Air Temperature Need For Effective Solar Heat

[Xndman]

Hi,
I have an above ground pool with 2 4x20 solar panels. Here in MD, the air temp has only been in the mid to low 60s, so naturally the sun has not been very hot. Only once out of the past week has the water returning from the solar heaters been warmer than the rest of the water. My question is how hot does the air and sun temp need to be before the solar heater will raise the temp of the pool > 72? If the outside air is only 65, is it crazy to think the pool could get anywhere near 72?

Thanks!!

[CrisL]

The temperature in the sun at the panels needs to be about 4-6 degrees warmer than the water temp for solar heating. Usually, on a sunny day, the solar panels will work even if the general air temp is a little colder than the water. This is because the black panels in the sun are a little warmer than general air temp. If the panels are on the roof, there will be an even bigger swing (the panels can work even if the general air temp is 5-10 degrees colder than the water because the temp on the roof in the sun may be 10-15 degrees warmer than the ground air temp). Of course, the warmer the air, the more punch you get out of the heater.

[mas985]

The only thing that really matters is the panel temperature and it needs to be above the pool temp to add heat. There is a correlation between panel temp and air temp but it also depends on air speed, humidity and other factors. I have seen panel temps lower than the air temperature (cloudy days) and up to 20 degrees hotter than the air temp so it is difficult to say exactly what you should expect. The best thing to do is put a thermometer on the panels to see what the temp is.

[Xndman]

One last question, I assume I should run the water through the heater on low speed (not high).

Thanks!!

[mas985]

Actually, heat transfer is better at higher flow since less heat is lost to the air when the panels are cooler. However, for energy reasons, you may still want to operate at low speed. If the panels are not excessively hot, then this is still efficient.

[StoshJ]

As was stated, there are many factors, but I can give me recent experience. I have 6 4x20 panels (It think 6).. Anyway, the temp out here in So Cal has been 78-81 the last week. Cloudy till noon. My pool is sitting at 88. Neighbors pool at 74 (no solar). So it's very possible to get the water temp above the ambient outside temp.

[Xndman]

In regard to the speed through the panels, I would have thought that the water flowing through the panels more quickly would have less time to "heat-up" from the panels. Also, I do have my panels on the ground. I assume they'll work better if I get them off the ground.

Thanks!!

[Xndman]

One last question. Does anyone have any thoughts regarding good materials to use to build a rack for 3 2x20 panels? I could always use plywood, but that would require an extremely large piece. Are there any other materials people have used? I'd like something I could angle up the side of the pool (like is often seen in pictures of above ground solar heaters).

Thanks!!

[mas985]

In regard to the speed through the panels, I would have thought that the water flowing through the panels more quickly would have less time to "heat-up" from the panels. Also, I do have my panels on the ground. I assume they'll work better if I get them off the ground.

Thanks!!

Maximizing heat transfer is key for a solar panel. According to thermodynamics, heat transfer is proportional to the temperature differential between surfaces or materials. Therefore, the higher the temperature difference between the water and the panel, the higher the rate of transfer (BTUs/hr) will be.

Another way to think of it is that if the water is moving too slowly in the panel, it will heat up to the panel temperature before leaving the panel. At this point it is not transfering any more heat to the water. Ideally, you want the water to leave the panel only slightly warmer than entering so that heat is continually transfered to the water at the highest rate. This why when the panel is 10 degrees warmer than the pool water, raising the pool temperature the first 5 degreees is much quicker than the last 5 degrees.

Having said all that, higher flow is only better for heat transfer. To lower your electric bill, you may still want to run at low speed. Even though the heat transfer is less, it may still be more than enough.

Also, too much flow can be damage a panel as well but that is usually at pressures exceeding 25 psi.