There are two main wind turbine generator designs: horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWT).
horizontal axis wind turbine generator is the conventional wind turbine generator that needs to face into the wind to generate electricity.
Vertical axis wind turbine generator are Omni-directional and can capture the wind in whatever direction it comes from. horizontal axis wind turbine generator is more efficient in the main, while vertical axis turbines can cope with turbulent air, gusts and sudden changes of wind direction more effectively.
Horizontal Axis wind turbine generator (HAWT)
Horizontal Wind Turbines consist of a rotor and generator on the top of a tower. They must be pointed into the wind to produce power. Most have a gearbox, converting the slower turning of the rotor blades into the higher revolutions required by the generator.
As the body of the turbine creates turbulence, the rotors are usually placed upwind of the generator. The turbine blades are designed, so they are not bent back by the wind. On small wind turbines, this is not a significant engineering challenge. As the turbines get larger, however, the weight of the rotor blades and the structure to support them rapidly becomes more industrial. A small wind turbine, such as the 500w Biard wind turbine kit as shown on the right, weighs 9kg (20 pounds) and can be installed by one or two amateurs following a set of instructions with a few hours of work. Scale this up to a 1okW system, and you are looking at a system weighing around 600kg (1320 pounds), requiring a professional installation for a team of wind engineers and electricians and taking several days to install.
The utility-scale wind turbines are huge: each rotor blade can measure 40 meters (131 feet), while the tubular steel towers are often 90 meters (goo feet) tall. The rotor blades rotate at between 10 and 22 revolutions per minute (rpm). At maximum speed, the tip of the blades can travel at over 200rnph.
Wind turbines for micro-power applications are far smaller, but they are still a substantial size, as the chart below shows. Note that the figures show a minimum height — in many cases, the height of the towers may be much higher: Turbine ROILM Diameter Minimum Height:
The scale of a wind turbine comes as quite a shock to many people. Particularly if you were originally assuming you could install a small, unobtrusive wind turbine on the roof of your house to generate all your domestic electricity needs.
The reality is quite different. The scale and the height of even a small wind turbine mean that you are going to need a fair amount of open space to install a wind power system. The following diagram gives some idea of the scale of different sizes of wind turbine:
Vertical Axis wind turbine generator (VAWT)
Vertical Axis Wind Turbines have a vertical rotor shaft and rotor blades that can pick up the wind from any direction. Principal components are typically located at the base of the turbine, facilitating repair and maintenance.
The main advantage claimed of the VAWT conﬁguration is that they are: one-directional and do not need to track the wind. They do not need to yaw, and the blade pitch does not need to be adjusted. They perform better than horizontal turbines in slightly turbulent or gusty winds but have a lower output than HAWTs in clean air. They generally require a stronger wind than HAWTs before they start generating any power at all.
VAWTS tend to rotate slower than the equivalent sized HAWTs. A 500w Savonius wind turbine, like the one on the right, for example, will tend to rotate at around 200 rpm, compared to around 1,000rpm for an equivalent HAWT. Virtually all VAWTs have a gearing system to generate power.
Because VAWTs can handle slightly turbulent air better than HAWTs, smaller systems have been installed in built-up areas that traditionally cannot be used for HAWTs. However, because of their lower outputs, they have not been particularly successful commercially.
VAWT Designs Savonius wind turbine generator
Finnish engineer and 1930s race drivers, Sigurd Johannes Savonius, invented the Savonius wind turbine in the early to mid-19205. It is one of the simplest turbines to build but extracts much less power from the wind than other types of turbine. However, their simplicity makes them ideal for small off-grid applications where only a minimal amount of electricity is required. Their simplicity also makes them suitable for home enthusiasts wishing to build their own wind turbines from scratch.
Diagram of a three-blade Savonius wind turbine, as viewed from above, facing down
Instead of rotor blades, Savonius wind turbines have multiple scoops to catch the wind. Each scoop is curved to reduce drag when they face into the wind, enabling them to spin freely. Savonius wind turbines have two, three, or four scoops, which catch the wind and spins the rotor shaft. Twin scoop systems are rare as they can experience problems with self-starting if the wind is in the wrong direction.
Darrieus wind turbine generator
The Darrieus turbines are nicknamed ‘eggbeater’ turbines due to their distinctive shape which is often likened to a kitchen whisk. The turbine consists of many curved rotor blades mounted onto the rotor shaft at the top and the bottom.
Darrieus turbines range in size from 500 watts, such as the Harvistor DARWIND5 pictured on the right, to 4MW turbines used for utility-scale energy production, although these larger designs are extremely rare. They tend to have two, three or five-bladed rotors, which can either be straight, as shown on the right or twisted through 60 degrees.
Darrieus turbines often require quite strong winds to start turning. Many commercially available turbines incorporate a small motor to get the turbines moving initially so that the wind can catch the blades.
Darrieus turbines have traditionally struggled in high winds, due to the angle of the rotors constantly changing. This can lead to vibrations at certain speeds that can eventually lead to the rotor blades to fail. Consequently, most Darrieus turbines have a built-in braking system to stop the turbine from operating in high winds. Besides, because the mass of the rotor is at the periphery of the rotor shaft rather than at the hub, there is additional strain on the hub bearings, which can lead to premature failure.
For these reasons, most Darrieus turbines tend to be comparatively small systems, capable of providing power up to 15kW, sufﬁcient for a small arable or sheep farm or a ﬁfty-person open plan ofﬁce.
A Cycloturbine is a variant on the Durieus turbine that uses adjustable rotor blades to adjust the angle of the rotor to ensure it picks up the wind effectively. This enables them to self-start in lower winds than with traditional Darrieus turbines and resolves the problem with vibrations that have affected the Durieus turbine designs.
Cycloturbines are the most efﬁcient vertical access turbines available today. Unfortunately, their complexity and design make them difﬁcult to scale up. Consequently, most commercially available cycle turbines are rated between 100w and 10kW. “While they are expensive when compared to HAWT of a similar size, cycle turbine manufacturers claim that their beneﬁts means that their designs can be slightly more efficient than conventional wind turbine designs.
Vertical or horizontal: which is right for me?
Horizontal wind turbines
There is a reason why horizontal wind turbines are appearing everywhere, while vertical wind turbines remain in obscurity. Horizontal turbines are quite simply better power producers.
As a general rule of thumb, Savonius wind turbines are only around half as efficient as horizontal turbines, while Darrieus turbines are around 90% as efficient as a horizontal turbine.
‘While cycle turbine manufacturers claim that their turbines are much more efficient than conventional designs, there appears to be little or no independent research that veriﬁes these claims.
Vertical wind turbine
If you like the idea of building your wind turbine from scratch, a Savonius vertical wind turbine is worth considering. The simplicity and inherent safety of this design make it more suited to DIY construction and while you will never generate huge amounts of power from your DIY wind turbine, you will have a higher chance of generating some power from home-built Savonius turbine than from many other turbine types.
For everyone else, horizontal wind turbines are almost certainly the better solution. There are turbines available from many manufacturers, there is far greater expertise available to help you, the performance is better, and the cost of the turbines themselves is signiﬁcantly less.