Update from Tom after first full season of use -- October 2009:
Pool Solar collectors:
The system worked without a hitch all summer long. The 8 panel array yielded about 160,000 BTU's per day, which on a pool the size of the one I was working on amounted to one degree a day. Unfortunately, we had one of the coldest summers on record, and the cold nights robbed heat faster than we could generate it. Overall, the pool owner (a friend) was happy with the system and noticed every sunny day he was gaining heat. About the only drawback is the "unglazed collectors" collected pine needles from nearby pine trees. The system was drained and prepped for winter last week.
I've designed an unglazed swimming pool
collector, pretty much the same design
as yours, but keeping everything as cheap
as possible. I'm using 4' x 8' x 1/2" treated plywood for the back, a border of
treated lumber that recesses the "edge" of the plywood for weather protection,
and 1/2" polyisocyanurate under the absorber. I like the insulation on top of
the board for two reasons. No thermal loss between the absorber/hose area and
the wood, and the exposed back is a durable material (the plywood). I like the
manifold design for drainback, and it works nicely for a "modular" panel (each
4' x 8' panel is a separate module). With this concept, I can add or subtract
modules based on the pool size.
The "T"s I am using for my connections are a "Lasco" irrigation fitting, actually a 1/2" injection molded plastic fitting which works great with 5/8" PEX. I just heat the end of the PEX for 15 seconds with a heat gun and push the softened PEX onto the fitting. It grips the PEX so well no clamps are required. I found another source for the fittings (at a local plumbing distributor) that's cheaper than the Lasco ones I've been getting at Home Depot. Apparently there are several vendors making the black irrigation hose and fittings.
The plastic (injection molded) irrigation "T"s are 1/2" all three directions. For my last order on those, I got the cost down to $.33 a piece. I'm using 5/8" PEX for both the manifolds and the risers. I had about 800'-900' of the 5/8" PEX left over from another project. I would have liked larger manifolds, but everything was working out so cheap I thought I would give this design a try.
The websites for the cheaper "T"s and
the Lasco fittings are listed below. Either the Lasco or "Seal Well Products"
will work. It just depends on where you can find your best pricing in you local
area. Home Depot stocks the Lasco product, and the "T"s ran about $.62 a piece
there. The Boshart, known as B.I.I. product (Seal Well) show list pricing on
their web site. I purchased these for $.33 each locally, so the list pricing is
quite a bit higher than you will pay if you can find them at a plumbing
Four completed collectors
I think the PEX will work good for the pool collector. The absence of glazing should eliminate much possibility of over heating it. I plan to design an on/off switch for the array, and then add an over-ride control that turns the pump on if the absorbers hit 200 degrees. You can always run the array at night or leave the pool cover off if the water is actually getting too warm (don't think that's going to be a problem up here).
Tom will answer email questions -- you can reach him at: toms1 AT chartermi DOT net (replace At with @, DOT with .)
Tom has sent in several great renewable energy projects ...
I'd offer one small alternative to Tom's design to think about.
The alternative would be to leave the insulation board out and apply the aluminum fin directly to the plywood. The thinking behind this is that 1) for most pool collecting applications, the pool water will be cooler or nearly as cool as the ambient air, and when this is the case, the insulation is not really needed because there is no heat loss, and 2) by having the fin in direct contact with the plywood, it will be easier to staple it down so that it fits very tightly against the PEX riser tubes -- this is important to get good heat transfer from the fin to the riser tube.
Part 2 is not so much of an issue for Tom, in that his press forms a very good fin groove that snaps on the tube, but it is more of an issue for fins formed with this sledge hammer method . Along the same line, I would use a bead of silicone in the groove before stapling it down -- the silicone fills any tiny air gaps that might remain between the aluminum and the PEX. The silicone is 10 times more conductive than the air it replaces, but any remaining silicone filled gaps must still be very very thin for good heat transfer.
Gary March 31, 2009