Tzern T. Toh

Performance limits of the three MEMS inertial energy generator transduction types

In this paper, trends from the last 10 years of inertial micro-generator literature are investigated and it is shown that, although current generator designs are still operating well below their maximum power, there has been a significant improvement with time. Whilst no clear conclusions could be drawn from reported fabricated devices with respect to preferred transducer technology, this paper presents operating charts for inertial micro-generators which identify optimal operating modes for different frequencies and normalized generator sizes, and allows comparison of the different transduction mechanisms as these parameters vary. It is shown that piezoelectric generators have a wider operating range at low frequency than electromagnetic generators, but as generator dimensions increase, the frequency to which piezoelectric transducers outperform electromagnetic transducers decreases.

Design and Fabrication of Heat Storage Thermoelectric Harvesting Devices

Thermoelectric energy harvesting requires a substantial temperature difference ΔT to be available within the device structure. This has restricted its use to particular applications such as heat engine structural monitoring, where a hot metal surface is available. An alternative approach is possible in cases where ambient temperature undergoes regular variation. This involves using a heat storage unit, which is filled with a phase-change material (PCM), to create an internal spatial temperature difference from temperature variation in time. In this paper, key design parameters and a characterization methodology for such devices are defined. The maximum electrical energy density expected for a given temperature range is calculated. The fabrication, characterization, and analysis of a heat storage harvesting prototype device are presented for temperature variations of a few tens of degrees around 0 °C, corresponding to aircraft flight conditions. Output energy of 105 J into a 10- Ω matched resistive load, from a temperature sweep from +20 °C to -21 °C, then to +25 °C is demonstrated, using 23 g of water as the PCM. The proposed device offers a unique powering solution for wireless sensor applications involving locations with temperature variation, such as structural monitoring in aircraft, industrial, and vehicle facilities.

A continuously rotating energy harvester with maximum power point tracking

In this paper, we analyse and demonstrate an energy-harvesting generator powered by continuous rotation, using gravitational torque to enable a single point of attachment. The electro-mechanical behaviour of the generator is presented, alongside experimental results from an implementation based on a conventional dc motor. The off-axis performance is also modelled. The design and experimental results for an adaptive power processing circuit are also presented, as the first demonstration of maximum power point tracking for a motion energy harvester.

Tuning the Resonant Frequency and Damping of an Electromagnetic Energy Harvester Using Power Electronics

In order to maximize power density, the resonant frequency of an energy harvester should be equal to the source excitation frequency and the electrical damping set equal to the parasitic damping. These parameters should be adjustable during device operation because the excitation characteristics can change. This brief presents, for the first time, a power electronic interface that is capable of continual adjustment of the damping and the resonant frequency of an energy harvester by controlling real and reactive power exchange between the electrical and mechanical domains while storing the harvested energy in a battery. The advantages of this technique over previously proposed methods are the precise control over the tuning parameters of the electrical system and integrated rectification within the tuning interface. Experimental results verify the operation, and the prototype system presented can change the resonant frequency of the electromechanical system by ±10% and increase the damping by 45%. As the input excitation frequency was swept away from the unmodified resonant frequency of the harvester, the use of the tuning mechanism was shown to increase real power generation by up to 25%. The prototype harvester is capable of generating 100 mW at an excitation frequency of 1.25 Hz.

Performance of phase change materials for heat storage thermoelectric harvesting

Heat storage energy harvesting devices have promise as independent power sources for wireless aircraft sensors. These generate energy from the temperature variation in time during flight. Previously reported devices use the phase change of water for heat storage, hence restricting applicability to instances with ground temperature above 0 °C. Here, we examine the use of alternative phase change materials (PCMs). A recently introduced numerical model is extended to include phase change inhomogeneity, and a PCM characterization method is proposed. A prototype device is presented, and two cases with phase changes at approximately −9.5 °C and +9.5 °C are studied.

Aircraft Strain WSN Powered by Heat Storage Harvesting

The combination of ultra-low-power wireless communications and energy harvesting enables the realization of autonomous wireless sensor networks. Such networks can be usefully applied in commercial aircraft where wireless sensing solutions contribute to weight reduction and increased ease of installation and maintenance. This paper presents, for the first time, a complete energy-autonomous wireless strain monitoring system for aircraft. The system is based on a multimode wireless time-division multiple access (TDMA) medium access control (MAC) protocol that supports automatic configuration and a time-stamping accuracy better than 1 ms. The energy supply depends solely on an innovative thermoelectric energy harvester, which takes advantage of the changes in environmental temperature during takeoff and landing. The system was successfully integrated and passed the functional and flight-clearance tests that qualify it for use in a flight-test installation.

Sensorless Estimation and Nonlinear Control of a Rotational Energy Harvester

It is important to perform sensorless monitoring of parameters in energy harvesting devices in order to determine the operating states of the system. However, physical measurements of these parameters is often a challenging task due to the unavailability of access points. This paper presents, as an example application, the design of a nonlinear observer and a nonlinear feedback controller for a rotational energy harvester. A dynamic model of a rotational energy harvester with its power electronic interface is derived and validated. This model is then used to design a nonlinear observer and a nonlinear feedback controller which yield a sensorless closed-loop system. The observer estimates the mechancial quantities from the measured electrical quantities while the control law sustains power generation across a range of source rotation speeds. The proposed scheme is assessed through simulations and experiments.

Scaling of dynamic thermoelectric harvesting devices in the 1-100 cm3 range

Aircraft sensors are typically cable powered, imposing a significant weight overhead. The exploitation of temperature variations during flight by a phase change material (PCM) based heat storage thermoelectric energy harvester, as an alternative power source in aeronautical applications, has recently been flight tested. In this work, a scaled-down and a scaled-up prototype are presented. Output energy of 4.1 J per gram of PCM from a typical flight cycle is demonstrated for the scaled-down device, and 3.2 J per gram of PCM for the scaled-up device. The observed performance improvement with scaling down is attributed to the reduction in temperature inhomogeneity inside the PCM. As an application demonstrator for dynamic thermoelectric harvesting devices, the output of a thermoelectric module is used to directly power a microcontroller for the generation of a pulse width modulation signal.

Nonlinear observer and controller design for sensorless operation of a continuously rotating energy harvester

Abstract This paper presents the design of a nonlinear observer and a nonlinear feedback controller for sensorless operation of a continuously rotating energy harvester. A mathematical model of the harvester with its power electronic interface is discussed. This model is used to design an observer that estimates the mechanical quantities from the measured electrical quantities. The gains of the observer depend on the solution of a modified Riccati equation. The estimated mechanical quantities are used in a control law that sustains power generation across a range of source rotation speeds. The proposed scheme is assessed through simulations and experiments.

Inductive energy harvesting from variable frequency and amplitude aircraft power lines

This paper presents a non-contact method of harvesting energy from an aircraft power line that has an AC current of variable amplitude and a frequency range of 360-800 Hz. The current and frequency characteristics of the aircraft power line are dependent on the rotation speed of the electrical generators and will therefore change during a flight. The harvester consists of an inductive coil with a ferrite core, which is interfaced to a rectifier, step-down regulator and supercapacitor. A prototype system was constructed to demonstrate reliable output voltage regulation across a supercapacitor that will supply a peak power of 100 mW under duty cycled load conditions. The system could fully charge a 40 mF supercapacitor to 3.3 V in 78 s from a power line current of 1.5 Arms at 650 Hz.