Low Energy Pool

Backyard science is cool. For the last few months I have been working on a theory to reduce the energy consumption of my pool. I started with a theory, and tested it with an experiment. It failed a few times, so I re-worked my theory and have now reduced my pool energy consumption by 75%. That’s a saving of around 7.5 kWh/day in summer. Now I can write it up and share my results with others. I think it’s just great that the Internet enables all of us to do real science on this scale.

Pools suck from an energy point of view, as I explained a few years ago in this popular blog post on the Floatron. The Floatron allowed us to reduce our filter run time from 10 hours day in summer to 4 hours. In addition every few weeks we would add solid chlorine tablets to boost the chlorine level a bit. However I wanted to go a step further, 4 hours is still around 4kWh/day, a big chunk of our household power consumption.

With the Floatron system some residual chlorine is required to keep the pool clear, about 0.5ppm. When this chlorine levels drops, the pools becomes cloudy, and slightly off color. However the cloudy appearance is not a full blown “green pool event”, as the copper ions keep the algae at bay. It can be simply corrected by raising chlorine level to 0.5ppm for a few days (you don’t need a legion of chemicals).

Maintaining or raising the chlorine level often means running the pool pump, which is connected to the salt cell. The pump pushes water through the salt cell,which injects the actual chlorine via a current passed through the cell. So to raise the chlorine level, you need to run the 1kW pump. A salt cell connected to the pool pump looks this this:

The actual power required to run the salt cell is much lower that the pump. Mine had a power input of 150W, and I measured 7.1V at 9.4A or 67W at the cell.

So I decided to try running the salt cell outside of the pump system. Under normal usage salt cells die every few years, so I had an old one lying around. They don’t usually die completely, they just become less effective and need to be replaced. So I took an old cell, waterproofed the electrical connections with some tubing and silicon, and threw it in the pool:

The idea is that with both ends of the cell open, natural convection currents would spread the chlorine around. The pump would still run a few hours a day and occasional pool use would also stir up the water.

I connected the wires to a 20W solar panel, via a Maximum Power Point Tracker (MPP) developed by Elektra. The MPP enables the solar panel to operate at it’s maximum power point (e.g. 17V) while allowing the load (in my case the cell) to find it’s own operating voltage, about 6V for my salt cell. A MPP is like a switch mode power supply in reverse. It allows the output voltage to vary, which keeping the input voltage fixed. They can double the efficiency of your solar panel.

Using the MPP I managed to get a maximum of 6.8V at 2.3A (15.6W) into the salt cell in the middle of the day. Plenty of tiny bubbles coming out of the cell. So I backed off the filter run time to 2.5 hours, then waited a few days to see what would happen to my chlorine levels.

The pool went cloudy and a little green. My good wife was not happy, and muttered something about “Honey, I shrunk the kids” experiments. I reasoned that the panel was too small, not enough current (or power, whatever makes the bubbles) was being supplied. It could probably only supply current for an average of 8 hours/day, or (125Wh/day).

So I connected a lab type power supply to the cell, and put 3.2A at 8V through it for 24 hours a day. After a few days the pool was clear and the measured chlorine was 0.5ppm and rising each day. Backing it off to about 15W kept the chlorine level constant and pool nice and clear.

OK, so that gave me a ball park figure of around 15W for 24 hours/day, or 45W if solar powered for 8 hours/day. Adding a bit of overhead for losses lets say a 60W panel. They cost around $500 here which was vetoed by the financial controller.

For a permanent power supply I bought a cheap 12V, 2.5A car charger. I chose a cheap charger as I wanted a simple unregulated transformer type design. Buying a charger and hacking it was cheaper and more fun that buying all the parts, drilling a box etc and building my own power supply. Looking inside I could see it was a simple fullwave rectifier using two diodes and two 12V windings configured as a center-tapped transformer. My first attempt was simply to remove one of the diodes to make a half wave rectifier. This would give an average DC voltage of half of 12V which is 6V. This worked, but the transformer ran hot and consumed 44W of AC for a power output to the cell of about 13W.

