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between 1/50th and 1/5th of the full size.
Tis depends on the capability of the wave
tank and the intended size of the fnal WEC.
Scaling dimensions of the experimental model
means that the lengths reduce by a geometric
scale and the ratio of chosen dominant forces
remain constant (similarity of forces). Froude
number scaling preserves the ratio of inertial
to gravitational forces. Froude scaling is
typically used for WEC wave tank testing,
ships and other ofshore structures, as waves
are gravity-driven. However, viscous efects
like turbulence and wave breaking will not
scale correctly. Additionally, it is very difcult to provide the correctly scaled material
properties, particularly for the mooring lines.
Power take off
A power take of (PTO) uses the relative
motion between WEC bodies, the seabed
or partially enclosed bodies of water to gen-
erate power. Tese are typically a hydraulic
system of valves, cylinders, accumulators and
motors; a wave-air interface with pressurized
air driving a turbine; a pneumatic system;
or a directly driven linear generator. Under
scaling laws for a typical WEC that has a
capacity of 200 kW, a 1/20th scale WEC
would generate a tiny 6 W. Hence, being able
to physically scale the fnal full-scale PTO
system is virtually impossible (i.e., developing
a hydraulic system with valves, accumulators,
a variable speed motor and a generator that
acts exactly like a scaled full-scale system, but
only generates 6 W).
Attempting to replicate a full-scale PTO
using a representative PTO with this power
level is challenging and may not match the
intended PTO’s responses. Frictional efects, in
the PTO or elsewhere, become signifcant and
can dominate resulting signals. Additionally,
due to space constraints within experimental
facilities or the issues associated with using
scaled moorings, the degrees of freedom that
the experimental model is allowed to move are
limited (i.e., a 6-degree of freedom WEC system is often tested with a scaled, single-degree
of freedom system). Tis introduces additional
friction from restraining the model and limits
the applicability of the resulting knowledge
for the fnal system design. It is not unusual
for an initial scaled WEC experimental setup
for the WEC to be barely able to move due
to excessive friction.
To compliment physical testing regimes,
numerical simulations can overcome many
of the limitations of scaled sea trails and
wave tank testing. Numerical simulations
can directly model the diferent forces and
associated rotational moments (torques)
acting on the WEC hull. Tese forces and
moments result from incoming waves, the
WEC interacting with the waves, the PTO,
the moorings and other environmental efects.
Within a numerical simulation, similar to
those WCWI tests, all the dynamic interactions of separate WEC hull components are
included. Tese forces and moments act in 6
degrees of freedom and excite motions of one
or more WEC Hull components.
Within WCWI, the P TO is typically modeled using hydraulic cylinder accumulators
and motors, a thermodynamic and turbine
model or a direct-drive linear generator. Te
P TO system can easily be changed and altered
as required and appropriate. Te efciencies,