Water Wheels in the Past
Water wheels have been used for thousands of years to pump water, forge iron, grind grain, saw wood and many other applications. The most ancient water wheels exploited the kinetic energy of flowing water and were called stream water wheels. Water wheels that used the potential energy of water were introduced later and were called gravity water wheels (the water exerts a pressure on the blades due to its weight). Water wheels were in widespread use until the end of 19th century, when large hydropower plants replaced them.
Can Water Wheels Be Reinvented?
Nowadays, water wheels are being recognized once again as a promising technology, especially in the micro hydropower field for electricity generation (installed power typically lower than 100 KW). Water wheels work well with very low heads and flow rates, for example in irrigation and mill canals, where conventional turbines (such as Kaplan turbines) are economically not viable. Water wheels are eco-friendly machines because fish can pass through them unharmed. Furthermore, their payback periods are shorter (7-12 years) concerning Kaplan installations. The European Small Hydropower Association estimates that in Europe about 350,000 sites suitable for similar micro hydropower plants exist.
Are Water Wheels Ancient or Attractive Machines?
Water wheels can be considered as old machines, with low efficiency and simple design. Therefore, with the aim of both filling the gap of engineering information on water wheels and shedding more light on their hydraulic behavior and efficiency, we have built up an experimental channel to test them (Politecnico di Torino, Turin, Italy).
We firstly investigated a breastshot water wheel, installing in the laboratory a 1:2 scale model of an existing one as seen in image accompanying this article. In breastshot wheels, water enters into the buckets from the upstream side of the wheel, near or under the rotation axle. We found a maximum hydraulic efficiency of 75% constant over a wide range of flow rates; the efficiency can improve up to 80% using an inflow weir instead of a sluice gate at the inlet. Theoretical and dimensionless models to predict the power losses and the power output were also elaborated as engineering tools, and Computational Fluid Dynamic (CFD) simulations were performed to investigate the effect of the blade number and shape on the performance. These results have shown that the filling process of the buckets is the most important process to optimize in the engineering design.
We then investigated the efficiency of overshot water wheels. In this kind of water wheel, the water enters into the buckets from the top, exerting the pressure which drives the wheel on the blades along the downstream side of the wheel. We again found that the maximum efficiency was constant over a wide range of flow rates and rotational speeds, with a maximum of 85%. We also developed a theoretical model to estimate the power losses and the efficiency, with significant similarities between the conceptual and experimental results. The experimental results have shown that at flow rates higher than the maximum optimal one, a big amount of flow rate is not exploited by the wheel, generating a decrease in efficiency. Therefore, we are currently performing CFD simulations to investigate a new design to improve the efficiency in these conditions.
To conclude, we can say that water wheels are competitive and efficient hydropower converters in low head applications. The vast diffusion of suitable sites and their low cost make them very attractive and promising. The design of a water wheel is not banal: an optimal one is what distinguishes a good water wheel plant from an ancient one.
Learn more about PreScouter at www.prescouter.com.
