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Build it Solar Collector- General Discussion - Page 2

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Posted by gary on October 4, 2008, 6:14 pm
Hi Mike,
I'd also like to hear any comments/suggestions on the two collectors,
but since there has not been much, I'll put in my 2 cents ...

The efficiency of all of these collectors depend on the solar
radiation level, and on the temperature difference between the fluid
going through the collector (storage temperature) and ambient
temperature -- so there is an efficiency curve, not a single point.

The SRCC website has certification data for many dozens of collectors
they have tested that you can download, and this includes the
efficiency curve parameters for each collector.

Most commercial flat plate collectors are similar in efficiency.  For
practical purposes, they have an efficiency under normal collection
conditions in the 50 to 60% area.  As it turns out, evac tube
collectors are roughly the same.
I suppose there are always more things that can be done to improve
this, and there is no fundamental Physics reason you can't approach
100%.  But, very little has changed in the efficiency levels of these
collectors since the 80's, so I am thinking that further improvements
are not currently cost effective.

The one feature that is included in many of the commercial collectors
is a selective coating on the fins.  This might improve the efficiency
by about 5 percentage points compared to a non-selective coated fin.
There are some DIY selective coatings, so you can get some of that
back if you want to, but they are not as effective as the commercially
applied coatings.

I think so.

This depends on the width of the fin.
Some of the solar reference books define a "fin efficiency" that is a
measure of how well a particular width, thickness, and material fin
does compared to a perfect fin.
I'll see if I can dig this up for the 0.018 fins I use, but if I'm
recalling correctly the fin efficiency for 6 inch wide by 0.018 thick
fins is good.

The diameter is not much of a practical factor -- a lot of commercial
collectors use 3/8 inch risers, and they work fine.
Its the fin width and thickness that limit performance.

Well, it won't do as well, but not sure how much of a hit you take.

No, the 6 inch spacing is already on the high side.

yes -- very thick.
This is also bad from the point of view that as you make the fin
wider, you are trying to push more heat into each inch of the copper
tube through a contact area that is not increasing as you increase the
fin width.  Since this thermal connection of the fin to the copper is
very important in the performance, this is a bad thing to do.

If the tests are right, you are really looking to get that 4% worse
performance for this homemade version improved -- it seems like there
is not a lot of room for improvement?
The other thing to consider is that silicone is a very good material
to use inside collectors -- it has a high temperature capability, its
glues the parts together in a way that will still accommodate some
differential thermal expansion, and it has a good track record for
providing a long life in this tough environment.  Not that the other
compounds might not do just as well or better, but it seems to me
there is some risk.  There are some silicone caulks that are filled
for higher thermal conductivity, and this might be a good thing to try
-- I've not found a good source for these yet.


Posted by amdx on October 7, 2008, 7:58 pm

Hi Mike,
I'd also like to hear any comments/suggestions on the two collectors,
but since there has not been much, I'll put in my 2 cents ...

Do you think there would be any advantage to a variable speed pump that
would adust its flow rate to keep some (unknown to me) minimum temperature
differential between the bottom and top of the collector?

I'll look for the SRCC website.

Regarding 3/8" vs 1/2" copper pipe. I have to wonder about about the pipe
1/2" pipe has 25% more surface area to absorb the collector heat and 25%
more boundary area to get that heat into the water.

So, do you think the aluminum to copper to water heat path is not a
bottleneck or limiting area?
Sorry to ask so many questions, at one time I worked with a physisist and I
was suprised how often
we worked near the edge of one parameter or another.

Probably now a moot point for me, I was at my local metal scrap yard
and found hundreds of 0.022" x approx. 2' x 3' aluminum sheets. I think they
might be used on
some type of rolling printing press.

Thanks Gary,
    Looks like you have put a lot of effort into this collector, I would bet
there are a whole bunch of us that appreciate your time.

Posted by Robert Scott on October 7, 2008, 8:52 pm

Well, it certainly is beneficial to take into account the temperature
differential when deciding when to turn the pump completely off.  It is less
clear when partial speed is preferable to either extreme.  If full speed is not
economical in some certain scenario, then I guess the the heat energy you would
have collected at full speed is less than the cost of the electricity to run the
pump (assuming pump wear and tear is not an issue).  So what is likely to happen
if the speed dropped to half?  Because the effect of viscosity is non-linear,
the energy required to push the liquid around at half speed is less than half
the energy required to push it around at full speed.  And if dropping to half
speed allowed the temperature differential to rise to the magic threshold, then
you would be getting exactly half the heat energy as you would at full speed
(with the same temperture differential).  So here you are spending less than
half the electrial energy to get exactly half the heat energy.  So yes, lowering
the speed would be cost-effective in theory.

Having said that, there are other practical considerations.  A variable speed
control is a one-time expense that needs to be amortized.  In calculating the
pay-back period of a variable speed control, you have to ask yourself how often
it would be called upon to work in variable speed mode.  And when it is working
in that mode, the amount of heat energy it is collecting is only slightly more
than the electrical energy used to run the pump.  So while there is a benefit,
it is quite small.

Robert Scott
Ypsilanti, Michigan

Posted by Jeff on October 8, 2008, 5:52 pm
 amdx wrote:

   Well, the fins are collecting the heat. As far as the contact area,
this is of much less importance for copper/aluminum as you can get
pretty good bonds (I use heatsink compound), at least compared to the
thermal resistance of the fin itself. For plastic piping, this is a
different story.

   Also note that you have much less flow resistance with 1/2" over
3/8", that will be more important in thermosyphon systems. But you will
find that the flow through the system, to get the temperature as low as
possible, has a fair size impact on efficiency.


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