flat plate collector specs [RE-wrenches]

Wed Oct 23 07:17:04 PDT 2002

High Quality Flat Plate Collector Specifications

The technology in flat plate collectors has not improved since the 1940's, except in three areas - the glass, the insulation, and the use of selective absorber coatings. The glass is better today - special tempered glass lets through 10 to 15% more sunlight than the transmissity of the glass in the 40's. The insulation is much better than the that available in the 40's. The selective coating used on absorber plate are better than the common flat black stove coatings in colder winter months, especially for space heating. The basic heat transfer from copper sheet to copper tube has not changed. The galvanized sheet metal boxes lasted 30 years - the 6063-T5 aluminum alloys used since the 1970's for framewall extrusions should last 100 years. It is still critical that there be at least 1" of air space and preferably 1 ½" between the absorber surface and the glass to reduce heat loss. The Morningstar designs for indented and flat framewalls are included in this chapter. These collector specifications are ideal for open loop drainback and closed loop glycol systems. This design evolved from several early collector designs that were similar - Gulf Thermal, US Solar Corp, and Morningstar Corp. The indented framewall was used in the Morningstar and Gulf Thermal designs and now is used by SunEarth in their Imperial Series. The flat framewall, used by Alternative Energy Technology (AET), Heliodyne, US Solar Corp, and SunEarth, was typical of designs used by Colt in California in the 1980's and Morningstar in Florida for their Cornerstone Model. The flat framewall integral mounting bracket design was common to these manufacturers and is the most common flat plate collector framewall design. It uses an integrated hinged mounting brackets to lock into the framewall. The hinged bracket allows the collector to pivot to the correct tilt angle for your latitude on a bolt through the hinge. The hinge attaches to an aluminum channel with a 3/8" SS bolt.

My ideal collector would have 1 ½" of thermomax high temperature stable polyisocyanurate foil-faced insulation with a 80 mm protective glass mat barrier between the absorber plate and insulation. The type L copper header would be 1" minimum with a .010" copper absorber and ten ½" ID type L copper risers per 44" of absorber plate with a black crystal coating. The glass would be 5/32" thick for a 4' x 8' collector and 3/16" thick for a 4' x 10' collector. These criteria with the modern aluminum framewalls will last over 120 years with the absorbers being replaced every 30 to 60 years. The best designs have aluminum mounting brackets that are extruded as a hinge to integrate with the framewall of the collectors. The hinge fits into an aluminum channel. The U shaped channel is usually 1/8" to 3/16" thick with two holes on each side wall for a 3/8" stainless steel bolt to hold the collector hinge in the channel. 

In drainback systems, the absorber plate will only have to be replaced once every 120 years. In glycol systems that are well maintained the absorber plate will have to be replaced two or three times in 120 years. The absorber plate must be all copper at least .01" thick. The glass must be tempered and the framewall must be mill finished anodized, anodized colors, or electrostatic painted aluminum for the collector to last 120 years. Check the thickness of the framewall. A critical detail is that the screws that hold down the brackets that hold the glass in place must be high grade stainless steel. Screws that are not stainless steel will make the job of replacing the absorber plate extremely difficult. Never use only fiberglass insulation behind the absorber, unless you live in desert conditions. Flat plate collectors are impossible to completely seal from outside conditions and WILL have condensation collection on the inside during rainy weather. Some manufacturers have condensation vents for rainy climates - you may need to add extra vent holes in these type of climates. It also allows the water vapor to be quickly removed from the box. In rainy weather, fiberglass will soak water like a sponge and greatly increase condensation problems. The best flat plate collectors have at least 1 1/4" to 1½" of closed cell poly- isocyanurate insulation below the absorber with a 60 to 90mm glass mat. Collectors that use only polyisocyanurate insulation will result in charring and degrading the R-value of the insulation during long-term stagnation. The glass mat helps prevent scorching the polyisocyanurate during collector stagnation. It allows the water vapor to be quickly removed from the box. I have always believed that collectors should be tested by labs after one year of full stagnation tilted at the latitude. The collector should have an aluminum back plate, especially if it is raised above the roof surface on struts. Connect collector header pipes with couplings - never use unions.

zNever mix metals in the absorber. Use all copper plates and copper tubing -aluminum fin with copper tubing is an inferior product. Aluminum will migrate from the fin to the copper tube. Selective coatings are difficult to put on aluminum fins. High absorbitivity and low emissivity selective coatings (black chrome or black crystal) are better than black paint as an absorber coating in cold winter climates or heating water over 160EF in the summer.

zNever use smaller than ½" outside diameter tubes or 13/32" ID in the risers. The inside diameter should be over 3/8" ID or the collectors will not drainback when used in a drainback system, and can create circulation problems in antifreeze systems. All open loop systems should use ½" ID risers due to water scaling problems. Ask for type L copper if you are installing commercial systems. Beware of manufacturers who use type M refrigerator pipe for risers. The wall thickness is thinner than type L or type M plumbing pipe and should not be used in the collectors that pump glycol antifreeze solutions due to the possibility of extremely high stagnation temperatures. The collector absorber will not last as long when type M copper plumbing pipe is used instead of L copper. 

zNever use anything other than one sheet of low iron tempered glass as the outer glazing. Tempered glass will typically last over 120 years if it is over 1/8" thick and will not lose its transmissivity over time. All alternative covers used from 1977 to 1986 had serious problems in either degrading in structural integrity AND rapidly losing transmissivity. Transmissivity will rapidly degrade over time on ALL glazing other than glass from the effects of sunlight, air pollution from cars, hail, wind storms, bird excrement, squirrels, pollen, sonic booms from jets, acid rain, a wide variety of industrial air pollution plus thermal expansion and contraction cycling of the cover material will seriously degrade all exterior covers other than tempered glass, causing the need for replacement in less than15 years. 

Try to get stippled glass instead of a clear smooth glass surface so that the absorber in the collector is not apparent. The patterned glass also prevents reflected glares from coming from the glass and improves aesthetics without diminishing performance.

Tom Lane
Energy Conservation Services
6120 SW 13 Street 
Gainesville FL 32608
352 377-8866
fax  338-0056

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