7.3.5 Attenuator devices
The Pelamis, or ‘sea snake’, consists of a number of floating cylindrical tubes, connected to each other by active joints ( see Figure 19). The tubes are arranged at a slight angle to the down wave direction and so act as attenuator devices. The wave-induced heaving and swaying of the tubes is resisted by hydraulic rams that pump high pressure oil through hydraulic motors via smoothing accumulators, and the hydraulic motors drive electrical generators to produce electricity.
The ability to withstand high wave power densities has been a key goal of the designers. The Pelamis is capable of inherent load shedding, i.e. the spine is not subjected to the full structural loadings that would otherwise be imposed on it during a storm. It sits down the waves rather than across them and so becomes ‘detuned’ in long storm waves, where the waves are much longer than the device.
The prototype 750 kW P2 devices are 150 m long, 3.5 m in diameter, and composed of five modular sections. To reduce risk, wherever possible use is made of existing technology that has been proven offshore. Three 750 kW devices were deployed 5 km into the Atlantic off northern Portugal in 2008. Testing of the Pelamis P2 units at the European Marine Energy Centre (EMEC) is contributing to the development of the Pelamis P2e, to be tested in 2015 leading to deployment in commercial demonstration arrays.
You’ll now look at the economics of wave energy – our topic for the next section.