About This Project
Wife and I had an adult “kiddie” pool last year during the heat of the summer and we spent a lot of time in it during the months of July and August. We upgraded to a larger pool (12’ in diameter, 2500 gallons/9600 L). But living at a northern latitude means keeping the pool warm enough to be comfortable anytime (shoulder season, or after a cool spell) and temperature was going to determine how many times we used it. So I thought of a solar pool project and here’s what I came up with.
- Raspberry Pi 3
- water temperature sensors x 4 (DS18B20)
- temp/atmospheric pressure sensor (BPM180)
- light sensor (TSL2561)
- power relay switch
- indicator LED
- ABS pipe and custom solar box
Schedules, Triggers & Alerts
- Daily pump on and off to circulate water for 2 hours regardless of available sunshine.
- If light sensor is above a specific lux value pump is turned on to circulate water through solar collector.
- If light sensor is below a specific lux value pump is turned off.
- Various email alerts for temperature targets.
- Lux light measurement. Had to cover the sensor with plastic garbage bag to keep the sensor from being overloaded. It has a nice range now of 1 through 1000.
- Ambient temp and atmospheric pressure. Read from behind the collector unit, much closer to what the pool environment is experiencing. Plus I can track pressure and do some rudimentary forecasting.
- Temp out and temp in are sensors sealed in the supply and return couplings of the collector.
- Top and Bottom temp are the in-pool sensors.
- Pump is the remote power switch for the pump and Power is an LED to indicate that the unit is powered and online just by looking at an external indicator LED.
Photos of the Project
Solar collector front - Cedar box, plexiglas front. 30’ of ABS pipe. Light sensor housing.
Solar collector back - Control box. Water in and out unions with temp sensors embedded. Light sensor feed. Sensor back is caulked with bathroom grade silicone sealant. Green power LED on side to indicate if unit is powered and online.
Early interior build of control box - Relay and power setup complete. Two temp sensors and ambient temp/pressure sensor. Relay controls bottom outlet for pump (fed through wire coupling). Upper outlet is constantly on for power to Pi.
Ambient temp/pressure sensor housing - fitted into PVC wire coupling and capped with multiple layers of wire screen (to keep bugs, etc out). Backside is caulked with bathroom grade silicone to keep interior of control box relatively sealed. Sits on the bottom of the control box to keep out water. Electronics are exposed to air so we’ll see just how long the sensor lasts (corrosion, etc.).
- Entire system is plugged into exterior grounded GFI outlet.
- 120V power isolated from RPi and sensors by sealed PVC outlet box and terminal strip connections are buried in hot glue.
Results & Future Considerations
In bright sunlight temperature differential of input and output is about 1/8th of a degree. Not amazing, but over a hot day this adds up and will assist in keeping the pool warmer for longer. During an early season heat wave we were able to get the pool to 29.5 C. Very comfortable with a beer in hand. Much hotter than our smaller non-solar pool the year before (max 26 C).
So far (1 month) interior of collection box and control box is dry and sealed after a couple of weeks of wet weather.
Next year I’ll add an additional solar panel in series to increase the heating output. Admittedly, the solar collector is not as efficient or capable as it could be for the volume of collection box. I may try different configurations.
Ideally, if the unit were on a steel pivot it would be easier to angel into the sunlight throughout the day (maybe automated).
Insulate supply & return pipes so the water does’t cool when the pump is off.
If and when Cayenne gets timed switches/triggers I would like to try cycling the pump during hot periods. If water is left to heat up in the pipes for even 10 min it gets substantially warmer.
One additional temp sensor in the collection box to see how efficiently the trapped heat turns into heated water.