Although the invention and use of water wheels dates back to Antiquity, with the current interest to produce power from renewable energy sources, there is nowadays a renewed interest in this type of hydraulic structure, in particular on small streams with low head difference (see Senior et al (2010) for a recent review of this type of hydraulic energy harvesters). Indeed, in many places along rivers and irrigation canals, one can find weirs, steps or ramps where unused hydropower potential exists. Similarly, old mills sites can be refurbished to exploit the head difference using new types of water wheels. The head difference that can be encountered on such sites usually lies in the range 0.5 m to 2 m, and most standard turbines are not economical in these conditions. Furthermore, it is highly desirable to promote hydraulics works and turbines that permit fish passing through the structure.

Recently, a collaborative study between the Irphé research laboratory (UMR 7342), specialized in fluid mechanics, and the PYTHEAS Technlogy company, developing energy conversion solutions for slow and intermittent energy sources, has been set up to examine a solution based on hydrostatic pressure machines (HPM). Their principle is based on exploiting the difference of hydrostatic pressure on the two sides of the blades and has been the subject of recent studies (e.g. Senior, 2009 ; Linton, 2013).

The objectives of the research program is to gain knowledge of the power production of such HPM in low-head difference conditions, its efficiency, and how the HPM's operation affects the hydraulic regime upstream and downstream of the HPM wheel. The current research combines three approaches: (i) the development and improvement of simplified mathematical models of the wheel under more or less idealized conditions (e.g. Senior et al., 2010), (ii) experimental tests performed with a scaled model of the HPM wheel in the HERODE flume facility of Irphé, and (iii) computational fluid dynamics (CFD) simulations of the flow around and within the wheel, by using the OpenFoam software suite.

In the presentation and the article, the experimental set-up in the Herode flume will be described, including the design of the wheel and the electronic chain that was designed to control the speed of rotation of the wheel by using an electromagnetic brake (simulating the presence of a power take-off system). The simultaneous measurement of the torque and rotational speed of the wheel shaft allows to estimate the power that can be harvested. The system is tested in a range of hydraulic conditions, i.e. various flow discharge rates and upstream and downstream water levels. The experimental results are then compared with predictions from semi-theoretical models. Overall, a good agreement is observed for the analyses performed up to now. In addition, the experimental results allow to adjust some empirical coefficients in these models for representing complex effects such a turbulence induced dissipation, water leak discharges around the blades, effects of air entrainment, etc.

As a complementary approach, the CFD calculations allow to improve the detailed description of the dynamics of the flow interacting with the rotating wheel, and to study alternative designs of the wheel (number and shape of the blades, dimensions of the wheel, effects of related works, ...).

An overview of the work carried out on two different models of wheels and the results obtained so far (both experimentally and numerically) will be presented at the CFM conference and in the associated proceedings paper, together with an outlook for the next steps of the research program.