evacuated tubes vs.flat plates [RE-wrenches]

[Conrad Geyser]

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I'd echo this conclusion that the evacuated tubes are only prescribed where 
a minimal gain is needed in a predominantly cloudy area.
K.I.S.S.
Conrad Geyser
Cotuit Solar


----- Original Message ----- 
From: "Jeffery Wolfe, Global Resource Options" 
<jeff at globalresourceoptions.com>
To: <RE-wrenches at topica.com>
Sent: Sunday, May 21, 2006 2:45 PM
Subject: RE: Drainback vs. Closed Loop Circulators [RE-wrenches]


>
>
> What we've seen with the evacuated tube collectors (which we have used,
> and don't any more) is the following:
> - they work better in cold, cloudy weather. But in the great northeast
> (and most of the rest of "cold country"), we don't get much solar
> irradiance during cold cloudy weather. (as a percentage of overall
> irradiance. For example, from Nov. 1 to Feb 28 we get about 20% of our
> annual irradiance.
> - the collectors are much more expensive than flat plate per BTU
> harvested on an annual basis
> - collecting a theoretical higher percentage of the energy when there
> is not much energy available does not result in that many more BTU's. It
> may result in more BTUs when you need them, but our experience does not
> bear that out to any large extent.
> - as was pointed out, they do NOT shed snow well. We've got some on our
> home, at a 50 degree installation angle, and they can stay covered for
> days (to high on the roof for me to get to, and those that do try to
> scrap snow of break tubes. We've had several customers learn the hard
> way even after our admonitions during training.)
> - while they are lighter to carry to the roof, there are more trips, by
> far, up and down the ladder. And carrying a box of 7' long tubes up a 2
> story ladder is no simple chore either. With proper training and proper
> tools, the installation time and safety has not proven to be any worse
> with flat plates. (We do not use 4x10's on standard residential
> projects. Why exacerbate the problem!)
> - the failure during warranty period has been high. Granted, one tube
> failure does not shut the system down, but it can create a radiating fin
> instead of a heat absorbing fin, reducing system capacity. The fact that
> we've had some failures, and that failures are fairly hard for an owner
> to see unless they pay careful attention to the system visually (the
> snow melts faster off a failed tube. That's about the only visual
> indication) they will not know about the failure. Where we have gone
> back for other reasons, we've too often found failure. This is reflected
> in other wrenches I know who have installed this technology for a large
> number of years in the past. The glass / metal seal is a weak point, and
> will always be a weak point.
> - there is one new evacuated tube on the market that has a glass to
> glass seal. That might work. Only problem is, they are incredibly
> fragile, and very difficult to ship. Not my recipe for a good product.
> - overall, we strongly believe (through modeling and anecdotal
> evidence) that high quality flat plate collectors perform about as well
> year round in our climate, have FAR fewer warranty issues, and cost no
> more to install safely.
>
> My 8 cents.
>
> Jeff Wolfe
> Global Resource Options, Inc., Solar Energy Solutions
>
>
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