Maximum Efficiency Point Tracking Research Articles

Maximum efficiency point tracking for an ocean thermal energy harvesting system

Limited energy is the most critical factor that restricts the persistent presence of underwater vehicles in the oceans; thus, harvesting the ocean's thermal energy that is stored in the water column between the sea surface and deep water is a particularly promising solution for the current power shortage. This paper has designed a new ocean thermal energy conversion system which using phase change material as energy storage medium, and proposed a novel maximum efficiency point tracking (MEPT) method for energy conversion. This new method, which is integrated with a radial basis function neural network (RBFNN), particle swarm optimization (PSO) and the proportion integration differentiation (PID) control method, could effectively improve the efficiency of energy conversion. Compared with the power generation system that does not use the MEPT method, experimental results show that the proposed method can improve the efficiency of the power generation from less than 19.05% to more than 34.3% and has higher stability (using this method: the efficiency changes from 34.3%-34.7%; without using this method: the efficiency changes from 13.56% -19.05%) when the load changes. This novel method can be used in many conditions, especially when the mathematical model of the generation system is unknown or researchers want to use fewer sensors for maximum efficiency point tracking.

A Bidirectional Wireless Power Transfer System Control Strategy Independent of Real-Time Wireless Communication

For the bidirectional wireless power transfer (BWPT) technology, the phase synchronization between the transmitting and receiving converters is very essential and difficult to achieve by wireless communication between two sides because the delay and data transfer interval of wireless communication severely influence the control performance. This article proposes a novel control strategy for BWPT, which can achieve the phase synchronization and maximum efficiency point tracking independent of the real-time wireless communication. To solve the overshoot and oscillation problem incurred by the proposed control strategy and the nature of the system itself, the dynamics of a series-series compensated BWPT system is then analyzed and a small-signal model of this system is derived based on the generalized state space averaging and extended describing function methods. Using the small-signal model, a BWPT control system based on the proposed control strategy is designed and optimized. The validity of the proposed strategy and the performance of the control system are verified experimentally using a 3.3-kW prototype system.

A Power Management System for Electromagnetic Energy Harvesters in Battery/Batteryless Applications

This article presents a miniaturized and cost-effective power management system (PMS) for low-voltage electromagnetic energy harvesters (EMEHs) operating in both battery-powered and batteryless applications. The PMS consists of two converters and their associated controllers. The first converter converts EMEH’s ac voltage into a dc one in a single step and provides a maximum efficiency point tracking (MEPT) feature to maximize the harvested energy and can be used alone in battery-powered applications. The second converter provides tight output voltage regulation for batteryless applications. The MEPT converter utilizes a topology that uses a single inductor shared between the positive and negative half cycles of the EMEH voltage, unlike the majority of existing solutions, hence achieving system’s cost and size reductions. In addition, its control architecture is based on low-power comparators and does not require zero-crossing detection (ZCD) block, in contrast to many existing solutions, allowing for further reduction in cost, losses, and size. Moreover, the shared inductor is not only used by the power stage but also as a key element of the bias supply scheme allowing for voltage stepping up and hence providing sufficient voltage to supply the control circuits. The voltage regulation converter features a highly integrated dc–dc converter where the power inductor serves as normal inductor and a substrate for the converter. Experimental results show the effectiveness of the PMS with an EMEH that has a damped ac voltage characteristics ranging from 1.0 V to 10 mV for battery and batteryless applications. The presented MEPT converter, switching at 40 kHz, achieves 32.3-mm3 magnetics volume, which is the biggest contributor to the real estate of the PMS. Also, it can harvest up to 17 mJ out of 20 mJ available energy while achieving a BOM cost less than 0.7€. Moreover, the highly integrated buck converter, switching at 4 MHz, provides a tight regulated voltage of 1.2 V at 5 mA for a group of wireless sensors while achieving efficiency of 80% for load currents between $100~\mu \text{A}$ and 200 mA.

Dual‐objective control strategy for maximum power and efficiency point tracking in wirelessly powered biomedical implanted devices

Wireless charging is widely considered as a safe and reliable way for powering biomedical implants, as it avoids problems like surgical infection. Wireless power transfer (WPT) systems are desired to work efficiently against variations in coil–coil distance or output load. On the other hand, to maintain the maximum overall efficiency of the WPT system, the high frequency (HF) power amplifier, used to feed WPT system, must operate at optimal zero-voltage switching (ZVS) condition. In this paper, an automated dual-objective control strategy adaptive to variations in coil–coil distance or output load is proposed which ensures both targets of tracking maximum power point and operating at optimal ZVS condition by adjusting operating frequency and duty-cycle of switching voltage of HF power amplifier, respectively; that is while there have been few studies which have addressed both the two targets. To evaluate the effectiveness of the proposed strategy, a PCB prototype, operating at 800 kHz, is fabricated. Experimental results, demonstrating the proposed strategy increases transferred power from 23 mW to about 45 mW, are in good agreement with theoretical predictions. Additionally, while implanting the receiver coil in a real biological tissue, experiments show only 2% of degradation in power transfer efficiency as well as no frequency shift.

