Heating Help

Drain back with evacuated tubes

I thought I read some time back here on the wall someone was using evacuated tubes for Drain back. Anyone?
I know the cons of doing that?
What positives?
all comments
Thanks
Michael

e-tubes w/ drainback

First off, only  "heat pipe" style e-tubes can be used.

One issue but not a deal-killer:

1.  Sunny day power failure.  When the power comes back on, the dry stagnating panels will get hit with water that will flash to steam.  Some water hammer will be heard, but SunMaxx, Apricus, and Sunda survive this event without problems.  The header is brazed copper and very robust, the thermal shock of going from 420F to 70F in seconds is OK.  The compression fittings at each end also  survive.  Customers may be concerned about the noise, so some controllers can be set up to prevent pump activation whenever the collector sensor is above 180F.  That setting will throw away  heat under certain conditions.

The manufacturers are mostly silent on this application, so I'm doing some of my own testing.  I've talked to an e-tube installer in NM who said "no reason to ever do anything but drainback" and he has a dozen or so systems installed.  It's worth looking at because it's cheaper, more efficient, and solves overheating issues without more equipment.

As you may know, most of the e-tube issues from 10+ years ago have gone away.  Reliability, hail resistance, and cost are now acceptable.  Snow is still an issue, with a different answer for different applications and local weather.

Here's an older thread:
http://www.heatinghelp.com/forum-thread/107359/Solar-panels

consider header geometry

If you are going to try to do drainback with a heat pipe type evacuated tube collector (Apricus, Sunmaxx, thermomax, etc), consider carefully the local geometry of the header and how well it will drain. Unlike the smooth headers in a harp style flat plate collector, most the ET headers have local high and low spots that don't necessarily drain as you might hope.

We have had some experience with collectors installed in a drain back where the conventional 1/4 inch per ft or more manifold slope was not sufficient to prevent local pooling of water in the header and subsequent system failure due to freeze. Antifreezing the system would have solved the issue, but this was a direct system with several hundred gallons of storage so the cost would be prohibitive even to get to 20%.

Before you try it, get your hands on a manifold, fill it with water, tilt it and watch it drain.

Good luck,

~Fortunat

www.revisionenergy.com

other e-tube drainback advantages

Good points, Fortunat.  The Viessmann/Thermomax header won't work.
The others I mentioned are a 3/4" (22mm) tube with 1/2" vertical internal cross-pipes that block some water from leaving, but not enough to cause a problem.

If you only have one panel, you probably don't need to tilt it, because it's draining from both ends.  That keeps things looking better.

Another advantage is in the typical attic piping.  A 4x10 flat plate collector can be difficult to plumb because the lower header winds up way out by the eave.  An e-tube would have both pipe penetrations much closer to the peak of the roof, making access much easier.  It also easier to achieve the proper slope on the supply and return from that point over to the pipe chase.

Drain back with evacuated tubes

Kevin
How about the stagnation heat on the insulation in the collector header?

The other problem I have heard is when there is stagnation the tubes tend to lose their vacuum?

These are things I heard.
Thanks for the tips. Does anyone on the wall actually have one installed on their home?

Michael

header insulation

The SRCC test includes dry stagnation, and this will weed out problems.  At COSEIA last week, Tim Delaney showed us a cutaway of the Sunda header.    The header pipe is supported by a ceramic spacer, then glass wool, then foam.  That collector had overheated, and some of the foam was brownish, but only where the ceramic spacer directly touched the foam.  So yes, there could be trouble in a design where the copper header touches foam.

That reminds me of an instance in 1982 where a flat plate collector actually caught fire.

Prior to the Apricus design, which is a double glass wall, most e-tubes had a glass to metal seal.  Daily dry stagnation over a long period would cause that seal to fail due to slight differences in thermal expansion.  Sunda thinks they've solved this with a more robust seal that stays much cooler.  Apricus and Sunmaxx don't have any glass bonded to metal.  Sunda is sticking with their design because a single glass wall lets 6-10% more energy inside the tube than a double wall.  The double wall tube is also much cheaper to replace after glass breakage.

Kevin

That's great info.
Much thanks

Correction: Apricus Header Draining

Upon further inspection, the Apricus header needs to be sloped in order to drain properly.   Here's a drawing of it, it isn't just a straight tube with cross pipes like Sunda and SunMaxx.
http://www.apricus.com/html/solar_collector.htm

More e-tube drainback testing

I have one Apricus AP-22 collector that I'm putting through its paces.
After a half hour of dry stagnation under full sun, when I turn on the pump I don't get any boiling or water hammer with 100F inlet water. This is a pressurized drainback system at 60 psi, but I was surprised. I'll try it at hotter inlet temps and with 2 collectors.

