Hacker News Reader

Comment by Qem | Hacker News Reader

> The United Nations (UN) believes there are 4B buildings on Earth. This week, a dataset called "GlobalBuildingAtlas" (GBA) was published by researchers at the Technical University of Munich (TUM) that attempts to estimate this number at being closer to 2.75B.

If we painted the roofs on all of them white, by how much would the temperature of the planet drop?

View on HN

Probably not as much as if we “painted” them black with solar panels and used the resulting electricity to displace fossil fuel burning.

A good percentage of Pakistan did this recently and removed 35% of demand off of their grid https://www.ted.com/talks/jenny_chase_solar_energy_is_even_c...

I would love to have an air conditioning / cooling solution that is directly linked to solar panels with no batteries involved. Like the sun shines, we get electricity, we do the work. My main goal for this thought experiment is to come up with uses of solar electricity that is resilient to the unpredictable and unreliable energy generation from solar. Thoughts?

Though experiment? This is already a thing in the off grid community. In practice you need at least small battery to smooth out the power, but it doesn't take much home automation to kick on the mini split when the panels/inverters have power to spare.

> I would love to have an air conditioning / cooling solution that is directly linked to solar panels with no batteries involved.

I think that would be very viable with fridges, that represent a large share of electricity consumption among the poorest. Before electricity, people powered fridges by constantly buying ice blocks. They were just isolated boxes where food was stored together with the blocks. Perhaps it's just necessary to go back at the roots, and make fridges that take energy from solar panels and generate a lot of ice by day, and uses it to keep cold at night, with no need for batteries.

I love this idea but I don't know enough about the specifics. Isn't it really bad(TM) for the pump or compressor or something I don't know about for the input power to be variable like this? Like there might be an errant cloud somewhere.

The whole point of my thought exercise is to see if we can somehow make the cost go down. My understanding is that the panel can easily last twenty five years but the battery you'd be lucky to go beyond eight?

Edit: good news / bad news

Bad news: this is not an original thought

Good news: smarter people than me are already working on this. See solar Variable Frequency Drive (VFD). Basically, my thought is a pump is a pump. if you can build a pump to pump water to irrigate poppy fields, you can use the same pump to drive refrigerant in a refrigerator/ freezer / heat pump.

> I love this idea but I don't know enough about the specifics. Isn't it really bad(TM) for the pump or compressor or something I don't know about for the input power to be variable like this?

No, modern refrigerators and other white box appliances with variable speed motors use electrically commutated (aka EC or brushless) motors that allow for motor speed control. Larger three-phase induction motors can have their speed controlled by a variable frequency drive (VFD).

> Basically, my thought is a pump is a pump. if you can build a pump to pump water to irrigate poppy fields, you can use the same pump to drive refrigerant in a refrigerator/ freezer / heat pump.

You’re close, but it’s more ‘a motor with enough power can drive any pump (or fan)’ than ‘a pump is a pump is a pump’, as there are many different kinds of pumps for various working fluids (water, glycol, oil, refrigerant, etc)

Regarding the fully solar powered A/C, you can smooth out power generation and consumption using capacitors (aka batteries)

Supposedly water heaters are an amazingly good target for variable power. Water heating takes 18% of home energy use[0] and is already a well insulated storage device. Just heat it up as you are able with the day's sun.

If you are looking for a cooling solution, you could go the other way and make water chillers through a dedicated water tank. You would tie the HVAC to pipe air through a heat exchanger. Seems like all of that is well established engineering.

[0] DOE link on water heating https://www.energy.gov/energysaver/water-heating

I'm now wondering if painting a roof white is better than covering the same roof with solar panels and using that to drive air conditioning (in the house). My intuition is that painting it white must be vastly more environmentally friendly, although it probably doesn't reduce the temperature very much compared to aircon.

It's worth factoring in that solar panels also provide solar insulation to the roof by effectively shading it, ie the sun shines on the panels and there is an air gap between the panels and your roof.

In cooler climates, you are now taking a hit on heating costs. That now blocked Winter sun would have provided some amount of free warmth.

If it's that cold, you probably have a fair amount of roof insulation anyway that's preventing heat absorbed by your roof from being conducted to the conditioned space.

Sure, but the physics still cuts both ways. Either sun hitting the roof causes measurable heating or it does not. The temperature gradient must flow.

Sun heating in Winter = awesome. Sun heating in Summer != awesome.

I am not arguing the strength of the effect, only that you must account for all seasons.