Maximum efficiency point tracking control method for series–series compensated wireless power transfer system

The efficiency of wireless power transfer system is significantly affected by variations in the system's load resistance and the coils’ coupling coefficient. For obtaining the maximum possible efficiency in different load resistances and coupling coefficients, a new maximum efficiency point tracking (MEPT) control method is proposed in this study. The proposed method realises the MEPT by converting the load impedance in real time and making the primary and the secondary resonant currents be a specific ratio. This specific ratio is determined by the equivalent series resistances of the primary and secondary circuits, which is independent of the coupling coefficient. The experimental results show that the maximum efficiency point can be tracked and the system efficiency can be improved by the proposed method.

Maximum Efficiency Point Tracking for Multiple-Transmitter Wireless Power Transfer

We propose a maximum efficiency tracking for a multiple-transmitter system that optimizes both the current ratios of transmitter (Tx) coils and the effective Rx load. It is found that unlike the single-Tx case, the optimal load impedance of multiple-Tx system depends on the current ratios among Tx coils. Meanwhile, the Tx current ratios should be controlled to be the same with magnetic coupling ratios between each Tx and Rx. Hence, we propose a two-step tracking method that first optimizes the Tx current ratios and then optimizes the Rx load impedance. To match the Tx current ratio with the magnetic coupling ratio, it is analyzed that the reflected resistances at every Tx become identical to each other when the Tx coil current ratios are equal to the magnetic coupling ratios. It is also revealed that the reflected resistance at specific Tx is reduced (increased) when its Tx current is increased (reduced) relative to other Tx currents. The proposed feedback equalizes all reflected resistances based on this property. The second step of the proposed tracking is to optimize the Rx load while fixing the Tx current ratios by globally scaling up or down the strengths of whole Tx currents. Maximum of 478 W is delivered at 79% efficiency. The algorithm does not have a limit on the number of Txs except the small standby power consumption. The proposed theoretic analysis and control method enable the first-ever maximum efficiency tracking for multiple-Tx system, where the first step optimizes the relative Tx current ratios and the second step optimizes the load impedance.

Pulse Density Modulated ZVS Full-Bridge Converters for Wireless Power Transfer Systems

Pulse density modulation (PDM) is an advanced technique for maximum efficiency point tracking of wireless power transfer (WPT) systems. By using PDM, both voltage regulation and efficiency maximization can be achieved without dc/dc converters. PDM is also compatible with the dual-side soft switching technique that utilizes resonant tanks and synchronous rectification. However, this soft switching technique depends on coupling and load conditions. Hard switching may occur when the coupling of coils gets stronger or the equivalent load is not properly controlled. To eliminate the dependence and ensure the soft switching under various operating conditions, this paper proposes a PDM zero-voltage-switching (ZVS) full-bridge converter for WPT systems. The converter employs a ZVS branch between switching nodes to provide a ZVS current, and uses a specially designed modulator to obtain the valid ZVS current waveforms. Experimental results verified the proposed operating principles and showed that the additional power loss caused by the ZVS current is insignificant. The overall efficiency of the WPT prototype was 93 ∼ 73% when the power transfer distance was 0.1 ∼ 0.4 m, among which up to 85% efficiency was observed when the distance equaled the coil diameter.

Pulse Density Modulation for Maximum Efficiency Point Tracking of Wireless Power Transfer Systems

Maximum efficiency point tracking (MEPT) control has been adopted in state-of-the-art wireless power transfer (WPT) systems to meet the power demands with the highest efficiency against coupling and load variations. Conventional MEPT implementations use dc/dc converters on both transmitting and receiving sides to regulate the output voltage and maximize the system efficiency at the expense of increased overall complexity and power losses on the dc/dc converters. Other implementations use phase-shift control or on-off control of the transmitting side inverter and the receiving side active rectifier instead of dc/dc converters but cause new problems, e.g., hard switching, low average efficiency, and large dc voltage ripples. This paper proposes a pulse density modulation (PDM) based implementation for MEPT to eliminate all the mentioned disadvantages of existing implementations. Delta-sigma modulators are used as an example to realize the PDM. A dual-side soft switching technique is proposed for the PDM. The ripple factor of the output voltage with PDM is derived. A 50 W WPT system is built to validate the proposed method. The system efficiency is maintained higher than 70% for various load resistances when the power transfer distance is 0.5 m, which is 1.67 times the diameter of the coils.

