The following question was recently posted on Quora:
“What is the most efficient design for a wind turbine?”
Check out the responses below from energy industry experts:
Suchit Meshram, wind turbine manufacturing professional:
In a nutshell, the best design of a wind turbine for maximum efficiency would be the one which is tailor made for the site location on which it is going to be installed.
Some of the aspects which are considered in industry for designing an efficient wind turbine are:
- Aerodynamic Efficiency: In Layman terms it means how much energy one can extract from a given volume of wind at a particular height at a given temperature and pressure. Popularly known as Betz’ law, which says maximum possible extractable energy is 59.3% (Cp value 0.593) of total available wind energy from a give volume of wind. Wind turbine designers around the world try to design their wind energy extractor (Blades) to catch up with this limit. presently available blades have Cp of around 0.35 to 0.45. While maximum claimed Cp is 0.52.
- Low cut in speed: Speed at which wind turbine starts is desired to be very low, that is usually the wind speed of 2 to 4 m/s.
- Reliability and Availability: Wind turbines which stay alive during peak windy days is desirable. 90 to 98% is generally claimed for any WT manufacturer.
- Service life: Most of the wind turbines installed around the world are designed to sustain for maximum 20-25 years of service during which they should only need minor maintenance such as grease oil replacement, paint systems etc.
- Lighter rotor blades: Lighter the better, directly related to cut in speed of wind turbine. Weight increases as blade length increases. A general relation of weight -Length is weight is proportional to (Length)^2.4. But design modifications and material selection demonstrate possibility of lower weights.
- Designed for a wind farm : Considering turbulence from neighboring wind turbines, the design for the 2nd row and the later installed turbines should be tweaked considering this interference.
- Bend-Twist coupling (BTC) and Sweep: Let us consider that blades are made of infinite aerofoils joined together adjacently. So during operation each of these aerofoil face different relative wind velocity (relative wind velocity is highest at the tip and lowest at the root section). Because of this, the blades has to be twisted to some angle(relative to root) so that aerofoils have different angle of attack for different relative wind velocity. [they might use different aerofoils altogether to take care of that but twisting also gives them an added advantage while designing]. Now, during operation, being flexible the blade itself bends to some extent, which leads to bend-twist coupling(BTC). Bend-twist coupling is “Twist due to bending and bending due to twist”. For blades made by composite material coupling is possible by altering the fiber angles in the material making up the bending stiffness of the wind turbine blade. Similarly sweep (forward and backward) can be provided to increase the aerodynamic efficiency.
Bend Twist coupling:[courtesy: 2012 sandia lab presentations]
Swept blades[[courtesy: 2012 sandia lab presentations]]
- Other aspects such as Wind Shear, Tower Shadows, Anticipative pitch/Yaw control,
Generally, Wind turbine manufacturers around the world mostly prioritize COE (Cost of energy) rather than turbine efficiency. COE means the amount of money spent to generate one unit of electricity. This covers all aspects including design of machine, Tower, Grid connectivity, Machine availability, maintenance etc. Of course, a highly efficient machine may lead to a lower COE but that is not the case always.
Mike Barnard, energy expert:
Summary: A modern horizontal-axis, triblade wind turbine would generate the most electricity. Claims of superior performance by alternate technologies accompanied by requests for investment should be viewed extremely skeptically.
Maximum potential generation from a volume of wind is determined by Betz’ Law (alternately known as Betz’ Limit). Betz calculated that the maximum power that could be gained from the wind was 59.3% of its total energy. [1]
Wind generation devices include:
- Triblade horizontal-axis wind turbines
- Vertical-axis wind turbines
- Cabled, flying wind generators (prototypes at present)
[9]
- Horizontal-axis wind mills of various types (no aerodynamic component to the blades)
- Vertical-axis wind mills of various types such as the Savonius wind mill
- Various flakey inventions that look like jet engines, or jet engines with big funnels or cones with pistons (the Saphonian [5])
- Towers that use passive solar heating around their base to create winds flowing up the tower past wind turbine blades spinning in the tower [4]
There are about 200,000 triblade horizontal-axis wind turbines generating power today.[7] They are the winning form of generation because they are the most effective. The reasons are easy to explain:
- Aerodynamic blades add a component of lift-related force to drive the blade faster. This is a significant advantage over windmills whether horizontal- or vertical-axis. Any even adequately designed wind turbine will always generate more electricity than the best windmill.
