
Solar panels help keep buildings cool, reducing A/C needs - MikeCapone
http://www.treehugger.com/renewable-energy/bonus-solar-panels-help-keep-buildings-cool-reducing-c-needs.html
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ChuckMcM
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.

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YokoZar
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.

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paulyg
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.

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

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MikeCapone
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.

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kalleboo
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℃)

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cscheid
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](https://en.wikipedia.org/wiki/Rankine_scale)

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mchannon
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.

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thekingshorses
This remind of the Solar panel on top of canal.

[http://en.wikipedia.org/wiki/Canal_Solar_Power_Project](http://en.wikipedia.org/wiki/Canal_Solar_Power_Project)
[http://www.sunedison.com/wps/wcm/connect/b16fe5ba-78b4-4f1a-...](http://www.sunedison.com/wps/wcm/connect/b16fe5ba-78b4-4f1a-a733-21bd056dfe7e/Case_Study_Narmada-
FINAL.pdf?MOD=AJPERES&CACHEID=b16fe5ba-78b4-4f1a-a733-21bd056dfe7e)

* 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.

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outworlder
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.

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

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darkarmani
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.

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sigkill
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.

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sp332
The body of a car is pretty insulating. The windows are the problem when it
comes to heat.

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gocard
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.

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

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

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zxcdw
> _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. :)

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waivej
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.

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huherto
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.

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mcantrell
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?

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

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tmuir
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?

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kijin
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).

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

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RachelF
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?

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

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

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

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Splendor
Doesn't this also mean they increase heating needs?

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MikeCapone
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..

