Please Help ! Looking for a Horizontal Stainless Steel Solar Tank about 80 Gallon

[juakali10]

Hi folks - a newbie here so I apologize if asking a dumb question but I am stuck !?!
I have two flat plate solar collectors that I want to connect to a stainless steel tank, the system runs off mains pressure. I am based in the Turks & Caicos Islands so its a direct system but the water is RO and poor quality, my last mild steel tank (enamel coated) but rotted through in 6 years so now looking for a stainless steel tank 316 stainless (with about 2 inch insulation). Importantly the tank must sit horizontally on my roof, above the panels. I can find exactly what I want in Australia http://www.ecosmart.com.au/products/roof-mounted but was hoping to find an American version. Does anyone know if this is possible ? Or, if anyone has any advice I am all ears to listen and learn. Thanks very much ! Mark

[J.P.M.]

Are you considering replacing only the tank or the entire system ? The link you included is for an entire system.

If you haven't done so alrady, I'd make sure the problem was a leak in a tank, and not someother problem (BTW, most often, tanks leak at a seam).

If I was considering replacing just the tank, and depending on the PITA factor(s) associated with a tank changeout, and after comparing the relative costs of a st. stl. tank vs. a more std. c, stl. tank, I'd consider investigating the idea of staying w/ a more std. (c. stl.) material tank and planning on a changeout at some regular interval, say, 5-6 yrs.

As for an American sourced vs. Australian or other origin tank, as long as there is a way to verify that the tank was designed, mfg'd., Q.C.'d and tested to a recognized code, the safety and quality of pressure vessels such as tanks are pretty much the same. Just get a tank that comes with a birth certificate (also known as a code stamp).

Q: Has the tank actually failed (sprung a leak ) ? Or, has the system simply stopped working or just overheating and so lifting the Pressure/Temp. relief valve from low/blocked flow due to fouling/crud buildup in the system ?

Thermosiphon type systems such as you seem to have work well, but the flowrates are usually low relative to pumped systems. Depending on the application, local conditions including H2O quality, and other details, such low flowrates can cause things to crud up more quickly.

All that said, SDHW service for batch heaters is not considered a severe or particularly critical application.

If the tank has indeed failed from fouling/corrosion of some sort and failed, while you're at it and before getting a new tank, it may be worth checking out the condition of the rest of the system components such as valves, elbows, the flat plate collectors other system components or places where dissimilar metals are joined for other possible and similar problems.

Just a note: 304 st. stl. has most all of the same corrosion resistant characteristics as 316, particularly for less severe duty such as solar DW heating, but may be slightly less expensive.

Also, and although probably not a consideration for this application, where using st. stl. might be considered by some as analogous to killing flies with a howitzer, note that st. stl. is not corrosion resistant to everything.

[juakali10]

JPM - thanks a lot for your ideas, I appreciate your time.
I will be changing the whole system, the previous tank was from Turkey and not the best quality, it corroded (several leaks on the metal pipes I connect to) for hot water out. Plus the crud in the pipes has also cause 50+ pin hole leaks in the solar panels so now my panels leak about 5 gallon a day into the gutter. The system is on its last legs but still gives me enough hot water to survive but we have no electric back up and the wife is complaining about luke warm showers !?!
A 5 to 6 year change out seems quite often to me, I was hope to find a 10+ year solution but maybe I am being naive ?
If I could find that Australian solution in the States or even who makes it in China then I would buy it in a second. Or if anyone has any other ideas I would love to learn. I just saw a video on sopal tankless system - not sure if anyone used them in a warm climate ?
Cheers,
Mark

[J.P.M.]

