Integrated woodburner, solar tubes, oil combi and underground heatstore.

[KLD]

Noel, List,

If the store fluid is stagnant, and frequently heated up to quite high temperatures, then wouldn't you hope that the dissolved air that was initially present in the fill water has gased out? The drop in pH looks worrying, though, but could more easily be corrected. My memory of chemistry is in a similar state as Noel's "water", but I think just adding NaOH will not help for long. On the contrary, high concentrations of NaOH can be corrosive by themselves. You need a buffering agent, like sodium phosphate. Any chemists around to substantiate this guess? Noel, could you take a sample of the liquid, adjust the pH, and then test corrosivity?

Any idea how to remove the current gunk? A re-fill with fresh water will also introduce fresh oxygen, but is probably easier then trying to filter out that sludge.

Klaus

[noelsquibb]

Thanks everybody,

Clearly we are discussing chemistry and I cheerfully admit mine is about as advanced as my lektronic knowledge.
I do have a pH meter that also measures electrical conductivity and dissolved solids, that I bought to monitor the acid mine water, that is now my liability (-;

Interesting that the browning / acidifying started when the store was left for the summer full of cold water though.
There had been a recent discussion on a navvy thread regarding putting inhibitor into thermal stores and the Finns apparently dont bother, as the damaging oxygen bubbles off somewhere, once the waters been hot for a while ....
Sounded like the right thing to do anyway.

Lightfoots links are interesting but dont quite get to cover metal sat in hot stagnent water and there may be issues with using the wrong resin but I did explain its end use to the suppliers tech support before I ordered. Could low pH attack the polyester resin ?  I will try to track down the people I ordered from.

It sounded like the caustic soda ( would that be NaOH ?) was the way but Klaus advises caution.
Could I monitor / dose to target a pH of 6-7  ?

I thought about inhibitor and or antifreeze but would need lots to dose 1200 litres and WTF do I do with it if Ive got to empty the store  ? I could dumper it over to the mine settling ponds I suppose .....

Strangely ( to me anyway ) the biological process seems to be triggered by the chemistry in stagnant water, perhaps  a little UV lamp might help ?

The best I could come up with from tonights particularly deep bath, was to treat it homeopathically or herbally,

How best to dose though  -

[Greenbeast]

Yes its NaOH

Try biodiesel supply places, i don't know how much you'd need but bought a 25Kg bag for £22

[lightfoot]

It's true that many European systems with large thermal stores/accumulators may not be treated with inhibitor/antifreeze, and also that after the water has been heated a number of times much of the dissolved oxygen will be released and can be vented off - but most of these systems are sealed/pressurised and therefore don't have a F&E tank etc (or in your case just a large open store), which can allow more oxygen to be reintroduced into the system, both when it's topped-up to make up for any evaporation etc and it may also be reabsorbed through the surface of the water in the F&E tank etc.  

Another reason for not using a antifreeze mix, apart from the cost etc, is it has lower specific heat capacity than water and would therefore require a larger store and higher flow-rates to hold and transfer the same amount of energy - also the systems/houses are generally well insulated and any underground pipework is kept below the frost level etc, plus the water in a large well insulated store will take a lot of freezing and pumps and maybe electric immersion or trace heating can be switched on if the system temp was to drop below say 5 degC etc, to help guard against freezing.  

However, your system/conditions/water etc maybe a little different and for the reasons given in my earlier post/links (lots to read and take in - but should cover most bases), I would recommend you consider using a suitable inhibitor and if necessary a antifreeze mix etc.  

[wexford]

Most of your oxygen gassing off will easily be replaced by the convection currents bringing water up to the surface and absorbing oxygen.

Contrary to popular belief the bubbles you see in an aquarium don't impart a whole lot of oxygen to the water, what they do is break the surface tension and agitate the water allowing more surface area in contact with the air. 90% of the gas exchange in an aquarium is at the surface with the remainder being the bubbles, plants and fishes.

However the higher the temperature of the water the less dissolved oxygen water can hold.


Other factors including turbulence are discussed here http://www.omega.com/techref/ph-1.html

Wexford

[KLD]

Wexford,
I agree that the concentration of dissolved oxygen is more or less in equilibrium the atmosphere. The average oxygen concentration will follow the average tank temperature. As long as the tank is kept hot, I can't see how oxygen is being "replaced" beyond the equilibrium?
The situation would be different if you were to introduce fresh water at a different temperature, but that's not the case here.

ON the other hand, if the tank does undergo major temperature fluctuations, then it would be a good idea to slow down the uptake of oxygen during the cooler part of a cycle. Maybe just decreasing the available surface, ie. interface area between water and air, would suffice? A floating piece of PUR or similar?

