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.