> “cool pavements” [..] reflect more solar radiation and emit less heat than conventional paving surfaces.
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. ;)
> “Conversely, by reflecting light — called incident radiation — onto nearby buildings, cool pavements can warm structures up, which can increase AC usage in the summer and lower heating demand in the winter.”
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 also like urban trees but remember they cost a fortune to maintain! For example, urban trees have to be expertly managed for disease along transit lines.
They require water, which requires irrigation (may not already be in place and of course has logistical problems), trees also root systems can lift and crack sidewalks/pipes/other things, you now have biological contaminants, there's bugs and fungus and things that can spread, and so much more.
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.
Just because biological contaminants exist doesn't mean adding more creates more issues. More trees also means more rats and pigeons. What's your argument? That just because we already have a bad thing doesn't mean that a similar bad thing isn't bad? That doesn't make much sense and is a pretty lazy argument.
How much irrigation that is needed very much depends on where you are and what kind of tree. After the first few years it take to settle, trees have deep roots and rarely need watering.
And I have never heard of trees contribution negatively to "biological contaminants", whatever that means. Rather, trees contribute to a cleaner and healthier environment.
Tree limbs die all the time. Normally when disease or damage kill a tree branch, it isn't a big deal because the tree is in a forest and probably won't fall on anybody or damage any property under it. In a city, you can't ignore a widowmaker. The risk of it damaging property or flat out killing somebody is too high, you have to pay to get it safely removed. Often this becomes "end-of-life removal" because it's deemed safer or cheaper in the long run to remove the tree entirely. So now you have a green mostly alive tree being chopped down because one of the limbs died, and everybody in the neighborhood thinks you're a tree butcher but nobody else wants to chip in to have it maintained.
The pavement replacement won’t be free either, I suppose.
I’ve never heard of a government cutting trees as a cost saving, however I didn’t look for that either.
I think that, once trees are there, people will be happy to pay the few dollarcents per year for their maintenance.
from one LA resident to another, amen to that. we were just walking down our street this morning and discussing, for the umpteenth time, whether our neighbors would actually be willing to take care of trees if we went out and bought them and planted them. the problem, as other commenters have pointed out, is that they do require upkeep, but since they're living/breathing things the more hardy ones can really be abused and still survive. The question is how do you incentivize the people who benefit from the trees--the residents, businesses, other local stakeholders--to actually maintain them?
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.
LA has an woefully underpublicized street (and lawn) tree program, where you can get free trees if you agree to take care of them (which is mostly just periodic watering), so you wouldn't even have to buy them yourself. for street trees on the parkway (the patch of ground between the sidewalk and street), you don't technically even need the permission of the owner, although the owner might not be amenable to watering them then. apartment dwellers (like me) can get trees planted on the parkway just by agreeing to take care of them on behalf of the owner.
it's true, that's exactly how we got our street tree, but a) it took an extremely long time to get the tree in the first place and b) the tree itself was rather lackluster, though now we're on year 3 of it and it's... ok. if I knew then what I know now I would've just gotten my own tree (one of the ones on the approved list) to get a nicer tree up front.
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.
ha! i’m glad you tried it! only one of those (san pedro) is in the city of LA, which i should’ve been more clear about. but those places do indeed appear to have more trees than the vast urban expanse. sadly san pedro does not rate as an above average area (rolling hills would). that said if you want to waste another 5 minutes of your life on this go to google maps again, look at the area between “downtown los angeles” and “santa monica” and find the green areas. really love that you actually tried to do it once already though!
> people who have never been here can point out the expensive neighborhoods because they are the greener ones
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!
While I initially would prefer the idea of more street trees the nagging voice in the back of my head continues to worry about unintended consequences.
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.
I agree with you on the matter of lawns, but not on street trees. Yes, they require maintenance, but they make a massive difference in the comfort of people walking, which is a big factor in people deciding to walk rather than drive, as well as increasing the general attractiveness of the area to potential residents who might otherwise live in a car dependent suburb of something. When you add up all the benefits, I'd consider street trees to be analogous to brushing your teeth-takes a bit of effort but the cost of not doing it is far greater.
