Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> This paper deals with small-power energy harvesting from heat. It can be achieved using both thermoelectric and pyroelectric effects. In the first case, temperature gradients are necessary. The main difficulty of thermoelectric energy harvesting is imposing a large temperature gradient. This requires huge heat flows because of the limited surface heat exchanges and the large heat conductivity of thermoelectric materials. This results in a drastic decrease of power and the efficiency of conversion. In case of pyroelectric energy harvesting, a time varying temperature is necessary. Although such a temperature time profile is hard to find, the overall optimization is easier than the thermoelectric strategy. Indeed, it depends much less on heat exchange between the sample and the outer medium, than on heat capacity that dimensions optimization may easily compensate. As a consequence, it is shown that the efficiency and output power may be much larger using pyroelectric energy harvesting than thermoelectric methods. For instance, using a limited temperature gradient due to the limited heat exchange, a maximum efficiency of 1.7% of Carnot efficiency can be expected using a thermoelectric module. On the contrary, a pyroelectric device may reach an efficiency up to 50% of Carnot efficiency. Finally, an illustration shows an estimation of the output power that could be expected from natural time variations of temperature of a wearable device. Power peaks up to 0. 2m W cm −3 were found and a mean power of 1 μ Wc m −3 on average was determined within 24 h testing. <s> BIB001 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> Abstract Pyroelectric cells based on fabricated screen-printed PZT and commercial PVDF films are proposed as thermal energy harvesting sources in order to supply low-power autonomous sensors. The cells are electrically modelled as a current source in parallel with output impedance. Heating and cooling temperature fluctuations generated by air currents were applied to the pyroelectric converters. The generated currents and charges were respectively in the order of 10 −7 A and 10 −5 C for temperature fluctuations from 300 K to 360 K in a time period of the order of 100 s, which agrees with the theoretical model. Parallel association of cells increased the generated current. The dependence of the generated current on relevant technological parameters has been also characterized. Finally, current from cyclic temperature fluctuations was rectified and stored in a 1 μF load capacitor. Energies up to 0.5 mJ have been achieved, enough to power typical autonomous sensor nodes during a measurement and transmission cycle. <s> BIB002 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> Abstract 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 isoelectric 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. <s> BIB003 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> ABSTRACT 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 compe titive 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 recyc ling 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 efficien cy 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-to-electrical energy conversion devices. Key Words: Energy harvesting, pyroelectric, bimorph cantilever, MEMS, surface micromachining <s> BIB004 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> Harvesting thermoelectric energy mainly relies on the Seebeck effect that utilizes a temperature difference between two ends of the device for driving the diffusion of charge carriers. However, in an environment that the temperature is spatially uniform without a gradient, the pyroelectric effect has to be the choice, which is based on the spontaneous polarization in certain anisotropic solids due to a time-dependent temperature variation. Using this effect, we experimentally demonstrate the first application of pyroelectric ZnO nanowire arrays for converting heat energy into electricity. The coupling of the pyroelectric and semiconducting properties in ZnO creates a polarization electric field and charge separation along the ZnO nanowire as a result of the time-dependent change in temperature. The fabricated nanogenerator has a good stability, and the characteristic coefficient of heat flow conversion into electricity is estimated to be ∼0.05–0.08 Vm2/W. Our study has the potential of using pyroelectric ... <s> BIB005 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> We demonstrate a pyroelectric nanogenerator (PENG) based on a lead zirconate titanate (PZT) film, which has a pyroelectric coefficient of about −80 nC/cm 2 K. For a temperature change of 45 K at a rate of 0.2 K/s, the output open-circuit voltage and short-circuit current density of the PENG reached 22 V and 171 nA/cm 2 , respectively, corresponding to a maximum power density of 0.215 mW/ cm 3 . A detailed theory was developed for understanding the high output voltage of PENG. A single electrical output pulse can directly drive a liquid crystal display (LCD) for longer than 60 s. A Li-ion battery was charged by the PENG at different working frequencies, which was used to drive a green light- emitting diode (LED). The demonstrated PENG shows potential applications in wireless sensors. <s> BIB006 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> We demonstrated the first application of a pyroelectric nanogenerator as a self-powered sensor (or active sensor) for detecting a change in temperature. The device consists of a single lead zirconate titanate (PZT) micro/nanowire that is placed on a thin glass substrate and bonded at its two ends, and it is packaged by polydimethylsiloxane (PDMS). By using the device to touch a heat source, the output voltage linearly increases with an increasing rate of change in temperature. The response time and reset time of the fabricated sensor are