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Chapter 1 Introduction
Sensor Networks’ (AEASN) project. The AEASN project was funded jointly by the UK
Ministry of Defence and General Dynamics UK. The project had two main aims:
1. To allow wireless sensor nodes to efficiently manage their own energy resources,
using energy harvested from their environment where appropriate. This should
permit nodes to be energy-aware and facilitate sustained operation.
2. To permit groups of nodes to negotiate and co-ordinate their sensing activities,
taking account of their energy status, to fulfil the requirements of the network.
The work presented in this thesis focuses on the first of these aims – delivering energyaware nodes that are able to manage their energy resources efficiently and accept energy
harvested from the environment. The research carried out by the author has fed into this
project, and three of his conference publications have also been presented as deliverables
of the AEASN project.
1.3
Justification for this research
Wireless sensor nodes must be capable of operating autonomously, and for many this
means they must operate without the constraint of a wired power supply. Conventionally, such devices have been powered by non-rechargeable batteries which are replaced
when depleted. However, energy harvesting (also known as energy scavenging) technology now offers the potential to sustain the operation of sensor nodes indefinitely.
In this process, environmental energy is converted into electrical energy which is then
used to directly power the sensor node, or is buffered (in rechargeable batteries or supercapacitors) for use later. Recent progress in the development of energy harvesting
technologies (harvesting electrical energy from such sources as light, vibration, or temperature difference) now permits sensors to be free from the limitations of operation from
non-rechargeable batteries. Indeed, the lifetime of the node will ultimately be limited
by its other components, such as sensors or flash memory.
To deliver a long operational lifetime, nodes must ensure that they use energy carefully.
Non-rechargeable batteries store a finite amount of energy which cannot be replaced,
so nodes must operate efficiently in order to achieve their designed lifetime. Similarly,
where nodes are powered from harvested energy they must, on average, use less power
than is generated in order to deliver sustained operation. Energy is arguably the most
critical resource for wireless sensors, and careful control of node activity to conserve
energy is essential. Energy harvesting is often, by its nature, an inconsistent operation
and the management of energy is a non-trivial task. Where nodes harvest energy from
their environment, their energy status may change rapidly and adaptive behaviour is