evacuated tubes vs. flate plates [RE-wrenches]

Tue Nov 20 11:09:55 PST 2007

Thanks Tom,

I was wondering if we were going to hear from you on this one...

Do you have any data on comparisons of FP vs. ET collectors when mounted
at a high angle of incidence (ie near vertical)?  Or know where some
reliable data might be foraged?  My client is interested in a near
vertical mounting if it isn't too inefficient.

And does anyone have any hard data as to the durability of various ET's?
(sorry for repeating this question - I just haven't found any references
to this kind of data anywhere, but many have suggested that ET's have a
shorter lifespan.

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 Tue, November 20, 2007 10:51 pm, Tom Lane wrote:
>
> The mature 12 year old German marketplace on the same latitude as Central
> Canada - now has a 90% flat-plate market versus 10% evacuated tubes.
> Evacuated tubes have serious problems shedding frost until very late in
> the
> day, qnd even more problems shedding snow.
>
> Anywhere in the U.S.A. flat plates will outperform evacuated tubes in the
> winter for heating water to 140 degrees F and to 160 degrees F in the
> summer.  Flat plates will not shatter in hail storms which easily destroy
> evacuated tubes.  Many contractors make a critical mistake in choosing
> evacuated tubes to heat hot water to 140 degrees F based on low-sun
> conditions which favor evacuated tubes.  Actual collector output is input
> times efficiency.  If input is nothing to start with (cloudy conditions)
> your efficiency may be high, but you still do not collect much energy.
> Based on your system performance on sunny days, when there is something to
> gain, and the best flat-plate collector will outperform the best evacuated
> tubes by 22% on anual savings, and by at least 6% under winter conditions
> in
> heating water to 140 degrees F.  The obvious reason to use the much less
> expensive, lower maintenance, longer lasting flat-plate collectors vs.
> evacuated tubes in that overall performance depends on efficiency when the
> sun is availableand the system should be "cooking" as opposed to when you
> can't see your own shadow in the middle of the winter. There is no single
> collector that can be labeled "the most efficient", for this depends on
> the
> water temperatures the system is required for storage.  In most cases,
> evacuated tubes collectors will be required when you need to heat water
> over
> 160 degrees F in the winter and over 180 degrees F in the summer.
> Evacuated
> tubes are a good choice when you need 180 degree F to 211 degree F
> temperatures.
>
> Even at extreme mountain altitudes in cold windy conditions, selective
> coated flat plates will usually outperform or equal evacuated tube
> collectors in heating water from 120 degrees to 140 degrees F at 0 to 32
> degrees F amient.  Evacuated tubes will heat water much better under
> extremely cloudy conditions.  They collect some heat during theses
> conditions in tubes, however, few BTU's will be transferred to storage.
>
> Evacuated tubes can produce dangerously scalding temperatures in storage
> well over 200 degrees F in the summer.  Most flat-plate systems will reach
> 180 degrees F to 190 degrees F and stop except in extreme desert climates
> like Arizona.
>
> Gatortom
>
> ----- Original Message -----
> From: "Hugh Piggott" <hugh at scoraigwind.co.uk>
> To: <RE-wrenches at topica.com>
> Sent: Tuesday, November 20, 2007 3:39 AM
> Subject: Re: evacuated tubes vs. flate plates [RE-wrenches]
>
>
>>
>>>>  ETs are inherently more intricate, delicate and complicated.  They
>>>> are
>>>> the Porsche and the FPs are the Toyota.
>>
>> I recently installed some chinese evac tubes on my own roof.  I am very
>> pleased with the output and the price was not bad, but I don't have much
>> experience of solar HW.  I do wonder how long they will last.
>>
>> What I was going to say was, I read in a recent edition of Renewable
>> Energy World, that most solar HW in the world is actually evac tube and
>> the reason is that the vast majority of Chinese solar HW uses them.  Not
>> so common elsewhere, but china has by far most of the installed capacity
>> of solar thermal systems.
>>
>> So I guess they are more than a gimmick if they have most of the market
>> already.
>> --
>> Hugh Piggott
>>
>> Scoraig Wind Electric
>>
>> http://www.scoraigwind.co.uk
>>
>>
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