Introduction - Energy Harvesting Autonomous Sensor Systems - page 13

1.2 Problems in Powering Wireless Sensor Nodes
As the WSN becomes dense with many sensor nodes, the problems in power-
ing the wireless sensor nodes, namely (1) high power consumption of sensor
nodes and (2) limitation of energy sources for sensor nodes becomes critical
and is even worse when one considers the prohibitive cost of providing power
through wired cables to them or replacing batteries. Furthermore, when the
sensor nodes must be extremely small, as tiny as several cubic centimetres,
to be conveniently placed and used, such small volumetric devices are very
limited in the amount of energy that the batteries can store, and there would
be severe limits imposed on the nodes' lifetime powered by the miniaturized
battery, which is meant to last the entire life of the node.

1.2.1 High Power Consumption of Sensor Nodes

Based on the breakdown of a wireless sensor node illustrated in Section 1.1.3,
the information about how much electrical power a sensor node consumes
during operation is determined. The power consumed by each individual
component (i.e., microcontroller, radio, logger memory, and sensor board in a
sensor node) is tabulated in Table 1.2 . It can be observed from Table 1.2 that all
the components in the sensor node consume a milliwatt level of power during
the active mode of operation and then drop to microwatts of power when in
sleep or idle mode. If the sensor node is set to operate at full duty cycle, that
is, 100%, the current and therefore power consumption of the sensor node
would be as high as 30 mA.
For most practical WSN applications described in Section 1.1.2, duty cycling
of the sensor node's operation is a common method discussed in the literature
[12-14] to reduce its power consumption and therefore extend the lifetime of
the WSN. During one operational cycle, the sensor node remains active for a
brief period of time before going into the sleep mode. During the sleep period,
the current consumption of the sensor node is typically in the microamper
range as opposed to the active period in the milliamper range. This results
in the sensor node drawing very little current, in the range of 2 to 8 A for

the majority of the time, and a short duration of current spikes in the range
of 5 to 17 mA while sensing, processing, receiving, and transmitting data as
illustrated in Table 1.2 . For the Xbow sensor node given in Table 1.2 , operating
at a duty cycle of 1% as opposed to 100%, the average current consumption
of a node with a supply voltage of 3 V is significantly reduced from 30 mA to
around 0.3 mA. Referring to Figure 1.11 , it is clearly illustrated that there is a
drastic difference in the battery life of the wireless sensor node operating at
duty cycles of 1% and 100%. This shows the need for duty cycling during the
operation of the sensor node.
Even with the help of duty cycling in the operation of the sensor node, the
power consumed by the wireless sensor node has been decreased by around 75
times, but the power density of its battery is still not high enough to support