Environmental Engineering Reference
In-Depth Information
Fig. 8.6 Total
(sensible + latent) heat flux
versus melting time for a 1
mm soft rime ice layer at
different initial ice
temperatures
1
−
mm so
ft
rime i
c
e
3000
Initial ice temperature of −30
°
C
Initial ice temperature of 0
°
C
2500
2000
1500
Melting time of 1−mm ice for
a 500 KW wind turbine
1000
500
0
0
100
200
300
400
500
600
Melting time (sec)
melting time increases. Assuming thermal resistive heating with a maximum heat
flux generation capability of 2,000 W/m
2
(approximately 500 W/m
2
for the
summation of latent and sensible heat fluxes and 1,500 W/m
2
to compensate for
the convection loss at the leading edge), this calculation shows that 6 min is a
reasonable melting time for a 500 KW wind turbine.
It is shown in Fig.
8.5
that the required heat flux for de-icing significantly
decreases from the blade tip to the blade root. Experimental data is reported in the
literature on the required heat flux for de-icing at different blade regions, as tested
by VTT (The Technical Research Center of Finland) and Kemijoki Ltd. in 1993.
They tested a de-icing system using a thermal resistor on a 450 KW turbine in
extreme conditions in Finnish Lapland in order to keep the rotors free of ice. This
is likely the world's first documented test of a heat-based de-icing system for
commercial wind turbines [
15
,
20
]. Their data shows de-icing power consumption
of 5 % of the 450 KW turbine's rated power with peak heating power of 4,500 W/
m
2
at the blade tip versus 350 W/m
2
at the blade root (13/1 ratio). Heating ele-
ments covered a maximum of 15 cm from the leading edge. Heating power was
sufficient in most cases to keep off rime ice in the winter; however, this system was
unable to prevent icing in the (rare) cases of freezing rain.
8.5 Direct Optical Ice Sensing
As discussed earlier in Sect.
8.3.1
, indirect ice sensing methods do not have
enough spatial resolution and accuracy for active localized de-icing. Still there are
no implementations to directly measure the presence and type of ice accurately on
the blade.
We have applied a newly developed optical sensing technique to enable detecting
early stages of ice formation, including both the type of ice and the thickness (to an
