There are over 1,000 old mills spread over the Holland countryside, Last year, one of them was the first to be retrofit with an add-on package for generating electrical power.
The idea makes sense since experts figure it takes about 10 kW to turn a mill stone. And mills are typically overbuilt: Most generate about 240 kW.
The mill chosen for retrofit dates to 1851. It turns at 4 to 24 rpm, and this gets sped up by a factor of 10, thanks to a special wheel. The mill still uses wooden gears, and though many have been replaced over the years, they are still built to original specifications. So the first step was to replace one of those gears with a stronger version that used reinforced plastic rollers.
This new gear attaches to a rightangle gearbox that lets the new components fit inside the mill, which is crowded with wooden beams. It sends power to a 10-kW generator through an Ogura, Somerset, N. J., electromagnetic clutch sized to handle the anticipated torques and speeds The generator starts sending out electricity when its input shaft turns at anywhere from 5 to 240 rpm. The mill creates an average of 955 Nm of torque for the generator, and the generator can only handle up to 540 Nm at 240 rpm. The clutch has a static torque of 700 Nm so it can easily handle the loads. Cycle rates and heat dissipation are not a concern for the clutch. The clutch can also disconnect the generator if power is not needed, if the miller needs power for grinding corn, or if strong winds cause an overspeed (exceeding 240 rpm at the clutch), at which point the clutch automatically disengages.
There are no batteries, so power gets used in an attached meeting room and restaurant. If these establishments do not need power, it goes into the national grid. But before the 50-Hz power gets used or sent to the grid, it goes through a grid-feed inverter for matching and synchronization.
The entire retrofit cost about $55,000 to $67,000, but the “free” power replaces electricity that costs $0.26/kW-hr in Holland, and that’s without the 19% VAT.