evacuated tubes vs. flate plates [RE-wrenches]

[Ken Schaal]

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Thanks Conrad

That sums it up pretty well !

Anyone care to comment on the " good and bad" among the new crop of ET's ?

Ken
CommonWealth Solar
"Solar Energy--The Fuel That Lasts Forever ! "


----- Original Message ----- 
From: "Conrad Geyser" <conradg at cape.com>
To: <RE-wrenches at topica.com>
Sent: Sunday, November 18, 2007 8:20 AM
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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