What kind of torque can slow moving water induce on an impeller shaft? I expect that it depends on the diameter of the impeller and perhaps the depth of the impeller, but I am out of my, er, depth here.
Thanks,
Rob
If you can provide even a ballpark estimate of the size of impeller you have in mind, I can easily run you through the calculations to get an order-of-magnitude estimate.
But let’s go for huge just so you can get an upper bound. Anything you might be envisioning will surely be less than this:
Let’s imagine that a simple impeller spans the width of the Mississippi River somewhere in Missouri. At this point, the river is 0.75 miles wide and flows at 2.5 miles per hour. (In units that make calculations easier, that’s 3960 ft wide and 3.7 ft/s)
Let’s say this hypthetical impeller is designed so that only one blade is submerged at any time. And each blade, when plunged into the water, is submerged to a depth of 20 ft. The axle of the impeller is another 20 feet above the surface of the water.
So the force exterted on the blade is the dynamic pressure of the water multiplied by the area of the blade. F = qA = ½ρV[sup]2[/sup]A = 0.5 * 1.94 slug/ft[sup]3[/sup] * (3.7 ft/s)[sup]2[/sup] * (20 ft * 3960 ft) = 2.10 million pounds.
Since that force effectively acts at the centroid of the blade (10 feet below the water, 30 ft from the axle), that resuls in a torque of 2.10x10[sup]6[/sup] lb * 30 ft = 63 million lb-ft of torque.
Consider that an upper bound for your impeller, and only then if you plan on spanning the Mississippi.
I ask because I was reading an article that mentioned in passing that although water contains as significant amount of energy, its low speed makes it impractical to use. It seems to me that if it is generating sufficient torque on an impeller shaft that it could be geared up for more practical speeds. Since they aren’t doing that, I wanted to know what the issue was.
Thanks,
Rob
That’s a big twinkie…
You are not thinking in the right units. Torque is really not important. Power is what you need to think. The problem is that to generate any significant amount of power you need to harness a lot of slow moving water or much less fast moving water.
The calculation is not difficult. To extract energy from a moving fluid you use an impeller which slows the fluid and extracts energy in the process. How much kinetic energy does X amount of water moving at 4 MPH? Suppose the impeller slows it to 3.5 MPH. How much energy does it have then? The difference is the theoretical maximum you can extract.
The torque, like the plumage, don’t enter into it.
Here’s an interesting article on a promising technology that can generate electricity in water flow as slow as 1.5 knots. The article states that turbines and water mills need an average of 5 or 6 knots to operate efficiently.
The device consists of vertical cylinders which should have little detrimental effect on marine life. Let’s hope it works.