Introduction - Energy Harvesting Autonomous Sensor Systems - page 15

Lifetime vs. Battery Capacity

14.00

Duty cycle = 100%

12.88

12.00 Duty cycle = 1%

10.00

8.69

8.00

6.55

6.00

4.39

4.00

2.21

2.00

1.11

0.02 0.05 0.07 0.09 0.14

0.01

0.00

0 500 1000 1500

Battery Capacity (mA-hr)

2000 2500 3000 3500

FIGURE 1.11
Expected battery life versus system current usage and duty cycle. (From Crossbow Technology
Inc., “MPR-MIB Users Manual,” Crossbow Resources, Revision A, 2007 [21].)

Another issue associated with the high power consumption of a wireless
sensor node is the need for equivalently high power density energy storage
or energy source. As can be seen in Figure 1.12 , the instantaneous current
drawn by the sensor node communicating with the gateway is as high as 0.35
A. This means that for a 3-V battery supply, 1 W of electrical power is required
at the first instant to wake up the sensor node, as illustrated in Figure 1.12 .
Similarly, for the subsequent operations of the sensor node, that is, transmit
(CSMA/CA TX) and receive (CSMA/CA RX), the sensor will demand 1 W
of electrical power from the power source to complete its RF communication
with its base station or gateway. Under this circumstance, the powering of
the sensor node becomes a potential critical problem to be addressed.

1.2.2 Limitation of Energy Sources for Sensor Nodes

In many application scenarios, the lifetime of the sensor node typically ranges
from 2 to 10 years depending on the requirement of the specific application.
For the case of deploying sensor nodes on a mountain to detect the thickness
of the ice on the mountain, it will take years for the melting process to be mea-
surable. Hence, the lifetime of the sensor nodes must be several years before
they go into an idle state. If that is the case, energy supply is one of the major
bottlenecks to be addressed. Many different types of energy storage technolo-
gies are already available; as illustrated in Figure 1.13 , alkaline/rechargeable
batteries and supercapacitors are the most portable and popular energy sup-
ply option for powering the sensor nodes in WSNs. Batteries convert stored