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Diagram of a Cross-flow turbine
1 — air-venting valve
2 — distributor
3 — turbine casing (all thick grey)
4 — runner
5 — removable rear casing
7 — water flow
8 — shaft

A cross-flow turbine, Bánki-Michell turbine, or Ossberger turbine[1] is a water turbine developed by the Australian Anthony Michell, the Hungarian Donát Bánki and the German Fritz Ossberger. Michell obtained patents for his turbine design in 1903, and the manufacturing company Weymouth made it for many years. Ossberger's first patent was granted in 1933 ("Free Jet Turbine" 1922, Imperial Patent No. 361593 and the "Cross Flow Turbine" 1933, Imperial Patent No. 615445), and he manufactured this turbine as a standard product. Today, the company founded by Ossberger is the leading manufacturer of this type of turbine.

Unlike most water turbines, which have axial or radial flows, in a cross-flow turbine the water passes through the turbine transversely, or across the turbine blades. As with a water wheel, the water is admitted at the turbine's edge. After passing to the inside of the runner, it leaves on the opposite side, going outward. Passing through the runner twice provides additional efficiency. When the water leaves the runner, it also helps clean it of small debris and pollution. The cross-flow turbine is a low-speed machine that is well suited for locations with a low head but high flow.

Although the illustration shows one nozzle for simplicity, most practical cross-flow turbines have two, arranged so that the water flows do not interfere.

Cross-flow turbines are often constructed as two turbines of different capacity that share the same shaft. The turbine wheels are the same diameter, but different lengths to handle different volumes at the same pressure. The subdivided wheels are usually built with volumes in ratios of 1:2. The subdivided regulating unit, the guide vane system in the turbine's upstream section, provides flexible operation, with 33, 66 or 100% output, depending on the flow. Low operating costs are obtained with the turbine's relatively simple construction.

## Details of design

Ossberger turbine section

The turbine consists of a cylindrical water wheel or runner with a horizontal shaft, composed of numerous blades (up to 37), arranged radially and tangentially. The blade's edges are sharpened to reduce resistance to the flow of water. A blade is made in a part-circular cross-section (pipe cut over its whole length). The ends of the blades are welded to disks to form a cage like a hamster cage and are sometimes called "squirrel cage turbines"; instead of the bars, the turbine has the trough-shaped steel blades.

The water flows first from the outside of the turbine to its inside. The regulating unit, shaped like a vane or tongue, varies the cross-section of the flow. The water jet is directed towards the cylindrical runner by nozzle. The water enters the runner at an angle of about 45/120 degrees,transmitting some of the water's kinetic energy to the active cylindrical blades.

Ossberger turbine runner

The regulating device controls the flow based on the power needed, and the available water. The ratio is that (0–100%) of the water is admitted to 0-100%×30/4 blades. Water admission to the two nozzles is throttled by two shaped guide vanes. These divide and direct the flow so that the water enters the runner smoothly for any width of opening. The guide vanes should seal to the edges of the turbine casing so that when the water is low, they can shut off the water supply. The guide vanes therefore act as the valves between the penstock and turbine. Both guide vanes can be set by control levers, to which an automatic or manual control may be connected.

The turbine geometry (nozzle-runner-shaft) assures that the water jet is effective. The water acts on the runner twice, but most of the power is transferred on the first pass, when the water enters the runner. Only ⅓ of the power is transferred to the runner when the water is leaving the turbine.

The water flows through the blade channels in two directions: outside to inside, and inside to outside. Most turbines are run with two jets, arranged so two water jets in the runner will not affect each other. It is, however, essential that the turbine, head and turbine speed are harmonised.

The cross-flow turbine is of the impulse type, so the pressure remains constant at the runner.

The peak efficiency of a cross-flow turbine is somewhat less than a Kaplan, Francis or Pelton turbine. However, the cross-flow turbine has a flat efficiency curve under varying load. With a split runner and turbine chamber, the turbine maintains its efficiency while the flow and load vary from 1/6 to the maximum.

Since it has a low price, and good regulation, cross-flow turbines are mostly used in mini and micro hydropower units of less than two thousand kW and with heads less than 200 m.

