Most domestic swimming pools need the water to be fully recirculated and filtered at least every 24 hours. To do this, conventional pools have 240 volt induction motor pumps that can be driven by solar but doing so is as inefficient as it is costly.

We'd intended to obtain our own pool recirculating system via the remote area rebate scheme but all suppliers approached insisted on installing a freestanding bank of solar modules, regulator, inverter and batteries. Their averaged quote (prior to rebate) was around $60,000. I knew there were more appropriate approaches but as no supplier was prepared to consider any we decided to forego the rebate. I designed, built and installed the extra low voltage system myself. Here's how it works.

This pool is solar powered for its filtration system. Note solar panel on left.

Starting by assuming it is unnecessary to recirculate water after dark removed the need for batteries (saving some $10,000 plus infrastructure). Calculating the pumping capacity needed to recirculate the 35,000 litres of water indicated it could be done by using an ultra-efficient pump of about 500 watts powered directly from solar during the Kimberley's typical 10 to 12 hours of sun (about 6 PSH).

The solution was within the German-made Lorentz Badu top range of ultra-low voltage pumps. The unit chosen runs from a nominal 48 volts using an MPPT controller (see page 38) to drive its brushless dc motor.

	Lorentz Bodu pump - this one runs on 48 volts dc. It pumps about 35,000 litres a day using four dedicated 120 watt solar modules.

Pumping losses were minimised by using oversize filters and piping. The pump maker's graphs showed it needed about 450 watts, so four dedicated 120 watt Kyocera modules were installed on cyclone-proof mountings at ground level just to the north of the pool. Pumping trials showed that water pressure and flow were almost exactly as indicated on Lorentz's graphs.

Keeping it clean
Pool water is normally sterilised by adding chloride directly, or by passing lots of amps through salt water to produce chloride. The former did not appeal and the latter was impossible anyway. I thought laterally and diverted the existing irrigation system to flow via the pool. This was done via a Nelson one-shot valve that opens 30 minutes each day. About 2000 litres or so flows into the southern end of the pool. A 50 mm pipe runs from close to the bottom of the pool's northern end and extends vertIcally to normal water level. It then runs horizontally for a few centimetres and then down, via a filter, into the existing irrigation system (see sketch below).

When the valve opens, the water level slowly rises to the horizontal section where it then flows down into the irrigation system over the course of the following two to three hours. The horizontal pipe has an essential opening in its upper surface to allow air to enter. Without it, once water starts flowing, it continues to syphon until the pool runs dry. As I found out the first day! This technique requires so little chloride that it does not bother plants.

This system has worked well since 2004. Nothing has needed changing or modifying since it was installed. The total cost, including concrete and steel for the cyclone-resistant solar mountings and all the electrics etc came to less than $8000. A far cry from the quoted $60,000! As the system runs at a nominal 48 volts dc, I was legally able to do everything myself Nothing has been or will be patented. Feel free to copy it - some already have!

A valve (not shown) allows irriation water to fall into the pool for about 30 minutes. as water level rises above the bottom of the U-shaped pipe at the right, it flows down and into the irrigation system over about three hours.