Roofing material converts sunlight to electricity

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Rooftop photo voltaic, roll roofing does look slightly more attractive than any kind of SPS in the sunniest locations. Let's assume 30% efficiency (optimistic) For new construction the cost including some batteries, inverters, and some extra wiring averages $10 per square foot more than a conventional roof on the portion of the roof that faces South or South-west. We get 10 watts per square foot, 5 hours per day. The sun angle is unfavorable the other hours of daylight unless the roof is steerable. We do get a tiny amount of power the other hours, especially early afternoon in late June when the sun is about perpendicular to a steep roof pitch. Figure an average of 500 square feet as some roofs are shaded by trees and/or taller buildings. That's 25000 watt hours per day = 25 KWH worth about one dollar per day at medium wholesale Electric prices. The $5000 extra cost is payback in 5000 days = almost 14 years. Somewhat less for the few roofs that have more than 1000 square foot of suitable roof area. A new subdivision could be planned with maximizing usable roof area but typically other considerations have priority unless the developer receives a large inducement.
Battery replacement may be $1000 over the 14 years. But the home owner may requard that as reasonable as the family has some electric power during electric utility failure. Payback is forever, unless the utility is willing to pay an average of 4 cents per kilowatt-hour for electricity put on the grid before the peak demand period begins. Pay back is typically forever, installing photovoltiac roll roofing on existing buildings that need the roof replaced. In some locales roll roofing is illegal for single family homes, so there is a cost in getting the code changed and doubling the typical roof pitch. Almost always the family will be a net user from the grid during the peak demand period, which will almost always be after the 5 hours when the sun angle was favorable.
Perhaps a million new buildings are built per decade in the sunniest locations in the USA, so we are adding 5000 megawatts = 2 large power plants to the generating capacity of the sunniest locations at a time of day when the energy is not needed. In locales other than the sunniest, which would average perhaps 15,000 watt hours per day, either the owner or the government is providing a subsidy of several hundred dollars per year, except for installations which are never shaded by trees or taller buildings during the 5 hour period of favorable angle.
80% of the USA population lives in locations that are cloudy more than 20% of the midday hours, so these probably should not attempt solar voltaic, even if the roll roofing panels only cost one dollar per square foot = $500 for 500 square feet. Presently the photovoltaic roll roofing costs about $15 per square foot plus the cost of installation. Neil

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Now, this one actually sounds practical.

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Even a standard black shingle roof absorbs a lot of heat. Too bad we have to spend all this money (on photovoltaic cells) converting that heat into electricity, just so we can turn most of it back into heat again to heat the house.

Isn't there a way to store the sunlight induced heat of the day so that it can be used at night without converting it to electricity at a 70% efficiency loss? What are these "heat sink" systems I've heard about?

Then if every house simply saved that much energy the primary grid would have that much more excess to work with.

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Well, here in Germany, this (rooftop phto-voltaic, I mean) is quite common meanwhile, since roofs really don't look different to 'normal' ones:

http://www.solarserver.de/solarmagazin/images/laumans1.jpg

Rooftiles in the upper part are with photo-voltaic, lower part normal ones.

http://www.solarserver.de/solarmagazin/images/ziegelbraun.jpg

http://www.solarserver.de/solarmagazin/images/ziegel.jpg

Two kinds of photo-voltiac rooftiles (in the middle: normal)

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2004 2:43 PM

Hi Walter: I can tell you really want USA etc to start putting the
photo voltaic on roofs. I do too, but we should not go beyond
some pilot programs, if we have to ignore the real costs and
other problems. You did not address most of my objections,
perhaps because there are few sources of detailed information
without lots of false inferences. Suppose Uncle Sam offers one
dollar per square foot cash for roofs that face between South to
South-west that have a slope over 12 degrees, plus an additional
30 cents per degree of slope up to 45 degrees. Over 50 degrees of
slope is counter productive, so we will subtract $30 per degree
of slope over 50 degrees. That will give the builder up to about
$1500 per building for optimizing the layout of the subdivison and
the roof designs. He may spend all of the $1500 on extra
materials for the steeper roofs, and another $400 per unit for
workmen's comp for the workers who fall off the steeper roofs.
Will he optimize even though it means a loss? Perhaps the $5000
extra cost is not pessimistic
Will some of the buyers balk at roll roofing? How do we expect
to interconnect the wires coming from each one meter strip of
roll roof material? Worse if there are 400 separate shingle type
photovoltaic panels, how do we make the electrical connections?
Perhaps best is up to 1000 tiny holes through the wood roof boards so the
wires can be connected inside the crawl space and repaired if a
bad connection occurs in a week or a decade. Parts of the crawl
space are over ten feet tall (3 meters) so finished rooms would be logical.
How do we retain access to the connections? Some buyers will not
want the battery bank inside the house or even the high current
wiring to the inverters. Is this going to be a 220 volt dc, 120 volt dc
system or some lower voltage? Can we repair roof leaks without
damaging some solar panels? Neil

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Well, due to German and EU-laws (you can't built a house as you want to - there are strict rules Twisted Evil

), there aren't many problems with such roofs .... besides the cats, of course :wink:

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Here's a secret - the best photo electric cells are hideously expensive per square metre but only 30% efficent. A recent discovery might boost this closer to 50% in a year or two reading Australian Scientist a month ago.

