Introduction - Energy Harvesting Autonomous Sensor Systems - page 16

0.4

1 2 3 4 5 6

0.35
0.3

(1) Waking up
(2) Synchronizing clocks
and processing of data
packet
(3) CSMA/CARX-listening
(4) CSMA/CA transmit
(5) CSMA/CARXACK
(6) Packet processing and
preparing for deep sleep

Current Drawn
by Sensor
Node

0.25
0.2
0.15

0.1
0.05

0

0.05 0.055 0.06

Time (seconds)

Sensor Node Gateway

FIGURE 1.12
Current consumption profile of a sensor node communicating with a gateway.

chemical energy directly into electrical energy. They are generally classified
into two groups: single-use/primary and rechargeable/secondary batteries.
The distinction between the two groups is based on the nature of the chemical
reactions.
Primary batteries are discarded when sufficient electrical energy can no
longer be obtained from them. Secondary batteries, on the other hand, con-
vert chemical energy into electrical energy by chemical reactions that are
essentially reversible. Thus, by passing the electrical current in the reverse
direction to that during discharge, the chemicals are restored to their original
state, and the batteries are restored to full charge. A supercapacitor (or super-
cap), is another electrochemical energy system other than batteries that has
been gaining its presence in powering wireless sensor nodes. There are sev-
eral reasons for this phenomenon to occur. One reason is that a supercapacitor
is very scalable, and its performance scales well with its size and weight. An-
other reason is that a supercap has many desirable characteristics that favour
the operations of the sensor nodes, such as high power density, rapid charg-
ing times, high cycling stability, temperature stability, low equivalent series
resistance (ESR), and very low leakage current [23].
Referring to the Ragone plot shown in Figure 1.13 , which consolidates var-
ious energy storage technologies and compares their power density and en-
ergy density characteristics, it can be identified that a supercapacitor has
much higher peak power density than the other energy storage device, like
batteries and fuel cells. This means that a supercap can deliver more elec-
trical power than batteries and fuel cells within a short time. As shown