Variable Power Demand Research Articles

Control System Design of Power Tracking for PEM Fuel Cell Automotive Application

AbstractOwing to load variations, the power tracking of fuel cells in automotive applications is emphasized. We address the control strategies to meet varying power demands while optimizing system efficiency. Based on the control‐oriented proton exchange membrane fuel cell model, the relative gain array is analyzed to determine variable parings in control design. As the dynamics of two control variables are different, a cascade control architecture is adopted. A feedforward‐feedback composite control is used to achieve fast response and eliminate the stable error. To avoid oxygen starvation at power transients, an oxygen excess ratio constrained strategy is proposed to limit the load current if there is lack of reaction oxygen. A mixed‐sensitivity synthesis is applied to suppress parameter perturbation and improve system robustness. To further suppress the fluctuations of the oxygen excess ratio and achieve the maximum system efficiency, an oxygen excess ratio invariant strategy is developed, which coordinates load current with airflow. A comparative study is conducted, with two scenarios of power tracking and fuel cell degradation. The results show that the mixed‐sensitivity strategy has a fast response in power tracking and effective perturbation suppression performance, while the oxygen excess ratio invariant strategy maximizes the system efficiency.

Stochastic modelling of aggregated thermal loads for impact analysis of demand side frequency regulation in the case of Sardinia in 2020

This paper proposes a model of the thermal dynamics and of the end-use of domestic refrigerators (fridges) and water heaters (boilers) in the foretasted scenario of the Sardinian electric network in 2020. This model is employed to evaluate the potential variations of power demand of the aggregates of fridges and boilers during one year in the considered scenario. The resulting quantities can be considered as a form of power reserves to be used for contributing to the frequency regulation through a proper demand side response strategy. The particular case of the system for frequency control proposed by the European Network of Transmission System Operators for Electricity (ENTSO-E) is then analysed by simulations in order to show advantages and drawbacks.

Dynamic power supply by hydrogen bound to a liquid organic hydrogen carrier

Liquid Organic Hydrogen Carriers (LOHCs) are able to store hydrogen in a dense and safe form at ambient conditions. While storage of electrical energy in these carrier systems is one possible and attractive application, the dynamics of the load profile has been believed to represent a major challenge for this storage technology. Conversely, we report here that storage systems based on the LOHC technology are indeed able to deal with significant variations in power demand. This is due to the significant free volume in the LOHC release unit offering the opportunity to handle dynamic behavior by pressure changes. While pressure changes allow quick adaption of the power release on demand, changes in the reactor temperature lead to slow modification of the power output, as demonstrated in this work for hydrogen release from perhydro-dibenzyltoluene (H18-DBT).

Nonlinear control of coal-fired steam power plants

This work proposes a nonlinear control strategy for steam power plants that efficiently controls the superheated steam temperature while accommodating large and frequent variations in power demand. The variables to be controlled are the pressure in the boiler, power generation, and superheater/reheater temperatures. The proposed strategy decomposes the overall plant into three separate subsystems and applies decoupling with deadtime compensation for each one of them. The derived strategy is implemented within a MATLAB/Simulink environment for different setpoint tracking and disturbance rejection cases, showing excellent performance and robustness.

Introduction of a New Numerical Simulation Tool to Analyze Micro Gas Turbine Cycle Dynamics

