Solar flat vs. vacuum tubes

Hi Tom,
NRT is installing solar water tubes in my new house in Steuben..We are starting to
put it all together and expect to pore the slab in a week or two.I called
my contractor friend about your article on flat tubes being the best for
domestic and he informed me NRT is not using the flat plates and he said
the round tubes are better.My question is since I am using solar water
tubes for both domestic & radiant floor heat are the flat plates still
superior to the vacuum tubes or does the duel use change the choice back to
vacuum tubes.My friend has been listening to you for years and says you are
well informed in solar etc. and that you might be designing or selling the
flat plate system in the near future.Is that true? Also when is your next
column?
Thank you.
Bobby

Hi,
Vacuum tubes work well. They are not as effective as flat plates for
domestic hot water systems.They are more efficient per NET square foot of
surface. The issue here is the black surface that actually intercepts the
sun. The fin inside a flat plate is a full sheet that mostly fills the
case. The vacuum tube has a fin inside each tube. And there are spaces
between the fins inside and the tube that contains it. And then there are
spaces in between the fins. The net area is a lot less on a vacuum tube
collector as opposed to a flat plate of the same surface area. The
additional efficiency of the vacuum tube does not compensate for this.
You need not take my word for it, check out this site: www.solar-rating.org/
Vacuum tubes look cool and cost more per square foot. Flat plates usually
produce about 20% more than tubes for a given gross surface area.
In your application, vacuum tubes will be a little bit better, but if I was
buying collectors, I would opt for flat plates.
Since you already own them, go for it. Install the tubes. Have realistic
expectations, though! Most solar installations will not yield the kind of
results for space heating that many people anticipate unless the heat load
of the building is fairly low.
Now for the real skinny on vacuum tubes: they are all produced overseas.
The reason they are being made instead of flat plates it that they are
easier to knock down and ship.
I used to be one of the biggest vacuum tube distributors in the US. We
stopped selling them because of breakage issues and cost.
They are not horrible, you should, however, know the facts!
Tom