Swimming pool salt chlorine generator economics

The following is a description of installing a salt chlorine generator for a swimming pool, mostly dealing with the economics, and which is an update to my page at http://www.truetex.com/poolcontrol.htm dealing with
controlling pool control chemistry automatically with a computer.
======================================================================In June 2007, I dispensed with all the injected solution apparatus described above, and installed a Hayward Goldline Controls Aquarite (AQ- RITE) electronic chlorine generator. To decide this, I performed an analysis known in engineering economics as the "make or buy" calculation: whether it is cheaper to make your own chlorine with an expensive up-front investment in equipment, versus buying it one jug at a time from a pool store.
If you can make your own chlorine at an equal or lower price than buying hypochlorite, then you can get rid of the big chore of hauling and dosing chlorine by hand. Can we even save money doing so, like the salt chlorinator brochures boast? Will this claim stand up to hard-nosed engineering scrutiny of the true costs?
The up-front costs of a salt chlorinator consist of the unit itself and its installation, plus the initial dose of salt for the pool. The Aquarite model I chose varied in delivered price from $645 (discounted on eBay) to $1278 (retail pool store). I installed the unit myself in about 4 hours, with another 4 hours for start-up and performance tuning. I would estimate this electrical and plumbing work to cost about $400 in time and materials, if hired out.
My 26,000-gallon pool theoretically required 693 pounds of salt to create the target 3200 ppm salinity. Bags of salt locally varied in price from $5.20 (Sam's Club) to $8.52 (retail pool store) per 40-pound bag (all prices including local sales tax). The initial dose of salt thus costs from $90 to $148 depending on the store. I hauled mine from Sam's Club. The sales clerk insisted on moving my neat pile of 20 bags of Morton Pool Salt (800 pounds, mind you) off the flatbed hand truck I pushed up to the checkout lane onto a sloppy pile on another truck with a bad wheel. She may have been concerned I was a clever shoplifter, hiding a big LCD TV inside that mountain of salt, like Pharoah's mummy locked inside the Great Pyramid. From there I had to push that miserable load like an ant pushing a brick, all the way to my vehicle at the end of the parking lot. Once I hoisted these into the back of my van and drove home, I recruited my kids to carry them over to the pool, setting the bags evenly around the edge. Then I would cut a bag open with scissors, pour about half the bag into a dry bucket, and fling that bucket's contents with a mighty "HUNHH!" across the pool surface with as much spread as possible, progressing around the perimeter to evenly disperse the undissolved grains along the bottom. It got to be like a golf stroke, making a fling that would throw a well-spread, horizontal sheet of flying salt nearly across the pool. Then I set the trusty Polaris 180 pool monkey to work, minus the collection bag, which would speed around the pool bottom, sucking up the salt and jetting it up into the water where it would finish dissolving. All told I only spilled a few ounces of salt onto the pool deck, making for about 99.96 percent efficiency. The rest of the flavorful granules got whisked in.
(Note to anyone facing the same task: I made the mistake of assuming my old pool water had no salt in it, when in fact it had a significant existing salt concentration from years of sodium hypochlorite chlorination. I overshot the target salinity of 3200 ppm, and instead got 4700 ppm, which cost me some extra time and money to dilute out with city water. The Hayward manual isn't clear that you should install the unit and turn it on before adding all the salt, to see if you have a significant initial salt concentration to consider and compensate for. This is complicated because you want to hurry up and get things going, but it takes all day for salt to dissolve and disperse evenly in a big pool. And the manual isn't clear that your unit isn't going to be damaged by powering on to test the low salt concentration of the undosed pool.)
The up-front capital investment for the unit, installation, and initial salt dose is thus from $735 to $1826, depending on your do-it-yourself skills and shopping acumen. This investment will have to be amortized into the lifetime of the equipment using some assumptions about lifetime and depreciation rates. Below we'll assume a 5-year, straight-line depreciation.
