Darrieus wind turbineTheDarrieus wind turbineis a type ofvertical axis wind turbine(VAWT) used to generateelectricityfrom theenergycarried in thewind. Theturbineconsists of a number of curvedaerofoilblades mounted on a vertical rotating shaft or framework. The curvature of the blades allows the blade to be stressed only in tension at high rotating speeds. There are several closely related wind turbines that use straight blades. This design of wind turbine was patented byGeorges Jean Marie Darrieus, aFrenchaeronautical engineerin 1931. There are major difficulties in protecting the Darrieus turbine from extreme wind conditions and in making it self-starting.

In the original versions of the Darrieus design, the aerofoils are arranged so that they aresymmetricaland have zerorigging angle, that is, the angle that the aerofoils are set relative to the structure on which they are mounted. This arrangement is equally effective no matter which direction the wind is blowingin contrast to the conventional type, which must be rotated to face into the wind.When the Darrieus rotor is spinning, the aerofoils are moving forward through the air in a circular path. Relative to the blade, this oncoming airflow is added vectorially to the wind, so that the resultant airflow creates a varying small positiveangle of attack(AoA) to the blade. This generates a net force pointing obliquely forwards along a certain 'line-of-action'. This force can be projected inwards past the turbine axis at a certain distance, giving a positive torque to the shaft, thus helping it to rotate in the direction it is already travelling in. The aerodynamic principles which rotate the rotor are equivalent to that in autogiros, and normal helicopters in autorotation.As the aerofoil moves around the back of the apparatus, the angle of attack changes to the opposite sign, but the generated force is still obliquely in the direction of rotation, because the wings are symmetrical and the rigging angle is zero. The rotor spins at a rate unrelated to the windspeed, and usually many times faster. The energy arising from the torque and speed may be extracted and converted into useful power by using anelectrical generator.The aeronautical termsliftanddragare, strictly speaking, forces across and along the approaching net relative airflow respectively, so they are not useful here. We really want to know the tangential force pulling the blade around, and the radial force acting against the bearings.When the rotor is stationary, no net rotational force arises, even if the wind speed rises quite highthe rotor must already be spinning to generate torque. Thus the design is not normally self-starting. Under rare conditions, Darrieus rotors can self-start, so some form of brake is required to hold it when stopped.One problem with the design is that theangle of attackchanges as the turbine spins, so each blade generates its maximum torque at two points on its cycle (front and back of the turbine). This leads to a sinusoidal (pulsing) power cycle that complicates design. In particular, almost all Darrieus turbines have resonant modes where, at a particular rotational speed, the pulsing is at a natural frequency of the blades that can cause them to (eventually) break. For this reason, most Darrieus turbines have mechanical brakes or other speed control devices to keep the turbine from spinning at these speeds for any lengthy period of time.Another problem arises because the majority of the mass of the rotating mechanism is at the periphery rather than at the hub, as it is with a propeller. This leads to very highcentrifugalstresses on the mechanism, which must be stronger and heavier than otherwise to withstand them. One common approach to minimise this is to curve the wings into an "egg-beater" shape (this is called a "troposkein" shape, derived from the Greek for "the shape of a spun rope") such that they are self-supporting and do not require such heavy supports and mountings. See. Fig. 1.In this configuration, the Darrieus design is theoretically less expensive than a conventional type, as most of the stress is in the blades which torque against the generator located at the bottom of the turbine. The only forces that need to be balanced out vertically are the compression load due to the blades flexing outward (thus attempting to "squeeze" the tower), and the wind force trying to blow the whole turbine over, half of which is transmitted to the bottom and the other half of which can easily be offset with guy wires.By contrast, a conventional design has all of the force of the wind attempting to push the tower over at the top, where the main bearing is located. Additionally, one cannot easily use guy wires to offset this load, because the propeller spins both above and below the top of the tower. Thus the conventional design requires a strong tower that grows dramatically with the size of the propeller. Modern designs can compensate most tower loads of that variable speed and variable pitch.In overall comparison, while there are some advantages in Darrieus design there are many more disadvantages, especially with bigger machines in the MW class. The Darrieus design uses much more expensive material in blades while most of the blade is too close to the ground to give any real power. Traditional designs assume that wing tip is at least 40m from ground at lowest point to maximize energy production and lifetime. So far there is no known material (not evencarbon fiber) which can meet cyclic load requirements

