A centimetre-scale bi-directional wind turbine for energy harvesting applications: Design and experimental tests

In this work, new cm-scale wind turbines for powering sensor nodes devoted to monitoring/diagnostic of railway vehicles are designed, built and tested in a wind tunnel. Two rotors, having four airfoil blades with a diameter of 3 and 4 cm, are designed following the blade element momentum-based approach. The two rotors, tested in a wind tunnel, reach a power coefficient (without considering the losses in the conditioning circuit) ranging between 20% and 30%, in line with the maximum values of the Schmitz theory; the correspondence between the numerical estimates and the experimental data is discrete. The 4 cm diameter rotor, once connected to a conditioning circuit, reaches a maximum net efficiency of about 15%, equal to the maximum value found in the literature, but with a 13,5 cm diameter rotor. Considering the practical application on trains and the fact that the rotor has to work for two opposite velocity directions (it must be bi-directional), a 4 cm diameter symmetrical rotor with a constant blade pitch and flat plate blade section is also built. With the symmetric rotor, we can observe that the generated power is about 50% lower than that produced by the same rotor with airfoil blades. Finally, the turbine is set inside a duct to protect it and increase performance. The tests performed on this ducted turbine (4 cm diameter, symmetric) allow us to understand that the duct significantly increases the performance of the rotor in downwind conditions, from 1% to 6%, while it is almost irrelevant in upwind conditions. Moreover, the diffuser increases the efficiency by about 20%, while the contraction cone does not significantly modify the performance of the turbine.