evacuated tubes vs. flate plates [RE-wrenches]

Kelly Keilwitz kelly at whidbeysunwind.com
Fri Nov 23 12:08:57 PST 2007



Hi Howie,
We've mostly installed Thermomax & Apricus ET's, but do some flat plates as
well:

On 11/20/07 11:09 AM, "Howie Michaelson" <howie at suncatchervt.com> wrote:

> 
> Thanks Tom,
> 
> I was wondering if we were going to hear from you on this one...
> 
> Do you have any data on comparisons of FP vs. ET collectors when mounted
> at a high angle of incidence (ie near vertical)?  Or know where some
> reliable data might be foraged?  My client is interested in a near
> vertical mounting if it isn't too inefficient.

The angle of incidence improvement claim for ET's is more for azimuth angle,
than tilt, and is based on 2 factors:
1. The tubes tend to refract light at an angle onto the collector plate, and
2. The Apricus collector plate is completely tubular. With the tubes spaced
there is no aperture cosine loss (i.e. it's just as if the collector was
facing the sun) for up to around 45 degrees off of the collector azimuth
direction.

My experience is that I think there is some validity to effect #1. An array
of each ET type on my own home definitely confirms the validity of #2. For
these reasons we think ET's work better on roof pitches more than 45 deg off
of South than the equivelant-sized FP's. Also the Thermomax tubes can be
mounted with the (flat) collector plate angled up to 15 degrees to one side,
which helps.
> 
> And does anyone have any hard data as to the durability of various ET's?
> (sorry for repeating this question - I just haven't found any references
> to this kind of data anywhere, but many have suggested that ET's have a
> shorter lifespan.

We've never seen a broken Thermomax tube. We've seen a fair amount of
shipping breakage in Apricus tubes (although haven't yet seen breakage after
installation). Thermomax tubes must be entirely replaced. Apricus tubes can
have only the glass replaced, reusing the collector and heat pipe, at
reasonable cost.
A colleague (who used to post on this list) who installed many Thermomax
systems in southern Oregon once reported that a big hailstorm that severely
damaged roofs, cars, and skylights didn't break a single tube in any of his
systems.

I can confirm that the frost and snow tend to stick around a long time on
ET's. Here in western WA we don't have much of an issue with snow, but do
see frost a fair amount. Today is a clear, cold day with plenty of frost. My
collectors were frost free by 10A. If it weren't so sunny today it might
take until noon.

The differences in ET design - and cost - are as big as the differences
between ET's and FP's. Thermomax Mazdon and Apricus ET's are heat pipes -
meaning each tube is a closed loop. This allows the complete installation
and charging of the manifold & system before the tubes are installed - good
for new construction before the homeowner moves in. Thermomax Solamax ET's
have direct flow into each of the tubes - which requires plumbing
connections at each tube, requiring significantly more time, and tube
installation before charging with HT fluid. Thmax Mazdon tubes each have a
thermostatically controlled valve that shuts down heat transfer at high
temps (~220 F). There is no heat control for the Apricus or Thmax Solamax
ET's. Careful, conservative design of storage size and/or heat diversion is
needed for Apricus and Solamax. We don't use (and our Thmax distributor no
longer provides) the Solamax tubes, as overheating can cause the HTF to boil
out and lock up/overpressure the system with steam until it cools down
overnight.

FYI, we've seen excessive condensation/frosting on some Thmax Mazdon ET's
that we think is related to QC issues with the thermostatic valves. We've
recorded the shutdown of heat at lower temps - causing less heat gain. Our
Thmax distributor is very squirrelly with information on this, but confirmed
there was an issue but that it is resolved. However, we've replaced a set of
10 "bad" tubes in one collector 3 times without the issue being resolved.

We can provide a 30-tube Apricus system for about $1K less than a 30-tube
Mazdon system. Apricus is made in China, Thermomax has been made in the UK
(but the company was recently purchased, and production may move to China).
We've been getting both out of nearby BC, Canada, so shipping has been
reasonable. FP's are heavy and bulky, and come from CA, so shipping is more
significant. Considering shipping, Apricus system materials have cost a
little less than FP's. When installation costs are included (even
considering high temp insulation, etc required), Apricus FP's usually have
had a clear cost advantage. However, with the dramatic fall of the US$ (and
homeland "security" issues at the border) our costs on ET's have been
increasing dramatically. Thus, we're starting to plan for, design and
recommend more FP systems, where getting the beasts to the roof isn't a big
hassle. If FP costs don't rise as much as the ET's we'll probably be
installing more of these in the future.

We have year-round cool ambient temperatures and windy conditions, which
probably aren't typical around the rest of the country. These conditions
have led me to consider ET's to generally be a more effective collector,
here. I haven't seen any data on it, but it logically it makes better sense
to me that ET's will do better in windy conditions than FP's, with their
single pane of glass. Tom, I'd sure like to see any data you have that
supports your contention that black chrome FP's will do better than ET's in
windy conditions. I'm not aware that SRCC data take this into account.

There are a lot of issues to consider when deciding what's the best SHW
collector.

-Kelly

Kelly Keilwitz, P.E.
Whidbey Sun & Wind, LLC
Renewable Energy Systems
NABCEP Certified PV Installer
987 Wanamaker Rd, 
Coupeville, WA 98239
PH & FAX 360-678-7131
sunwind at whidbeysunwind.com
 


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