[Greenbuilding] Evacuated tube collectors?

[Corwyn]

On Jul 19, 2007, at 14:40, Speireag Alden wrote:

> In the few years that he has owned them, 3 out of 4 of his Thermomax
> tubes have failed.  (I think they've all been replaced under
> warranty.)  He attributes this to the wide temperature swing, which
> is partly simply climate, and partly, at least when he first
> installed them, due to a solar-direct pump which did not pump soon
> enough or late enough in the day.

Distressing, even if the cause is known, and avoided.  I really don't 
want to mess with things on my (steep, slippery) roof.  So, even 
warranty replacements are a pain.  Nor do I want to be stuck with 
broken tubes and an out of business manufacturer.
>
>      He says that the glass is thicker on the Sunda tubes, and they
> seem to last better.
>
>      A neighbor of ours also has Thermomax tubes, and he put two El
> Sids in series, each hooked to a different panel, one pointing
> southeast and the other southwest, to get the early and late sun.
> This works well, but it also doubles the cost.

Why not put the panels in parallel rather then the pumps?
>
>      My best idea to date is this:  Put the tubes inside a box, with a
> removable glass frame.  You could even paint the back of the box
> black, if you wanted.  I'll have a loop running through a very large
> heat sink, the earth under the house and insulation umbrella, which
> will always be well above freezing.  What if I run the return loop
> back and forth under the tubes, inside the box?  Make it copper
> painted black and after the snow sheds it will even help gather heat.
> Then, after a snow fall, I simply start the pump and let it run, and
> the 55°F to 70°F antifreeze coming out of the ground will easily melt
> the snow.  Yes, you lose a bit of heat out of the ground, but once
> you get the snow off of the collectors, you get it back and then some.

Sounds like you are getting close to flat panels here.  Why not just 
get flat panel collectors?  All the dealers talk about efficiency, by 
which they mean BTU/ft^2.  I tell them I couldn't care less how 
efficient they are.  I care about BTU/$, as I have plenty of room on 
the roof.

>      This would raise cost, but defeat the snow issue and combine,
> perhaps, the best aspects of a flat plate collector and an evacuated
> tube collector.

The EOS people I spoke to did not claim their tubes gathered much more 
BTUs than a comparable flat panel.  They talked about more even heating 
(which since we both are heating a large store, doesn't matter).   Any 
references on how much better tubes are in marginal conditions?

Thank You Kindly,

Corwyn