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re: well when should i take my pool cover off??!!
7 oct 1996
ken tait  wrote:
>john.harrison@mailbox.uq.oz.au (john harrison) wrote:

>... hope this reply does not bring on the rath of god (np)...

he (god) is a merciful god, but he does not suffer fools gladly.
especially they who purveyeth, and acteth as if they knoweth,
but knoweth not, and will not accepteth truth.

>i have been following this group for a year now and frankly have been 
>intimidated to such a point that i thought it best not to participate.

welcome to the group :-) i've been told it can be a good idea to keep one's
mouth closed and not reveal one's ignorance. i'm still learning about this.

>>i am still not sure of the facts about solar blankets for the pool.
 
perhaps you have been talking with experts?

>>obviously i leave it on at night. 
 
>yes an no! 

yes, i'd suggest. but hey, i'm not an solar pool expert.

>>when the sun comes up and starts to warm the air, ground, and water at
>>what point should i take the cover off to let the rays do their work
>>directly on the water, or does the evaporation always suck out more
>>energy then can be provided directly

i'd guess that almost always happens.

>(meaning i only take the cover off to actually swim!)

good idea.

>the heat load or loss on a pool is affected by many variables.  water
>temp., air temp., ground temp., water table level, humidity, wind
>speed, construction of pool, type of pool (above, on or in ground)
>etc., etc.
 
since you are a p. ing, ken, perhaps you could easily come up with a
precise calculation indicating when it would make sense to leave an
r1 transparent pool cover off the pool, when the sun is shining.

>and of course most of these are changing varibles depending on time of
>day, time of season, climate, weather etc.  so its quite clear, there
>is no real answer to your question.

from a thermal point of view, it's clear to me: unless the pool is too hot,
leave the cover on, except for swimming. with insignificant exceptions.
also i'm beginning to think that solar pool experts can't spell.
do solar pool experts have silly walks (tm) too?

>common sense will have to prevail.

and perhaps a little arithmetic?

>a pool blanket reduces heat loss by   a) reducing evaporation.

enormously, in most cases. here's one version of a formula for the ratio
of heat loss by convection to the heat loss by evaporation, for a pond,
independent of windspeed, from page 664 of duffie and beckman's 1991
_solar engineering of thermal processes_:

qc   0.001(tp-ta)     where tp and ta are pond and ambient temps (f) and
-- = ------------           pwp and pa are vapor pressures at the pond
qe     pwp - pa             surface and ambient vapor pressure in inches
                            of mercury, 29.6" being 1 atmosphere. 

what else? clausius-clapeyron: ln vp = 17.863-9621/tabsolute,
and humidity ratio w = 0.62198 pw/(p-pw). and qe is roughly 
(tp-ta)(2+v/2) btu/hr-ft^2 with a windspeed of v mph. and a
clear sky can suck heaps o' radiative heat out of a pool.

>and   b) reducing heat transfer out of the water (to the air)

about as much as a single pane window reduces heat loss from a house,
for small temperature differences, ie not much, as we have discussed. 

>a pool blanket reduces heat gain by   c) shading the pool.

it might "shade" the pool by reflecting and absorbing 10% or 20% of the sun
that falls on the cover, depending on the color (blue being a stupid color,
i'd guess) and glazing thickness and material. some of the reflected solar
power is lost to the sky, and some eventually ends up in the pool, likewise
absorbed power. so we might take the pool cover off when the sun is shining
if we believed that the increased gain (30 btu/hr-ft^2, ie 10% of the peak
solar input?) would be more than the 1 btu/hr-ft^2-f increased loss by r1
conduction and evaporation (which might be much more than that.) 

>examples:  assume pool is at 80f.

with what air temp (80 f?) and humidity ratio (0.0133, like philadelphia,
in july?), and how much sun, and clouds and wind? lacking more numbers,
this example doesn't make much sense to me.

>	its raining and cloudy		-on

is the air warmer than the pool? does it rain much when it is not cloudy? 
are there people in the pool? ("look out, here comes ken, trying to save
more btus..." :-)

>	its sunny, warm and no wind	-off

this would hardly ever make sense, i'd guess. 

>	its sunny, hot and humid, but thunder
>	 showers called for all afternoon 	-god knows?

god might say:

         leave the cover off, if the rh is 100%, and
	 the air is at least as warm as the pool.
         and get out of eden, for dumbth.

>>is there some quirky relationship between air temp and water heat
>>transfer that will enable me to know when its beneficial to take the
>>cover off at other times?

i think so, but it seems to me that doesn't matter,
since those conditions hardly ever happen...

>now the answer to your first assumption. --"blanket on at night".

"a no-brainer," god might say, muttering something about earthworms
being able to figure that out, and wondering why he gave us ascii.

>a side effect of a pool blanket is that it screws up the water flow in
>the pool, particularly at the surface.  this causes stagnant areas
>that breed algae and bacteria.  bubble pack covers are the worst for
>this.  leaving the blanket on all the time is not good!!!

perhaps the answer to this is 5 parts per billion of cu ion, via a smidge of
cuso4 in the water (which turns blonde hair green in larger quantities, and
causes human death in larger ones) or an aqua-air cu/ag ion electrolyzer,
((410) 489-5288) or a small air bubbler under the cover. (god: "and don't
pee in the pool.")

>so another judgement call.  if you leave the cover off at night you
>will lose tons of heat.  i mean tons, were talking millions of btu's.

unless it's a warm humid night, in which case the pool might gain heat.
millions of btus, even, on a really hot night.

>you will either have a cold unusable or at least unpleasant pool if
>you dont heat it, or a costly proposition if you heat it with oil,
>gas, elec. 

or a solar pool heater?

>however if your smart and heat it with solar, who cares if the cover
>is off or not.

right, who cares, if it's solar heat! solar heat is special heat. it never
leaves the pool! solar btus are different from oil or gas btus. they are
special permanent btus. and commercial solar pool heaters are inexpensive,
and they will make you feel ecological _all over_!

>the heat you lose at night will go back in the pool costing you nothing,

solar is truly magical! pool size, weather, covers, none of those matter,
if you have enough solar pool heaters! now where was that order form...?

>limiting the algea problems and reducing the
>cover on, off, on, off--you know what i mean-- problem. 

right...

>additional
>advantages are that you wear out the cover less and it will last
>longer and of course all the tons of heat are now in your pool and not
>in your attic so if you have ac your saving again.

makes perfect sense.

>when the variables
>are such that the loss is bigger than you can gain during the day,
>start using the cover.

that almost makes sense...

>i hope this was simple because it sure was long---sorry about that.

it was a long response, ken, but it was also obtuse. 

nick



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