Energy scavenging, or energy harvesting, which involves the conversion of ambient energy sources of energy such as mechanical vibrations, heat and light in useful electrical energy continues to receive significant industrial and academic interest. This stems from the ability of harvesting systems to provide a route for the realization of autonomous and self-powered low-power electronic devices. Among the wide variety of potential sources of ambient energy, the harvesting of energy from vibrations, movement, sound and heat are considered to be highly promising energy harvesting technologies.
Waste heat is a necessary by-product of all thermodynamic cycles implemented in power, refrigeration, and heat pump processes and recovering even a fraction of this energy has the potential for an economic and environmental impact. The conversion of heat directly into electricity can be achieved by thermoelectricity or pyroelectricity. The advantages of pyroelectric materials, compared to thermoelectrics, is that they do not require bulky heat sinks to maintain a temperature gradient, and have the ability to operate with a high thermodynamic efficiency for converting temperature fluctuations into useable electrical power. Energy conversion by heating and cooling a pyroelectric material therefore offers a novel way to convert heat into electricity and the approach has attracted interest in application areas such as low-power electronics and battery-less wireless sensors. Since all pyroelectric materials are piezoelectric it is of significant interest to utilize piezoelectric and pyroelectric materials for energy harvesting applications with electrical properties that are readily adjustable and tailorable to suit the harvestable energy source and are also sufficiently robust to survive any applied mechanical loads and thermal strains.
In order to assess the properties of materials for energy harvesting a variety of performance figures of merit containing combinations of physical properties have been developed to describe the ability of materials to generate energy for practical applications. For piezoelectric and pyroelectric energy harvesting applications, the important requirements for improved performance are low relative permittivity, low specific heat capacity and high piezoelectric and pyroelectric coefficients.
It is difficult to obtain the material with both high piezoelectric/pyroelectric coefficients and low relative permittivity by traditional methods. Efforts on improving performance often focus on dense materials which undergo chemical modifications (doping, substitution) or utilise the single crystal materials. Due to the complexity of developing new formulations with high cost, and the low Curie temperature and poor mechanical properties of single crystals, the applications of pyroelectric harvesting materials are currently limited. In this fellowship, the Fellow exploited her processing skills to develop novel pyroelectric materials with high figures of merit, high Curie temperature (operating temperature) and good mechanical properties, which have been readily manufactured in a low cost process.
This research has developed porous pyroelectrics for energy harvesting to generate electric power from thermal fluctuations, such as natural temperature fluctuations (e.g. wind, day/night, human body), exhaust gases, manufacturing processes. This work has provided the basic understanding of the effects of controlling pore orientation, surface area, porosity shape and fraction on the pyroelectric coefficients, figures of merit, electro-thermal coupling factors, rate of temperature change and piezoelectric performance, to create novel materials for harvesting applications.
