conductor, energy doesn’t move easily through it.
2. The Delta between ambient air temp and pavement temp low. Generating power would most likely include moving energy between the higher temp pavement to the lower temp air and capturing some of it during the transition. This is possible at any delta, but were really talking about a maximum of about 30 degree C temperature delta.
3. There are more efficient ways to generate power. Build wind turbines, solar panels, dams, etc. These are all very good ways of generating power.
4. There are more efficient ways to keep pavement cool. Change the pavement materials, plant trees, build shade sources for the pavement.
Oh dear I'm certainly not proposing running electricity through the pavement itself. I'm suggesting you harvest the heat from the asphalt with e.g. a thermocouple and use that to produce a voltage.
>The Delta between ambient air temp and pavement temp low.
Perhaps, but not between the pavement and the soil a foot or so underneath.
This would be important as if you moved the energy from the hot pavement to a cold source you're cooling the small part where you're taking heat from. As pavement is a poor heat conductor all the energy in the surrounding mass of pavement would flow only very slowly to the patch you were using. This would severly limit your ability to move heat and therefore generate electricity.
This is also why using the ground as the cold sink isn't a great idea. Once you have moved the heat energy to the ground it sits there only slowly moving away. This is why the ground is noticeably cooler just a foot beneath the hot pavement. Once your cold source is warm, the ability to move heat is again limited.
Air is also a bad conductor of heat, but as it's a gas it can move. So once you heat up some cool air it will become less dense and move away. This is convection.
You could theoretically build a power plant by building a pipe through the pavement (does this reduce the structural integrity of the pavement?) or directly underneath and touching (surface area and high heat transfer would be the desired quality of the pipe). Inside the pipe is a liquid and a set of conditions such that as the liquid heats up next to the pavement it will turn into a gas. You could then capture the extra pressure generated by the gas via a turbine. There would be a condensing section of pipe (typically above the boiling section so gravity can move the cool liquid back down) so it’s a closed system. The primary problems with this setup are:
1. You’d have to change the liquid or change the pressure inside of the tube to adjust for the difference in temperature. Imagine a place like Houston where the pavement might be 150F or it might be 50F. The conditions of the power plant vary.
2. The material cost is stupid high for the amount of power it would generate. It would probably be a better use of materials to capture the heat waste of kitchens boiling water.
3. The labor cost is stupid high for the amount of power it would generate.
4. I’m really concerned about how this will change fundamental goal of pavement (to provide a solid, long living, smooth material for roads and building foundations.). It’s actually possible that reducing the size of the heat fluctuations will make the material more stable. I think it’s just as likely that adding complexity to the system will make the pavement less stable, but if you are going for the 2% of doubt a reduction in maintenance cost would be able to modify the $/Watt function you’d be fighting wind/solar/hydro for.
I’d really like to stress than 2 and 3 will combine to make Wind 10x-1000x cheaper.
Because of the hat, I like to think the two characters are Heisenberg and Pinkman.
"Lisa, in this house we obey the laws of thermodynamics" -- Homer Simpson
Darn, I thought they were going to talk about using heat pipes driven deep into the ground to turn the pavement into a heat sink and was wondering how they were going to get the pavement to conduct heat to the heat pipes. Now I'll never know. ;)