Evacuated Tubes [RE-wrenches]

Wilco Vercoelen w.vercoelen at dommelvalley.ca
Sat Nov 17 18:59:14 PST 2007



Dear Howie,

We believe that there is a great difference in quality of these tubes.
So basically saying, evacuated tubes are better or worse than flat plate is
not recommended. 
However...
In a snow belt region we recommend flat plates over evacuated tubes, because
like the others wrote, there is very little heat loss, so snow cover is an
issue and you want the heat during the winter, unless you put them
vertically or at a steeper angle which would be preferable, the sun's angle
is lower in the sky in Fall and Winter anyways and this way it reduces the
overheating in the summer.
 
Generally you can say that ET performance (Net area) is much better, not
less, check SRCC & SPF, but to achieve the same results with flat plates,
from a financial point of view it is often wise to install more net surface
area of flat plates, because ET systems generally costs more, but it also
depends on what is available in your area and what the costs are.
There are many different manufacturers' out there and there good and bad
apples.

Wilco Vercoelen
Dommelvalley Green Power


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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