Bridport Renewable Energy Group

Bridport Food Festival 2013

On June 21, 2013, in Anaerobic Digestion, Biogas, PV, by David Neylan

BREG at the Food Festival with Anaerobic Digestion, solar hot water and the new solar PV power station.

The sun was out for us in the morning but it clouded over just as we had set up! We did manage to provide some hot water to the cafe next door with the little sun and biogas that we had.

Although the main point of the festival was food BREG was able to not only demonstrate renewable energy but make a small contribution of energy. Our efforts with Anaerobic Digestion are focused on small producers allowing them to cycle nutrients from plants and animals back to the soil as well as gain some energy in the form of gas. (more…)


Demonstration DIY digester

On June 13, 2013, in Anaerobic Digestion, by David Neylan

During the winter we have been busy designing and building our DIY digesters for testing over the summer of 2013.  Using the experience gained from our work in 2012 with the Methanogen 700 liter digester we have tried to simplify the equipment and produce a system from off the shelf components that is easy to use, robust and low cost.

Pictured are the 30 litre digester in an insulated box and a 100 litre bell over water gasometer gas store.

The first of the series of DIY digesters is the demonstration 30 litre model that has to be portable so that we can demonstrate it to various interested audiences. As the gas produced is roughly the same as the digester volume this 30 liter unit is too small for domestic use but it does show all the principles involved in this type of design.

We have given a few ‘shows’ already to groups such as Turn Lyme Green and the Schumacher College and we have an upcoming event at the Bridport Food Festival if you want to see for yourself.



The quiet period over the year end has provided an opportunity to review where we’ve got to, and to plan for the new year.  I’ve tried to capture the main points in the following, some of which you may already have seen, and it’s been helpful to me to get clear on our priorities.  I want to say a big thank you to everyone who has helped get us this far.

The objectives of the project.

The Bridport Renewable Energy Group believes that anaerobic digestion (AD) could add value to small scale food production, and has designed this project to explore the hypothesis.  Funding has been secured to purchase a proprietary digester and to develop the process in the field over 2 years.  The initial site for the activity has been at Ourganics in West Dorset, an off-grid market garden and educational centre run on organic principles, and the project has now come to the end of the first year. The digester has been decommissioned for the winter, and this note records the learning that is influencing the second year of the programme.  There will be three main areas of activity:

-         the development of the process on the basis of a simple low cost DIY design;

-         integration into the business of Ourganics, in particular an exploration of the benefits coming from the application of digestate to the crops;

-         the sharing of information and knowledge gained from the programme.

Process development.

Various aspects of the process have been investigated using the proprietary digester shown in the following figure:

Site at first gas July 2012

Proprietary digester of 700 lt, bell over water gas holder of 1500 lt, and pv system rated at 180 Wp.


Minimising parasitic load.

The AD process needs energy to operate – primarily to heat the digester, but also electrical energy to run ancillaries.  In this project where the process is operating ‘off grid’ the electricity is provided by a photovoltaic (PV) power station, and heating comes from a gas fired boiler currently running on bottled propane.  There is a plan to move to biogas as a fuel, and this parasitic load will have to be met before any net energy is available for use elsewhere.  For the process to be an attractive proposition it should produce more energy than it consumes.  This gives rise to the notion of Coefficient of Performance, the ratio of energy produced to energy consumed.  A CoP of 1 would mean that all the energy produced would be used to heat the process.  A CoP of 2 would mean that for every two units of energy produced, one would be used by the process, and one would be available for use elsewhere eg for cooking.  For the process to be an attractive proposition the CoP should be greater than 1, and a key requirement for the second year and for the DIY design is to achieve an acceptable CoP.

Understanding different feedstocks

The process has been run predominantly on grass cuttings.  These are available from the maintenance programme at Ourganics where the grass is cut with a motor mower.  Other than grass it has been possible to provide the process with comfrey, weeds, apple pressings and mint.  These have had an impact the nature of the products, but without any facilities to carry out chemical analysis it has not been possible to say what has changed and by how much.  As a result it is proposed to purchase a gas analyser and to develop a working relationship with an organisation with digestate analysis capability.

Reducing the operating workload.

In general, energy supplies are made available at their point of use, without any need for the consumer to be involved in how the energy is produced or delivered.  However with micro scale AD (mAD) the consumer is involved in the entire supply chain, requiring knowledge skills and time to operate and maintain the process. This represents a significant additional workload for people who are already fully occupied producing food.  Therefore an objective of the DIY system is to reduce the operational burden, and to develop simple procedures so that the process can be safely tended by individuals for example WWOOFs with little knowledge of the process.

