Waste heat can be directly converted into electrical energy by performing the Olsen cycle on pyroelectric materials. The Olsen cycle consists of two isothermal and two iso-electric field processes in the displacement versus electric field diagram. This paper reports, for the first time, a procedure to implement the Olsen cycle by alternatively placing a pyroelectric material in thermal contact with a cold and a hot source. Poly(vinylidene fluoride-trifluroethylene) [P(VDF-TrFE)] copolymer thin films with 60/40 VDF/TrFE mole fraction were used. A maximum energy density of 155 J/L per cycle was achieved at 0.066 Hz between 25 and 110°C and electric fields cycled between 200 and 350 kV/cm. This energy density was larger than that achieved by our previous prototypical device using oscillatory laminar convective heat transfer. However, it was lower than the energy density obtained in previous “dipping experiments” consisting of alternatively dipping the samples in cold and hot silicone oil baths. This was attributed to (1) the lower operating temperatures due to the slow thermal response achieved using heat conduction and (2) the smaller electric field spans imposed which was limited by the smaller dielectric strength of air. However, the proposed procedure can readily be implemented into devices.

This paper is concerned with the direct energy conversion of waste heat into electricity using pyroelectric materials. It reports detailed numerical simulations of a prototypical pyroelectric energy converter assembled experimentally and previously reported. The device consisted of a hot and cold source separated by a series of microchannels supporting pyroelectric thin films. A piston was used to vertically oscillate a working fluid back and forth between the thermal sources. The experimental device was fully instrumented with thermocouples and pressure sensor. The transient two-dimensional mass, momentum, and energy equations were solved numerically to determine the local and time-dependent temperature at various locations inside the device for operating frequency varying from 0.025 to 0.061 Hz. Excellent agreement was found between the simulated and experimentally measured local temperatures at all operating frequencies considered. These results confirm our previous numerical results and the simulation tool can now be used to design the next generation of pyroelectric energy converters.

This study is concerned with pyroelectric energy conversion to directly convert waste heat into electricity. The pyroelectric effect refers to the flow of charges to or from the surface of a material upon heating or cooling. A prototypical pyroelectric energy converter was designed, built, and tested. It performed the Olsen cycle consisting of two isothermal and two isovoltage processes in the charge-voltage diagram. Co-polymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] thin films sandwiched between metallic electrodes were used as the pyroelectric elements. Their temperature oscillation, charge, and voltage along with the overall heat input and output were measured experimentally. Then, the electrical power generated and the energy efficiency of the device were computed. The effects of channel width, frequency, and stroke length on temperature swing, heat input, and energy and power densities were investigated. Reducing the channel width and increasing the stroke length had the largest effect on device performance. A maximum energy density of 130 J/L of P(VDF-TrFE) was achieved at 0.061 Hz frequency with temperature oscillating between 69.3 and 87.6°C. Furthermore, a maximum power density of 10.7 W/L of P(VDF-TrFE) was obtained at 0.12 Hz between 70.5 and 85.3°C. In both cases, the voltages in the Olsen cycle were 923 and 1732 V imposed on a 45.7 microns thick 60/40 P(VDF-TrFE) films. To the best of our knowledge, this is the largest energy density achieved by any pyroelectric energy converter using P(VDF-TrFE). It also matches performances reported in the literature for more expensive lead zirconate stannum titanate ceramic films operated at higher temperatures between 110 and 185°C.

Pyroelectric energy conversion offers a novel approach for directly converting waste heat into electricity. This paper reports numerical simulations of a prototypical pyroelectric energy converter. The two-dimensional mass, momentum, and energy equations were solved to predict the local and time-dependent pressure, velocity, and temperature. Then, the heat input, pump power, and electrical power generated were estimated, along with the thermodynamic energy efficiency of the device. It was established that reducing the length of the device and the viscosity of the working fluid improved the energy efficiency and power density by increasing the optimum operating frequency of the device. Results show that a maximum efficiency of 5.2% at 0.5 Hz corresponding to 55.4% of the Carnot efficiency between 145 and 185°C can be achieved when using commercial 1.5 cst silicone oil. The maximum power density was found to be 38.4 W/l of pyroelectric material.