Wednesday, August 7, 2013

Solar pool heating

"Rushforth Solar" <ar@...> wrote:

> I would think your quickest cheapest pool heater would be covering the pool
use with black plastic.

Clear plastic works better, since it's hard to warm water from above.

For one thing, warm water rises.

For another,

Tr = (1/a)(2s/Pi)^2ln[8(Ts-Ta)/(Pi^2(Ta-Tb)],

(dividing by (Ta-Tb) on the right), where

Tr = time to obtain average temp Tb
a = thermal diffusivity
s = slab thickness
Ts = constant surface slab temp
Ta = initial slab temp
Tb = average slab temp at time Tr

In their example, warming a 2" air slab would take 25.3 seconds. says water has a 0.143x10^-6
m^2/s thermal diffusivity,

So warming a 2" water slab would take 3920 seconds (65 minutes.)

And warming a 60" water slab would take 980 hours.