Hot water compost … Free energy from biological processes.

Two weeks ago we made a hot aerobic compost. But it was no ordinary compost… We wanted to explore whether we could attain heat from purely biological processes – in other words, infinitely sustainable processes.

Jean Pain building a compost heap

In the 1970s, French inventor and innovator Jean Pain designed a system which provided his home with free hot water and combustable energy using the heat and gas generated through the composting process. Using machines and the scrub he cleared from a nearby forest, he made compost heaps so large they provided heat for an entire year.

But could we build one without machinery and zero budget? How much material could we gather? How much water could we heat? Our experiment had to be realistic – how about enough water for someone to take a bath?

Turns out we got enough heat for four people, and we had to add cold water to get in! Even after going through the inefficient garden hose from the compost to the bath tubs, the water came out at 44°C. Damn thats hawt!

Piping hot at 44 degrees Celsius

Here’s Leo running us through the setup.

The compost we built was originally about 3 cubic metres, shrinking to about 2 once it peaked. We scored a water drum from the good people at the Te Maunga transfer station and built the pile around it. We layered in the funky food waste from our compost collection with a variation of different materials, all contributing different elemental or structural benefits. These include: Hardwood sawdust, scrub and dead grass, horse manure, cow manure, leaf litter, clay, cardboard, old compost, new compost, fungal compost, lime and paramagnetic rock dust.

Building the compost around the water drum

There was also a hose running through the compost, but the compost got so large that we lost it. 

finished compost heap rugged up to keep the heat in

The trick is to catch it at the peak of it’s heat profile. When a compost peaks, the heat inside reaches a high temperature point and drops back off. This signifies a lack of oxygen, which slows down microbial activity. At this point it’s a good idea to turn your compost to aerate it again. Because we were keen for a bath, we pre-empted our composts peak at 65°C in the processes of draining 100 litres of heat out of the compost. Not ideal, but the heat bounced back pretty easily. Worth noting that it is important to never let your compost get to 70°C as this is the point when all your thermophilic bacteria die and precious nitrogen converts into a gas and escapes into the atmosphere.

Jean Pain also captured methane from his compost, does that mean our next experiment can involve fire?