However, plants also reflect a lot of energy, unlike black/asphalt roofs that most people in the US install.
Reflects 98.1% of the suns rays.
However if an entire downtown or subdivision does it? Possibly.
The solar panels will still absorb and radiate heat back so you won't get as much cooling as you would have just with passive radiative cooling paint, but it'd be better than just solar on a regular roof in both ways (solar output and thermally).
So just put the PV panels on top, and a reflective layer below.
https://creativelyunited.org/wp-content/uploads/2019/02/temp...
Those nice trees take a few decades to grow, need serious pruning, will cause foundation problems next door, and increase building setbacks anyway. We also waste two entire lanes for street parking, have basically no businesses on either side anyway, and the street is still too wide. It might be urban, but it's still a car centric hellscape, just with some shade.
An ideal mirror reflects 100% of the light, and so does an ideal white surface. The white surface just spreads out the light.
If you light a mirror evenly from all angles, it will be indistinguishable from a white surface.
By making the sun hit at a layer above the ceiling (and assuming you have proper air circulation below), your attic area will heat less, hence your house will get less hot.
(This is similar to the effect you get by staying under a tree shadow)
As per the article, yes, if everybody had air conditioners city wide that would increase the city temperature. But that would require everyone to blast it at the same time
You've just made a ~100% efficient heat capturing machine. The 98.1% reflecting paint will be much more efficient, with the added bonus of not being dependent on the grid.
It would be nice if the cells were highly reflective at a range of wavelengths between the range they can actually use and the range where they radiate waste heat.
https://news.ycombinator.com/item?id=27782370 ("Passive radiative cooling below ambient airtemperature under direct sun")
Some work on surfaces of this kind have shown promising results, being able to fall below ambient temperature even in full sunlight.
However... that means they don't radiate well at night (they have low emissivity and are not well thermally coupled to the roof). Painting the roof white is ideal.
It tends to be newsworthy when it goes wrong, because the results can be things like buildings cooking nearby objects at certain times of day. Spicy stuff.
https://www.upi.com/Odd_News/2013/09/03/Skyscraper-melting-c...
"All will be simultaneously unveiled at 12 Midday tomorrow, citizens, so, ...take care out there!".
(shared link, readable even without a sub)
https://www.nytimes.com/2021/11/10/realestate/cool-roofs-cli...
Your reasoning is a bit like assuming solar panels stop flooding because they prevent the roof from getting wet.
Indeed it would be nice to power the AC with solar power, but how does this help the city to cool down? You would cool down the houses, but the heat from the houses would remain in the city, it would even get hotter.
As others have pointed out, you might do better bouncing the incoming radiation back out of the city with white roofs. And still others say you can do a bit of both.
Also, you could definitely do well to reduce your roof albedo by switching to white/reflective roofs. This point has been made endlessly over the last 30-40 years. No one actually does it. People are actually installing solar panels.
To the contrary -- at least in NYC it's extremely common. The vast majority of "tenement" apartment buildings have silver roofs, for this reason.
Just fire up Google Maps in satellite view over Manhattan and see for yourself. White-looking roofs everywhere (but silver in reality).
New construction with roof decks tends to use things like white-ish floor squares as well. They're obviously much cooler to walk on in the summer.
https://flatroofdoc.com/silver-coating-protecting-a-rubber-r...
Elsewhere I find a claim that the aluminum paint used was a 20th century thing, but now white paint is preferred as being more reflective.
Los Angeles has building codes for cool roofs:
https://www.ladbs.org/docs/default-source/publications/ordin...
Black is hotter in the day and colder at night. In the desert you want light colors, which do the opposite.
https://creativelyunited.org/wp-content/uploads/2019/02/temp...
It seemed a bit odd, because my roof is fully lit up, but only for a few hours a day- one half is covered most of the morning, and the other half most of the evening.
Sadly, it's not even really nice trees, just gigantic cottonwoods that litter leaves and twigs and sticky leaf pods like crazy.
The problem with solar is that unlit panels behave like a diode and no current flows. Chain together a lit and unlit panel and you get nothing. Put the inverter ahead of the interconnects and that's not a problem.
I'd love to do solar, but the only way I can make the numbers work is if I DIY'd the full install.
At the office building, the estimated payback was all of two years.
US skilled labor costs are insane. It makes all construction costs ridiculously high.
People also stuff like leasing that adds overhead.
Your 5-10k range installs will be $10-20,000 AFTER the tax credit, which might be a little higher now than when that graphic was put together.
Of course, this doesn't account for things like needing to re-do existing electrical work to get affected areas up to code, or if you need to replace the shingles on your roof while you're at it because it's typically recommended to do both at the same time if you're going to add solar.
Then again, you may also have the problem of ensuring your roof will withstand the extra weight, depending on the system. Most people are probably fine here, but for older houses in snowy climates it's worth thinking about as well.
I've looked into solar for my house. Aside from not getting enough sun throughout the day due to tree shade, there's also the problem that every solar calculator available wants to pretend that energy prices are going to go up 5% a year. Well, I've lived at my current house for 5 years and they haven't gone up once. The real payback for me is 20 years, if I'm lucky, even without the trees in the way.
For that matter, can solar panels be wired in parallel instead of in series?
The more immediate problem with climate change isn't that there won't be enough places to live/farm, it's that it's all going to move causing destabilization of societies.
The long term effects of running out of arable land are a considerable distance in the future.
Also your framing of the question *you* are bringing up is weird. Climate change will make for climate conditions that will make agriculture *harder* in general, I don't think your model of "Oh well we'll go to cold places that have become warmer" is really relevant and by the way arable land is already a resource some states are competing for.
