Solar panels help keep buildings cool, reducing A/C needs | Hacker News Reader

Solar panels help keep buildings cool, reducing A/C needs

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65 pointsMikeCapone42 comments

When we were installing the system on our home back in 2003 the fact that it would keep the attic cooler was part of the literature. It wasn't a 'new' thing. But nit picking aside, the panels actually work less well when they are hot than when they are cool, so having air flow around them is very helpful. We also did an experiment where, using a drip irrigation system, we ran water down the roof behind the panels. On hot days it evaporates off the roof cooling the air between the roof and the panels still further. It was only a modest gain in efficiency though if you include the energy to pump water up to the roof in the first place. Being in the Bay Area the big win is when a cool breeze comes off the bay on a clear day. Cools the panels for best efficiency. On a good day we can do 36kWh for the day.

On an unrelated note. the 25year warranty on panels is problematic, we lost a panel and Sharp (the manufacturer) couldn't replace it because they no longer make the 185W panel in the same form factor (only the 165W panel) and since we're using 2nd gen series system that would reduce the whole string down to 165W equivalents. Fortunately we replaced it with a Chinese panel that fit on into the same mounts and had the required electrical characteristics. The bottom line was that you need a 'system' warranty not a 'parts' warranty if you want to be sure that you won't be buying all new parts when something breaks.

If you happen to own a swimming pool, you can also try running the water from there to radiators in the attic, cooling the house and heating the pool.

"if you include the energy to pump water up to the roof in the first place"

Most places (every place?) I've ever been has had enough mains pressure to get water to a rooftop without an additional pump. I like your idea!

Those who think the loss of solar heating on the roof in winter is an issue seem to assume the radiative heat transfer from the sun is constant between summer and winter. Winter has shorter days. And the sun is at a more acute angle in winter. So the amount of radiative heating from the sun is much less.

It reminds me of farmers planting deciduous trees near the house: shade in the summer, sun in the winter.

Exactly. Roof overhangs can provide the same kind of benefit if well designed. The sun is "higher" in the sky during summer so the overhangs provide shade for a building's windows, and the sun is "lower" during winter, allowing the sun's rays to hit the windows, providing some heat for the buildings. Gotta love low-tech self-adapting systems like these.

To anyone who doesn't think in Fahrenheit, a 5°F difference is 2.7℃ (took me a a few seconds of of thinking to get that conversion, since I couldn't just stick "5 F in C" in Google, since that yielded -15℃)

You were looking for the Rankine scale: 0°R = 0K, and n °F - (n-1) °F = m °R - (m-1) °R

https://en.wikipedia.org/wiki/Rankine_scale

Although you get less solar input to the shaded roof during the winter (the sun would be at such a low angle there'd be precious little anyway), the PV panels themselves put out more efficient power when it is cold out. Almost all solar panels are most efficient in the dead of winter, mostly due to less of a bandgap mismatch between silicon and terrestrial sunlight (note that efficiency and energy generation are entirely different from each other, and often inversely correlate). CIGS, GaAs, and CdTe panels don't have this benefit but they represent a minority of the panels being sold.

The argument for putting up shade structures instead of solar panels is pretty spurious- shade structures that last 30 years cost almost as much as PV panels these days, particularly after installation.

This remind of the Solar panel on top of canal.

http://en.wikipedia.org/wiki/Canal_Solar_Power_Project http://www.sunedison.com/wps/wcm/connect/b16fe5ba-78b4-4f1a-...

* Government owns canal, so they saved money by not buying expensive land.

* Solar panel helps reducing evaporation of water

* Water will cool solar panel, and it will increase production by 16% (heard on tv).

* They also wanted to install hanging micro hydro turbine under panels to produce electricity through flowing water.

I've always found it weird that we are burning fossil fuels, in order to generate energy, which will in turn be used to remove excess energy.

Running air conditioners from them seems like a better proposition. I didn't even know about the passive cooling provided by the panels themselves.

For places where panels are expensive, what about a radiator system? Circulate a coolant, remove heat from the roof of the house, release somewhere else. Might be cheaper than removing heat from the air inside the house.

Seems overly complex - just paint your roof white and reflect most of that light and heat back into the void of space.

You need white paint that works for infrared as well as visible light. But along those lines, instead of mounting solar panels, just mount something cheap to shade your roof.

And if someone figures out such a magical paint it's going to be an great business idea/overnight success story because you could put it on cars and reduce the afternoon-oven characteristics.

The body of a car is pretty insulating. The windows are the problem when it comes to heat.

And make sure it's properly insulated to keep the heat that is not reflected outside

Would just installing propped up reflective (or something painted white) panels above a roof have the same effect? Prop them up so that air can efficiently flow underneath to further reduce the heat your actual roof absorbs. The cost seems like it could be very minimal (as opposed to costly solar panels). The benefit over trees is that you don't get leaves stuck in your gutters or branches falling and damaging your home.

It doesn't look as though the authors factored in any increased heating costs in wintertime. Is this effect negligible?

I spoke with a LEED guy once about this.

Heating efficiency is more a function of heat escaping the house through poorly sealed doors and windows, poor insulation, etc. You lose heat from every side/top of the house. The heat gained inside a house from winter sun on a roof in a cold environment is negligible. The best strategy for maximizing heating efficiency is to get better windows/doors and insulation.

This compares to the summer, in which the majority of heat is gained by solar radiation on roofs and walls, with a much smaller amount being obtained through heat leaking into the house or radiating through doors/windows (assuming somewhat modern doors/windows).