Klaus

[wexford]

I was thinking of something cheaper and easier.  Dump a little mineral oil into the store, it'll form a gas impervious layer on the top. A cap full of fully synthetic car oil would do the trick I think?

Wexford

EDIT Maybe wait until the store is hot first so as not to trap oxygen in the store?

[noelsquibb]

I don't see how unpressurised heatstores can be sealed from atmosphere as theres bound to be thermal expansion and to ensure no air can get in, you would end up with a pressurised container, unless there was some kind of floating cover that fitted the store pretty well.

The oil slick seems like a neat idea though. Be interesting to see if it could subsume the current surface material.
Presumably any air that wanted to bubble out could get up through the oil layer if the bubble was big enough.

Is there any risk of the oil vaporising when the water its sitting on is around 70c ?

Is the entire sequence of chemistry triggered by oxygen in the water or is the problem more to do with steel and copper joined in series and sat in hot oxygenated water ?

The circulating water in the heat exchangers does have inhibitor added , just don't ask me what %age cos there have been too many  'release of liquid events'  since the inhibitors were added.

With regard to trying to stop water penetrating the GRP layer, I wondered if I could hang sheets of bentonite matting. This stuff is used to form waterproof layers in landfill sites and for creating ponds. Once wet, the bentonite flakes swell up and seal the layer. Not easy to ensure good laps and coverage 2m deep though.

I think in the short term, to get through to a time when we do not need heating,  based on the suggestions here, I will do the following -

Dose with NaOH to neutralise the pH, possibly with a slight bias to alkaline.
I will sample water from the bottom, middle and top of the store ( easy enough with a long pick up pipe on the Subaru fuel pump ) and see if there is a consistency in the chemistry or if there is a contamination gradient.

I will then mix equal amounts from the samples to get a 1 litre blended sample. I will carefully measure how much NaOH is required to raise the pH to 7.0 then multiply this by 1400. This amount will be mixed with warm water taken from the heatstore until it is all dissolved.
The NaOH solution will be injected at varying depths, using a funnel with a suitable length of pipe attached.

4 cans of Fernox inhibitor will be added to the above ( just seemed like a good number )

Try out oil slick air seal.

Don't think the antifreeze will bring much to the party unless its around 20% by volume and that's err ...
62 gallons ......

The water leaking into the insulation layers will be pumped out weekly, the water level topped up in the heatstore and the pH checked.

Any suggestions for sacrificial material to reduce electrolytic transfer from the heat exchangers

Probably an idea to run these additions past the GRP tekkies at http://www.cfsnet.co.uk/ in case any of em will attack the resin.



Then review the situation in spring and consider options -

Rebuild the incoming heat exchanger using only copper ? 
The radiators do a great job of transferring the heat and seem act as excellent stratifying baffles.

Recoat the GRP layer with  bentonite matting ?
More GRP ?
2 Pack chemical resistant paint ?

As I recall, the tekkies I spoke to regarding using a made up butyl rubber liner said the rubber was ok to about 80c but the unavoidable joints would not take the heat.

Get a very expensive stainless steel tank built to fit the hole ?



Get a really big heating oil tank and fill it while I can still afford to ?

[Brandon]

zinc

[dhaslam]

Zinc should take most of the galvanic action away from the  iron.  The action is caused by  a galvanic action between  copper and iron.   The zinc will be completely corroded away in time.   The water should not have high salt content, not sure how you can avoid this when neutralizing an acid.   

[Greenbeast]

please be very very careful when mixing and moving the lye with water, it will burn you and the fumes will attack your throat and eyes

i'm careful enough dissolving 100g at a time into 300ml water, but dosing that amount of water will no doubt require more than that.

i would offer some of my lye but i expect we're too far apart to make it sensible and it requires a hazardous materials courier

[dtl]

KLD,

The oxygen is the water is probably getting used up in whichever process is producing the sludge.

The depleted oxygen is then being resupplied at the fluid surface to replace that which has been taken from the saturated solution.

[dtl]

Try fitting a zinc (as previously recommended) water heater sacrificial anode rod in your store.

[Brandon]

I was thinking more of a ship sized anode

here

[GMEAT]

Hi, First time post from a long term 'lurker'

I was prompted to register when reading about your (fascinating) project because I have been thinking of something along the same lines ( just theory so far!) but using Phase Change Materials.

So my somewhat radical suggestion is to increase the surface area of the heat extraction pipework and replace the water with wax.

I'm fully prepared to be 'shot down in flames' but would be interested to hear the opinions of others!