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
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
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
Basically, the major things to come out of the prototype results
- 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
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
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.
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
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
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
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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