Solar and Hydronics

Kevin_in_Denver

My background is nearly identical to Siggy's except I bailed out of solar and HVAC in 1984 and didn't look back much until recently.

I see one aspect of this system that could be a problem and could explain why the tank blew its gasket.

The use of a pressure rated tank and closed system is desirable for several reasons, but the tank is very expensive. Expense isn't the issue (maybe a used propane tank could save money)

During a sunny, warm summer day, solar can heat the tank up to the controller's high limit (typically 180F). During periods of low usage, the collector pumps would shut off on high limit and the collectors will drain (this is good, as collector stagnation is the most reliable way to shed excess heat).

Then if someone uses a bunch of DHW, the collector pumps kick back on, sending 170F water up to a bank of collectors that have been stagnating. In Colorado, a flat plate collector can stagnate at 215F and higher. The water flashes to steam and pushes the tank pressure way above the the PRV setpoint, and it burps some water and the whole system rattles like mad (not good).

John, did you ever witness this condition? The insolation rates in northern NY may be low enough to prevent it, but it did happen to us in CO.

Maybe the newer controls could prevent it by never turning the collector pumps on if the panels are at 195F or above, but they didn't used to offer that feature.

Here's a reason to try to keep the whole system at 15psi pressure: An unexpected phenomenon we saw when you operate the closed system at atmospheric pressure. As the water flows up the collector array and starts down the return pipe, it creates a siphoning effect. At the top of any siphon is a partial vacuum, that's how it pulls water up and over. Now the pipes rattle 10am-2pm due to low temperature boiling/water hammer at the top of the collectors.

hr

This question comes up a lot

here is a great example with a proven track record. Thanks, as always, to Siggy for the concept, explanation and drawings, and to PM magazine for sharing

hot rod

http://www.pmmag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2379,130447,00.html

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siggy

solar system pressure

Kevin,

No, I've not had this situation occur to the best of my knowledge. The tank does have a 15 psi PRV at the bottom connection. To my knowledge it has never opened.

The turn on differential is around 5 to 8 F and seems to keep the collectors from stagnating unless something is broken.

BTW, the PM Hydronics Workshop column for September will be a continuation of the solar / hydronics discussion.

Stay tuned.

Siggy

hr

Solar info at Mark Eathertons

monthly column in Contractors mag also.

http://www.contractormag.com/articles/column.cfm?columnid=323

hot rod

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Kevin_in_Denver

More solar system design discussion

Since this is one of my favorite topics, bear with me as I try to keep the ball rolling. Solar could soon be the fastest growing segment of the hydronics industry, so heck, it may even be worthwhile!

Mark's column discusses antifreeze systems. Summer stagnation should have effectively killed this design long ago.

Bray oil which had a very high boiling point was perhaps the best attempt to address the problem of "bulemic" systems. It has its own bunch of problems, but I guess it still is used.

If you tried a glycol/water mixture, summer stagnation would eventually boil every drop of liquid in the collectors after storage high limit was reached. The expansion capacity required to absorb this much expansion is much larger than normal boiler design calculations would predict, but that's not the problem.

During these conditions, as soon as some of the vapor tries to migrate out of the collector array, it cools below the the boiling point. Then this pocket of vapor collapses on itself, causing those water hammer noises. These are very loud and very annoying, and not good for any of the system components.

Does anyone know of a glycol system design/control strategy that sucessfully addresses this?

hr

I've used only glycoled

systems in all my own solar installs. But mine are pressurized to 12psi. I have had only meltdown when I foolishly used pex to connect directly to the panel. Pump quit on a sunny day and the pex burst.

Other than that I have a 15 year track record with glycoled solar systems.

I am intrigued with the drain back system simplicity however. Dave from Metro Solar, Denver has a design with a locally fabed stainless tank that's sounds nice.

It was nice to see a large solar display at this years Homebuilders show in Vegas. This solar module caught my eye www.solargenix.com

If you are shopping for some panels, here is a good deal (shameless plug)

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=41981&item=5913652368&rd=1

Delivered by yours truly

hot rod

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David Woycio

solar

The trick to closed loop glycol system is to size the pump, pipe, and expansion tank correctly. For example and 4 x 8 flat plate collector needs .75 gpm of flow. Add up the collectors, factor in the pipe length (feed and return) and pressure drop of heat exchanger for your final gpm. Use a pump that will give you this and then some. Running these systems too slow causes all kinds of problems. We usually use a butterfly valve or ball valve the adjust flow at the return. The expansion tank needs to be able to handle 250 deg. fluid temp plus the volume of the system. The old Extrols were great because they had a EPDM bladder and lasted forever.


Anybody know where you can get EPDM #15, #30, and #60's?

Most times the knocking is from air in the loop. The ONLY thing I like about glycol systems is that they're quiet. To get all that air out of the sysem we usually use an auto air vent. Never found an auto air vent that wouldn't leak. If it leaks, you lose pressure. We'll ALWAYS valve them out of the loop. Just let the system run for a couple of weeks and then isolate.

A good glycol like DOW-Frost inhibited should be able to handle the temps from stagnation.

Kevin, where are you in Denver?

hr

Now if high temperature fluids

are in order.. The synthetics used at the Kramer Junction parabolic site run up to 735°F. Goes to a HX to produce superheated steam to power turbines generating something like 165 megawatts!

hot rod

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Kevin_in_Denver

synthetics

Sounds like pretty hot stuff. Oils tend to have higher pump and parasitic pumping costs, much reduced collector efficiency, leak where water won't, and sometimes incompatible with ordinary plumbing parts.

Anybody using them in residential?

