This was as bizarre as gift wrapping an elephant and almost as difficult.
I was an engineering student in the in the 1970s when the price of crude oil went from “cheap” to “expensive”. The assumption was that “proper” fuel was oil, gas or coal and this was consumed either directly or indirectly by “sensible” machines like reciprocating engines or turbines. The lecturers thought we needed to broaden our knowledge and we were formed into groups and told to find alternative fuels. For no obvious reason my group opted for chicken poo and suggested anaerobic digestion as a solution to the oil crisis. As we were all clueless about anything biological, the presentation was not much more than a neatly drawn cross section of a dumpster, I seem to remember a prolonged discussion on the appropriate shading for poo. Until I purchased a composter earlier this year, that project was my only exposure to a biological conversion processes and I am still clueless but the link at the bottom of the page is informative.
At the beginning of September, I had cause to put my hand into the composter to recover something which had been dropped whilst disposing of kitchen waste and it was noticeably warm in there. A week or two back I made a 1 meter spear thermometer by poking a thermistor into a length of copper tube and the result was this graph:
This suggested that the composting process was generating heat and dissipating it to the surrounding air. I guess it was only a few watts, but it was heat which was being produced without burning anything.
On 25-Nov-15, the maximum temperature in the composter was 18 deg. C, which may be less than optimum. There is an annual rhythm to composting, the greatest availability of composting material is in later summer and autumn when there is stuff around like wilting tomato plants, windfall apples and fallen leaves. For me, early spring is a convenient time to dig compost into the soil before anything is planted. Ideally, the composter should work hardest over the winter when the air temperature is 5 – 10 deg. C. Being both a novice composter and tight with money, I did not want to spend £50 on a thermal jacket, £5 of bubble wrap seemed like a good solution, at least for this year. the temperature has now stabilized at 22 deg. and there is greater uniformity within the composter. Over the next few days I will compete the insulation by adding some polystyrene foam to the lid.
I have read horror stories of composters getting too hot and ceasing to work and I guess that in extreme cases, there is a risk of spontaneous combustion, so some form of monitoring is essential, e.g. looking for steam or smoke.
This exercise has made me curious about the amount of heat the composter is dissipating. A lot of discussion takes place on the generation of electricity from sustainable sources, whilst a typical household might consume 4,300 kwh or electricity it also uses 15,000 kwh of gas for heating of which maybe 1,000 – 2,000 kwh (a guess) is used for heating shower and bath water. A gas boiler heats water from from 10 deg. C to 50 deg. C. “Low” grade heat is often disregarded, but if heat from the composter can boost the feed water temperature from 10 deg. C to 20 deg. C there is the potential for a 25% reduction in the energy needed to produce a warm shower. Exploring that concept would be a good use for first year engineering students.