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which solar thermal system is the best? - Page 6

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Posted by nicksanspam on July 20, 2006, 3:47 pm
 


Did you do an incidence angle modifier (IAM) calc for the tubes?

I'm not sure how to do that.

Nick


Posted by Gary on July 20, 2006, 10:18 pm
 
nicksanspam@ece.villanova.edu wrote:

Hi Nick,

I have not been accounting for the IAM at all.

Using the IAM corrections in the SRCC directory,

    The Sunda performance is unaffected, since its S coeficient is zero
    The Heliodyne would be slightly down.
        0% at 0deg incidence
        -1% at 30 degree incidence
        -4% at 45 degree incidence

By eyeball, it looks like it might drop the Heliodyne solar faction about about
a half of a point?  For Billings, from 0.39 down to 0.385  ??

It looks like the IAM on the Apricus would actually help it, maybe bringing it
up a bit closer to the other two?

Or maybe I'm getting something wrong?

Gary



--


Gary

www.BuildItSolar.com
gary@BuildItSolar.com
"Build It Yourself" Solar Projects









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Posted by Alan on July 21, 2006, 4:41 pm
 This effort is very impressive.  All things being equal though, the
marginally higher price of the evacuated tubes could be reflected in
their longer life, no?  It seems to me that with that maze of copper
tubing in a flat plate collector, like the arteries in a human body,
some kind of build-up of minerals will reduce performance and cause a
need for repair.


Gary wrote:
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Posted by Gary on July 21, 2006, 6:51 pm
 Alan wrote:

Hi Alan,
Flat plate collectors have a reputation for very long lives -- 30+ years, and
they are repairable.
Evac tubes may do just as well.
I've heard of early failures in both cases, but (hopefully)those are the
exception.

If mineral build up is a problem, it can be addressed by using a closed system
with an antifreeze and water mix, as well as other means that have been
discussed here lately.  Right off hand I'm not sure why it would be any more or
less of a problem on the two types of collectors?  Although, I guess the buildup
would be limited to the header on an evac tube setup.

I've not seen the "marginal" difference in pricing you mention -- maybe you can
tell us where the good deals on evac tubes are :)  The prices I've been able to
find have the evac tubes selling for substantially more than flat plates.
Its important to compare prices at equal areas.
For me, evac tubes have the added awkwardness of being available in only a very
limited set of sizes that don't fit my space very well, where as the flat plate
collectors and absorbers can be ordered to match almost any space.  But, this
may not be a problem for most people.  For me, flat plates have the added
advantage that you can build all or part of them yourself at a substantial
saving.

The wide differences in the efficiency curves for the evac tubes that the SRCC
has tested would make me uneasy about buying an evac tube that has not gone
through the SRCC certification to establish its efficiency (or, at least some
form of independent testing) -- how would you know you are not getting one of
the low efficiency designs?

I'm coming across here like I'm anti evac tubes, and I am definitely not.  I
just think they need to show that their price/performance is better, and I'm not
seeing that yet.

Gary








losses

temperatures

the

tube

SRCC

getting

panel

less

partly

with

News==----

Newsgroups

www.BuildItSolar.com
gary@BuildItSolar.com
"Build It Yourself" Solar Projects









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Posted by Alan on July 21, 2006, 4:32 pm
 I've see these discussions before.  There are a couple of problems that
I've been toying with for years, but this is what I've observed:  1)
Flat Panel for a simple open system tends to be more problematic in
terms of mineral build-up, since the copper tubing, especially when
hot, is an ideal place for calcium and magnesium deposits.  Hence, the
limited lifespan of the system.  Evacuated Tube systems don't run water
through a maze of tubing, only through a header, and the header could
be oversized to make this problem one many generations away.  2) Solar
water heating and space heating really aren't a good pairing, although
I've been trying to figure out a way to make them so.  For either flat
panel or evacuated tube design, the companies want an estimate of the
peak summertime hot water usage, and then they calculate the solar
array to handle 90% of that.  In California, at least, summertime
radiant is zero, and summertime DHW is negligible in terms of natural
gas cost.  So, neither solar system can't put out the heat when it's
needed most, in the middle of a cold January night.  The solar storage
tank for either system needs to be gigantic, to store heat, and reduce
summer time excessive heat build up.  For any sizeable system in
California designed to collect heat for radiant floor, a summer heat
dump will be needed, and who wants to dump a large amount of heat into
their swimming pool or spa in summer?
3) Price comparisons of the two seem to be equalizing, and contrary to
popular myth, the Chinese appear to be more advanced in terms of
technology than the USA in the solar water heating industry.  I've got
a radiant floor system and have plenty of sunshine though, so I'm still
working on this system design.  The best plan so far for reducing
summer heat is to cover the panels with a tarp.


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