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Composting and Heat Extraction Experiment
August 27th, 2009 by peter
We are often asked the question “Can I warm water (or my tack room or my greenhouse) using the heat from my compost pile?” Intuitively, my answer has been “Yes, of course”, but the question that remained was how to best do this.
As an experiment, I assembed a heat exchanger using 50 feet of 1/2-inch diameter copper tubing mounted to a wire mesh frame, as shown here. My thought was to circulate water from an insulated barrel through the copper tubing using an inexpensive, low-volume fountain pump.
Heat Exchanger Unit
Using our new Macro-Bin, I laid the heat exchanger horizontally on a fresh bed of horse manure, approximately 2/3 up from the bottom of the bin. I then covered the heat exchanger with about one foot of raw manure and six inches of finished compost.
Then I started the fountain pump to purge air from the system and test the apparatus – everything worked well, as I had hoped. I turned the pump off and turned the blower on using an aeration cycle time of 30 seconds ON and 30 minutes OFF. Within 24 hours, the composting manure came up to a temperature of 155ºF at the height of the heat exchanger and when I turned on the water pump, that water coming from the hose was too hot to touch for several seconds. “It Worked!”… or so I thought at the time.
After allowing the water to circulate for several minutes, the temperature of the water coming out of the heat exchanger dropped off considerably to within 2º of the temperature in the barrel. This puzzled me, so I kept the water pumping overnight to see what would happen. The following morning, I discovered that the difference in water temperature was still only a few degrees.
In an effort to understand what was happening in the bin, I inserted a temperature probe down to the exchanger and then through the wire mesh to approximately 12 inches beneath the coils. The compost temperature at this depth was 165ºF. As I extracted the probe, the temperature stayed steady to within 2 inches of the frame and then dropped quickly to approximately 85ºF. As I continued to pull up the probe, the temperature stayed low to within 2 inches above the coil and then quickly climbed back up to approximately 155ºF.
What appears to be happening is that the heat immediately around the coils is being extracted but the heat outside of this area is not “flowing in” to replace it. We know that compost is self-insulating and this yet another example of this phenomenon. Does this mean that compost cannot be used to heat water? No, but it is reasonable to conclude that the compost pile and heat extraction system would need to be much larger than my test apparatus to create a significant warming of pumped water.
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