Heating with Compost
Heating a building with compost sounds like a new, innovative idea – but it’s nothing new. In the past, farmers have employed this heating method by allowing the bedding and manure from livestock to compost in the barn over the winter to help keep their animals warmer. The technique only added a few degrees to the barn’s indoor temperature – but that makes a big difference on a cold winter night.
As the cultural shift toward sustainable living has grown, composting has experienced a surge in popularity. As a result, the practical advantages of composting are being re-addressed, particularly in regard to energy savings – hence the interest in heating with compost. In this post, we discuss compost heating methods and their practicality. We’ll also ask you what you think.
The concept. I recently saw a concept for a yurt with compost bins around the perimeter. The idea was that the compost would help heat the building – but I never saw how well it worked, and it looked a bit cumbersome. Then again, if you live in a yurt, “cumbersome” probably has a different meaning to you. Perhaps it’s more practical than I’m imagining.
This got me thinking about heating my wife and I’s, future home and some of the issues involved in a compost heating system. Once I can experiment, I’ll post more in-depth information, but to whet your appetite, here’s what I’m thinking so far: aerobic compost produces tons of heat, water vapor, and carbon dioxide. And when I say heat I mean it – even a small pile can reach 140F. Did I mention that compost can spontaneously combust? Yeah, we’ll have to deal with that too. It should be possible to use the heat generated by the compost to heat a home safely and effectively. Even if the compost only offsets some of the fossil fuel heating costs, it could be a huge win.
Venting the CO2. I grew up in Vermont, where winters are dry and dusty, leading to a host of health issues. While North Carolina winters are much more temperate, they’re still a bit dry. For a successful compost heating project, it’d be nice to use some of the water vapor to humidify the dry winter air. Still, that would have to be a secondary goal, because it seems that the biggest challenge to compost heating is venting the CO2 and smelly gases without losing all of the heat that’s generated. The CO2 is dangerous, and the smell from trace gases is unpleasant. Somewhere along the line, the gas must pass through a biofilter (fancy talk for “finished compost”). I’d like the design to be oil/electric-power-free, but even if some is needed it’s still more energy efficient to run a blower than a heater. I’m guessing the best design is a forced aeration in vessel system (put a bunch of composting material in a barrel and blow air through it).
Coil-based heating. One approach would be to heat water by placing a coil at the core of composting material and circulating water through. As I found out in cooking with compost, opening up the pile causes a huge drop in temperature and it’s slow to reheat. In addition, I’d fear wicking away too much heat would drop the core temperature and slow the thermophilic bacteria (compounding the heat generation issue). This approach may still be worth investigating, though.
Using a heat exchanger. The heat is best captured by venting the hot compost gases through an air-to-air heat exchanger. The moisture may be a problem, so perhaps it should first run through a dehumidifier. Keeping a well-sealed air envelope inside the house and providing additional heat from compost is feasible; whether it’s economically viable remains to be seen. If you remove cheap oil/coal-based energy from the equation, a lot of things become economically viable.
Feedback needed. Has anyone tried heating with compost? Thoughts? At least I’m not the only one.
UPDATE Feb2011: This has been a much more popular post than I expected. I have an experiment designed but am traveling (work, no fun I promise) and will have some results in the coming months along with a prototype design.