The hot dirt on compost

By Su Wild-River

It’s well known that compost generates heat. Clever people even heat their houses, run cookstoves and cars off compost. Here’s a little look at a humble backyard compost system through the lense of a thermal imaging camera.

A thermal imaging camera is really a heat sensor, not a camera which shows what you see with your eyes. In these photos, the hottest areas are shown in white, and a rainbow spectrum going through red, to yellow, green and finally blue, show colder areas.

The photographs were taken on a cool autumn morning, with an ambient temperature around 14 degrees Celsius (about 57 Fahrenheit). The yellow glowing thing is a ‘compost dalek’, or plastic drum with air vents at the top, and compost inside.

Its interesting to see in the first pair of photos, that the water-filled pot at the front, and the two ceramic pieces behind are the coolest spots around. These are the high thermal mass areas of the photographs, and I was surprised at first to see that they were so cold, as I usually think of thermal mass as warm. Of course the main feature of thermal mass is not that it stays warm, but that it moderates temperature changes. Large thermal mass elements, like water tanks insulated slabs change temperature so slowly that they stay around the long-term average ambient temperature.

Compost by Kath McCann. Photos by Su Wild-River

Compost by Kath McCann. Photos by Su Wild-River

The outside of the compost dalek registers a temperature of around 16.5 degrees Celsius. It is much warmer than everything around it, and only the vents are colder.

Looking inside without turning the compost gives us a look at even higher temperatures. The hottest parts here are about 20 degrees Celsius. They are the deeper parts of the compost, so we dug a bit further.

Compost by Kath McCann, Photos by Su Wild-River

Compost by Kath McCann, Photos by Su Wild-River

Only an inch or so down, the hottest temperature was 26.4 degrees Celsius. This is more than 10 degrees above the background temperature.

Compost by Kath McCann, Photos by Su Wild-River

Compost by Kath McCann, Photos by Su Wild-River

Now this compost is owned by Kath McCann, a keen gardener, and a good composter, but not someone who is using the stuff to heat her house. It seems as if even humble compost could make enough heat to warm us up, if only we plan and use it wisely.

So there you go. Compost really is warm. How could you be using yours?

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