The Wells turbine is a low-pressure air turbine developed for use in oscillating-water-column wave power plants to avoid the need to rectify the air stream by delicate and expensive valve systems. It keeps its sense of rotation in spite of the changing direction of the air stream, which is driven by the rising and falling water surface in a compression chamber. Its blades feature a symmetrical airfoil with its plane of symmetry in the plane of rotation and perpendicular to the air stream. Its efficiency is lower than that of a turbine with constant air stream direction and asymmetric airfoil. One reason is that symmetric airfoils have a higher drag coefficient then asymmetric ones, even under optimal conditions. Also, in the Wells turbine, the symmetric airfoil is used with a high angle of attack (i.e., low blade/air speed ratio), as it occurs during air velocity maxima in volatile flow. Then the air stream stalls and lift collapses. The efficiency of the Wells turbine in oscillating flow reaches values between 0.4 and 0.7. Another disadvantage is the absence of self-starting capability. For starting, the generator has to be used as motor, which consumes energy. This simple but ingenious device has been developed by Prof. Alan Wells of Belfast University in the late 1980s.
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Another solution of the problem of stream direction independent turbine is the Darrieus rotor.
