Active Energy Harvesting

Open Access
Liu, Yiming
Graduate Program:
Electrical Engineering
Doctor of Philosophy
Document Type:
Date of Defense:
October 23, 2006
Committee Members:
  • Heath Hofmann, Committee Chair
  • Qiming Zhang, Committee Member
  • George A Lesieutre, Committee Member
  • Jeffrey Scott Mayer, Committee Member
  • Energy Harvesting
  • Piezoelectric
  • Electrostrictive
Harvesting energy from the ambient environment is an enabling technology for wide deployment of wireless sensor networks. Converting mechanical energy to electrical energy using piezoelectric and electrostrictive materials has been the choice for many energy harvesting applications. The energy harvesting circuit is the interface between a piezoelectric/electrostrictive device and electrical load. A conventional view of energy harvesting circuitry is based on power conditioning concepts, which often involve AC to DC conversion and voltage regulation. In fact, an energy harvesting circuit also applies electrical boundary conditions to the device during energy conversion which are crucial for optimizing the harvested energy. This thesis presents a study of a relatively new type of energy harvesting approach: active energy harvesting. In this thesis, energy harvesting using both piezoelectric and electrostrictive materials is investigated. For each type of material, a theoretical model of energy conversion process is established, based on the electro-mechanical boundary conditions applied to the device by different energy harvesting circuits. This modeling technique has certain advantages over a harmonic analysis approach. First, it gives a more intuitive picture in terms of understanding the energy harvesting process than the harmonic analysis approach. Second, it is more general in its conclusions, that is the input mechanical excitation and electrical boundary conditions are not constrained to sinusoidal form but instead represented by several critical states of the electro-mechanical boundary conditions. Finally, for nonlinear materials, such as electrostrictive polymer, a linear harmonic analysis is no longer applicable, while the presented technique does not have this limitation. As a result of better understanding the importance of electromechanical boundary conditions in the energy conversion process, questions were raised: what is the best electrical boundary condition for a given mechanical excitation? And how to achieve the maximum power conversion? This thesis answers these questions by presenting the relatively new concept of active energy harvesting, which uses switch-mode power electronics to control the voltage and/or current of the piezoelectric/electrostrictive devices. Two control strategies, voltage control and charge control mode of operation are presented. Practically, power electronic circuits are not 100 percent power efficient, which greatly influences the performance of active energy harvesting system. We also address this issue by taking into account the loss due to reactive power flow between the piezoelectric/electrostrictive device and the energy storage unit. Experimental results of active energy harvesting are also presented for both piezoelectric and electrostrictive polymers. The model is validated by comparing theoretical prediction with experimental data. The experimental results also demonstrated superior energy harvesting performance over conventional diode rectifier circuits.