Comparing Solar Air Heater Designs & Performance

In my last post – Solar Heat: Free for the Taking, I covered some of the background information on how to take advantage of the sun’s energy to help heat your home. Building a solar air heater is an easy and rewarding project for both beginner or experienced DIYers and there are all kinds of different designs and plans floating around – just ask Mr. Google.

The most popular and flexible DIY solar heater projects seems to be the self-contained unit which can be attached to a wall or roof for supplementary heat. Today I’m going to look at 4 of the most popular variations of these units. And thanks to Gary & Scott, a couple of dedicated solar enthusiasts, I can share a brief summary of the comparable performance you might expect from these units.

Design basics

All of these units share common features and can be built with basic power and hand tools. Many of the self-contained solar air heaters I’ve run across are based on a 4′ x 8′ frame, although other sizes may be just as effective depending on your specific design and site.

In all cases these are the key features:

• Frame – The frame is typically made of 1 x 6 or 2 x 6 lumber. The interior depth is usually about 3 to 4 inches depending on construction.
• Insulated Back – This is where most of the heat can be lost. 1 to 2 inches of Polyisocyanurate is recommended. Side insulation is a bit less important.
• Matte Black Interior – All interior surfaces should be painted with a heat tolerant matte black paint to absorb as much of the sun’s heat as possible.
• Solar absorber – This is the heart of the unit.The absorber collects the heat which is transferred to air travelling across and through the heated surfaces.
• Air intake/outlet – Cooler air enters the unit (usually at the bottom) and after picking up the heat from the absorber, exits the top of the unit. This happens either through a natural process (Thermosiphoning) or assisted by a thermostatically-controlled fan.
• Glazing – The front of the unit is sealed with clear material to allow the sun to shine on the solar absorber and build up the interior temperature. Typical glazing materials are polycarbonate (Lexan or twinwall type), acrylic, or tempered glass.

Solar absorber

All else being equal, the solar absorber material and airflow within the “box” is where the designs below differ. This can have a big impact on the efficiency and effectiveness of the unit as a whole. Finding the right combination of heat gain and air throughput may require a bit of experimentation. A solar heater that can move a lot of 120F air is more effective than 160F air moving too slowly. High interior temperatures lead to much more heat loss through the glazing. Fan speed and duct size will affect the air flow.

The designs described below do not show the fan, which is usually located on the outlet end to pull air through the unit. It’s recommended to provide a flap of some sort to automatically close off the outlet when the interior temperature of the unit drops below room temperature to avoid reverse siphoning of warm air into the unit. A layer of lightweight plastic works well to seal the opening if the outlet has some hardware cloth over it. Although these units are shown tilted to face the sun, they can be installed vertically in northern latitudes as well.

Back-pass Type

The back-pass collector has been around for a long time and there are a few variations in designs. The basic idea is that the air is heated as it moves upwards behind the heated solar absorber. Alternating baffles may be added to slow or disrupt the airflow to increase the heat transfer.

Some window-mounted back-pass systems allow cool interior air to enter through an isolated chamber at the back.The air gains heat as it rises, travelling behind the solar absorber. The absorber may also be situated to allow air to travel on both sides for more surface contact.The heated air exits from the top of the unit.

 

Double Screen Type

The screen collector is another common type that’s frequently used and it’s the easiest and least expensive to build. The black mesh screen provides lots of contact surface for transferring heat to the moving air, while adding very little resistance to the air flow. In most instances, the screen is tilted within the box so the screen is closer to the glazing at the top of the unit. A layer of typical black window screen can be stapled to each side of a wooden frame and mounted within the box.

In the tests that Gary and Scott ran there seemed to be no appreciable difference in performance between metal and fibreglass window screen material. As with all solar heaters, try to keep as much air as possible away from the glazing to reduce heat loss.

 

Aluminum Soffit Type

The Aluminum soffit absorber is essentially a variation on the screen absorber and functions on the same principle. The solar absorber is made from panels of commercially available perforated soffit material. The absorber panel is constructed by mounting perimeter cleats on the inside of the box with the bottom cleat against the back of the unit and the top one close to the glazing. The side cleats run diagonally to provide a continuous mounting surface for the perforated soffit. The rising air picks up heat as it scrubs the heated surface, passing through the perforations to exit through the top vent. The materials cost are higher for this type compared to the screen absorber.

 

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Tube Type (Aluminum pop cans or downspout)

The “pop can” solar heater has gained popularity in recent years and a close cousin, using aluminum downspouts has entered the field. Both of these collectors work on the same principles so I’ll address them together. The solar absorber in these units is essentially a series of metal tubes that the air travels through, picking up heat along the way.

The unique feature on the tube type collectors is that they use sealed plenums at the top and bottom in order to direct the air through the tubes. The air enters the bottom plenum, usually near the center of the unit. Some builders add deflectors to help spread the airflow more evenly across all the tubes. Since the plenum is sealed and isolated from the glazing, the only way the air can travel is up through the tubes, picking up heat from the surface as it moves. The heated air exits the tubes into the upper plenum where a fan pulls it out into the room.

The main differences that I can see between using pop cans and downspouts is the materials cost vs. your labour. The pop cans are cheap and easy to collect, but it takes a lot of work to clean, cut out the tops and bottoms, stick together with silicone and then paint a couple of hundred of them. The downspouts would be very fast and easy to cut, paint and install in the unit, but cost more. I haven’t seen any comparison data between the two types to see if one is more efficient than the other.

Which collector type is more efficient?

Comparing the efficiency of DIY solar heater designs is a pretty sketchy area at best. Every builder uses their own methods of measuring temperature, airflow and efficiency so the short answer is – no one REALLY knows for sure.

On the plus side, in the winter of 2010-2011 solar enthusiasts Gary Reysa and Scott Davis put in the time and effort to run some side by side comparison tests on a few of the designs described above. Even though Gary and Scott live in different parts of the US, they used the same materials and designs for their tests and came up with similar results.You can check out their comprehensive comparison test which includes methodology, graphs, thermal pictures and other details at BuildItSolar.com.

 

So what did they find?

In a nutshell:

Screen Type:

Best performance overall as well as the cheapest and easiest to build. Both Gary and Scott were surprised and used this design for reference when testing the others.

Aluminum Soffit Type:

Performance essentially tied with the reference screen type – but a bit more complicated and expensive to build.

Back-pass Type:

High pressure drop (bad). Performance -10 to -20% off reference screen type. Improvement possible though redesign. (See thermal image above)

Tube Type – (Aluminum Downspout tested)

As tested, performance was -40 to -50% of the reference screen type. It’s the most expensive to build and Gary felt there was room for improvement – specifically in equalizing airflow across all the tubes. Future tests will likely show improved performance.

My solar heater plans

For the past year or so I’ve been thinking that a pop-can heater was my best option based on cost and effectiveness. After pouring through Gary and Scott’s test results I’m reassessing my plan. Motivated by the superior performance, lower cost and easier construction of their screen type collector, I’ve started working on a “portable” screen absorber unit to address a specific situation in my home.

Once I’ve got it finished and give it some test runs I’ll share the results.