It never fails. Whenever I install a new windmill, I can almost guarantee that it will be referred to as an old windmill. When I assure people that it is newly manufactured, the doubt is apparent. “Surely it has been modified to be more efficient,” is a standard comment. “Not really,” is my answer.
There’s no need to reinvent the water-pumping windmill. It’s a workhorse that has been mostly unchanged since before World War II. About one million windmills are pumping water in the world today. The most common application is to install a windmill directly over a drilled or dug well. Pumping water from an aboveground source is also an easy task for a windmill. If you need to pump water on your property and the site has access to reliable winds, a water-pumping windmill may be a good option.
Water-pumping windmills are simple devices. I always enjoy pulling the sheet-metal cover off the gearbox and letting folks see just how a windmill works. I encourage them to spin the hub and watch the internal parts interact. I’ve heard the action of a windmill motor described as “using a big wheel for leverage, like the steering wheel on a big pirate ship.” People also liken it to “a big jack that lifts water.”
No matter how you describe it, the water-pumping windmill is a simple machine that uses mechanical advantage in multiple ways. It’s a direct-drive device that transfers energy via gears, rods, simple valves, and a piston in a cylinder—and uses high torque to move water. In contrast, wind-electric turbines use electrical generators coupled to high-tech airfoils that require high speed to do their job.
This difference becomes evident when you compare the eighteen or so large blades on windmills to the two or three sleek blades on wind-electric turbines. The wide blades on the water-pumper are designed for low start-up wind speeds and slow-speed operation, as opposed to the electrical generator’s thin blades, which are designed to run at higher rpm.
The blades of the windmill wheel catch the wind—just like the sails on a sailboat—which turns the wheel (rotor). The wheel is attached to a shaft by long arms. The shaft has small pinion gears at the other end, inside a gearbox. The pinion gears drive larger bull gears, which move pitman arms. The pitman arms push a sliding yoke up and down, above the bull gears (much like a crankshaft, connecting rod, and piston in a standard vehicle engine). The moving yoke lifts and drops the pump rod to do the work down below.
The pump rod goes down the tower through a watertight seal at the top of the well’s drop pipe, and to the pump cylinder, the part that moves the water. The cylinder is attached to the bottom of the drop pipe below the water level, and has a simple piston and two check valves.
As the piston rises, water moves up the pipe above it. At the same time, water is sucked through a screen and the lower check valve below the piston, into the lower section of the pump cylinder. When the pump rod reverses and begins to descend, the lower check valve closes and the piston check valve opens. This allows water in the cylinder to pass through, and the water that is trapped above the piston to be pushed up out of the cylinder and ultimately to its final delivery height. One might think of the pump as a cup with a trap door in the bottom that opens when the cup falls and shuts when the cup rises. This cycle is constantly repeated as the wind wheel turns to move the pump rod up and down.
If the wind wheel is moving, the pump piston is moving. As the wind speed increases, the speed and frequency of the piston stroke increases, so more water is pumped. But the windmill’s efficiency drops because the airfoil is not optimized for higher wind speeds—it doesn’t make as much use of the cubic effect of wind power as a wind generator does. (The power available in the wind is proportional to the cube of the wind speed.) But then, water needs do not increase in proportion to the wind speed either, so this is not a major impediment. In fact, water pumpers do the job they are designed for efficiently and well.