Micro gas turbine (MGT) technology is evolving toward a large variety of novel applications, such as weak gas electrification, inverted Brayton cycles, and fuel cell hybrid cycles; however, many of these systems show very different dynamic behaviors compared to conventional MGTs. In addition, some applications impose more stringent requirements on transient maneuvers, e.g., to limit temperature and pressure gradients in a fuel cell hybrid cycle. Besides providing operational safety, optimizing system dynamics to meet the variable power demand of modern energy markets is also of increasing significance. Numerical cycle simulation programs are crucial tools to analyze these dynamics without endangering the machines, and to meet the challenges of automatic control design. For these tasks, complete cycle simulations of transient maneuvers lasting several minutes need to be calculated. Moreover, sensitivity analysis and optimization of dynamic properties like automatic control systems require many simulation runs. To perform these calculations in an acceptable timeframe, simplified component models based on lumped volume or one-dimensional discretization schemes are necessary. The accuracy of these models can be further improved by parameter identification, as most novel applications are modifications of well-known MGT systems and rely on proven, characterized components. This paper introduces a modular in-house simulation tool written in fortran to simulate the dynamic behavior of conventional and novel gas turbine cycles. Thermodynamics, gas composition, heat transfer to the casing and surroundings, shaft rotation and control system dynamics as well as mass and heat storage are simulated together to account for their interactions. While the presented models preserve a high level of detail, they also enable calculation speeds up to five times faster than real-time. The simulation tool is explained in detail, including a description of all component models, coupling of the elements and the ODE solver. Finally, validation results of the simulator based on measurement data from the DLR Turbec T100 recuperated MGT test rig are presented, including cold start-up and shutdown maneuvers.

Analysis of energy access and impact of modern energy sources in unelectrified villages in Uttar Pradesh

Bringing access to modern energy sources to the poorest in society is a key goal of many policymakers, businesses and charities, but in order to be a success projects and schemes must be founded on accurate data. We undertook a survey of energy demand and usage patterns in households in unelectrified villages in Uttar Pradesh, India to assess access to and utilisation of energy sources for lighting and cooking. The times of usage were recorded and analysed and the effect on usage patterns of transitioning from traditional to modern energy sources is assessed. We quantify the cost and greenhouse gas emissions of current energy use in order to provide a benchmark of potential mitigation through the use of renewable energy technologies: a typical household with kerosene lamps only for lighting spends INR 3243 (US$50.67) and emits 381 kgCO2eq per year; households with modern cooking energy spend 17% more through increased usage, but emit 28% less greenhouse gases compared to those with traditional stoves only. Cell phone ownership was found to be 50% amongst adults. We use demographic and utilisation data to construct an hourly demand profile of basic electricity demand extrapolated to each month of the year, and present an example of aspirational demand assess the impact of desirable appliances. A Monte Carlo simulation is used to highlight the daily and seasonal variation in total energy and power demand. A hybrid system, with solar power and battery storage meeting daytime demand and higher-capacity diesel- or biomass-powered generation meeting the remainder during evening peaks and winter months, would satisfy demand most effectively.

Simplification of a Mechanistic Model of Biomass Combustion for On-Line Computations

Increasing utilization of intermittent energy resources requires flexibility from energy boilers which can be achieved with advanced control methods employing dynamic process models. The performance of the model-based control methods depends on the ability of the underlying model to describe combustion phenomena under varying power demand. This paper presents an approach to the simplification of a mechanistic model developed for combustion phenomena investigation. The aim of the approach is to simplify the dynamic model of biomass combustion for applications requiring fast computational times while retaining the ability of the model to describe the underlying combustion phenomena. The approach for that comprises three phases. In the first phase, the main mechanisms of heat and mass transfer and limiting factors of the reactions are identified in each zone. In the second phase, each of the partial differential equations from the full scale model are reduced to a number of ordinary differential equations (ODEs) defining the overall balances of the zones. In the last phase, mathematical equations are formulated based on the mass and energy balances formed in the previous step. The simplified model for online computations was successfully built and validated against industrial data.

A statistical analysis of energy and power demand for the tractive purposes of an electric vehicle in urban traffic - an analysis of a short and long observation period

The article presents the results of a statistical dispersion analysis of an energy and power demand for tractive purposes of a battery electric vehicle. The authors compare data distribution for different values of an average speed in two approaches, namely a short and long period of observation. The short period of observation (generally around several hundred meters) results from a previously proposed macroscopic energy consumption model based on an average speed per road section. This approach yielded high values of standard deviation and coefficient of variation (the ratio between standard deviation and the mean) around 0.7-1.2. The long period of observation (about several kilometers long) is similar in length to standardized speed cycles used in testing a vehicle energy consumption and available range. The data were analysed to determine the impact of observation length on the energy and power demand variation. The analysis was based on a simulation of electric power and energy consumption performed with speed profiles data recorded in Poznan agglomeration.

