Bi-directional Energy Conversion Research Articles

Development and research of a hybrid power unit for ultralight aircraft: an innovative approach to energy efficiency and operational flexibility

This scientific article presents an innovative concept of a hybrid power unit designed for ultralight aircraft, with the aim of improving energy efficiency and operational flexibility. As part of the development of the system, the construction of the combustion unit and the electric motor / generator, which are the key elements of this solution, was described. The advanced internal combustion engine controller and the bi-directional energy conversion converter have been developed and built to enable optimal cooperation of both energy sources. In order to carry out experimental research on the developed system, a special test stand was built on which a prototype drive unit was mounted. The results of the research include preliminary performance characteristics of the prototype drive unit and an analysis of the achievements that indicate the potential benefits of using such a hybrid drive unit. The article also summarizes the conclusions and recommendations for further work on improving this innovative solution.

Time-sharing orbit jump and energy harvesting in nonlinear piezoelectric energy harvesters using a synchronous switch circuit

Nonlinearity has enabled energy harvesting to advance toward higher power output and broader bandwidth in monostable, bistable, and multistable systems. However, operating in the preferable high-energy orbit (HEO) rather than the low-energy orbit (LEO) for making such advancement has restricted their applications. Based on a monostable nonlinear system, this paper proposes a self-contained solution for time-sharing orbit jump and energy harvesting. The joint dynamics of an electromechanical assembly consisting of a nonlinear energy harvester and a switched-mode piezoelectric interface circuit for high-capability energy harvesting is studied. The proposed solution is carried out by utilizing a cutting-edge switched-mode bidirectional energy conversion circuit (BECC), which enables time-sharing dual functions of energy harvesting and vibration exciting. A theoretical model is established based on impedance analysis and multiple time scales method to analyze the stability, frequency response, and phase evolution of the autonomous and nonautonomous nonlinear energy harvesting systems. In particular, the detailed dynamics for the orbit jumps with the vibration exciting mode of BECC are studied. Experiments are performed to validate the full-hysteresis-range orbit jumps with the monostable nonlinear energy harvester. The harvested power after orbit jumps yields a nine-fold increase, compensating for the energy consumption under vibration exciting mode quickly. The proposed solution also refrains the system from extra mechanical or electrical energy sources for orbit jumps, which leads to the first self-contained solution for simultaneous energy harvesting and orbit jump in nonlinear piezoelectric energy harvesting. This work enhances the practical utility of nonlinear energy harvesting technologies toward engineering applications.

Function-space optimization to coordinate multi-energy storage across the integrated electricity and natural gas system

The synergy between electric power systems and natural gas systems brings significant energy storage potentials yet challenges the modeling and solution techniques of partial-differential-equation (PDE) constrained optimization. This paper proposes a function-space optimization to coordinate multi-energy storage across the integrated electricity and natural gas system. Compared to general algebraic-space optimization, the proposed function-space optimization (FSO) uses continuous spatial–temporal functions as variables, for which time and space can be explicitly expressed and the PDE constraints of gas dynamics can be directly embedded into optimization. Considering the bidirectional energy conversion enabled by gas-fired units and power-to-gas, linepack gas storage and battery energy storage are coordinated through FSO to improve the operational economy. For tractable calculation, the Bernstein polynomial spline-based solution space transformation is adopted to transform FSO into mixed-integer linear programming. Case studies on a single-pipe example and two practical test systems are conducted and the simulation results validate the effectiveness of the proposed FSO method.

