Whilst browsing through a 1953 edition of ‘The Engineer’ newspaper last week, I was intrigued to read about the development of a 100kW wind driven generator built in St. Albans that made use of the ‘Andreau’ principle in which the wind-driven propeller blades were hollow.
The trailing tips of the blades were provided with openings through which the air was expelled by centrifugal action as the propeller was rotated by the wind.
According to the old newspaper, ‘air flow was induced up a vertical converging duct to the hub of the propeller. The air was admitted through vents at the lower end of the duct and passed through an air turbine which drove a vertical shaft alternator mounted at the base of the duct.’
An intriguing concept! But why has such a design now been abandoned in favour of more modern propeller designs?
Dr. Jim Platts at the University of Cambridge Institute for Manufacturing in Cambridge, England, knew the answer.
‘For decades, all electrical machinery has been high speed whereas wind turbines have very slow rotational speeds. And for many wind turbine manufacturers, this means (they need to use) a high ratio gear box and then a conventional generator. For others, it means a large diameter direct drive generator,’ he said.
The approach explored in the Hatfield machine, however, was to use the centrifugal flow of air through the hollow blades to suck air up the tower and use that to drive a high rotational speed turbine and generator at the base of the tower. Hence, the air was being used as a gearing mechanism.
‘But there are system matching problems and probably efficiency issues, and I don’t know of anyone exploring this route currently, at least not to generate electricity,’ concluded Dr. Platts.
Sara Alan-Smith at National Wind Power concurred with Dr. Platts after conferring with some of her colleagues. She told us that: ‘the performance data for the Enfield Andreau machine in question, cited in the proceedings of a UN conference on renewables in the late 1950’s, showed that its efficiency was very low – only about one quarter of the efficiency of a modern wind turbine.’
Henrik Stiesdal at Bonus Energy A/S also cited the lack of efficiency and came up with some figures to back up his argument.
‘For obvious reasons,’ he said, ‘the rotor efficiency did not exceed the efficiency of a rotor driving a normal transmission system consisting of a gearbox and a generator. The efficiency, however, was far lower than the efficiency of a normal transmission system. In the 1950s, the total efficiency of a 100 kW gearbox and generator would be slightly above 85% at rated power, somewhat lower at partial power. In comparison, the efficiency of the Andreau Enfield turbine would be the product of an air flow efficiency in the inlets, ducts and outlets (unknown, but most likely on the order of 50-70%), high-speed turbine efficiency (most likely on the order of 70%) and generator efficiency (92-93%), totalling an overall efficiency of about 40%.’
Consequently, the type of turbine would need a swept rotor area more than twice the size of a comparable turbine with a traditional transmission system.
The very low efficiency explains then why it was abandoned. Internal air duct friction losses of the machine were large enough to minimise any advantages achieved by the elimination of a mechanical coupling.
‘At a more practical level,’ added Stiesdal, ‘it turned out that the turbine was very noisy due to the high air velocity at the outlets in the blade tips. Therefore, the operational success was very limited. As far as I know, it was taken down and exported to, of all places, Algeria!’
Despite the failure at Hatfield, all’s well that ends well. Professor Stephen Salter at Edinburgh University has recently put a very interesting twist to this ‘hollow’ line of thinking. For some time, he has been working on solving the problem of how to increase moisture content in the air and then feed it inland where it can create rainfall.
Salters’ proposal is that we should develop vertical axis wind turbines of the ‘egg-whisk’ variety floating out at sea that suck up water through hollow blades and spray out small droplets from their trailing edges. These drops would evaporate, salt would fall back into the sea and a cloud of water vapour would then drift inland, increasing the probability of rain.
So despite their earlier lack of success at Hatfield, hollow blades may find a use at last. What goes around comes around. Literally.
Golding, E. W. (1976): The Generation of Electricity by Wind Power. 2nd edition. Halsted Press (Wiley), New York. (First published in 1955. 1976 edition is with an additional chapter by R. I. Harris).
Noyes Data Corporation (1975). Energy Technology Review No.6. Wind Power. Published in the USA by NDC, Noyes Building, Park Ridge, New Jersey 07656.