Reducing the cost.

Small scale food production is economically marginal, and for mAD to become widely accepted it would need to be cost effective to set up and run.  This is not the case with the proprietary system being demonstrated, which was initially purchased to reduce the technical risk and provide a basis for learning.  The basic cost of the digester was inflated by the costs associated with the ancillary systems: gas storage, electricity supply, heating system, water pumping, set up on site and so on.  For the process to become widely adopted it will need to be considerably cheaper and simpler, and an objective of the second year is to demonstrate a simple low cost DIY system.  A prototype design is shown in the following figure.


DIY digester in domestic setting.

 DIY digester of 30 lt capacity with gas holder of 115 lt working volume.  System pressure is 13 mB and heating demand 17W.  The digester is fed on kitchen waste and the gas is used to brew up.


Use of the digestate.

With the relatively late start of the project, there has not been time during this first year to explore the beneficial effect of applying the digestate to the land.   It has been suggested that this might be the more valuable product to come from the process, so that a monitored programme of growing food is a priority for the second year.   This will not be as straightforward as measuring the volume of gas produced, and it is proposed to design a range of  experiments that will clearly reveal the benefits coming from use of the digestate.  This work will be supported by the digestate analysis referred to above.

Use of the biogas

The surplus biogas has so far been used for heating water.  For the second year, operating with a better CoP, it is anticipated that additional gas will be available for cooking and baking, and suitable equipment is to be developed.

Combining the mAD energy systems with those at Ourganics.

Already there has been some success in providing biogas to the outside kitchen at the site, where it has been used for heating water for cooking, laundry and washing.  More biogas from a more efficient process will increase the range of activities and further reduce the need to buy-in bottled gas.   The digester will be heated electrically from a photovoltaic system and operated mainly during the growing season; at other times excess electricity will be made available to the proprietor’s house, to supplement the existing generation.

Sharing the learning.

A key theme in the business plan of BREG is to share knowledge about renewable energy.  When selecting the site for the mAD project, preference was given to a location running an educational programme.  This has brought a regular audience to the project, and together with three workshops, and visits to other sites, has enabled the team to speak to something in excess of 325 people.  Further workshops are planned for 2013, and the project will continue to blog their learning here.

Food producers at one of the AD workshops.

The workshops were very popular, with our favourite student being Poppy (the dog).

In the new year it is proposed to assemble a mobile digester, based on the diy design, together with a basic all-weather demonstration area which will be available in time for the Bridport Food Festival in June.   This will require the purchase of a trailer and gazebo and associated facilities, and thereafter the equipment will be available for demonstration at events elsewhere.

During the past year the project has focused on developing an understanding of the process, and producing biogas.  The biochemistry has proved to be very reliable, and with development of the low cost DIY system it is hoped to draw in other food producers, in particular the research group of the Permaculture Association.  If you would like to try some digestate, please get in touch.


Richard Toft, mAD Project Manager.

This project has been funded by DECC LEAF and part financed by the European Agricultural Fund for European Development 2007-2012: Europe investing in rural areas.




Thermal images; seeing the heat

On October 13, 2012, in Heat loss, by David Neylan

Earlier in the year we borrowed a thermal imaging camera to view the digester while the heating was being tested.  While the amount of gas used to maintain the digester at 40C was being measured, the camera was used to find the greatest losses.

Thermal Imaging is now a much more common tool for assessing the heat losses from buildings to aid the improvement in insulation and save energy.

Though the images are revealing there has to be an understanding of what is happening to gain useful results.  An example of this would be the time of day that a building was surveyed; if the camera was used in the evening after a sunny day the walls would appear to be emitting heat which on the face of it would show heat loss from the building, but if the wall was made of masonry it may have absorbed heat during the day and be re-radiating it.  (more…)


Yes we have gas!

On September 19, 2012, in Biogas, by David Neylan

The digester has been producing gas reliably since mid July.   We used a good inoculum (cattle slurry and digestate from another digester) and it was no surprise that the fresh green biomass produced burnable biogas.

It is quite difficult to see any gas flame in daylight as even when it is not sunny the polytunnel is very bright.  By making a dark background and blocking out some light we have this picture.


The picture shows the biogas flame heating a kettle.

The amount of gas we have been producing has been linked to the amount of grass we have fed.  Initially we wanted to make as much gas as possible but we are still developing the kitchen to make it useful so we have been in the situation of having to burn the gas it off when we tend the digester.  This has led us to ease off the feeding to reduce the gas production, not an ideal situation.  The aim of this project was not to just (more…)