Particularly with small run-of-the-river plants, the flat efficiency curve yields better annual performance than other turbine systems, as small rivers' water is usually lower in some months. The efficiency of a turbine determines whether electricity is produced during the periods when rivers have low flows. If the turbines used have high peak efficiencies, but behave poorly at partial load, less annual performance is obtained than with turbines that have a flat efficiency curve.

Due to its excellent behaviour with partial loads, the cross-flow turbine is well-suited to unattended electricity production. Its simple construction makes it easier to maintain than other turbine types; only two bearings must be maintained, and there are only three rotating elements. The mechanical system is simple, so repairs can be performed by local mechanics.

Another advantage is that it can often clean itself. As the water leaves the runner, leaves, grass etc. will not remain in the runner, preventing losses. Therefore, although the turbine's efficiency is somewhat lower, it is more reliable than other types. No runner cleaning is normally necessary, e.g. by flow inversion or variations of the speed. Other turbine types are clogged more easily, and consequently face power losses despite higher nominal efficiencies.

## References

1. ^ E.F. Lindsley, Water power for your home, Popular Science, May 1977, Vol. 210, No. 5, 87-93.
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 17 news items
 HydroWorld Alaskan 455-kW Gartina Falls small hydroelectric project to reduce diesel ... HydroWorld Mon, 24 Aug 2015 14:11:15 -0700 The powerhouse will contain one 455 kW horizontal cross-flow turbine/generator set, inlet piping, guard valve, switchgear, and controls. The dam and intake structure is being constructed above Gartina Falls and a steel penstock, access road, and the ... Agriland Deutz-Fahr unveils C6000 series combine Agriland Fri, 31 Jul 2015 23:11:15 -0700 The cleaning effectiveness of the C6205 is boosted by a ventilation system with a cross-flow turbine this blows air across the entire width of the combine harvester. The Deutz-Fahr C6205 can be also fitted with a high-performance straw chopper and ... WCPO 9 Q&A: Hamilton startup harvests the power of water to create clean electric ... WCPO Thu, 05 Mar 2015 09:02:22 -0800 Startup kWRiver Hydroelectric manufactures its patented Williams Cross-flow Turbine, a device that uses falling water to create electric energy. The WCT is designed to be installed at low-head dams (defined as dams with a waterfall of less than 15 feet ... HydroWorld French city seeks construction, operation of Taconnaz small hydro project HydroWorld Thu, 03 Sep 2015 13:03:45 -0700 French hydro engineer Hydrostadium called for bids by September 10 to supply a cross-flow turbine-generator for the 1-MW Brides-les-Bains hydropower project in France's Savoie Department. French hydro operator Societe Hydro-Electrique du Midi took ... The Guardian New uses for old mills and farms The Guardian Sun, 05 Oct 2014 21:30:05 -0700 Over at Cole, where the streams now wind through beautifully landscaped gardens beside welcoming buildings of honey-coloured stone, a sophisticated cross-flow turbine puts the Pitt to modern use, supplying the property with power, and passing any ... Micro hydro in Afghanistan International Water Power and Dam Construction Fri, 19 Sep 2014 08:56:15 -0700 It is encouraging local turbine manufacturers to become involved and, after scouring the local market, discovered that cross-flow turbine production capacity is limited to 40KW. Therefore, NSP has decided to limit each turbine unit to 40KW: if a higher ... HydroWorld Study of the potential for micro-hydropower development in North Carolina HydroWorld Tue, 15 Oct 2013 12:03:24 -0700 The powerhouse configurations included a simplex, cross-flow turbine and an asynchronous generator housed in a non-insulated cinder block powerhouse on an engineered concrete slab. The penstock configuration was assumed to include a buried PVC ... Full speed ahead on Eastport area tidal energy efforts Bangor Daily News Sun, 18 Dec 2011 13:59:26 -0800 The core technology, Ferland said, of ORPC's proprietary “Tidgen” systems is a rectangular, cross-flow turbine that resembles a paddle wheel. In its current configuration, each unit is 98 feet wide and 17 feet deep. When submerged in a six-knot tidal ...
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