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Looking into the options of adding and retaining heat in the outdoor pool (and saving wallet) - heat inverter, roof solar water heater, gas heaters etc. the pool cover seems to rule them all (at least here in Australia).

There is a very nice (though pretty old - 1978) paper by CSIRO on this topic:

SWIMMING POOL HEATING BY SOLAR ENERGY

This paper very well proves the efficiency of solar covers (blankets) in retaining heat, that part is very clear. What I am not able to figure out from this paper, and could not find answer to anywhere else is:

From heat absorption and retention point of view, is it better to keep cover (blanket) on through the day or take it off?

Please if you are going to answer this question in other way than finding/recommending paper on this topic, consider there are many factors like day vs water temperature, sunny vs cloudy, humidity, wind, clear (opaque) solar blanket vs dark vs two layered (top layer clear, bottom layer), so you most likely will need to pick some most common scenario unless you need to cover all the factors and their combination of effects in general.

E.g. sunny day in Sydney, air temperature between 25° - 30°, water temperature 20°, standard humidity, two layered blanket with top layer transparent (light blue, bottom opaque silver) - for some reason this is the combination I see the most in e-Shops.

Also there seems to be a lot of bias about it including claims like bubbles work like magnifying glass so they transfer more heat from sunlight into the pool, or that double layered (combination of transparent layer over opaque bottom layer) absorbs more heat into the water than fully transparent layer which allows most sunlight go through into the water - which seems highly unlikely to me.

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I have invested more time into research on this topic, and the conclusion is that generally it is better to keep cover on all the time when pool is not used. Explanation:

  • Outdoor pools can gain a significant amount of heat from the sun, absorbing 75 to 85% of the solar energy striking the pool surface.

  • Transparent bubble cover may reduce solar energy absorption by as little as 5%, dark cover can reduce it by as much as 40%.

  • Almost all of a pool’s heat loss - about 95% - occurs at the surface, about 70% through evaporation, 9% radiation to the sky, 20% convective loss mostly to the air

Pool blankets (covers) depending on many factors block efficiently evaporation, standard is over 90%, often above 95%. Reducing heat loss through the day by keeping blanket on - blocking evaporation (and other effects), outweighs the increased effect of heat absorption from sun light under absolute majority of conditions. Especially in cases of transparent blankets where the difference of absorption with and without blanket is only few percent, while difference in heat loss by evaporation is under most weather conditions significantly higher than that.

Plus keeping cover on avoids possible accidental flows of water over cover when manipulating with cover causing increased evaporation and heat loss even when cover is on - until water above cover evaporates. And well, it is less work too. So keeping cover on it is.

References:

Czarnecki, J T, (1978) Swimming Pool Heating by Solar Energy, CSIRO Division of Mechanical Engineering Technical Report No TR 19

Conserving Energy and Heating Your Swimming Pool with Solar Energy, US Department of Energy, National Renewable Energy Laboratory, July 2000

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