Low-Cost Maximum Efficiency Tracking Method For Wireless Power Transfer Systems

This paper presents a novel maximum efficiency point (MEP) tracking method for wireless power transfer (WPT) systems. It is proved that at the MEP for a typical system containing a boost converter on the receiving end, the derivative $dD/d{V_{{\text{in}}}}$ ( ${V_{{\text{in}}}}$ being the inverter's dc input and $D$ being the duty cycle of the converter) is equal to or smaller than a constant $\beta $ that is determined by system parameters. By tuning ${V_{{\text{in}}}}$ and comparing $dD/d{V_{{\text{in}}}}$ with $\beta $ , the MEP can be tracked without a power or current sensor, hence reducing the tracking cost. A WPT system is demonstrated experimentally to verify the method. When the diameters of and the vertical distance between the transmitting and receiving coils are 4.3 and 2.35 cm, respectively, without horizontal misalignment, high efficiency is maintained even when the load ranges from 5 to 100 Ω, and the efficiency improvement reaches 41.2% when compared to the measured efficiencies without the proposed method. The differences between the maximum efficiencies tracked by the proposed method and the manually searched maximum values are at most 1.8%. Extension of the proposed method to WPT systems employing other popular forms of inverter, dc/dc converter, and coil topology is also presented.

Modeling and Dynamics of HTS Motors for Aircraft Electric Propulsion

In this paper, the methodology of how a dynamic model of a conventional permanent magnet synchronous motor (PMSM) may be modified to model the dynamics of a high-temperature superconductor (HTS) machine is illustrated. Simulations of a typical PMSM operating under room temperature conditions and also at temperatures when the stator windings are superconducting are compared. Given a matching set of values for the stator resistance at superconducting temperature and flux-trapped rotor field, it is shown that the performance of the HTS PMSM is quite comparable to a PMSM under normal room temperature operating conditions, provided the parameters of the motor are appropriately related to each other. From these simulations, a number of strategies for operating the motor so as to get the propeller to deliver thrust with maximum propulsive efficiency are discussed. It is concluded that the motor–propeller system must be operated so as to deliver thrust at the maximum propulsive efficiency point. This, in turn, necessitates continuous tracking of the maximum propulsive efficiency point and consequently it is essential that the controller requires a maximum propulsive efficiency point tracking (MPEPT) outer loop.

Achieving Efficiencies Exceeding 99% in a Super-Junction 5-kW DC–DC Converter Power Stage Through the Use of an Energy Recovery Snubber and Dead-Time Optimization

A highly efficient 5-kW bidirectional dc–dc converter power stage operating from a 400-V supply implementing super-junction (SJ) metal–oxide–semiconductor field-effect transistor (MOSFETs) is presented. SJ MOSFETs have low on-state resistances and low switching losses. However, their application in voltage-source converters can be compromised by the reverse recovery behavior of their intrinsic diodes and their highly nonlinear output capacitances. A series switching-aid circuit is used to control the output capacitance charging current. The dead times between switching transitions are assessed and optimized in order to deactivate the intrinsic diodes. The combination of these techniques enables very high efficiencies to be attained. Calorimetric measurements indicate a full-load efficiency of 99.1% for the prototype 5-kW dc–dc converter power stage. A loss reduction of approximately 50% is achieved with the prototype converter power stage when compared to an equivalent insulated-gate bipolar transistor (IGBT)-based power stage. Lastly, a loss versus duty cycle function is experimentally determined, which can be used to inform the design of a maximum efficiency point tracking system.

Megahertz Multiple-Receiver Wireless Power Transfer Systems With Power Flow Management and Maximum Efficiency Point Tracking

This paper presents systematic analysis and design of a megahertz multiple-receiver (RX) wireless power transfer (WPT) system driven by a Class E power amplifier. Both circuit-level and system-level analyses are conducted to discuss the characteristics of the WPT system. In order to simultaneously manage the power flow and maximize the overall system efficiency, a new control scheme is proposed that combines maximum efficiency point tracking and time-division multiplexing. Through the proposed scheme, RXs with different loading and coupling conditions can be charged by the same transmitter with simplified design and control strategy. Finally, the proposed control scheme is validated using an example 6.78-MHz three-RX WPT system. The experimental results show that various power requirements can be quickly and stably satisfied, and the overall average system efficiency can reach as high as 71.7%.