So I partially unwound one of the 12V windings until the unloaded AC voltage was 7V. I then built a bridge rectifier for just this winding and obtained around 5.4V and 1.9A at the cell (10.3W). This is a bit low, but I’ll see how it goes in the pool for a few weeks. I have the other winding spare so I can try unwinding again, this time leaving a few more turns on. The power input is about 23W so it’s not as efficient as I would like. A switch mode power supply configured as a current source would be ideal.

My filter run time is now 2 hours day. This filters the pool water, and injects extra chlorine from the conventional in-line salt cell. Total power consumption is 2kWh/day for the pool plus 600Wh/day for the continuous chlorination gadget. Thats a total of 2.6 kWh/day, down from 4 kWh/day with just the Floatron, or 10 kWh/day pre-Floatron. The 1.4kWh/day saved is enough to run my fridge. In addition we no longer seem to need supplementary chlorine in the form of tablets (saving maybe $60/year). The pool is crystal clear, best we have ever seen it:

Curiously, the measured chlorine level is quite low. I measure it every few days and it’s hovering around 0.2ppm. With the pool looking this good I had figured the chlorine should be higher. Maybe its concentrated in pockets that I am not measuring, as with my “outboard” salt cell chlorine distribution is likely to be less uniform.

Is it safe?

My understanding of how the copper ions (from the Floatron) and the chlorine remove algae and bacteria is weak. I am not even sure what all those bubbles are coming out of the cell. Are they chlorine gas, or some other by-product? I am pretty sure the Floatron takes care of algea, as we haven’t had a runaway green pool event in years. The Floatron site claims that it can also kill bacteria, and has a test report you can read. However the sample size is small (one pool). Other sources maintain that a high chlorine level is required to kill bacteria.

I had some interesting comments on my Floatron post from Nick work works for a state Water board (here and here). He makes safe water for a living, and also posted some interesting links on pool sanitisation.

So I am not sure if my system is taking care of bacteria. Bacteria are invisible so there are no obvious signs like water clarity. Swallowing pool water with bacteria can make you ill, in particular if you are sick already, or very old or very young. People are the main source of bacteria in pools. Like many pools, ours spends 355 days a year people free. Of course I don’t want anyone to get ill in our pool, but to be honest my main motivation is to make sure the damn thing doesn’t go green, while wasting minimal energy (both electrical and human).

From my previous pre-Floatron days, I know that pool chlorine levels vary wildly as most people (like me) aren’t likely to closely monitor the pool every day. This usually means multiple “green pool events” a year, and many hundreds of dollars in chemicals. These chemicals and high chlorine levels also have nasty side effects, like itchiness and sore eyes. So with a crystal clear pool that requires hardly any maintenance I do feel I am ahead of the pack in safety. To double check I will get a professional bacteria test done on my pool water, to see if my system is taking care of bacteria.

17 thoughts on “Low Energy Pool”

  1. Let me get this straight. You still use 2.6kWh/day and shitloads of water and generate shitloads of chlorine for the sake of swimming 1 day a year (I hope that is some kind of exageration)?


    May I remind you we live in a country with perpetual water problems and with the highest per capita energy usage in the world.

    Sorry, the decadence of private pools just shit me off. Surely there’s a public pool near your place.

  2. Yes, thats about right, except it’s 10 days a year, my pool is (mostly) refilled from rain water, and Australia has the highest GHG emissions per capita, not energy used. Although we are up there for sure.

    The government allows me to fill my pool (or install a new one) during a water shortage but won’t let me wash my car or water my vegetable garden. Curious.

    Like many people, we bought a house with a pool, so (short of filling it in at great expense, or letting go green and stinky) we are stuck with it.

    So we do the best we can. And I have done more than most – check out the Renewables and Electric Vehicles links to the right.

