Converting heat to electricity with non-linear pyroelectrics: A review

The efficient conversion of waste thermal energy into electrical energy is of considerable interest due to the huge sources of low-grade thermal energy available in technologically advanced societies. Our group at the Oak Ridge National Laboratory (ORNL) is developing a new type of high efficiency thermal waste heat energy converter that can be used to actively cool electronic devices, concentrated photovoltaic solar cells, computers and large waste heat producing systems, while generating electricity that can be used to power remote monitoring sensor systems, or recycled to provide electrical power. The energy harvester is a temperature cycled pyroelectric thermal-to-electrical energy harvester that can be used to generate electrical energy from thermal waste streams with temperature gradients of only a few degrees. The approach uses a resonantly driven pyroelectric capacitive bimorph cantilever structure that potentially has energy conversion efficiencies several times those of any previously demonstrated pyroelectric or thermoelectric thermal energy harvesters. The goals of this effort are to demonstrate the feasibility of fabricating high conversion efficiency MEMS based pyroelectric energy converters that can be fabricated into scalable arrays using well known microscale fabrication techniques and materials. These fabrication efforts are supported by detailed modeling studies of the pyroelectric energy converter structures to demonstrate the energy conversion efficiencies and electrical energy generation capabilities of these energy converters. This paper reports on the modeling, fabrication and testing of test structures and single element devices that demonstrate the potential of this technology for the development of high efficiency thermal-to-electrical energy harvesters.

Pyroelectric thermal energy harvesters are intriguing alternatives to thermoelectric devices due to their high thermodynamic efficiency and reduced heat sink requirements. We report a concept for pyroelectric energy harvesters that utilize liquid-based switchable thermal interfaces to achieve thermodynamic cycling frequencies and hence high-power densities. Pyroelectric energy harvesting in thin films of 56/44 P(VDF-TrFE) copolymer is demonstrated at thermodynamic cycle frequencies of the order of 1 Hz and material-level power densities of the order of 100 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . The present work demonstrates the viability of high-power density pyroelectric thermal energy harvesters based on liquid-based switchable interfaces and identifies a need for optimized electrode arrays to maximize their potential.

Non-linear pyroelectric energy harvesting using ferroelectric thin films exhibits high energy conversion, primarily due to their large breakdown field compared to bulks. Here, we report the pyroelectric energy conversion potential of lead scandium tantalate, Pb(Sc1/2Ta1/2)O3 (PST) thin film fabricated on a c-sapphire substrate using chemical solution deposition. To enable the application of high electric field and to assess the pyroelectric energy conversion performance, interdigitated electrodes were deposited on the PST thin film. A maximum harvested energy density of 9.1 J cm−3 per cycle was deduced from polarization measurements in films undergoing an Olsen cycle between 0 °C and 150 °C when the electric field was varied between 50 and 1500 kV/cm. Furthermore, PST thin films can reach up to 27 % of Carnot efficiency for a temperature interval of 10 K between 30 °C and 40 °C. This study highlights the significance of PST thin films for electro-thermal energy harvesting and promising opportunities for enhancing the conversion efficiency and power density using thin films or thin film multi-layer capacitors in the future for thermal energy harvesting.

Among the various forms of natural energies, heat is the most prevalent and least harvested energy. Scavenging and detecting stray thermal energy for conversion into electrical energy can provide a cost-effective and reliable energy source for modern electrical appliances and sensor applications. Along with this, flexible devices have attracted considerable attention in scientific and industrial communities as wearable and implantable harvesters in addition to traditional thermal sensor applications. This review mainly discusses thermal energy conversion through pyroelectric phenomena in various lead-free as well as lead-based ceramics and polymers for flexible pyroelectric energy harvesting and sensor applications. The corresponding thermodynamic heat cycles and figures of merit of the pyroelectric materials for energy harvesting and heat sensing applications are also briefly discussed. Moreover, this study provides guidance on designing pyroelectric materials for flexible pyroelectric and hybrid energy harvesting.

