Anyone with a compost heap knows it can get pretty hot right in the thick of fermentation. That thermal energy normally escapes the pile, but with a relatively low-tech installation, the passive energy generated by decomposing biomass can be used as a source of indoor heating.
The Germans call this sort of installation a biomeiler, which roughly translates to “bio-kiln” or “bio-reactor.” For ease of construction, a biomeiler is usually built with straw or wood chips instead of just any old compost. The typical design can provide passive heat for 18 to 24 months, and more advanced configurations can also harvest biogas.
Basically, it works like this: Biomass is heaped either on the ground or onto a polyethylene tarp in layers. In between the layers, coils of hosing or piping are configured to harness the heat from the center of the pile. These coils are completely filled with water, which runs through a heat-exchange mechanism, like a radiator, and back to the biomeiler in a closed loop. In cooler climes, it is sometimes necessary to insulate the exposed connections in the piping from frost.
The biomass is usually irrigated for the first few days, in order to kickstart the bacterial growth within that will create temperatures of 120 degrees F or more within the pile. Often, biomeilers are covered thereafter in order to avoid sustained rains that will wash away all the good decomposing bacteria that take up residence.
As with calculating the optimal volume of compost required for hot composting, using some basic principles of physics, there are certain constants at play in calculating the necessary size of a biomeiler for your particular thermal energy needs. Height, area and volume of biomass can be easily customized to correspond to the number of kilowatts desired.
Although some standard physical laws dictate the size and shape of a biomeiler, the idea is both simple and open to innovation. In theory, any managed compost pile could be turned into a source of sustainable, passive, eco-friendly heating.