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.
mainly cost. trees are cheap (~$100-300) compared to tarps or other fixed shade options, though i'm down for any and all shade. for comparison, bus shelters are estimated to cost $5K-30K each ( https://pedestrianobservations.com/2019/04/12/little-things-... ).
i mean, leaf collection for street trees is like 10-15 minutes of work a week during growing season? you could give a neighborhood kid $5 to do that along with the occasional watering (we actually have a volunteer organization in my neighborhood that does this, though not weekly). artificial shelters need cleaning/maintenance too, so it doesn't seem to be a significant net difference.
This is where it’s more open to interpretation. Wet leaves on pavement are a significant fall hazard and should be removed more frequently, but that’s optional.
Tree trimming and eventually removal can cost 500-1000+$ 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.
While I generally agree, you also have to be careful not to go too far in the other direction. My cousin's car was totaled while driving in Philadelphia. A large tree, that was not actively maintained, fell over when the ground was too wet. A foot difference and he would have been killed.
primarily by choosing an appropriate tree that doesn't do that. countersinking the tree can work in some cases too (often with a metal grating around the base of the tree so people don't fall in and hurt themselves). i don't know that you can "fix" the roots once the tree has become well established, without seriously damaging the tree and potentially creating additional hazard.
You also have to be careful of mainlines and turning that into swiss cheese with the root system. Ultimately it takes a fair amount of research to pick the right location and tree.
like parent commenter said LA actually has a list of acceptable “street trees” that they’ve approved so they don’t kill the sidewalks. some of the most glorious trees you’ll see here are ficus, but those are not on the list because they do blow away sidewalks, streets, utilities. unbelievable shade trees though that require very little water.
i just learned that a huge, grand old tree in my neighborhood is a species of ficus, and indeed, the sidewalk next to it is a buckled, gnarled mess. but i’d rather deal with that problem than harm the tree (it seems like it would be pretty straightforward to build a concrete ramp over the roots, like a mini-bridge).
that's neat, like the temporary flooring used for outdoor 3-on-3 basketball tournaments. a concrete bridge would probably be smoother, more durable, and require less maintenance, but the interlinked rubber segments would be easier to reuse/reposition over time and being semi-permeable is a useful property for other reasons.
ah super cool. fully agree that taking a tree like that out would be a travesty. and I'm not insensitive to the infirm or disabled when having to deal with broken sidewalks, but taking those trees out would be a sin when there are other ways to address the root damage like the ramp you suggest or those rubber tile things.
Ooooh, countersinking a tree to get its roots below the pavement, with a metal grating to keep people out.. thanks, I used to wonder about this and had no idea why some one would do that.
So maybe the tree is more valuable than the pavement? It is a living being, it has lived there for many years. Maybe we can show it some respect? Learn to live with it, as it lives with you? Appreciate the many benefits it brings, direct and indirect?
We aren't separate from nature, killing it is killing ourself.
I would add that city people underestimate or simply do not know about the ability of vegetation to cool off its environment. My city has areas (few abandoned plots) with a wild high grass and bushes on them. When it’s hot and you walk nearby, like 2-3m close, it is as if you went from the street to under AC. I don’t know how they do it, but my guess is that photosynthesis is a hell of an endothermic process. And you’re right, modern “city trimming” is just a disgusting non-functional nonsense.
TIL. But that means plants move heat somewhere, and not pack it into chemical bonds. This maybe defeats the idea, because these areas are cold because they are rare. If ubiquitous, in a still hot day they would just raise humidity and probably make things worse?
FYI I've brainstormed on an innovation use case once that would allow citizens to use an app to suggest things like planting trees everywhere using AR etc.
When discussing with academics, I was flabbergasted to find out that in street canyons, trees can have a negative effect because they reduce air flow.
So, apparently it's not as simple as "just plant some trees"...
