> "Solar Air Heater Plans" by Ray Wolf, ISBN 0-87857-369-3. Published by Rodale Press, 33 East Minor St., Emmaus, PA 18049 in 1981.

I think Wolf's book is very nice in its completeness of detail, but technically out of date.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 should pass an "overnight oven test," ie put the material inside a 350 F oven overnight and see how it fares... But Kornher and Zaugg's book also seems out of date.

They describe the ways many people in southern Colorado have made 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 air heaters is a local hobby, or at least it used to be. This is not too surprising, since that part of Colorado is very 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." The home of an orphan handcrafted technology...

Perhaps Wolf's design can be improved. For one thing, it should be larger-- a 4' x 8' collector is a toy, providing a very small fraction of the heat for an average house. For another, building one of these sounds like fine cabinetmaking--screwed and glued half-lap joints, etc. Wolf uses a 1/2" CDX sheet of plywood for the back of the collector, with a 4 x 8 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 4 x 8' 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' x 24' collector, over 90% of the heat loss is through the glazing. And Wolf's flat fiberglass glazing may now be obsolete. Greenhouse suppliers are dropping Filon, etc, because in five years or so, it yellows and gets fuzzy on the outside. Thin clear 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, using 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 shadecloth, not alongside it, will "collect heat," especially if the air is pushed by a fan. I'd use black aluminum screen for a solar closet air heater, and less-expensive greenhouse shadecloth for a lower- temperature sunspace air heater. (Boiling one shadecloth sample for a few minutes shrank it by about 20%, altho it stayed strong.)

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 of the unit. 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.

Robert Sanders continues:

> They estimate that the installed cost is approx. US$250. I priced the main materials using clear pine rather than #2 grade and came up with US$279, plus paint, etc. I estimate that total cost will run about US$370.

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. His chart says that in my area, this 32 ft^2 system would save 30 gallons of oil, and the annual electricity consumption in active mode is listed as 731 kWh. I pay about 64 cents per gallon of oil and 11 cents/kWh, so the net annual savings for one of these collectors is 30 x .64 - 731 x .11 = $19.20 - $80.41 = - $61.21, ie I might invest $370 and 40 hours of labor, and lose $60/year on my investment, at best, if I used this collector in active mode. So I wouldn't do that.

Instead, I'd buy 9 20' curved galvanized steel pipes from Stuppy or X. S. Smith in New Jersey for about $250, put them up on 4' centers, burying the straight end of each pipe in the ground and attaching the other end to a horizontal board under the eave of the house. Then I'd attach a large sheet of 5 cent/ft^2 3-year greenhouse poly film over that grape-arbor-like curved steel pipe frame, and hang a piece of 15 cent/ft^2 black shadecloth inside, to make a 16' x 32' solar air heater and 10' wide lean-to sunspace along the south side of my house. Warm air would flow out of the house through a passive plastic film damper in a first floor window, between the shadecloth and the glazing, and back through the shadecloth into the house through a second floor window with another damper. I'd hang the shadecloth over the outside of the glazing in summertime.

This would take a day or two 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. It wouldn't overheat my house, since it has stone walls, bare on the inside and polyurethane-foamed and stuccoed on the outside. Other people might need to open a window on sunny winter days, or turn on an exhaust fan with a thermostat, or add some thermal mass to the inside of the house. Another layer of drywall, 2-liter soda bottles or drums full of water, concrete furniture... :-)

Other people might use a clearer plastic with a longer life, perhaps Jade Mountain's 5-year, 43 cent/ft^2, Tuff-Glass, which comes in 48" x 144' rolls. This would be have to be joined at each curved steel pipe, vs. the large poly film sheet, which need only be attached along the 16' and 32' edges. Another glazing option is Dupont's heat-sealable, clear, UV-transparent, Tedlar PVF film, which is very strong and light, and should last about 10 years. I think this is available in 5' wide rolls for about 15 cents a square foot, with minimum orders from Dupont of about $3000. Perhaps Real Goods could distribute this product, heat-sealed into larger sheets. UV transparency would allow tanning inside the sunspace. Young urban professionals might like that.

> The book is well laid out and can easily be followed by a novice. The collector has internal baffles to increase efficency and a vent to allow air to pass during summer months. They suggest the addition of 2 4" muffin fans (the kind used on computers) along with a thermostat which is a snap disc design to turn on at 110 degrees F and off at 90 degrees F.

Wolf says that adding the two 105 cfm, 24 watt (ie 4.4 cfm per watt in free air) Grainger fans almost doubles the efficiency of the collector. When I look in a Grainger catalog these days, the most efficient 4" muffin fan that I see is their stock number 4C827, which delivers 70 cfm at 8 watts (8.75 cfm/watt in free air), and costs $20.50, and has a maximum temperature rating of 120 F. I prefer Grainger's 4C688 10" diameter fan, which delivers 560 cfm and uses 36 watts (15.5 cfm/watt in free air) and costs $60.75, and has a maximum temperature rating of 149 F. One can add a speed control (Grainger SC343, $16.02) to move less air with less electrical power. For moving more air, I like the $12 K-Mart 3-speed slimline HABF-20 20" box fan made by Holmes (1-800-5-HOLMES) in China. I'd put the fan in series with a room heating thermostat (Grainger 4E036, $15.39) and a sunspace attic fan thermostat (Grainger 2E340, $15.48.) For moving serious air, I like the idea of Emerson HF industrial ceiling fans, or Grainger's 60" fan, the redoubtable 4C721, at 315 rpm, 46,000 cfm, 160 watts, max, and $168.50.

Nick
From: nick@vu-vlsi.ee.vill.edu (Nick Pine)