António F. de O. Falcão, IDMEC, Instituto Superior Técnico, Technical University of Lisbon.
In general they stand on the sea bottom or are fixed to a rocky cliff. Shoreline devices have the advantage of easier maintenance and installation and do not require deep-water moorings and long underwater electrical cables. The less energetic wave climate at the shoreline can be partly compensated by natural wave energy concentration due to refraction and/or diffraction (if the device is suitably located for that purpose).
The typical first generation device is the oscillating water column (OWC). Another example is the overtopping device Tapchan (Tapered Channel Wave Power Device), a prototype of which (rated 350 kW) was built on the Norwegian coast in 1985 and operated for several years. The oscillating water column (OWC) device comprises a partly submerged concrete or steel structure, open below the water surface, inside which air is trapped above the water free surface.
The oscillating motion of the internal free surface produced by the incident waves makes the air to flow through a turbine that drives an electrical generator. The axial-flow Wells turbine, invented in the late 1970s, has the advantage of not requiring rectifying valves. It has been used in almost all prototypes.
Full sized OWC prototypes were built in Norway (in Toftestallen, near Bergen, 1985), Japan (Sakata port, 1990), India (Vizhinjam, near Trivandrum, Kerala state, 1990), Portugal (Pico, Azores, 1999), UK (the LIMPET plant in Islay island, Scotland, 2000). The largest of all (2MW), a nearshore bottom-standing plant (named Osprey) was destroyed by the sea (in 1995) shortly after having been towed and sunk into place near the Scottish coast. Smaller shoreline OWC prototypes (also equipped with Wells turbine) were built in Islay, UK (1991), and more recently in China. The Australian company Energetech developed a technology using a large parabolic-shaped collector to concentrate the incident wave energy (a prototype was tested at Port Kembla, Australia, in 2005).
In the present situation, the civil construction dominates the cost of the OWC plant. The integration of the plant structure into a breakwater has several advantages: the constructional costs are shared, and the access for construction, operation and maintenance of the wave energy plant become much easier. This has been done successfully for the first time in the harbour of Sakata, Japan (in 1990), where one of the caissons making up the breakwater had a special shape to accommodate the OWC and the mechanical and electrical equipment. The option of the “breakwater OWC” was adopted in the 750 kW OWC plant planned to be installed in the head of a new breakwater in the mouth of the Douro river (northern Portugal) and in the newly built breakwater at Mutriku port, in northern Spain.
