By Gaelan Brown
Properly managed compost gets really hot. Experienced composters know that pile temperatures can exceed 160° F if the material is properly aerated. You may have wondered if there are ways to capture that heat in a cost-effective way, and the answer is yes.
This winter both Vermont Compost Company and Jasper Hill Farm are heating large greenhouses through Vermont’s harsh winter entirely with energy captured from compost. These systems are capturing up to 1 million BTUs of energy per ton of material composted, saving thousands of dollars on heating and hot water fuel and enabling year-round vegetable growing in Vermont.
A ton of quality compost can sell for $30 to $100 depending on how it’s marketed. And one million BTUs of energy captured per ton is worth up to $35 in terms of replacing propane, electricity or fuel oil. By comparison, the energy value of a ton of coal is $30.
Recent advances in compost heat recovery are the result of many years of research and collaboration among organizations like Highfields Center for Composting, CompostPower.org, Yestermorrow Design/Build school, Agrilab Technologies, Acro Labs, Ltd, Bruce Fulford, Vermont Compost Company, and the Universities of Vermont and New Hampshire and others.
The keys to success have been new methods of managing aeration of the compost to keep the microbes happy and hot, combined with innovative heat exchangers. The result has been predictable cost-effective heat recovery and improved compost production processes in general.
Two critical factors for commercial, municipal and farm-scale compost operations are managing aeration and controlling odors. Turned windrow or other mechanical tumbling methods of aerating the compost are expensive in terms of equipment and energy costs. And a few days after material is turned it usually goes anaerobic, meaning there’s not enough oxygen to keep the composting microbes happy, which causes problems in terms of compost temperature, quality and odors.
Aerated static pile (ASP) composting uses fans to move air through the material via pipes or channels underneath it. ASP composting ensures that material is always being properly aerated, maintaining ideal conditions for the thermopilic composting that kills pathogens and weed seeds and breaks material down quickly. ASP has been shown to use less than half of the energy of turned windrow operations, with better speed and quality results.
Negatively aerated static pile systems (pulling air down through the top of the material into channels underneath it) enable heat to be captured by running the hot vapor pulled from the compost through a heat exchanger. Negative aeration also supports easy ways to capture C02 emissions and odors from compost.
At Jasper Hill the negative aeration compost heat recovery system is exhausted into a simple bio-filter inside the greenhouse, so that the C02 emissions from the compost feed the plants while the compost odors are captured. This reduces the amount of cold fresh air that must be brought into the greenhouse for C02 replenishment, which is essential for plant growth.
Vermont-based Agrilab Technologies and Acro Labs in Canada developed the patented process with negative aeration and heat exchangers for compost heat recovery systems like the ones at Jasper Hill Farm and Vermont Compost Company.
The new mobile version of Agrilab’s system, the Compost Heat Wagon, is now running at Vermont Compost. The Compost Heat Wagon is a space-efficient and mobile “plug-and-play”-style compost aeration and heat recovery system that can capture up to 150,000 BTUs per hour continuously. It includes a high power aeration fan with variable-speed drive and computerized controls, data monitoring, condensate collection and the heat exchange system that makes hot water which is used for space heating or washing.
Hot water from the Isobar heat exchanger is circulated from the Compost Heat Wagon to two 350-gallon tanks in Vermont Compost’s greenhouse. Heat loss from the tanks adds warmth to the greenhouse and water from those tanks is circulated into radiant heated seedbeds and irrigation lines.
Since the Compost Heat Wagon is mobile it’s easy to setup quickly, and it comes with lease-to-own financing options from multiple banks. Other Agrilab systems that have been operating for several years are capturing up to 200,000 BTU per hour continuously through winter months from relatively small 200-ton compost piles.
Compost heat recovery is a simple concept for cost-effective base-load renewable energy that doesn’t need subsidies to have great economics. For more information about compost heat recovery, visit www.compostpower.org, www.agrilabtech.com, or search for the book, The Compost Powered Water Heater, by the author of this article, Gaelan Brown.