Solar Sponge
The DIY Solar Air heater

By David L. Jones & Nicole Jones

What can you do after you have applied some basic energy saving techniques to your house, such as CF lighting, reducing latent power, insulation in the walls and roof, and switching to "Green Power"? We asked ourselves this same question, and looked at our electricity bill. It was obvious that heating was the dominant factor in winter time. Could we heat our house better and cheaper than our current oil heaters, or do something to supplement them?

We did some research and found that Solar Air Heaters looked quite promising. The drawback was that prices for commercial units were prohibitively expensive, approaching $3000. This seemed excessive for what looked like a metal box with a couple of fans, so we decided to research this field further and see if we could design and build our own for significantly less. If anything, it would be a load of fun and educational!

The result is the Solar Sponge, an active Solar Air Heater which can be built for under $500 installed with parts from the local hardware store. As a bonus its performance equals or outperforms commercial units of the same size!

 How It Works

Solar Air Heaters come in two varieties, active and passive. The Active types use fans to force the heat through the collector and ducting, while passive types rely on thermal syphoning to circulate air through the house. In both cases cooler air is extracted from either outside or inside the house (at floor level), heated, and then pumped back into the house. The passive type have very specific requirements for mounting (usually outside a window to the ground) so we opted for the more versatile active type which could be mounted on the roof.

To start with you need a collector, a box which collects the heat. The traditional design for a solar collector comprises a black metal plate which absorbs the solar radiation and converts it into heat, and a sealed cavity on top to trap the heat within. This is essentially how a “green house” works. The heat is extracted from the back of the plate (via another sealed cavity) and circulated by fans. This is often known as the “dead air space” design.

That’s all there is to it. Such designs have been around for many decades, and commercial units are very popular in the US and Europe.

There are variations on this basic design in which there is a single enclosure of a metal box and a glass plate on top, and air is pumped through this single sealed enclosure. However, this design intrinsically has higher losses through the front plate, which requires the use of expensive and fragile glass. Pilkington SunPlus glass is designed for this purpose but is very expensive, more expensive than our entire Solar Sponge design and installation!

We opted for a cheaper and simpler solution based on the traditional dead-air-space design, but it was the choice our materials which would prove to be the deciding factor in cost and ease of construction.

Galvanised steel is the traditional material used for such designs, but we opted for Aluminium due to it having 4 times the thermal conductivity of steel. In fact we built our entire collector box out of aluminium, which made it easy to work with various pre-made sheets and tubing sizes from the local hardware store.

Another key aspect to the Solar sponge design is the use of very thin (<1mm) polycarbonate sheeting for the front cover. This drastically reduced our cost compared to glass or thicker 3mm polycarb sheeting. Polycarbonate sheeting has similar transmissive properties (around 90%) of the purpose designed low-iron SunPlus glass, but at a fraction of the cost. It is also extremely tough and can withstand hail. The thin material does however warp a fair amount compared to thicker sheeting, but this had no noticeable effect on the performance, and aesthetics weren’t important to us.

The design of the collector box was obviously crucial to the design. It needed to be a certain minium size, and efficiently transfer the heat to the air as it passes through. The design of any solar collector will always be a rather delicate balance between air flow rate, internal surface area, and losses to the outside world (back through the polycarb cover plate and out the sides of the box). Pump the air too fast through the collector and it won’t have time to heat up, pump it too slow and it’ll get hot but the volume won’t be there to heat your room, and heat losses will be greater.

Somewhat contrary to common sense, the hotter the collector plate the greater the overall losses will be. So you want to actually minimise the collector plate temperature by having sufficient air flow rate and “heatsinking” within the design. The aluminium construction of the solar sponge is fairly optimum at doing this. But you can go overboard and have too much heat sink material, as being too cool is bad too!

It was important to have as long an air flow path as possible within the collector, and also have the cold air being drawn in from the bottom of the (angled) collector and coming out the top of the collector. Hot air rises and we want to make use of that fact to improve our efficiency. So the Solar Sponge was designed to be rectangular, with the longer sides on the horizontal, the inlet port on the bottom, and the outlet port on the top. Internal channels were added to “snake” the air around the box, picking up heat as it goes. With a 1.5m long box, this equates to a total air path of 4.5m, a length long enough to pick up sufficient heat. Holes were also drilled on the bottom side on the internal square channel walls to help break up the airflow.