  3. David Hi,
    very intersting story .. i have a pool and i am looking to reduce my power bill . i have a couple of questions if i may . Firstly i should start with the floatron , i saw an article in the ATA mag , but i dont recall if anyone mentioned where can we buy it in Australia ? secondly , regarding running the cell in the pool , is fair to say that i need a panel 60W and above ( i have a spare 120W panel ) and a MPP ? do you recommend any particular one ? how much do they go for ? anything else ?

    thanks for your help

    Andrew – SYDNEY

  4. Hi Andrew,

    Re where to buy the Floatron, check out the comments on the Floatron post. Yeah a 60W and above panel should be OK, depending on your pool size and what condition your salt cell is in. I bought my MPP from a friend in Germany (can send you her email), but I think other commercial units are available here in AU.

    – David

  5. I wonder if you could use 2 salt cell chlorinators in parallel to produce the chlorine faster? Maybe then you could halve the pump run time depending on pump capacity and filtration requirements. Or use a 2nd and much smaller pump just to operate the chlorinator. Perhaps they would last longer or operate more efficiently if less water passed though it (wishful thinking :-)

  6. I wonder if the performance of the submerged salt cell would benefit from further water movement, but not supplied by the pump? I was thinking something along the lines of an aquarium power head ( http://www.aquariumguys.com/maxijet1200.html ): 300GPh, 20W (or 200GPh, 8w). 300GPh isn’t going to move all 60,000 gallons, but it quite likely mike make the submerged cell more effective. They’re only $30 USD–the financial controller might even let you buy two of them, which is a tiny fraction of the 1kW pump…

  7. You want to be careful immersing mains powered equipment in pools. There are specific isolation requirements for equipment in electrical contact with the water of your pool, aimed at eliminating the chances of 240V being applied to the water. For example, if you open up the controller for your salt cell, you will see that it contains a type of transformer known as a “safety isolating transformer”. Such transformers have extra isolation between the primary and secondary compared to the type of transformer in a battery charger. I’ve got a feeling that they are also built to be fail safe in case of a primary-secondary insulation breakdown?

    It’s also worth mentioning that pool electrical circuits must contain an residual current device, but you would almost certainly have one of those if you are plugged into an existing mains circuit.

    I’m fascinated with your experiments, as I too have a power/chemical hungry pool. Any ideas on how to avoid having to regularly add acid to the water of a salt pool? Have you ever peeked inside your Floatron? It would be an interesting experiment to see what could be done with some electronics and surplus copper mains cable.

  8. Still thinking about the Floatron… Presumably it has two electrodes? It would be interesting to see an oscilloscope plot of the open circuit voltage between them. A second interesting plot would be a plot of the voltage with some sort of load resistance between the electrodes (or plot of short circuit current if you are confident that it won’t be damaged). From there (if the power supply is linear), it should be possible to derive a Thévenin equivalent and design an equivalent supply. Another interesting number would be a rough surface area of the electrodes (12cmx3cm dia.?), which will give a current density. Final step would be to build a system that has copper electrodes with a similar area and current density, and see if it works. Maybe the Floatron electrodes are more sophisticated than just a lump of copper? The references (1970s NASA) given in their documentation seem to be based around using silver ions.

  9. Hi John,

    Thanks for your comments on safety isolation, that was one area I was concerned about. It’s one of the reasons I went for a chunky linear power supply (rather than switch mode), I felt a bit safer with an old-school big transformer. The combination of mains electricity, salt water, and people is potential dangerous. Yes we have an earth leakage detector, but now that you mention it I’ll double check it covers that circuit.

    I tend to pull my chlorination cell out of the water when the pool is use, this is actually for the safety of the cell in a pool full of teenage kids :-)

    The Floatron is just a small solar panel connected straight to the electrodes. The open circuit voltage across the electrodes (i.e. out of the pool) measured 5.9V with the panel facing the morning sun. The electrode measured 30mm diameter by 120mm long. I am pretty sure that any small voltage source applied across a copper electrode would do the same job. I read somewhere the electrode is mainly copper with a few % silver. Not sure how to test that, but yes it would be an interesting experiment.

    The electrode is a rod that starts at about 25mm in diameter and perhaps 130mm long.

