Horsepower

yes.

In general, torque will increase with the wind speed squared. (HP therefore increases with the wind speed cubed.) Probably the easiest way to measure torque is by powering an alternator or generator with the turbine shaft, and measuring the power output (providing an electrical load such as water heaters as required). Then the torque can be calculated from power and rpm. (This is basically the inverse of what you plan to do on your bench.)

You say the turbine is driving a shaft directly with no load. I assume you are aware that you must load the shaft to measure torque.

Ken, thanks for your reply. I am really trying to find out what HP the turbine will produce at different wind speeds. The reason being that I have a range of alternators which are driven by petrol engines. We know the hp of these engines and what the alternators can give out at certain revs. I still dont know if my turbine would be strong enough to match the engine hp.

At the moment there is just a shaft coming off the centre of the turbine. I was hoping to load the shaft with some kind of torque reader maybe something like is used to test tractors using the PTO. Is there any simple way to do this.

The standard, mechanically simple dynamometer is a disc brake with the caliper mounted on a torque reaction arm to which a scale is attached (or scale equivalent, such as a strain gauge). However, by the time you come up with the mounting arrangement, and means for varying the brake force remotely, and the means for reading the strain gauge remotely, you could have already set up an alternator as the load, I think.

Hydraulic pumps can make good dynamometer loads. To keep the pump small, you'd drive the pump via a chain or gear step up transmission, and monitor pump pressure as the indicator of torque. You'd have a lot of hose to run, however.

If you use an automotive alternator, (or other engine-driven alternator) you will need to gear it up (increase its speed relative to the turbine shaft) to get a reasonable output.

The power of a windmill, in general terms, is somewhat less than:

r² x V³

Power is in watts, with the blade radius (r) in meters, and the wind speed (V), in meters per second. So with 3 meter blades and a 10 meter per second wind, power would be 9000 watts. With a 5 m/s wind, the power would be 1125 watts.

Tip speed is generally 5 times wind speed, so in the 10 m/s wind, the tips would be moving at 50 m/s (don't get your head in the way!). The circumference is about 19 meters, so the rpm would be about 160. Torque would then be expected to be close to 400 lb ft or 540 Nm. In a 5 m/s breeze, torque would be about 135 Nm, and rpm would be around 80. Depending on the prevailing wind speed, you might want something like 1:20 step up gearing.

Thanks Ken. If I used your theoretical calcs which come up with ft lbs of torque, how do I equate that to hp. Also if I had a pump driven by the turbine how do I convert the psi readings to torque or hp.

It is our ultimate aim to have an hyd pump on the mast ,piped down to a hyd motor on the ground, driving an alternator. At the moment I know that the alternator needs around 8 to 10hp to drive it at its most efficient speed.It would still be charging at lower speeds but not as much.

If you are planning to use hydraulics anyway, then using a pump to measure torque would clearly be the way to go.

To go from kW to hp, divide by .746. So, for instance, 9 kW = 9/.746 hp = 12.06 hp.

To go from hp to torque: hp/5252 * rpm. So 12 hp*5252 /160 = 394 lb ft.

As you probably know, the voltage produced by the alternator must be higher than the battery voltage to charge batteries. Voltage will change with rotation speed. So being able to increase the rotation speed in low wind speeds would be beneficial. Hydraulics are able to do this easily. You may be familiar with hydrostatic transmissions on tractors. They can produce a wide range of wheel speeds from a constant engine speed. Typically the wheel torque, even for a smallish (15 hp) tractor is extremely high at low wheels speeds, because of the effective gear ratio. (A 3600 rpm engine will sometimes be powering the wheels at only 60 rpm during tilling etc: 60:1) So for your application, you might want to experiment with a hydrostatic transmission from a tractor, which would have the pump, motor, and controls. (You'd be using it "backwards" with the wheel end at the windmill, and the engine end at the alternator.) (In the US these units are usually available from surplus outlets.) HOWEVER: most of these units are quite fully integrated, with the pump and/or motor being built right into the housing -- so you would want to make sure you know what you are getting. You could find it very difficult to be allow the motor and pump to be in different locations. (If cost is not an overriding concern, you'd just buy an appropriately sized variable displacement pump or motor -- they tend to be quite expensive, however.)

In the interests of efficiency, you'd want the lines to and from the pump and motor to be large diameter and straight. There are some quite efficient variable displacement pumps and motors available. I doubt that the hydrostatic transmissions in lawn tractors are especially efficient -- they are built mainly to a price, and mainly for flexibility in speed control rather than for minimizing fuel consumption. But if you read about the HyDrid, (you can download a brochure) you can see that at least the developers believe that hydraulics can be efficient.

Another possibility for hydraulic drive would be to use gear (or geroler) pumps (which are cheaper but less efficient than piston pumps) and then use a belt drive CVT (which are inexpensive) at ground level to automatically keep the speed relatively high when the wind drops. These units generally vary over a range from about 1:1 to 2.5:1, (which is not quite enough to work in both high ans low wind speeds) but could perhaps help keep the system generating over a wider range of wind speeds. The bulk of the reduction would be handled by a difference in displacement of the pump at the turbine vs the one at the ground.

Hydraulics, incidentally, work just like gearing for your purposes: the speed of the driven unit varies directly with the volume of the pump, and the torque varies inversely -- you trade one for the other, just as you do in gearing.

The hp of a hydraulic motor is: psi * gpm / 1714. From hp you can calculate torque, per the previous formula.

Other possibilties: chain drives are very efficienct. A chain drive could go down the center of the tower, to power a hydrostatic unit at ground level. This would avoid the losses of pumping fliud through long lines, and would keep the complex parts at ground level, where they are easier to service. It would also allow you to use an integrated unit like the tractor unit. Of course you'd have the problem of how to enable the windmill to align with the wind without twisting the chain drive. (This is a concern with hydraulic lines too.)

Ken I commend your knowledge on the stuff I am doing. I looked up your Web Site, very interesting and furturistic. I am also very interested in that kind of stuff.

I would like to email you directly about the exact nature of what I am doing if you would`nt mind. If you would be ok with it please let know on michael@roof2000.com

Quote:

The hp of a hydraulic motor is: psi * gpm / 1714. From hp you can calculate torque, per the previous formula.

Hydraulic motor torque (in-lb) = Pressure (Psi) x motor displacement (in³/rev) / (2 x Pi)

Hydraulic motor power (Hp) = Torque (in-lb) x RPM / 63025

From Surplus Center.

Ken's method is the easiest and most accurate. You could calculate torque using the aerodynamic efficiency of your blades and the aerodynamic conditions, but that is extremely difficult. Even if you have a standard blade with known performance parameters, the performance of the blades is dependent on blade angle of attack and apparent wind which is a function of blade angle, wind speed, and rotor speed.

These can all be calculated but there is a reason wind tunnels are still used in the field of aerodynamics.

Here is a good forumla :

scroll down the page

http://www.otherpower.com/windbasics1.html