Integrated planning of natural gas and electricity distribution systems for enhancing resilience

Integrated power and gas systems (IPGSs) have developed a critical infrastructure in multi-energy systems. The bi-directional energy conversion between the two systems may cause disturbances in one system spread to the other one by coupling components. Especially under extreme events, this coupling relationship can cause the system to collapse. Therefore, it is urgent to enhance the resilience of IPGS at the planning stage. Based on this requirement, the integrated planning model of IPGS considering cascading effects for enhancing resilience is proposed in this paper. Firstly, the two-phase framework contains phases of “demand reachability evaluation” and “integrated planning” is proposed. Furthermore, capacity accessibility considering load level is defined to evaluate the reachability of the power system and the natural gas system (NGS) under extreme events. In this period, the Monte Carlo simulation (MCS) technique is used to represent the random failure of components. Based on this, an integrated planning model considering cascading effects is developed to optimize the location and capacity of coupling elements for enhancing IPGS resilience. Numerical results on the IPGS test system demonstrate the effectiveness of the proposed methods.

Optimal Operation of Integrated Energy Systems Based on Multi-Energy Coupling With Power-to-Gas Technology

Abstract Natural gas and renewable energy sources make up an increasingly large proportion of power generation due to their being environmentally friendly. Because of the intermittent and fluctuating output of renewable energy sources, the emerging technology of power-to-gas (P2G) units is of great significance to alleviate. This paper focuses on the coordinated optimization of the combined gas and electricity network (CGEN) system with bidirectional energy conversion. Gas-fired power generation (GPG) plants and P2G are considered as linkages between the two networks. A unified CGEN mathematical model is established with the minimum operation cost as the objective function. The injection and production process of underground natural gas storage (UNGS) is also taken into consideration that is not available in other literature. Variables such as the output of P2G and gas-fired power generation plants, the supply of UNGS, and the wind curtailment are optimized correspondingly. The improved 24-node power grid (PG) and Belgium 20-node natural gas pipeline network (NGPN) are adopted to test the validity and capability of the proposed model, and then we compare the results CGEN model with the single Belgium NGPN to analyze the impact on the NGPN. Moreover, we adopt a coupled operation system of a 6-node power system and a 7-node NGPN to further analyze the influence of P2G on the CGEN. The results show that compared with the system without P2G, the total operating cost of the system is reduced by 9.39%, the natural gas load shedding is reduced by 26.1%, and the wind curtailment is reduced from 326 MWh to zero.

On the positive and negative damping effect of piezoelectric structure using switched-mode bidirectional energy conversion circuit

The introduction of a switched-mode bidirectional energy conversion circuit (BECC) facilitates the development of piezoelectric devices toward integrated and energy-efficient multi-functional designs. These new designs realize the organic combinations of two or more functions among energy harvesting (EH), vibration excitation (VE), and dynamic sensing. Yet, the structural effect after applying the BECC was not comprehensively investigated and strictly quantified. This letter analyzes the electromechanical joint dynamics of a piezoelectric structure using the BECC, which was developed after the synchronized triple bias-flip (S3BF) technique, under different EH and VE operation modes. It shows that the EH modes electrically induce an extra positive damping effect, while the VE modes induce a negative one. A lumped model and impedance analysis are used to evaluate the electrically induced damping and the energy conversion efficiency by the BECC. The closed-form expression of the vibration displacement under different operation modes is derived. Experiments are carried out under different operation modes and frequencies. The theoretical and experimental results show good agreement. They validate the damping tuning capability of BECC in either positive or negative damping directions. This switched-mode interface circuit offers a promising solution for building an adaptive dynamic control of piezoelectric structures with high energy efficiency.

A Modeling and Analysis Framework for Integrated Energy Systems Exposed to Climate Change-Induced NaTech Accidental Scenarios

This paper proposes a novel framework for the analysis of integrated energy systems (IESs) exposed to both stochastic failures and “shock” climate-induced failures, such as those characterizing NaTech accidental scenarios. With such a framework, standard centralized systems (CS), IES with distributed generation (IES-DG) and IES with bidirectional energy conversion (IES+P2G) enabled by power-to-gas (P2G) facilities can be analyzed. The framework embeds the model of each single production plant in an integrated power-flow model and then couples it with a stochastic failures model and a climate-induced failure model, which simulates the occurrence of extreme weather events (e.g., flooding) driven by climate change. To illustrate how to operationalize the analysis in practice, a case study of a realistic IES has been considered that comprises two combined cycle gas turbine plants (CCGT), a nuclear power plant (NPP), two wind farms (WF), a solar photovoltaicS (PV) field and a power-to-gas station (P2G). Results suggest that the IESs are resilient to climate-induced failures.