One that wouldn't work. Two, those trees would have fallen on his house in ten to fifteen years because his house was east of the trees, and 'lions tail' tree pruning creates a giant lever arm and the roots will lose that battle in the first wind storm after a rain storm. That pattern had just played out ten years prior all over town. Big pruned Doug Firs falling onto people's houses.
What you want is to be able to be able to walk under a tree with your arms over your head and not hit branches. No more, no less.
My neighbor shields much of my house from the sun. We had issues when his branches started touching my roof, but we made sure it was trimmed in a way to still give us the shade it has all these years.
Of course, this is spot heating, and over a whole house the effect will be diminished, but it tells you how quickly things can get hot.
We've got plenty of freshwater in these parts, but in other places freshwater availability will be the limiting factor. Famously, swamp cooling works great in the desert if you can spare the water.
Trees are great, let's just be real about what they can and can't do.
(Of course those are being watered regularly.)
Leaves block the Sun relatively high off the ground, meaning people are in the shade during the hottest part of the day and feeling as much radiation off buildings and streets in the evening.
Beyond cooling they’re also good for water management. Most street trees don’t need any watering (at least in the Midwest) they live off rainwater. When it rains, water pools on the leaves and evaporates from there, reducing the amount of run off and keeping the street drier and less humid on days with light rain.
In cooler months (and places where this is relevant) the leaves fall and you can still get the warming benefit of sunlight.
Maybe it’s a disagreement of terms? I guess they aren’t technically “cooling” the area in an active sense, but they absolutely keep it cooler even when it’s 95 and the dew points over 70
To be fair, things like insulation are also good here. You want something outside to get hot when the sun shines on it instead of whatever space you're cooling.
(There are, of course, many heat sources inside that are annoying. I was running some fuzz tests yesterday and was annoyed to watch my air conditioner scale up in power to remove the heat that was wasting. I guess that's why people use The Cloud for everything. You can heat up someone else's city while enjoying the comforts of home!)
I live in Vietnam. It is 8am right now and the dew point is 80. It only 80 at the moment but is supposed to get up to 90 later in the day. I assume the dew point will also go up to around 90.
AccuWeather says the "RealFeel" will be 97 and "RealFeel Shade" will also be 97, so being in the shade does almost nothing, since it is overcast and raining very slightly at the moment.
We're not in a heat wave or anything, this is just normal weather basically year round. If anything it's been especially rainy and cool the past few weeks.
Trees are nice but are not remotely a substitute for air conditioning. Trees do not lower humidity enough to stop the average European/American from sweating constantly just sitting still here.
that's not to say that the heat under a trees' shade cannot still be oppressive. merely that shade matters, as the existence of two "RealFeel" measurements you shared indicates
Well the whole thread was something arguing they are.
First post: "I love trees and want more of them in our cities, my only point is that they are not usually a substitute for HVAC"
Reply: "I guess we disagree on what trees do well"
I don’t think anyone (certainly not me) has intended to argue that trees can replace AC, just that they make the city around them (including indoors in many cases) much cooler than they would be without the trees
Of course, trees are great for many other reasons like air quality, but for temperature fundamentally they're just big evaporative coolers. I wish cities in more water-stressed areas would consider arcades + rooftop solar.
Which makes it a good solution only to suburban heating.
It's not that there's no trees, but you'll see them in parks and boulevards that might as well be narrow parks.
If anything, I see far more trees in the American midwest. Here the streets are very wide, with basically no foot traffic to speak of, and too much road for the number of cars that use the street in urban areas. You can definitely fit trees here, but that's because so much of that concrete is waste. The extra trees just mean more distances, and more distances means more parking lots and more concrete.
Paint roofs white - and then go plant some trees.
Some modern neighborhoods are trying living roofs with grass. Haven't taken off and the grass hasn't survived either.
Lastly, other day saw post about leaving grass uncut reduced surface temps notable amount.
Since the person you're responding to is talking about solar panels on roofs and you're suggesting trees instead?
As always it's likely a combination of solutions, and rooftop solar is a good idea for more reasons than just heat.
As it turns out, solar panels are often more reflective than the typical roof of a house in North America. So solar panels can actually reduce the heat island effect. But it is mostly independent of whether they are hooked up at all.
Reflective of what wavelengths? Isn't that an odd characteristic of something designed to absorb electromagnetic energy?
I think reducing the absorption of the roof and the walls would help, but even then you have still have a house sitting in 34C+ air temps for at least 10 hours per day, so it gets hot. But when I consider my dads house, which has 2m eaves all around, high ceilings, and massive roof space - it still gets hot. His house might feel nicer without a/c during early summer, but by mid+late summer it is hot too.
Maybe better insulated doors+windows would help? My house is double brick walls. Maybe the "outer layer" needs to be able to get hot, and then the "inner layer" insulated from the outer as much as possible. That's a lot of change though. Instead I just run the a/c all day "for free" off my solar panels.
You’re not reducing energy here. If you gather and then release the energy again (make it do work) it’s simply being re-released.
Do they behave like heat pumps in some fashion where that energy is converted?
Oh I see now you say 20% so okay, some but definitely not even half.
Still, we're going to need every bit of help and I guess that 20% might be 30 or even 40% decades down the road.
A typical solar panel absorbs ~20% of the sun's energy blasted on to a city rooftop. If that's used to power air conditioning it's doubly effective. You're reducing the heat urban heat island effect from the sun's energy, and you're also not adding additional heat from external power sources in to the city.
On a large scale this would really help make cities more livable. On a hot summer day here in northern Kentucky I noticed a massive difference (I've roughly measured it at a 5-7F difference) going from less developed areas to the more urban areas.