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This is one reason I heavily push for roof top solar, even if it is a bit more expensive than utility scale. In many areas air conditioning is pretty much a giv
by reportingsjr 3y ago
This is one reason I heavily push for roof top solar, even if it is a bit more expensive than utility scale. In many areas air conditioning is pretty much a given at this point.
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.
- guerrilla 3y agoWhat % do trees and other plants absorb?
- reportingsjr 3y agoBelow 10%. The most efficient plants are food crops we've genetically modified. I believe corn absorbs around 10%. However, plants also reflect a lot of energy, unlike black/asphalt roofs that most people in the US install.
- guerrilla 3y agoWhen you say efficient it makes me suspicious. Just because a plant doesn't absorb the energy for conversion doesn't mean it doesn't absorb it as heat. Remember, they're quite connected to the ground so they could be good heat dissipation (I have no idea, just speculating.)
- tonfa 3y agoOne thing trees/plant help with (instead of concrete/asphalt in cities) is with the added impact of evapotranspiration (besides providing shade).
- ars 3y agoPlants also evaporate water from the extra light energy, which further cools things.
- wolverine876 3y agoBut they reflect green energy.
- Ekaros 3y agoWouldn't it be more effective to surface the roof tops with some reflective material?
- _vertigo 3y agoNot if you want to use the sun’s energy for power
- stephen_g 3y agoYou can do both - having solar panels over a reflective white roof (like the fancy newly discovered passive radiative cooling paints) helps increase PV output by cooling the panel. 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).
- amelius 3y agoThermal radiation is just EM waves, which you can reflect. So just put the PV panels on top, and a reflective layer below.
- gamerDude 3y agoYes! In fact there is a new paint that was developed with exactly this in mind. https://www.purdue.edu/newsroom/releases/2021/Q2/the-whitest-paint-is-here-and-its-the-coolest.-literally..html https://www.purdue.edu/newsroom/releases/2021/Q2/the-whitest... Reflects 98.1% of the suns rays.
- artursapek 3y agois that dangerous to aircraft?
- mitthrowaway2 3y agoAircraft can safely fly over snow, which has albedo if 90%, so it's probably ok.
- raverbashing 3y agoThere's one other minor factor that helps even if the solar panel did nothing: 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
- _Algernon_ 3y agoConsidering that solar panels are close to perfectly black, they will also close to perfectly absorb the remaining 80%. The 20% of solar electricity that is then used for AC, will be released as heat as well. 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.
- pfdietz 3y agoAre PV cells really that black for photon energies below their bandgap? 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.
- MichaelZuo 3y agoYes they are very black in the visible spectrum. Like the parent said a highly reflecting white paint for rooftops would certainly be the smarter choice.
- pfdietz 3y agoI wasn't talking about the visible spectrum, I was talking about near IR below the bandgap (which is just below visible for Si cells.) This light does not contribute to the output of the cells, so reflecting it back into space would reduce heat generation at no loss of efficiency. If anything, by keeping the cells cooler it would slightly boost efficiency.
- perihelions 3y agoThere were HN threads about this (?) https://news.ycombinator.com/item?id=27782370 https://news.ycombinator.com/item?id=27782370 ("Passive radiative cooling below ambient airtemperature under direct sun")
- aeternum 3y agoYou can do both, typical rooftop solar installations typically don't cover the majority of the roof. Make the rest reflective and/or make it a space for people. Rooftop area has some of the greatest views in any city. Kinda crazy that we don't use it as space for humans and greenery in most cases.
- y42 3y agoWait what? An AC does not simply turn hot air into cold air. It "moves energy" from one place to another. 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.
- robotresearcher 3y agoThe argument is that you aren’t moving additional energy into the city from power stations to run the AC, which ends up as heat. Instead you’re using energy that’s already radiated into the city from the sky. 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.
- reportingsjr 3y agoExactly correct! 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.
- crazygringo 3y ago> No one actually does it. 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. (While, sadly, contrary to your assertion, virtually nobody is installing solar panels in NYC. Landlords don't seem very interested so far.)
- wolverine876 3y agoWhy silver and not white?
- 7e 3y agoWhat is the effect of this approach in winter? The opposite? Losing heat from the sun that would otherwise help heat the building?
- chiefalchemist 3y agoTrees also "capture" heat. Whether urban or suburban, we'd be wise to plant more anywhere we can.
- slt2021 3y agojust wanted to add illustration https://creativelyunited.org/wp-content/uploads/2019/02/temperatures-with-and-without-trees.jpg https://creativelyunited.org/wp-content/uploads/2019/02/temp...
- slt2021 3y agoTrees are a much better solution to urban heating. Proven with millennia, eco friendly, and makes cities beautiful and desirable to live https://creativelyunited.org/wp-content/uploads/2019/02/temperatures-with-and-without-trees.jpg https://creativelyunited.org/wp-content/uploads/2019/02/temp...
- jjoonathan 3y agoTrees are swamp coolers. Consuming freshwater for outdoor air conditioning isn't necessarily a bad idea, but you aren't cheating thermodynamics.
- ComputerGuru 3y agoNative, grown trees need no watering, even in a (one- or two-off) drought.
- jjoonathan 3y agoSure, but less evapotranspiration = less cooling. You aren't cheating thermodynamics.
- ComputerGuru 3y agoApparently a good part of the reason (parts of) the Midwest can get uncomfortably humid some summers despite being so far from large bodies of water is the evapotranspiration from the crops (mostly the corn). So too much evapotranspiration is not always a great thing! (Of course those are being watered regularly.)
- kgermino 3y agoTrees are so much more than swamp coolers. 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.
- vbezhenar 3y agoYou can also insulate roof and it'll just heat air and radiate energy in cosmos. Heating air is not a problem because it's already somewhat high and will only move higher.
- sidewndr46 3y agoIt can't be "doubly effective". The energy cannot simply disappear into the void. If it is used to run something electrical in the house, that energy is still there. If you ship it into the grid and across the country, less of the heating is localized. But it is still heating as it travels through the wires. 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.
- wolverine876 3y ago> solar panels are often more reflective than the typical roof of a house in North America Reflective of what wavelengths? Isn't that an odd characteristic of something designed to absorb electromagnetic energy?
- TylerE 3y agoHonestly you'd get a much more effective reduction in AC needed, for much less money, by putting a decent ceramic tint on all the glass. There are quality tints these days that have minimal effect on visible light while blocking 90%+ of IR.
- neRok 3y agoI have roller shutters on my east and west facing windows, and my north facing windows (this is in Australia) get no direct sunlight because they are shaded by my patio. So no direct sunlight is coming into my house through the windows. Still in summer after ~2pm the house heats up. I think overall my house is decently insulated from the sun because in winter my house is extremely cold (what little heat the sun could give doesn't get in to my house). Recently in early spring there were days when the outside air temp was hotter than in my house. 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.
- mlindner 3y ago> 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. 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.
- deleted 3y ago[deleted]
- ck2 3y agoDo Solar Panels themselves not get glowing hot from hours of direct sunlight? 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.