JPM - thanks a lot for your ideas, I appreciate your time.
I will be changing the whole system, the previous tank was from Turkey and not the best quality, it corroded (several leaks on the metal pipes I connect to) for hot water out. Plus the crud in the pipes has also cause 50+ pin hole leaks in the solar panels so now my panels leak about 5 gallon a day into the gutter. The system is on its last legs but still gives me enough hot water to survive but we have no electric back up and the wife is complaining about luke warm showers !?!
A 5 to 6 year change out seems quite often to me, I was hope to find a 10+ year solution but maybe I am being naive ?
If I could find that Australian solution in the States or even who makes it in China then I would buy it in a second. Or if anyone has any other ideas I would love to learn. I just saw a video on sopal tankless system - not sure if anyone used them in a warm climate ?
Cheers,
Mark

You're most welcome.

If you are in a warm climate, that is to say, one where freezing WILL NOT occur, a batch heater or a thermosiphon heater of the type you seem to have or have in mind is a good choice, and about the only choice if power for pumping is not available. Anyway, if local conditions or water quality causes problems such as corrosion, it will happen with any type system.

I might suggest a look at "builditsolar.com". Lots of ideas for DIY solar water heaters and a good primer on the subject. I've built a few solar water heaters in the past including a "breadbox" heater that used a recycled domestic water tank. Low tech. Just a few principles to keep in mind. Relatively easy to make from used/surplus material. Cheap. Appropriate technology (a term that seems to have become an anachronism).

Good luck.

[LucMan]

Sounds like you have extremely corrosive water, or possibly salt content. Replacing the tank with stainless is not going to solve all your issues especially if salt is the main corrosive, 304 or 303 stainless are preferable for salt, but that still leaves mostly copper collectors and headers. There may be some evacuated tube solar systems available with SS headers, and the corrugated SS tubing is available for connecting the header to the tank. Check Alibaba.
Good luck in your search.

[Ampster]

If I understood the OP correctly he also mentioned RO (reverse osmosis) and that can leave the water slightly acidic.

[J.P.M.]

If I understood the OP correctly he also mentioned RO (reverse osmosis) and that can leave the water slightly acidic.

If the OP's statement is correct and if the reverse osmosis system is functioning nominally, and the inlet water has (among other things) high alkalinity, the outlet of an R.O. water system can be somewhat acidic, particularly if the source H2O also has dissolved gases, particularly a lot of CO2.

The PH levels usually observed in potable H2O from properly functioning R.O. systems under such conditions are not usually a problem for most materials used in potable water systems, at least not for the ones I've designed.

I suspect the corrosion problems the OP reports have more to do with other conditions such as depositional and chemical precipitation fouling and probably galvanic corrosion from dissimilar metals.

Water PH in/of itself - either high or low - may be contributing a bit to the corrosion which usually is caused by a combination of factors. Hard to tell w/out more information.

I'd get the R.O. system checked out.

[LucMan]

I have never seen an RO system used in any other than a point of use location IE a kitchen spigot for drinking water. That doesn't mean that they don't exist for residential applications, but to treat a whole house would be cost prohibitive. They require constant maintenance, waste a tremendous amount of water (4 to 1 ratio) and require the re-addition of minerals to make the water palatable. The water needs to be tested to determine exactly what is causing the problem. RO is commonly used when salt is present.

[Ampster]

I have never seen an RO system used in any other than a point of use location .........

RO is commonly used when salt is present.

I saw a whole house RO system at a friend's house in Austin Texas. He was on a well. It was an expensive home and it was a first for me although I did supervise the installation of one for a medical research facility. The OP hasn't clarified yet if that is what he meant when he used the term RO, but he was located on an island in the Caribean.