Does the house already have AC or not? And are you talking about indoor temperature or outdoor temperature? Also many places are cold. Running AC would make it worse.

There are certain materials that aren't just white but also specifically radiate heavily in IR bands that the atmosphere is particularly transparent to and so can actually stay sub-ambient temperature while in sunlight. See, for example, https://onlinelibrary.wiley.com/doi/full/10.1002/eom2.12284

But adding conventional AC is never going to cool down the world.

AC doesn't make it cool it merely transfers the heat to the outside in an energy efficient manor. The emerging tech is special coatings that convert heat to an infrared frequency that easily escapes the Earth's atmosphere.

Most manors aren’t very energy efficient. Take Downton Abbey for example.

Thank you!

man·ner /ˈmanər/ noun 1. a way in which a thing is done or happens.

Given the era it is about, Iy could be considered energy-efficient, as the carbon footprint of individuals and households was much lower a century ago.

There was a good study on this a few years ago that ran the numbers on this and landed on white paint for residential homes as the best option, for a few reasons, if I remember correctly:

- Installation, maintenance and transmission costs are lower when solar is aggregated on farms - Solar offsets air conditioning, but that moves the heat outside. White roofs reduce the need for AC, which helps significantly with urban heat scenarios

A quick search yields a UCL study, which supports the lower claim: https://phys.org/news/2024-07-roofs-white-city.html

I would guess not that much - certainly fractions of a degree - because the proportion of the planet's surface is still relatively low, and as much as 60% of that reflected light would be scattered and absorbed by the atmosphere.

It'd have a much bigger impact if all those roofs had solar panels, and the resulting electricity was used to replace carbon-emitting energy sources.

Paris and other climate accords have much ado about fractions of a degree, leading me to believe that that's highly consequential.

I think we should do more albedo engineering. like white roads and car roofs.

White roads could potentially be blinding, but yeah something lighter than what we do currently could be very worthwhile. It'd have much higher nighttime visibility too.

> White roads could potentially be blinding

Hell I was just walking down the street a minute ago and thinking the same! It's October ffs! (It IS October right?)

I'm thinking about light colored roads that seem to be made of concrete. See them here and there. Seemed to be more of them when I was a kid.

Wonder if that would make a substantial difference? Much brighter than asphalt but not bright enough to bother drivers.

I was thinking you could potentially engineer them to reflect IR light, but I feel like dust and dirt would probably quickly eat into the effectiveness. The question is whether it'd really make a difference on a global scale at all.

There are materials that do what you're looking for surprisingly effectively, staying many degrees sub-ambient even in direct sunlight. (See https://onlinelibrary.wiley.com/doi/full/10.1002/eom2.12284 for example). But a roadway is a really difficult use case for a surface coating. With something like 50% of the surface area of many American cities being road or parking, there's a lot of potential room for effectively mitigating the urban heat island effect, but I think roofs are a better target. They don't have cars sitting on top of them blocking sightlines nearly as often.

Back-of-the-envelope - Road area. World road length ≈ 60–70 million km. Using an average paved width of ~8–10 m ⇒ area ≈ (0.9–1.3)×10¹² m². Earth’s surface is 5.1×10¹⁴ m², so roads cover ~0.09–0.13% of the planet.

- Albedo change. Dark asphalt is ~0.05–0.10. “White” coatings can push toward ~0.4–0.6 (fresh), but weathering quickly dulls them. So a plausible Δalbedo for roads is +0.2 to +0.5.

- Global albedo change. Δα_global ≈ (road fraction) × (Δalbedo_road) ≈ (0.001)×(0.2–0.5) ≈ +0.0002 to +0.0005.

- Radiative forcing. Globally averaged incoming sunlight ≈ S₀/4 ≈ 340 W m⁻². Forcing from an albedo change is ΔF ≈ −Δα_global × 340 ≈ −0.07 to −0.17 W m⁻².

- Temperature response. Using a standard sensitivity ~0.8 °C per W m⁻² (≈3 °C per CO₂ doubling): ΔT ≈ −0.05 to −0.14 °C at equilibrium.

Teslas will start crashing like there’s no tomorrow.

So nothing much will change?

there is a cool project that explores this topic: https://coolcolors.lbl.gov/

Painting roofs (or even all man-made surfaces) white wouldn't do much globally at all, but it might reduce the urban heat island effect. But probably (speculating a bit here) not quite so much as creating more green spaces (or green roofs, for that matter).

Reply on news.ycombinator.com