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Cooling rate of still pool vs circulating pool? 1

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Tomfh

Tomfh

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Feb 27, 2005
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I have a 50,000L swimming pool with rooftop solar heating.

Im trying to understand the rate at which it cools overnight with the pump on, compared to off.

I’m running a variable speed pump and tossing up between 24 hr running on very low flow, vs 8-12 hrs at medium to high speed during the day.

Running overnight would presumably mean more heat loss, reducing heating efficiency.

Supposing the water is 28 degrees C at the end of the day, and average night time temperature is say 15C, how much heat loss difference between still water and circulating water?

Edit: there are two pumps. The second pump, the solar boost pump, only comes on when the roof is hotter than the pool water.

 
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Measure for and inlet and outlet temporaries through your roof units.

Our measure delta T and as soon as it goes negative, turn the pump off.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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The temp drop overnight probably will depend more on how clear the weather is than the temp.
If the night is clear then the radiant cooling effects will likely dominate over the simple convective cooling.
Running at low speed overnight likely helps keep the system primed and prevent air entrapment.
It might be worth putting a couple of sensors in the system (RDTs or similar) so that you can see the inlet and outlet temps.
Then you would actually know.


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P.E. Metallurgy, consulting work welcomed
 
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Seconded on radiant losses. Somewhere I read that water will freeze in the desert at temps above freezing due to the higher loss to the open, unblocked sky than is made up for from heat conveyed by the air. A search turned this up:
Pulling an insulated cover over the pool would do better to retain heat.

Even without the pumps natural circulation will happen as conductively and evaporatively cooled water will sink and still-warm water will rise, creating convective cells.
 
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Littleinch said:
Measure for and inlet and outlet temporaries through your roof units.

Our measure delta T and as soon as it goes negative, turn the pump off.
Click to expand...

I have two pumps. The solar boost pump only circulates via the roof if the the roof is hotter.

Im talking about the main pump. Will circulating these pool water cause faster heat loss than if the pool was static overnight.
 
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3DD, We used to dig a hole in the ground (12" dia x 18" deep), put in a container of water down in it set up on rocks off of the ground. Leave it uncovered at night and cover it during the day. With 40F at night, 60F daytime, and clear night skies you would have a container of slush in 4 or 5 nights.

Tom, I believe that any conductive/convective losses will be greatly outweighed by radiant cooling effects.

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P.E. Metallurgy, consulting work welcomed
 
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That’s really interesting about the freezing water. I have a thermal camera and I’ve noticed how cold the sky is. Often well below zero C.

Will the radiative heat loss be largely unaffected by whether the water is circulating?
 
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Those are two separate things. Heat loss for a pool is a combination of radiation, convection, and conduction
> radiation -- in to the cold, cold, sky
> convection -- into the ambient air
> conduction -- into the ground

In all actual heat loss mechanisms, the loss is from the edges of the volumetric surface boundaries of the pool, so the remaining heat must conduct/convect through the volume of the water to get to the outer surface. Pumping circulates the water, so ostensibly, warmer water will reach the outer surface quicker, thereby increasing the heat loss. However, water is fairly efficient at conduction and convection, so it might be a tiny difference. Note that convection in air is less efficient than convection in water, so that interface is not constrained by the water.

TTFN (ta ta for now)
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IRstuff said:
Pumping circulates the water, so ostensibly, warmer water will reach the outer surface quicker, thereby increasing the heat loss.
Click to expand...

That’s what I was imagining. The pumps helping to transport the internal heat to the outside of the “block” of water, allowing it to escape more easily, as opposed to the heat having to work its way out to the edges.
 
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Well your pool will be circulating naturally as cooler water at the surface becomes denser and sinks to the bottom. The flow of that is likely to be similar or larger than your pump flow.

Quite variable dependant on cloud cover.

But sounds like a good excuse for a set of experiments...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
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You really do not have a pool cover? That would probably prevent 80% of the losses.

IR; There are four methods of pool heat loss, you left out refrigeration! Evaporation hauls away a huge amount of the heat. (Blocked by the most basic pool cover!)

Keith Cress
kcress -
 
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We used a floating 'bubble wrap' cover for years.
Had an Al pole across the pool to roll it up.
Easy and great for trapping heat, saving water, and saving chemicals.

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Get the pool cover.
When these guys do the model, they assume turnover via pump or natural convection, uniform temp and densities.

In my scuba experience water in lakes in moderate to warm climates tends to stratify with a marked thermocline at around 25ft depth transitioning to a temperature equal to the average yearly earth temperature below 40ft depth. Above 20ft its close to average monthly temperature. Typical pools are not that deep, so uniform temperature of a pool should be a reasonable assumption.

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For 16 different combinations of sheltering, shading, landscaping and use of solar blankets and solar pool heaters, the results show that the major energy loss from pools is by evaporation during the day and by long wave radiation by night, and these losses can be effectively cut and hence water temperature raised by decreasing the wind, or using solar blankets or solar pool heaters. In the summer climate of Toronto, the best of these methods could increase maximum pool temperature by 3 or 5 K and lengthen the comfortable swimming season from an average of 40 days to 70 or 80 days.

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