  10. One way to figure out composition is to measure the volume of the electrode, whether that be by direct measurement/calculation or by dipping it in a liquid and measuring its displacement (weigh the amount of water displaced, water has a density a shade over 1kg/L?). Next weigh the electrode. The atomic mass of silver (107.8682) and copper (63.546) are known, so from there it should be possible to calculate the relative proportions of silver/copper, assuming that only those two elements are present. Potential spanners in the works include additional elements being present, or there only being trace amounts of silver, beyond the accuracy of the measurements, though a sanity check is that both ratios should be measured in the range 0<=n<=1.

    Perhaps another way is to use the electrode in a galvanic cell and do some sort of measurement of the potential of the cell? Perhaps the potential is a linear combination of the potentials for silver and copper, depending on their concentrations?

  11. Hi David,

    Do you know of anyone who has kept their pool clean using fish? Although many people would I’m sure find it a little odd, I think that finding ways to bring ‘nature’ back into our lives is a good thing. To take it a step further and actually produce edible fish, that keep your pool clean… perhaps I am asking to much :)

    Your thoughts?

    Warm regards,

  12. My crystal clear clorinator has the copper ioniser in it,looks like pure copper bars to me.
    Looked at transformer,9.6V@11A &10.5V@0.5A, assumeD from what you have found the clorinator is the bigger.
    Tried dipping in my pool, two nice clean long stripped copper 6mm2 cables, connected to a 12V@18AH deep cycle battery,obviously keeping wires apart at about 50mm,danger Will Robinson and measured terminal voltage dropped to around 11V,lots of small bubbles,don’t know chem but saw little blue floaties happening,copper chloride? presipitate and ions happening,sparkly surface above,can you see the effect of copper ions?
    Think might use 10W waterproof amorphous rubber panel and LED driver circuit from ATA,
    and maybe solar battery charging module could float,no pun,small lead acid supplying the driver circuit,think they are MPPT in operation correct me if wrong, otherwise a modded silicon chip circuit, would all need good potting though in the pool and some copper busbar.
    Hey maybe a 6V float battery could simplify your circuitry.
    Is the floatron just simply asolar panel and battery chager? as all solar panels are generally manafactured to float lead acids with minimum circuitry


    1. Hi Jeff – I think the Floatron is just a small solar panel connected to the electrodes, very low current like a few 10’s of mA.

  13. Apologies about the Floatron sounds like you’ve already had it apart,that’s a very expensive small 6V panel and no constant current circuit.
    Looked at the 5 star pool pumps Drew mentioned.
    Sorry Drew but star ratings are relative. When the appliance is doing the task, like an electric heater is nearly 100% efficient it does’nt mean its going to save you energy, those pumps may have slightly higher efficency due to design than a normal pump, but with multispeed that’s task specific,with a pool you don’t want speed control
    even soft start what you need is feed back e.g. sensors is the pool fitered, etc, then stop.

  14. Hi David – thanks for your interesting blog. I hope you’re still listening. Like you I have an pool that I bought with the house. Its rainwater filled (and a fire fighting reservoir) – great for really hot weather swims for me and a few neighbor kids. I plan to turn into an aquaponics system one day – with my veggie garden up the slope. In the meanwhile I hate using Chlorine – so made my own DIY Solar Powered ionizer, guided by a great Blackwaters YouTube video. It consists of 2 pieces of copper tubing (electrodes) separated by plastic spacers and powered by a Solar Battery Charger for a Motorbike (Cost A$23). I have it floating in an old kalite fruit box, with the 2 electrodes protruding below the box. Total cost $25. Actually it puts out too many copper ions on a hot day – so I take it out for much of the time. I have the water tested occasionally, but will get a cheap test kit for copper (keep at o.3-0.4 ppm). I pump for filter only for 8-12 hours per week, at off peak times, and still use a little chlorine (this is a fresh water system not Salt) but my plan is to replace this (when my chorine supply runs out) with hydrogen peroxide (another oxidant, but not as carcinogenic as chlorine) – to handle the occasiona water bug or tadpole. Having spent much of my youth swimming in farm dams and rivers, I dont need crystal clear water, but this pool is as clear as we need it – can see the bottom clearly.

  15. The bubbles coming off the reaction are Hydrogen. Make sure you don’t allow this to accumulate in a closed system.

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