Optimised operation of an off-grid hybrid wind-diesel-battery system using genetic algorithm

In an off-grid hybrid wind-diesel-battery system, the diesel generator is often not utilised efficiently, therefore compromising its lifetime. In particular, the general rule of thumb of running the diesel generator at more than 40% of its rated capacity is often unmet. This is due to the variation in power demand and wind speed which needs to be supplied by the diesel generator. In addition, the frequent start-stop of the diesel generator leads to additional mechanical wear and fuel wastage. This research paper proposes a novel control algorithm which optimises the operation of a diesel generator, using genetic algorithm. With a given day-ahead forecast of local renewable energy resource and load demand, it is possible to optimise the operation of a diesel generator, subjected to other pre-defined constraints. Thus, the utilisation of the renewable energy sources to supply electricity can be maximised. Usually, the optimisation studies of a hybrid system are being conducted through simple analytical modelling, coupled with a selected optimisation algorithm to seek the optimised solution. The obtained solution is not verified using a more realistic system model, for instance the physical modelling approach. This often led to the question of the applicability of such optimised operation being used in reality. In order to take a step further, model-based design using Simulink is employed in this research to perform a comparison through a physical modelling approach. The Simulink model has the capability to incorporate the electrical and mechanical (Simscape) physical characteristics into the simulation, which are often neglected by other authors when performing such study. Therefore, hybrid system simulation models are built according to the system proposed in the work. Finally, sensitivity analyses are performed as a mean of testing the designed hybrid system’s robustness against wind and load forecast errors.

Configuration and coordinated-distributed control strategy for a hybrid combined cycle

This paper is about an innovative type of power plant, called Hybrid Combined Cycle (HCC), and its control, by means of a coordinated-distributed strategy. The considered HCC plant consists of a Combined Cycle Power Plant (CCPP), a concentrated solar plant (parabolic trough) and a thermal storage system (a molten salt two-tank indirect sensible thermal storage), connected together in an original form. The proposed control strategy for such an interconnected plant corresponds to a distributed one, since it relies on pre-specified controllers for each subsystem, and is coordinated meaning here that a coordinator is designed to give the setpoints to each local controller, on the basis of global information and subsystem modeling. After its presentation, this strategy is applied to the considered HCC plant, including its modeling step and then its coordinated control. The approach is illustrated in simulation considering three coordination schemes: one based on LQ design, another one on MPC, and a third one on H∞, and the results are promising in terms of gas consumption reduction while accommodating to power demand and renewable source variations.

Modeling and Analysis of a Multilevel Parallel Hybrid Active Power Filter

This paper introduces a new control approach for the Multilevel Parallel Hybrid Active Power Filter (M-PHAPF) which can compensate harmonics and variable reactive power demand of loads by controlling the DC link voltage adaptively in medium voltage applications. By the means of this novel control method, M-PHAPF obtains a better and more efficient performance in the compensation of harmonics and reactive power compared to when using conventional control methods. The performance and stability of the proposed method are verified with a simulation model realized in PSCAD/EMTDC with different case studies. The simulation results demonstrate that harmonic compensation performance meets the requirements of the IEEE-519 standard.

Design of a SOFC/GT/SCs hybrid power system to supply a rural isolated microgrid

The aim of this research study has been to design a Hybrid Power System (HPS) which works with biogas and whose main components are a Solid Oxide Fuel Cell (SOFC), a Gas microTurbine (GT), and a module of SuperCapacities (SCs). The HPS is the only power source of a rural isolated microgrid. Its structure, operating strategy, and controller have been designed considering the following criteria: efficiency, power quality, SOFC lifetime and robustness in stability and performance.The HPS structure includes a unique power converter, a 3-Level Neutral Point Clamped (3LNPC) inverter that connects the HPS to the AC microgrid. Regarding the selected operating strategy, it consists in regulating the SOFC power output to its rated value. Thus, the SCs and the GT must respond to the power demand variations. On the other hand, a study of the HPS shows that its dynamic behavior is not linear. Therefore, a special attention is put on designing a robust HPS controller. The control model is identified and the robust digital controller is designed using the “Tracking and Regulation with Independent Objectives” method. Simulation and experimental results show how the proposed structure, operating strategy, and controller allow ensuring a good behavior of the HPS from the point of view of the abovementioned four criteria.

