Introduction - Energy Harvesting Autonomous Sensor Systems - page 18

majority of autonomous sensor nodes in WSNs. The electrical energy neces-
sary for their operation is provided primarily by batteries. Although batteries
have been widely used in powering sensor nodes in WSNs presently, the
problem is that the energy density of batteries is limited, and they may not be
able to sustain the operation of the sensor nodes for a long period of time. Re-
ferring to the case scenario of an Xbow sensor node given in Table 1.2 with an
operating duty cycle of 1%, the average power consumption of the node with
supply voltage of3Visaround 1 mW. With reference to the highest reported
energy for current battery technologies that ranges around 3.78 kJ/cm 3 [1],
for the ultralow-power miniaturized wireless sensor node with a volumetric
size of around 10 cm 3 operating at an average power consumption of 1 mW
to have a 10-year life span, it needs a 100-cm 3 battery. The size of the battery is
10 times the sensor node's size. In fact, the calculation is a very optimistic es-
timate as the entire capacity of the battery usually cannot be completely used
up depending on the voltage drop. In addition, it is also worth mentioning
that the sensors and electronic circuits of a wireless sensor node could be far
smaller than 10 cm 3 .Inthis case, the battery takes up a significant fraction of
the total size and weight of the overall system and also is the most expensive
part of the system. Thus, energy supply is largely constrained by the size of
the battery.
In short, batteries with a finite energy supply must be optimally used for
both processing and communication tasks. The communication task tends to
dominate the processing task in terms of energy consumption. Thus, in order
to make optimal use of energy, the amount of communication tasks should be
minimized as much as possible. In practical real-life applications, the wireless
sensor nodes are usually deployed in hostile or unreachable terrains; they can-
not be easily retrieved for the purpose of replacing or recharging the batteries,
so the lifetime of the network is usually limited. There must be some kind
of compromise between the communication and processing tasks in order to
balance the duration of the WSN lifetime and the energy density of the storage
element. In summary, limitation in the device size and energy supply typically
means a restricted amount of resources, for example, for central processing
unit (CPU) performance, memory, wireless communication bandwidth used
for data forwarding, and range allowed. The need to develop an alternative
method for powering the wireless sensor and actuator nodes is acute. The
main research focus is to resolve the energy supply problems faced by the
wireless sensor nodes in a WSN.
1.3 Energy Harvesting Solution for Wireless Sensor Nodes
To overcome the major hindrance of the deploy-and-forget nature of wire-
less sensor networks due to the limitation of available energy for the net-
work constrained by the high power consumption of the sensor nodes and