Design of tunable optimal load circuit for maximum wireless power transfer efficiency

Maximum power transfer and maximum efficiency are two important design constraints in wireless power transfer applications. Several works have investigated the proper load and impedance match conditions to optimize either efficiency or power transfer. In this paper we show that the optimal load for maximum power transfer and maximum efficiency is the same (a conjugate matched load) when the source resistance is zero. This is important, as many WPT systems have a relatively low, unknown source impedance. Since the optimal load for both efficiency and power is the same as the source impedance approaches zero, the designer can use a bi-conjugate load for a near optimal design for both maximum power and efficiency. As the source impedance becomes significant, the bi-conjugate matched system provides higher power, but at the expense of lower efficiency. Maximum efficiency is achieved with a non-bi-conjugate load, when the source impedance is non-negligible.

We derive the Z-parameters for the two coupled antennas used for wireless power transfer under the assumption that the antennas are canonical minimum scattering antennas. Using the Z-parameter and the maximum power transfer efficiency formula, we determine the maximum power transfer efficiency of wireless power transfer systems. The results showed that the maximum power transfer efficiency increases as the mode number or the radiation efficiency increases. To verify the theory, we fabricate and measure two different power transfer systems: one comprises two antennas generating <TEX>$TM_{01}$</TEX> mode; the other comprises two antennas generating <TEX>$TM_{02}$</TEX> mode. When the distance between the centers of the antennas was 30 cm, the maximum power transfer efficiency of the antennas generating the <TEX>$TM_{02}$</TEX> mode increased by 62 % compared to that of the antennas generating the <TEX>$TM_{01}$</TEX> mode.

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In this paper, we show that there exists an optimal number of coils for maximum wireless power transfer efficiency through systems of identical cascaded resonators. We apply convex optimization to determine the ideal spacing of a system in which only the direct coupling between adjacent resonators is considered. We then model the high-order coupling as a perturbation to this system and solve for the optimal spacing. We show that as the number of coils in a system increases, the efficiency for a given source-to-load distance reaches a peak and then starts to drop. Beyond this point, the losses introduced by each additional resonator begin to outweigh the high efficiency achieved by increasingly tight resonator coupling.

Sadi Carnot's theorem regarding the maximum efficiency of heat engines is considered to be of fundamental importance in thermodynamics. This theorem famously states that the maximum efficiency depends only on the temperature of the heat baths used by the engine, but not on the specific structure of baths. Here, we show that when the heat baths are finite in size, and when the engine operates in the quantum nanoregime, a revision to this statement is required. We show that one may still achieve the Carnot efficiency, when certain conditions on the bath structure are satisfied; however if that is not the case, then the maximum achievable efficiency can reduce to a value which is strictly less than Carnot. We derive the maximum efficiency for the case when one of the baths is composed of qubits. Furthermore, we show that the maximum efficiency is determined by either the standard second law of thermodynamics, analogously to the macroscopic case, or by the non increase of the max relative entropy, which is a quantity previously associated with the single shot regime in many quantum protocols. This relative entropic quantity emerges as a consequence of additional constraints, called generalized free energies, that govern thermodynamical transitions in the nanoregime. Our findings imply that in order to maximize efficiency, further considerations in choosing bath Hamiltonians should be made, when explicitly constructing quantum heat engines in the future. This understanding of thermodynamics has implications for nanoscale engineering aiming to construct small thermal machines.

This article presents a maximum efficiency tracking method for an inductive power-transfer (IPT) system based on a Boost dc-dc converter. Impedance matching (IM) is achieved by maintaining the equivalent load impedance to be constant against load variations. The expression of the optimal equivalent load impedance corresponding to the maximum system-transfer efficiency is derived, and the impedance-transfer characteristics of the Boost converter in both continuous conduction mode (CCM) and discontinuous conduction mode (DCM) are analyzed. The duty cycle of the Boost converter is controlled to maintain the optimal equivalent load with extended range in DCM mode, and the switching frequency of the Boost converter is further adjusted to improve the accuracy of the IM. Both simulation and experimental results have demonstrated that the proposed IM method can help achieve the maximum power-transfer efficiency under a wide range of load variations.

This paper presents a microwave power transmission scheme in fully enclosed space using reconfigurable reflecting elements (i.e., elements terminated with tunable reactive loads). The scheme is modeled by a multi-port network, based on which the expression of power transmission efficiency in terms of scattering parameters is obtained. The maximum achievable power transmission efficiency and the optimal reactive loads of reflecting elements can be found by solving a simple optimization problem. The model is verified by simulation and experimental results, which demonstrate that the scheme can improve power transmission efficiency significantly in a fully enclosed environment.

