Solar Sponge
The DIY Solar Air heater

Project Update

I get quite a lot of email about the Solar Sponge, mostly from people wanting to know how it all went, what improvements I've done, what lesson I learned and so forth.

So rather than answer the same emails over and over again, I thought I'd write an update with some random responses to some common emails I've received. I'll tidy it all up as time permits.


As bad luck would have it, once we built and installed the original prototype, the following winters in Sydney Australia have all been  record high temps, almost summer like, so the Solar Sponge has been hardly ever needed since the end of that first winter it was installed. Not that we were really complaining about warmer winters though.

I didn't use insulation because I wanted to see what was possible using the lowest cost no-frills approach.
Insulation of the collector box will help, but there are much larger losses in the system that are more important to spend effort on than the collector box, for instance:
1) Fan capacity
2) Collector transfer efficiency (how it can provide maximum heat transfer to the air, whilst providing minimal air resistance)
3) Ducting insulation
4) Collector size
5) Ducting length

Of of these are more important than collector box insulation, especially in full sun.


Results though show that a much larger collector is needed than our 1.6m2 for the size living room we have. It was great as a prototype study, but not hugely practical in it's installed configuration.

Basically, the major things to come out of the prototype results were:
- A much bigger collector is needed
- There is a lot of loss in the ducting. Insulated ducting and keeping it  as short as possible is very important.
- An improved design to increase the collector-to-air transfer efficiency would help a lot. But there is a trade-off here with air resistance vs transfer efficiency.
- A lot of fan power is required. If you do the calcs of how much air volume you have to move per hour in order to exchange the air for heating, you'd be surprised at the high numbers. Our system (4 fans) had at least twice the volume rate of the commercial Sun Lizard design and it still wasn't enough to adequately exchange the volume of air in the large open room. No wonder the Sun Lizard hardly works at all!
- Separate collectors for each room is much better than one big one and leave the doors open.
- The thin clear polycarb sheeting warps a LOT with time. This makes it very hard (read impossible) to seal properly. I wouldn't use it again.


Many thanks for your reply David. I suspected as much about the polycarb sheeting. I have many panes of window glass left over from a renovation and will probably try them ... even with the risk of hail damage (it has to rain before hail and at this rate we are not likely to have that for a while yet).
Only problem with that might be that it could block too much of the UV.
Special "low iron" glass is used in commercial collectors like the Sun Lizard, but it's fairly expensive I believe.
Certainly worth trying the stuff you have though.


As for the need for improved air/aluminium contact, I was considering pop rivetting short right angled pieces (maybe 40mm square) to the back plate to force the air around them. Was also considering having the aluminum sheet suspended in the middle of the box and thereby having two sides in contact with the air flow. Will probably construct a very small unit to refine the physics ...
I had one suggestion of using steel wool wedged between the top and bottom plate. Don't know how that would work, but worth investigating.


Q) Can you tell me what the backing of the box is made of? There is no picture of the underside of the box. It sounds like it too, is made from aluminum, but this can't be, as you'll loss all your heat to the outside.... is it plywood? Is there any insulation? If the bottom, inside, of the box, was covered with a 1 inch (2 cm) piece of  rigid foam, would this not make a big improvement (reduction of loss).

The entire box is aluminium, as shown here:

An entire metal box is needed in order to get as much thermal transfer as possible to the passing air inside, which is the most important aspect of solar air heater design. The losses externally are not as great as that encountered by coupling to the passing  air inside. The external box gets to well over 100degC without any air flow, and almost 70deg with full airflow.
This graph shows the losses from collector to outlet:

Yes, insulation would help a bit, but probably not nearly as much as you would think, thermal coupling is still the most important thing. The concept of the solar sponge was simplicity and low cost, designed to see what's possible with a minimalist design. That's why no insulation was added on the prototype.

Insulation is *much* more important on the ducting from the unit to the outlet in the house than it is on the unit itself.
Basically, heat losses externally on the collector are not that important because it is continually receiving energy input from the sun. Remember, the collector gets hot, very hot anyway, without any insulation.
So system importance of losses/inefficiencies are in this order:
1) Air coupling & air volume (these are the two biggies)
2) Ducting losses
3) Collector losses.


Some key things I leaned from the Solar Sponge development were:
1) Solar Air Heaters do work, but you need LARGE arrays to be really useful. My 1.6m2 prototype wasn't all that practical.

2) You need a LOT of air volume circulation to even moderately warm an average size house room over the course of a day. In the end my 4 fan system was not enough.

3) Losses are HUGE, not only in the collector but in the ducting and in the room being heated. When you actually do the calculations for what it takes to displace an actual room full of air you'll be very surprised at how much fan power you actually need. And that doesn't include the losses.

Don't bother with the Sun Lizard. Small collector size and small air volume mean it practically useless. Rob Kemp's website has info on it for his modest home (I've been there to see it) and it basically did nothing for him. Classic case of over-engineering a basic idea without actually knowing what the real fundamental issues are.

The thin polycarbonate sheeting warped a lot, I wouldn't use it again. Very difficult to keep the seals intact. Thick or corrugated polycarb is your best bet. The special glass is expensive.

Talking with some US developers I gather that the vertical wall mounted passive heaters are really really good. If you have area below your window then I would recommend trying a passive wall heater where the hot air rises naturally out the top and into the window under convection . Actually you don't need a window, just a large wall with a hole in it.
No fans losses, little ducting losses, little radiative losses out front of the panel (due to it being vertical), and pretty much most of the heat gets into the house. Small fans can be used to supplement this though.
Very cheap to make too I've heard.
Although in the US they like to have those big "barn" style houses with large high walls where this is possible, we don't tend to have those much here.


My first idea for the Solar Sponge was indeed to put aluminium heatsinks on the underside of the collector plate, like the ones used for electronics power devices. Of course they are expensive if you use a few, and I tried to come up with many low cost solution but didn't come up with much. So the first prototype simply had the snaked channel, so I could see how much performance would be gained from a simple system.

I love your scourer idea, didn't think of that one!
It would indeed increase the surface many orders of magnitude and "disrupt" the airflow to allow time for it to gather more heat. I like it.
My initial thoughts are:

1) Scourers are usually stainless steel which is a relatively poor heat conductor (and hence radiator), but they would eventually get to the same temperature as the collector I assume. Copper would be much better, but likely more expensive?

2) There would be a trade off between blocking the airflow and the fan power required to force the air through. My current 4 fan system already struggles. But if you can get more heat transfer then that might compensate for the lower volume rate. But from my measurements, volume is probably the most important variable as the looses in a typical room are quite large.

3) They wouldn't add much "sinking" ability to the collector due to the relatively high thermal resistance of the stainless steel. Good or bad?, I'm not sure.

Although I'm an electronics designer, so have little theoretical background on the actual thermodynamics involved with this sort of stuff. It would come down to actual measurements. There are an awful lot of trade-offs in a system like this.


I added a thermal switch directly to the underside of the collector plate to automatically switch the fan on and off when a certain temperature is reached. These are cheap and simple to fit. Jaycar and many other suppliers have them:


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Copyright(c) 2008 David L. Jones

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