Maximum Effectiveness of Electrostatic Energy Harvesters When Coupled to Interface Circuits

Accepted versio

Micromechanics for energy generation

The emergence and evolution of energy micro-generators during the last two decades has delivered a wealth of energy harvesting powering solutions, with the capability of exploiting a wide range of motion types, from impulse and low frequency irregular human motion, to broadband vibrations and ultrasonic waves. It has also created a wide background of engineering energy microsytems, including fabrication methods, system concepts and optimal functionality. This overview presents a simple description of the main transduction mechanisms employed, namely the piezoelectric, electrostatic, electromagnetic and triboelectric harvesting concepts. A separate discussion of the mechanical structures used as motion translators is presented, including the employment of a proof mass, cantilever beams, the role of resonance, unimorph structures and linear/rotational motion translators. At the mechanical-to-electrical interface, the concepts of impedance matching, pre-biasing and synchronised switching are summarised. The separate treatment of these three components of energy microgenerators allows the selection and combination of different operating concepts, their co-design towards overall system level optimisation, but also towards the generalisation of specific approaches, and the emergence of new functional concepts. Industrial adoption of energy micro-generators as autonomous power sources requires functionality beyond the narrow environmental conditions typically required by the current state-of-art. In this direction, the evolution of broadband electromechanical oscillators and the combination of environmental harvesting with power transfer operating schemes could unlock a widespread use of micro-generation in microsystems such as micro-sensors and micro-actuators

Thermal and Mechanical Energy Harvesting Using Lead Sulfide Colloidal Quantum Dots

The human body is an abundant source of energy in the form of heat and mechanical movement. The ability to harvest this energy can be useful for supplying low-consumption wearable and implantable devices. Thermoelectric materials are usually used to harvest human body heat for wearable devices; however, thermoelectric generators require temperature gradient across the device to perform appropriately. Since they need to attach to the heat source to absorb the heat, temperature equalization decreases their efficiencies. Moreover, the electrostatic energy harvester, working based on the variable capacitor structure, is the most compatible candidate for harvesting low-frequency-movement of the human body. Although it can provide a high output voltage and high-power density at a small scale, they require an initial start-up voltage source to charge the capacitor for initiating the conversion process. The current methods for initially charging the variable capacitor suffer from the complexity of the design and fabrication process. In this research, a solution-processed photovoltaic structure was proposed to address the temperature equalization problem of the thermoelectric generators by harvesting infrared radiations emitted from the human body. However, normal photovoltaic devices have the bandgap limitation to absorb low energy photons radiated from the human body. In this structure, mid-gap states were intentionally introduced to the absorbing layer to activate the multi-step photon absorption process enabling electron promotion from the valence band to the conduction band. The fabricated device showed promising performance in harvesting low energy thermal radiations emitted from the human body. Finally, in order to increase the generated power, a hybrid structure was proposed to harvest both mechanical and heat energy sources available in the human body. The device is designed to harvest both the thermal radiation of the human body based on the proposed solution-processed photovoltaic structure and the mechanical movement of the human body based on an electrostatic generator. The photovoltaic structure was used to charge the capacitor at the initial step of each conversion cycle. The simple fabrication process of the photovoltaic device can potentially address the problem associated with the charging method of the electrostatic generators. The simulation results showed that the combination of two methods can significantly increase the harvested energy

Energy harvesting from human and machine motion for wireless electronic devices

Published versio

The Design and Effect of Power Electronics on Vibration-Based Energy Harvesting Methods.