Dimension effects of a magnetoelectric gyrator with FeCoSiB/Pb(Zr,Ti)O3 layered composites core for efficient power conversion

It is well known that a DC or fully tuned inductive power transfer (IPT) system operating at its maximum power output point results in a low power efficiency being equal or below 50%. Based on the analyses of the maximum power transfer condition against the detuning factor of an LC resonant circuit, this paper proposes a Capacitive Power Transfer (CPT) system which can track the maximum power output using a DC-DC converter, at the same time achieve a high-power efficiency when the circuit is not fully tuned. The method is applied to a voltage-fed inverter at series tuned off-resonance operation for reduced power losses. A prototype system is built with a half-bridge inverter to drive the capacitively coupling circuit, and a buck-boost converter is designed for impedance matching. A perturb and observe algorithm is developed for controlling the duty cycle of the DC-DC converter to track the maximum power against load variations. It is found that under a fixed electric field coupling condition, the system can maintain the maximum power transfer operation at 10W and achieve a power efficiency of about 70% over a wide range of load variation from $5\mathbf{\Omega}$ to $500\mathbf{\Omega}$ .

Chapter 3 - Antenna Fundamentals II

This paper presents a resonant wireless power transfer method that leverages a 90-degree voltage phase shift between the transmitting and receiving coils to enhance efficiency and maximize power transfer. When the resonant coupling is achieved, the secondary coil with an adjustable capacitor forms a tuned LC circuit. If the primary coil is driven at the resonant frequency of both the primary and secondary sides, the system can transmit 250W of power between the coils over a distance of 50 cm. Using a single power transmitting unit (PTU) board with multiple paralleled gallium nitride high-electron-mobility transistors (GaN HEMTs), the system achieves a maximum power transfer efficiency of 88%, highlighting the effectiveness of the design in high-efficiency, long-distance wireless power transmission. The key to the success of high-power, high-efficiency RWPT is in exhibiting the imaginary turn ratio presented on the air transformer. The imaginary turn ratio can realize the negative impedance conversion that converts the positive resistance on the power-receiving unit into a negative one, and thus, the damping of the resonance oscillation becomes negative and positively encourages more power to be delivered to the power-receiving unit (PRU) load. This paper derives the theory of the imaginary turn ratio and demonstrates the implementation of the RWPT system that exhibits the imaginary turn ratio effect.

A stationary irreversible regenerative Stirling cycle model, with two sources of irreversibilities (finite rate of heat transfers and heat leak), is analyzed. The aim of this work is to obtain the criterion of partial optimization presented in [3]. Since with this criterion, the optimal relations for the allocation and effectiveness of the heat exchangers of Carnot-like power plant are obtained; when two design rules are applied, alternatively: internal thermal conductance fixed, or areas fixed. These optimal relations are the same for maximum specific power and efficiency. As an instance, after the substitution of these optimal values in the specific power and efficiency, the maximum specific power and efficiency are obtained. Then the maximum efficiency and specific power are compared, and it is found that the maximum efficiency is greater than maximum specific power for both design rules.

This study focuses on the design, simulation, and load power optimization for the development of a novel frequency-tunable electromagnetic vibrational energy harvester. The unique characteristic of a magnetorheological elastomer (MRE) is utilized, that the shear modulus can be varied by changing the strength of an applied magnetic field. The electromagnetic energy harvester is fabricated, the external electric circuit is connected, and the performance is evaluated through a series of experiments. The resonant frequencies and the parasitic damping constant are measured experimentally for different tuning magnet gap distances, which validate the application of the MRE to the development of a frequency-tunable energy harvesting system. The harvested energy of the system is measured by the voltage across the load resistor. The maximum load power is attained by optimizing the external circuit connected to the coil system. The analysis results are presented for harvesting the maximum load power in terms of the coil parameters and external circuit resistance. The optimality of the load resistance is validated by comparing the analytical results with experimental results. The optimal load resistances under various resonance frequencies are also found for the design and composition of the optimal energy harvesting circuit of the energy harvester system.

An irreversible model of a Brayton cycle, which includes a regenerator, an intercooler, and a reheater, is presented in this paper. The power output and the thermal efficiency are optimized with respect to different pressure ratios and cycle temperatures for a typical set of operating conditions. It is found that there are optimal values of the turbine outlet temperature, reheat, intercooling, and cycle pressure ratios at which the cycle attains the maximum power output and the maximum thermal efficiency. Also, the optimal values of the above-mentioned parameters are different for different parameters. The maximum power point and the maximum efficiency point exist but the power output corresponding to the maximum efficiency is found to be lower than that which can be attained. The optimum criteria for different pressure ratios and the turbine outlet temperature corresponding to maximum power output as well as corresponding to maximum thermal efficiency are obtained and discussed in detail.

Maximum power and efficiency of an irreversible thermoelectric generator with a generalized heat transfer law