All I want for Christmas is a working Anaerobic Digestor....
We have long felt that it is important to try to minimise our carbon footprint and also to generate our own renewable energy.
In 2007 the community owned wind turbine was commissioned, and before that we installed solar hot water panels on most of the houses on the farm.
I think it is fair to say that we have come to know the good and bad points of them both. Suffice to say that they obviously need specific weather conditions before they work.
So we decided to install an anaerobic digester(AD) with our eyes wide open. After much research, we decided on a version that would be ‘farmer friendly’ and would only need the slurry from the dairy cows topped up with low grade silage (weeds!). A ‘farmer friendly’ anaerobic digester is one that doesn’t need a lot of attention and maintenance. Farmers are busy enough as it is, without having to spend a couple of hours every day tending a digester. We’d heard stories of digesters needing constant attention from a full time engineer with alarms installed all over the farm (including the bedroom). So a ‘farmer friendly’ one which you only had to tend a couple of hours a week seemed the way to go. We also didn’t want to follow the German route of adding lots of maize to the slurry. It just seems mad to be using crops that humans can eat to produce energy. But if government’s incentive system is more geared to encouraging farmers to produce renewable energy than to grow food, as it was in Germany, then that’s what will happen.
The advantages of AD over wind or solar just seemed a ‘no-brainer’. Unlike wind and sun, we have a plentiful supply of slurry (and weeds – we’re an organic farm after all). Warming the waste from cows to maximise the methane production and then using that methane to produce electricity, whilst at the same time reducing the methane to carbon dioxide is also less damaging to the environment (methane is over 20 times more harmful than carbon dioxide).
Our Combined Heat and Power unit (CHP) is basically a lorry engine that has been converted to run on methane instead of diesel. The engine turns a generator to produce electricity and just like a car or lorry engine, it is water cooled and therefore produces lots of hot water – combined heat and power. Now the beauty of this is that we are in charge. It’s not a case of ‘do we have wind today?’, instead it is ‘how much energy do we need today?’. Whilst the engine will run 24/7 if you need it, we can also opt to turn off the engine for a few hours and only run it when we are using electricity. The price you get when buying and selling electricity is a bit like the interest rate you get from your bank on a loan versus on savings. We are currently buying electricity (100% renewable of course!) for about 12p, but when we generate it and don’t use it ourselves it ‘spills’ on to the national grid. Then we are paid just under 5p for it. It makes sense to use as much as possible of the electricity that you generate yourself. So the ability to switch on and off to match when you need it, was a big plus.
We still had a few concerns, the main one being that of the 4 anaerobic digesters we had visited we’d only seen one that was actually working. There always seemed to be a good reason for it not working, and we were reasonably confident that we wouldn’t make the same mistakes……
So where are we now? We were right, we didn’t make the same mistakes – we made different ones! I won’t go in to every detail, that would just be embarrassing for several people (mainly us). But the main things along the line have been:-
1. Small scale renewable companies often go bust. It happened with the company who supplied the community wind turbine 9 years ago and it happened again with one of the companies in this project. All the people we’ve worked with in the Renewables sector are fantastic, genuine, innovative individuals. But they are usually more interested in the technical side of what they are doing than the financials. And as we all know, banks in recent years have not been known for supporting businesses without a strong financial plan. So the end result is that the company you start the project with either disappears or gets subsumed by a bigger company who isn’t interested in small scale renewables. Your technical support has gone.
2. Although this has caused a couple of long delays in the project, on the positive side, we have found a couple of local guys who were prepared to ‘have a go’. One sorted the mechanics of the CHP engine and the other re-programmed its software. So we have the beginnings of some local expertise for our system.
3. For the slurry to maximise its methane output it’s best kept at a temperature of around 35oC. Our slurry tank is insulated and has underfloor heating. Once the slurry reaches about 22 oC, there is enough methane to run a boiler that heats the underfloor heating system, but first you have to get the slurry to over 22 oC. That cost over £2000 of LPG. Yes, we get the irony!
4. We like to think that we are doing the ‘right thing’ by our cows from an animal welfare point of view. Whilst the bigger herds are now housed 365 days a year, our cows still go out to grass for the better half of the year. It depends on the weather, but this year they went out to lush new grass in mid April and came back to their comfy shed mid November. This means that we can collect and store the slurry during the winter months, but will get very little additional slurry during the summer months, as they are only indoors being milked for 2 hours a day. As well as the slurry from the dairy cows, we store the slurry from the younger cattle which are also housed during the winter. That slurry is kept in a separate store from that of the milking cows as they are in separate sheds. The plan was that we would use the slurry from the dairy cows during the winter, then when the cows went out to grass, we would use the slurry from the younger stock that had been building up all winter. This year has been our first full year of operating the AD. The methane production was working well. We cut the grass for silage in June. Immediately following that farmers always apply the slurry as a fertiliser. One of the great things about the digestate is that it is a much better fertiliser than raw slurry. It doesn’t smell (as much), because the ammonia has been removed in the digestion process. It is also not as toxic to the environment as raw slurry, and if applied correctly it releases less nitrogen to the atmosphere – a win-win-win. And the good news is that in practice all this appears to be true. We are truly happy with the digestate as a fertiliser. After we had applied the digestate, we added more of the old winter slurry from the young cattle and it quickly produced enough methane to fire up the boiler and away it went again. Our second cut of silage was at the end of August, and again we spread the digestate as a fertiliser on the newly cut grass. We transferred more of the old slurry into our digestore – and nothing - no gas! We added the weeds to it (loads of them), we brought in LPG again to try to get it heated and still nothing! We talked to a microbiologist who told us that we had dead slurry, no matter what we added to the slurry, it would not act as an effective medium. We should empty the tower and get some fresh slurry in as soon as we could.
5. It is now December. We have fresh slurry. ‘This should only take a few days’ said David. That was until he went to check on it one evening to see flames coming out the sides of the boiler! Just a little alarming – maybe that alarm in our bedroom wouldn’t be such a bad idea. This was caused by the boiler becoming completely gunged up with sulphur and carbon deposits. So he spent a day cleaning that only to discover a burned out wire connected to a thermostat. The next job today is to google for a replacement cable with correct thermostat connection.
6.To be continued……………..