First Water Draw Test on Prototype 3 of DHW Pond


This is the data for the first water draw test from Prototype 3 of the solar pond water heater.


The pipe coil is 200 ft of 1 inch dia HDPE pipe.  This pipe coil holds about 8.2 gallons of water.  The pipe coil is surrounded by 165 gallons of water, which is heated by the sun.


The draw test plot below shows the effect of taking out 16 gallons of water at a flow rate of 1.6 gpm.

The initial 8 gallons (approximately 5 minutes starting at 7:13) comes out right at tank temperature (116F in this case), since it is coming from the already heated water in the pipe coil.

At approximately 7:18, the outlet temperature drops, because the hot water that was already in the coil is exhausted.  The output temperature drops from around 116F down to about 100F due to the inefficiency of the heat transfer from the pond water to the water flowing through the pipe coil -- better than I expected -- this might be improved some by more spacers between the pipe coils.




Notes on the test:

Draw Test 1: 6/21/2007

Flow rate: 37.6 secs/gal or 1.6gpm

16 gallon draw @ 1.6 gpm:
Start Gallons: 329.0
End Gallons: 345.95

Logger has the temperature record.
Should have done a button push at start and end of draw, but:
Start time aprox: 7:13 pm
End time aprox: 7:23

Initial draw of water from the pipe coil:
Start temp: 116.6F
End of the 8 gal: 115.8F

2nd 8 gal -- heat transfer from pond to coil:
Start temp: 115.8F
Initial drop over 1 min to: 100.5F
Final temp at end of draw (7:23): 99.07F

So, at a flow rate of 1.6 gpm, the output water will be about 15F below the pond temperature after the pipe coil has emptied out -- not so bad.
During this time, Tin = 52.8F, Tout = 99.5F, and Tpond = 113.2F,
Tpond - Tin = 60.4F and Tpond - Tout = 13.7F
so, after the initial 8 gal is drawn, the pipe coil acts as heat exchanger that delivers 77% of the delta from Tin to Tpond.

Pond temps recorded off the temperature gage on the pond:

Time    Tank Temperature
0 116F
@ 10 gal 114.6F
@ 11.2gal 113.2F
@ 12.9gal 112.6F

End draw 111.4F
2 min after 110.3F (2 minutes after end of draw)
4m 30s after 109.4F
6m 30s after 109.0F
7m 33s after 108.9F
52 min after 108.1F

Heat out of pond:
Qpond = (165gal)(8.3lb/gal)(116F - 108.1F)(1 BTU/lb-F) = 10820 BTU

Heat from pipe:
Qpipe out = (8 gal)(8.3 lb/gal)(116.5F- 52.5F) + (8 gal)(8.3lb/gal)(99.5F-52.8F) = 4250 + 3100 = 7350 BTU

So, these don't agree to well -- not sure why -- maybe the pond temperature is not done adjusting, or there the heat withdrawal results in an uneven temperature distribution in the pond.


Pipe coils installed in the pond -- two 100 ft coils connected end to end.



Prototype 3 of solar pond water heater.