Ahmadreza Tabesh

A Low-Power Stand-Alone Adaptive Circuit for Harvesting Energy From a Piezoelectric Micropower Generator

An adaptive energy-harvesting circuit with low power dissipation is presented and demonstrated, which is useful for efficient ac/dc voltage conversion of a piezoelectric micropower generator. The circuit operates stand-alone, and it extracts the piezoelectric strain energy independent of the load and piezoelectric parameters without using any external sensor. The circuit consists of a voltage-doubler rectifier, a step-down switching converter, and an analog controller operating with a single supply voltage in the range of 2.5-15 V. The controller uses the piezoelectric voltage as a feedback and regulates the rectified voltage to adaptively improve the extracted power. The nonscalable power dissipation of the controller unit is less than 0.05 mW, and the efficiency of the circuit is about 60% for output power levels above 0.5 mW. Experimental verifications of the circuit show the following: 1) the circuit notably increases the extracted power from a piezoelectric element compared to a simple full-bridge diode rectifier without control circuitry, and 2) the efficiency of the circuit is dominantly determined by its switching converter. The simplicity of the circuit facilitates the development of efficient piezoelectric energy harvesters for low-power applications such as wireless sensors and portable devices.

Application of Fractional Calculus Theory to Robust Controller Design for Wind Turbine Generators

This paper presents a robust controller design method for wind turbine generators using the concepts of fractional calculus. It also compares features of fractional order control systems with those of classic integer order controllers. The proposed method uses isodamping feature, which desensitizes the phase-frequency variations about a gain crossover frequency. This increases the robustness of a fractional order control system against uncertainties. In conventional integer order control systems, realization of isodamping feature requires a controller with very high-order transfer function whereas a fractional order system can readily realize this feature in a compact form. The proposed method is applied to a study system consisting of a permanent magnet wind turbine generator. The test system investigates the tracking performance of the control system considering the backlash and aging phenomenon within the dynamic model of the wind turbine generator. The study results based on a time-domain simulation show the superior capabilities of fractional order controller compared with classic controllers in the presence of model uncertainties. It has been shown that the concept of fractional calculus can be used as a promising robust control approach for the future high performance feedback control systems with application to wind energy systems.

Dynamic Model and Control of DFIG Wind Energy Systems Based on Power Transfer Matrix

This paper presents a power transfer matrix model and multivariable control method for a doubly-fed induction generator (DFIG) wind energy system. The power transfer matrix model uses instantaneous real/reactive power components as the system state variables. It is shown that using the power transfer matrix model improves the robustness of controllers as the power waveforms are independent of a dq frame of reference. The sequential loop closing technique is used to design the controllers based on the linearized model of the wind energy system. The designed controller includes six compensators for capturing the maximum wind power and supplying the required reactive power to the DFIG. A power/current limiting scheme is also presented to protect power converters during a fault. The validity and performance of the proposed modeling and control approaches are investigated using a study system consisting of a grid-connected DFIG wind energy conversion system. This investigation uses the time-domain simulation of the study system to: 1) validate the presented model and its assumptions, 2) show the tracking and disturbance rejection capabilities of the designed control system, and 3) test the robustness of the designed controller to the uncertainties of the model parameters.

Topology Design for Collector Systems of Offshore Wind Farms With Pure DC Power Systems

This paper presents an analytical design method for the dc collector systems of offshore wind farms and proposes an improved topology to overcome the limitations of the conventional series-parallel (SP) topology. During a severe failure in the SP topology, bypassing the faulty units imposes overvoltage on fault-free units and leads to the disconnection of the series branch from the collector system. By providing auxiliary connection paths, the suggested topology reduces the overvoltage of the units and hence maintains fault-free units operational following a severe failure. Hence, the efficiency of a wind farm increases considerably upon any failure occurrence. To show the merits of the suggested topology, the captured power during a failure has been calculated and fairly compared with that of the conventional SP topology, considering the excess cost of the suggested topology due to additional connection paths. To investigate the performances of both topologies and their feasibility, a collector system including 12 wind turbine generators has been simulated in time domain software environment. Then, various failure scenarios have been investigated to evaluate the transient behavior of the dc collector system following slight and severe failures. The study results reveal the feasibility of the proposed topology as a promising structure for future dc collector systems.

Economical Design of Utility-Scale Photovoltaic Power Plants With Optimum Availability

This paper presents an algorithm for the economical design of a utility-scale photovoltaic (PV) power plant via compromising between the cost of energy and the availability of the plant. The algorithm inputs are the plant peak power and the price of inverters with respect to their power ratings. The outputs are the optimum inverter ratings and the interconnection topology of PV panels. This paper introduces the effective levelized cost of energy (LCOE) (ELCOE) index as the core of the proposed design algorithm. ELCOE is an improved index based on the conventional LCOE that includes the availability of a power plant in economical assessments. The conventional LCOE index determines centralized topology (e.g., 1-MW inverter for a 1-MW PV power plant) for minimizing the energy generation cost, whereas based on ELCOE, a multistring topology (e.g., a 1-MW PV plant consists of fifty 20-kW inverters) despite of higher investment cost becomes the economically winning topology. Given the price of commercially available PV inverters at present, the case studies in this paper show that, for 0.1-100-MW PV power plants, the economical ratings of inverters range from 8 to 100 kW. The recently installed PV power plants confirm the feasibility of the calculations based on the suggested algorithm.

