DIY Solar Heater Performance
Updated: May 6, 2019
Wave Energy is a solar power (Solar PV) consulting company located in Central Florida. For more information on how to get started with solar power, START HERE.
There are quite a few good DIY solar pool heater videos already out there, but what I wanted was some solid data on the actual performance of these Do-it-yourself solar heaters and then compare those results to solar heaters available commercially. The video provides some useful talking points on the system and below you will also find the calculation sheet, formulas, and references.
For a good video on the DIY Solar Heater build, click here. His is a much cleaner design, better job on the acrylic cover, and overall more efficient build but overall surface area to water is less. I used 1/2 inch tubing rather than 3/4.
The test was performed over a period of 5 hours on 1/31/2017. With 3 out of 5 of those hours being peak sun, 2 being partial shade or cloud. It was cool out and the pool temp was near 57 degrees at the start of the test.
I used a digital thermometer to check water temperature of the pool and the outlet from the solar array. I also used a La Crosse Alert digital thermometer and humidity sensor to check ambient air temperature inside the array using a probe and a sensor outside as reference. With no water flowing through the heater, the ambient air heated up somewhere past 140 degrees inside (The probe stopped recording due to high temp). With water flowing through the heater, the probes max temp reached 106 degrees.
I used a submersible 1/5 HP Utility pump made by Superior Pump, and 1/2 inch rainbird irrigation semi-rigid poly tubing. The tubing is clamped in a circle within a black box (as much as possible ensuring as much surface area as possible is exposed to sunlight), and the box was covered with 1/16 inch clear acrylic using 2 2x4' sections. 1 4x4' section would have been better, but couldn't source locally. The flow rate was a steady 2.5 GPM. Measurements were taken in 30 minute increments
To calculate the performance you first need to understand the units of measure being used. BTUs or British Thermal Units, is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. A gallon of water weighs 8.33 lbs. Since to measure energy delivered, we need an interval of time, we will use the BTUH, or BTUs delivered in an hour interval.
(60 X 8.33 X GPM X DeltaT) or shortened: (500 X gpm X DeltaT)
60 minutes X Weight of 1 Gallon of Water X Flow Rate in GPM X (The difference in Inlet Temp vs Outlet Temp)
To determine GPM you can use the GPM chart that comes with your pump and then calculate in the discharge height, resistance of tubing, friction, diameter and length of tubing or you can simply take a measurement: Time how long it takes to fill up a gallon container. Repeat a few times and then take the average. Divide 60 by the number you got. That is your GPM.
The following spreadsheet has some of these calculations already built into the cell formulas. Feel free to use and modify as needed. We also calculated how long to raise the temperature by one degree and how long it would take to raise the entire pool volume to 80 degrees in a 10,000 gallon pool. I did not calculate in any thermal conductive loss or gain from the surface area of the pool and air temperature/wind. Generally, if its windy and cooler, you will lose heat, if its warmer and very little movement, you could gain energy. Even your pool jets breaking the surface can impact this number.
At its peak, the solar heater generated a DeltaT of 4 degrees with an average DeltaT of 3.2 degrees. MAX BTUs delivered were 5000 with an average of 3986 BTUS delivered. This means the system delivered, on average 250 BTUs a sq/ft.
For comparison, we used the Heliocol HC-50. It's one of the higher rated commercially available systems on the market and it also provided solid datasheets for reference. Click here for the reference data.
The heliocol HC-50 is 4' X 12.5' while this DIY solar heater is only 4' X 4', or three times smaller. We can assume if we ran continuous tubing through 3 arrays, the DeltaT Max would be 12 degrees Fahrenheit and our results would be around 750 BTUs sq/ft. Not quite as good as the HC-50's 956 BTUs sq/ft, but not bad for a garden hose timber block design. With some modifications, updates and better materials this greenhouse design could easily outperform the typical roof top solar design available commercially.
Now with that said, this raised another question:
What if we used an electric heater that was powered by Solar PV? How much roof top solar would it take?
For this, I used a Eco-Smart 11kW tankless water heater. Small unit, but it converts 99.8% of the electricity consumed into heat. This system averaged around 45,000 BTUs. This system heated the entire volume of water, exposed to the elements, 16 degrees in just under 48 hours.
My initial assessment was that this heater system could be powered by 3 standard 300W solar panels enough to heat the pool this much 4-5 times a year. What was not calculated in was thermal conductive loss between the pool and the air. In fact, it would take a little longer and more than 1800 kWhs annually. We will review performance of systems designed ot heat pools and comapre to see if we can get more efficiency from an electric heater powered by solar PV vs rooftop solar pool heaters.