The power that can be produced by Pelamis at a given site depends upon the response of the machine and the local wave climate. The machine response is known from our sophisticated numerical model of Pelamis that is run in-house and validated against measurements made on scale models and our full-scale machines. We construct a power table – i.e. the power output of the machine in typical sea-states (characterised by the average height and the average period of the waves).
For a given potential wave farm site, the wave climate is established from historical data over a representative period of time – the longer the better and ideally several years. The average wave height and average wave period of each observed sea-state are derived from satellite measurements, or from data from wave-buoys in the vicinity, or they may be calculated (hindcast) from wind-wave numerical models that can determine wave conditions from known windspeed and direction over the relevant area of ocean. The record of the wave conditions over the representative period builds up a picture of the wave climate, i.e. its variation with the seasons and from year to year.
The Pelamis power table and the wave climate are combined to give the electrical power response over time and from that, its average level and its variability. These in turn will help determine the revenue stream from the sale of that electricity for a wave farm at that location, and thus its commercial attractiveness.
Once the general area of the wave farm site has been determined more work can be done to pick the best site within that area. For example, the mean wave direction and its variability should be determined. Pelamis is relatively insensitive to changes in wave direction because it ‘weathervanes’ i.e. it swings to face the waves. However, the optimum orientation for the mooring is to be aligned to the mean wave direction. A detailed study of the wave climate and the bathymetry may point to a particular location where there is favourable local focusing of waves. Other essential tasks related to the resource assessment include the calculation of ‘weather windows’ that allow sufficient calm weather to carry out maintenance and other operations, and, of particular importance, the statistical determination of the extreme waves that can be expected at the site over the lifetime of the machine, which the machine is designed to survive.