Comparisons between self-rectifying air turbines and self-rectifying water turbines for OWC wave energy converters

The oscillating water column (OWC) with a self-rectifying air turbine has been the most popular type of wave energy converter since the 1980s. Several types of such turbines have been developed and used. The Wells turbine has been the most popular, largely because of its mechanical simplicity, but others, mostly of impulse type, have also been developed. More recently, since the early 2010s, some authors proposed replacing self-rectifying air turbines by self-rectifying water turbines submerged in the lower part of the water column. Such turbines were mostly of Wells type or a self-rectifying version of the classical cross-flow hydraulic turbine. For a given OWC converter and a given turbine geometry (Wells turbine or other), the paper uses dimensional analysis to establish relationships between air and water turbines: the water turbine is much larger (diameter ratio 5.36) and its rotational velocity is much smaller (ratio 153.8). This shows the unrealism of papers on OWC converters reporting performance comparisons of the same turbine at the same rotational velocity in air and water. It is shown theoretically how a reduction in OWC diameter (as a way of lowering the water turbine size and cost) results in a decrease in wave energy absorption. The spring-like air compressibility effect in the air chamber is discussed. The aerodynamic noise, a major disadvantage of the air turbine near inhabited areas, is discussed in some detail. Finally, more efficient (and less noisy) self-rectifying air turbines developed since about 2010 are discussed as well as their replication in water.