Wind Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 62

Unperturbed
Wind Power,
P o

heoretical
Mechanical
Power, P T

Raw
Electrical
Power, (AC)

Raw
Electrical
Power, (AC)

Processed
Electrical
Power, (DC)

Stored
Electrical
Energy, (DC)

Electrical Energy
(Regulated DC
power)

Wind
Turbine

Electric
Generator

Passive
Rectifier

DC-DC Boost
Converter

Energy Storage
Supercapacitor

DC-DC Buck-
Boost Converter

Wireless
Sensor Node

4V, 1.5 F
t charge = 85 mins
τ = 65 mins

η rect = 88%

η wt = 39% η gen = 15% η rect = 50% η conv = 85% P = 3.6 mW

t lifetime = 0.8 hrs

82 mW 32 mW 4.8 mW 2.4 mW 2.04 mW 12.00 J 10.56 J

FIGURE 2.25
Line diagram of the power distributed in the WEH system without an active rectifier and MPPT
scheme.

2.1.3.2 Power Conversion Efficiency of the WEH System
The WEH system as a whole is a complex system that is made up of many
different subsystems. In order to understand how to improve the overall
efficiency of the WEH system, it is important to study the performance of each
of these subsystems and perform some power analysis to better understand
how power is distributed at each power conversion stage of the WEH system.
Line diagrams are drawn in Figures 2.25 and 2.26 to illustrate the input and
output power available for each subsystem so that the power conversion
efficiency of the subsystem can be determined.
Referring to Figure 2.25 , the line diagram starts from the input with a wind
speed of 3.62 m/s where 82 mW of raw wind power is supplied to the wind
turbine with an efficiency of 39%, and 32 mW of mechanical power is avail-
able for harvesting. Using a standard diode-based full-bridge rectifier, 50%
of the raw electrical power (AC) generated at the output of the wind turbine
generator of approximately 4.8 mW is converted into raw electrical power
(DC) of 2.4 mW. With a standard boost DC-DC converter being used in the
WEH system, it is observed in Figure 2.25 that the electric generator has a
relatively low efficiency (15%) because its internal source impedance is not
properly matched with the subsequent subsystems. Hence, the processed DC
power to charge the supercapacitor is only 2.04 mW.

Unperturbed
Wind Power,
P o

heoretical
Mechanical
Power, P T

Raw
Electrical
Power (AC)

Raw
Electrical
Power (DC)

Processed
Electrical
Power (DC)

Stored
Electrical
Energy (DC)

Electrical Energy
(Regulated DC
power)

Active
Rectifier

Boost Converter
with MPPT

DC-DC Buck-
Boost Converter

Wireless
Sensor Node

Wind Turbine Electric

Generator

Energy Storage
Supercapacitor

5.5 V, 1.5 F
t charge = 85 mins
τ = 17 mins

η wt = 39% η gen = 41% η rect = 72% η conv = 84%

η rect = 88% = 3.6 mW

t lifetime = 1.5 hrs

P

82 mW 32 mW 13 mW 9.36 mW 7.86 mW 22.69 J 19.97 J

FIGURE 2.26
Line diagram of the power distributed in the proposed WEH system.