|This page provides plans for a simple thermosyphoning air collector. The diagram shows how the collector operates -- very simple. Cool air from the living space enters the collector space through the lower vent. As it is heated by the sun, it rises (warm air being lighter), and exits the upper vent back into the living space. No blowers, wiring, ducts, or controllers.|
The characteristics of the Thermosyphoning Air Collector as compared to a direct gain window:
It has roughly the same collection efficiency as a direct gain window.
Night heat losses are less than for a direct gain windows.
Very simple -- no fans, no ducts, no controllers -- uses natural convection.
Does not provide daylighting or views as a direct gain window would -- this can be an advantage or disadvantage.
The inside surface of the wall can be used for shelves etc. -- unlike a direct gain window.
No structural changes to the wall framing are required, since no structural members are cut -- this can make it an easier retrofit than a direct gain window.
These are very cost effective energy collectors. For example, a PV panel of the same size would cost 5 to 10 times more, and produce about one fifth as much energy! Depending on the materials used, and how good a scrounger you are, the payback period will usually be a year or two at 2006 fuel prices.
The collectors can be used in new construction, or as a retrofit.
These plans are excerpted from the book "Passive Solar Energy" by Bruce Anderson and Malcolm Wells. The full book is available for free download here.
This retrofit passive space heating device, called a thermosiphoning air panel (TAP), uses the existing house wall as the major structural element. The exterior finish is removed, new ThermoplyŽ structural sheathing added over the existing wall, and wood framing added to support the ribbed aluminum absorber plate (industrial siding material) and to support the field-installed insulated glass units. The system shown uses three patio door replacement units as the aperture, creating three areas of absorber plate, each of which requires a high and a low vent through the house wall to allow the thermosiphoning action to occur. (See pg. 57 for damper construction tips.) The weight of the added glazing is carried by brackets at the base of the panel to a continuous ledger strip bolted to the house wall. After flashing is added, the exterior siding materials are patched around the unit to complete the installation. (Construction details, New England SUEDE.)
Some additional notes:
For some reason, the plan above shows no real detail on the vents that connect the collector space to the living area that is heated. You need to provide inlet vents from the living space to the collector along the bottom of the collector to let room air into the collector, and provide outlet vents from the collector space to the living area to let heated air back into the room. The total area of the lower vents should be at least half the cross sectional area of the collector. Same for the upper vents.
The vents must include a back draft damper to to prevent backflow of cold air from the collector into the living space at night. A very simple (but effective) back flow damper is shown in the solar barn collector article. You should also provide for a way to close the top vents off for the summer, so that you don't get unwanted heat from the collector.
This collector uses a solid sheet of metal roofing for the absorber, with the air flowing behind the metal roofing. An alternative, which is generally thought to be more efficient, is to use metal lath or metal screen for the absorber, and to arrange the airflow so that the air flows through the mesh. The solar barn collector provides some details on this type of collector in case you want to use it instead of the solid absorber.
If the aesthetics of the collector are an issue, you may want to consider using a dark color (e.g. dark green) for the absorber instead of black. This will look better, and will reduce the efficiency only slightly. Another option is to use glass that is textured, so that you don't clearly see the absorber through it. Using double wall polycarbonate glazing would be another option to diffuse the view (its also easier to work with).
It is a good idea to include an overhang above the collector that blocks the summer sun. This is not strictly necessary, since the sun is higher in the summer, and you can shut off the airflow from the collector to the living space. You can find a good tool to determine how deep your overhang should be in order to block summer sun, but let in winter sun here.