about 0.9 and 3 s, respectively. The minimum detecting limit of the change in temperature is about 0.4 K at room temperature. The sensor can be used to detect the temperature of a finger tip. The electricity generated under a large change in temperature can light up a liquid crystal display (LCD). <s> BIB007 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> The pyroelectric effect offers an intriguing solid-state approach for harvesting ambient thermal energy to power distributed networks of sensors and actuators that are remotely located or otherwise difficult to access. There have been, however, few device-level demonstrations due to challenges in converting spatial temperature gradients into temporal temperature oscillations necessary for pyroelectric energy harvesting. We demonstrate the feasibility of a device concept that uses liquid-based thermal interfaces for rapid switching of the thermal conductance between a pyroelectric material and a heat source/sink and can thereby deliver high output power density. Using a thin film of a pyroelectric co-polymer together with a macroscale mechanical actuator, we operate pyroelectric thermal energy harvesting cycles at frequencies close to 1 Hz. Film-level power densities as high as 110 mW/cm(3) were achieved, limited by slow heat diffusion across a glass substrate. When combined with a laterally interdigitated electrode array and a MEMS actuator, the present design offers an attractive option for compact high-power density thermal energy harvesters. (C) 2012 Elsevier B.V. All rights reserved. <s> BIB008 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> With the ever-growing demand for renewable energy sources, energy harvesting from natural resources has gained much attention. Energy sources such as heat and mechanical motion could be easily harvested based on pyroelectric, thermoelectric, and piezoelectric effects. The energy harvested from otherwise wasted energy in the environment can be utilized in self-powered micro and nano devices, and wearable electronics, which required only µW–mW power. This article reviews pyroelectric energy harvesting with an emphasis on recent developments in pyroelectric energy harvesting and devices at micro/nanoscale. Recent developments are presented and future challenges and opportunities for more efficient materials and devices with higher energy conversion efficiency are also discussed. <s> BIB009 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> Abstract We deal with the thermal energy which is one of the ambient energy sources surely exploitable, but it has not been much interest as the mechanical energy. In this paper, our aim is to use thermal energy and show that it’s an important source for producing the electrical energy through pyroelectric effect which is the property of some dielectric materials to show a spontaneous electrical polarization versus temperature. In this context, we present a concept to harvest a thermal energy using infrared rays and pyroelectric effect. The pyroelectric material used in this work can generate an electrical voltage when it subjected to a temperature change which will be ensured by the use of infrared lamp. Our experimental results show that the electrical voltage, current and harvested power increased significantly when increasing the area of the pyroelectric element. The experimental results show also that with this simple concept we harvested a heavy amount value of power which will certainly be useful in an extensive range of applications, including sensors and infrared detection. These results shed light on the thermoelectric energy conversion by Ceramic lead zirconate titanate (PZT) buzzer having the pyroelectric property. <s> BIB010 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> There is widespread interest in finding sustainable replacements for fossil fuels, and hence hydrogen production via water splitting has received much attention. However, finding efficient water splitting methods remains a challenge. We use first principles theory to design a catalytic cycle for water splitting that employs the pyroelectric effect in ferroelectrics. Taking PbTiO3 as an example, we show that actively controlling the ferroelectric polarization via cyclic temperature modulations can catalyze the splitting of H2O into O2 and H2. In practice, the energy needed to drive this cycle may be provided by low/intermediate grade heat. Since no precious metals are used in this catalytic cycle, this approach may lead to viable water-splitting methods that employ earth-abundant elements. <s> BIB011 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> Wasted heat is one of the most abundant and widely available energy sources in our living environment and industrial activities. A lot of attentions have been paid on harvesting ambient wasted thermal energy. In this work, a flexible PG (pyroelectric generator) based upon a thin PVDF (polyvinylidene fluoride) film has been fabricated. Based on the pyroelectric effect, the PG can harvest thermal energy arising from the time-dependent fluctuating temperature with spatial uniformity. At the temperature change of 50 K, the PG can deliver an open-circuit voltage of 8.2 V and a short-circuit current of 0.8 μA, respectively, with the maximal output power of 2.2 μW on a load of 0.1 MΩ, which can be utilized to directly drive a LCD (liquid crystal display) or LEDs (light emitting diodes), or to charge a commercial capacitor for subsequent usages. Moreover, the PG can be used to construct a self-powered thermosensor as a result of the linear relationship between the output voltage and the temperature change. Our study promotes the development of the PG for scavenging wasted thermal energy and opens up plenty of potential self-powered applications. <s> BIB012 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> Low temperature thermal to electrical energy