Wind Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 41

Wind
Speed

P wind P T P elec

P wind =
ρAv 3 /2

Aerodynamic
Eciency, C p

Generator
Eciency, η gen

v

FIGURE 2.3
Functional block diagram of a wind turbine generator.

the power available in the wind is harvested by the wind turbine to gen-
erate electricity. It is experimentally tested that the aerodynamic efficiency
(also known as the power coefficient C p )ofthe wind turbine with a wind
speed of 3.62 m/s is around 39% ( C p = 0.39). Using all the technical infor-
mation gathered for the wind turbine generator, the theoretical power P T
generated by the wind turbine at an average wind speed of 3.62 m/s can be
calculated as

P T, 3 . 62 m / s = C p P wind (2.2)

1
2 (0

03 2 )](3 62 3 ) = . 39)(1 . 225)[ (0 . . = 32 mW

The estimated power of 32 mW is the mechanical power available at the
rotor shaft of the wind turbine based on the aerodynamic blade effect as
well as the rotor shaft of the direct-coupled electric generator. Referring to
Figure 2.3 , taking the generator efficiency

gen into the power flow calculation,
the amount of electrical power harvested by the wind turbine P elec can be
described in terms of the incoming wind speed by the following equation:

P elec, 3 . 62 m / s =

gen P T, 3 . 62 m / s

(2.3)

= . = .

(0 41)(32 mW) 13 12 mW

The electrical powers P elec obtained experimentally from the wind turbine
over a range of simulated wind speeds of 1.3 to 8.5 m/s were tested with
different resistance loadings, and the power curves of the wind turbine are
shown in Figure 2.4 . For wind speeds available at the deployment site ranging
from 2.3 to 7 m/s as shown in Figure 2.2 , Figure 2.4 shows that the harvested
electrical power ranges from 2 to 70 mW, respectively. For the average wind
speed at the target site of 3.62 m/s, the electrical average output power gener-
ated by the wind turbine is 13 mW. This maximum electrical power of 13 mW
can only be harvested from the wind turbine at a matching load resistance
of 100 .

2.1.2 Design of an Efficient Power Management Circuit

In the WEH system, the power management unit is used to take care of the
dynamics of both the wind turbine generator subjected to environmental con-
dition variations such as varying wind speeds as well as the power-aware
wireless sensor node operating at the deployment site. Inside the power
management unit, a supercapacitor, which has a fast dynamic response, is