A comprehensive review on small-scale thermal energy harvesters: Advancements and applications

Time-sharing orbit jump and energy harvesting in nonlinear piezoelectric energy harvesters using a synchronous switch circuit

Solid state generators and energy harvesters for waste heat recovery and thermal energy harvesting

The nonlinear energy harvester has become a hot topic due to its broad bandwidth and lower resonant frequency. Based on the preliminary test and analyses in our previous work, further analyses and tests on the influence of parameters, including the nonlinear magnetic force of the hybrid energy harvesting structure on its output performance under harmonic excitation, are performed in this paper, which will provide powerful support for structural optimization. For designing a nonlinear piezoelectric-electromagnetic hybrid energy harvester, the state equation of electromechanical coupling, the harmonic response and average output power, voltage, and current of a nonlinear hybrid energy harvester under harmonic excitation are derived by the harmonic balance method. The effects of the excitation acceleration and the external load on the output performance of the nonlinear hybrid energy harvester are verified through experimental tests. The results showed that the output power of the nonlinear hybrid energy harvester increases with the increase in the acceleration of harmonic excitation, and the increase is affected by external load. When the piezoelectric-electromagnetic hybrid harvester operates at the optimal load and the resonant frequency, the average output power reaches its maximum value and the increase of the load of the piezoelectric unit makes the resonant frequency of the energy harvesting system increase. Compared with linear harvesting structures, the nonlinear hybrid harvester has better flexibility of environmental adaptability and is more suitable for harvesting energy in low-frequency environments.

In this paper, we design and experimentally validate a new auxetic nonlinear piezoelectric energy harvester for broad working bandwidth and high power output, which combines a clamped-clamped beam with multiple rotating square unit cells. On one hand, the key structural parameter of the square rotating unit cell is adjusted to obtain desired broad working bandwidth. On the other hand, the number of the unit cells is increased to improve the power output of the energy harvester with minor influence on the working bandwidth. Therefore, based on the parameter adjustment and unit cell number increment, the proposed energy harvester can obtain both broad working bandwidth and high power output, which can solve the trade-off between these two aspects in previous auxetic nonlinear energy harvesters. Finite element analysis is performed to analyse the characteristics of the energy harvester. The lumped parameter model is utilized to predict the performance of the energy harvester, which matches well with the experimental results. In the experimental validation, under 0.3g base acceleration, the working bandwidth and power output of the auxetic nonlinear energy harvester are increased by 14% and 268%, respectively, compared with the conventional nonlinear energy harvester.

Harmonic balance analysis of nonlinear tristable energy harvesters for performance enhancement

Equivalent damping and frequency change for linear and nonlinear hybrid vibrational energy harvesting systems

Energy harvesting from vibrations is a popular research topic. However, it is difficult to change the values of its dynamic stiffness characteristics during the operation of device. This paper analyzes the nonlinear dynamical behavior and energy harvesting performance of an energy harvesting device with a gear unit and an inertial amplifier. The Melnikov method is used to discriminate the chaos behavior of the system and is illustrated by numerical simulations. Chaotic dynamics analysis provides a simplified analytical idea that offers more insight into the performance of this energy harvesting device. In addition, the basins of attraction of the coexistence solution of the system are plotted, and the trajectory and energy harvesting efficiency of the system are analyzed for changes in the initial value. The results show that the device has the convenience to change the form and efficiency of system energy harvesting. The theory of Melnikov functions recognizes the presence of chaos and can provide solutions for the parameterization of the system. The choice of initial value greatly affects the energy harvesting efficiency.

Harvesting electrical energy from thermal energy sources using pyroelectric conversion techniques has been under investigation for over 50 years, but it has not received the attention that thermoelectric energy harvesting techniques have during this time period. This lack of interest stems from early studies which found that the energy conversion efficiencies achievable using pyroelectric materials were several times less than those potentially achievable with thermoelectrics. More recent modeling and experimental studies have shown that pyroelectric techniques can be cost competitive with thermoelectrics and, using new temperature cycling techniques, has the potential to be several times as efficient as thermoelectrics under comparable operating conditions. This paper will review the recent history in this field and describe the techniques that are being developed to increase the opportunities for pyroelectric energy harvesting. The development of a new thermal energy harvester concept, based on temperature cycled pyroelectric thermal-to-electrical energy conversion, are also outlined. The approach uses a resonantly driven, pyroelectric capacitive bimorph cantilever structure that can be used to rapidly cycle the temperature in the energy harvester. The device has been modeled using a finite element multi-physics based method, where the effect of the structure material properties and system parameters on the frequency and magnitude of temperature cycling, and the efficiency of energy recycling using the proposed structure, have been modeled. Results show that thermal contact conductance and heat source temperature differences play key roles in dominating the cantilever resonant frequency and efficiency of the energy conversion technique. This paper outlines the modeling, fabrication and testing of cantilever and pyroelectric structures and single element devices that demonstrate the potential of this technology for the development of high efficiency thermal-toelectrical energy conversion devices.

Modeling of a honeycomb-shaped pyroelectric energy harvester for human body heat harvesting

Multi-solution phenomena and nonlinear characteristics of tristable galloping energy harvesters with magnetic coupling nonlinearity