[juakali10]

Hi there, sorry for the delay, I was flat out at work. . .
You guys are correct Reverse Osmosis is how we get our water. That is how the mains water is treated for the whole island ! As you say its an expensive solution, burning diesel, to generate the electricity to the the RO plant which is probably why mains water here is 4 cents a gallon !?! This is the water company on the island https://www.provowater.com/my-water-supply/
The water is safe enough to drink but everyone does complain at how quickly things corrode (an electric water tank does between 2 and 4 years) never more before they leak.
That is why i figured 316 stainless was the way to go (I thought that was marine grade) so would be the best for our environment ? But if 304 is better I do see some tanks made from that.
What i am after I though was quite simply but obviously not as I can not find one very easily. I just want a direct feel tank, ideally made of stainless but prepared to go (steel with enamel) that will sit horizontal above two 4' by 10' flat plate solar collectors.
Unless you guys can put me right, I figured I can not go evacuated tubes as they are too fragile come hurricane season (but maybe they are stronger that I am guessing as I have never actually seen one !?)
As I am sure you can tell I don't really know what I am up to, but I do know that I pay over 40 cents per kWh and we have over 300 sun days a year in the Turks & Caicos so if solar isn't viable here it isn't viable anywhere !?! My last system was called ezinc from Tallahassee, Florida it was from Turkey I think and not very good quality but quite cheap 1200 complete kit and worked well for 6 years. But I think they have gone out of business, probably because of quality control issues.
Like I say... all advise welcome, but I really don't want to build my own. If I could buy a complete unit (actually I need two) so two tanks and four collectors for about $3000.00 first cost in America (or China) then I would be happy ! ! ! Thanks guys. Mark

[Ampster]

Nobody has talked about sacrificial anodes to extend the life of an enamel tank. I am more familiar with their use in the marine environment but there has to be a tradeoff in cost unless the issue is clearly not galvanic corrosion.

[LucMan]

First step is to get your water tested to find out exactly what you are dealing with.
You could always go with an inexpensive standard closed system with a Titanium heat exchanger for your main water. Your power consumption would go up because of the pumps used but at least you wouldn't have to replace the system every six years.

[J.P.M.]

Nobody has talked about sacrificial anodes to extend the life of an enamel tank. I am more familiar with their use in the marine environment but there has to be a tradeoff in cost unless the issue is clearly not galvanic corrosion.

While that's correct and a good point for the tank corrosion, if they're still there at all, they appear to not be doing their job.
If the tank is enamel lined, the tank designers may have figured that would make anodes superfluous and not used them.

Also, mostly because the electrical currents generated by galvanic cells are pretty small, they don't travel far. So, while anodes in tanks are usually useful if water can get to the galvanic cell sites, anodes in tanks primarily protect tanks and not other components farther away, and in any case only do so as long as the anode material lasts. They work reasonably well for c. stl. tanks, less so or maybe not at all to protect dissimilar metals in contact via soldering or joining or other components farther removed from the tank. Anodes also need inspection, particularly in the duty/environments as described by the OP. Sounds like maybe a couple/3 yrs. might be all that might be expected in this service. A blinding flash of the obvious: If they are sacrificed as is their function, and not replaced, anodes cannot do their job.

To my experience and training, the presence of corrosion is always assumed/expected to be present. It may/probably/usually will have more than one cause and may manifest in different ways in different parts of a system. Different causes of corrosion may act together or against one another.

Galvanic corrosion is but one form of many forms of corrosion, albeit a common one. Sacrificial anodes, while a good and useful tool, are not the only tool. Galvanic corrosion is best/first handled at the system design stage with material selection with reference to the electromotive series in mind, proper system grounding, particularly for motors on pump,considerations for bridging around flanged/gasketed joints and other measures. there's also the idea of inducing small electrical currents to counter act the effects of galvanic corrosion but such measures are not common and pretty tricky/expensive to pull off. Most of that stuff is usually unknown to homeowners or solar installers and probably unnecessary for residential solar thermal systems.

This application however, may warrant some further investigation. Still, local (tank) anodes are a good adjunct/backup, but not the only line of defense, or even the first line of defense in the overall battle against corrosion.

Given what the OP describes with respect other corrosion in other parts of the system, I'd think there's other corrosion factors at work along with galvanic corrosion. Some other causes of corrosion might be, among many such causes, stress corrosion cracking, hydrogen embrittlement and corrosion/deposition/precipitation fouling, the last cause being present primarily in the collectors and the system piping.

