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re: best hot air panel?
12 apr 2003
madra rua  wrote:

>i just came across a small book called solar air heater written by ray wolf
>and published by rodale plans, in 1981. it includes drawings and construction
>details for a 4' x 8' unit.  it's probably lurking in thousands of libraries.

ah yes. "solar air heater plans" by ray wolf, isbn 0-87857-369-3...
rodale press, 33 east minor st., emmaus, pa 18049, 1981.

wolf's book is very nice in its completeness of detail, but technically
out of date. you might also look for _the complete book of solar air heating
systems_ by steve kornher and andy zaugg. rodale press, 1984, which is more
general and has lots of practical hints. for instance, they suggest that
any material used in a solar air heater pass an "overnight oven test," ie
a night inside a 350 f oven to see how it fares. but kornher and zaugg's
book is also out of date.

they describe the ways people in southern colorado have home-built solar
air heaters (think how even a little mass-production would help...) under
the guidance of s. k. ramstetter, "the father of low-cost solar heating
in the san luis valley." building solar air heaters is a local hobby, or
at least it used to be. that part of colorado is cold and sunny, and the
people are not wealthy. they care about their propane heating bills. kornher
and zaugg say "this high mountain valley has more owner-built collectors
per capita than anywhere else in the world, and is often described as the
most solarized place in the u. s." 

wolf's design can be improved. it could be larger--a 4'x8' collector is
a toy, providing a very small fraction of the heat for an average house.
and building one of these sounds like fine cabinetmaking--screwed and
glued half-lap joints, etc. wolf uses a 1/2" sheet of plywood for the back
of the collector, with a 4x8 sheet of 5/8" insulation board over that,
which he says has an r-value of 5. why does the collector need a plywood
back? why not just add a perimeter frame to the existing house wall? and
why insulate the back of the collector, ie the surface between the house
and the collector? the heat losses from the back of the collector will
make the house warmer, and the house wall itself is already insulated.

wolf uses another 4x8' sheet of insulation board for the sides of the
collector. why insulate the wooden sides of a large shallow collector?
this does not seem cost-effective. in a 6" deep, 16'x24' collector, over
90% of the heat loss is through the glazing. wolf's flat fiberglass glazing
is now obsolete. greenhouse suppliers are dropping filon, etc, because
in five years or so, it yellows and gets fuzzy on the outside. thin
polycarbonate plastic seems like a better choice. 

wolf, or perhaps "robert flower, thermal engineer" also uses two layers of
black aluminum window screen, "to increase the amount of absorbing surface
inside the collector box without greatly inhibiting airflow," but in his
design, the air does not flow _through_ the screen, it flows along the back
of the screen, on the north side. it is thermally more efficient to have the
cooler air from the house rise up on the south side of the screen, between
the screen and the cool glazing, and flow through the screen, from south to
north, where it cools the screen and heats itself, and then flows back into
the house. this is called a matrix or transpired absorber collector.

that's how the conserval air heater works: a blower draws outside air
from south to north through a sheet of aluminum, painted black, with 1/32"
holes making up about 2% of the surface area. the absorber plate heats the
air as it flows through the plate. the conserval heater has an efficiency
of 80% without any glazing. adding glazing lowers the efficiency and raises
the price, but allows warming inside air from say 68 f to 130 f, instead of
just preheating lots of outside air from 32 f to 40 f to ventilate a building.

steve baer has been building passive solar air heaters like this for 20 years
with 5 layers of black painted wire mesh for the absorber. wolf says over and
over, "get black aluminum window screen. do not get fiberglass screen; it
will not collect heat." steve says the same. they probably mean that it won't 
collect as much heat in their designs. my guess is that a layer or two of
black fiberglass screen or black plastic shadecloth with air flowing through
the screen, not alongside it, will "collect heat," especially if the air
is pushed by a fan.

i do like this quote from page 8 of wolf's book:

  when talking about the efficiency of a solar collector, you have to
  consider not only how much energy you collect, but also how much it
  costs to collect it.

  let's look at two parked cars, each collecting solar energy in a
  parking lot on a sunny summer day. car a is a used vw costing $2,500,
  while car b is a "previously owned" deluxe rolls royce costing $42,000.
  let's say our vw collects the solar equivalent of 1 gallon of gasoline,
  while the rolls collects the equivalent of 1 1/2 gallons of gasoline
  during the day. which is the better deal? the cost per gallon of the
  vw gasoline is a fraction of that of the rolls; thus, although the
  cheaper collector doesn't collect as much energy, the cost per gallon
  is far below the more efficient rolls royce collector.

  we feel like we've designed a collector that works like a rolls royce
  at a vw price. our collector is the most cost-effective solar collector
  we know of. if you build your unit with entirely new materials it should
  cost you no more than $250, installed. the cheapest comparable-size
  commercial unit sells for slightly over $400, delivered but not installed.
  both units qualify for a 40% federal tax credit [no longer true.] 

  when you talk to solar salesmen, they will talk one of two numbers,
  depending on which favors their collector: cost per square foot of
  collector surface or btu's delivered per square foot of collector
  surface per year. the first is an indicator of the cost of the unit,
  the second, an indicator of the overall effectiveness... a combined
  figure gives you the cost-effectiveness of the unit--sort of a solar
  "mpg" rating. very few salesmen want to talk these numbers. 

wolf estimates that the 1981 installed cost is approximately $250...

chart 1-1 on page 8 of wolf's book gives an estimate for the annual fuel
savings for his unit, which varies from 25 to 50 gallons of oil per year.
in my area, this 32 ft^2 system saves 30 gallons of oil, and the annual
electricity consumption in active mode is listed as 731 kwh. at $1.50
per gallon of oil and 10 cents/kwh for electricity, the net annual savings
for one of these collectors is 30x1.50 - 731x0.10 = $45 - $73.1 = - $28.10,
ie i might invest $250 and 40 hours of labor and lose $28/year at best,
if i used the collector in active mode. so i wouldn't do that.

instead, i might buy 9 20' curved galvanized steel pipes from stuppy or
x. s. smith for about $250 and put them up on 4' centers, burying the
straight end of each pipe in the ground and attaching the other end at
the eave of the house, then attach a large sheet of 5 cent/ft^2 4-year
greenhouse poly film over that grape-arbor-like curved steel pipe frame,
then hang a piece of 15 cent/ft^2 black shadecloth inside to make a 16'x32'
solar air heater and 12' wide lean-to sunspace along the south side of
the house. warm air would flow out of the house through a passive plastic
film damper in a basement window, between the shadecloth and the glazing,
and back through the shadecloth into the house through a second floor 
window with another damper. this would take a few days to install, more
like a tent than a building. it would collect the heat equivalent of
about 500 gallons of oil a year, at a cost of about $350...

clear corrugated dynaglas polycarbonate glazing is another alternative for
the roof of the sunspace. the south wall might be covered with excel clear
flat polycarbonate, at $308 + $10 ups shipping for a 49"x50' roll from rimol
greenhouse systems. it's about r1, with a solar transmission of about 90%,
and it has a 10-year light transmission guarantee and a 20-year expected
mechanical lifetime.


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