ESF Green Campus Initiative

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.

“People used to joke with me that I should use the heat generated by that huge compost pile to heat my house. By why couldn’t a bin heat a greenhouse?”

Your question about extracting heat from the compost pile is very timely. I am currently constructing a heat collector using 50 feet of 1/2″ diameter copper tubing. Using an 80-gallon per hour fountain pump, I will circulate water through the copper tubing, heating it as it goes around the loops. Based on my calculations, water should cycle 200 times per hour thorugh the system.

My plan is to lay it horizontally about 18 inches from the top of the bin so that it is free to settle along with the compost. When the batch is batch is done, I will scalp the compost off the top and lift the collector off to use in another pile. I plan on heating water but it could also be used to heat a radiant floor. We are talking with a group in Alaska about heating their greenhouses in February and March. I also have a client in New York who has been experimenting with heat extraction to warm the seed trays in his greenhouse.

I will post updates on my findings as I move forward with my own heat transfer experimentations.

Quite often, I am asked the question, “Can I recover some of the heat from my compost pile?” The simple answer is “Yes”, but then we must also answer the questions “How can this be done easily and inexpensively?”

Gregg Twehues at Stone Barns Center in Tarrytown, New York, has been experimenting with heat recovery from his O2Compost Micro-Bin system with a good deal of success. In Gregg’s case, he wanted to use the recovered heat to warm seed trays in their propagation house as a means of reducing their overall power consumption. Stone Barns Center has extensive greenhouses and they grow organic herbs and vegetables year round for use at Blue Hill, a world renowned four-star restaurant located on-site: www.bluehillfarm.com.

With his first test, Gregg used a long coil of plastic irrigation tubing buried in the Micro-Bin, and he was able to heat the circulated water to 84 degrees Fahrenheit for approximately 5 days. In his second test, he “sweetened” the mix by adding high nitrogen feedstocks and he was able to extend the hot phase of composting to nearly 3 weeks. In future tests, he plans to add 30 feet of copper tubing to the coil to improve heat transfer to the water.

This seems to be a growing area of interest. O2Compost was recently contacted by the Soil and Water Conservation District in Alaska to help develop a method of heating greenhouses in conjunction with composting horse manure and other feedstocks at temperatures below minus 20 degrees Fahrenheit, for weeks on end. This could be our ultimate challenge – we’ll keep you posted.

In an effort to continually refine the Micro-Bin approach to small scale on-farm compostnig, we have been conducting our own research to “learn by doing”. The following discussion pertains to the first of three tests that we are currently running.

Test #1 – Retention of Nitrogen with Sweet PDZ

As described on their website (www.sweetpdz.com), “Sweet PDZ is the leading stall freshener on the market and is the odor control and deodorizer of choice for thousands of horse, pet and livestock owners. Sweet PDZ is an all-natural, non-hazardous and non-toxic material (i.e. Zeolite, a naturally occurring volcanic mineral). It captures, neutralizes, and eliminates harmful levels of ammonia and odors. Sweet PDZ is a far superior alternative to lime products for ammonia removal and moisture absorption. Don’t risk compromising your horse’s performance.”

In discussing this product with their Sales Manager, Tom Menner, I asked if the ammonia (i.e. nitrogen) that is captured by their product is retained in the compost, thereby improving the quality of the finished product. Tom’s response was, “That’s a good question, I don’t know. How can we find out?”

To answer this question, we have set up one of my Micro-bins with 2.5 cubic yards of horse manure mixed with sawdust bedding. At the 3-foot horizon (about a foot down from the top), I placed five nylon mesh bags with different mixes of manure, bedding and Sweet PDZ (see illustration and table below). Each sample was prepared in an identical manner with the exception of the type and quantity of Sweet PDZ. The objective is to compost these samples for 60 days and then send representative portions of each sample to the laboratory to evaluate the carbon to nitrogen ratio (C:N). The samples will be pulled around Halloween and the lab test results will be posted here as soon as we receive them.

Plan View (Looking Down)