expected temperature rise on new solar system

[dwichman]

Just installed 6 Aquasol 4x12' panels on the roof for my 14x36' IG kidney shaped pool. Want to make sure they're doing their job and don't have any air pockets, blockages, etc. The panels are on a south facing roof and at about 40-42 GPM I'm getting a 4 degree rise in the temp entering and exiting the panels during full sun, 80 -> 84 degrees. I assume there are formulas for calculating what I should get based on the BTU ratings of the panels which are 47K BTU/day, but couldn't quite figure out how to calculate based on instantaneous temperature readings like this.

Any advice?

Thanks.

 

[mas985]

40 GPM * 8.34 lbs/g * 1 BTU / (lbs - F) * 4F = 1334.4 BTU / Min ~ 80k BTU/hr

47k BTU/Day must be for one of the 4x12' panels at about 980 BTU/Day/sq-ft which is close to the standard average rate. The total output would be 282 BTU/Day. Note that the hourly rate will change during the day so it may look like you are doing better than the daily average. What is the temperature rise over the course of a day?

 

[Molson]

I have a 16x40 IG pool, also have 160 sq feet of panels on a south roof. I see 4-6 degrees a day on hot days. we had a hot spell for a week a while back, I had to dial down the solar has we were creeping towards 90 degrees.

 

[dwichman]

Temperature rise over the day is about 5 degrees, but we did have some slight haze and mixed clouds, although mostly sunny.

Could have also been 3 degrees change in temp as there's probably some margin of error in my method of reading the pool thermometer and assuming that as solar inlet, and the lcd on the heater that is after the solar outlet and assuming that's the delta T.

So based on that formula and a 3-4 degree change between the inlet/outlet, that would be 480K-640K BTU over 8 hours. But as you said that was high noon in full sun, so the real total should look more like 288K when averaged.

Is it more accurate to calculate # of gallons and degree rise in pool temp over the day?

 

[JohnT]

You've got to realize that the pool itself is a large solar collector and the temperature would have risen even without the panels. The only way to estimate the solar gain is to measure GPM and delta T, but that will vary from minute to minute with clouds, wind, air temperature, sun angle and water temperature. If you are concerned about air in the panels, get an infrared thermometer and get up there and go over them. Air pockets will be hot compared to the rest of the panels.

 

[mas985]

THe IR thermometer is a great idea!

80k BTU/hr is probably too much as the maximum raditiation from the sun is about 300 BTU/Hr/sq-ft or 86k BTU/hr so your panels would have to be close to 97% efficiency which is unlikely. If the temperature difference was 3.5 degrees, the output would be about 70k BTU/hr and about 81% efficient which more typical. So I think you are right in the ballpark of where it should be.

 

[dwichman]

Got a hold of a infrared thermometer from a friend, what a cool tool. Think I'll need to get one of these for my own entertainment...er, use. The collector surface temps were pretty consistent, within about 2 degrees of each other. At the time 92-94 near the top header, and the roof itself was about 145. Naturally the bottom of the panels (inlet) were a few degrees cooler. So it sounds like they're flowing pretty well.

I was concerned because the roof on our 1970 home sags a bit in the middle, probably about 1" over a ~30ft span, and I thought that I might have an air pocket in the top header opposite the outlet. I did position the anchor point for this side about 1.5" lower measured from the ridge of the roof as instructed. I guess a combination of that and the 2" header is probably allowing the panels to flow well across the top despite the slightly sagging roof.

Thanks for all the info. I'm happy now that I don't have to go back up on the roof and try and reposition them. I about died the first time putting these up in 85 degree weather!