evacuated tubes vs. flate plates [RE-wrenches]

Travis Creswell tcreswell at ozarkenergyservices.com
Sun Nov 18 09:41:47 PST 2007


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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