Back of the prototype with the ports in its wooden frame

We based our prototype collector size on two 900x900mm pre-cut aluminium sheets, giving us a total collector area of 1.6m2. Although this is on the small side, it was suitable for a first prototype.


After initial testing it was obvious that fans are crucial to the design. They must be the right capacity, use a minimum of power, and must be configured correctly based on the total ducting length.

Fans can be used in parallel and in series. Fans in parallel will give an increase in total airflow rate for low pressures (short ducts), but show little gain at high pressures. Conversely, fans in series will give a greater airflow rate at high pressures (long ducting), but show little gain at low pressures. So there is no point in having 4 fans in parallel on a long duct, as you won’t see much improvement over 1 fan. The fan graphs shown illustrate this concept. Fans are also rated into free air, so a 100CFM fan does not produce 100CFM into a duct! In fact it will most likely be 1/10th of that value or less. This is important to know when it comes to energy and efficiency calculations.




A typical solar heater installation will have at least 5 meters of inlet and outlet ducting, plus the collector itself (another 5m say). That length of ducting is going to be “high resistance”, leading to high pressure. So we need fans in series to increase our airflow rate.




We constructed two “fan boxes” wrapped in aluminium sheet each using two 120mm x 38mm 12VDC 107CFM fans. There must be a minimum distance between series fans to avoid any loss due to air vortexes.




We used all the fans on the outlet side ducting, but ideally they would be on the inlet side to increase operational life (lower temperature).


Toxin Free

From the outset it was important to us that the entire system be free of toxins. After all, there was no point sitting in a warm house if you were breathing in toxins!

This meant that the entire air path (with the exception of the fans themselves) had to be made of metal, including the collector box and the ducting. Although we chose an all-aluminium construction of the collector to improve thermal performance, this also happens to be the perfect toxin free environment. Even the neutral cure silicon used to seal the aluminium box is toxin free, and we used Aluminium rivets to join it all together. The front of the collect plate and the wooden frame are painted, but they are not part of the air collection path.

Solar Power?

We (obviously) thought about powering our fans from solar cells, but then realised we already had “Green Power” so were already getting solar and wind power. Not to mention that the cost of solar cells (50W) would be prohibitive, with a payback period in the order of 50 to 100 years.

What about Heat Extraction?

Some designs allow you to extract heat in summer as well. We looked at this but decided that the extra complexity was not warranted considering you can buy a simple exhaust fan for this purpose.


All of the parts to build the Solar Sponge are available from your local hardware store.

The mechanical drawing and the photos should have enough information to construct your own units.


The angle of the collector to the sun is most important. However, as most installations would be fixed, compromises have to be made. Some designs have angled collector fins inside the box to supposedly improve performance, but this is a false assumption. Angled fins do not increase the total surface area relative to the sun. Solar radiation will be absorbed the same from any angle.

We fixed the Solar Sponge to the roof beams using 4 bendable metal mounting strips that protrude from under the tiles. Although we found we needed an extra wooden frame to extend the height of the collector. Tile-tites were used to replace the roof tiles and allow the ducting to penetrate the roof.

During testing we found that having our inlet on the ceiling along with the outlet (even with them in different rooms) we got a laminar flow across the ceiling which was very inefficient. Ideally you want the inlet on the floor level, but our house design did not allow this.

So in the end we simply disconnected the inlet duct and drew air in from the outside. As a bonus we now have shorter ducting, fresh air circulation, and no laminar flow problems.

This however may not work in all climates.


Based on initial result graph I don’t think we have yet reached the maximum air flow rate for our particular collector size, some more research will have to done in this area.

The design at present does not have a controller for automatic operation. But we plan to add a simple temperature sensor to the collector plate to automatically switch on the fan when the sun hits the collector.

Unfortunately we just finished the Solar Sponge at the end of winter, and the warmest winter on record to boot! So we didn’t have much time to put it to real use. However, the results we got were quite encouraging for such a small collector. Whilst the prototype Solar Sponge worked fairly well, we feel that it was a little on the small side (1.6m2) for our large living area.  Typical heat output was in the order of 500W on an average day. Not bad for a mere 25W of fan power!

This Graph shows a 3°C increase in our main living room temperate which is quite impressive for such a small collector.

The prototype has shown us that Solar Air Heaters are a viable technology and can be made cheaply. It sure is nice sitting under a vent pumping out 40°C air. We can’t wait for next winter!

Return to the Main Page



Copyright(c) 2005 David L. Jones

EMAIL Contact:  david AT alternatezone DOT com (do the anti-spam thing)