Contra-rotation, using two rotors one behind the other, driving the two halves of an electrical generator. Thus creating a true "Wind Turbine".
Wind Energy Collection using Contra-Rotation
A F Stobart
|Contra-rotation, using two rotors one behind the other, driving the two halves of an electrical generator. Thus creating a true "Wind Turbine".|
|by A F Stobart BSc.Chem.Eng|
The basic mechanical principles of the currently popular three bladed large Wind Energy Collectors (they are not strictly turbines having only one rotor) go back to 1185 or before when Nicholas rented the "milling by the wind" (Molendium venti) from the Knights Templars in E. Yorkshire. And possibly as far back as when the "slave skilled in the manufacture of windmills" slew the Sultan Omar of Baghdad in 644 AD. A good history of wind energy developments from 1905 to 1972 can be found in [Ref.1].
A friend of Betz who is sometimes described as the "father of modern wind energy collection theory" [Ref.2], Hans Honneff, wrote a book on the use of contra-rotation, using two rotors one behind the other, driving the two halves of an electrical generator. Thus creating a true "Wind Turbine" [Ref.3] The concept was for very large wind turbines, three to a tower, 150m, dia, each, generating a total of 21 MW. The two rotors for each of the three turbines had a 120m, dia. set of magnets and coils as the electrical system, each rotor revolving separately from the other. Small (10m. dia.) models of the above were tested by the Third Reich Wind Energy Ministry, [1935-45].
In fan and turbine engineering the energy collected by two or more rotors in series is additive.[Ref.4] This point was confirmed for wind energy collection with model trials in 1976 by Simon Bromet (unpublished manuscript). Further confirmation of this fact was published recently following some trials by the California Energy Commission. But using a conventional alternator. [Ref 5] An attempt to the "additive" effect of contra-roation was made by Trimble Windmills in the period 1976-82. A small (5kW rated at 10 m/sec windspeed) machine was built, and a number sold to farmers and isolated sites for heating. [Ref.6] With some tap off for local electric power. The machines had a contra-rotating permanent magnet alternator, and sail wing blades attached to each half. Current collection was through slip rings. While the full additive effect shown by Mr Bromet was not achieved, probably due to the two sets of blades being too close together, a respectable Cp of 0.37 was achieved. [Ref.7] Blade spacing requires further research and development. Note that contra-rotation electricity generation does not require a gearbox, thus saving costs and energy loss.
Later the concept of "ganging up" many contra-rotating rotors was conceived due to the discovery of German work by the 3rd Reich Wind Energy Ministry (1934-45) indicating an interesting synchronous non electronic method of linking contra-rotating turbines. [Ref.8] Other work on this may be contained in a FIAT report, not yet traced. [Ref.9] This concept was called an Integrally Linked Multiple Array (ILMA) which was chosen for display at the Rolls Royce sponsored poster session at the 1988 Techmart Exhibition.. [Ref.10] The system has yet to be tried out, albeit several wind energy collectors on one tower (single rotors) had been proposed earlier for Marine work [Ref 11] and tried out on land.[Ref.12] The latter unit had six rotors to give a total rating of 400 kW.
The rationale for many rotors, provided they can be cheaply mass produced and electrically linked is that as a Wind Energy Collector is an area for income, but a volume for cost, above a certain diameter is seems probable that the cost of many rotors will be less than one large rotor of the same rated output. This principle of "many circles" is found in piston engines. The original ILMA concept was put forward in 1983. [Ref 13] One potential advantage of this approach for marine work (and for wind turbines on buildings [Ref.14) is that rotors can be dispersed over the supporting structure, eg a redundant Oil Platform, without the need to have means to support the considerable overturning moment of a single large rotor. Losses due to breakdowns are minimised as not all the small rotors are likely to fail at one time. Due to rotors being point suspended (as per Honneff), loads on supporting structures can be considerable reduced, together with overturning moments, as this suspension converts side wind forces into a downward load on the supporting structures.
An important matter for future developments is to change the alternator design for ILMA from a radial air gap, as is found in most electrical generators, to an axial air gap. A generator in this configuration has been reported. [Ref,15, 29] The original Trimble alternator, designed and manufactured by Clarke Chapman at Gateshead was robust, virtually trouble free (one ran for over 10 years with no maintenance attention), efficient, but very heavy and costly.