Maximum Efficiency Point Tracking (MEPT) for Variable Speed Small Hydropower Plant With Neural Network Based Estimation of Turbine Discharge

The paper presents the control system of a small hydropower plant (SHP) working at variable speed operation (VSO). In general, VSO increases the turbine operating discharge and gives higher efficiency over a wide operating range. It is particularly advantageous under conditions different from the original design conditions. Changing of the hydrological conditions requires adjustments to the control system; therefore, the new maximum efficiency point tracking (MEPT) algorithm is proposed. The MEPT algorithm implemented in the load controller employs the gradient method. The water level governor based on the PI controller combined with the adaptive load controller is analyzed. The troublesome measurement of the turbine discharge is replaced by the neural-network-based estimator. The correlation analysis of the system parameters was performed by using principal component analysis. The proposed algorithm was implemented in the programmable logic controller and tested on a real low-head SHP of 150 kW power with two single-regulated propeller turbines.

Standalone fuel cell generation system with different tracking techniques: economic analysis

The proton exchange membrane fuel cell (PEMFC) can be operated at different points such as the maximum power point (MPP) to extract maximum power and the maximum efficiency point (MEP) to operate at maximum efficiency. However, the different tracking techniques influence the cost of electricity (COE) of the fuel cell generation system. In this paper, the economic analysis of the PEMFC with the MPP tracking (MPPT) and MEP tracking (MEPT) techniques using the HOMER energy system analysis software is presented. A detailed comparison of the economic impact of the tracking techniques for ten load configurations of a standalone fuel cell generation system, which includes combined heat and power (CHP) loads is presented and discussed. Finally, based on economic considerations, a procedure to select a suitable tracking technique for particular requirements of the standalone PEMFC application is proposed. It is found that in the case of CHP configuration, the MPPT technique is the preferred technique to achieve low COE.

Improving a free air breathing proton exchange membrane fuel cell through the Maximum Efficiency Point Tracking method

Abstract This work investigates the design and validation of a fuel cell management system (FCMS) which can perform when the fuel cell is at water freezing temperature. This FCMS is based on a new tracking technique with intelligent prediction, which combined the Maximum Efficiency Point Tracking with variable perturbation-current step and the fuzzy logic technique (MEPT-FL). Unlike conventional fuel cell control systems, our proposed FCMS considers the cold-weather conditions, the reduction of fuel cell set-point oscillations. In addition, the FCMS is built to respond quickly and effectively to the variations of electric load. A temperature controller stage is designed in conjunction with the MEPT-FL in order to operate the FC at low-temperature values whilst tracking at the same time the maximum efficiency point. The simulation results have as well experimental validation suggest that propose approach is effective and can achieve an average efficiency improvement up to 8%. The MEPT-FL is validated using a Proton Exchange Membrane Fuel Cell (PEMFC) of 500 W.

A Maximum Efficiency Point Tracking Control Scheme Based on Different Cross Coupling of Dual-Receiver Inductive Power Transfer System

One of the most promising inductive power transfer applications is the wireless power supply for locomotives which may cancel the need for pantographs. In order to meet the dynamic and high power demands of wireless power supplies for locomotives, a relatively long transmitter track and multiple receivers are usually adopted. However, during the dynamic charging, the mutual inductances between the transmitter and receivers vary and the load of the locomotives also changes randomly, which dramatically affects the system efficiency. A maximum efficiency point tracking control scheme is proposed to improve the system efficiency against the variation of the load and the mutual inductances between the transmitter and receivers while considering the cross coupling between receivers. Firstly, a detailed theoretical analysis on dual receivers is carried out. Then a control scheme with three control loops is proposed to regulate the receiver currents to be the same, to regulate the output voltage and to search for the maximum efficiency point. Finally, a 2 kW prototype is established to validate the performance of the proposed method. The overall system efficiency (DC-DC efficiency) reaches 90.6% at rated power and is improved by 5.8% with the proposed method under light load compared with the traditional constant output voltage control method.

