| Summary: | It has been suggested that biofuels produced from microalgae may be a more sustainable alternative to other types currently produced, although currently the production of microalgae for this purpose has the potential for the overall energy balance to be negative. Microalgae are today cultivated in oval ponds of up to 10 m width and 500 m length, with water depths of 200 to 300 mm. The water must be kept in motion to avoid sedimentation. This is usually done with paddlewheels which have 6 to 8 blades, and a typical diameter of around 1.20 m. The energy demand for the continuously running wheels is one of the main cost factors, whilst the wheel efficiency is typically estimated as only 10 %. Very little is known about the effect of blade number or rotational speed on wheel efficiency. This research aimed to improve the paddlewheel as a propulsion mechanism in order to reduce the energy required. Theoretical work and 1:5 scale physical model tests were conducted to analyse the parameters affecting paddlewheel performance, to develop a consistent and improved model of the hydraulics of algae ponds, to define optimum configurations and to develop appropriate design tools. The results indicate that the number of blades, rpm and immersion depth have a great effect on the efficiency, with optimum values of over 60 % being achieved with higher blade numbers and lower rpm. Using an insert to reduce the backflow around the blades increased the efficiency and discharge of a 12-bladed wheel especially for the lower rotational speeds tested. A new theoretical equation to calculate the efficiency of the wheel was derived and validated against the physical model. It was found that the leakage of the fluid beneath the blades was the main loss factor in the efficiency of the wheel and should be minimised by using the insert where possible.
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