Economics of Saltwater Chlorine Generators

[chem geek]

The following table shows the economics of saltwater chlorine generators and why they save money IF one uses a larger (often oversized) unit and is in an area where electricity costs are not very high.

Pool Size	18,000 Gallons
Power Cost	$0.10/kWh

Name	Model	Output (lb/24hr)	FC/hr	Life (hrs)	Power (Watts)	Power $/lb Chlorine	New Price	Labor	Total New	New $/lb Chlorine	Replace Price	Labor	Total Replace	Replace $/lb Chlorine	Total 1repl	Total 2repl
Pentair
Intellichor	IC15	0.60	0.17	10,000	65	$0.39	$460	$200	$660	$2.64	N/A	N/A	N/A	N/A	$3.03	$3.03
Intellichor	IC20	0.70	0.19	10,000	220	$1.13	$720	$200	$920	$3.15	$455	$200	$655	$2.25	$3.83	$3.68
Intellichor	IC40	1.40	0.39	10,000	220	$0.57	$787	$200	$987	$1.69	$513	$200	$713	$1.22	$2.02	$1.95
Intellichor	IC60	2.00	0.56	10,000	220	$0.40	$995	$200	$1,195	$1.43	$770	$200	$970	$1.16	$1.70	$1.65
Hayward
AquaRite	T-CELL-3	0.60	0.17	7,560	150	$0.90	$655	$200	$855	$4.52	$219	$200	$419	$2.22	$4.27	$3.89
AquaRite	T-CELL-9	0.70	0.19	11,800	150	$0.77	$765	$200	$965	$2.78	$335	$200	$535	$1.54	$2.94	$2.72
AquaRite	T-CELL-15	1.45	0.40	8,640	150	$0.37	$845	$200	$1,045	$2.00	$409	$200	$609	$1.17	$1.96	$1.82
AquaRite	T-CELL-15LL	1.45	0.40	10,577	150	$0.37	N/A	$200	N/A	N/A	$641	$200	$841	$1.32	$2.03	$1.92

The unusual cell lives for the Hayward cells are calculated from this document where they list "equivalent pounds of Trichlor produced over the lifetime of the cell" where I assume 90% Available Chlorine. So, for example, 0.9*210/(0.60/24) = 7,560.

The above does not account for the cost of salt which for an 18,000 gallon pool would be 450 pounds of salt (if starting with no salt and getting to 3000 ppm). Morton Pool Salt at Walmart is $6.38 for 40 pounds so 16 cents per pound. This is $72 so not that much compared to other initial costs and maintenance. I am also not including the cost of acid that usually needs to be added to the pool. That could be as much as $15 per month if the pool isn't managed well to reduce this need, but people can usually get this down to $6 per month or less. This acid cost is significant since $6 per month at 1.5 ppm FC per day for an 18,000 gallon pool is roughly $1 per pound chlorine (i.e. increases the above numbers by $1).

Note that the cost for chlorinating liquid or bleach is usually $4 per pound (chlorine gas equivalent) or less and Trichlor is about the same price but requires a base as well. At discounted prices in some areas the chlorinating liquid or bleach might get down closer to $3 per pound or even less when on sale. For reference, 12.5% (trade) chlorinating liquid at $3 per gallon is $2.88 per pound; at $4 per gallon it is $3.83 per pound.

So it should be very clear from the above table that to get significant savings one really has to use the larger cell sizes of the IC60 (or at least the IC40) for Pentair Intellichlor and the T-CELL-15 (not LL) for the Hayward AquaRite. This assumes that the cells actually last as long as indicated based on usage and that they do not degrade at all based on time alone. [EDIT] Note that the larger cells will output higher FC out of the returns (see this post) so for lower pump flow rates and/or smaller pools that could be too high an FC. [END-EDIT]

Note that the economics change considerably if electric rates are high. Instead of the 10 cents per kilowatt-hour I used above, if I assume 40 cents per kilowatt-hour then the IC60 cost is $2.85 assuming 2 cell replacements before the entire system needs replacing while the T-CELL-15 cost becomes $2.93. Add in the cost of salt and especially acid and this may get to roughly the same as or more than chlorinating liquid or bleach.

What has changed over time is that the life of these cells has increased so they are more economical now than in the past. Note that at 2.0 ppm FC per day in 18,000 gallons, the IC60 lasting 10,000 hours in theory would be able to operate for (10,000 hrs)/((2.0 ppm FC/day)/(0.56 ppm FC/hr running)) = 2800 days or many, many years (over 7 years if used all year long).

I made assumptions for labor for installation and cell replacement maintenance at $200. In practice, many people don't buy their equipment at the discounted online prices I show above and instead the installer charges retail and pockets profit from it and charges labor on top of that. In that scenario, the economics will be a lot worse than shown above.

