Vibration energy harvesting from rotorcraft drive systems using thermoelectric conversion of heat generated by mechanical damping

Open Access
Author:
Mahmood, Raheel Sheikh
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
December 02, 2014
Committee Members:
  • George A Lesieutre, Thesis Advisor
  • Edward A Smith, Thesis Advisor
Keywords:
  • energy harvesting
  • vibration
  • rotorcraft
Abstract:
Energy harvesting from ambient structural vibrations on rotorcraft continues to generate increasing interest due to its potential use in powering wireless sensors for applications such as structural health monitoring. Vibration energy harvesters developed in the past have used piezoelectric or electromagnetic devices to convert mechanical work into electrical energy for use by accelerometers, strain gauges, or other devices. Vibrations from rotorcraft drive systems typically occur within a frequency range of 500 to 3000~Hz as a result of transmission gear meshing. An alternative method for vibration energy harvesting employs a thermoelectric generator which converts thermal energy dissipated by a damped mechanical oscillator installed on the vibrating structure to electrical energy. Damping of the oscillator is provided by either a viscoelastic spring or by forces due to eddy current induction in a metal plate moving relative to a magnetic field. Two harvester designs implementing oscillators damped by either viscoelastic or electromagnetic forces are described, and one-dimensional analytical models are developed to model heat generation and conduction for each design. Test articles are designed for experimental evaluation of the two heating methods, and the analytical models are used to predict the behavior of the test articles. An analogous thermal circuit model is used to model transient heat flow, and a finite element model is used to model magnetic flux distribution to aid in prediction of temperature response of the test articles. Experimental characterization of a stiff, low damping elastomer indicated decreasing stiffness and loss factor with increasing base acceleration. The average measured stiffness was found to be slightly lower than the expected value based on the material specification, and the average loss factor was also lower than expected but aligned with the expected value at low acceleration levels. An increase in the temperature of the viscoelastic test article driven at resonance was recorded at a slightly lower rate than predicted. Induction heating measurements of aluminum plates of varying thicknesses showed an increase in surface heating with plate thickness. The measured temperature over the test period increased quadratically with relative velocity, as predicted by the analytical model. Measurement of the power output of two thermoelectric generators with changing surface temperature revealed differences in the response for upward and downward temperature change, indicating nonlinearities in the behavior of these units. Results of the study showed limited applicability of the considered approaches for high frequency vibration energy harvesting applications due to the long time required for heating of the elastomer, or the high velocities required for magnetic induction heating. Suggestions for improving the performance and efficiency of the proposed harvesters are provided along with recommendations for future work.