Converting slurry into electricity has paid handsome dividends for one dairy business which threw open its doors to offer a glimpse of how an anaerobic digester works. Wendy Short was joined visitors
Four generations of Chippendales occupy the family farm at Kirkby Stephen and while father, Mark was prepared to give up dairying when milk prices reached their low point, son, Adam was determined to push on.
Adam was, however, keen to escape the ties of twice-daily milking for the 170-cow Holstein Friesian herd and four robotic systems were installed at the 300-hectare Wharton Hall in 2011.
This has allowed him more time to oversee the management of the cows, although their electricity usage is higher, compared to a conventional parlour.
The herd is housed all-year-round, with permanent access to outdoor loafing areas, and there are plans afoot to move up to 200 milkers. In addition, the farm rears bullocks from the dairy herd, which are sold as stores, as well as running a small Aberdeen Angus suckler herd and a flock of more than 1,000 ewes.
As well as looking for time savings, the Chippendales also wanted to improve the environmental credentials of their farm and they first looked at the installation of solar panels, which were refused on the grounds on the farm’s grade one architectural listing.
Wind turbines were another potential option, but the steading’s highly visible position and the likelihood of local objections were the main reason for the rejection of the idea.
A more viable solution was to tackle the methane emissions produced by the herd, but the family was initially under the impression that anaerobic digestion was only suitable for large enterprises and would require multiple feedstocks.
After some research, they discovered that a small-scale digester capable of converting slurry into electricity was available on the market.
It would produce electricity to run the farm and provide an income, via environmental payments.
After extensive research, they selected a 44kW plant, which was designed by Bioelectric and fitted by South Yorkshire-based Solcap Energy.
Installed in April 2017 at a total cost of about £300,000, it can handle 20,000 litres of slurry per hour.
The material is heated to 35-37C to encourage the production of methane.
“At some point, we are hoping that we will also use the electricity produced by the farm in our houses, but the cabling is expensive and therefore we have installed a domestic biomass boiler for our own heating and hot water,” says Adam.
“At present, we use about 75 per cent of the electricity that is produced by the digester in our dairy, which should bring an annual saving of roughly £40,000; robots also tend to use more water, compared with traditional parlours.
“The remainder of the electricity that is generated is being sold to the National Grid on a 20-year agreement, as part of the Government’s renewable energy incentive payments.
“The liquid by-product of the process, digestate, will be spread on the land,” he said.
“The anaerobic digester is part of our commitment to dairying and has also offered us considerable saving, so that we can reduce our milk production costs.
“On the figures that have been provided, we should have recouped our investment within a maximum of seven years.”
Anaerobic Digester
THE anaerobic digester plant requires a 250 cubic metre site, in fairly close proximity to the slurry store.
The minimum recommended herd size is 80 cows; enough to supply the smallest-sized slurry-only plant, which is 11kW.
Slurry to fuel the system must contain no rain water, wash-down water or antibiotics.
Access to three-phase electricity is another requirement.
It is claimed that the digestate will allow a longer spreading window, but this has not yet been proven at Wharton Hall and the material has yet to be formally tested for its soil nutritional properties.
How It Works
THE fully automated installation pumps a set quantity of digestate from the reactor to the storage area and replaces it with fresh slurry from the store. Using anaerobic fermentation, the reactor converts the slurry into gas, which is subsequently purified and combusted in the engine, to convert it into green energy.
Biomass Boiler
FOUR generations of Chippendales live on three houses on the site, with the biomass boiler supplying all of the heating and hot water.
The farm woodland produces roughly 50 per cent of the wood that is utilised, with the remainder bought in.
“In the future, we may use heat from the digester to dry the logs for the biomass boiler and there may also be the potential to divert some of the energy for grain-drying,” says Adam.
“This would give us another saving on bills and also a possible income, through the Renewable Heat Incentive payment scheme.
“Energy bills are a constant worry and while the bills are expensive at present, it is likely that they will only increase in future years.”
Herd Performance
THE herd yields 11,000kgs from a total mixed ration which includes 34kgs of grass silage, 10kgs of home-grown triticale produced as wholecrop and 5kgs of a protein blend.
The calving interval is less than 380 days.
Each cow visits the robot an average 3.7 times a day.
Calving is all year round and the milk, which averages 4.1 per cent butterfat and 3.4 per cent protein, is sold to Arla.
Dairy Cow Slurry/Biogas Facts
ONE cow produces 26 cubic metres of manure per year. It can be converted to 650 cubic metres of biogas annually.
Three cows will produce enough manure to provide enough energy to support one small family for a year.
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