was then computed by summation of these cylinders along an entire line of interest (such as a 50 m depth contour or a 50 nautical mile line).
The products of the wave resource assessment include a database of 51-month time series at 3-hour intervals of wave parameters that can be used to reconstruct the fitted frequency spectra, although directional spreading information is not available. In addition, the group provides maps of annual and monthly average wave conditions (such as wave power density, wave height, period, direction, shown in three-dimensional plots) in a geographic information system (GIS)1 presented by the National Renewable Energy Laboratory’s (NREL’s) Renewable Resource Data Center. Bulk numbers for the total available theoretical wave resource and the total technical resource for different regions and for the entire United States are presented in the assessment group’s written report (EPRI, 2011).
To produce an estimate of the technical wave resource, the assessment group adopted an approach based on analyzing the cumulative probability density function (PDF) of wave power converted by a wave-energy device of prescribed capacity as a function of wave height. For a given threshold operating condition (TOC) and maximum operating condition (MOC), the percentage of the wave power that can be recovered can be estimated as a function of the rated operating condition (ROC). Note that this approach considers several extraction filters (e.g., TOC, MOC, and ROC constraints) and simplifies or neglects others (e.g., efficiencies, device spacing). The group generated cumulative PDFs for sites along the U.S. coastline and estimated the technical wave resource using the TOC and MOC values specific to three devices (Archimedes Wave Swing, Pelamis, and Wave Dragon) for various ROC values.
Compared to the more rigorous approach taken to compute the theoretical resource, the technical resource estimate relies on considerably looser assumptions. In the report, many of the factors are bundled into a single “packing density” of power per kilometer of installed system and some simple assumptions about the range of conditions in which the installed system can operate. Inaccurate or overly optimistic assumptions in these evaluations could create misleading estimates of the technical resource. In fact, the numbers used by the wave assessment group indicate that the technical resource is between 30 percent and 90 percent of the theoretical resource, depending on location. These concerns are addressed in more detail below.