Now let's calculate the variable costs of operation per unit of chlorine delivered. Hayward's documentation on the Aquarite claims that the unit "produces 1.45 pounds of pure chlorine" per day. Just what they define by the ambiguous term "pure chlorine", they don't say. They make several such casual statements in their documentation for this very technical product, which as an engineer I find annoying; no doubt some marketing genius made them dumb-down the precise language. It seems likely they mean the chlorinating equivalent of 1.45 pounds of gas chlorine, which as we've seen elsewhere on this page, is the "available chlorine" figure which actually doubles the weight compared to the elemental weights in hypochlorites or cyanurated chlorine. In short, the most economical competing chlorinator is the 2.5 gallon just of sodium hypochlorite sold at the local pool store for $4.05/jug which yields about 2 pounds of "available chlorine". To this price we must add about $1.25 for make-up acid to neutralize the alkalinity of the jug product and its sodium hydroxide stabilizer, for a total cost of $5.30. So the buy-it chlorine is $2.65 per pound, to which we want to compare the make-it cost per pound. (Let me add that I am suspicious about the pool store's claim for potency of their chlorinating solution, since my laboratory found that their jugs of pool acid were watered down by about 20 percent of what's claimed on the label, and sometimes the jugs of chlorinator don't seem to work very well. So they may be selling something that is significantly more expensive than the label would indicate. I contacted the weights and measures authority for the State of Florida, who gave my report about product dilution some serious consideration, but in the end they decided not to pursue it.)
Likewise, we should confirm by some kind of measurement how much chlorine the Aquarite generator actually puts out, and the power consumption of the unit, rather than just taking Hayward's claim as truth. After the unit was installed and running, I metered the power consumption at 1.2 amps at 240 volts, or 288 watts, significantly more than Hayward's claim of 1.0 amps or 240 watts. The other thing we have to assume is that Hayward's claim for 1.45 pounds of daily chlorine output must be based on running the thing flat-out at 100 percent capacity for 24 hours. Residential electric power here in SE Florida mid-2007 is running about $0.124/KWH (actual bills I receive, not the phony "average bills" the utility publicizes). To run the Aquarite at 100 percent for 24 hours is thus 288 watts times 24 hours, or 6.91 KWH, whichs costs $0.86. So the power costs only about $0.59 per pound, but we haven't added in the amortization. Later we're going to assume 8 hours per day generating chlorine at 100 percent, which per year will cost $105 to generate 176 pounds of chlorine. I confirmed the basic performance of the unit when new by setting it to 100 percent capacity and running for 8 hours (after dark to avoid sunlight losses), which should have generated 0.48 pounds of chlorine into my 26,000-gallon pool, for a concentration of 0.48/26,000 x 1/8.34 gallons/pound = 2.2 ppm, which an OTO colorimetric test approximately confirmed.
Let's consider make-up salt as an amortized cost, since rainwater tends to dilute the salt out of the pool, requiring more salt to be added. My location on the planet (indeed, you can even see my pretty blue pool here on Google Maps, by zooming all the way in on the satellite view, not quite where the Google arrow points but just east of the S-bend of the road, where they must have caught me on a good day when the pool was in shape) gets about 50 inches of rain per year, which is about 20,000 gallons over the area of my pool. If that all went to diluting salt due to overflow, then the annual salt cost would be $72 to $118. I would estimate the annual cost to be perhaps 1/3 of that, or $24 to $39. That is, you have to heave in a bag of salt every few months, which is consistent with what other pool owners report as their experience.
To amortize, we must make some assumptions about how much per day the unit will run, since the object is not to run at 100 percent capacity, but to merely meet the pool's chlorine demand for sanitation. My current strategy is to run from about 4 p.m. to midnight daily, which avoids producing chlorine merely to lose it quickly from daylight. Enough ORP to sterilize the pool and kill algae is achieved after a few hours, and the pump runs during this time to complete about one turnover of the pool volume. This keeps the pool looking good without having to run it half the day or add expensive algaecides. On those occasions when there is a big bathing load or other need for midday sterilization, we run the unit continuously. I am still a refusenik when it comes to cyanuric "stabilizer", even though every salt chlorinator manufacturer follows the herd on this issue.
The amortization of the original installation costs requires that we assume some lifetime for the equipment. The salt chlorinator consists of two subsystems, one being the digital controller and power supply, and the second being the titanium-ruthenium-oxide electrolytic-catalytic cell with temperature and flow sensors.
