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

>"mike hardy"  wrote:

>>...what am i missing?

>well, assuming we're still refering to the post by on
>20 jul 1998....

hey, that's me...

>first the numbers in the original post were off, the amount of evaporation he 
>expected (35 lbs per hour) is about 10 time too high for an 8 x 16 foot pond. 

it depends on more than the pond size, no? those numbers seemed reasonable
for the average nighttime july temperature and humidity and windspeed and
air pressure in abilene, texas. i wonder where you get your numbers...

>but here, i'll assume the same pond water temp. about 77 deg f.  


>you want to extract heat from your house. what temperature do you want
>the air in your house to be?

80 f sounds good.

>since the pond is 77 f. your house can't be any cooler than that.

not much, without some extra stuff, like a dessicant system or heat pump.

>the heat influx to your house, (in the summer - hot outside air,
>sunshine, cooking, lights, etc) is probably about 40,000 to 60,000 btu/hr. 

wouldn't that depend on where the house is? (picky, picky.) and other stuff
like how big it is and how much insulation it has and how air-leaky it is
and which way it faces and how the windows are shaded?

>that is the size of a/c unit you would need.

i guess you'd need a bigger pond, in that case.

>let's say you could stand having the house 80 f.

ok. you might care about the humidity too. drip, drip, drip...

>lets assume that a heat exchanger can take 81.5 f air from the house,
>cool it to 78.5 f. and at the same time 77 f. water is warmed to 80 f.

ok. (if we can cool air to 78.5 f with 80 f water.)

>you will need to move 1600 gph of water, no problem for a 1/3 hp pump.

or maybe less, with big pipes and no change in water level, but that's a
lot of flow. suppose we warm water from 77 to 83 f, whilst cooling house
air from about 83 to 77? or use a larger pond (ponds are pretty and cheap)
and warm water from 75 to 85. we might flood a 1,000ft^2 pond or flat roof
with 2" of water at night and drain it back at dawn into 24 55 gallon drums
with 600 ft^2 of surface in a 6x8' closet inside the house, and vice-versa
in wintertime, with a greenhouse over the pond.

>now how much air do you need to move? air weighs about 0.076 lbs per cu ft.
>its specific heat is 0.241 btu/lb/deg f at constant pressure.  so you need
>12,000 cu ft/min of air flow.  

i guess you are figuring 12k x 0.076 x 0.241 x 3 f x 60 = 40k btu/h.
how about                 4k x 0.076 x 0.241 x 9 f x 60 = 40k btu/h?

>if the heat exchanger has a low pressure drop (less than 0.5 in sp),
>the air blower requires a motor with about 5 hp capacity.

or, we might use a few fan coil units, like magicaire's (940) 767-8333
$150 all-copper 2'x2' shw 2347 duct heat exchanger, which transfers 45k
btu/h between 125 f water and 68 f air at 1400 cfm with a 0.1" h20 drop.
we might hang 4 of these under one of grainger's $110 86 watt 4c853 48"
21k cfm ceiling fans. hmmm. i guess that won't cool 85 f air to 75 f with
75->85 f water. we could use 8 of them, in some sort of counterflow series/
parallel arrangement... what sort of heat exchanger did you have in mind?

>that is about the same power as the compressor and air blower (combined)
>for a conventional a/c. a slightly larger a/c unit will cool your house
>to 75 f, which the pond could never do.

there must be a better way.

>if, however, you have a large cold lake or river near your house or access to 
>cold well water then you could use a much smaller fan.  

a house with lots of cold well water might not need ac.

>if you have a cheap source of durable pipe, you could use your ceiling, walls 
>and floor as the heat exchanger, and not have any fan.

i used to work at a place (moore products, home of fine pneumatic controls
and well-aged management) with fin-tube pipe in the ceiling. sometimes,
it dripped...

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