Pool Solar Questions

I have a solar heater on the pool. We live in Las Vegas. It is hot here now, between 115 and 120 the last week.
My pool water is 94, an uncomfortable temperature.
I know that if I run my system at night, it will circulate the water through the collectors, and though night time temps are 90 right now, it should bring the temp down. Plus, not running it during the day will keep the circulation from going through the panels and picking up more heat.
My questions:
Does not having the water on and running through the panels in the day when it is hot damage them? It seems like they would get awful hot. When I cut down the flow to the panels, the water in there reaches 160 degrees. That can't be good.
Should I just run it night and day? To keep the panels from getting too hot during the day, and at night to help dissipate some heat?
We like the temp around 85, and that seems to be about 15-20 below ambient. But right now, ambient is so high that it comes out at 94. Normal temps for right now is 104.
Any suggestions or information by solar gurus would be appreciated.
Steve
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SteveB wrote:

The solar systems I see all have a thermostat to set the pool temperature. When the pool reaches the set point, the panels are bypassed. AFAIK, the temp in the panel then rises but does no harm. We have the system sold by United Solar and the pool temp is set by our Aqualink controller inside the house. For people without the Aqualink integrated system, United Solar provides a separate box that controls pool temps.
I was told that the United Solar panels are just fine without circulating water when being bypassed.
Nobody
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We are really considering a thermostat, but now they have to come and cut all the pipes to install one retro.
I am just looking for something to regulate it a bit before we have it changed out. And I didn't want to hurt the system in the meantime by running it dry.
Any idea how much a thermostat and installation cost?
Steve
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On Sun, 17 Jul 2005 11:50:31 -0700, "SteveB"

Call it a hot tub and it will be too cool.[g] My spa temp is set to hold 99 but the water is 102 here in Tucson.

You can actually cool to below air temperature at night, just as you can get water hotter than air temp in the sunlight. Trust me, I've had two different houses with solar hot water heating (neither of which I had installed, what a dumb idea) that I abandoned because of winter freeze ups when the air temp was above freezing. The clear night sky has a black body temperature very near absolute zero (0 Kelvin) so an Earthly black body collector at 0 deg C is 273 degrees -hotter- than absolute zero. It radiates energy back into space and cools off.

It depends on the construction but most panels -should be- designed to run in a stalled condition.

Run it at night.

Tell me about it. We set a new -high- "low" temp the other night: 89 F.

Not a guru, but did take a college course in solar heating and cooling a long time ago.
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A couple of nights ago, it was 101 at the 11:15 PM weathercast. I get up early. Been doing some wrought iron welding. I start at 0430 and kick off about 0900. At 0900, it was 100 degrees.
Every year I say I am going to get out of here next summer, but I never do.
Steve
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It's not that cold. There are approximate formulas for "effective sky temperature." Here's what Duffie and Beckman say on pp 157-8 of the 1991 edition of "Solar Engineering of Thermal Processes":
To predict the performance of solar collectors, it will be necessary to evaluate the radiation exchange between a surface and the sky. The sky can be considered as a black body at some equivalent sky temperature Ts... The net radiation from a surface with emittance E and temperature T to the sky at Ts is Q = EAs(T^4-Ts^4).
The equivalent blackbody sky temperature... accounts for the facts that the atmosphere is not at a uniform temperature and that the atmosphere radiates only in certain wavelength bands. The atmosphere is essentially transparent in the wavelength region from 8 to 14 um, but outside of this "window" the atmosphere has absorbing bands covering much of the IR spectrum. Several relations have been proposed to relate Ts for clear skies to measured meterological variables. Swinbank (1963) relates sky temperature to the local air temperature, Brunt (1932) relates sky temperature to the water vapor pressure, and Bliss (1961) relates sky temperature to the dew point temperature. Berdahl and Martin (1984) used extensive data from the United States to relate the effective sky temperature to the dew point temperature, dry bulb temperture, and hour from midnight t...
Ts = Ta[0.711+0.0056Tdp+0.000073Tdp^2+0.013cos(15t)]^0.25,
where Ts and Ta are in degrees Kelvin and Tdp is the dew point temperature in degrees Celsius. The experimental data covered a dew point range from -20 C to 30 C. The range of the difference between sky and air temperatures is from 5 C in a hot, moist climate to 30 C in a cold dry climate.
Clouds will tend to increase the sky temperature over that for a clear sky... the sky temperature is critical in evaluating radiative cooling as a passive cooling method.
Let's see. Philadelphia has an average daily minimum Ta = 67.2 F (= (67.2-32)/1.8 + 273 = 293 K) in July, with an average humidity ratio W = 0.0133 pounds of water per pound of dry air, which corresponds to a dew point of about 65 F (18 C), so at 6 AM, when cos(15x6) = 0,
Ts = 293[0.711+0.0056(18)+0.000073(18^2)]^0.25 = 280 K or 45 F.
Albuquerque, NM has a min Ta of 64.4 F (291 K) in July with W = 0.0092 corresponding to Tdp = 55 F or 12.8 C, so at 6 AM on an average day,
Ts = 291[0.711+0.0056(12.8)+0.000073(12.8^2)]^0.25 = 275 K or 35 F.
Nick
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wrote:

Don't know about all that. I ran my pool pump all night last night through the panels, and my pool only dropped two degrees.
Steve
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On 18 Jul 2005 11:02:43 -0400, snipped-for-privacy@ece.villanova.edu wrote:

You are correct of course. My radio frequency background (no pun intended) was in play. Even there, there are absorption bands but the quiet sky (away from radio sources) is very cold.
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On Sun, 17 Jul 2005 11:50:31 -0700, "SteveB"

Lucky you....My pool is UP to 84 today.... BUT the few times it got up much higher it sure was not refreshing to jump in.... IF my pool got up to the high 80's it would be completely useless ...
In you case I would install a BYPASS valve and not run the system at all....until the water temp drops down ...a heck of a lot... if needed drain the collectors...
I used a couple of lawn springlers connected to my returns (Rube Goldberg connections...since I was just just experimenting .. BUT running the sprinklers during the night did lower the water Temp quite a bit... about 6-7 degrees ...BUT with the air temp in the 90's at night...you will not drop it any lower then that....
Bob Griffiths
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hmmm.. i would have though evaporative effects might cool it further.
Is it possible to shade the solar collectors?
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be safe.
flip
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