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Solar Air Heaters

Overview - 

Solar air heaters are systems that collect solar energy and transfers the heat to passing air, which is either stored or used for space heating. The collectors are often black to absorb more of the sun's energy and a conductive material, often metal, acts as a heat exchanger. There are many different designs and systems may include fans to increase the flow rate of air. Alternatively, a passive collector can be built such that when the hot air rises it draws fresh air through the bottom. Fans can often increase the performance of the system, but require additional parts and adds complexity. Solar air heaters can compliment traditional indoor heating systems by providing a free and clean source of heat (after initial costs). While clouds effect the energy output of the system, the metal will store energy on a hot day and will reduce the impact of momentary cloud cover. To achieve best results, the system should be unshaded and facing the general direction of the sun (south for the northern hemisphere, north for the southern hemisphere)^[1].

In the UK, 40% of all energy consumption was dedicated to space heating^[2]. This percentage is the largest component of energy use and can be reduced by simply using solar air heaters as a means of enabling the sun to passively heat our in door areas without consuming the Earth's finite resources.

The financial and environmental benefits of implementing solar air heaters can be estimated through the use of this RETScreen tool that was created by the government of Canada and made freely available to the world.

Materials

There are two common methods used to create a solar air heater. The first is a pop can design and the other directs the air through a channel in a "snake" pattern. Both methods use similar materials and tools with only a few exceptions. 

Both designs require the following materials: ^[3]  [4]          

• Plywood 
• Wood - 2"x4"
• Glass or plexiglass
• Sealant
• Insulation
  
• Black paint
• Aluminum "L" trim
• Screws
• Drill

To build a "snake" design, a piece of sheet metal for the base would help increase thermal mass and improve heat transfer to the passing air. 

Alternatively, building the pop can design requires a large number of pop cans and either a mortar or glue to connect the cans. 

Two basic forms of solar air heaters

Pop can

Materials:

http://greenterrafirma.com/solar-air-heating.html

Solar ”Can” Air Heater

This simple small passive solar heater, made from recycled aluminum drink cans, can be used to heat an insulated garage or small room. A larger heater or a number of similar heaters can be used to heat larger spaces, or to heat smaller spaces to a higher temperature.

The box is built from 2″ x 4″ studs and a sheet of plywood, measured to tightly hold 5 rows of 10 black-painted aluminum drink cans. The inside of the box is sealed using caulk to prevent hot air from escaping. Cold air is drawn in from a hole at the bottom of the box and heated air emerges from the top passing through a pipe into the space to be heated. A Plexiglas sheet is glued to the top of the box to let sunlight in but not let the hot air escape.
This solar space heater works by drawing the air to be heated into the bottom can of a column of cans. The air is then heated inside the cans by the sun’s energy and the hot air within them rises upwards (thanks to convection) to be fed into a pipe which re-enters the building to be heated.

For the air to pass through a column of cans, holes must be drilled into them. There is already a hole at the top of each can out of which the drink is poured. That just leaves holes at the bottom of each can to be drilled. In the bottom can of each column, a 1 inch hole is drilled in the side.

The cans of each column are glued together using caulk or silicon adhesive and painted using black paint to help them absorb the sun’s energy. Barbecue,fireplace or stove paint is excellent for this as it will not flake off. Make sure it has a totally matt finish.

The inside of the box must also be painted with the same paint before the columns of cans are glued into position using caulk or silicon adhesive. The outside of the box should be treated with preservative, varnish, or paint to help it survive the elements for many years.

Ideally the whole unit will be sealed with a sheet of tempered glass. However, tempered glass (unless you can find and recycle a sheet) is also very expensive. Therefore Plexiglas (plastic) can be used, but it will degrade far more quickly and become opaque blocking out the sunlight.

A hole at the top of the box acts as the hot air outlet and can be connected to the building/room to be heated using an insulated pipe.

Required Improvements to this Design

“Snake” the cans together so the air travels farther and has more opportunity to take the heat from the aluminum cans.
Add a fan to both the intake and output sides of the unit, to move more air.
Insulate the box - bottom, sides and ends. One inch rigid foam is easy to work with.
Keep the input and output pipes short and super insulated.

"S" collector

      Regional considerations

skills and knowledge required

Solar data (maps)

http://www.meteonorm.com/media/maps_online/world_global_8100.png

angle of collector - General rule of thumb is to place the collector at an angle equivalent to the lattitude of your location. For more details, please follow this link

http://www.solarpathfinder.com/industry

technical specs (schematic)

Estimated costs

The cost for to build a solar air heater varies significantly depending on whether the materials are purchased "new" or if they were salvaged. In addition, the decision between glass and plexiglass has a very large impact on total cost of the system. Finally, there are other factors such as the use of a fan, solar panels, back-up battery, wires and sheet metal that could be incorporated in the design and would certainly increase the budget.

Overall, a basic system can be constructed for approximately $95, which could easily be reduced by being resourceful when gathering the components. 

Alternative designs

http://solutions.3m.com/wps/portal/3M/en_US/Window_Film/Solutions/Markets-Products/Residential/Safety-Security_Window_Films/

http://www.solarsponge.com/article.htm

http://www.solarsponge.com/images/SolarSpongeMechanicalDrawing.gif

Videos

Here are a couple videos to help you build a solar air heater and maybe even get inspired to improve the designs.

Footnotes