Savonius wind turbineSavonius wind turbinesare a type ofvertical-axis wind turbine(VAWT), used for converting the force of thewindintotorqueon a rotatingshaft. The turbine consists of a number of aerofoils, usuallybut not alwaysvertically mounted on a rotating shaft or framework, either ground stationed or tethered inairborne systemsThe Savonius wind turbine was invented by theFinnishengineerSigurd Johannes Savoniusin 1922. However,Europeanshad been experimenting with curved blades on vertical wind turbines for many decades before this. The earliest mention is by the Italian Bishop of Czanad,Fausto Veranzio, who was also an engineer. He wrote in his 1616 bookMachinae novaeabout several vertical axis wind turbines with curved or V-shaped blades. None of his or any other earlier examples reached the state of development made by Savonius. In his Finnish biography there is mention of his intention to develop a turbine-type similar to the Flettner-type, but autorotationary. He experimented with his rotor on small rowing vessels on lakes in his country. No results of his particular investigations are known, but the Magnus-Effect is confirmed by Knig.[1]The two Savonius patents:US1697574, filed in 1925 by Sigurd Johannes Savonius, andUS1766765, filed in 1928.The Savonius turbine is one of the simplest turbines.Aerodynamically, it is adrag-type device, consisting of two or three scoops. Looking down on the rotor from above, a two-scoop machine would look like an "S" shape in cross section. Because of thecurvature, the scoops experience less drag when moving against the wind than when moving with the wind. The differential drag causes the Savonius turbine to spin. Because they are drag-type devices, Savonius turbines extract much less of the wind'spowerthan other similarly-sized lift-type turbines. Much of the swept area of a Savonius rotor may be near the ground, if it has a small mount without an extended post, making the overall energy extraction less effective due to the lower wind speeds found at lower heights.The maximum power of a Savonius rotor is given by, whereandare the height and radius of the rotor andis the wind speed.[citation needed]Theangular frequencyof a rotor is given by, whereis a dimensionless factor called thetip-speed ratio. The rangevaries within is characteristic of a specific windmill, and for a Savonius rotoris typically around 1.For example, an oil-barrel sized Savonius rotor withh=1mandr=0.5munder a wind ofv=10m/s, will generate a maximum power of180Wand an angular speed of20rad/s(190 revolutions per minute).

Savonius turbines are used whenever cost orreliabilityis much more important thanefficiency.Mostanemometersare Savonius turbines for this reason, as efficiency is irrelevant to the application of measuring wind speed. Much larger Savonius turbines have been used to generateelectricpower on deep-waterbuoys, which need small amounts of power and get very little maintenance. Design is simplified because, unlike with horizontal axis wind turbines (HAWTs), no pointing mechanism is required to allow for shifting wind direction and the turbine is self-starting. Savonius and other vertical-axis machines are good at pumping water and other high torque, low rpm applications and are not usually connected to electric power grids. They can sometimes have longhelicalscoops, to give smooth torque.The most ubiquitous application of the Savonius wind turbine is theFlettner Ventilator, which is commonly seen on the roofs of vans and buses and is used as a cooling device. The ventilator was developed by the German aircraft engineerAnton Flettnerin the 1920s. It uses the Savonius wind turbine to drive an extractor fan. The vents are still manufactured in the UK by Flettner Ventilator Limited.[2]Small Savonius wind turbines are sometimes seen used as advertising signs where the rotation helps to draw attention to the item advertised. They sometimes feature a simple two-frameanimation.