Our Solar-Heated Spa Pool


When Fill and I went to buy a spa pool, we went to the local farmers' supply store, and tried out a cattle water trough for size - by sitting in it - much the the enjoyment of the salesperson!

This 600 litre black polythene tank is now sitting on our verandah being heated by a large solar panel sitting on the ground below.


To get the maximum efficiency, theory says that the panel should face due NORTH, and should slope Upwards so that in midwinter, the sun's rays strike the panel at 90 degrees at midday. This would be the optimum arrangement, but as you'll see below, I've chosen NOT to have the best slope, because of the problem I had with the water pressure. (Note that if you're in the Northern hemisphere, you'll have to face the panel SOUTH!)


Water circulates through the collector panel and the spa pool by thermo-syphon, so there is no pump to worry about. This is because the collector is placed BELOW the pool, and at an angle (about 25 degrees) so that the hot water (which is less dense than the cold water) will be pushed up into the pool by the colder water (more dense) coming into the collector from below. Trust me, it works!


As you can see from the diagram above, the pool itself is insulated to keep the heat in. There is a layer of polystyrene around and below the tank, as well as a thick blanket of fibreglass (pink batts) insulation. The pink batts are first enclosed in a u-shaped layer of black polythene film, then wrapped around the tank as shown. Any water spilling from the pool will fall to the ground without wetting the fibreglass.

There is also a floating cover, made of four layers of foam plastic sheet. (The same stuff from which jandals or thongs are made.)

The inlet and outlet pipes are 20mm diameter, and are spaced as shown.

(Note: I think the bottom pipe should have been closer to the bottom of the tank - this would prevent the bottom layer of cold water from "stagnating" as it does at the moment.)


The diagram above show how the panel itself was constructed. It consists of a pair of 2400mm x 1200mm galvanized steel sheets (0.55mm thick), spaced apart by an array of bolts and spacers to allow an 8mm water space between the sheets. The edges of the sheets are bolted and sealed with 20mm x 3mm steel strips between the sheets and on the outsides. One outer steel strip is actually 20mm x 20mm x 3mm angle iron, to provide rigidity. I also used Galvanized Iron Sealer (a silicone-based product) to help seal against leaks.

The entire top surface of the panel is painted matt black, to increase the absorption of heat. (Black is the best colour for this)

The inlet and outlet pipes are 20mm galvanized steel, with steel mounting strips welded on both sides. The steel sheet was bent around the pipe, and bolted in place with plenty of sealer to seal the (inevitable) gaps.

The entire panel is enclosed in a wooden box made from 100 x 50 treated timber framing, with thick cardboard heat insulation under the panel. (I haven't drawn this - it's fairly straightforward for anyone with a few nails, a hammer and a saw!)

This box is covered with a layer of clear polythene plastic film, tacked down on the outside edges of the box. The inlet and outlet pipes are connected by short pieces of rubber hose to lengths of 20mm alkathene (black plastic) pipe, to connect to the spa pool inlet and outlet.


The finished project works really well - there were a couple of minor leaks, but those were sealed by sloshing a litre of acrylic paint around the inside of the panel. (Dried by using a vacuum cleaner in "blow" mode...)

In Summer, if we fill the pool with COLD water (15 degrees), and leave it for three days, (full sunshine), it's too hot to get into! Then we add cold water every night and use it for about a week or ten days (until it gets too disgusting to get into!), then we empty it, wash it out, and fill it up again!

I worked out the overall efficiency the other day, and was astounded! It's about 50% (give or take 10% or so). Assuming 1 kW per square metre solar radiation, and 2.8 sq m total surface area, and 4 hours per day of full sun, the amount of heat which falls on the panel from the sun is about twice the amount of heat which actually gets into the water (measured as the temperature rise for a volume of 600 litres over 3 days - the arithmetic is fairly basic - I can send you a copy if you ask for it.) This means that we're getting an effective heating power of around 1.5kW in the Summer.

The WINTER was the really telling time, though... The results are not so good, mostly because of the amount of heat lost overnight, and because of the fact that there is no insulation under the bottom of the solar panel. At the end of each day, we lose as much heat as we gain, resulting in water which gets up to about 30 degrees (not quite hot enough for a comfortable soak), so we add hot water from our larger storage system (see my Thermonuclear Power project ... ), or we fire up a small chip heater which is connected to the spa pool, and bring the temperature up to 40 degrees that way.

A WORD OF CAUTION!
I didn't realise how careful I had to be about PRESSURE! The head of water (measured from the top of the spa pool to the bottom of the collector panel) is about 1.5 metres, and when I filled it up, the galvanized steel sheets BULGED alarmingly! The whole panel is as tight as a drum, and is visibly straining at the bolts which space the sheets apart.

A friend of mine worked out that each bolt is holding an area of 100 square inches, and that if I'd set it up with the panel at an angle of 55 degrees (as originally intended) we'd have had a head of 3 metres, and there would have been a force of about 250 kg on each bolt! So I made sure that the head was as small as possible, but at the same time ensuring that the TOP of the collector panel was BELOW the BOTTOM of the spa pool. (This is to prevent reverse-syphoning at night, which would cool the whole thing down - not a good thing...)

POSTSCRIPT:
Now that this project has been going for a few years, it's clear to me that this type of solar panel isn't really adequate for the task. It's too small (3 sq.m.), and to heat up 600 litres of water in a reasonable time, I really need more than the 1.5kW of power which it provides. If I did it again, I'd build a 15 sq.m. panel with a couple of hundred metres of black polypipe as the collector. This type of panel is around 25% efficient, so will provide almost 4kW of power. The one which I built on our cottage roof would work just fine, although it would need a small recirculating pump - thermosyphon is NOT an option with the thin polypipe collector. Cost? Around $100 for the pipe, $100 for the pump, and $100 for the framing and fittings. Not much more than I spent on the 3 sq.m. panel originally. And it will last MUCH longer, I'm sure.
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