Two engineers, Wallace Bester and Francois van Tonder, made a clever and simple plan for their friend – a dairy farmer – to generate electricity from biogas for his small dairy and to save cost in this way.
It makes sense to make your own electricity if you have a large dairy farm of 600 cows or more, since the electricity savings are so big that that it could pay off the infrastructure costs in no time. But at smaller dairy farms it takes a bit more effort to get the numbers to add up.
This didn’t scare off Wallace Bester and Francois van Tonder of Africa Thermal Technologies. Fanie Basson asked them to make a plan with the slurry dam on his farm, Welgemeend, near Malmesbury in the Western Cape province of South Africa. Fanie milks about 300 cows, makes his own silage and also farms with wine grapes.
Since the equipment is expensive, they tried alternative sources like the website classified advertising website Gumtree.co.za to keep the costs down for this fairly simple system which catches biogas through means of a sheet and transforms it into electricity that is fed into the farm’s power network.
A similar system with pumps, regulators, filters and imported engines could cost anything from ZMW 2 million to about ZMW 2,8 million. But they managed to install it for less than ZMW 70 000 by using second-hand equipment, a few pipes and two buckets.
“This was an existing slurry dam of about 50 m x 10 m. Therefore it wasn’t necessary to set up the cement structure. We pulled a PVC sheet (10 m x 4 m) over a part of the dam to catch the biogas,” says Francois.
It’s much more cost effective than having to build a concrete well or set up a stainless steel tank. The sheet was just about the most complicated part of their task, he says. They used a PVC sheet, but a high density polyethylene sheet would have worked just as well, but it is more expensive.
“The wind flattened it, the rain made it wet and it was pulled away by storm water. The problem was hanging the weights in the right way to anchor it.”
The weights are cement blocks that are attached by means of a rope to the bottom of the sheet. Some of the weights stand at the bottom of the dam and other hang a bit higher to keep the sheet under tension.
“The sheet catches about 40m³ methane gas per day, which delivers 50 kWh worth of electricity. It represents a saving of about a ZMW 680 per month on the electricity bill.
“If we could pull a sheet over the entire dam, the engine would run all day and save up to ZMW 6 800 per month,” says Wallace.
When the sheet stands at about 1 m above the water (like in the picture) the engine is switched on, which sucks out the gas through a normal 40 mm irrigation pipe.
This is how the system works
The pipe from the sheet runs upwards first (see diagram) so any water that gets sucked up with it, can run out. It then leads to a 20 l plastic bucket which catches the rest of the water. The pipe connects the methane gas with the engine, about 30 m away.
A second plastic bucket keeps a minimum counterpressure to prevent the gas from escaping.
“It also serves as a flame trap, which prevents a fire spreading from the engine to the dam,” says Wallace.
They used a second hand Ford 3 l V6 engine with a 45 kVA AC dynamo which they found on Gumtree. It’s equipped with a truck radiator.
“We used our own mixing system which combines the air and gas so that it burns just right,” says Francois.
One part biogas is mixed with about eight parts air and then sucked into the engine. The mix is set alight by the spark of the spark plugs (similar to what happens in a standard petrol engine). The engine then runs a three phase alternator to generate the electricity.
“A lot of guys that do this need high pressure to mix the gas and air. Because we are using the engine’s sucking power to get the gas from the source, we lose about 1 % to 2% of efficiency, but it cuts out the need for a gas pump, which is expensive and uses a lot of electricity,” says Wallace.
The last step was to install an intelligent generator control which could safely sync with the power network.
“It controls how the power flows into the network and that the system doesn’t make more electricity than is needed.”
To do this, a few sensors were installed that sends signals to a control system. A speed sensor was installed on the engine’s flywheel so the control system could adapt the air valve to keep a constant speed of 50 Hz.
Thanks to an oxygen sensor in the exhaust pipe, the control system adapts the
gas valve accordingly and ensure a good burn rate. The engine’s temperature and oil pressure are also monitored and the engine can be switched off if necessary.
Consider options for financing
Wallace says they’d like to cover the entire slurry dam and are considering options to finance it. In the meantime they are extending the generator so that they can still automatically milk the cows and cool it when there are power failures.
Potential for heating
According to Francois, biogas’s real potential lies in heating, not necessarily power generation. “If a dairy farmer wants to use it to heat chicken coops or tunnels for example, the sheeting is a good option. Anyone with a dam of dung that is bubbling, should be putting a sheet over and using it as electricity or heating.”
Wallace says it would cost an estimated ZMW 280 000 to cover the entire dam and to complete the project. The energy saving would then come to about ZMW 6 800 per month. The system could therefore pay itself off within three and a half years. “Anything that can pay itself off within five years or less, is a good investment.”