Kevin_in_Denver

glycol systems

Did you ever run into the problem that Mark Eatherton described? That is, a space heating system burping in the summer with no load?

Kevin_in_Denver

Solar in Denver

Still in Southeast Denver. I was the design/development engineer at Mountain Mechanical Sales, Solar Shelter, then Solaron.

My passive solar home is nearly finished at 1491 So. Gaylord. ME actually stapled down my radiant tubing.

Larry Weingarten

passive solar

Hello Kevin: From reading your posts, it's clear you have a lot of real world solar experience. (I've been involved since 1978) I'd be really interested to know some of the design ideas in your house... You probably know the solar home tour is coming up. Might be a good time to show others too!

Kevin_in_Denver

Passive

Is a new thread, so I'll start it.

David Woycio

solar

Kevin,
If youre looking for some used solar thermal collectors for your house get a hold of me. We have close to 100 flat plates at the shop that are in very good shape. Our new shop address is 4060 S. Clay St. near Englewood golf course.

I have been installing and designing solar heating systems since 1982 in Colorado. We work on a lot of glycol systems and the "burping" in these systems is usually a sign of not enough room for expansion. We usually install the expansion tank on the return for this very reason. I know that it's oposite of the "pumping away" method but I like watching the pressure guage move when the pump activates. It's a good way to tell if you have circulation. A mistake some techs make is not pressurizing the expansion tank to match the fluid charge pressure. We charge our closed loop systems to 32psi @ 70 deg. F.

I also feel that solar thermal heating sytems are making a come back. With the radiant heating system technology available today coupled with what we have learned over the years in the solar thermal industry we can install a system that will perform for years without any major service and save the customer 50 - 75% on his heating bill.

Good luck with house project.

Joe_32

Solar stagnation.

Yes Tilt the collectors up if system only used for space heating to vertical or less ie 70 degress from horizontal if dhw is used. Temps stay low in summer as input is reduced. The government has rating for input on flat plate collectors all over the country set at different angles from horizontal to vertical. I have seen banks of ten 4x10 collectors in this configuration. For three years now with no over heat problems.

Joe

Carl PE_2

Hey Metro Man!

> Kevin, If youre looking for some used solar

> thermal collectors for your house get a hold of

> me. We have close to 100 flat plates at the shop

> that are in very good shape. Our new shop

> address is 4060 S. Clay St. near Englewood golf

> course.

>

> I have been installing and designing

> solar heating systems since 1982 in Colorado. We

> work on a lot of glycol systems and the "burping"

> in these systems is usually a sign of not enough

> room for expansion. We usually install the

> expansion tank on the return for this very

> reason. I know that it's oposite of the "pumping

> away" method but I like watching the pressure

> guage move when the pump activates. It's a good

> way to tell if you have circulation. A mistake

> some techs make is not pressurizing the expansion

> tank to match the fluid charge pressure. We

> charge our closed loop systems to 32psi @ 70 deg.

> F.

>

> I also feel that solar thermal heating

> sytems are making a come back. With the radiant

> heating system technology available today coupled

> with what we have learned over the years in the

> solar thermal industry we can install a system

> that will perform for years without any major

> service and save the customer 50 - 75% on his

> heating bill.

>

> Good luck with house project.

Carl PE_2

Hey Metro Man!

I might be interested in a few of those collectors. Drop me an email (anonymous at warpnet dot net) with mfr/part#/$ and we'll talk.

Thanks!

Carl

Mark Eatherton1

Oohh Oohh, I do (hand waving wildy in back of class)

Barring power failures, this method has proven bullet proof over the years. The problem with typical closed loop solar systems is that the system expansion tank is not designed to handle the temperature differential associated with stagnation conditions. Even if it were capable of handling the fluidic expansion, the glycol is incapable of handling stagnation temperatures at the absorber. At temps above 250 degrees, glycol pyrolizes and breaks down into an organic glycol based acid, known as glycolic acid. It can do the same thing at lower temps, but is accelerated at elevated temps.

The solution, is to install a snap disc temperature sensor on the solar return loop going back to the storage tank/heat exchanger. It is a normally closed configuration. You wire it in series with one lead going to the STORAGE input to the differential controller. If the sensor senses temps above 180 degrees F, it opens the storage sensor, which sends a signal to the controller telling it the storage tank is ice cold. This keeps the solar fluid moving. The solar storage tank will peak out below boiling (in most cases) and the pump keeps the fluids moving, keeping them from boiling off or flashing to steam. The system runs into the night, giving up heat from the collectors back to the sunless sky, thereby cooling the storage tank down to around 175 deg F, and allowing the cycle to begin again the next day. This obviously only applies to properly sized solar systems where the occupants have gone on vacation. If a power failure occurs during peak insolation, the loop WILL dump.

It is EXTREMELY important that the solar pre heat system for the DHW portion of the system be equiped with an anti scald tempering valve to avoid a scalding situation. This tempering device MUST be placed between the solar pre heat and the auxilliary, or you WILL blow the ECO on the auxilliary if someone draws hot water during peak temperatures. If not so equipped, DO NOT attempt this quick fix solution.

Even with this "system", it is a requirement that fluid conditions be maintained annually. If you ignore them, the system WILL go away...

A Graingers part number 2E366 will fill the bill quite nicely...

ME



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Kevin_in_Denver

Murphy's Law

Well, I'm sure that I'd eventually have that power failure, or my pump would fail, or the collector loop would airlock, so eventually the glycol would burp and/or turn acid.

Joe's method would be helpful, but those hot, late September days would still have way too much insolation to entirely prevent the possibility of burping.

Metro Man, I believe, at 250F and 32 PSI is trying to keep the collector fluid from burping out by brute force. What is the PRV setpoint?