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re: shaded ponds for cooling houses
11 aug 1998
t. postel  wrote:

>water in a calm body is always considered a smooth surface.
>if the suface is made rougher, then whatever wind is blowing
>will tend to drag the surface along and reduce the relative wind velocity.  

i guess that wouldn't happen if the extra surface were reeds.
they could also provide shading and help prevent loss of coolth
by wind convection from the pond surface to the warm air during
the day. circulate pond water through a heat exchanger in the house
during the day, and run it up over the roof or "cascade" at night? 

a 12x12x4' deep pond of 70 f water that supplies 40k btu/h for 16
hours a day to a house would gain 640k btu/day. with 12x12x4x64
= 37k btu/f, the pond temperature would rise 17 f during the day,
not counting daytime evaporation. i guess evaporation from reeds
just cools the surrounding air, vs cooling the pond directly, so
a high bank or rim board at the level of the plant tops might help
trap the cool air and keep that pond/bog/artificial wetland cooler. 

>>currents and waves reduce evaporation? 
>yes, it is the same as in convection.  if the body of water
>is large enough to have wind generated waves then the wind
>is moving the water, so the velocity of the wind relative
>to the water is reduced.

i see your point. makes sense to me...

>...evaporative cooling can be increased by increasing the surface area
>and by increasing the relative wind velocity.  the most common solution
>is to use a vertical spray.  the water can be made into a fine mist,
>and the wind can be taken as the speed of the droplets... 

well, that's a guaranteed wind. how much wind? v = sqrt(2(32)10) = 25
feet per second or 17 mph, when the droplets hit the pool? what size 
droplets? what's the heat loss from each droplet? how much by convection
and how much by evaporation?

but that takes more energy, vs very low-power cooling solutions
with a very high cop... 

>i have no sources for figuring cooling spray efficiency...

i guess they are around, eg ashrae. baruch givoni gave a paper
describing the performance of a cooling tower with downward natural
air convection in an arid climate at the '96 world renewable energy
congress in denver. 

>...i have found an estimate that natural cascades lose about 3%
>of their flow to evaporative heat loss in normal daytime conditions
>per 100 feet of vertical drop.

can this be more precise? what shape is the "cascade," 100:1 w/t?
what are "normal daytime conditions," 4 am on an average july night
in abilene, tx, ie 73 f air with a 65 f dewpoint and a 10 mph wind?
how about july, 1998? 

>if the water flow is 1600 gph, after the water comes out of
>the heat exchanger in the house, drop it off of the roof.

from a gutter or pvc pipe with holes in the bottom,
into a deep pond with some plants below...?

>a 10 foot cascade, well broken, should lose about 40k btu/hr.

sounds good, at an energy cost of 100 watts, for a cop of 120?
how many 1/16" diameter droplets will 1600 gph make as it descends
10' in mid air, and what is their total surface area and average
"wind," and how many btu/h will they lose if considered as tiny
ponds of 70 f water exposed to 73 f air with a 64 f dewpoint?

>and it will sound really neat!

that might appeal to people...

nick



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