Water wheels are today often considered to be relics from the beginning of the industrial revolution; romantic but inefficient hydraulic machines often made of wood and belonging to the past, e.g. Smith (1980), Reynolds (1983).

It is generally believed that turbines evolved from water wheels, that they are much more efficient and subsequently replaced them as hydraulic power converters. A closer look at the statistics however reveals a slightly different picture. In the 1850’s, an estimated 25-30,000 water wheels were operated in England alone, McGuigan (1978). In Germany 33,500 water wheels with power outputs ranging from 0.75 to 75 kW were licensed as late as 1925, Müller (1939). The water wheels were almost exclusively employed as mechanical power sources driving grist-, powder-, and mineral mills as well as textile and other machinery, mostly in small businesses. With the advent of the electric motor as a cheap power source however in the 1940’s and 50’s the water wheels disappeared virtually completely.

Most ‘modern’ water wheels employ the potential energy of the water wheel only, see Figures 1a to c. Only the stream wheel, which was developed for very low head differences of 0.3 – 0.8 m uses the kinetic energy (Figure 1d).

Today, the requirement for the utilisation of low head hydro power sources for electricity generation is greater than ever. The currently unused low head micro-hydro potential is estimated as 500 MW in Germany and around 600 – 1000 MW in the UK. In developing countries, the rising demand for electricity in combination with large distances means that decentralised electricity generation has a high priority. Most low head, low flow hydro power sources however are today not exploited since standard turbines can not be employed economically in such conditions. Consequently, there exists a demand for a cost-effective low head hydraulic energy converter, which still could not be met.

Water wheels may offer an attractive solution to this problem. A small number of companies in Germany and the USA are again manufacturing water wheels for electricity generation. The water wheels themselves are however based on design manuals from the late 19^th Century. The performance characteristics of such wheels appear however still to be largely unknown, so that the assessment of the available power potential, comparisons with other turbine types such as the Kaplan or the Ossberger (crossflow) turbine and even the determination of optimum operating conditions for water wheels relies on estimates.

back to top

Turbines are usually not considered viable for low head hydropower sites since their unit costs per kW installed capacity are too high (10 – 12,000 £/kW)> In addition, there are concerns about the effect of turbines on the fish population (mechanical damage and damage by very low pressures). Water wheels, with costs of 3 – 7.5 £/ kW can be considered an economically interesting solution. The effect of water wheels on the fish population is currently being studied; their slow motion and the operation at atmospheric pressure are however thought to reduce impact on the fish population significantly.

During operation, water wheels emit a low frequency thumping noise similar to a steam engine. In particular in urban settings this can cause problems. The low frequency sound also has propagation characteristics which are not well understood, so that complaints can come from unexpected corners (a water wheel in Emmendingen/Germany e.g. drew a few complaints from nearby houses, which were addressed, and from a street cafe located 150m downstream).

back to top

The Overshot Wheel:

The overshot wheel has been researched quite thoroughly already; there even was a PhD-thesis on overshot wheels done at Stuttgart University in 1935. Undershot and breast shot wheels however have not been investigated; a small research project into the performance of characteristics of breast shot wheels has therefore been conducted, see Müller & Wolter (2004).

The Breast Shot Wheel:

A 1m diameter model of a breast shot water wheel designed after Bach (1886) and Müller (1899) was built and tested. Figure 2 a shows the inflow, Figure 2b the wheel itself.

The model tests showed that the ‘traditional design’, based on Bach’s 1886 book, contained some unfavorable features and assumptions. The original maximum efficiency of 76% could be improved to 87% by modifying the inflow detail, and by sloping the tailrace channel. The breast shot water wheel appears to be a very interesting energy converter. Currently, the micro-hydro group is involved with a full scale study of a newly built breast shot wheel at Lemsford / Hertfordshire.

Fig. 2c / Fig. 2d - Efficiency curve for traditional geometry with a coulisse inflow. / Efficiency curve for improved inflow and geometry.

Although stream wheels could be potentially quite interesting since e.g. their application would not constitute a major change of the river, no such wheel has been built for more than 100 years and very little is known about the performance characteristics and design requirements for such wheels. Currently, a joint research project with the Universities of Southampton and Berlin is under way in order to investigate the characteristics of this wheel type. Figure 3a shows a 500 mm model wheel at TU Berlin.

Figure 3b shows the theoretical power output curve for a stream wheel of 3.40 m width, running in a 5m/s flow. The maximum efficiency is 40% of the total or, to be in line with the definition used in turbine design, 80% of the usable energy at a wheel speed of 60% of the free stream velocity of the water. This does not appear to be unfavorable when compared with the most advanced Pelton turbines, which can reach 92% efficiency.

back to top

The micro-hydropower group at Southampton University is currently involved with two prototype projects; conducting research and advising on design of water wheels. A longer-term research project looking at the performance characteristics of water wheels, improved geometries with respect to efficiency and power rating and at environmental aspects such as fish compatibility and noise emission is under way. Some wheel types which were specifically developed for low heads between 0.8 and 1.2m, e.g. the Poncelet wheel, still require fundamental research.

Aspects of this work have been published in:

Müller G. and Kauppert K. (2002). Old water mills: Britains new source of energy?. Proceedings of ICE, Civil Engineering, Volume 150, Issue 4, 178-186. view paper

back to top

• Theoretical Considerations on the Hydraulic Pressure Machine