evacuated tubes vs. flate plates [RE-wrenches]

[Tom Lane]

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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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>
> 


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