Design of a Bidirectional CL3C Full-Bridge Resonant Converter for Battery Energy Storage Systems

In this study, a bidirectional CL3C full-bridge resonant converter was developed using a bidirectional active bridge converter as the main framework to improve conventional LLC resonant converters. A resonant inductor and resonant capacitor were installed at the secondary side of the developed resonant converter. The bidirectional operation of this converter enables zero-voltage switching at the supply-side power switch and zero-current switching at the load side. The aforementioned phenomena enhance the overall circuit efficiency and enable the resonant tank voltage to be increased in the reverse mode, which cannot be achieved with conventional bidirectional LLC resonant converters. The electrical equipment isolation function provided by a transformer made electricity usage safer, and digital control technology was adopted to control electrical energy conversion and simulate bidirectional energy conversion. Specifically, the experiment and simulation emulated how the developed converter enables energy transmission from a DC grid to a battery energy storage system through constant current–constant voltage charging and energy transmission from a battery energy storage system to a DC grid through constant power discharging.

Optimal dispatch for reversible solid oxide cell-based hydrogen/electric vehicle aggregator via stimuli-responsive charging decision estimation

The ongoing growth of green vehicles had led to an increase in demand of cost-effective and driver-satisfactory hydrogen/electric vehicle aggregators (HEVAs). However, existing approaches for cost minimization of HEVA can lead to poor performance due to the inaccurate modelling of power–gas exchange system and neglection of schedulable characteristics of loads. Furthermore, the behaviour of drivers was rarely considered from a psychological perspective. To resolve these limitations, the optimal dispatch scheme of HEVA, equipped with reversible solid oxide cell (rSOC), is investigated by quantifying drivers’ charging decision response toward pricing stimuli. As the core of the bi-directional energy conversion, rSOC is modelled by considering the climbing power constraints and time-dependent restart-up cost. At the driver side, EVs are aggregated as clusters for efficient computation. Two charging modes are designed for drivers with incentive discounts. To measure the relationship between external factors and charging decision response, the stimuli-responsive charging decision estimation is proposed by introducing Weber–Fechner law (W–F Law). To minimum operation cost, a mixed integer nonlinear programming (MINP) method is presented. The results validate that the operation cost of HEVA can be decreased by 19.37%, and the maximum utilization of energy is realised in the proposed scheme. Additionally, the impacts of sizes of power–gas exchange devices are investigated for practical reference. Under a given charging demand, the proposed dispatch scheme can realise installation of smaller devices, and thereby, resulting in lower construction cost.

Research on Hysteresis Modeling and Compensation Method of Giant Magnetostrictive Force Sensor

Linearity is an important index for evaluating the performance of various sensors. Under the Villari effect, there may be some hysteresis between the input force and the output voltage of a force sensor, meaning that the output will be multivalued and nonlinear. To improve the linearity and eliminate the hysteresis of such sensors, an output compensation method using a variable bias current is proposed based on the bidirectional energy conversion mechanism of giant magnetostrictive material. First, the magnetization relationship between the input force, bias current, and flux density is established. Second, a nonlinear neural network model of the force-magnetization hysteresis and a neural network model for the compensation control of the force sensor are established. These models are trained using the magnetic flux density-force curve and the magnetic flux density-current curve, respectively. Taking the optimal linearity as the objective function, the bias current under different input forces is optimized. Finally, a bias current control system is developed and an experimental test platform is built to verify the proposed method. The results show that the proposed variable bias current hysteresis compensation method enables the linearity under the return of the force sensor to reach 1.6%, which is around 48.3% higher than under previous methods. Thus, the proposed variable bias current method effectively suppresses the hysteresis phenomenon and provides improved linearity for giant magnetostrictive force sensors.

