Cow Power
Effluent to electricity
Effluent from 900 cows is collected from a concrete pad waiting area...
...and passes into the biodigester.
There are two herds of cows on the 360-hectare property; one is milked twice per day and the other herd of older cows is milked only once. The cows generate approximately three cubic metres of manure at each milking and the same amount of digestate flows out of the biodigester in a flow-through system. Effluent from the 900 cows is collected from the concrete pad waiting area outside the milking shed and washed into a central drain using a dilute mixture of liquid manure that gradually thickens. As the last cows enter the shed, fresh water is sprayed from nozzles on the backing gate to clean the pad. Recycling the liquid saves on water use while at the same time keeping the effluent as concentrated as possible for use in the biodigester.
The collected effluent passes into the biodigester – a PVC-lined tank made of tanalised pine and insulated with extruded foam, with a capacity of 160 cubic metres. A 16-kilowatt dual-fuel diesel generator driven by a three-cylinder engine heats hot water and circulates it through piping immersed in the tank, heating the biodigester mixture to 35 degrees Celsius. Bacteria break down the manure in the biodigester, generating biogas, which is 65 per cent methane and 35 per cent carbon dioxide, while a mat of sulphur-digesting bacteria floats on the surface, removing sulphur dioxide from the gas mixture. Heat recovered from the hot oil of the engine when it is running and from the exhaust is put through a heat exchanger to heat the water used to keep the tank's contents active.
A small pump operates for about five minutes every hour to mix the contents by removing a portion from the sump level and returning it to the tank through two entry points, one at floor level and one in the headspace. "We wanted to avoid penetrating the tank as much as possible so we used existing injector ports," Mr Bywater explains. "We consider that the hydraulic principle works best, rather than introducing a stirring mechanism into the tank."
When the generator control receives a signal that there is a full headspace of gas, the generator starts up on diesel fuel and synchronises with the grid (mains power). The system then sucks the biogas out of the headspace and switches the generator to dual fuel, running 85 per cent biogas and 15 per cent diesel. The power it generates is fed onto the dairy shed switchboard and then, depending on the local load at the time, it is either used onsite or exported into the network.
"We have exported some power to date," says Mr Bywater. "We export it between milking times when the load is low. When the cows are being milked the electrical load is greater than the generator output so there will be some importing at that time. We always planned to provide about one third of the energy requirement from this particular site and the amount of material that we can get."