Absolute Stability Analysis Using the Liénard Equation: A Study Derived From Control of Fuel Cell Ultracapacitor Hybrids

Load-following in solid oxide fuel cells (SOFCs), hybridized with an ultracapacitor for energy storage, refers to an operating mode where the fuel cell's generated power follows the variable power demand, delivering the total demanded power at steady-state. Implementing this operating mode presents a rich set of problems in dynamical systems and control. This paper focuses on state-of-charge (SOC) control of the ultracapacitor during load-following, under transient constraints, and in the presence of an unknown nonlinearity. The problem is generalized to stabilization of a plant containing a cascaded connection of a driver and a driven dynamics, where the former is nonlinear and largely unknown. Closed-loop stability of the system is studied as a Lur'e problem and via energy-based Lyapunov equations, but both impose conservative conditions on the nonlinearity. An alternate approach is developed, where the closed-loop dynamics are formulated as a class of Liénard equations. The corresponding analysis, which is based on the nonlinear characteristics of the Liénard equation, yields more definitive and less conservative stability criteria. Additional conditions that lead to limit cycles are also derived, and a bifurcation pattern is revealed. The generality of the proposed approach indicates applicability to a variety of nonlinear systems.

A robust estimator-based optimal algebraic approach to steam generator feedwater control system

Feedwater control systems are used to maintain the steam generator water level within prescribed narrow limits and to provide constant supply of steam during power demand variations. Current feedwater control systems are often found to be unsatisfactory during startup and low power operations. A robust nonlinear estimator-based optimal algebraic control is developed for feedwater control systems to solve the water level tracking problem during power demand variations. It is shown that the proposed control provides an optimal and robust water level tracking with a single automatic controller over the complete range of power operations in the presence of plant uncertainties and noisy measurements.

Production Scheduling of an Air Separation Plant

Cryogenic air separation plants consume a large amount of electricity to produce various gaseous and liquid products; they can reduce their operational cost by proper exploitation of energy contracts and intelligent utilization of liquid products. In this paper, we propose a State Task Network(STN) based model which replicates a representative air separation plant. This STN based representation owing to its significantly more granular modelling can reap higher benefits in terms of improved decision making than the approaches proposed in literature. Production Scheduling based on this model computes the optimal operating conditions for the plant under variable power pricing options and demand scenarios. The modelling framework is very rigorous and includes almost all the real world limitations and constraints in air separation plant operation. This unit wise scheduling approach provides more flexibility and also pave the way for integration of the scheduling layer with the lower layered (hierarchically) control system. The proposed framework has been evaluated on the representative air separation plant and the results demonstrate the benefits of optimal plant operation.

Thermodynamic and economic analysis of diesel engine based trigeneration systems for an Indian hotel

The paper proposes an engine based trigeneration system that would aid in harnessing the energy from engine jacket water and engine exhaust gases for cooling loads via an absorption chiller. A thermal stabilizer is proposed for stabilizing the temperature fluctuations at the desorber inlet of absorption chiller. Two systems are considered for meeting variable thermal and power demand. One configuration includes only absorption chiller with an auxiliary water heater while the other has a compression chiller in addition to absorption chiller and the auxiliary water heater. A comparison of both these alternative options is attempted on thermodynamic and annualized life cycle costing basis for a typical hotel. It is seen that trigeneration systems with compression chillers are more efficient than trigeneration systems without the use of compression chillers for a hotel located in inland peninsular region of India.

