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Wind-Powered Ships

Throughout history sailing has been instrumental in the development of civilization, affording humanity greater mobility than travel over land, whether for trade, transport or warfare, and increasing the capacity for fishing. The earliest representation of a ship under sail appears on a painted disc found in Kuwait dating between 5000 and 5500 BCE. The first navigators began to use animal skins or woven fabrics as sails affixed to the top of a pole set upright in a boat, these sails gave early ships range. From the 15th to the 19th centuries, ailing technology continued to develop throughout the European "Age of Sail", in which international trade and naval warfare were both dominated by sailing ships. The technological advances of the Industrial Revolution soon replaced the sail with steam powered ships, and by the 20th century sailing vessels were primarily associated with racing and pleasure craft. During the late 20th century to the present major maritime commercial companies have begun to experiment with various forms of wind power to assist ships propulsion in an effort to reduce the cost and environmental impact associated with burning fossil fuels. The following are four types of wind powered technology currently undergoing experimentation.

Flettner Rotor

A rotor ship, or Flettner ship, is a ship designed to use the Magnus effect for propulsion. To take advantage of this effect, it uses rotor sails which are powered by an engine. The Magnus effect is a force acting on a spinning body in a moving airstream, which acts perpendicularly to the direction of the airstream. The overall behavior is similar to that around an airfoil with a circulation which is generated by the mechanical rotation, rather than by airfoil action. German engineer Anton Flettner was the first to build a ship which attempted to tap this force for propulsion. His first idea was to produce the propulsion force by using a belt running round two cylinders. Later Flettner decided that the cylinders would be better rotated by individual motors, thus avoiding power losses from the main engine. Flettner applied for a German patent for the rotor ship on 16 September 1922. Flettner constructed an experimental rotor vessel, and in October 1924 a large two-rotor ship named Buckau set out on her first voyage in February 1925, from Danzig to Scotland across the North Sea. The rotors did not give the slightest cause for concern in even the stormiest weather, and the rotor ship could tack (sail into the wind) at 20-30 degrees, while the vessel with its original sail rig could not tack closer than 45 degrees to the wind. Unfortunately it was found at the time that the rotor system could not compete economically with the diesel engines that were also being developed for ships in this era. Due to the high cost of fuel and the environmental impact of global warming, there are multiple companies currently experimenting with improved versions of the Flettner Rotor with the goal of reducing fuel consumption by at least 25%.

Crosswind Kite

A tethered wing, flying in a crosswind at many times the wind speed, harvests wind power from an area that is exceeding the wing’s own area. Experiments using this concept have been conducted since the early 20th century. Crosswind kite power was brought again into focus when Miles L. Loyd carefully described the mathematics and potential of crosswind kite power in 1980. However, with the available technology it was not possible to create an economical automatic control system to control the wings of a kite system harnessing that much kinetic energy. With the advance of computational and sensory resources, fine control of the wings of a kite system became not only affordable, but cheap. In the same time significant progress has been made in synthetic materials suitable for the wing and tether have become affordable. Among those materials are high-performance polyethylene, carbon fiber, and rip-stop nylon. Multiple companies and academic teams work on crosswind kite power. Most of the progress in the field has been achieved in the last 10 years. A large number of people currently enjoy the sport of low altitude kite surfing, kiteboarding, snowkiting, and power kiting. Advances in computers, sensors, kite steering units, and servo-mechanisms are being applied to attain full autonomy of the launching, flying, and landing of crosswind-kite-power source that are aiming for the utility-scale energy-production market. But the sectors of high altitude larger CWKPS aiming for utility-scale electrical production to compete against other forms of energy production must overcome various challenges to achieve mainstream acceptance. The cost associated with the equipment required to safely launch, fly and recover crosswind kites is currently the greatest drawback.

Inflatable Kite Sails

The Inflatable kite sail concept has recently received a lot of interest. This rig consists of flying a gigantic kite from the bow of a ship using the traction developed by the kite to assist in pulling the ship through the water. Other concepts that have been explored were designed to have the kite rig alternately pull out and retract on a reel driving a generator. The kite used in this setup is similar to the kites used by recreational kiteboarders on a much larger scale. This design also allows users to expand its scale by flying multiple kites in a stacked arrangement. The idea of using kites is currently the most popular form of wind assisted propulsion on commercial ships, largely due to the low cost of retrofitting the system to existing ships with minimal interference with existing structure. This system also allows a large amount of automation using computer controls to determine the ideal kite angle and position. Using a kite allows the capture of wind at greater altitudes where wind speed is higher and more consistent. This system has seen use on several ships recently with the most notable being the MS Beluga Skysails, a merchant ship chartered by the US Military Sealift Command to evaluate the claims of efficiency and the feasibility of fitting this system to other ships.

Airborne Wind Turbine

An airborne wind turbine is a design concept for a wind turbine with a rotor supported in the air without a tower, thus benefiting from more mechanical and aerodynamic options, the higher velocity and persistence of wind at high altitudes, while avoiding the expense of tower construction, or the need for slip rings or yaw mechanism. An electrical generator may be on the ground or airborne. Challenges include safely suspending and maintaining turbines hundreds of meters off the ground in high winds and storms, transferring the harvested and/or generated power back to earth, and interference with aviation. Airborne wind turbines may operate in low or high altitudes; they are part of a wider class of airborne wind energy systems (AWES) addressed by high-altitude wind power and crosswind kite power. When the generator is on the ground, then the tethered aircraft need not carry the generator mass or have a conductive tether. When the generator is aloft, then a conductive tether would be used to transmit energy to the ground or used aloft or beamed to receivers using microwave or laser. Kites and 'helicopters' come down when there is insufficient wind; kytoons and blimps resolve the matter. Also, bad weather such as lightning or thunderstorms, could temporarily suspend use of the machines, probably requiring them to be brought back down to the ground and covered. Some schemes require a long power cable and, if the turbine is high enough, a prohibited airspace zone. As of April 2014, no commercial airborne wind turbines are in regular operation.

References

1. Flettner Rotor, http://www.thiiink.com/flettnerrotor/

2. Crosswind Kite Power, http://homes.esat.kuleuven.be/~highwind/wp-content/uploads/2011/07/Loyd1980.pdf

3. Kite Powered Yachts, http://www.peterlynnhimself.com/Kites_For_Yachts.php

4. Airborne Wind Turbines, http://blog.environmentalresearchweb.org/2014/04/12/flights-of-fancy-airborne-wind-turbines/

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