Solar Pool Heater

This started out as an idea to take the edge off the cool water in our small raised swimming pool. It's an Intex EasySet 8' diameter, above ground pool.

Pool Volume

Volume = πr² x depth = 3.142 x 1.2 x 1.2 x 0.55 = 2.5m³
This has a mass of about 2.5 million grams.

Solar Energy

The maximum heat input collected from the sun is down to the size of the collector which is 1.2m². So potential heat input is 1.2 x 1.2 x 1.3KW/m², so approximately 1.9kw. Allowing for inefficiencies, this is probably a usable 1KW (being pessimistic).

Heat capacity of water is 4.2 Joule/gram per °C, so the heat required to raise pool by 1 °C is 2.5 million (grams) x 4.2 = 10.5 million Joules

The heat input of 1KW equates to 1000 Joules/second.

So time to raise pool 1°C = 10.5 million / 1000 = 10500 seconds = 175 minutes (~3 hours). So over the course of a sunny day, the pool heater should raise the temperature noticably.

Construction

The solar collector is made from a 1.6mm thick sheet of steel, which is 1.2m². To this I've attached a 25' coil of 8mm copper pipe, which forms a spiral. Cool water is fed in at the top and spirals towards the centre of the collector, getting warmer as it does so. It then exits through the back of the collector and back into the pool.

The collector is mounted in an insulated wooden case. It will have a glass front soon.

A 24V pond pump is used to pump water through the collector and this is powered by two 12V, 5W photovoltaic panels, connected in series.

Results

So far I've run the solar heater with no glass sheet on the front and on a fairly windy day, so I expect it to be much more efficient once the glass is added. I started it off at 11am and the pool temperature was 18.1ºC. The temperature of the water leaving the collector varied depending on the strength of the sunlight but was exceeding 27ºC. The collector surface was getting hotter than my digital thermometer could read (maximum is 50ºC).

By 4.30pm the water temperature was up at 21.4ºC, which is a pretty good increase. Where the pipe leaves the collector, I had to bend the pipe at quite a tight radius and its a bit constricted. These is reducing the flow slightly, so I plan to fix this later on.

I'm running the pump using a mains PSU as it is 24Vac and I've yet to find a solar cell that outputs ac voltages :-). I'll see if I can find a 12Vdc pump later on, to run via solar cells.