Optimal Control of a Thermoelectric Generator Attached with a Latent Heat Accumulator

Generally MPPT control (maximum power point tracking) is adopted to control of a thermoelectric generator. However, in the case of generation by use of a heat accumulator MPPT control cannot obtain maximum whole electrical output during the operation period. This is because the amount of heat stored in a heat accumulator is limited and easy to be exhausted rapidly by MPPT. Therefore MEPT (Maximum Efficiency Point Tracking) control should be developed to obtain maximum power from limited heat stored in the heat accumulator. When thermoelectric generator is used for waste heat recovery, conversion efficiency is quite difficult to be measured. This is due to time delay between the change of temperature profile in the thermoelectric generator and the change of heat medium temperature. Decrease of output current is desired to enlarge output because decrease of current decreases Peltier heat and improves efficiency of heat recovery. The experimental results indicate that current fluctuated by MPPT control causes loss of power output. We proposed the optimal control in which current is 10% smaller than one of MPPT control and evaluated it experimentally. We call this control scheme MEPT control. In this evaluation 500 W class thermoelectric generator, latent heat accumulator and the test facilities included 30 kW electric heater are utilized. Experimental result shows MEPT control exceeds MPPT in total electricity recovered from heat accumulator.

Energy control strategies for the Fuel Cell Hybrid Power Source under unknown load profile

Four new energy control strategies are proposed here for the Fuel Cell Hybrid Power Source (FCHPS) used in stationary and mobile FC application (such as the FC backup source for a smart-house and FC vehicle, respectively) based on the Load Following (LF) control and Maximum Efficiency Point Tracking (MEPT) control of the fueling rates. The LF control approach is used to design simple strategies of the Energy Management Unit (EMU) that will assure a charge-sustaining mode for the batteries stack of the Energy Storage System (ESS). If a fueling rate is controlled based on the LF strategy, then the other is controlled based on MEPT strategy in order to maximize the FC net power available. The advantages of the proposed EMU strategies during an unknown load cycle are comparatively shown.

A Maximum Efficiency Point Tracking System for Wireless Powering of Biomedical Implants

Inductive power transfer systems are widely used to supply the current generation of biomedical implants. However, the power transfer efficiency is highly dependent on both the mutual coupling of the coils and the load impedance of the implant side. To overcome the challenge of time-dependent coil orientation and load impedance changes due to different power consumption levels, a maximum efficiency point tracking (MEPT) system for inductive links is presented within this work. After characterizing the momentary mutual inductance of the transmission channel, the designed system calculates the optimum load impedance and performs a dynamic impedance matching by emulating this optimum load with an input voltage-controlled switching regulator. Therefore, the presented system is capable of extracting energy at the best case efficiency for a wide range of real loads. The implemented system features a small system size (14 x 14 mm2) and a low self-consumption of only 250 μW, while providing up to 100 mW to the load.

Efficient energy control strategies for a Standalone Renewable/Fuel Cell Hybrid Power Source

In this paper, four energy control strategies are proposed and analyzed for the standalone Renewable/Fuel Cell Hybrid Power Source (RES/FC HPS). The concept of the load following (LF) and Maximum Efficiency Point Tracking (MEPT) is used to control the fueling rates. A standalone RES/FC HPS uses at least one Renewable Energy Sources (RES) and a Polymer Electrolyte Membrane (PEM) Fuel Cell (FC) as backup source. Photovoltaic (PV) array and wind turbines (WT) farm are used as RES and the surplus of energy during light load stages is stored in hydrogen tank via water electrolysis to fuel the PEMFC. Small-scale RESs and commercially available PEMFCs are interfaced to the common DC bus via power converters and then to the single-phase distribution grid through a voltage source inverter. RES/FC HPS seem to be an efficient alternative for supplying smart houses and isolated sites. This paper proposes a new supervision strategy of the Energy Management Unit (EMU) based on the LF control approach that assures a charge-sustaining (CS) mode for the Energy Storage System (ESS). So, the capacity of the batteries stack can be reduced at minimum if it is directly connected to the DC bus. The ultracapacitors stack compensates dynamically the power flow balance on the DC bus, regulating the DC voltage via a bidirectional buck-boost power converter. Thus, a semi-active hybrid topology is adopted for the ESS having the batteries stack. The MEPT loops ensure an optimized energy management of the RES/FC HPS. The LF strategy designed based on the power flow balance guarantees the load demand through the efficient management of the power flow from RES, FC and ESS, adapting FC energy production to load profile. The FC energy production is controlled via the fueling rates (one is controlled in the LF loop and the other is controlled in the MEPT loop) to maximize the energy consumption indicator: the ratio of the produced energy and the fuel consumed by the FC system during a load cycle. Thus, this paper evaluates the control performances of the four control topologies to fuel the FC stack during an variable RES and load profile. The MATLAB-Simulink® software package is used to model the RES/FC HPS and develop the four EMU strategies. Simulation results shown comparatively the performance of the EMU strategies proposed under different scenarios of RES power generation and load demand.