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Posted by Fredrick Skoog on May 29, 2008, 8:38 pm
 
Hi All,

I am considering getting a solar heated hot water system and have been
quoted for either the flat panels or the tubes. The guy who quoted for the
tubes said that they were more efficient, less susceptable to dirt (i.e.
bird shit was more likely to splat between them rather than all over them)
and any damage was more likely to mean the replacement of a tube or two
rather than then whole panel (he obviously had a thing against birds as he
cited an example of a bird dropping a stone or some such)

The next guy was all for the flat panels reckoning that the difference in
efficiency was marginal, that the tubes had some inherent issue with
overheating the heat-transfer fluid and that they were more expensive with
no real gain.

I would be extremely interested in other people's experiences and comments
to try and sort the wood from the trees.

Cheers


Posted by Solar Flare on May 30, 2008, 12:36 am
 
Winter cold weather demands vacuum insulation techniques. If you live within
25 degrees of the equator, forget the cost of evacuated tubes.



Posted by Steve on May 30, 2008, 1:34 am
 
I don't have tubes or a flat panel, but I do have an opinion, that is
largely influenced by where I live (a little over 45 degrees latitude).

Heat transfer occurs by one of 3 methods:
  Convection
  Conduction
  Radiation

When vacuum tubes are used, the first of these two methods of heat loss are
minimized.

With the flat panels I can probably meet my DHW (domestic hot water) needs
for 5 months of the year.  With vacuum tubes I can probably meet my DHW
needs for 9 months of the year.

A local installer told me he is a fan of drainback systems, which drain all
water from the panels under freezing conditions to avoid damage.  On those
systems regular water may be circulated to collect heat.  He told me that
you can't use Vacuum tubes on a drainback system since there could be
problems with the dry system overheating.  So you would have to use a glycol
system and all of the associated expense.

Personally I would prefer to have a vacuum tube system and use regular water
for heat transfer.  If there is a concern with freezing I could run a heat
tape along the water lines to keep them from freezing, it wouldn't take much
heat and it seldom gets that cold here anyway.  The extra months of meeting
my DHW needs would (I think) be worth the hassle.

I hope this helps,
Steve



Posted by AstickfortheMULE on May 30, 2008, 5:13 am
 Where are you located?

I am not sure that there is a big difference between flat panel and
Vacuum tubes system.  Tubes tend to be more expensive but more
efficient.

Have you considered the unglazed swiss panels from energie solaire?

that are less expensive and also are a dynamite roof with 3 layers to
it, all are vented.

You might want to check them out. I put them on my roof in Seattle and
I am floored by the performance.  My roof is 450 sf and it is angled
only 22 degrees due south and on a recent day with clouds, 40 degrees
the panels were reading 120 deg F.

I have combined these panels with a large 800 gallon tank from SSTS

I also have a 5000 gallon and 8000 gallon tanks tanks that will be
used with a water to water heat pump (solar heated GEOTHERMAL) and
this winter I will find out if pumping 35 degree water through the
panels on a cloudy winter day will be able to get heat out of the
December sun (non existent mostly).

Eric

Posted by dold on May 30, 2008, 6:57 pm
 
The local installer said that evacuated tube systems had to be undersized
somewhat, to prevent boiling the water during periods of overproduction.

He liked flat panels and the $000 heater/storage tank from Phoenix
http://www.htproducts.com/phoenixsolar.html

He was so full of bull on most items that I discarded a lot of what he
said, but the overheat idea left me thinking.

What about the Solahart systems?

--
Clarence A Dold - Hidden Valley Lake, CA, USA  GPS: 38.8,-122.5

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