Apricus in drainback

Kevin,

That cutaway is a great illustration of what I'm talking about. Our experience with these collectors in drainback here in the northeast is not good.

It takes a lot of collector pitch to ensure that the last little pocket of water drains out of the manifold sufficiently. 1/4 in per ft won't do it (i have a couple of destroyed manifolds to prove it).

Apricus still carries drainback as an option in their literature, so it must be doable, and it sounds like you are doing a bunch of research so I'm sure you'll have it covered...

by the way, from another thread...what 'cracked' when you shocked the system thermally? a heat pipe? or part of the header?

~Fortunat
www.revisionenergy.com

Apricus Thermal Shock

Test Conditions:
Full sun.

Solar Panel is an Apricus evacuated tube AP-22

The near-panel piping is corrugated Stainless Steel, the
rest of the solar supply & return is PEX.  (We are also testing the limits of  PEX in a pressurized drainback collector loop.)

After 2 hrs of stagnation, the copper header measured 375F,
but the actual temperature of the absorber could be much higher.

Bottom of tank temperature was 80F

The pump was turned on to test for system boiling and thermal
shock.  Once the pumping started after
about a minute, the boiling sounds were minor.

The maximum return temperature measured was 160F.

There was no spike in the collector loop pressure.

Flowrate: 1.5 gpm

After the test, I noticed that tube #3 was shattered just
below the midpoint.

Even though the cold collector fluid doesn’t circulate
directly through the tube, the heat pipe can conduct so much heat away from the
absorber that is causes localized cooling that stresses the glass of the
evacuated tube, which may break.  The Apricus design apparently puts the heat pipe too close to the glass absorber.   SunMaxx had an older design
like that , but their current design has the heat pipe on axis.   Stay tuned.

stagnation temperatures

here is a formula Siggy worked up to show potential stagnation temperature in a typical flat plat collector. Plug in the Y-intercept and slope from the collector you are using to get an idea of what you may be dealing with.

I have heard of tube cracking in the sunny, hot Arizona conditions also. Lots of temperature swing at work there.

hr

Stagnation temperatures

Rod,

That equation seems to make sense. The other place to get stagnation temperature is either from the collector spec sheet or from a testing lab. SPF (the european equivalent of SRCC) tests stagnation temperature as part of their test protocol and lists it on the test report.

Their database is available online just like SRCC and I tend to find the SPF numbers more accurate in lots of applications.

A typical test report is attached in case anyone wants to see one and is too lazy to search for their website. Comparing the published tested stagnation temperature to the theoretical temperature derived by Sigenthaller is left as an exercise for the reader...

Great solar day today....gotta get out of the office to go enjoy it!

~Fortunat
www.revisionenergy.com

nice data

I notice the formula they use produces a curved line. I like that they show Gross and Apertue area outputs. That's where the evac tube number can get skewed.

More Stagnation Testing AP-22

I'm using this thread as my lab notebook.

After a full day of dry stagnation, the collector sensor reads 385F. (70F ambient). For ease of attachment, I use a bronze 3/4" Ford-style compression fitting with a silicone seal. It works well on the 22mm Apricus header. The Ford fitting surface temp maxed out at 200F.

From there, I use a Falcon corrugated stainless water heater connector with a silicone O-ring. The thermal conductivity of the SS is poor enough to drop the pipe surface temperature to ambient in just 3 inches from the threads of the Ford fitting.

http://www.falconstainless.com/Benefits_Features.html#pagetop

These look just like the stainless piping that many manufacturers are pushing for retrofit installations. The trouble is, that pre-insulated stuff runs $20/ft., and I think flexible collector loop lines are tough to keep straight enough to ensure constant slope and drainback.

Pex forced to fail

As I mentioned earlier, I'm also testing PEX in the pressurized drainback loop.

I was able to get the PEX to fail by closing a valve on the collector loop return line, and turning the pump on. The AP-22 generated enough steam to blow the supply line. The return line did not blow because it has a 36" Falcon fitting and 36" of galvanized pipe before joining to PEX. The drainback collector loop was also pre-pressurized with compressed air to 100psi.

The steam was going either way, and found the supply PEX first.

In general, you should never have a valve on the return line of a drainback system. All other possible failure modes that I tried didn't cause any PEX failures.