Air turbine optimization for a bottom-standing oscillating-water-column wave energy converter

Abstract

The oscillating-water-column wave energy device equipped with an air turbine is widely regarded as the simplest and most reliable, and the one that was object of the most extensive development effort. The aerodynamic performance of the air turbine plays a major role in the success of the technology. A case study was selected to investigate these issues: the existing bottom-standing plant on the shoreline of the island of Pico, in Azores Archipelago. The overall performance of the OWC plant was modelled as an integrated hydrodynamic and aerodynamic process. The hydrodynamic modelling was based on linear water wave theory. Published results from model testing of Wells and biradial turbines, together with well-known tools from dimensional analysis, were employed to determine the aerodynamic performance, and to optimize the turbine size and rotational speed. Singleand two-stage Wells turbines were considered. Unlike the biradial turbine, the performance of the Wells turbine, especially the single-stage version, was found to be severely affected by the constraints on rotor-blade tip speed (to avoid excessive centrifugal stresses and shock waves in the more energetic sea states). A power law, relating instantaneous values of the electromagnetic torque and the rotational speed, was found to apply to both types of turbines as a rotational speed control algorithm. The large runaway speed of the Wells turbine was found to be a risk to turbine integrity by excessive centrifugal stresses that requires safety measures. The much lower runaway speed of the biradial turbine is not expected to be a major problem.

The influence of scale on the air flow and pressure in the modelling of Oscillating Water Column Wave Energy Converters

“…Eq. (10) is essentially the same as the differential equation for pressure fluctuations in [27], derived in a similar line of reasoning and used for 2D and 3D cases.…”

Well-posedness of a nonlinear shallow water model for an oscillating water column with time-dependent air pressure

“…With this type of hypothesis on the air pressure inside the chamber, it is possible to find in ocean engineering literature an evolution equation governing the dynamics the pressure variation P ch (t). For instance, we refer to [11,15]. It is derived for oscillating water column with Wells turbines [29], for which the relation between the pressure drop and the velocity of the air in the resistance layer is linear.…”

“…In the last decades oscillating water columns have been widely investigated both experimentally and numerically in the hydrodynamical engineering literature for the sake of understanding how to increase the performance of these wave energy converters in order to facilitate a real installation. For instance, we refer to [11,14,15,24,30,31,32] and references therein. In particular, in [30] Rezanejad, Bhattacharjee and Soares showed that the inclusion of an artificial step at the sea bottom may lead to a significantly increased efficiency, that is the capacity of power absorption of a wave energy converter.…”

Wave power extraction from multiple oscillating water columns along a straight coast

“…Numerical and physical studies on a coast/breakwater-integrated OWC can be found in Morris-Thomas, Irvin & Thiagarajan (2007), He, Huang & Law (2012), Zhang, Zou & Greaves (2012), López & Iglesias (2014), Elhanafi et al. (2016), Falcão, Henriques & Gato (2016), López et al. (2016), He, Leng & Zhao (2017) and Howe & Nader (2017).…”