Compact wind acceleration turbine

Compact wind acceleration turbine

Compact Wind Acceleration Turbines (CWATs) are a class of wind turbine that uses structures to accelerate wind before it enters the wind-generating element.[1] The concept of these structures has been around for decades [2] but has not gained wide acceptance in the marketplace. In 2008, two companies targeting the mid-wind (100 kW-1 MW) marketplace have received funding from venture capital. The first company to receive funding is Optiwind, which received its series A funding in April 2008, and the second company is FloDesign Wind Turbine Corp, which also received its series A funding in April 2008. Optiwind [1] is funded through Charles River Ventures and FloDesign is funded through Kleiner Perkins.[3]

Other CWATs under development include the Windtamer and Enflo turbines.

CWATs are a new acronym that encompasses the class of machines formerly known as DAWTs (diffuser augmented wind turbines). The technologies mentioned above all use diffuser augmentation that is substantially similar to previous designs as the primary means of acceleration.

DAWTs were heavily researched by K. Foreman and Oman of Grumman Aerospace in the 1970s and 1980s and Igra in Israel in the 1970s. At the end of a decade of wind tunnel research and design funded by Grumman, NASA, and the DOE, it was determined that the DAWT system's economics were not sufficient to justify commercialization. In the 1990s the Grumman technology was licensed to a New Zealand company, Vortec Wind. The attempt to commercialize the Vortec 7 in New Zealand from 1998 to 2002 proved it to be economically untenable when compared to the dominant HAWT (horizontal axis wind turbine) technology.

The historic failure of DAWTs to gain acceptance has to do with the economics of these systems. DAWT systems have not shown improvement in the economics of HAWTs. This lack of advantage has centered around two key factors affecting the development of this type of machine.

The first factor regards power increase and the method of comparison used by DAWT (and more recently CWAT) designers to determine the whether the system is worth developing. Grumman and other attempts to commercialize these machines compare their machines to HAWTs based on a rotor area to rotor area comparison. As Van Bussel of Delft (The Science of Making More Torque from Wind: Diffuser Experiments and Theory Revisited, G.J.W. van Bussel, Delft, 2007) (http://repository.tudelft.nl/view/ir/uuid%3A2f19000e-f2b5-468e-8d10-5e338e1be888/) pointed out, this is an inaccurate comparison and the comparison of power multiples should be made on the basis of the exit area of the diffuser or shroud not the rotor area.

Grumman claimed a 4× increase over an unshrouded turbine based on an acceleration of 1.6 times the ambient wind velocity (An Investigation on Diffuser Augmented Turbines, D.G. Philips, 2003 (reference materials compiled from K.M. Foreman)). A 1.6 acceleration is in fact 2.6 times the power of a HAWT if the ratio of the shrouded rotor to the exit area is 1.6. If however the rotor to exit area ratio is 2.75 (as it was in the Grumman case), the actual power increase over a HAWT with the same swept area as the diffuser exit area is only 1.4× the power. Given that the DAWTs with this ratio have roughly a solidity of 60+% when the blades and the diffuser are accounted for and the solidity of the HAWT is roughly 10%, the cost and amount of material needed to produce the 40% gain outweighs the increase in power.

Second is the structural requirement in terms of resisting overturning and bending in extreme wind events which all wind turbines must be designed for by ISO standard. The DAWT structure has poor drag characteristics (see D.G. Philips). That combined with higher solidity leads to substantially greater structural costs than a HAWT in the support structure, the yaw bearing, and the foundation.

For these reasons, low power increase when referred to exit area and structural costs, DAWTs have not been a solution to improving wind energy at utility scale. The power increases thus far have proved insufficient to offset the structural costs. In small wind applications where structural issues are lessened they may be better than HAWTs if it can be definitively shown that they can improve output by 40% for the same cost.

The recent attempts mentioned above to improve on the DAWT type (the CWATs) appear to have similar issues.

In the case of Optiwind a 3 times increase is claimed for their 150 KW and 300 KW based on the horizontal dimension of their machine. If the power these machines produce is referred to the stack height (the vertical height - the tower height) of their system, it is only roughly 80% of the power produced by a HAWT of that diameter. The solidity and drag properties of the Optiwind are again problematic in extreme wind events and it seems unlikely that the system represents a cost improvement over a HAWT.