 

[Mike_B]

I put off using a SWG this season in my pool currently being built to allow me plenty of time to research. This helps out tremendously. Thanks chem geek!

 

[wpturner05]

This is great chem geek!! I switched last year to SWG but still used pool store testing and advice. THIS year I switched to TFP method and I can truly see and FEEL the cost savings in my wallet. I think the ultimate savings is when a pool owner uses these or similar methods. When I converted last year to a SWG, my pool store manager was non-too-happy. Now I understand why. He did manage to still sell me a lot of chemicals last year though.

 

[JoyfulNoise]

Great analysis. My next SWG will be an IC-60.

I think one also has to factor in some liquid chlorine usage. I have used some LC in my pool chemistry ritual as I prefer to bump the FC up for high bather loads using LC rather than running the SWG harder. Then there's also the occasional SLAM (once per season, never more than 2.5 days to complete) so far that I have had to do on the occasions when I felt like my pool water was about to go sideways. I admit my pool is complicated by an attached spa which sees excessive human usage and rock waterfall (that has four little pools of water in it each holding ~ 2-5 gallons) which sees excessive avian usage (bird poop complicates my life).

As well, SWG pools use higher CYA levels (almost twice as much as non-SWG pools). My pool sees very little inherent change in CYA levels over a typical season even with splash out and regular backwashing. So, for me, that seems to be more of a limited, one-time cost. Others with significant water exchanges (rainier climates) might need more CYA to keep their SWG pools happy.

So does the analysis change much if you add in offsets for those two scenarios?

 

[chem geek]

Most everything else adds in more costs. If you have more water dilution, then you'll need more CYA to replace it at 80 ppm compared to a non-SWG pool with 50 ppm. The same is true for the salt level. However, I think the main cost difference is in the acid and that depends on the specifics of the pool. While many people are able to minimize acid usage via a lower TA, higher pH target, and higher CYA (so lower SWG on-time), there are still some SWCG pools with noticeably higher pH rise, possibly due to short pipe runs with undissolved chlorine gas outgassing.

If there were a 50% water dilution from either winterizing the pool or significant winter rain overflow then replenishing the salt would be around $36 and replenishing the incrementally higher CYA (30 ppm in half of 18,000 gallons so 2-1/4 pounds) would be around $8. If we assume 2 ppm FC per day for 5 months (and that may be on the higher side for SWCG usage that might be closer to 1.5 ppm FC per day given the lower FC/CYA ratio and higher CYA level), then in 18,000 gallons that is 45 pounds of chlorine so so that comes to an incremental cost of around $1 per pound of chlorine.

So yes, if one needs to replace a lot of salt and some CYA from significant water dilution then the economics get closer between an SWCG and non-SWCG and if one also has to add more acid it can make the SWCG be more expensive than non-SWCG pools especially where electricity costs are higher.

The bottom line is that one doesn't normally purchase an SWCG to save money. One purchases one for the convenience of automated dosing and not having to buy/carry chlorine. My main point for doing the chart was to show that one should most definitely buy a larger unit even if significantly oversized.

It is also interesting how the Wattage rating doesn't change with the cell size. Though there may be significant overhead and losses from the power electronics (though it's just a transformer so I don't think there are), I find it hard to believe that the system with the cell with 0.7 pounds per 24 hours uses the same power as the system with the cell with 2.0 pounds per 24 hours. For the cell by itself, the electrochemical energy depends on the current and voltage. Modern cells use plate stacks that are in series with plates in the middle being passive (not electrically connected) but generate chlorine due to the potential differences between each plate (it behaves as if the two sides of a middle plate are really two plates electrically connected). For the Intellichlor, they say that the voltage output from their IC20/40/60 power center is 35V (ranges from 22V to 39V depending on conductivity so voltage drop and has a 6 Amp maximum). 2.0 pounds per 24 hours is 0.0105 grams per second so we have ((0.0105 g/s) / (70.906 g/mole Cl₂)) * (6.022x10²³ e^-/mole) * (2 e^{-/Cl₂) * (1.602x10-19 Amp-sec/e-) = 28.6 Amps and while with 35V that would appear to be 1000W, this current is split among plates so the voltage is not 35V between each plate. For 2 active and 4 passive plates (so 5 sections so 7V per section) this would be 200 Watts. This implies 90% transformer and other efficiency compared to their 220 Watt spec and that might be correct. However, the IC20 should be using 35% of the power in the cell compared to the IC60 so that part I cannot explain unless perhaps the IC20 has only one passive plate so it's (0.7/2.0)*28.6 = 10.0 Amps is over 17.5V so 175W and the IC40 has only three passive plates so it's (1.4/2.0)*28.6 = 20.0 Amps is over 8.75V so 175W. [EDIT] However, as I note in this post the power supply spec says it is limited to no more than 6 Amps so the plate stack is probably constant with 2 active and 4 passive plates, but the plate area is reduced for the smaller cell size and the current is lower. [END-EDIT]}

 

[JoyfulNoise]

The bottom line is that one doesn't normally purchase an SWCG to save money. One purchases one for the convenience of automated dosing and not having to buy/carry chlorine. My main point for doing the chart was to show that one should most definitely buy a larger unit even if significantly oversized.