An improved small-deflection electromechanical model for piezoelectric bending beam actuators and energy harvesters

The analytical model presented in this paper describes the energy conversion mechanism of a piezoelectric beam (bimorph) under small-deflection static and vibrating conditions. The model provides an improved approach to design and analyze the performance of piezoelectric actuators and energy harvesters (sensors). Conventional models assume a linear voltage distribution over the piezoelectric beam thickness, which is shown here to be invalid. The proposed modeling method improves accuracy by using a quadratic voltage distribution. The equivalent capacitance of a beam shows a 40% discrepancy between a conventional model and the proposed model for PZT5A material. This inaccuracy level is not negligible, especially when the design of micro-power electrical energy harvesting is concerned. The method solves simultaneously the solid mechanics and Maxwells equations with the constitutive equations for piezoelectric materials. The paper also proposes a phasor-based procedure for measuring the damping of a piezoelectric beam. An experimental setup is developed to verify the validity of the model. The experimental results confirm the accuracy of the improved model and also reveal limitations in using models for small deflections.

An Efficient Piezoelectric Windmill Topology for Energy Harvesting From Low-Speed Air Flows

This paper presents a topology for micropower piezoelectric wind energy harvesting useful for developing self-powered wireless sensor nodes. The features of the proposed topology, as compared with the existing piezoelectric/electromagnetic topologies, are as follows: 1) delivering power at high voltage levels, particularly at low-speed air flows; 2) starting operation at low cut-in speeds (about 1 m/s); and 3) robust structure for operating at high-speed wind flows practically tested up to 20 m/s. The proposed topology consists of a small fan with embedded permanent magnets (PMs) and a piezoelectric beam with a PM proof mass, which interacts with the PMs in the fan to harvest wind power. This paper also presents an analytical model and a design procedure to determine the number of PMs in the fan and their arrangements to maximize the captured power and minimize the cut-in speed. Using a prototype of the proposed topology, it is shown that the device starts capturing wind power at the wind speeds above 0.9 m/s. It is also shown that the suggested topology is at least 10% more efficient than the existing topologies in using piezoelectric materials and that its total volume power density is higher than those of the other topologies.

A Topology and Design Optimization Method for Wideband Piezoelectric Wind Energy Harvesters

This paper presents a topology for wideband piezoelectric wind energy harvesting and a design optimization method for capturing maximum available energy considering the wind speed distribution model. The proposed device is useful for powering of wireless sensor nodes in smart monitoring applications. The device includes a piezoelectric beam that vibrates due to the interactions between miniaturized permanent magnets (PMs) embedded in a small turbine and a magnet at the tip of the beam. The design method determines the resonance frequency of the beam with respect to the wind speed distribution model. Compared to the conventional wideband topologies that include multiple beams for resonating at different frequencies, the suggested topology uses a single beam and achieves the wideband feature via the arrangement of PMs within the turbine. This feature enhances the power density of the piezoelectric wind energy harvester. The wideband feature of the device is experimentally verified using a device prototype in which the beam resonance occurs at the fan rotational speeds of 155.0, 193.5, 257.9, and 387.0 rad/s. The maximum power density, maximum efficiency, and cut-in wind speed of the device are 0.59 mW/cm3, 24%, and 2.1 m/s, respectively.

Magnetic Field Energy Harvesting from AC Lines for Powering Wireless Sensor Nodes in Smart Grids

This paper presents an electromagnetic energy harvesting device for capturing power from ac power lines using a miniaturized linear permanent magnet (PM) synchronous generator. The suggested device is useful to power wireless monitoring sensors in emerging smart grids. Conventional methods for energy harvesting from power lines use current transformers which structurally need to form closed magnetic paths around the lines. However, the suggested device can operate in the proximity of the lines due to using an independent PM generator. This feature facilitates installation of the device particularly in smart grids that require ample of easy-to-install and maintenance free monitoring sensors along the power lines. The proposed energy harvester uses a magnetically driven vibrating beam around the line that is attached to a linear PM synchronous generator (PMSG) including an array of small PMs with opposite polarities. This PM arrangement improves the induced voltage of the generator due to increasing the flux gradient within the linear PMSG. Experimental test results based on a cm-scale prototype show that an average power of 0.16 mW at 50 A (i.e., power density of 3.2 mW/kA) is achievable, equivalent to a 1038-mAh/2.7-V re-chargeable battery per year for a 100-A line.

Dynamic Modeling and Analysis of Cascaded DFIMs in an Arbitrary Reference Frame

This paper presents a comprehensive investigation of various reference frames and their relative speeds in cascaded doubly-fed induction machines (CDFIMs). Then, a compact space-phasor model of CDFIMs in the natural and arbitrary reference frames is presented that is useful for performance analysis of CDFIMs based on rotating phasors. The merits of the proposed model compared with existing models are using a common (single) reference frame for all machines and parts of CDFIMs, and the model formulation is independent of the rotor angle position. The reference frames in the suggested model can be fixed on the common rotor of the CDFIM machines for dynamic analysis or it can be fixed on the stator/rotor flux of each cascaded machines useful for CDFIMs controller design. A two-axes equivalent circuit for a CDFIM is also presented, which enables analysis of CDFIMs in commercially available power circuit simulation software tools. The validity of the model is verified via the investigation of an experimental setup and consists of two mechanically coupled 4-kW DFIMs and a 4-kW dc machine as a load. The setup is investigated under various test scenarios, including free acceleration, load (torque) change, and frequency step change conditions.