<P> Recent research by John Dabiri of Caltech suggests that vertical wind turbines may be placed much more closely together so long as an alternating pattern of rotation is created allowing blades of neighbouring turbines to move in the same direction as they approach one another . </P> <P> Wind turbines need regular maintenance to stay reliable and available, in the best case turbines are available to generate energy 98% of the time . </P> <P> Modern turbines usually have a small onboard crane for hoisting maintenance tools and minor components . However, large heavy components like generator, gearbox, blades and so on are rarely replaced and a heavy lift external crane is needed in those cases . If the turbine has a difficult access road, a containerized crane can be lifted up by the internal crane to provide heavier lifting . </P> <P> Installation of new wind turbines can be controversial . An alternative is repowering, where existing wind turbines are replaced with bigger, more powerful ones, sometimes in smaller numbers while keeping or increasing capacity . </P>

Where does the earths wind power come from