A Bidirectional Energy Conversion Circuit Toward Multifunctional Piezoelectric Energy Harvesting and Vibration Excitation Purposes

Piezoelectric transducers provide a bidirectional channel for converting energy from mechanical form to electrical or in the opposite direction. Various applications were developed with either of these two energy-flow processes. For example, energy harvesters take advantage of the energy flow from mechanical to electrical domains, while vibration exciters utilize that from electrical to mechanical domains. However, few designs have integrated the bidirectional energy-flow processes in a single device in the past. Such an integration is beneficial to some multifunctional applications. The key obstacle of integrated bidirectional design results from the interface circuit design, rather than the transducer. This article makes a breakthrough by proposing a bidirectional energy conversion circuit (BECC) solution for the time-sharing energy harvesting and vibration exciting purpose. It is realized by developing a new control strategy on the synchronized triple bias-flip (S3BF) piezoelectric interface circuit, which was formerly used for energy harvesting enhancement. The control logic for energy harvesting and vibration exciting modes are discussed in detail. The time-sharing multi-functional ability is validated in experiments. It is the first time to realize a compact and integrated self-excitable energy harvester by using a single interface circuit. Experiments are carried out for validating the feasibility of the BECC toward future versatile engineering designs.

Edge‐cloud collaborative architecture based multi‐time scales rolling optimization of regional integrated electrical and natural gas energy system considering wind power uncertainty

With the serious energy crisis and speedy development of natural gas utilization, regional integrated electrical and gas energy system is an efficient way to improve energy potency. This paper studies the energy scheduling of integrated electrical and gas energy system with bi-directional energy conversion at distribution network level, considering wind power uncertainty. To realize independent autonomy of energy networks, meantime adjusting scheduling strategies of integrated energy system from system-level perspective, innovative edge-cloud collaborative operation architecture is proposed. In edge servers, electrical and gas networks are optimized severally for scheduling strategies. In cloud server, the uncertainty of wind power are processed with robust optimization method, strategies of energy conversion units are updated for re-optimization. Cooperated with edge-cloud architecture, a novel multi-time scales rolling optimization framework is proposed, electrical and gas networks are optimized under day-ahead time scale in corresponding edge servers, and electrical network is further adjusted under intraday time scale in electrical edge server. Case studies show that edge-cloud collaborative operation architecture can ensure the practical feasibility of optimization, which is more suitable for energy scheduling of integrated electrical and gas energy system. Multi-time scales rolling optimization framework have significant efficiency on reducing the impact of wind power uncertainty.

A multi-slack bus model for bi-directional energy flow analysis of integrated power-gas systems

The bi-directional energy conversion components, such as gas-fired generators (GfG) and power-to-gas (P2G) has enhanced the interactions between power and gas systems. This paper focuses on the steady-state energy flow analysis of an integrated power-gas system (IPGS) with bi-directional energy conversion components. Considering the shortcomings of adjusting active power balance only by single GfG unit and the capacity limitation of slack bus, a multi-slack bus (MSB) model is proposed for integrated power-gas system, by combining the advantages of bi-directional energy conversion components in adjusting active power. The components are modeled as participating units through iterative participation factors solved by the power sensitivity method, which embeds the effect of system conditions. On this basis, the impact of the mixed problem of multi-type gas supply sources (such as hydrogen and methane generated by P2G) on integrated system is considered, and the gas characteristics-specific gravity (SG) and gross calorific value (GCV) are modeled as state variables to obtain a more accurate operational results. Finally, a bi-directional energy flow solver with iterative SG, GCV and participation factors is developed to assess the steady-state equilibrium point of IPGS based on Newton-Raphson method. The applicability of proposed methodology is demonstrated by analyzing an integrated IEEE 14-bus power system and a Belgian 20-node gas system.