But 8 years ago Canon Research (and they are very big and well funded) developed a material like aluminium that is 15% energy efficient. So what you say - half as good as the best. Well this substance would only cost $2 a square metre to produce to final manufacture. So you could cover you entire house with a substance like this for the same cost of 1 or 2 metres square of top end material.

They never brought it to market - pisses you off big time hearing this doesn't it!

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yes

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rosborne979 wrote:

......Isn't there a way to store the sunlight induced heat of the day so that it can be used at night without converting it to electricity at a 70% efficiency loss? What are these "heat sink" systems I've heard about?

Then if every house simply saved that much energy the primary grid would have that much more excess to work with.

There is indeed, and it has been around for many years; it is called a tromb wall - a heavy wall made of a dense material (usually concrete) that is placed inside the home, away from exterior walls. This 'tromb' wall is hollow inside forming a kind of chimney.
Solar heat gain warms the wall, and the air contained inside the cavity; this air can be evacuated in the summer, to cool the home and recirculated in the winter to heat the home; the heat trapped in the wall also helps to heat the home at night.

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Hi BoWoGo: I had not heard of the Tromb wall before, but it should be quite helpful if big enough. Most of the passive (and almost passive)systems take up much floor space and/or require the house to be an unconventional shape, with not much flexibility.
When rosborne first asked, my thought was a dozen or more one or 2 millimeter tubs running the entire length of specially made roll roofing. Connections that didn't leak at the edge of the roof would be challenging, and the roll roofing with interior tubes might be costly even mass produced. A useful amount of warm water would be produced averaging 10 degrees c = 18 degrees f warmer than the air temperature under low wind, bright sun conditions. Much better performance would occur with a plastic tarp over, but not quite touching the roofing. That would be like a green house or the solar collectors used for solar hot water heaters. I can't think of any way to make the tarp stay put in high winds, Any ideas? Water stores more heat than stone and concrete, but a big leak can do much damage to the house. Neil

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BoGoWo wrote:

There is indeed, and it has been around for many years; it is called a tromb wall - a heavy wall made of a dense material (usually concrete) that is placed inside the home, away from exterior walls. This 'tromb' wall is hollow inside forming a kind of chimney.
Solar heat gain warms the wall, and the air contained inside the cavity; this air can be evacuated in the summer, to cool the home and recirculated in the winter to heat the home; the heat trapped in the wall also helps to heat the home at night.

Sounds good. Why aren't there more of these things in more houses?

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I've heard of the tromb wall but haven't followed all this for years. Had batches of info in my bookshelf in the early eighties. Still it makes sense, just in materials absorbing and letting go heat.

We have a project where next phase will involve solar panels... and our job is to hold them up on the house, by some sort of post and beam structure or corbels we devise to hold a frame. We are rather looking forward to it, our client is an environmental atty.

I guess I should explain that we are not housing architects - I am a landscape architect.

Anyway, I'm here listening.

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Will this work in locations with large pigeon populations? :wink:

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Hi Tailbone: The average watts out of the solar panels will fall as the pigeon poop accumulates. At some point, someone will have risk life and limb to scrub off the poop and other crud, before useful power reaches zero. You have identified a significant overhead cost often over looked in calculating years to payback. Neil

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rosborne979 wrote:

Even a standard black shingle roof absorbs a lot of heat. Too bad we have to spend all this money (on photovoltaic cells) converting that heat into electricity, just so we can turn most of it back into heat again to heat the house.

Isn't there a way to store the sunlight induced heat of the day so that it can be used at night without converting it to electricity at a 70% efficiency loss? What are these "heat sink" systems I've heard about?

Then if every house simply saved that much energy the primary grid would have that much more excess to work with.

I think what you are referring to might be heatpump systems.

A transfer fluid, such as the well-known freeon (old skool), is pumped through a cycle where it is compressed to extract heat and expanded to extract heat. So heat is pumped from a source to a sink.
The heat is free, you only pay for the electricity to run the pump.

When one side of the system in put into the ground (GEOTHERMAL HEAT PUMP SYSTEM) the ratio of energy output to energy input (CoP or Coeficcient of performance) can be around 5. In other words your energy bill for heating drops to 1/5. The Ground becomes your heat reservoir and heating as well as cooling modes are possible.

Ontarios hydro one is loaning people money to install this system, since their grind is getting overloaded.

My analysis such a system in the mild Vancouver climate showed a payback period between 6 and 8 years.

There is lots of resources for GHP all around the world, it's definitly worth considering for any new home (retrofitting will be more expensive and needs to be looked at case by case).

Cheers

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Roofing material converts sunlight to electricity

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