One related but more obscure observation: As I'm thinking about this, if the tank does have provision for anodes in it, and they are still there, and they are magnesium, as seems more common for DHW tanks, there's a small but non zero chance that being magnesium might be part of, or a contributor to, the corrosion problem or it's aggressiveness with respect to time. While probably not the largest contributor, the magnesium (as opposed to something less anodic like zinc, for example) may increase the probability or rate of stress corrosion cracking and that may be adding to the corrosion rate.

Reason why magnesium might accelerate corrosion rates: If the H2O has high alkalinity as the OP reports, and if the R.O. treatment does make the water slightly acidic (that is, cause a higher hydrogen ion concentration) as can happen with R.O. systems in such alkaline service, the higher negative potential of the magnesium anodes - if anodes are/still present - may add to the hydrogen ion concentration and lead to an increasing time rate of something called hydrogen embrittlement in system components which can manifest as cracks/failure in materials, primarily of the (steel) tank. Copper is also susceptible but usually less so unless a lot of chlorine and/or ammonia is present. Hydrogen embrittlement is a different corrosion mechanism than galvanic corrosion but may be one of the causes of another corrosion mechanism called stress corrosion cracking or can be one of the causes that can create an environment where stress corrosion cracking can be more likely. While Stress corrosion cracking usually manifests at much higher temps., (if it's going to happen, it usually shows up much more quickly with increasing/very high temps.), it can/does happen at lower temps. - it just takes longer. Anyway, zinc anodes may be a better anode choice - if a good choice is even available.

The deposition and/or precipitation fouling has a better chance of happening in fluid conduits where fluid velocities are low - thermosiphon water heaters are a close to perfect example. Stuff either collects in some spots where fluid velocities are low/zero, or where corrosion sites (microscopic nicks or metallurgical anomalies exist) promote chemical reactions and stuff to grow. The growing stuff can corrode the metal its growing on/stuck to leading to holes.

A couple or so questions for the OP:
Do you know/can you find out what the water supplier does to the water, if anything, besides R.O.'ing it and then adding some chlorine ? Specifically, any PH monitoring/adjustment(s) of the PH ?
You mention other users. What types of equipment do other residential water users in the area use to meet their domestic water requirements ? Do they have the same corrosion situation(s) you seem to have ?
Any conversations with them/information/anecdotal stuff you can/care to relate ?

BTW, 304 /316 and the L grades of each being austenitic are much less susceptible to galvanic corrosion but are still susceptible to hydrogen embrittlement and stress corrosion cracking, but probably less so than most c. stl. materials. Martensitic stainless (generally the 400 series stainless) are less susceptible, but I sure wouldn't want to go looking for a 400 series st. stl. tank, much less pay for one.

Just some thoughts/observations.

Take what you want of the above. Scrap the rest.

[juakali10]

First step is to get your water tested to find out exactly what you are dealing with.
You could always go with an inexpensive standard closed system with a Titanium heat exchanger for your main water. Your power consumption would go up because of the pumps used but at least you wouldn't have to replace the system every six years.

Happy to research what you suggest. To point me in the right direction do you have have a make in mind - that people have been satisfied with. I don't mind using pumps if I have to. . .

[juakali10]

JPM - other people also have their appliances wearing out at a much faster rate. You are doing well to get 4 years from a standard electric water heater. Which is one of the reason I want to go solar. Our land fill here must be full of leaking water heaters (and we are only 30,000 people !?!)
I am sure, my anodes are well gone, to be honest I cant get the socket open. On my new tank (if it has anodes) I will be sure to release, grease the threads and tighten back up so I can change the anode out after 18 months - or at least check it.
From what I know, the water company just runs the RO and dumps plenty of chlorine in to the finished product. Most people here drink bottled water which also drives me crazy. I put a filter on my kitchen sink and the water is perfect ! I was thinking if I put a filter on my supply to the house would that help the situation ?