Green Water, This can't be good

todd
todd Registered Users Posts: 26 ✭✭
in General Solar Power Topics #1
Hey all,

I have 6 flat plate 3x6ft solar collectors directly plumbed into a hot tub and it has worked well for a few years now.
I easily get 100+ tub temps with the suplus heated water dumped through a ground based heat dump.

Last night I did a part water change and refil, the ph was a tad low so I added bicarb of soda to bring the level to normal 7-8 range.

So long story short in the course of an hour the clear water turned a slimy green color and could only see about 4 inches below the surface.
The green equals copper compounds being formed with the collectors and valves are copper and brass based.

Oh and yes I use, however sparingly, the various hot tub chemicals, di-chlor and bromine tablets, both which are known to react with copper.

Questions:

should I add a stainless steel heat exchanger and convert the solar loop to a closed system or
should I rebuild the collectors and replace the copper tubing with stainless?
(reason, the collectors are old, purchased used from the now defunct aaa solar once in NM.)

If I go with a heat exchanger, I'll probably build one as they are quite expensive,
any one ever see plans on the net?

how does one properly size a heat exch. anyway ?

I retro fit the collectors, will stainless cause much of a loss in heat collection and transfer?

thanks
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Green Water, This can't be good

    Some of the rule of thumbs I have seen:

    50% of pool surface area in solar panels

    20 gallons of hot water per person per day. Roughly 0.5 to 1.0 sqft per gallon of hot water per day. (nominally, cold regions need more panel area. Hot areas may want to mount panels near vertical to reduce summer time boiling water.

    Regarding your system--I wonder if the interior of the condenser had various scale formations internal to the piping (mineral and copper salts)--And your PH/Chemical change just "flushed" the plumbing.

    Which is kind of always the problem with copper and water--Chemistry + heat seems to be a big unknown what will happen over time. I guess you can add issues of mixed piping (i.e., copper+steel or copper+iron pump body) and galvanic corrosion. Probably need to use plastic or metal isolation couplers to reduce current flow in the piping system.

    I had a plumbing contractor tell me that using a foot of brass pipe between the copper and iron pipes works too (not sure I believe it--but brass is supposed to pretty neutral relative to others metals).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #3
    Re: Green Water, This can't be good

    I am not sure it is copper that is the major issue. My guess is bacteria or algae bloom due to incorrect PH or Alkalinity. Try a major flush with shock, get the PH and Ank. right, then re-flush the system.

    Tony
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  • todd
    todd Registered Users Posts: 26 ✭✭
    #4
    Re: Green Water, This can't be good
    icarus wrote: »
    I am not sure it is copper that is the major issue. My guess is bacteria or algae bloom due to incorrect PH or Alkalinity. Try a major flush with shock, get the PH and Ank. right, then re-flush the system.

    Tony

    This is definately a copper compond being formed due to the chemicals used to maintain water chemistry and the reaction with the copper tubing used in the solar collectors.
    I did a second partial water change and this time checked & cleaned the filters. They were quite filled with the copper gunk. After about 10 minutes the remaining green was gone.

    the hot tub uses all plastic PVC plumbing and the solar collectors are connected using hi temp radiator hose so no metal to metal electrolysis issues.

    back to the original questions rebuild the panels w/ stainless or add a heat ex.?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: Green Water, This can't be good

    It is a tough call... Copper is "easy" to bond with metal plates used to increase surface area/increase heat transfer/efficiency. Stainless is difficult to get a thermal bond with so efficiency falls/array size increases.

    A heat exchanger does allow you the optimum water chemistries for each use (tub/pool vs thermal panel loop). Obviously, you have added expenses--need a 2nd pump+controller. Some sort of pressure tank for the heat exchanger loop. Loss of efficiency due to the second heat exchange interface. And you may choose to add antifreeze or drain bank to the panel loop.

    If the current system is unusable because of the problem--then any solution will probably be better.

    Your choice may be between a plastic solar thermal collector (may not be efficient enough for hot tub), or go with the heat exchanger and see what happens (you may be able to keep the current thermal panels and even add a couple if needed).

    This may be the type project where a good installer or equipment retailer can give you more accurate information/choices.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • CDN_VT
    CDN_VT Solar Expert Posts: 492 ✭✭✭
    #6
    Re: Green Water, This can't be good

    Oh and yes I use, however sparingly, the various hot tub chemicals, di-chlor and bromine tablets, both which are known to react with copper.
    ""di-chlor and bromine""

    So the Chemist says these mixing's are unstable , but not a cancelling effect.