You are forgetting some heat sources already present that can have a big effect:

Humans - yes humans

From http://aheatingguysblog.blog.com/2009/09/23/human-body-btu-o... :

"The human body at rest will produce about 250 BTU while sleeping, At idle about 400 BTU, doing light work about 650 BTU, and with heavy work up to 2400 BTU. That means 25 people dancing in a 2000 square foot home with temperatures of -10 degrees outside would never need a furnace. This is why bars and misc other places tend to have swings in temperatures more frequently."

Electronics and lighting

Incandescent and halogen lighting while dropping in usage is still huge. Computers or any other machinery.

Insulation

Not a heat source but slows down the heat transfer.

As someone who has mostly lived in hot climates, I have never understood why all our homes here in the South (Gulf Coast) have black roofs. I want to paint mine white badly, but I am afraid it will look terrible.

I'm also confused by my HVAC return air duct opening at floor level instead of ceiling. I only use heating a few weeks out of the year. But the climate is such that A/C is needed often for dehumidification as well as temperature control.

For one thing, because the sun is lower in the winter time, the solar gain deflected in the summer is greater than that lost in the winter.

Heat gain/loss through the roof and other opaque parts of the building shouldn't be an issue because insulation is cheap and about 200mm of modern insulation is all you need before air tightness and AC efficiency become the major factors for energy loss. Also, cubicle farm style office buildings are typically in cooling mode, i.e. they are rejecting excess heat for at least 3/4 of the year even in a temperate climate because of the heat generated by people, IT, lighting etc. Therefore reducing solar gain through windows is pretty critical to improving energy efficiency by reducing the load on the cooling systems. So using solar panels as part of a brise soleil to provide shading to windows would be the optimum solution but this is a more expensive facade structure to build and more expensive and less safe to maintain, so it's typically only done on high spec buildings which need some eco bling.

Residential buildings have different factors in play. Residential efficiency is all about retaining as much heat as possible in winter through air tightness, winter solar gain through windows and insulation. With good insulation and solar shading, summer heat gains are not so much of an issue for residential in temperate climates; for example most people in Northern Europe don't have AC because there are only a few weeks of the year when you would have it switched on.

I think it depends where you are, but the extra insulation could help:

> For example, in winter, the panels would keep the sun from heating up the building. But at night, they would also keep in whatever heat accumulated inside. For an area like San Diego, the two effects essentially cancel each other out, Kleissl said.

If you are a homeowner, you want your attic to be as similar to the outside air as possible, especially in winter. A warm attic and a freezing outside is a recipe for ice dams.

The solar heat gain is close to 1kw per square meter. The wood stove that heats half of our house is only 6kw.

> The solar heat gain is close to 1kw per square meter.

Where, for how long and when? Certainly not when you need to regularly use a wood stove for heating, at least not around here in northern Europe. :)

I'll have to check the numbers. The 6000W came from the stove's rating tag. It's a small stove that supplements an oil burner to make the living room and kitchen comfortable. The oil burner keeps the whole house warm enough to not burst the pipes.

I'm not sure where I read 1000W but it is for a square meter perpendicular to the sun's angle. Here in North East USA, solar installation folks say we get "3 hours" of direct sunlight per day in winter and 6 hours in the summer. I think that accounts for shorter days and weather.

I have been thinking about it for a while. In a hot and dry environment (az, nm, tx) most of the heat is gained by the sun. It is easy to see. If you leave your car in the sun, it will be extremely hot. But if you leave your car in the shade of a tree it will be ok.

You don't even need the solar cells, all you need is to create a shade for your house. Something that covers the sun, but still allows air to go through. Trees is probably the best option, if you have the space. Other materials could be some sort of ceramic tiles that rest about 10 cms above the ceiling of the house or use a shade cloth. I have tested the shade clothes, they seem to be very thin and they let a lot of light go through, but if you touch the concrete floor underneath it is not too hot as opposed to the place where the shade is not covering the floor.

I always wondered if that was the case, so it's nice to see someone actually checking it out. I do wonder why some of the numbers weren't more specific. A 5% discount on the solar panels over their lifetime is kind of vague. What was the price? What is the panel lifetime?

Doesn't this argue for just mounting some kind of cheap shade on your roof?

I've thought about this before, but on a large scale. As in, millions of square miles of solar panels. Could you effectively regulate the temperature of the earth by modulating the amount of sunlight that is converted to heat by turning the excess in to electricity?

But the electricity will be used to power all sorts of appliances, most of which generate heat. So I don't think there will be a noticeable difference on a planetary scale unless you use most of the electricity to ship energy out to space. If there is any change to the global temperature, it will probably be due to the albedo change (amount of sunlight that gets reflected off all those shiny panels of glass).

You could always use that excess electricity to run energy-intensive processes to store the power chemically. For example, split water.

No, since electricity is just another form of energy that will sooner or later be turned into heat. You'd have to remove the energy or store it to regulate the temperature, so either mirrors or any form of long-term energy storage would work.

Or turning it into chemical bonds like plants do? It reminds me of Buckminster Fuller describing fire as "sunlight unwinding from tree trunks". Or burning oil as unwinding sunlight from lots of prehistoric plants and animals.

I don't know much about solar panels, so a silly question:

If the panels are 15% efficient, what happens to the remaining 85% of the sun's energy?

It's either reflected or turned into heat within the panel.

So the solar panel itself heats up the environment, even if the roof is in shade?

In the same way that a normal roof heats up the environment, yes.

Doesn't this also mean they increase heating needs?

There might be ways to mitigate this if you are in an area that gets very cold for a large portion of the year, such as having the panels flat against the roof so that they conduct heat into the roof.

But in most places that get that cold, roofs are covered with snow, so I'm not sure how that would work since most of the sun's rays would be reflected anyway (though the snow provides extra insulation..). Hmm..

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