I’ve been busy gathering materials and building four test panels to see which design is the best one for our solar water panels.
The Test Panels
Each test panel is 2 × 4 ft. The equal size will allow us to compare per-foot heat generation directly. The panels are made of 2-×-6-in nominal lumber. The bottom is 1/2-in plywood. The top is double-walled clear poly with 82 percent light transmission. A single-walled option was available with 90 percent light transmission, but the insulation factor on the double wall should make up for the difference and more closely mimic what our final arrays will carry. Additionally, the installation is easier as the double-wall is flat while the single wall has waves that must be specially sealed. The front of the panel contains the copper-pipe collector arrays and the back is insulated with high-heat-rated insulation. In this case we’re using 3.5-in-thick recycled mineral wool with an R-15 insulation rating. These panels are not sealed to the elements with cladding and flashing because they are just test arrays and meant to be disassembled for demonstrations. In total, each test panel costs about $100, or $12.50/ft². The larger panels should be even more cost effective.
The Four Designs
Inside the identical panel boxes are four different array designs. All four use approximately the same amount of copper pipe and metal, except for the “trickle down” as noted below. Three of the four designs are shown in the diagram below. The first design — “Manifold” — is a series of four vertical 1/2-in pipes that rise up from a 3/4-in pipe. The risers are attached to an aluminum flashing back. As the sun strikes the metal, the heat is transfered to the pipe, through which water is pumped. The second design — “Circuitous” — is a single length of pipe that cuts back and forth across the face of the panel in horizontal passes, also connected to a metal backing. The “Semi-Concentrated” collector is set up similar to the manifold collector’s pipes, except instead of being attached to a metal backing, they sit in front of a parabolic mirror, which concentrates the sun’s rays onto the pipes. The “Trickle Down” collector (not pictured) distributes water from a pipe along the top of the panel. Water is spread over a corrugated sheet metal panel, picking up heat as it flows down to a gutter at the bottom, where it is collected.
Much of this construction is fairly straightforward: straight cuts on the box components, gluing and screwing the pieces together, cutting insulation batts, and slicing clear plastic with a utility knife. The only more complicated part of the construction is sweat soldering of copper pipe joints. It would have been nice to use PVC or threaded black pipe, but these would either fail in the heat or are not flexible enough for this application. Unfortunately, this skill may be a stumbling block for some. It is my first time making these joints, and it is taking a bit of practice to get leak-free seals. If you have a friend with any of these skills, ask him/her to help you and learn something new.
“Lab Notes” are a new series of posts chronicling the daily progress our research projects. Research Project No. 1 is the testing and installation of a solar heating system for domestic water and space heating. These notes may be useful for anyone interested in building such a system at home. Others might prefer the more succinct guide to solar heating, videos, and other formal publications that will result from this research project and be posted to the website as they are available.