Rules Of Thumb For Solar Air Collectors

Getting ready to build? The following guidelines have been developed by the Small Farm Energy Project and others working to optimize air-heating collectors. These calculations are designed to  give prospective solarizers a much-better-than-ballpark set of numbers for sizing the collector  area, air gap, fan, vent openings and, if used, storage. 

1. To build a collector that operates without the need for storage, don't let the collector area exceed 20 per cent of the house's heated floor area, if the house is reasonably well insulated.

2. Baffle layout should be such that no single "air run," the distance between an inlet and outlet,  exceeds 32 feet. Larger collectors such as the Stark's are divided into two separate zones, though  both are powered by a single fan. A multi-zone collector can also have outlets opening onto different parts of the house.

3. Fan-powered air flow should equal an "actual" two cfm per square foot of collector at sea level, and 3 cfm per square foot at an altitude of 7000 feet, because of decreasing air density. ("Actual" flow is the fan's or blower's rated capacity [cfm] less the effects of resistance to air flow [called static pressure drop] caused by the friction of moving air against solid surfaces.)  SFEP has calculated the average pressure drop through these collectors to be from .3 --.5 inches on an instrument that measures pressure change in "inches of water" in a column. (It gets complicated; just take us at our word.) Most fan and blower cfm ratings are given for a range of pressure drops, so choosing the right units won't be difficult. SFEP recommends direct drive blowers for small collectors (less than 150 square feet) and belt drive units (squirrel cage) for larger systems to allow for experimentation and fine-tuning with different  pulley sizes. The goal for both of these is to produce the minimum temperature rise through the collectors -- usually at least 20F.-- while still maintaining an outlet air flow that gives off enough warmth. The cooler-running collector is more efficient because it transfers that much more heat to the house (and less back out through the glazing).

4. The air gap is a function of the air flow (volume over time) and the air velocity (speed over time). The optimal air flow is 800 feet per minute (fpm). Divide the calculated cfm by 800 fpm to get the area (in square feet) of the air gap cross-section.  The gap is then found simply by dividing the cross-section area by the width of the collector air way in one direction of air flow. In the Phelp's straight-through collector the total width is 45 inches and the designed air flow is 300 cfm. The area of the air gap is therefore 300 cfm/800 fpm = .375 square feet or 54 square inches. The air gap width is then found by: 54 sq. in./45 inch width = 1.2 inches. The Phelps settled on an air gap of 1'/4 inches. The Stark's collector has a variable width of 11 -- 16 feet due to the presence of windows. The average width was figured to be 13.5 feet but be- cause the air flows through the collector in two directions, up and then down, the actual collector width in one direction is half the average width, or 6.25 feet (75 inches). At a design air flow of 550 cfm the calculation proceeds: 550 cfm/800 fpm = .6875 sq. ft. or 99 sq. in. Air gap width  is: 99 sq. in./75 inch width = 1.32 inches. Again a 1'/4 inch actual gap is close enough.  

5. The collector inlets and outlets must be of a size equal in area to the air way (between baffles) they serve. For example, the Stark's air ways were generally 16 inches wide with a 1.5-inch air gap. That means an area of 16 X 1.5 = 24 square inches. ,Thus, each vent opening was cut to measure 4 X 6 inches. 

6. When duct work is needed to get a more extensive distribution of solar heat, the design velocity for ducting is 500 fpm. Thus, a 200 square foot collector designed for a 500 cfm flow rate would need duct work with a cross-sectional area of 500 cfm/500 fpm or one square foot. A '12 X 12-inch duct could be used, or one 6 X 24 inches. It even is possible to build a duct onto an exposed basement ceiling using the floor joists and subfloor for three sides and thin plywood for the fourth side. Sealing air leaks with caulking and duct tape is crucial, along with providing duct insulation. 

7. Storage. A rule of thumb on storage sizing calls for 50 -- 60 pounds of rock per square foot of collector. Working with Btus, the specific heat of rock is such that one cubic foot stores 20 Btus for every 1 Degree F. it rises in temperature. In the case of a 40 Degree F. rise, a cubic foot would  store 20 X 40 or 800 Btus. Let's say also that the collector output is 150,000 Btus per day. In order to store that much heat (at a 40F. design temperature rise): 150,000 Btu/day/800 Btu/cubic feet of rock = about 187.5 cubic feet of rock needed, or about 18,750 pounds of the stuff. That's roughly seven cubic yards, one cubic yard weighing 2700 pounds. The storage bin also should be proportioned for minimum surface area to minimize storage heat loss. It should be stressed that incorporating storage into the collector system is no simple task, and because of space limitations we've by no means included all the information needed to do the work. Only the skilled craftsman who has some experience with forced-air heating systems should make the attempt. A good source of information on these air collectors and storage systems is the Domestic Technology Institute. Their publications available through Solstice Publications, Box 2043, Evergreen, 80439. Ask for publication g BP-044 "Solar Forced Air Heating System Plans," which is a set of six 18 X 24-inch blueprints available $16. Another source of plans for an air heating collector and rock storage system is the Ayer's Cliff Center for Solar Research, Box 344, Ayer's Cliff, Quebec, Canada JOB 1 CO. Phone: 819-838-4871. 

The North Collector:
Construction Plans

Solar space heating systems are expensive. Sometimes. But, according to the San Luis Valley Solar Energy Association, a 64-square-foot North collector, complete with electric fan, can be built for less than $250. And that's the deluxe model, made from high-quality, store-bought materials. 1f you use scavenged materials, the price could be much lower. Hundreds of North collectors -- small ones and giant ones -- are bolted to walls and roofs in the San Luis Valley. Most were built according to plans supplied by the Association. Each collector is a wooden box. Its walls are 2 X 4s, the back is plywood, the glazing is fiber-glass-reinforced plastic. A fan draws cool air from the house into the collector, where it is heated by the sun's rays. Wooden baffles force the air to travel a long, zigzag path on its way through the collector. This gives the sun enough time to heat the air. The fan then sends the air back into the house. A sheet of corrugated, black-painted aluminum is attached between the glazing and the baffles. It acts as an absorber plate, gathering the sun's warmth. If you have the funds, you could add thermal mass to your system: a bin full of rocks, for example, or 55-gallon drums filled with water. That would increase your expenses, but it would provide solar heat 'round the clock. The hot air from the collector would be blown onto the thermal mass, where some of the heat would be stored for release during the dark hours. You might decide to channel heat from the collector to various rooms in your house. The ducts that you install for this purpose would also raise the cost of your system. A typical arrangement would consist of ducts leading from the collector to floor vents on the north side of the house, where there's no direct gain through windows. The San Luis Valley Solar Energy Association (512 Ross Avenue, Box 1284, Alamosa, CO 81101) sells North collector plans, including instructions for making an all-metal version. Price of the booklet: $3

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