Radiation/Evaporation Cooling Pond


Latest pond run -- July 29, 2011...

Small Test Pond

This is a small test pond to see how much cooling one could get from a shallow pond exposed to the air and night sky.

The pond is 45 inches by 93 inches (29 sqft).  It built on a 4 by 8 sheet of 1 inch thick polyiso. 

Pictures of the pond below.

Hard to find a spot that is totally level, so depth varies some, but average is 1.8 inches.
A total of 45.7 gallons. 

First night log:

radiation pond temperature log

The blue line is the water temp.

This is water just out of the well, so start temp is a fairly low 51F -- In the cooling application, it would probably start at more 65F, and this would probably make the cooling go faster.

So it cooled  from 51F down to 42F.   

The water temperature averages about 7F below ambient air temp. 

Cooling in BTU was (51F-42F)(45.7gal)(8.3 lb/gal) (1 BTU/lb-F) = 3413 BTU (1 KWH), or 118 BTU/sqft of pond area.

This is substantially less than the earlier scheme of flowing the water down a sheet of metal, which achieved 467 BTU/sqft, but the starting temps were not comparable, and the pond one involves much less pumping energy.  Will see better on the next night with a higher starting temp. 

Even from this little test, its apparent that there is some potential -- a 20 by 20 pond would store 14 KWH of cooling overnight.

Radiation cooling pond
The pond is a 2 by 4 frame over a 1 inch thick sheet of polyiso, then cover with 4 mill poly film -- took all of 10 minutes to build.

radiation cooling pond
Filling with water.

2nd Night

plot for 2nd night

This is the plot for the 2nd night.

There were more clouds, so may not be as good a test.

The big drop in tempat 7:30pm was the addition of some new water from the well to make up for water lost to evaporation.


Night of July 29, 2011

Dark blue, solid, heavy line is pond temperature

Other blue line is relative humidity

Green line is ambient temp at 1 ft above ground

Violet line is dew point temperature

The poly film cover was off during the day to keep the pond from heating too much -- the cover was put back on at 8:15pm (at triangle mark).  There is a change in rate of cooling when the cover is put on -- from about 3.5F per hour with cover off to 1.8F per hour with cover on.  Presumably this is mostly due to losing the evaporative cooling.

Looks like there might have been some clouds moving through around 3 am.

The cover was put on with a pond temp of 70F, which is about where it would normally be if the pond had been used for floor cooling the day before.  Over the night, it cooled down to 52F.

If the same amount of cooling were attained with the 450 gallons of available storage tank water, it would be (450 gal)(8.3 lb/gal)(70F-52F)(1 BTU/lb-F) = 67K BTU --  Not so bad -- probably enough for our needs.

Going to run this again a couple times alternating between cover and no cover.












Gary July 24, 2011