Thursday, July 30, 2009
Water typically has about 780 times the density of air (if my math is right). That means that a small drag type or savonious turbine could generate some usable power. Not a lot, but usable non the less. Using 4 inch pvc for the floats and the blades gives us a blade set that is about 7 inches wide and in the drawing I made each blade set about 7 inches tall. But you could make longer if you have deeper water. If you make each blade 14 inches long and have two of them like the picture shows, then you should get somewhere between 2 watts and 300 watts. That is if the water is flowing between 2 mph and 10 mph. At about 5 mph you would get a consistent 40 watts. You could also build a few of these with those cheap treadmill motors. The motor could be housed in a wood box to keep the rain off and open on the sides at the bottom to let the belts connect. Multiple units could be connected in parallel to charge a battery bank.
So, the floats are just 4 inch pvc with end caps. The platform is 1/2 inch plywood painted with good marine paint. The generator is a small DC motor. The blades are just a section of the 4 inch pipe cut to length and cut in half and then slid apart. You could have a tail on the back so it always lines itself up into the water flow like a windmill would. Or you could have a rope that holds it into position. Also, if you slide the blades further apart from the axis, you effectively make a bigger blade set and can get much more power. But rememer that it will spin slower and the homemade wooden pulleys will have to be bigger. To see how to make a wooden pulley, check here and a lot more detail here.
So, if you have a river or stream on your property and the water flow is between 2 and 10 mph then you could use this. You could even use this for tidal as well.
How good is this? Well, let's say you get 50 watts on average, 24/7. And into a 12 volt battery that is about 4 amps. That is what a 60 watt solar panel does, except that is only about 5 hours per day. So you would need about 5 of those 60 watt panels that cost about $400 each. So, $2,000 worth of solar power PV panel equivalent. Don't under estimate continuous power. It can be very deceiving. And this could be built for $50 to $100 each depending on the motor you use or if you have one already.