Solar Pond Water Heater -- Water Draw Test

This is a test of a double water draw from the solar pond Domestic Water Heater. For this water heater, a 200 ft coil of 1 inch ID HDPE pipe is immersed in the solar heated water tank. The cold well water enters the 200 ft coil, is heated by the solar pond water, and exits the pipe coil heated up. Since the pipe coil holds about 8 gallons of water that starts at full pond temperature, the first 8 gallons is delivered at full pond temperature -- after that, the water temperature drops down to a value somewhat below pond temperature.

This test is for two 16 gallon draws that are spaced about 10 minutes apart. The idea is to see how fast the coil temperature recovers after an initial draw.

Draw Test Plot:

Temperature of pond -- black solid line

Temperature of water in -- blue dash-dot line

Temperature of water out -- red dash line

This was a set of two 16 gallon water draws with a 10 minute wait between the two draws.

The pipe coil itself holds 8 gallons, so by using a 16 gallon draw, the first half comes from the water that was already in the pipe coil and the 2nd half comes from cold water that has traveled through the pipe.

The idea of the 2nd draw was to see how fast the water in the coil comes back up to pond temperature after an initial draw that has lowered the pipe coil temperature.

The numbers at the top of the plot show what's going on:

1 at about Minute 26 -- start of the first draw -- initially the outlet water comes out at the full pond temperature of about 140F.

Water Meter: 345.95 gallons, accumulated flow 0 gallons.

2 -- at about Minute 29.5 -- This is where the 8 gallons that was already in the pipe coil runs out. The inlet water is heated by the single pass through the pipe coil. The outlet temperature drops to about 115F, or about 21F below the pond temperature.

3 -- at about Minute 35.5 -- Stop water draw at 16 gallons. Between 3 and 4 (about 10 minutes) there is no flow through the coil. The aim is to see how if the pipe coil gets back up to pond temperature in these 10 minutes.

The Twater in and Twater out temperatures have no meaning during this time, since there is no flow through the pipe.

Water Meter: 362 gallons, accumulated flow 16 gallons.

4 -- at about Minute 45.5 -- Start of the 2nd 16 gallon draw.

Its clear from the water outlet temperature that the pipe coil has nearly fully recovered to pond temperature in the 10 minutes. If you look at the pond temperature line, it levels off after about about 5 minutes, possibly indicating that the pipe coil has made a lot of its recovery in 5 minutes.

The high inlet temperature at the start of the draw is due to the fact that a garden hose was being used to supply the water, and it had heated up in the sun.

Water Meter 362 gallons

5 -- at about Minute 51 -- this is where the 8 gallons that was already in the pipe coil runs out. The inlet water is heated by the single pass through the pipe coil. The outlet temperature drops as in the interval between 2 and 3 above.

6 -- at about Minute 55.5 -- end of the 2nd 16 gallon draw and end of test.

water Meter 378.08 gallons, total accumulated flow 32 gallons.

I did not regulate the flow rates too well in this test.

Rough flow rates on first draw:

First 8 gallons at 2.3 gpm

2nd 8 gallons at 1.3 gpm

Rough flow rate for the full 2nd draw was about 1.6 gpm

Ambient temperature was about 90F.

Conclusions:

The heater will deliver the first 8 gallons at full pond temperature (this may satisfy most draws).

After the initial 8 gallon draw, the pipe coil acts as a fairly efficient heat exchanger, and continues to deliver water that is about 15F below the pond temperature. If the pond is up in the 130+F area, then the outlet temperature will be around 115F, and no further heating is likely to be needed.

After an initial draw that lowers the pipe coil temperature, the pipe coil temperature comes back up to pond temperature in 10 minutes (probably with most of this gain occurring in the first 5 minutes).

Gary 7/6/07