Linear network model for integrated power and gas distribution systems with bidirectional energy conversion

With the advancement of emerging power-to-gas (P2G) technologies, the integrated power and gas distribution system (IPGDS) with bidirectional energy conversion is becoming a promising measure to promote the integration of renewable-based distributed generation. This study proposes the linear network model for the IPGDS and takes the reactive power consumption of P2G into account. The linear model considering the network constraints of the IPGDS is implemented combining the Wendroff difference for gas network equations and the pyramidal approximation for power network equations. With the proposed linear network model, the economic dispatch of the IPGDS is studied considering distribution network reconfiguration. Both the overall positive role of P2G and the negative effect of P2G's reactive power consumption are analysed. The former helps to mitigate the voltage violations and to improve economic efficiency, while the latter reduces these benefits. Case studies in the integrated 33-bus power and 12-node gas distribution system validate the effectiveness and applicability of the proposed linear network model of the IPGDS and the necessity of considering P2G's reactive power consumption.

A Non-Dissipative Equalizer with Fast Energy Transfer Based on Adaptive Balancing Current Control

In this study, an active inductive equalizer with fast energy transfer based on adaptive balancing current control is proposed to rapidly equilibrate lithium-ion battery packs. A multiphase structure of equalizer formed by many specific parallel converter legs (PCLs) with bidirectional energy conversion serves as the power transfer stage to make the charge shuttle back and forth between the cell and sub-pack or sub-pack and sub-pack more flexible and efficient. This article focuses on dealing with the problem of slow balancing rate, which inherently arises from the reduction of balancing current as the voltage difference between the cells or sub-packs decreases, especially in the later period of equalization. An adaptive varied-duty-cycle (AVDC) algorithm is put forward here to accelerate the balance process. The devised method has taken the battery nonlinear behavior and the nonideality of circuit component into consideration and can adaptively modulate the duty cycle with the change of voltage differences to maintain balancing current nearly constant in the whole equilibrating procedure. Test results derived from simulations and experiments are provided to demonstrate the validity and effectiveness of the equalizer prototype constructed. Comparing with the conventional fixed duty cycle (FDC) method, the improvements of 68.3% and 8.3% in terms of balance time and efficiency have been achieved.

Error minimization based on multi-objective finite control set model predictive control for matrix converter in DFIG

Matrix Converter (MC) has many advantages such as capability of bi-directional energy conversion, no need for DC bus and high value filters. However, modulation methods are very difficult and require high computational effort. Therefore, Multi-objective Finite Control Set Model Predictive Control (MFCS-MPC) has been implemented to simplify the complexity of this topology. However, the weakness of this control method is using mathematical model and affected by parameter mismatches due to the external disturbances, frequency and temperature variations. In this study, past errors of rotor current are added to the cost function and the weighting factors are adjusted with the Fuzzy Decision Making (FDM) approach. Due to the combination of these two methods, a robust system is obtained that can operate even in model-plant mismatch. The proposed algorithm is implemented using the TMS320F28335 Digital Signal Processor (DSP), a 32-bit floating-point processor operating at 150 MHz. Results obtained from simulation and experimental setup are compared with the current literature studies and the performance of the proposed method is confirmed.

Integrated Deterministic and Probabilistic Safety Analysis of Integrated Energy Systems with bi-directional conversion

The rapid development of the technology of energy conversion is changing the global energy landscape. In this study, a reliability analysis framework for Integrated Energy Systems (IESs) is developed, based on the concept of Integrated Deterministic and Probabilistic Safety Analysis (IDPSA). A bi-directional energy conversion system model is developed to simulate the deterministic evolution of the IES. Then, the dynamic event tree (DET) analysis technique is used to describe the stochastic evolution of IES, based on the physics of IES. Given the scenarios from the DET, the probabilistic safety margin method is used to evaluate the reliability of IES. The contract pressures at delivery points are adopted as critical safety parameters for the evaluation of the safety margins. To reduce the computational burden, Order Statistics, combining the Bracketing and Coverage approaches, are used to obtain the percentiles of the probabilistic distribution of the safety parameters. An application of the developed framework is performed on an IES. The results indicate that the framework can evaluate the reliability of IES from the perspectives of dynamic failure probability and operation performance. Furthermore, the impact of P2G (power to gas) on the energy supply and operation security of the IES is analysed in detail.

