Add-on Solar Collector Heater

Heat from the sun: I added a solar collector to the south wall of my bilevel suburban Denver house in the fall of 2005. It's a little like a section of greenhouse stuck to the side of the house. Since there was an upstairs window practically right over a downstairs window, I didn't have to make any permanent modifications to the house to allow air circulation through the collector. On sunny winter days, I just open the upper and lower windows and set up a portable fan in one of them. Passive convection would circulate some air from the lower to upper level, but adding the fan greatly increases the heat output. collector in winter, fan running 8mm twin wall polycarbonate

Construction:

The structure is modular; a rectangular base of 2x6 redwood (tapered to conform to the irregular concrete/brick patio) sits on the concrete patio and is bolted to the house foundation. The side and front frames that sit on the base are made of ~2" x 2" weather-resistant wood -- redwood and cedar, most of which I scavenged from my scrap collection. A couple of the thin cross piece supports are cut from salvaged 1" oak flooring. All the wood panels screw together and could be disassembled in an hour or two. The translucent panels are 8 mm. twin-wall polycarbonate (Verolite brand), which is widely used in greenhouses, very strong, UV resistant, and heat transparent, though it's not cheap. The polycarbonate can be cut with a fine-toothed, plywood-type or thin carbide blade if you're careful. I mounted some lengths of sheet metal I had (salvaged baseboard radiator backs), on to the house wall, with stand-offs so air can circulate in front and back. These darkly painted steel pieces help the efficiency of transferring solar heat to the air circulating through the collector. All wood pieces were primed and painted with exterior latex paint to match the house exterior.

Winter use: Around mid-October, it becomes cold enough and the sun angle low enough to get significant supplemental heat from the collector. I monitor the air temperature in the collector with a remote sensing thermometer; around mid-morning, as the collector air gets above indoor room temperature, I open the two windows and turn on the fan, blowing air from upper into lower level. Since I'm almost always home, controlling it all manually works OK. If I weren't almost always here, I would have to install a thermostatic controller for a fan and some electrically operated vents in the windows, or figure out some other circulation system, because temperatures can vary a great deal as the cloud/sun conditions change. E.g. it can be cloudy and freezing cold at 09:30, and the collector temperature might be 28 F. degrees; if the sun comes out at 10:00 it can rise to over 110 degrees in 1/2 an hour. On the typical sunny winter day here, I'll heat my house up to 66 F. degrees (with the natural gas furnace hydronic system) when I wake up. Then, starting mid-morning, I will operate the collector about 5-6 hours, with the furnace turned off. Depending on outside temperature, I may or may not need to turn the furnace on again in the evening.

Summer setup: By early June, I don't need to heat the house, and slide out the large polycarbonate panels, and replace them with sections of redwood lattice held in by screws into the frame. I insert a custom-fit rectangular planter box in the base, and plant vines and flowers: morning glories and various beans, which over the summer, climb up the trellis and provide shade for the windows. (By mid-summer, the sun elevation is such that very little direct light would come in these windows anyway, but the trellis and vines are a nice decoration anyway.) The photo below shows the vines in mid-summer, halfway up the trellis. By late summer the vines totally cover the trellis.

Costs and savings: With all the custom fitting and angles, this was a complex carpentry project that took ~40 hours to build. I used a lot of surplus wood and hardware I had on hand, but bought a 10' cedar 4x4 that I ripped down to make the long pieces for top section and 3 4x8' polycarbonate pieces (about $2.25/sq. ft.). My total material cost was ~$300, but you'd spend about $60 more if you had to buy all the wood and hardware. How long did it take to recoup the cost in energy savings? Comparing my heating costs with the previous years', I'm estimating that I saved enough natural gas to pay for the project, more or less, in its first winter of use. Now beginning its third winter of use (2007-8) it's still working fine and I'd guess saving me a few hundred dollars a year in fuel costs.

mid-summer trellis setupcollector side detail, summer