Solar Air Heater #2

I recently completed the build of a screen based solar air heater that is now attached to my shop next to the soda can based solar air heater that I built last year and detailed here.  It is based on designs from BuildItSolar and the Yahoo Simply Solar group. 

Screen and Soda Can Heaters

 

Construction

• Frame:
  • Constructed from ¾" x 4 ⅝" oak. This size was selected to match the finished depth of the can based collector that it is mounted next to.
  • Outside dimensions of the box are 96" x 49"
  • The back of the box is a piece of ¼" plywood.
  • The outside of the box was painted with exterior latex paint that matches the paint on the building.
  • There is a cap on top of the collector box that is sloped to shed water.
  • There are trim strips on the sides to close off the gaps between the box and the siding so that the local wasp population can't build nests behind the collector.
• Insulation
  • The back and sides are insulated with ½" R3 polyisocyanurate. ½" is the thickest any of the local home centers in this area carry.
• Screen:
  • Two layers of 18 x 16 x 0.011 mesh aluminum insect screen spaced 1 ¼" apart and painted black with high temp grill paint.
  • The screen is mounted on a ¾" wide by 1 ¼" deep frame that fits just inside the insulated sides of the box.
  • The wood for the screen frame was not perfectly straight and required that a horizontal spreader be placed across the screen frame at the mid-point to keep the frame from bowing in.
  • The spreader is ¾" x ¾" and will block some of the vertical flow of air that is between the 2 layers of screen. This might possibly force some of the air back through the screen. Is this good or bad? After completion I realized I could have used a much smaller spreader, maybe 3/8 dowel.
  • The screen is mounted parallel with the glazing. It is not tipped outwards from the bottom to the top as some collectors I have seen.
  • The screen frame is held up off of the back insulation with 4 small wood blocks that are sized to center the screen in the box between the back insulation and the glazing.
• Glazing
  • The glazing is 2 pieces of Suntuf from Home Depot.
  • There is a vertical support bar in the center of the box where the 2 pieces of glazing overlap. At first I tried just sealing the 2 pieces together with silicone but this failed to hold up while moving of the glazing panels prior to installing them and I realized it wouldn't hold up during expansion/contraction of the glazing panels either. The glazing is now screwed to the support bar to hold the 2 pieces together. There is a bead of silicone run between the 2 pieces to help with sealing.
  • The glazing is currently not sealed to the collector frame. It is screwed to the frame at every valley location on the wiggle strips. It is also screwed to the stiles of the collector box about every 15 inches.  This was done so that the glazing could easily be removed if I decide to add a third layer of screen.
• Fan and Ducting:
  • The air inlet and outlet ports are 4" round duct. They are centered in the box horizontally and are offset from the top and bottom by 2".
  • The fan is a 125 cfm fan. General guidelines from the Yahoo Simply Solar email group suggests 3 cfm per sqft of collector.
  • This is the same model fan used in the soda can based collector.
  • The fan is mounted at the lower duct location and pushes air through the collector instead of being mounted at the top and pulling air through the collector.
  • The temperature control is provided by a 90/110 snap switch. It is mounted to the back right next to the outlet port.

 

Thoughts After Construction

• Twin-wall polycarbonate looks much better.  However, 
  • The 2 pieces of Suntuf cost $42.
  • The closest place I can get twin-wall is 100 miles round trip and is $90 for a 4' x 12' sheet. 15 mpg in the truck and $3.15 for gas adds another $21 to the twin-wall price.
• I was surprised that with 2 layers of screen how easy it is to see the insulation on the back of the collector.  Based on Gary Reysa’s tests on BuildItSolar it appears that 2 layers of screen will absorb approximately 52% of the incoming light.  This collector has a reflective back so the screens will get a chance to absorb some light on its way back out for a total of about 80% absorption.  While the collector seems to be performing well, I’m not pleased with having possibly 20% of the light reflected back out of the collector.  I need to investigate adding a third layer of screen or painting the back of the collector black.