- The blades of the triblade design are always flying through clean air. The turbulence of the previous blade’s passage has been carried downwind by the time the next blade passes the same point. Vertical-axis wind turbines are flying through turbulent air a significant percentage of the time. The clean air allows the triblade HAWTs a sizeable advantage. [2]
[8]
- The blades of the triblade design are always presented at the optimal angle to the oncoming wind. Vertical-axis wind turbines change their angle constantly, and only a portion of even the best designs are at an optimal angle at any given time. Aligning the blades to the oncoming air requires trivial amounts of energy compared to this advantage. [2]Savonius windmills are even worse as they capture wind in the concavity in half of their surface area and shed wind on the convex portion with attendant drag and additional turbulence on the other half of their surface area. (I recently analyzed a potential investment for a small firm in micro-generation capability and saw that the inventor had created 5 ‘innovations’ around the basic savonius premise that took it from a cheap form of energy sufficient for minor irrigation uses to a very expensive form of generation of energy sufficient for minor irrigation uses.) For context, here is a cost-effective savonius irrigation windmill made out of an old plastic barrel and some scrap lumber.
- Triblades scale up well. One of the biggest advantages is that you can put a very big set of blades on a very tall tower and gather lots of wind above the point where it slows down due to contact with the ground.Many ‘innovative’ designs have been proposed that use some sort of venturi effect in combination with turbine rotors, but the fundamental problem is that in order to gather sufficient wind, you have to scale the outer shell up to the point where weight and material costs become prohibitive. An outer shell has to scale up at least to the square of the diameter and likely more. A 3 MW wind turbine with 80 meter blades can catch a subset of the energy from 20,096 square meters of air. A venturi shell at that scale would have a circumference of 251.2 meters, would likely have to be at least 10 meters in breadth before noticeable effects started kicking in and would weigh an enormous amount.Other ‘innovative’ designs fly wind-capturing devices of some sort or other -- blimp-shelled turbine blades, frames with turbines, kites with turbines -- into wind that’s more constant and higher off of the ground. The problem is that these are constantly running into scale limits. The blimp-shelled wind generator starts having rigidity problems long before it gets to utility-scale, generation. The flying kites with blades start requiring massive and very long cables in order to resist the forces. Generally these prototypes are very interesting and never see the market. All of them start requiring massive ground installations with extraordinarily large winches when you want utility levels of generation. When you start thinking ship-hawser levels of strength multiplied by kilometres of cable you start realizing that the weight and expense of the cable alone becomes prohibitive at any useful level of generation.[3]
- Triblades just sit in one place on a big pillar when they are generating electricity. This is very efficient, which is one of the reasons that they payback the energy used in construction faster than any other form of electrical generation. [6] One wind farm in Australia generated 302 times the electricity that was used to start them, brake them and turn them into the wind over a year. Compare this to the requirements for a flying wind turbine which has to be hauled in when the wind doesn’t blow, launched when the wind starts and has a heavy cable potentially kilometres long adjusted to maximize generation regularly.
- The solar-wind tower with turbine blades comes closest to being an interesting technology, however maintenance is never explored. The turbines are usually envisioned as being stacked 3-7 or more horizontally up the length of the tower. They will be operating in very hot winds, likely 45 degrees Celsius or higher at velocities of 50 kph and higher. Effectively, the device is a convection oven that would cook a human in short order. Working inside of the tower would require refrigerated suits and breathing gear, if the wind velocity made that feasible at all. Pulling the turbine head or blades out of the tower to service them would be an extraordinary job. Closing the wind input would require closing gates on a diameter of five kilometres.
[1] http://en.wikipedia.org/wiki/Bet...
[2] Why aren’t vertical-axis wind turbines more popular?
[3] Are airborne wind turbines a plausible source of cheap energy?
[4] http://www.dailymail.co.uk/scien...
[5] http://www.energymatters.com.au/...
[6] How long does it take a typical wind turbine to generate more power than what was used to create it?
[7] http://www.gwec.net/global-figur...