Thanks for the additional analysis. And I absolutely agree with your above quoted statement! There are many times I see posts of people asking if the SWCG will save them money. Most often the general reply is "It's a wash. You're paying for the convenience upfront." But now there is a great post to point folks to who want to see actual hard numbers and analysis.

Great post, thanks for putting it together!

And, as a plug for reducing acid consumption, I have been using a lower TA target (60ppm) and have 50ppm borates in my water. With a little fine-tuning of the water features I have been able to arrest the rise in pH in my pool to the point where the water pH hangs at 7.6-7.7 for almost a week with no acid additions. I can then easily drop it down to 7.4 and not have to add acid for almost 10-12 days straight. I'm toying with going lower on the TA (perhaps 50ppm) but I do have to be careful and watch the CSI as it can go sharply negative pretty quickly.

 

[JoyfulNoise]

In most electrochemical cell baths I worked on (Cu, Ni, PdCo and Au), the power supply is a constant-current PS with a floating voltage output (with a voltage clamp to avoid electrolysis of water). That way you can use the plating current (C/s) to theoretically derive the mass plating rate. Actual film thickness and area measurements would then tell you how your plated film density compared to bulk.

In the electrolysis cell for chlorine production, are you saying then that the power supply is constant voltage with a current that is allowed to float based on cell conductivity and other solution parameters?


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[chem geek]

Not necessarily. They give a voltage range so I suspect they try and keep the current constant. That's not the issue in the calculations since I just used the middle voltage that they "spec'd". The question is how many passive plates they are using and why they claim to use the same power regardless of cell size (except for the smallest size which uses a different power supply).

 

[JoyfulNoise]

Not necessarily. They give a voltage range so I suspect they try and keep the current constant. That's not the issue in the calculations since I just used the middle voltage that they "spec'd". The question is how many passive plates they are using and why they claim to use the same power regardless of cell size (except for the smallest size which uses a different power supply).

Pentair does say that they periodically (default is every 4 hours, I think) reverse their anode/cathode configuration as a "self cleaning" function to avoid calcium build up. So the internal electronics must have the ability to control/select the current on different plates.

As for constant power, isn't the relevant metric for electrochemical reaction rates current-density? So they could use a fixed current supply and then vary the plate size to achieve a faster reaction rate? There are practical limits to current density based on the coatings used and how sensitive they are to Joule heating effects, etc. I only ask because again, in my electrochemistry days, we had to be very careful to balance the anode and cathode area ratios to not only keep below a specific current-density limits for the bath chemistries but also to maintain coating uniformity.

 

[chem geek]

Yes, I'm sure they are careful with plate distances and use coatings to have a high but not too high current density. Again, I just think that the actual power used in the IC20/40/60 are not all exactly 220 Watts. For the smaller units they can choose smaller plate areas or they can use a stack with fewer plates. I would think though that they'd lower the voltage with fewer plates if they were at the same distance to keep the same current density.

You know, now that I read their spec again, it says "6 amps maximum" from the power supply so maybe they aren't changing the stack size and instead just changing the plate area and using less current. So for their power supply output they have around 35V with 6A which is 210W (so their 220W makes sense with 95% efficiency). With a plate stack of 2 active and 4 passive, the voltage between the plate pairs is 7V and the current between each is 5.72A. The effective current in terms of electrolytic generation is 5 times higher than this because in effect the same electron is used 5 times in pairs of electrolytic reactions (it's not literally the same electron since it's given to hydrogen ions and taken from chloride ions). However, this would mean that the IC20 only has a current from the power supply of 2.00A and would only be 70W, not 210W. I think they're spec about 220W for the IC20 and IC40 is just wrong and the power usage is actually lower for these. If that is the case then the $/pound chlorine for electricity would all be around $0.40 which makes more sense.

This wouldn't change the advice of getting the largest unit because the cell cost still dominates in that and the pricing is not anywhere near proportional to the output, but it makes the pricing somewhat closer between them. There is one side effect to oversizing a unit which is that when it is on it is outputting higher FC levels (out of the returns) and this would be exacerbated at lower flow rates. The IC60 at 0.56 ppm FC per hour would be 3.4 ppm FC incremental out of the returns in an 18,000 gallon pool if the turnover time were 6 hours and in a smaller 7500 gallon pool it would be 1.3 ppm FC per hour so with 6 hour turnover that's 8.1 ppm FC. So there are some practical limits to oversizing especially in smaller pools and longer turnover rates. Since the IC40 is still fairly efficient, it may be a more practical choice in smaller pools with 30% lower FC levels coming out of the returns.