They can also be a hazard as they can crash much easier during a storm, earthquake or hurricane. Especially if they are dry and not alive any more.
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.
Less population density means more transport, though, as the city is so spread out. More cars, more roads, more parking, more infrastructure generally.
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.
It's interesting that the effects are so complex and nuanced. I also wonder - they didn't specify - just how reflective these surfaces need to be, and how "bright" they are to look at. I could see glare being a potential safety hazard if the surface is reflective enough, but I'm assuming that any surface rough enough to make good pavement won't be _too_ bad to look at.
I would doubt they went far enough to get meaningful measurements. I think this is just more of the Ivy League enriching itself at the bottomless climate feeding trough. The discovery that light colors absorb less heat isn’t remarkable. I would be embarrassed to see this if I were an MIG alumnus. Statements about replacing all pavement in a metropolitan area (that doesn’t even patch its potholes) are so out of touch that they discredit the whole article. Just replace all that asphalt with concrete filled with, I guess, prismatic beads which would be like staring at the sun? Brilliant! Oh, or use “lighter colored aggregates”? Right. We I never thought to just ask the local geology to rearrange itself a bit so my local quarries can produce a different color product. I’ll give it a go!
Several here have made the much more obvious observation: just use plants.
This totally nerd-sniped[0] me, and now I'm going down the rabbit hole wondering if there's a business opportunity for generating electricity via the thermoelectric effect[1] and thus literally cooling the pavement by pulling some energy out and putting it to good use.
I’m in the 98% not feasible category for a few reasons. I’m open to new information or arguments, but I can’t see a way over a few very large hurdles.
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.
>Pavement is a very poor conductor, energy doesn’t move easily through it.
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.
Conductor can be used to describe more energy types than electricity. In this case they were referring to heat energy.
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.
Confirming that I was using conductor to mean heat transfer instead of electrical transform.
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.
Why wouldn’t you just plant some trees. You don’t need any fancy pavement if it is below a tree..
European cities are filled with trees to cool the streets below them.
Obviously, you’d need smaller streets that fit pedestrians and bicycles instead of massive urban highways for trees to be effective.
US cities have trees, too. One downside, though, is the roots can grow and break up the sidewalk. There are some sidewalks in Seattle that are completely inaccessible to wheelchairs or even folks with walkers.
If roots were the biggest obstacle for wheelchair use, I'm sure many a wheelchair user would rejoice. I doubt that's anywhere near the case. Cars parking on the pavement, silly delivery robots, shops with oversized signs, all kinds of rubbish, inaccessibly high kerbs, are likely bigger issues.
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 :-(
In Poland, there is a trend (referred to as "betonoza") to replace city squares which had trees with concrete squares devoid of any vegetation. There are several reasons for that. Main reason seems that it's easier to get government/EU money for such 'renovation'. Second reason is that it's considered more prestigious to have a concrete square, because that's what other famous cities have.
My city recently renovated a huge square, got rid of all the trees and shrubs and plants, and turned the whole thing into a giant concrete wasteland. The mayor was quoted as saying that "not every square needs trees" ... It's mind boggling.
I wonder if the increased reflectivity could be a visibility issue for drivers, especially when wet (which can already be an issue with regular asphalt). Certainly sounds good otherwise though.
This is good scientific writing. The article discusses some nuance and hints at the complexity of the topic, but presents results in a widely understandable way.
79 comments
[ 2.8 ms ] story [ 149 ms ] threadDarn, 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. ;)
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.
[0] https://imgur.com/a/k8OUF84
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.
Several here have made the much more obvious observation: just use plants.
[0] https://xkcd.com/356/
[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.
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 :-(
Not only is cooler, it looks much better than cement… It can be expensive as it uses a lot of hand labour.
It only requires political will.
Edit: A POV drive through the same place https://www.youtube.com/watch?v=qgbgu7Pv60A
"Lisa, in this house we obey the laws of thermodynamics" -- Homer Simpson