Combined MGT – ORC Solar – Hybrid System. PART A: Plant Optimization

The renewed interest towards the low range power plants operating solicits the search of optimal solutions in terms of several key aspects:- The optimization of the thermal cycle in relation with the heat addition source, say an externally fired combustor, solar energy collectors, or waste heat from topping processes, together with the choice of the most favorable operating fluid.- The realization of the power plant with affordable costs, so more suitable with its employment as a distributed energy systems.This paper deals, in particular, with a combined cycle plant consisting of a topping micro-gas turbine (MGT) and of a bottoming ORC system (fig. 1 and fig. 2). A hybrid energy source is considered by means of the integration of the conventional fuel supply with a solar field. The authors’ work proceeds with the optimization of several main parameters, say:- The location and the size of the solar field (with parabolic trough assembly) and, therefore, of the temperature limits of the compressed air in the MGT;- The more appropriate working fluid in the ORC bottoming plant;- The off-design response for the best adaptation of the system to variable heat and power demands and to changes in the solar radiation intensity.

Implementation of Hybrid Filter for 12-Pulse Thyristor Rectifier Supplying High-Current Variable-Voltage DC Load

This paper presents a parallel combination of a shunt-connected passive filter and a distribution static compensator (DSTATCOM) for a 12-pulse thyristor rectifier feeding a high-current variable-voltage electrolyzer load. Since the process requires variation of voltage and current over a wide range, the reactive power consumed by the thyristor rectifier also varies enormously. As the passive filter is only able to provide a fixed compensation, the DSTATCOM is used to meet variable reactive power demand to keep the input power factor high even at partial load. In order to keep the DSTATCOM rating small, the bulk of reactive power is supplied by the dominant harmonic passive filter. Moreover, DSTATCOM does not provide any harmonic compensation, which is also taken care by the passive filter. Design and control of this system are discussed briefly, and more emphasis is laid on the implementation of the hybrid filter. An experimental system is tested rigorously, and detailed results are presented.

An applied methodology for multi-objective optimum sizing of hybrid electric vehicle components

In recent years, automobile industry focuses on the reducing the imperfections of hybrid electric vehicles (HEVs), such as higher cost, added weight of batteries, limited range, recharging facilities and emission rates. In the view of the related studies, we can infer that fuel cell (FC)-Battery hybrid configuration is a viable alternative solution. Such that, FCs are eco-friendly and proper for long ranged drives, on the other hand batteries have fast response for varying power demands. Moreover, FC-Battery hybrid electric vehicle (FCBHEV) proposes solutions for the imperfections. So, we deal with optimal sizing of FCBHEV and propose a methodology for the optimization of HEV components using the multi-objective approach considering the minimization of operating cost, weight and volume simultaneously. To optimize the sizing of HEV components, the mixed integer linear programming (MILP) model is tested in GAMS v.24.1.3 using the solver CPLEX v.12 and optimization processes are performed for different range of drive cycles and the results are discussed to reveal the effectiveness of the approach.

Reliable and flexible steam and power system design

Steam and power systems should be designed with high reliability and flexibility to satisfy process energy and power demands and reduce penalty costs due to equipment failures and steam and power demand variations. Uncertainties of equipment failure and flexible process steam and power demands have different impacts on system reliability, steam and power generation, individual equipment operation performance, and process production loss due to utility deficits. Measures adopted to respond to uncertainties implementation include compensation options of equipment operating load sharing, equipment startup, and equipment (in failure) repair, and penalties both of electricity import from the grid and production loss. This paper has proposed a procedure of the system design based on simultaneously modelling and optimizing of the structure and operation with system reliability analysis, and a mixed-integer linear programming (MILP) model is formulated associated with compensation costs and penalty costs to obtain both system configuration with spare equipment (in hot or cold standby) and spare capacities, and operating scheduling specification to account for equipment failures and process steam and power demand fluctuations. In this optimization, the effect of equipment failures on system operation performance and costs is analyzed by system reliability, and uncertain steam and power demands are formulated by a multi-period stochastic programming. A case study shows the application and effectiveness of the proposed methodology.