Environmental Engineering Reference
In-Depth Information
different regions (by sending required heat flux to different blade regions) within a
reasonable amount of de-icing time (20 min-1 h) given the conductivity of the
blade. The local required heat flux is a function of blade location, pitch angle, and
environmental conditions (altitude, ambient temperature, wind speed, and direc-
tion). Twelve resistive heaters (ten 5 watt and two 10 watt) are mounted from the
leading edge up to 1/3 of the chord length of our blade with the higher power
resistors placed at the leading edge.
Optical ice sensors are distributed on the blade such that each thermal actuator
is surrounded by four sensors. Optical fibers terminate in GRIN lenses mounted
flush with the surface of the blade. Ice sensors (3 mm in diameter and 6 mm in
height) are installed in drilled holes of the same size and attached with water
resistant glues for environmental stability (Fig.
8.10
b).
At ice temperatures below 0 C, before any melting occurs, optical ice sensors
cannot provide enough information for control gain calculation since the ice
thickness and type do not change by heating. Therefore, combined temperature and
optical ice sensors are used for closed-loop thermal control before any phase
change of ice. Below the melting temperature, optical ice sensors determine which
actuator should be active while temperature information is used to calculate the
controller gains. Alternatively, ice thickness information provided by the ice
sensors can be used to calculate control gains during ice thickness variation. Four
thermocouples [
26
] are placed close to the leading edge ice sensors for gain
calculation of the PID control that is a function of the difference between desired
temperature (slightly above 0 C) and instantaneous local temperature. It is eco-
nomically preferable to use fewer temperature sensors on the blade. Hence, dis-
tributed temperature sensors are only installed on the leading edge area. Thin
temperature sensors are used to prevent aerodynamic degradation in this work.
Figure
8.11
shows distributed optical sensors and resistive heaters on the blade.
Ultimately for active de-icing implementation on a full-scale wind turbine, it is
necessary to have distributed resistors embedded inside the composite blade to lower
the risk of damage by lightning. As mentioned earlier, distributed resistive heating
provides an efficient local control of heat flux on the blade. However, due to a large
number of actuators and sensors in this methodology, the possibility of a fault in the
network should be carefully considered in the design as an area of future work.
Under some very cold conditions, instant freezing of the runback water might
occur at the edges of the heating elements or on some cold blade areas that are not
covered by heating elements. This could form a barrier at the edges of the heating
element. The edge barriers may tend to grow toward the leading edge as horns
without contact with a heating element. Those horns that happen to grow on the
optical sensors could immediately be detected and de-iced by locally increasing
thermal flux using the surrounding heating elements. Intuitively, using a staggered
array configuration of heaters instead of an aligned array can be more efficient
(Fig.
8.12
) for preventing these local horns to grow all the way toward the leading
edge where icing leads to large aerodynamic losses. In addition, placing resistive
heaters in both upper and lower surfaces of the blade close to the leading edge area
can be considered to even further reduce the potential threat of ice horn residues in
