Stream Water Wheels and Floating Mills

Stream water wheels are hydraulic machines that are installed in flowing water. The kinetic energy of the flow determines the rotation of the water wheel, generating mechanical energy and, eventually, electricity. When a stream wheel is supported by boats on its sides, it is called a floating mill (Fig.1). Water wheels that instead use the potential energy of the flow (the water weight) are called gravity water wheels, as in Quaranta and Revelli (2016).

Floating mills, very diffused in the past, usually contained the mill’s machinery inside a mill boat, and a water wheel which was connected to boats or floats on its sides. Floating mills were sometimes built with two symmetrical floating bodies, often with conically shaped bows to guide the water into the wheel, or with a central boat and two wheels on each side. From the Middle Ages onwards, floating mills were built in Europe especially in sites of rapid flow of up to 3.2 m/s. However, such velocities (needed to produce a reasonable amount of energy) are/were not diffused. Furthermore, the maximum efficiency of such machines is low (25-40%), since most of the flow kinetic energy is dissipated in turbulence and in the impact on the blades.

From Kinetic Energy to the Hydrostatic Force of Water

In a previous study, a stream water wheel was investigated in a channel which was wide and deep almost as the blades of the wheel. In this case, it was observed that the efficiency was higher, since a water wheel set closely along the channel performs like a weir; the difference of water level between upstream and downstream acts in addition to the stream kinetic energy. This obstruction effect has been later better investigated in Batten and Müller (2011). A body with a base plate and a bow section has been constructed around the wheel; the contraction region was designed for maintaining a constant flow velocity and for the development of a head in front of the turbine. Downstream of the wheel, there was a stern section with an expansion section that was designed for the flow to exit at a shallower depth and at higher velocity. There were also separators to generate a region of low pressure and to reduce the water level downstream of the rotor, facilitating the discharging process. This design generates the hydrostatic force (the higher water depth upstream of the blades) which drives the wheel; the maximum hydraulic efficiency is 80%.

The Hydrostatic Pressure Machine (HPM) in Zero Head Applications

The damming-up effect generated by the water wheel [5] is the principle which has also been effectively exploited in the rotatory Hydrostatic Pressure Machine (HPM) in Senior et al. (2007). HPM is an hydropower converter which acts as a weir, since the hub diameter is equal to the head difference, and the blades depth and width are similar to the downstream flow ones (Fig.1). The upstream flow depth ranges up to the top hub level. The water level difference generates an hydrostatic pressures which acts on the blades, with maximum hydraulic efficiency of 80%. This machine can be used in sites with maximum flow rate of 2 cubic meter for second per meter width, while the hydraulic head (generally from 1 m to 2.5 m) can be generated by the wheel itself. Therefore, the design has to take into account that the upstream water level increases when the wheel is in operation.

Applications in the future

The advantage of HPM is that it can be used in straight channel where bed drops do not exist, since the hydraulic head which drives the wheel can be auto-generated. This can be also a strategy in irrigation canals to rise the water level by the damming effect, carrying the water from a location to an higher one. At the moment, some HPM are installed mainly for scientific purposes, and experimental and numerical research is in progress.

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