Flodesign's MEWT (mixer-ejector wind turbine, another CWAT variation) is differentiated from previous DAWT's by using a lobed two stage diffuser (Grumman and Vortec machine were also two stage, but conical instead of lobed) to equalize the pressure over the exit area of the diffuser. The theory is that creating a uniform pressure distribution with the lobes and the injection of external flow will prevent boundary layer separation in the diffuser thereby allowing the maximum acceleration through rotor. Werle and Presz's (Flodesign's chief scientists) paper, AAIA technical note Ducted Water/Wind turbines revisited - 2007, details the theory behind their design. Maximum acceleration detailed in their paper is 1.8× the ambient velocity from which they derive that 3-5 times more power is available at the rotor than for an unshrouded turbine. When referred to exit area this multiple drops to between parity and 2.1 times the HAWT power. Flodesign's turbine based on released images and CAD's appears to be substantially similar to the Vortec and Grumman machines except for the lobed inner annulus. This would indicate that it's drag characteristics can be expected to be similar.

Again it is unclear that these machines can outperform HAWTs on either a power output or cost basis. Additionally many CWAT companies seem to misunderstand certain aspects of wind power. There are many claims that because these machines start at lower speeds and stop at higher speeds they can gather even more power. The fact is that there is very little power in winds below 5 meters per second (2% of annual production) and the percentage of annual wind that falls above the HAWT cut out speed of 25 meters per second is less than .05%. Many of these claims including the idea that these machines will use less material than a HAWT for the same rating seem somewhat poorly thought out.{ http://venturebeat.com/2010/01/20/flodesign-adapts-jet-engines-to-power-up-wind-turbines/ }

There is one recent DAWT design that appears to have a definitive positive power, if not cost, comparison to HAWTs. This is the Enflo turbine. Based on its rotor:exit ratio and the published power performance this turbine appears to have a confirmed 2 times increase in power output over a HAWT of the diameter of the exit area. It is still unlikely that this machine can scale to larger ratings but based on published data the Enflo appears to be the best performing DAWT/CWAT yet built.{ http://www.enflo-windtec.ch/english/turbine.html }

References

  1. ^ DeRosa, Ronald; "New company wants to harness area's wind power": Register Citizen, August 31, 2008
  2. ^ Leibowitz, Barry; Duffy, Robert, “Verification Analysis of the Toroidal Accelerator Rotor Platform Wind Energy Conversion System”, prepared for New York State Energy Research and Development Authority, September 1988
  3. ^ O'Brien, George; "FloDesign Has Innovation Down to a Science"; Business West, April 28, 2008

Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать реферат

Look at other dictionaries:

  • Wind turbine — Offshore wind farm using 5MW turbines REpower 5M in the North Sea off Belgium This article discusses wind powered electrical generators. See windmill for wind powered machinery used to grind grain or pump water. A wind turbine is a device that… …   Wikipedia

  • Unconventional wind turbines — Wind turbines History Design Manufacturers Unconventional As of 2010, the most common type of wind turbine is the three bladed horizontal axis wind turbine (HAWT). This article deals with various types of wind turbine that differ from the… …   Wikipedia

  • turbine — /terr bin, buyn/, n. any of various machines having a rotor, usually with vanes or blades, driven by the pressure, momentum, or reactive thrust of a moving fluid, as steam, water, hot gases, or air, either occurring in the form of free jets or as …   Universalium

  • Gas turbine — Microturbine redirects here. For turbines in electricity, see Small wind turbine. For turbines driven by the flow of gas, see Turbine. A typical axial flow gas turbine turbojet, the J85, sectioned for display. Flow is left to right, multistage… …   Wikipedia

  • technology, history of — Introduction       the development over time of systematic techniques for making and doing things. The term technology, a combination of the Greek technē, “art, craft,” with logos, “word, speech,” meant in Greece a discourse on the arts, both… …   Universalium

  • automobile — automobilist /aw teuh meuh bee list, moh bi list/, n. /aw teuh meuh beel , aw teuh meuh beel , aw teuh moh beel, beuhl/, n. 1. a passenger vehicle designed for operation on ordinary roads and typically having four wheels and a gasoline or diesel… …   Universalium

  • Business and Industry Review — ▪ 1999 Introduction Overview        Annual Average Rates of Growth of Manufacturing Output, 1980 97, Table Pattern of Output, 1994 97, Table Index Numbers of Production, Employment, and Productivity in Manufacturing Industries, Table (For Annual… …   Universalium

  • ship — shipless, adj. shiplessly, adv. /ship/, n., v., shipped, shipping. n. 1. a vessel, esp. a large oceangoing one propelled by sails or engines. 2. Naut. a. a sailing vessel square rigged on all of three or more masts, having jibs, staysails, and a… …   Universalium

  • energy conversion — ▪ technology Introduction       the transformation of energy from forms provided by nature to forms that can be used by humans.       Over the centuries a wide array of devices and systems has been developed for this purpose. Some of these energy …   Universalium

  • Compressed-air energy storage — (CAES) refers to the compression of airto be used later as energy source. At utility scale, it can be stored during periods of low energy demand (off peak), for use in meeting periods of higher demand (peak load). Alternatively it can be used to… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”