While the use of mechanical power transfer technology declined, it continued being developed in select applications that included the marine transport industry and in cable-hauled and cable-based transportation systems. Several recent developments in several renewable energy technologies suggest that mechanical power transfer may be applied to segment of the power generation industry. A research team at Technical University of Delft in the Netherlands is developing a kite-based airborne wind power conversion technology called a laddermill.
The laddermill involves a series of kites fly on a transverse axis in a loop. It is a cable-based airborne conveyor system that drives a ground-based generator via a mechanical cable power transmission. The research group at Delft estimates that the laddermill could generate up to 200MW and transfer that power to an alternator using a tension cable would around a drum. Ongoing research and development into ultra-long-chain, super-polymer lightweight fibers of extraordinarily high tensile strength such as Kevlar, carbon fiber and specialized steel alloys suggest that such materials may actually be capable of carrying the extreme tensile loadings of 100MW per cable.
There are several research groups around the world that are in the early stages of developing various forms of airborne wind conversion technology that involve ground-based power conversion and/or power storage technology. The ground-level technology may be a generator (alternator), a water pump, an air pump or a mechanical power transfer system that links multiple airborne power systems to a single ground-level power conversion or energy storage technology. The ground-level power transfer technology may involve already proven and existing technology such as aerial tramway cable systems that involve much weight and bulk.
Lightweight fiber material of ultra-high tensile strength may likely be applied to kites that are designed to change the angle of aerodynamic attack in relation to high-altitude winds so as to exert a sequence of alternating tensile forces on a series of lightweight cables that drive a ground-level equipment. Several kite-based wind power concepts incorporate winch-and-cable systems to convert large lateral or vertical displacements of the kite(s) to small displacements of a piston of a water pump or air pump. Other kite-based mechanical power transfer systems propose to combine winch-and-cable systems to drive a crankshaft or to drive through one-way clutches to produce unidirectional torque.
Given the generally unpredictable nature of wind, there may be economic merit in connecting airborne wind power technology directly to an energy storage system at numerous locations. One or more kites may activate a water pump technology in a pumped hydraulic storage system or an air pump in a compressed air energy storage (CAES) system. Precedents in the natural gas industry involve compressor stations up to 200-miles apart and pressure levels up to 1500-psia. Such precedent indicates that an airborne power technology that directly drives an air compressor technology using cable-based mechanical means may be located several miles from a CAES installation.
Laddermill of Holland is using an airborne wind power technology to produce rotary motion and generate torque using high-strength lightweight cable technology and a system of pulleys. An airborne concept proposed by the Selsam Superturbine group of California involves looping a lightweight high-strength cable around an airborne and a ground-level windlass system. The multiple windings of cable around the wide pulley of the windlass promises to reduce slippage and improve prospects for high levels of mechanical power transfer at high efficiency. Such a drive system may connect a co-linear battery of connected lateral-axis wind turbines installed across a valley where predominantly unidirectional winds blow to hydraulic or pneumatic pumping systems.
Advances in mechanical tension cable technology can be applied to evolving power conversion technologies. Precedents in aerial tramway development indicate that a pair of windlass pulleys with a looped cable can transfer power mechanically over an extended distance. Ongoing development in metallurgy and in steel alloys is steadily raising tensile strength, improving corrosion resistance and increasing wear resistance of cables destined for aerial tramways and other forms of mechanical power transfer. A windlass cable power transfer system may also offer the equivalent of a gear ratio that can connect engines and electrical, hydraulic or pneumatic loads that are designed to rotate at different speeds.
In underground pumped storage, a looped windlass cable would be able to transfer over 100MW of power from a low-speed engine at ground surface level to hydraulic turbines installed some 2000-ft (600m) or more below ground surface. A windlass-and-cable mechanism that includes pulleys to guide the cable could connect multiple vertical-axis wind turbines to a single electrical generator, hydraulic pump or air compressor. The cost of the mechanical cable power transfer systems may offer cost savings over a system that involves multiple electrical generators.
Rod-based drive systems that included roller bearings connected the driving wheels of steam railway locomotives and transferred power efficiency between parallel drive shafts. A series of 3 or 4 closely spaced tension cables with roller bearings at the crankshafts would be able to transfer power efficiently between parallel crankshafts that are placed at extended distances, including connect multiple vertical-axis wind turbines. The system of tension cables could incorporate automatic slack adjusters that may compensate for the cables stretching over time as a result of cyclical tensile loads.
A tension cable system may be able to transfer power efficiently from a low-speed engine located at ground level to pumping turbines located some 2000-ft (600m) below at an underground reservoir. It could also transfer power from a submerged battery of co-linear lateral-axis kinetic free-low water turbines to a single electrical generator located above water level. The tension cable system may also be able to connect different numbers of low-speed thermal engines and pumping hydraulic turbines or pneumatic pumping systems.
While the golden age of mechanical power transfer over an extended distance may appear be of another era, a series of modern developments in airborne and ground based wind power creates a modern application for efficient mechanical power transfer technology over much greater distances. There is a convergence of technologies that combines ongoing research and development into lightweight, high-strength polymers and high-strength metallic cables along with new developments in power generation and energy storage technology. It promises to reintroduce a power transfer technology from another era and advance that technology to serve the needs of a modern era.