    Acid / Soda = PH %

    That with "direct" oxygenated water (The worst type!!!) with heat can eat 316-408 Stainless, you have a terminable chemical effect , then there's algae growth ..

    As with anything , Change will happen slowly and over time to correct.
    Start fresh , and read the tables of PH , (pools out of use ) Quit eating skin!

    VT
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  • todd
    todd Registered Users Posts: 26 ✭✭
    #7
    Re: Green Water, This can't be good

    Between the di-chlor, bromine and lack of sunlight algae in not an issue.
    This is a covered hot tub not a pool.
    Tap water at my location is typically around 7.4 ph, "hot tub people" state 7.8 to 8.4 is ideal.
    So I usually add about 1/2 cup of bicarb to treat 400 gallons. 1/4 cup dichlor twice a month and the bromine tables are added as needed, using 1 or 2 a month.

    I thought stainless steel was more chemical resistant than pure copper tubing, I'll have to look into this, as it's a pretty spendy project to replace the copper in the collectors. And if so then sort of eliminates the idea of adding a heat ex.
    Maybe I might want to wait until the colectors fail and replace with evac tube type?
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  • fix_it_guy
    fix_it_guy Solar Expert Posts: 26 ✭
    #8
    Re: Green Water, This can't be good
    BB. wrote: »
    I had a plumbing contractor tell me that using a foot of brass pipe between the copper and iron pipes works too (not sure I believe it--but brass is supposed to pretty neutral relative to others metals).
    -Bill

    I can confirm that for you. I was an irrigation tech at a golf course that had an ancient irrigation system. The control valves were brass and had various stainless steel and aluminum springs, valves inside them. They had galvanized bolts holding them together and were threaded onto galvanized pipe. The brass doesn't react with these other materials, that is why they use it. I'd imagine it plays nice with copper also.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #9
    Re: Green Water, This can't be good

    I would suggest avoiding the Evacuated Tube type.

    They are great if you need HOT water (something like 160F+). But for pool/tub temperatures, the less costly panels are just fine. For pool temperatures (80F) the simple/cheap plastic mats are as efficient or even a bit more efficient (as I recall).

    Also, Evac Tube are easy to shatter and can have problems with overheating/boiling/mineral build up too (at the "hot spot").

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: Green Water, This can't be good

    Same process that makes batteries work, guys; electrolysis.
    Want to see a bad reaction? Connect copper to galvanized steel pipe and run water through it.

    Brass is made of copper & tin; ends up not having the electrical differential that copper has so won't react to the steel - not "dissimilar" metals.

    Putting plastic in as a "break" also works.

    The OP's problem is one of chemical erosion of the copper. The chlorine and bromine will eat through just about anything eventually. Keeping the pH around 7 is your best bet for longevity of pipes. Good luck doing that.
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  • todd
    todd Registered Users Posts: 26 ✭✭
    #11
    Re: Green Water, This can't be good

    Oh yeah, I forgot to mention there is brass valves and connectons in the heat dump, the solar circuit pump has stainless & brass materials, but no iron anywhere.
    and aside from this all the plumbing is a mix of pex, pvc & cpvc tubing, rubber hoses, clamps, chewing gums, a few 6 packs etc.

    I found a few stories on the net where roofs had collapsed and killed people in pool houses. The wrong grade of stainless was used as structural components and failed due to high levels of chlorine. My take is material selection will be quite important.
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  • CDN_VT
    CDN_VT Solar Expert Posts: 492 ✭✭✭
    #12
    Re: Green Water, This can't be good
    Cariboocoot wrote: »
    Same process that makes batteries work, guys; electrolysis.
    Want to see a bad reaction? Connect copper to galvanized steel pipe and run water through it.

    Brass is made of copper & tin;
    Brass IS Copper & Zinc
    Bronze IS Copper & TIN
    All vessels (boats) that have brass thru-hulls ,will not be insured if a claim is made 4 a flounder. Bronze fittings below water line.
    Galvanic corrosion tables , and if you use "stainless" there are many flavours 304-418 in plumbing.
    VT
    I run Evacuated Tube type collectors , hog-wash !!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: Green Water, This can't be good

    Right; brass & bronze other way 'round.

    Bud Abbott's shortstop. :p The effect is the same.
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