We have a 4.88m x 2.74m x 1.22m (13m³) Bestway swimming pool in the backyard. We only use the pool when the pool water temperature is > 25°C, but it only happens if there are intensive sunshine and high ambient temperatures for a few days in a row.
To speed things up and extend the pool usage time, I decided to create a solar heater using 16mm LDPE pipes. These are typically used in irrigation systems and are UV resistant, black in color and with thin walls thus ideal to heat up water. I purchased two rolls 100m each, totaling 200m and an additional 25m garden hose to connect to the pool. I bought a second-hand Grundfos circulation pump and curled up the black irrigation pipes on the roof of a nearby tool shack. The shack’s roof is covered with bitumen tiles, these heat up quite a lot under the sun, contributing to the heating effect. That is a quick and dirty way, it would be much better to spiral the pipe on some holder structure, but I admit that I was just too lazy and the pipes aren’t visible from the ground thus they posed no aesthetical annoyance.
That would pretty much do, but what about doing some measurements to assess this solution’s effectiveness? To provide metrics I hooked up three wireless temperature sensors: one measures air temperature, second measures pool temperature and the third measures the solar heated water temperature. A relay powers the circulation pump on and off based on a schedule.
To measure the harvested solar energy, I use the formula for volumetric flow rate in a heating system:
q = h / (cp ρ Δt)
q = volumetric flow rate (m³/s) h = heat flow rate (kJ/s, kW) cp = specific heat (kJ/kg°C) ρ = density (kg/m³) Δt = temperature difference (°C)
The circulation pump manages to pump constant 1.55 l of water per minute (q).
The specific heat of water (cp) is assumed to 4.186 kJ/kg°C for the temperature range I anticipate.
The density of water (ρ) is averaged to 998 kg/m³ for the temperature ranges I anticipate.
The temperature difference (Δt) will be provided by the difference of readings of the solar heated water and the pool temperature.
All of the above values can fluctuate based on temperature, pressure, water consistency and other factors, so we assume approximate values.
So solving the above equation for h with the constants above gives us:
h = q (cp ρ Δt) h = (4.186 kJ/kg °C)*(998 kg/m³)*(1.55 liter)*(1/1000 m³/liter)*(Δt °C)/((1 min)*(60 s/min)) h = (4.186)*(998)*(1.55 )*(1/1000)*(Δt) / ((1)*(60)) h = 0.107922 Δt
So for example, if the pool temperature is 20°C and the solar heated water temperature is 45°C, the momentary heat flow rate can be calculated as
h = 0.107922 * (45°C-20°C) = 2.69805 kW
On a sunny and hot day, the solar heated water can easily reach 55°C.
I record the metrics and calculated harvest using emoncms. I have a daily kWh feed that summarises the harvested energy by day and will have calculations on the savings compared to electrical heating at the end of the season. Currently, the system yields roughly 10kWh per day.
Below is a live dashboard:
• 200m 16mm LDPE pipes – 40 EUR
• 25m garden hose – 25 EUR
• Second-hand Grundfos circulation pump – 30 EUR
• Misc connectors, adaptors etc- 20 EUR
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