Preface

Energy harvesting is the conversion of ambient energy present in the environment into electrical energy. It is identical in principle to large-scale renewable energy generation, for example, solar or wind power, but very different in scale. While large-scale power generation is concerned with megawatts of power, energy harvesting typically refers to microto milli-watts, i.e. much smaller power generation systems. The development of energy harvesting has been driven by the proliferation of autonomous wireless electronic systems. A classic example of such systems are wireless sensor nodes which combine together to form wireless sensor networks. Each sensor node typically comprises a sensor, processing electronics, wireless communications, and power supply. Since the system is by definition wireless and cannot be plugged into a mains supply, power has to be provided locally. Typically such a local power supply is provided a battery which on the face of it is convenient and low cost. However, batteries contain a finite supply of energy and require periodic replacement or recharging. This may be fine in individual deployments but across a wireless network containing a multitude of nodes batteries are clearly not attractive. Furthermore, the need to replace batteries means the wireless system has to be accessible which may not be possible or may compromise performance. Finally, there are environmental concerns about disposing of batteries. Energy harvesting was developed, therefore, as a method for replacing or augmenting batteries. By converting ambient energy in the environment, the energy harvester can provide the required electrical power for the lifetime of the wireless system which is also free to be embedded or placed wherever it is best suited to perform its function. Energy harvesting typically exploit kinetic, thermal, solar sources, or electromagnetic radiation sources. Kinetic energy harvesting converts movement, often in the form of vibrations, into electrical energy. Thermal gradients can be exploited by using thermoelectric generators while solar energy is harvested using photovoltaics. Electromagnetic radiation can capture the energy from radio waves but unless this energy is specifically broadcast, power levels are typically very low. The challenges for energy harvesting are to maximise the available electrical power from the ambient energy found in the application environment. Vibration energy harvesters, for example, need to be tuned to match characteristic frequencies found in the environment which often means bespoke generator designs are required for different applications. It would be much better if such generators