Honnef placed his "generating ring" almost at the tip of the rotors, where the angular speed is greatest. A similar approach is required with ILMA units, especially as the centre one third of any WEC is useless for energy collection, and the wind in this area can with advantage be directed outwards with a nose come. A theoretical study of contra-rotating electrical machines is known of. [Ref.16]
The single rotor concept however has had it's advocates for Marine use [Ref 18]. Honnef envisaged very large rotors, barge mounted, with the energy collection at the base of the rotors rather than all round the edge. A similar proposal for a large single rotor made by Mullett in 1956, is quoted by Bockris. [Ref.11]
APPLICATIONS FOR MARINE RENEWABLE ENERGY USING CONTRA - ROTATION
The first application is clearly to bring it onshore.
Electricity from WEC's. The usual application with synchronous generation (feasible with the ILMA concept, if the Kloss electric linkage theory proves effective) is direct use, fed into the Grid in UK.
If generated as "crude" (asynchronous) current by allowing the WECs to "run with the wind", which improves energy collection efficiency, the current can be stored as high temperature heat from resistance heaters in Cowper Stoves, to provide Process Heat, which, as noted above, is a much greater energy demand in UK than electricity per se. [Ref.13]
This could be applied in the Chemical Industries, and in Ireland, Scotland and Wales for whiskey distilling. [Ref.17] It might also be used for powering a superheater section in a Nuclear Power Station's steam circuits, whose temperature conditions often leave something to be desired.
DC electric current can be used to fix atmospheric Nitrogen (Birkeland & Eyde process, pioneered by Norsk Hydro). Which has been examined on a small scale in Canada, using a "nitrolyser". [Ref.19]
An application for both on and off shore is Hydrogen production from sea water by Electrolosis. [Refs.11, 20] This gas is becoming increasingly considered as a Road Transport Fuel. (Second largest UK energy usage). It can also be used to convert Coal to Liquid fuels. [Refs.11, 21, 22 & 23] And has been considered for processing domestic waste to fuel.[Ref.24] This technology might provide an extended life for redundant oil platforms. Although existing oil pipelines might be metallurgically unsuitable for its delivery to shore. And it cannot be mixed with Natural Gas to add to the latter's energy content as all gas using appliances would have to be altered. [Ref.25]
Offshore, [Ref.11] offers an interesting concept of the electrolosis taking place at depth in sea water. Providing Hydrogen under compression for transport and storage, and a means of disposing of the chlorine produced though dispersion in the sea.
However extensive enquiries over the past few months in both the USA and the UK have found no references to this technology having been tried out. Albeit Hydrogen compression at the point of production by electrolosis is being investigated in the USA. [Ref 26] The chlorine might however have commercial uses too.
Not everyone is convinced of the economic viability of a "Hydrogen Economy". Ref 27] But the possible economic advantages of the technologies discussed above could change this view ? Certainly one island off Norway, Utsira, has recently gone to a Wind and Hydrogen energy supply system. Some of the wind energy is used to produce Hydrogen, which through Fuel cells produces electricity and heat when no wind power is available. [Ref 28]
1/. Warne, D F and Calnan P G, Generation of Electricity from the Wind, Proceedings of the Institution of Electrical Engineers, Vol 124, No.11R, November 1977, pp 963-985, 121 references.
2/. Betz A, Die Windmuhlen im Lichte neuerer Forschung, Die Naturwissenschaften, 18 November 1927
3/. Honneff H, Windcraftwerke, F.Vieweg, Braunschweig 1932, (copy in Science Museum Library). Hohenwindkraftwerke, Elektrotechnik und Maschinenbau, Vol 7, No. 41 & 42, 13 October 1939, pp 501-6, German Patent No. 871580 1953,
4/. Osborne,W C and Turner, C G, Woods Practical Guide to Fan Engineering, 2nd edition, July 1960, pages 134-5.
