Following the previous post we had five sunny days followed by a dull day.  The heating arrangement was the same 1 sq M solar hot water panel and single 2M long vacuum tube, and the resulting temperatures were as follows.

Plot of digester temperatures over six days in June. Green is the solar panel hot side, red the solar panel cold side and blue the digester internal temperature.

Each sunny day the digester temperature rose 4 to 6 degrees, and overnight lost 3 to 5 degrees.  On the sixth day, when it was overcast and the solar panel didn’t get above the digester temperature, the digester continued to cool, eventually by about 15 degrees.  At this point the weather forecast was for a month of sunny days, and we drew several conclusions:

- repeated sunny days would take the digester temperature into the thermophilic range, and a couple of dull days could bring it down through the mesophilic to the psychrophilic range.  The bacteria need time to acclimatise to the different regimes, a function of the ‘population doubling time’, and the fluctuations could kill off the digestion;

- penetrating the insulating jacket and introducing the vacuum tube appeared to have increased the rate of heat loss by about 30% to 14 – 18W;

- if the digester were to be put into the hands of people who are not technical experts,  the temperature would need to be controlled automatically.  This in turns requires energy storage, which although quite possible with solar hot water, quickly leads to the need for pumping.  The heating method of choice is electrical, with batteries for storage of course, and this calls for an electrical (pv) power station.

PV power station undergoing proving trials at Glastonbury festival 2013.

The outcome of these trials is that we have abandoned solar hot water heating, and modified the digester to be electrically heated with thermostatic temperature control.  The introduction of the power station has brought many other benefits in that it can provide surplus power for lighting, slide shows, phone charging, computer and printer operation etc, all of which it does at Ourganics where the digester is being developed.

The digester heating is now achieved using low voltage cabinet heaters:

Electric heaters mounted on copper plate.

The heaters are thermisters, with a nominal operating temperature of 40 degrees.  Here there are three, two rated at 10W and one at 20W and used to boost the heating rate, and they they are mounted on copper sheet cut from an old hot water cylinder.  Where mains power is available they can be powered from a car battery charger.

The digester has now been moved to Trill Farm where it will digest food waste during their Summer Festival over the weekend of 27/28th July.  We’d be delighted if joined us for all the latest news.