Vibration Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 130

Energy Stored vs. Capacitance

1.2

1
0.8
0.6
0.4
0.2
0

0 5 10 15

Capacitance/uF

20 25 30 35

FIGURE 4.21
Harvested energy for various capacitance values.

various capacitance values, 3.3 F yields the highest stored energy of 1.1 mJ

and so does its generated voltage of around 85 V. This high output voltage
might be a great challenge to the design of the power management circuit. As
such, it is necessary to make a compromise between the harvested energy and
the generated voltage to meet the energy requirement of the wireless load.

4.2.3 Power Management Circuit

In order to determine the technical specifications of the power management
circuit, the power requirements of the RF transmitter have to be determined.
The transmitter power requirement is more important as it is the load to which
the piezoelectric transducer supplies. Since the input voltage and current vary
according to the transmission range and data rate, there is a need to determine
the relationship between the input voltage and transmission range, as well
as the corresponding input current. An experiment was conducted to inves-
tigate the relationship as shown in Table 4.3 .

TABLE 4.3
Power Consumption of RF Transmitter Load

V ee I cc Minimum Transmission Range Power Energy

(V) (mA) Time (ms) (m) (mW) (mJ)

1.5 0.280 105 0.0 0.42 0.044

1.6 0.365 95 6.5 0.58 0.055

2.0 0.450 84 — 0.90 0.076

2.4 0.580 73 10.0 1.39 0.102

2.8 0.710 60 — 1.99 0.119

3.6 0.975 57 15.5 3.51 0.200

4.0 1.120 55 — 4.48 0.246

4.4 1.210 53 18.5 5.32 0.282

5.0 1.410 50 20.0 7.05 0.353

6.0 1.740 48 — 10.44 0.501

7.0 2.060 45 24.0 14.42 0.649