Sensitivity Analysis for the Encaged Turbine Concept in Oscillating Water Column Plants

Oscillating water column plants are one of the most popular wave energy device types. Prototype OWC units have been operating in various parts of the world since the mid-1980s and such developers have more field experience of this technology than any other relevant plant. The most common turbine used is the self-rectifying Well's turbine which has a rather low peak efficiency if compared to other designs but was preferred in terms of its simplicity and cycle performance. The present study exploits the merits of a new concept for the power extraction process, that of an encaged turbine for OWC plants, which allows conventional high-efficiency turbines to be employed in such plants. This is achieved by guiding the pressurized air into a sequence of three chambers, creating a unidirectional closed air circuit through the turbine. A theoretical model is deployed simulating the operation of the plant and a sensitivity analysis is carried out for the design and working parameters. Results indicate that the power extraction efficiency may exceed the 50% level in a real plant. 1. Introduction The exploitation of the energy contained in the earth’s oceans has been proposed several times in the past, especially the energy that is carried in the ocean’s waves. Several different wave energy devices for harvesting the wave energy have been proposed [1, 2], but the main concept under investigation are the oscillating water columns. The principle of operation of the traditional oscillating water column is rather simple, extracting useful mechanical energy by operating on the U-manometer principle, where water in one of the “legs” oscillates under the action of the waves, while that in the second acts as a piston on the air cavity above. In a real plant, a hollow reinforced concrete structure—pneumatic chamber—communicates with the sea (and the incident waves) by a submerged opening in its front wall [3, 4]. The incident waves act on the water column contained in the lower part of the capture chamber, which in turn causes alternate air flow to and from a duct that contains the turbine which drives the electric generator. Almost all prototype plants of OWC employ the Well’s turbine as the power delivering system, which has the advantage of not requiring rectifying valves because of its ability to rotate unidirectional despite the air flow direction. Though results in terms of efficiency so far have been rather disappointing by the usage of Well’s turbine, mainly due to hysteretic characteristics of the symmetrically blades in a reciprocating flow [5]. The turbine’s