The major breakthrough in developing our DIY digester has been in understanding and minimising the heat loss, so that our latest 200lt design is losing heat at the rate of 11W, and achieving a theoretical coefficient of performance of about 5. Electric heating has been the method of choice for ease of control, but it is questionable whether this is the best use of such a high value commodity, and we have been investigating the use of solar hot water heating.

The design has been modified to be solar heated, with a heating coil:

The digester vessel, a 220 lt blue plastic drum, sits within the coil. The two vertical pipes are air vents and aid the filling of the system.

and a DIY panel 1M2 in area:

Digester heated by a single panel operating under thermosyphon.

Temperatures have been monitored using an Eagle Tree data logger, and the digester heats very effectively when the sun shines, but cools rapidly for a period when the sun goes in. The heating circuit operates on the thermosyphon principle and it appears that the water continues to circulate for a while after its temperature has dropped below that of the digester, so that there is a cooling effect of up to 5 degrees. This has led us to explore the use of proprietary vacuum tubes to provide the heat. The preferred designs do away with circulating water, and incorporate a heat pipe which is extremely efficient, and has the added benefit that the heat transfer shuts off when the temperature drops

Vacuum tube inserted into digester. The wooden shelf supports the data logger on the left, and a gas meter on the right.

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At this stage, the digester contains only water. The next step is to monitor the response of the digesting bacteria to the fluctuating temperature that is a feature of solar heating in our variable weather. We plan to do this at Trill Farm where we will be running a demonstration of food waste digestion for the duration of their Summer Festival over the weekend of 27/28th July. Why not join us there and catch up on the digester development?