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Re: Solar Energy



Barry Smith  <barry@bluesky.com> wrote:
>lajoie@eskimo.com (Stephen Lajoie) wrote:
>>Nick Pine <nick@vu-vlsi.ee.vill.edu> wrote:
>>>Stephen Lajoie <lajoie@eskimo.com> implacably waxes:
>>>
>>>> ...giving solar every benefit of a doubt...
>>>
>>> Solar what?

>> Solar anything. I figured the area, then applied a thermodynamic 
>> efficiency to get the total area in shadow. It doesn't matter at all how 
>> you convert it. Someone threw out a 30% efficiency for PVs. Okay, so I 
>> used PVs. With an assumption of PVs, I showed that the cost would be 
>> far more than the country could spend.

In talking about costs, the efficiency and the technology do matter.
 
>By your statement, Edmund's price is $9375 per square meter, which most
>folks in the business would say is a bit high.  Your mark down brings
>that down to $4687.50, and your miracle factor of ten reduces that to
>$468.75 per square meter.

Which is about $40 per square foot. Polycarbonate glazing costs about
$1 per square foot in small quantities. You don't need much more than
that to collect solar hot air. In fact, you can do a good job collecting
solar hot air with some curved steel pipes, say 20' long, on 4' centers,
such as the ones used in commercial greenhouses, leaning against the south
wall of a two-story house, covered with a large piece of 5 cent/ft^2
polyethylene film, which is guaranteed for 3 years, with a large piece
of 15 cent/ft^2 greenhouse shadecloth inside the greenhouse. 

For $600 you can buy all the material for a 600 ft^2 sunspace from a
commercial greenhouse supplier like Stuppy: some curved steel pipes and poly
film that you can turn into a sunspace in a day or so. Three people can
put up a 3000 ft^2 commercial greenhouse in less than a day... This sort
of arched lean-to structure is nicely described on pages 34-36 of William
Shurcliff's 1978 Brick House book, _New Inventions in Low-Cost Solar Heating--
100 Daring Schemes Tried and Untried_. 

Dr. Shurcliff says "Here we discuss a scheme that may be ideally suited to
retrofit application to existing houses the south vertical faces of which
receive much solar radiation even in winter. The scheme is much like that
pioneered by S. R. Kenin and others of Solar Room Co./Box 1377/Taos, NM 87571.
There are already several buildings that use his scheme, or schemes reasonably
similar to it. An example is Egri House in Taos, NM. This house is described
in detail in my book _Solar Heated Homes of North America: 120 Outstanding
Examples_... ***It is hard to think of any other system that supplies so much
heat (to an existing house) at such a low cost***." He goes on to explain how
to make this a low-thermal-mass sunspace, eg by hanging a piece of greenhouse
shadecloth inside, which costs about 15 cents per square foot and comes in
rolls up to 30' wide and 300' long. (It comes in green too.)

In the summer, you might hang the greenhouse shadecloth over the poly film,
on the outside. This would prevent overheating and provide shade and make
the poly film last longer.

Dr. Shurcliff talks about using a thin layer of insulation on the north wall,
and says "Then, practically no heat is delivered to the massive components
of floor or wall; practically all of the heat is promptly transferred to
the air. And since the thermal capacity of the 100-200 lb. of air in the room 
is equal to that of one fourth as great a mass of water (about 25 to 50 lb.
of water), the air will heat up very rapidly. I estimate that its temperature
will rise about 40 F degrees in about two minutes, after the sun comes out
from behind a heavy cloud cover [unlike most solar collectors,] At the end
of the day, little heat will be "left on base" in the collector floor or
north wall and accordingly, the enclosure will cool off very rapidly [and
very little of that captured solar heat will be lost at night, unlike, say,
a Trombe wall.] 

This is more like a transparent tent than a structure, but the galvanized
steel pipes last at least 40 years, I'm told. You could also grow grapes on
them. Sounds great (or grape :-), doesn't it? Whatever happened to this idea?
Why aren't more people using it? Why isn't real Goods selling it, somehow?

I'm not a passive purist: I would probably put a $12 20" box fan in a second
floor window looking out into that sunspace, with a thermostat, and a plastic
film damper of course. This would make it work better, and it would be
cost-effective. (Fans move more air per watt than blowers, and larger fans
are more efficient than smaller ones. Norman Saunders says that the
efficiency of a fan is about the same as the blade diameter in inches.)

Why doesn't someone make some sort of modular solar air heating kit? A
well-thought-out, lightweight, large, inexpensive version that could be
shipped UPS? Plastic film, shadecloth, etc. No blower. I just got a 10 pound
UPS package with an 16' x 32' piece of 80% black shadecloth, with hems and
grommets on all four edges, a part of a 500 square foot solar air heater,
(that's about 50 kW, for all you PV people) from PAK Unlimited in Norcross,
Georgia at (404) 448-1917. The cost was $71.25, including shipping. 

Low-thermal-mass sunspaces seem like a good way to heat houses, perhaps
with the warmstores of Norman Saunders or my solar closets, ie insulated
boxes with containers of water inside them, heated by sun-warmed air.
With a fin tube just under the closet ceiling and a natural warm water
convection loop to a water heater upstairs that provides close to 100%
of the hot water for the house, even after a week or two of cloudy days,
while the solar closet provides 100% of the space heating...

Nick


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