Bridport Renewable Energy Group

Glastonbury festival 2014

On July 2, 2014, in Anaerobic Digestion, PV, by David Neylan

Following on from last years success; winning the Greenpeace Sustainable Green Trader Gold Award,  BREG teamed up with the Fat Belly Cafe for a second year in the Croissant Neuf field which is part of the Green feilds and is run on renewable electricity; principally Photovoltaics, PV.

The Off Grid Fridges were back in action as despite the rain, this years festival was a hot one. A 100W freezer was used to make ice to keep two well insulated fridges below the regulation 8ºC.

In addition BREG provided some Off Grid Gas using some of the food waste from the Cafe kitchen. (more…)



The demonstration.

In her quest for sustainability, the owner of Trill Farm has an aspiration to turn any waste product into a valuable feedstock.  To illustrate this principle she invited BREG to demonstrate anaerobic digestion of food waste during their Summer Festival over the week end of 26-28th July.  On our part, we were keen to put the process in the hands of people unfamiliar with the technology, and to serve the needs of a real live situation.  The catering kitchen on the farm provides food throughout the festival, and at its peak on the Saturday evening, served a meal to approximately 500 people.  That, and the campsites set up for the festival, provided the cooked food waste.

I’m stressing the cooked food waste because we hold AD as a sister process to composting.  Uncooked food waste eg peelings, skins etc are consigned to the compost heap, and the cooked food waste, which would normally be collected by the Council on a Monday morning, was sorted and made available to the digester.

The process implemented at Trill was illustrated as follows:

The pictogram of the process prepared for Trill Festival. At the Festival it was not possible either to burn the gas, which was stored, or to apply digestate to the crops - it went on the compost heap.

The main components were the digester,  gas bladder, and electricity generating station for heating the digester:

The AD system installed at Trill. The digester on the right is of 220 litres capacity, the bladder (behind the sun screen) a nominal 3 cu M, and the pv station 380 Wp.


The food waste was collected from the kitchen and from the recycling centres dotted around the campsite, transferred to a drum and kept under water, so that the acidification stage of digestion could commence:



The digestate may be the unsung hero in this process.   Our attention tends to be focussed on the biogas, but the truth is that at this scale it’s only ever going to be a modest amount, perhaps able to supply 80% of the gas used for cooking for a family of four.  On the other hand the digestate production can be quite significant, and some have suggested it may even be worth more than the gas.

To explore this we have been carrying out growing trials – applying digestate as liquid fertiliser to crops to see if there is a benefit.

Raised beds at the Bridport Community Allotment. Two beds were kindly made available by the Orchard Group. They were side by side and watered every two days. An average of 18 snails were removed on each visit.

We had 500 litres of digestate left at the end of last year, with an N:P:K analysis of 4.5:0.3:2.0 kg/te, and calculated that a dilution factor of 10:1 would be appropriate.

The growing trials were kept very simple in the belief that to be convincing the benefits had to be obvious.  In all cases two crops were grown – a trial and a control. (more…)


… more on solar heating of the digester.

On July 14, 2013, in Heat loss, PV, by admin

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.




Solar heated digester

On July 1, 2013, in Anaerobic Digestion, by admin

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.


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?