Recent advancements in communication and low-power sensor nodes have led to innovative data acquisition systems for applications such as heart monitoring and forest-fire detection. Often these systems are in locations characterized by limited access to electrical power, yet they are in the presence of ambient mechanical vibrations. Therefore, energy harvesting from mechanical vibrations is proposed as a solution for powering these wireless sensor nodes. There are two devices that are commonly used for vibration-based energy harvesting: piezoelectric devices and electrostatic devices. This dissertation focuses on the power electronic interface between vibration energy harvesting devices and electrical energy storage elements. By including power electronic efficiency as a parameter in the analysis of variable-capacitance energy harvesting, new fundamental properties of these devices are derived: a threshold efficiency necessary for energy harvesting, analytical solutions for optimal harvesting conditions, a comparison of energy harvesting methods at practical power electronic efficiencies, and a comparison of energy harvesting capabilities of various device architectures. Case studies are presented to illustrate practical applications of the theory presented in this work. One case study demonstrates the advantage of using the Charge Pump Method for MEMs applications, and illustrates the use of these new fundamental properties to aid power electronic architecture selection. Ultimately, the analysis-aided design produces more than twice as much power as previous implementations on the same device. Recently, the dynamic active energy harvesting method has been proposed as a way to widen the bandwidth of resonant piezoelectric energy harvesters; however, the bandwidth extension is dependent on power electronic efficiency. In this dissertation a new energy harvesting system is proposed that includes a resonant inverter topology, in conjunction with new low-power analog control circuitry, in order to produce the first wideband autonomous dynamic active energy harvesting system. Experimental results using the Mide Volture V20w piezoelectric device shows that the harvested power is up to twice that of the adaptive rectifier method. These results include previously ignored loss mechanisms such as control losses, gating losses, and phase detection losses; making this system the first autonomous energy harvesting system of its kind.PhDElectrical Engineering: SystemsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/120824/1/steinal_1.pd

Micro-mechanical sensor for the spectral decomposition of acoustic signals

An array of electret-biased frequency-selective resonant microelectromechanical system (MEMS) acoustic sensors was proposed to perform analysis of stress pulses created during an impact between two materials. This analysis allowed classification of the stiffness of the materials involved in the impact without applying post-impact signal processing. Arrays of resonant MEMS sensors provided filtering of the incident stress pulse and subsequent binning of time-domain waveforms into frequency-based spectra. Results indicated that different impact conditions and materials yielded different spectral characteristics. These characteristics, as well as the resulting sensor array responses, are discussed and applied to impact classification. Each individual sensor element in the array was biased by an in situ charged electret film. A microplasma discharge apparatus embedded within the microsensor allowed charging of the electret film after all device fabrication was complete. This enabled electret film integration using high-temperature surface micromachining processes that would typically lead to discharge of traditionally formed electret materials. This also eliminated the traditional wafer-bonding and post-fabrication assembly processes required in conventional electret integration approaches. The microplasma discharge process and resulting electret performance are discussed within the context of the MEMS acoustic sensor array.Ph.D.Committee Chair: Allen, Mark; Committee Member: Brand, Oliver; Committee Member: Michaels, Jennifer; Committee Member: Michaels, Thomas; Committee Member: Ready, Jud W

Power Management Electronics

Accepted versio

Energy Harvesting for Tire Pressure Monitoring Systems

Tire pressure monitoring systems (TPMSs) predict over- and underinflated tires, and warn the driver in critical situations. Today, battery powered TPMSs suffer from limited energy. New sensor features such as friction determination or aquaplaning detection require even more energy and would significantly decrease the TPMS lifetime. Harvesting electrical energy inside the tire of a vehicle has been considered as a promising alternative to overcome the limited lifetime of a battery. However, it is a real challenge to design a system, that generates electrical energy at low velocities while being robust at 200 km/h where radial accelerations up to 20000 m/s2 occur. This work focusses on developing different electromechanical energy transducers that meet the high requirements of the automotive sector. Different approaches are addressed on how the change of acceleration and strain within the tire can be used to provide mechanical energy to the energy harvester. The energy harvester converts the mechanical energy into electrical energy. In this thesis, piezoelectric and electromagnetic transducers are discussed in depth, modelled as electromechanical networks. Since the transducers provide energy in the form of an AC voltage, but sensors require a DC voltage, various common interface circuits are compared, using LTspice and applying method of the stochastic signal analysis. Furthermore, a buck-boost converter concept for the electromagnetic energy harvester is optimized and improved. Experiments on a tire test rig validate the theoretically determined output and confirm that well designed energy harvesters in the tire can generate much more energy than required by an TPMS not only at high velocities but also at velocities as low as 20 km/h