Wind turbine speed will play a big part in longevity. Just as you wouldn't drive your car down the interstate in first gear because of the increased engine wear and poor fuel economy, you don't want a wind turbine that spins at an excessively high RPM. The faster that rotor turns, the quicker the parts will wear out.
All the small turbines we discuss, except for the Jacobs, are direct-drive machines. They are called direct-drive because there is no gearbox between the rotor and the alternator. The alternator and rotor shaft spin at the same RPM. Fortunately most wind turbines marketed today will list the RPM which makes it handy for comparison purposes.
To make a connection, let's look at a gasoline or diesel powered generator. Typically they will operate at a speed of either 1800 or 3600 RPM. For a similar sized generator, the one that operates at 1800 RPM will use less fuel than the 3600 RPM unit. For one thing a faster turning engine will produce more friction which means it will use more fuel. This is one of the factors in the newer "inverter" generators that have a switch usually labeled as "Eco". These generators have the ability to run at a reduced engine speed when under a light load.
Bearing life can also be determined by wind turbine speed. Bearings are important as they are one of the few moving parts on a direct-drive wind turbine. Several factors play into the like of a bearing with heat, dirt comtamination and inadequate lubrication as the major culprits.
Another issue that can increase as RPM increases is vibration. As a machine wears and the blades get nicked and abraided, imbalances will become more pronounced at higher speeds. This will naturally put more stress on the turbine and tower which can lead to early failure in any of these components.
The problems such as excessive vibrations can occur in any turbine, high or low RPM. Something such as an out of balance condition will eventually be a problem for any machine. Saying that turbine RPM is directly related to increased failure may be a broad and unfair characterization, though it is our opinion that there is a correlation. When you are comparing turbines you may find that this is one of those "features" that increase the price tag but really do nothing for increased energy output. In the short term that may very well be true but a turbine is a long term investment so slow and steady may very well be a better choice.