converters have the potential to provide a route for recovering waste energy. In this paper, we propose a new configuration of a thermal harvester that uses a naturally driven thermal oscillator free of mechanical motion and operates between a hot heat source and a cold heat sink. The system exploits a heat induced liquid-vapour transition of a working fluid as a primary driver for a pyroelectric generator. The two-phase instability of a fluid in a closed looped capillary channel of an oscillating heat pipe (OHP) creates pressure differences which lead to local high frequency temperature oscillations in the range of 0.1–5 K. Such temperature changes are suitable for pyroelectric thermal to electrical energy conversion, where the pyroelectric generator is attached to the adiabatic wall of the OHP, thereby absorbing thermal energy from the passing fluid. This new pyroelectric-oscillating heat pipe (POHP) assembly of a low temperature generator continuously operat... <s> BIB013 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> Hydrogen fuel cells are a promising energy conversion technology due to its high energy density and zero greenhouse gas emission. As a result, the production of hydrogen from renewable and alternative resources has gained significant interest in recent decades. This paper demonstrates a new approach which uses a pyroelectric energy harvester for water splitting and represents a novel alternative hydrogen source. Pyroelectrics are attractive for harvesting waste heat due to their ability to convert temperature fluctuations into electrical energy. A range of pyroelectric materials and geometries for water electrolysis are analysed to determine, (i) the minimum material thickness to generate a critical potential to initialise water decomposition and, (ii) to maximize the charge and hydrogen mass production. We then successfully demonstrate that an appropriate pyroelectric material, when combined with rectification of the alternating current, can harvest heat fluctuations and generate a sufficient electric potential difference and current for water splitting. By harvesting the pyroelectric electrical energy, a continuous hydrogen bubble production was observed during thermal cycling. Practical routes to maximize the level of hydrogen production for this new concept are also explored. <s> BIB014 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> Abstract As one of the most important renewable and green sources, the development and utilization of waste heat have been received more and more attentions. A big part of waste heat comes from chemical process, which is ubiquitous in industry and laboratory. However, this form of waste heat is difficult to be utilized due to its low grade and easy dissipation. In this paper, we present a flexible pyroelectric device as a potential approach for effectively harvesting waste heat from chemical exothermic process. To achieve practical application, the pyroelectric device simply attaches to the outside of a beaker, in which various chemical exothermic processes happen. The output voltage (under input impedance of 100 M Ohm) and short-circuit current can be 9.1 V and 95 nA when the neutral reaction of sodium hydroxide and hydrochloric acid per amount-of-substance concentration proceeds in the beaker. The generated electricity can directly drive a liquid crystal display. Moreover, this pyroelectric device is also proved to be a self-powered temperature monitor reflecting chemical process in real time, as the calculated temperature variation of solution based on pyroelectric current well agrees with the measured one by thermometry reference. This work expands the development of pyroelectric device for harvesting chemical waste heat and opens up the potential applications on self-powered chemical process monitor. <s> BIB015 </s> Energy Harvesting Technologies for Achieving Self-Powered Wireless Sensor Networks in Machine Condition Monitoring: A Review <s> Pyroelectric Energy Harvesting <s> Abstract Conversion of temperature fluctuations to useable electrical energy is rendered by the pyroelectric effect. Waste of heat in our day to day environment and in industrial sector constitutes an abundant source of energy. Herein, we report a pyroelectric generator (PyG) that produces pyroelectric output up to 1.5 V and 1.5 µA. Its power density is 0.034 µW/cm2 upon exposure to heat-cool condition for a temperature variation from 310 K to 340 K. Due to the fast response time (121 ms) of the PyG, it is expected to be use as a self-powered temperature sensor. The generated electricity could also be stored in a capacitor up to 0.8 V in three heating–cooling cycles. It has been also demonstrated that PyG is possible to drive by water vapour where energy-consuming alternating devices is not necessary. The temperature oscillation achieved by spontaneous water condensation and evaporation from the surface of the PyG that produces open-circuit voltage of 1.6 V for a temperature variation from 303 K to 333 K. Thus the PyG driven by water vapour supports an efficient retrieval of energy from hot water vapour, which is wasted mostly. The linear increment of voltage as a function of temperature indicates PyG is also suitable to use a temperature sensor that may also work in self-powered mode. In addition, the PyG can also harvest the waste body heat, i.e., heat dissipation from human body surface and from the process of respiration. That promises an effective self-powered temperature sensor that might be useful in healthcare monitoring, safety and security sectors. <s> BIB016
