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re: heating a pool?
11 sep 1996
cathy woodgold  writes:
 
>i wonder whether painting the sides and bottom of the pool black
>would make more difference than some black hose etc.

we could just dye the water black... :-)

but most of the sun would be absorbed anyway, no?
here's a quote from duffie and beckman (1991), page 652:

  as radiation is transmitted through water, the longer wavelengths are
  absorbed near the surface. measurements by nielson (1988) indicate that
  little radiation at lambda > 0.7 micrometers is transmitted through a
  meter of clean water, but over 95% of the radiation in the wavelength
  range 0.3 to 0.6 micrometers is transmitted through this much water.

they also say solar ponds are of the order of 1 to 3 m deep.

table 2.6.1 of the book shows that about 40% of the energy in the terrestrial
solar spectrum has wavelengths < 0.7 micrometers. and an ordinary pool paint
might reflect half of the sun, and that reflection has to go back out of the
water again, giving the water a second chance to absorb it on the way out, so
maybe only 0.4x0.95x0.5x0.95 = 18% of the incoming solar energy escapes from
the pool, ignoring the 5% water-air reflections... 

>an expensive idea:  build the pool with three parts.

hmmm.

>the middle part is a regular pool. the other two parts are the same area,

why the same area? vot is the goal here?

>but only 4 inches deep, painted black.

why 4 inches deep? why not dark green, which can look very dark to the sun,
but bright to the eye, which is most sensitive to yellowish green? why does
the lord permit such suffering on earth, if he is omnipotent? what is truth?

>when the sun is not shining, drain some water from the pool so the
>shallow parts are empty.

to avoid losing heat at night...

>(how to get the warm water to mix into the rest of the pool, though?)

the filter pump?

>the shallow parts could be covered with bubble cover all the time, or never
>covered.

never seems simpler. if the shallow parts have some thermal mass (how much?),
and they are only covered with water once in a while (how often?) to remove
most of the collected and stored solar heat, evaporation loss won't be a lot.
how often should we spray a roof?
 
a square foot of 4" thick concrete weighing 48 pounds with a specific heat of
0.16 btu/f and a thermal capacity of 8 btu/f-ft^2 absorbing 300 btu/hr of sun
might heat up 40 f in an hour, if it lost no heat to the to the air or the
ground. suppose we covered it with 8 pounds (one gallon) of water per hour,
raising the water temp by something like 30 f. how long would the water have
to sit on the concrete, each hour? suppose the main thermal resistance is
the concrete, at about 0.2/inch, ie about 0.4 for half the slab thickness.
we want to transfer about 300 btu through a us r-value of 0.4 in t hours, with
an average temperature difference of 10 f? so, ohm's law for heatflow says
u = t delta t a/r, ie 300 = t (10) 1/0.4, so t = 12 hours. too long. what
to do? add lots of steel to the concrete to raise the thermal conductivity?

flood it more often, using less water each time, to remove the heat from
the surface, before it has a chance to propagate into the concrete? this
means more evaporation loss... a less porous surface like tile would help.

some people say thin-walled 4" pvc sewer pipe costs less than the volume of
concrete it displaces (but doesn't the slab have to be thicker, if it has
4" pipes inside?) maybe the water should flow through pipes in the slab.
maybe smaller pipes, eg polyethylene tubing, just inside a masonry wall
to the north of the pool, if we want to use the pool in the winter.

most pools have a shallow part already, a skirt, about 6' wide, with almost
the same area as a 24' x 32' pool surface. make it thinner, of reinforced
concrete? hard to do for existing pools. paint the skirt darker, and put some
pvc pipe with some holes in it around the outer perimeter, and add a solenoid
valve and controls to the filter pump to divert the flow to intermittently
wash the skirt with some pool water, if the pool were too cool and the skirt
were warm, washing dried cool-aid, cookie crumbs, etc., into the pool along
with the warm water. 

we might wash the skirt during the day to keep it cooler for feet, and
wash it at night sometimes, if the pool got too warm, automatically.

nick




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