 

[JoyfulNoise]

I suspect the rated/listed power requirement is worst-case scenario. Not sure what the U/L listing requirement is for that.

Also, do you think the cell plates can really support a 7V potential difference? I would think with that much over-potential other side reactions would come into play (electrolysis of water, etc). Again, from my plating days, over-potentials were really bad for both cell efficiency and plated film quality.

But I agree with your final conclusions, a bigger cell is definitely the right way to go (best bang for the buck) and I'll definitely be upgrading to the IC60 as soon as I need a replacement.


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[chem geek]

That's what the ruthenium coating is for. To significantly lower the overpotential of the chlorine so that it dominates over the generation of oxygen. They also add other oxides to the mix including selenium, iron, nickel, and lead in order to raise the overpotential of oxygen. Yes, there is probably still a little loss with some oxygen produced, but they need the higher voltage to overcome the resistance between the plates because 3000 ppm still isn't that conductive. Their spec clearly shows 35V coming from their supply so that is only cut down by the number of passive plates and the 6A number would be consistent with 4 such plates so effectively 5 cells in series each with 7V and 6A (for the IC60).

 

[mguzzy]

Thanks for the additional analysis. And I absolutely agree with your above quoted statement! There are many times I see posts of people asking if the SWCG will save them money. Most often the general reply is "It's a wash. You're paying for the convenience upfront." But now there is a great post to point folks to who want to see actual hard numbers and analysis.
...

I agree its a wash price wise.. And depending on the conversion/initial installation costs it may swing it one way or the other.. BUT this is the biggest thing I noticed with my SWG conversion.. and there is no way to put a column that figures this metric: I spent a HECK OF A LOT LESS TIME dinkering with the pool chemistry compared to pre-SWG days. My time is money.. so the less time I have to mess with it.. that more time I have to enjoy it. Once the pool is "stablized" at the beginning of the season and I found it to be a lot easier to maintain.. mostly just weekly tests, and turn the knob if there were more swimmers or hotter days. I attribute this to the fact the chlorine is created in small incremental amounts and "injected" into the water flow continuously when the pumps are running. So the change to the chemistry is more gradual and easier to maintain. Much less drastic than dumping a gallon or two of chemicals in the pool and then having to deal with the effects of that mini shock (fix the Ph, tweak the CYA, fuss with the TA.. it never ends) . As a kid I remember the days of the pool man showing up which meant we couldn't swim till later that day. Now chlorine is created all the time.

Next the quality of the water is better without the chlorine spikes. One of my kids has very sensitive skin and the saltwater is actually good for here. I have a never ending list of anecdotal stories of people coming to my pool and asking why its different and that their kids had an easier time with the water for one reason or another.

I can't put a number on those items.. but for me it makes the SWG conversion well worth it. If I were to put a number on it would be a savings on the order of hundreds of dollars a month. The simple fact that other of my friends were able to fire the "pool guy" since with a SWG.. all he really did was scoop leaves out of the pool. (sorry 'bout that you pool guys out there)
Mark

 

[Semi-retired]

I think the original post was excellent to get a good feel for the "economics" of SWG but also demonstrate that there is an advantage to spend a little more up front to "oversize" the SWG. I went with an IC 40 for a 15K pool, and certainly could have gone with the IC 60, despite my pool contractor trying to convince me that the IC 20 was fine. A caveat to some of the above comments is running a SWG with a variable speed pool pump. The energy savings of running this pump at very low speed is huge. I run my pump almost 24/7 (about 20 hours actually) but it only consumes ~150-170 watts, i.e. a light bulb. I keep it just above the low flow setpoint of the SWG and then dial down the SWG "on time" to 20%. Basically it equals the results of a higher "on time"/shorter pump run time set up (which would be much more expensive due to increased pump power usage when run at a higher RPM). But, has the advantage of always filtering, always skimming, always circulating and a near constant input of Chlorine.

 

[R.J.]

Not sure how the economics work with my Intex SWG, but I love it and think it's worth every penny. For these larger pools, if it's a wash financially, why not go with the more convenient option?

 

[Chuckiechan]

Chem Geek, I certainly appreciate your analysis, but for me it a matter of one less chemical to schlepp home. Bleach is heavy! I'm weak, old, and lazy!

And since I borated my pool, my acid use has dropped off quite a bit.

I see the expense as the same as buying a new filter. They aren't cheap, but you gotta spend what you gotta spend if you want to own a pool.