The cell lifetime is said to be five years, but during that time the capacity gradually degrades due to the loss of the catalytic oxide coating as the cell ages. While this aging gradually diminishes the maximum rate of chlorine production, I would expect that aging does not adversely affect the power efficiency in terms of watts consumed per mass of chlorine produced. So the cell lifetime question really amounts to how long can you run a new cell before the cell's aging capacity diminishes below the pool's need for chlorine. In short, how many years before the pool is turning green again although you're running the chlorinator flat out? The economics are further complicated by the ability to compensate for diminished cell capacity by running the chlorinator longer during the day, which also requires the circulation pump (and its power costs) be kept running. It is not unlike a car tire, which may have some expected lifetime, but which become progressively less safe on wet roads as the tread diminishes; such things tend to have less than the optimistic lifetime the manufacturer claims.
So one must guess about cell life, because these salt chlorinators using the ruthenium oxide coating are a relatively new technology, and this is the first such unit I have owned. The gradual capacity decline is what I hear from others who have owned them longer. So let's just be optimistic and believe the manufacturer that I will be generating chlorine for five years with the original cell. The manufacturer explicity makes no guarantee about performance after the first year, and pro-rates any replacements for a five-year expected lifetime.
The controller on the wall purportedly has a longer lifetime, and the manufacturer will sell you a plug-in replacement cell so you can renew your system for less than a whole new system. I am not so optimistic about the controller life, though, because I know what the outdoor climate does to electronics, and the Aquarite is an oddly built steel enclosure with no heat sinking on a 300 watt transformer. I measured the internal temperature at 180 degrees on a normal day, so I do not expect this controller to live a lot longer than the 5-year cell.
If we assume a 5-year lifetime with straight-line depreciation and 6 percent money, then our capital cost of $735 to $1826 yields an annual depreciation of $174 to $433. To this we add the $24 to $39 for make-up salt and the $105 per year for electric power. So our total annual cost is $303 to $577 for 176 pounds of chlorine, or from $1.72 to $3.28 per pound of chlorine generated. Note that the capital costs (purchase and installation) are greater than the consumables (electricity and salt), perhaps much greater if you're not a do-it-yourself'er or careful shopper. The marginal cost of making extra chlorine is a bit better, since we don't have to include the controller amortization or make-up salt, just the electric power and cell depreciation, amounting to about $1.20 per pound of chlorine.
The make-or-buy decision about chlorine thus comes down to several simple factors: (1) If you can avoid retail prices and do the installation yourself, you can make chlorine cheaper than you can buy it (although by no means as cheap in true costs as the salt chlorinator manufacturers claim), and reap a significant further savings in manual hauling and dosing of pool store chlorine, and (2) Even if you hire out all the installation and maintenance, you'll pay twice as much or more than doing it yourself, and the generated chlorine will be as much as 50 percent more expensive than hauling jugs of hypochlorite, you'll be buying a significant convenience for a reasonable price.
To conclude, the salt chlorinator approach is economical for both the do-it-yourself'er and the hire-it-out-for-convenience pool owner, although for different reasons.
I've not included some factors that may or may not be critical to the economics of a given situation: There is a performance risk to buying any piece of sophisticated equipment like this, should you have problems with it. There is a risk of choosing the wrong unit or the wrong manufacturer, and you're entering into a 5-year committment. Significant to me is the fact that Hayward derates the chlorinator unit from its alleged residential capacity (40,000 gallon pool) to 25 percent (10,000 gallon pool) for commercial or public pools in Florida. My economic calculations do not include Inflation in the price of consumables, or variations in the time value of money. A fair comparison to buying pool- store chlorine should account for the not insignificant chore of hauling and dosing the nasty stuff manually. Since this hauling of inventory is not something one always can keep up with, you occasionally have a neglected pool that costs you still more chlorine to clean up, and which costs you something in just being ugly to look at; the reliable supply from the salt chlorinator avoids this supply-chain risk. I've made my comparison to buying a type of chlorine (2.5 gallon jugs of 10 percent sodium hypochlorite) which is relatively cheap in my Florida market compared to elsewhere in the world. Finally, there are murmurs of salt in pools causing corrosion of metal fixtures and deterioration of natural stone masonry work, and this risk of structural damage should be considered in the costs of running a salt-chlorinated pool; I'll know more about this in a few years after some experience with the salt water on my natural marble coping and stainless steel fixtures.
I considered some other salt chlorinator units than Hayward's, such as the Pentair IntelliChlor, the Pool Pilot, and the Zodiac LM3. The decision for Hayward came down to it having a simpler controller that was less expensive and more rugged; with my computerized controls, I didn't need a more expensive controller that did timing.
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