[Greenbuilding] Evacuated tube collectors?

[Speireag Alden]

Hallo, all.

     Some of you may recall Niko Horster, who posts infrequently, but 
always has something interesting to say.  So he wrote me an e-mail, 
inviting me to call him on this topic.  I did, and here is a summary 
of the conversation.  If I get anything wrong, I hope that he will 
correct me.

Intro

     I am located in west central New Hampshire.  Niko lives near me. 
This climate enjoys at least four solid months with frequent snow 
each year.  The snow is often wet and sticky, and it often melts 
partially and then re-freezes.

     Niko is a technically capable fellow, and he has, on his own 
house, both Thermomax evacuated tubes and a flat plate collector.

Niko's Concerns

     Niko thought my ideas could be implemented, but that the details 
were potentially problematical.

Solar Direct

     First, he doubted that a solar-direct El Sid-type pump would work 
well.  The problem is that the collectors heat up too much before the 
pump starts running, and after the pump stops, due to sun angle.  He 
said that with decent sun, there is no doubt that a solar direct pump 
works fine, but near sunrise and sunset naturally there's less light 
on the panels.  On cloudy days, of course, the pump doesn't run, even 
though the tubes are heating up.

Heat Stress

     The result is that the tubes get mighty hot, especially at the 
connectors, because the fluid isn't running through and cooling 
things down.  They are designed to take this, but every mechanical 
design is stressed by temperature change, and these are no different. 
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.

     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.  Niko suggested 
running the pump straight off of house current, with a temperature 
sensor to turn it on when the panels heat up.

     Niko also said that if he were starting fresh right now, he would 
buy Apricus tubes.  He cited the fact that they don't rely on a 
copper-to-glass connection, which is where his Thermomax (and also 
Sunda, I think) tubes have failed.  He also pointed out that their 
connectors permitted swapping tubes out without depressurizing the 
system, and without disturbing the other tubes (points not in favor 
of Thermomax or Sunda, apparently).  They also have a much better 
warranty.

Snow

     Niko also pointed out that evacuated tubes are such good 
insulators that they don't shed snow.  They don't even shed frozen 
dew.  Get a thin layer of snow on them and you're done until you get 
the snow off.  (Ross MacLeod found the same thing in his research.) 
Apparently a British expert pooh-poohed this concern, saying that 
Britain gets snow, and then called Niko a month later with profuse 
apologies, saying that after getting caught in a snow storm in Maine, 
he now understood the kid of snow Niko was dealing with.  I spoke 
with the local dealer whom Karen Bushey referred me to, and among 
other things, he also said that they didn't shed snow, and you had to 
do it manually.

     Niko suggested heat tape running up the back of each tube, 
pointing out that it would take perhaps $0.15 to $1.00 per snowfall 
to get the tubes to shed the snow.  I'm leery; I know that they make 
heat tape for use on the surface of eaves, specifically to prevent 
ice dams, so no doubt I can find such heat tape, but that's making 
things complex, and eventually I'd like to .

     I suggested salvaged patio doors over the collectors, but he 
pointed out that they're usually low-e glass, and thus would be 
blocking insolation.  That's true, but I could make them removable, 
so that it would only be an issue during frost weather.  Come to 
think of it, I suppose I could set them up like shutters, paint the 
backsides black, and when it snows, close them over on top of the 
snow, and let them melt it right down.

     Or unroll black landscape fabric or plastic over the collectors 
after a snow fall, and roll it back out of the way when it has done 
its work.  But that requires manual intervention, which is always 
nice to avoid.

     I pointed out that I'll have a greenhouse attached to the house, 
eventually.  Niko was enthusiastic about the notion of mounting the 
solar collectors inside the greenhouse.  That way snow was a 
non-issue, since it would melt off the greenhouse, and they'd be in a 
warmer environment anyway, which simplified everything.

     However, the greenhouse ain't built yet.  And, if I put the solar 
collectors down where I want them, they can thermosiphon to some 
extent (though I don't know if they will, much).

     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.

     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.

-Speireag.

-- 
Fill the molten glass.
Sit with singing summer frogs.
Think on Jack's wedding.