On the Efficiency of Multi-Source Energy Harvesters

Energy harvesters can be used to provide small amounts of power in remote locations. Applications include powering wireless sensor networks and powering microelectromechanical systems. A wealth of different designs exists for harvesting energy from different sources, including designs which harvest from multiple sources simultaneously. However, there are no universally accepted metrics for assessing the performance of energy harvesters; this can make it impossible to compare designs in any meaningful way.

The first part of this thesis develops a domain-neutral framework for describing and analysing the behaviour of energy harvesters. This involves introducing a system of dimensionally consistent analogies into energy harvesting. Using this domain-neutral and dimensionally consistent framework, it is possible to come up with general expressions for the behaviour of single-source energy harvesting systems. This approach is then validated experimentally for single-source energy harvesters.

The second part of this thesis involves extending the theoretical analysis to multi-source energy harvesters. Using the system of analogies defined in the first part of the thesis it is possible to create an n-degree-of-freedom matrix representation of a multi-source energy harvester. This enables us to derive expressions which are valid for both single-source and multi-source energy harvesters. The expressions for the maximum power absorbed by an energy harvesting device are shown to be independent of the number of sources, as well as any static coupling or coupling through material effects (e.g. piezoelectric). Numerical simulations are used to explore the validity of these expressions for various system configurations driven with a mixed stochastic-deterministic input signal. From the results of these numerical simulations, a practical approach for estimating the efficiency of an energy harvester using the maximum power absorbed as a theoretical limit is described.

The third part of this thesis describes experiments which validate the theoretical analysis. These experiments are used to provide an example of how to calculate and compare the efficiency of energy harvesting designs.