Control de un sistema de energía basado en volantes de inercia para mitigar los huecos de tensión en el punto de conexión común

Este artículo presenta el diseño de un sistema de energía basado en volante de inercia para mitigar los huecos de tensión. Con el sistema se mejora la calidad de energía en un punto de una red de distribución, el cual está expuesto a la conexión aleatoria de máquinas eléctricas. Para ello, se modela el sistema de distribución de energía, el sistema de inyección de energía que está compuesto por una máquina eléctrica con volante de inercia, el sistema de conversión de energía bidireccional y el sistema de control de corriente, voltaje y velocidad. El sistema diseñado permite inyectar una potencia de 22.8 kW y capacidad de 1.2 Wh, compensando los transitorios producidos por las cargas conectadas a la red

A proton exchange membrane fuel cell-compound thermoelectric system: Bidirectional modeling and energy conversion potentials

Thermoelectric device may appear as thermoelectric cooling (TEC) mode or thermoelectric generation (TEG) mode when it is generally applied to recover the waste heat produced from proton exchange membrane fuel cell (PEMFC), typically operating in the range of 60–80 °C. Although PEMFC integrated thermoelectric cooler or generator has been investigated separately in the past years, researches regarding their simultaneous TEC and TEG modes are still not reported so far. In the present work, a comprehensive thermodynamic performance analysis of the fuel cell-thermoelectric hybrid (FC-TEH) system considering TEC and TEG models simultaneously is conducted to exploit the energy conversion potential of the electrochemical and thermoelectric coupling processes. Irreversible characteristics and exergoeconomic performance of the hybrid system are thoroughly analyzed through combining finite time thermodynamics and thermodynamic economics. Subsequently, parametric comparisons between the fuel cell-thermoelectric cooling (FC-TEC) and the fuel cell-thermoelectric generation (FC-TEG) models are sensitively identified in terms of the decision targets, such as power output, energy efficiency, exergy efficiency and unit exergy cost. In addition, operating regimes of thermoelectric models in FC-TEH system are further determined to reveal thermoelectric conversion conditions and ensure efficient operation of the thermoelectric device (TED). Present results further demonstrate that FC-TEH system firstly behaves as FC-TEC in the current density range of 0–1.2 A/cm2, then FC-TEG and ultimately FC-TEC mode; where, only the TEG mode has the positive influence on the power output of the hybrid system. In addition, effective ranges of current density for the FC-TEG mode and minimum unit exergy cost are also confirmed. Present research may be significant for fully enhancing the energy and exergy performance of electrochemical – thermoelectric process.

Optimal scheduling of bidirectional energy conversion units in energy and ancillary service markets for system restoration within MCESs

Multi-carrier energy systems (MCESs) can be formed by the integration of various energy infrastructures including power and natural gas systems. The proliferation of bidirectional energy conversion units in an MCES can set the stage for a more resilient and robust system. This study shows how bidirectional energy conversion units and storage devices can be optimally scheduled within an MCES for provision of various regulation services to the grid operator. To that end, a new model is proposed for optimal scheduling of power-to-gas (PtG), gas-fired generation, and gas storage units in an MCES. The model aims to facilitate integration of renewables, utilise gas, and power price arbitrage, provide regulation services to the real-time (RT) market, and contribute to the system restoration. New indices that quantify the contribution of the MCES operator to RT and ancillary service markets are proposed. The proposed model is validated technically and economically by using a test system historical operating data. Numerical results demonstrate that while the proposed model is technically feasible, it also enhances the economic viability of the grid operator.