Why can't a low solidity wind turbine be used in high torque applications through gearing? From my understanding, low solidity wind turbines (such as the three blade type) are more efficient due to a higher tip speed ratio, giving a higher coefficient of performance.  However, they are not well suited to high torque applications.
Whereas, high solidity turbines have a lower tip speed ratio hence they are less efficient, but they produce more torque, making them better suited for applications like pumping water.
This is what the text books tell me: low solidity for electricity production vs high solidity for pumping water.
My question; why not just use high speed, low solidity turbines and gear them to produce a low speed, high torque output?  This makes use of the more efficient turbine design and allows it to be applied to a high torque requirement.  There are other factors in choosing a turbine, such as start up speeds but surely in terms of power output gearing can be used to match a turbine to a load?
Thanks!
 A: High speed turbines have higher blade stresses, more stringent balancing requirements, and more design issues with the rotordynamics of flexible blades.
By comparison, a low speed turbine that works can be cobbled together with any sort of crude technology - it might not be very efficient, but it can do a useful amount of work and it won't fall apart in a high wind!
People have been making useful low-speed turbines for literally thousands of years - they called them "windmills." From an engineering point of view, you don't fix designs that aren't broken!
It might be worth commenting that large wind turbines (in the MW power range) are usually very low rotation speed and high torque devices (e.g. 5 or 10 RPM) compared with a typical small water pump turbine - but of course the blade tip speeds are high because the tip radius is very big.
A: Wind turbines are application-designed, and adding gears (to address different applications) costs money. In addition, a many-bladed turbine is efficient enough as-is to run a water pump given that the required work comes for free. You just scale up the blade disc diameter a bit if you want more work output and live with the inefficiency. 
