[Strawbale] Chinese-made evacuated tube collectors and boilers vs. hot wa...

[Wayneapple at aol.com]

Evacuated tubes actually work better in the winter than flat plate(providing 
the snow gets off of them) and flat plate panels work better in the summer. I 
have done computer simulations that back this up(Michigan climate.. The 
compromise with evacuated tube collectors is that there always only a small portion 
of the panels that is optimally oriented toward the sun. The larger the 
temperature difference between the desired fluid temperature and the outside air the 
better a evac panel works in relation to a flat plate.
 
Evacuated tube panels(even the Chinese panels) are more expensive per sf that 
flat plates. Also note that not all evacuated tube panels are created equal. 
Some perform better than others. The SRCC ratings have a "Y"  intercept which 
is the efficiency with no temp difference between fluid and air. The othe key 
rating is the slope of the efficiency curve. Most flat plates have higher "Y" 
intercepts but steeper curves than evac panels.
 
The evac panels also have spaces between the tubes so that there is some 
losses there when comparing gross sf with output.
 
Smooth flat plates shed snow sooner(they also lose more heat so they aid 
sliding) than the tubes do.
 
>From the standpoint of longterm survivability is concerned, the flat plates 
are tractor technology. Loss of vacuum and tube breakage may reduce the 
lifespan. If the manufacturer is no longer in existance, a broken tube could spell 
the end of a system. I have had flat plate panels stagnate for years with little 
degredation.
 
If you have the room for an additional 10-15% sf for flat plates(to 
compensate for lower winter efficiency) I'd go with the flat plates.
See comments below.
 

Wayne Appleyard
Architect
Sunstructures Architects
201 E. Liberty st. Ste. 1
Ann Arbor, MI 48104
(734) 994-5650
fax 994-5656  

Wayne Appleyard
Architect
Sunstructures Architects
201 E. Liberty st. Ste. 1
Ann Arbor, MI 48104
(734) 994-5650
fax 994-5656


 
 
 
In a message dated 4/23/2007 11:38:51 PM Eastern Daylight Time, 
fritter63 at charter.net writes:

> Hi,
> evacuated tube collectors are recommended for points
> tropical and were snow is not likely to cover the
> collector, blocking the sun.Not true. they are too delicate to
> scrap snow off of because they have glass tubes like
> old style florescent light bulbs sticking out of them.Who gets on their 
roof to scrape snopw off of their panels??
> I think it might be dangerous to walk around on the
> roof covered in snow trying to clean snow off light
> bulbs anyhow.
>
> Flat panel collectors are recommended for the North
> West US and temperatures around 70 degrees F or less
> because they are more effcient at heating water at
> those temps. the colder it gets, the better flat panel
> work than evac tubes, I think. not true see above. Perhaps the colder and
> hotter it is, Flat panel work best, mild temps evac
> tubes work better. Flat panel do not heat water as hot
> as evac tube collectors, so if you need 160 degrees
> evac tubes might be the best choice.  Evac tubes are
> better at collecting heat in between about 20 degrees
> and 90 degrees on cloudy days, if I am reading the
> graph correctly. Please do not use the figures I give
> here, I am giving them to show the types of things to
> consider, not because I am good at interperting black
> and white graphs that show 3 types of collectors on
> sunny days and cloudy days using solid and dashed
> lines. This has got to be the worst graph I have seen
> in at least 2 months!
> Flat panel are the most common type of collector.
> If the rubber seals in the glass tubes fail on evac
> tube collectors, they probably will work less well
> than Flat Panel, which does not have rubber seals, I
> think.
> Shody


 



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