In this regard, Google's energy chief literally "raised the hub" by commenting on the merit of mounting turbines on towers with hubs at elevations of 200m or 656-feet. For many years the hubs were placed at an elevation 80m or 262-feet with newer towers placing the hub at 100m (328-feet) and up to 128m (420-feet). Wind velocities are respectively 1.25 and 1.6-times the velocity at 80m elevation. While placing hubs at an elevation of 200m may seem daunting, there may be potential to borrow precedent from the bridge building industry.
Cable Suspended Turbines:
There has been recent research on installing suspension bridge type cable systems across windswept valleys and ocean inlets to carry turbines at locations where predominantly unidirectional winds blow. Such is the case in the southern Andes Mountains (southern Chile), the fiords of Norway and the Western Ghats of India during the summer monsoon. The boundary layer effect causes wind to interact with nature of the terrain, often "steering" winds into the valleys where the channel effect guides the wind through.
While cable systems derived from suspension bridge design have potential to carry multiple levels of groups of lateral-axis turbines across valleys, they limited to valleys that are often less than 2000m (6500-feet) across. Cable-based-windlass drive mechanisms may connect groups of turbines that rotate in the same direction at different elevations and drive into a single electrical generator. Such an approach promises to reduce the cost of generators and towers that are related to wind power.
An alternate cable-suspended wind technology combines aerial tramway and ski lift cable technology. Wind pushes cable-mounted airfoils in a loop back-and-forth across a valley and drives electrical generators mounted near the valley walls. Several research people are undertaking further research to develop such a concept. A cross-sectional area of wind of 5000-feet in width by 200-feet in height, blowing at 30mi/hr (50Km/hr) could generate a peak output of 46MW at 35% conversion efficiency with over 150MW of peak output in a wind of 45mi/hr.
Curved Span on Towers:
A revolutionary new design of bridge was recently opened to traffic in France and involves a curved span placed at an elevation of 780-feet or (237m) crossing a windswept valley. Tension cables installed inside the rectangular cross section maintain a compressive load across the curved span that is held aloft of comparatively slender bridge posts. The curved span is built into the valley walls on both sides and provides fore-and-aft stability and lateral stability to the bridge posts through a firm connection.
While bridge posts and wind towers differ greatly in design, there may be potential to borrow design precedents from one and apply it to the other. Example, 2 x offshore islands that are 2 to 3-miles apart may each protrude to a height of over 300m from the sea or a lake. The water between them may be sufficiently shallow to install towers for wind turbines and winds may be favorable. There may be potential for structural engineers to seek ways by which to combine a curved span structure with wind towers placed between the 2-islands, to increase the elevation of turbine hubs.
Wind velocity may increase by a factor of 1.7 to 2.2 over the velocity at an elevation of 80m (262-feet), with potential for the same diameter of wind turbine to generate 5 to 10-times the power output. For wind power generation, 2-curved rows of box-shaped trusses that connect the between the towers would form the spans that would be secured into islands or valley walls on either side. The trusses would be secured at 2-levels to each tower and be designed to manage tensile as well as compressive loads. Diagonal trusses would connect at 2-levels between the towers of the 1st and 2nd row.
The combination of the box-shaped trusses that form the curved spans and similar diagonal bracing trusses would duplicate the curved bridge span. The design would provide the lateral and fore-and-aft stability that would strengthen the towers against buckling and flexing. An offshore wind farm built using such technology could involves 50-turbines of 20MW to30MW each with hubs placed at elevations of over 200m. The array could generate a peak output of some 1000MW to 1500MW of output at lower costs than independently built towers.
The towers connected to and stabilized by a curved span may also carry vertical-axis turbines, with the potential to install groups of multiple lateral-axis turbines that drive into a generator above and blow the curved span. The vibrations generated by the lateral-axis turbines may mimic the vertical and lateral vibrations generated by a freight train crossing over a bridge. There may be scope to install a combination of tower mounted (axial-flow or vertical axis) and span mounted (lateral-axis) turbines to increase power output at competitive costs.
Future energy markets will likely demand technology that can generate electric power at competitive market prices and free from government subsidy. On this front, there is potential to develop terrain enhanced tower-mounted turbines that place hubs at elevations of 200m to 300m and incorporate turbines of 20MW to over 30MW output. While the curved span offers a method by which to stabilize towers and allow them to be built to greater height, there is also potential to use tension cable systems secured to valley walls to provide structural support to higher towers that are placed in windswept valleys.
There will likely be further development on turbines that dispense with the tower and that may be carried entirely by tension cable suspension systems for operation in windy valleys and ocean inlets. Several groups that are working at refining competing airborne wind power technologies that involve airborne and ground level electrical generators have already built and tested prototypes. The work of increasing power output while reducing the cost of wind power will continue, with the likelihood of breakthroughs also occurring in tower-mounted turbine technology.