5/. Kari Appa Contra-Rotating Wind Turbines, US Patents 6,127,739, 6,278,197, and California Energy Commission report on www.eisg.sdsu.edu/Far/00-09%20Far.pdf
6/. Stobart A F, Heating from Wind, Building Technology and Management, July/August 1981.
7/. de Paor A M, O'Malley M J, & Ollivier D, Identification and passive control of a wind driven permanent magnet alternator system for heating water. Wind Engineering Vol 7 No. 4 pp 193-207, 1983
8/. Kloss M, Der direkte Antreib von Synchrongeneratoren durch Grosse-Windkraftwerke in Parrallelbetreib mit einem taktgebenden Netz, Elektrotechnische Zeitsschrift 63 Jahrg Heft 31/32 August 1942
9/. Gaenger B, Foettinger, Robitsch, Teubert, and Witte, FIAT report No. 1111 on the work of the Reicharbeitsgemainschaft Windkraft (12 reports/papers)
10/. Sachs T, Christmas Tree Turbine, Electrical Review Vol 221 No 23, page 20, 30 November-13 December 1988.
11/. Bockris J O'M, Possible means of Large-scale use of Wind as a source of Energy, Environmental Conservation, Vol 2 No. 4 Winter 1975 [25 references].
12/. Van der Linden, K, Sixmaster Turbine, Holland (address known). Unit reportedly bought by The Municipal Energy Authority of Rotterdam about 1990. And www.superturbine.net
13/. Stobart A F, Wind Energy: some notes on its collection, storage and application. Energy World, pp 4-6, May 1983
14/. Campbell N S, and Stankovic S, Wind Energy for the Built Environment Report on Project WEB by BDSP Partnership Ltd, Imperial College London, and Mecal Applied Mechanics BV, Universitat Stuttgart, September 2001
16/. Bonwick W J, & Ah Fock A L D, Synchronous machines with contra-rotating armatures, steady state behaviour. Proceedings of the Institution of Electrical Engineers, Vol 132, Part B, No.4, July 1985.
17/. Stobart, A F, Cyfoeth Dichonadwy a Pheirianneg Gynaladwy, Cyfoeth Dichonadwy Cymru yn y Dyfodol (Renewable Energy and Sustainable Engineering, The Potential Future Wealth of Wales), Report submitted to Cynulliad Cenedlaethol Cymru, (The National Assembly for Wales), as a Report for the Consultation on Renewable Energy 25 June 2002.(For ease of access enter "a ferrand stobart" in Google search to locate the DTI website).
18/. Armstrong J R C, Wind Turbine Technology Offshore J A Consult, 76 Dukes Avenue, London W4 2AF (On a website)
19/. Nilectra Corp of Dayton Ohio, USA, Saginaw Valley State College, USA, and Wellington Windpower, Ontario, Canada. Undated press cutting of about 1982, (Work supported by a US Government grant). See also work by Alamaro M, of 73 Herzel Street, Hod, Sharon 45283, Israel, on a similar concept for Nitrogen fixation, circa 1982.
20/. Altmann, M & Richert, F Hydrogen Production at Offshore Wind Farms, Offshore Wind Energy Special Topic Conference, Bussels, Belgium, 10-12 December 2001. Website contacts www.lbst.de & www.geo-mbh.de
21/. Harrison J S, Merrick D, Smith M, & Rasmussen G, The use of non-fossil derived Hydrogen; a possible role for coal conversion, 3rd World Energy Conference 23-27 1980.
22/. Dainton, Dr A D, Prospects for Coal Liquifaction, Energy World, February 1984.
23/. Prospects for the use of Hydrogen in the UK, 1988 Civil Service report, (Private Communication)
24/. MANOIL project, Greater Manchester Refuse Disposal Department (Private communication 1986)
25/. Spare, P, Letter in the Professional Engineer for November 2003
26/. Avalence Federal Ultra-High Pressure Hydrogen Contract. Press release by www.fuelcellmarkets.com July 2004-08-31
27/. Bossel, U. Hydrogen, Why it's future in a sustainable energy economy will be bleak, not bright. Renewable Energy World, March-April 2004, pp 155-159
28/. Fuel Cell Power, No. 17 Summer 2004, page 7, www.fuelcellpower.co.uk
Design of a Small Contra-Rotating HAWT
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