Compared with thermoelectric energy harvesting, an indispensable prerequisite of pyroelectric energy harvesting is temperature fluctuation over time BIB001 . The phenomenon utilised is that the polarization intensity of some dielectric materials releases charges with temperature fluctuations; this is called the pyroelectric effect BIB002 . Normally, free electrons and holes generated by the spontaneous polarization of a crystal are respectively neutralized by the holes and free electrons on the surface of the crystal. In certain materials, for example gallium nitride, the polarisation changes as the temperature changes and a voltage appears across the crystal. The conversion efficiency of pyroelectric energy harvesting is much higher than that of thermoelectric energy harvesting, especially using new temperature cycling techniques (like the Olsen cycle BIB003 ). Hence, finding a source of time-varying temperature becomes one of the challenges of pyroelectric energy harvesting. From the references examined, it is clear that researchers are striving to capture temporal temperature gradients in different fields. It is shown in El Fatnani et al. BIB010 that energy in the form of temperature fluctuations generated by infrared radiation coming from an infrared lamp can be harvested by the pyroelectric effect. This approach also contributes to the enhancement of the autonomous infrared sensors by means of pyroelectric materials. In addition, Ayesha et al. BIB016 has investigated a commercial patch transducer that can be a self-sustaining pyroelectric generator by taking advantages of temperature gradients caused by the process of breathing. This may be the potential applications on the non-invasive human healthcare monitoring. Similarly, pyroelectric energy harvesting is also appropriate for water splitting through harvesting temperature fluctuations and generating sufficient power BIB011 BIB014 . In the field of chemistry, Zhao et al. BIB015 harvests energy from waste heat with a flexible pyroelectric device and successfully operates a temperature sensor to monitor this process in real time. In order to produce a greater temperature difference, some devices with different architectures are designed to directly capture the time-dependent temperature gradient from spatial temperature fluctuations through the movement of the pyroelectric materials. For instance, Figure 10a describes the schematic of a typical pyroelectric energy generator with a bi-metallic cantilever beam construction consisting of two metal electrodes of a capacitor and the pyroelectric dielectric material BIB004 . One end of the cantilever structure is connected to the hot surface via an anchor, and the other end is connected to two small masses. When the cantilever is heated and bends towards the cold surface, the structure rapidly loses heat and bends back towards the upper hot side. Then the structure rapidly heats and bends away from the hot side. This process subtly converts the spatial temperature gradients into temporal temperature fluctuations of the cantilever structure. To harvest more energy, millions of the millimetre sized converters can be fabricated in 2D arrays to create a much larger device. Figure 10b gives another pyroelectric harvester schematic designed by Cha et al. BIB008 with liquid-based switchable thermal interfaces for rapid cycling of the pyroelectric polymer film between hot and cold surfaces. Its power density is about 110 mW/cm 3 at an operating frequency of 1Hz. Zhang et al. BIB012 adopts a similar structure to fabricate a thin-film pyroelectric generator with the maximal output power of 2.2 µW when the external load was 0.1 mΩ. After reviewing some papers relating to such structures, it is concluded that the challenge of pyroelectric energy harvesting is mainly focused on high-frequency temperature fluctuations and high efficiency of energy extraction cycles. polymer film between hot and cold surfaces. Its power density is about 110 mW/cm at an operating frequency of 1Hz. Zhang et al. BIB012 adopts a similar structure to fabricate a thin-film pyroelectric generator with the maximal output power of 2.2 μW when the external load was 0.1 mΩ. After reviewing some papers relating to such structures, it is concluded that the challenge of pyroelectric energy harvesting is mainly focused on high-frequency temperature fluctuations and high efficiency of energy extraction cycles. With the development of nanotechnology, thin films, nanowires and nanofibers have been investigated for pyroelectric EH, which gives the opportunity for pyroelectric techniques to be incorporated with other EH approaches BIB009 . Yang et al. BIB005 has fabricated the first pyroelectric nanogenerator with the pyroelectric ZnO nanowire arrays to convert the thermoelectric energy into electricity. The designed pyroelectric nanogenerator can drive a liquid crystal display directly and a light-emitting diode with the assistance of a lithium battery, which demonstrated that pyroelectric nanogenerators are the potential applications in self-powered WSNs BIB006 BIB007 . Moreover, high-temperature resistant nanomaterials are appropriate for extremely harsh environments. Start-up transients or load changes of rotating machinery can supply the temporal temperature fluctuation for pyroelectric generators, but these changing operating states only last a comparatively short time, and storage devices cannot rely on short-term energy collection for a long period of condition monitoring. Hence, thermoelectric and pyroelectric EH techniques mainly focus on the spatial temperature gradient (available in oscillating heat pipes BIB013 , heat exchangers, marine boilers, engine, motors, pumps and compressors). For indoor machines, they play an important role in achieving self-powered WSNs for condition monitoring because of wasted energy existing in the form of heat and supplying sufficient energy for WSNs. Both thermoelectric generators and pyroelectric generators are maintenance-free and have the advantages of long lifetime, lightweight, durability and compactness. However, their cost is relatively high when compared to their power conversion efficiency. The development of nanomaterials may fill this flaw in the future.