POWER GENERATION EFFICIENCY OF PIEZOELECTRIC CRYSTALS

The purpose of this research is to explore, as a basis for an alternate energy source, the conversion of mechanical energy to electrical energy by means of the piezoelectric effect. The scope is limited to experimentally determining the energy conversion efficiencies of thickness mode vibrating quartz crystals. Because maximum conversion efficiencies are desired, measurements were restricted to the crystals' resonant frequencies since piezoelectric crystals behave as forced harmonic oscillators. As such they have a marked increase in efficiency at resonance.;It was found experimentally that X-cut quartz crystals with nominal resonant frequencies of 1 MHz possess an energy conversion efficiency of approximately 95 percent. Associated with this was an 18.6 percent experimental error of which approximately two-thirds came from errors in calibrating the ultrasonic probe.;The major conclusion reached in this experiment is that piezoelectric crystals in general and quartz crystals specifically are potential alternate energy sources. The realization of that potential will first require the development of practical and efficient ways to utilize primary energy sources, e.g. solar energy or wind energy, to drive piezoelectric crystals at their resonant frequencies.;The experimental procedure involved sequentially submerging the two receiving crystals in degassed water and driving them with acoustical waves. The conversion efficiencies were determined by measuring the acoustical power acting on the crystals and the corresponding electrical power generated in the attached electrical loads. The acoustical waves were generated by a third quartz crystal operating at the resonant frequencies of the receiving crystals and far enough removed to place the receiving crystals in the far field. A miniature ultrasonic probe was used to systematically measure the intensity of the acoustical waves. These intensities were then numerically integrated to obtain the acoustical power received by the crystals. The impedance-matched electrical loads attached to the crystals each consisted of a series RL circuit. The inductances were used to tune out the crystals' capacitances and the resistances to generate the electrical power.