evacuated tubes vs. flate plates [RE-wrenches]

Mon Nov 19 05:53:08 PST 2007

Thanks everyone for the feedback.  This generally confirms what I already
thought.

It does make me wonder if there are any wrenches out there that actually
prefer ET's to F collectors for any reason other than ease of installation
(not that that is insignificant, but it isn't what I want to make my
general recommendations based on)?  Or even use both types on a regular
basis and why?

I did see a chart on Apricus' website that seemed to indicated very little
production losses until the vertical angle of incidence was greater than
45-50 degrees.  Not sure if this is true, but if it is, then a near
vertical installation of ET's would be an advantage over FP's in
circumstances where vertical placement is necessary or preferential.  And
from what I've gathered, while ET's are more complicated and high tech (ie
less preferential), in circumstances where high angle of incidence is
required, then ET's seem to have an advantage.  And in snow country, a
near vertical position might help decrease the snow shedding problem
enough that it becomes an smaller issue.  Is this others' understanding?

I also second the question:  if there are big differences between ET's,
does anyone have recommendations or list of makes to stay away from and
why?

Hopefully this rehashing of this thread is helpful to others...

Thanks
Howie
-- 
Howie Michaelson
NABCEP Certified Solar PV Installer™

Sun Catcher, LLC
Renewable Energy Systems Sales and Service
http://www.SunCatcherVT.com
(cell) 802-272-0004
(home) 802-439-6096

On Sun, November 18, 2007 12:41 pm, Travis Creswell wrote:
>
> AET, a flat plate manufacturer in Florida, has link on their website to a
> side by side comparison that is worth reading.
>
> http://www.aetsolar.com/solar_comparisons.htm
>
> IMHO, flat plates perform fine for potable water heating and blend into
> the
> roof much better then ET's.
>
> I get a chuckle when ET's promoters talk about how much more efficient
> they
> are in cloudy weather.  Is that really a big deal?  To me it's like a wind
> turbine that claims to start producing power at 5mph...there's just not
> that
> energy there to capture.
>
> Travis Creswell
> Ozark Energy Services
>
> -----Original Message-----
> From: Conrad Geyser [mailto:conradg at cape.com]
> Sent: Sunday, November 18, 2007 7:21 AM
> To: RE-wrenches at topica.com
> Subject: evacuated tubes vs. flate plates [RE-wrenches]
>
>
> Howie et all,
>
> This is a topic near and dear to my heart having worked on both styles of
> collectors for the last 19 years.
>
> The graph that I've seen show that flat plates put out more BTUs per
> square
> foot than evacuated tubes at temperature differentials up to ~ 120F
> (collector inlet fluid to ambient air).  I haven't confirmed this, but
> this
> is what the SRCC numbers are supposed to show.  I can send you this chart
> if
>
> you contact me offlist, Topica won't allow it on the list.
>
> So the question becomes do you have a significant use for differentials
> greater than 120F?  For any basic solar thermal system that I've worked
> on,
> the answer has always been no.  Maybe in some cases in very cold regions
> that still have decent solar gain in the winter.  For example, if you had
> a
> space heating system in an area with a significant amount of solar gain
> days
>
> with daytime temperatures below zero, then ETs would be something to look
> into.  Or if you need a tiny bit of high differential heat on a cloudy
> day,
> ETs will give you that as well where FPs will give you zippo.  Perhaps
> this
> is why the Decathlon entries are using them.
>
> The dropping efficiency of FPs at higher differentials is usually a
> convenient thing as the systems are self limiting.  ETs will take you to
> the
>
> moon with the temperature if you don't shut them off or dump the heat.
> This
>
> suggests drainback systems, another topic near and dear for another
> thread.
>
> Then comes the question of KISS.  ETs are inherently more intricate,
> delicate and complicated.  They are the Porsche and the FPs are the
> Toyota.
> Hopefully quality ETs of the current era are more durable than the 80s
> versions.  All the 80s versions have we have come across have outlived
> their
>
> lifespans where most of the FPs are still going strong.  There has to be a
> reasonable benefit to justify the complexity of ET technology so don't
> forget to take this into account.
>
> I'm hoping that we see some Adsorption cooling or steam driven
> technologies
> come out of the huge ET development of later years.   I just can't seem to
> find a logical place for them in solar thermal systems in New England at
> this point.
>
> happy solarizing,
> Conrad
> Cotuit Solar
>
>
>
>
> ----- Original Message -----
> From: "Howie Michaelson" <howie at suncatchervt.com>
> To: <RE-wrenches at topica.com>
> Sent: Saturday, November 17, 2007 3:07 PM
> Subject: Evacuated Tubes [RE-wrenches]
>
>
>
> Hi All,
>
> I'm looking for experiential as well as "non-biased" info on Evacuated
> tubes.  I have installed them a few times in the past, but have pretty
> much steered clear of them since for several reasons:
>
> 1. Here in snow country, I've seen them not shed snow for significant
> periods of time, and have also seen them break from ice/snow movement down
> the roof.
>
> 2. From what I've seen/heard their output is in most cases not any better
> than flat plate collectors, and due to lack of snow shedding capabilities,
> sometimes substantially worse.
>
> 3.  They are more expensive than flat plates, and are rated lower by SRCC
> so in Vermont are given less incentive money (which is based on the
> ratings).
>
> A client saw the Solar Decathlon and noticed that almost all the
> competitors used evacuated tubes (ET's).  He also said that one
> manufacturer was whacking a tube very vigorously against hard objects with
> no damage resulting.  He wants to put in the reasonably best collectors he
> can with an eye toward Return on Investment, but is now enamored with
> Evacuated tubes since they would fit well on the south wall of his home at
> a very steep angle (near vertical), and was told that there is virtually
> no output penalty for doing so.  Placing them near vertical probably goes
> a long way toward answering the snow shedding issue, but I'm mostly
> ignorant about the rest of these issues.
>
> Any info about any of these concerns or leads to a place where I can get
> good comparative info on ET's vs. flat plate collectors would be much
> appreciated.
>
> Thanks,
> Howie
> --
> Howie Michaelson
> NABCEP Certified Solar PV InstallerT
>
> Sun Catcher, LLC
> Renewable Energy Systems Sales and Service
> http://www.SunCatcherVT.com
> (cell) 802-272-0004
> (home) 802-439-6096
>
>
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