In order to look at the performance that may be expected for a device deployed at sea, the measures of performance should be as close to wave-to-wire as possible.
The idea behind the OWC-WEC is that the energy from the waves is converted into useful energy in the form of electricity. Thus, the guiding factor is that the maximum possible energy be converted.
The amount of energy converted is very dependent on the energy that hits the WEC in the form of the wave motion. In designing the WEC only one structure may be built, but many sea states will be experienced. Therefore, the WEC should be able to change its operation with the incident sea.
As well as maximising the amount of energy converted, the WEC may have to operate with certain limits; for example, limits for turbine speed or maximum chamber pressure.
1.3.1 Annual energy conversion
Annual energy conversion can be a very useful measure of WEC performance as it averages over the various seasons within a year while also providing a physical value which can be understood.
The method of calculating annual energy conversion usually uses a numerical model in which the wave is described by a spectrum. The frequency of occurrence of such spectra are then given for different locations and the energy converted by the WEC in such conditions is multiplied by the frequency of occurrence. Of course, for a system in the real sea, the total energy converted over the year may be quoted.
Although having specific measurements for given locations is important for estimating energy conversion, and thus for deciding on the feasibility of different projects, the fre- quency of occurrence for spectra disguises the range of spectral shapes which are included within any frequency of occurrence measure. It is also the case that the difference between the frequency of occurrence from one year to the next can be very large (Guanche et al.,
2013; Neill & Hashemi, 2013).
Annual energy conversion may also suffer as a performance measure from uncertainty in energy conversion rates in large seas. Here numerical models are not at their most reliable, while the amount of energy converted is large. This difference can be made clear for the case of a WEC operating in a survival mode. For the sea state slightly below this survival mode, the energy conversion is likely to be large, while for the sea state just in the survival mode, the energy converted is zero.
1.3.2 Energy conversion in different sea states
The energy converted should therefore be presented for different sea states. This is often done as a power table (Tietje et al., 2011). This enables the WEC to be relocated for an annual energy conversion assessment. Alternatively, the difference in seasonal energy conversion may be investigated if this data is available. It also enables performance to be assessed with more confidence for those sea states where a model is deemed to act well, while allowing for uncertainty in large sea state operation.
1.3.3 Efficiency and capture width
One performance parameter which is often used in engineering application is efficiency. In the case of a WEC, this is not in fact of great importance. As the waves are free, “wasting” energy does not particularly affect the cost of the converted energy. Clearly anyone owning a WEC would like that WEC to convert more energy (and thus bring in more money), but having a very efficient WEC which does not convert a lot of energy is not useful.
Capture width, C, is one way of describing efficiency for WECs. Capture width is the width that a WEC capturing all of the wave power incident per metre would be to convert as much power as the WEC in question, and is given by
C = Pabs ˜ Pinc
(1.30)
where Pabs is the power absorbed by the WEC and ˜Pinc is the incident power per metre
of wave-front (Price et al., 2009). For example, if a WEC has a capture width of 4 m, this is equivalent to a perfectly efficiency WEC which is 4 m in width. If the WEC in question has a width of 4 m, the efficiency would therefore be 100%. If it had a width of 20 m, the efficiency would therefore be 20%. Capture width may be given as a function of
device size. The variables, C, Pabs and ˜Pinc may be taken for single frequency waves, for
individual irregular sea states or for average values in a given wave climate.
If capture width is used as a way of scaling between devices of different sizes, or for identifying performance areas which could be made more efficient it can be of use. However, the values for energy conversion (average power) should be given for each sea state in order to asses performance, and thus the use of capture width is additional in energy conversion performance assessment, rather than fundamental.
In this thesis the total energy converted in a sea state will be used as the ultimate measure, but the average mechanical power converted for each sea state will also be given so that the effectiveness of such an OWC in other conditions may be estimated. Thus the conversion of energy from the incident wave to the estimated mechanical power output may be traced.