Hacker News Reader

Comment by jl6 | Hacker News Reader

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

View on HN

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

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.

Reply on news.ycombinator.com