Direct Energy Conversion System Research Articles

Stability Analysis and Improvements for Variable-Speed Multipole Permanent Magnet Synchronous Generator-Based Wind Energy Conversion System

This paper discusses the stability issues of the permanent magnetic synchronous generator (PMSG)-based direct driven wind energy conversion system (WECS) and presents a torque compensation strategy to improve the system stability. The stability analysis shows that the drive train dynamics can significantly affect the system stability by introducing a low-frequency oscillatory mode and the stability margin can be enlarged with the slower generator torque control loop or the reduced power converter response. The torque compensation strategy contains a feed-forward compensator in the torque control loop, which is formulated based on the dc current injected into the dc link capacitor of the converters. With such compensation, the oscillatory mode is effectively suppressed and the stability of the WECS is improved. The simulation results verify the theoretical analysis as well as the small-signal and transient performance of the strategy.

Maximum efficiency of direct energy conversion systems. Application to fuel cells

This work develops a methodology to calculate the maximum First Law efficiency of any power device. To do so external irreversibilities due to heat transfer are suppressed by means of Carnot engines which mechanical work is added up to the one resulting from the device under hypothetical internally reversible mode of operation. By applying the developed methodology comparisons between Direct Energy Conversion Systems (DECS) and power cycles can be properly set on balanced assumptions in each type of device. In this paper, the main equations of the methodology are presented and the specific equations for fuel cells are derived. The potential of this methodology is illustrated by comparing fuel cells and power cycles First Law efficiency as a function of temperature.

Comparison of oscillation damping capability in three power control strategies for PMSG‐based WECS

AbstractWith the aid of small signal analysis and digital simulations, this paper compares the mechanical and power oscillation damping performances of three power control strategies for the multi‐pole permanent magnetic synchronous generator (PMSG)‐based direct driven wind energy conversion system (WECS). Maximal power point tracking (MPPT) control implemented in the generator side has inherent abilities on the oscillation damping. For the smoothed or constant power requirements, power oscillations are hard to damp, and additional active damping controller is required. Active damping can be achieved with power control on the generator or grid side and DC link voltage control on the generator side. With additional compensator in the power or DC link voltage control loop, a damping torque is produced to suppress the oscillations. An improved control structure, which has inherent oscillation damping capability, is proposed for the power control of WECS. Combined with different power control strategies, this structure can be applied to achieve different power outputs. The validation of the proposed control structure is verified by the simulations. Copyright © 2010 John Wiley & Sons, Ltd.

Possible developments in energy conversion using liquid metal magnetohydrodynamics

Liquid metal magneto-hydrodynamic-energy-conversion (LMMHDEC) systems have been a matter of great interest and research & development since 1960. The various states of design and development of such systems go through a step-by-step progress with time. This paper highlights the phenomenon of direct thermal energy conversion systems using liquid metal as an electrodynamics fluid and gas/vapour as a thermodynamic fluid. An analysis of the technological drawbacks responsible for low efficiency of these LMMHDEC systems along with possible R & D solutions have been discussed in this technical research paper. The separation of electrodynamics fluid from thermodynamic fluid at various stages of MHD conversion remained an efficiency challenge of the various types of systems. To meet this challenge, a Dual-cycle MHD system has been designed in this paper. Both the fluids viz. thermodynamic and electrodynamics go through a phase change in this cycle. The thermal efficiency is optimized when one fluid goes into a phase change during a cycle and another fluid does not experience any phase change. The information covered in this paper enables an overview of concepts and the background to choose a cycle for a given temperature range.

Direct electric energy conversion system for energy conversion from marine currents

Direct electric energy conversion without input power regulating system or gearboxes may offer a simpler and thus low cost way of converting the energy of moving water. The proposed electrical system consists of a permanent magnetized direct drive generator, a rectification step, transmission, an inverter, a transformer, and a grid connection. This paper focuses on the generator, and simulations of a 5 kW permanent magnetized direct drive generator have been performed. Operation of the generator at both part load and overloads are calculated and presented. It is found that this direct drive permanent magnetized generator provides a relatively high efficiency for both part load and overload conditions.

Particle Discrimination Experiment for Direct Energy Conversion

A direct energy conversion system designed for D-3He fusion reactor based on a field reversed configuration employs a venetian-blind type converter for thermal ions to produce DC power and a traveling wave type converter for fusion protons to produce RF power. It is therefore necessary to separate, discriminate, and guide the particle species. For this purpose, a cusp magnetic field is proposed, in which the electrons are deflected and guided along the field line to the line cusp, while the ions pass through the point cusp. A small-scale experimental device was used to study the basic characteristics of discrimination of electrons and ions in the cusp magnetic field. Ions separated from electrons are guided to an ion collector, which is operated as a one-stage direct energy converter. The conversion efficiency was measured for cases with different values of mean and spread of ion energy. These experiments successfully demonstrate direct energy conversion from plasma beams using particle discrimination by a cusp magnetic field.

The 20 year evolution of an energy conversion course at the United States Military Academy

Over the past several years, an energy conversion course offered by the Mechanical Engineering Program at the United States Military Academy in West Point, New York, has evolved into a cohesive series of lessons addressing three general topical areas: advanced thermodynamics, advanced mechanical system analysis, and direct energy conversion systems. Mechanical engineering majors enroll in Energy Conversion Systems (ME 472) during the fall semester of their senior year as an advanced elective. ME 472 builds directly on the material covered in Thermodynamics (EM 301) taken during the student’s junior year. In the first segment of ME 472, the students study advanced thermodynamic topics including exergy and combustion analyses. The students then analyze various mechanical systems including refrigeration systems, internal combustion engines, boilers, and fossil fuel fired steam and gas turbine combined power plants. Exergetic efficiencies of various equipment and systems are determined. The final portion of the course covers direct energy conversion technology, including fuel cells, photovoltaics, thermoelectricity, thermionics and magnetohydrodynamics. Supplemental lessons on energy storage, semi-conductors and nonreactive energy sources (such as solar collectors, wind turbines, and hydroelectric plants) are included here. This paper discusses the evolution of ME 472 since its inception and explains the motivations for the course’s progress.

Experimental Simulation on Discrimination of Charged Particles in a Cusp Direct Energy Converter for D-3He Fusion

A direct energy conversion system designed for D-3He fusion reactor consists of a CUSP direct energy converter (DEC) and a Traveling Wave DEC, where, respectively, electrons and thermal ions are separated from fusion protons and decelerated via Venetian blind type electrodes to produce DC power, and protons are velocity-modulated, bunched, and then decelerated by RF traveling waves to produce RF power. For the basic investigation of the CUSPDEC, a small scale experiment is conducted using a device consisting of a single cusp and low-energy plasma and ion sources. Performed are the measurements of basic characteristics of the behavior of charged particles in various conditions including magnetic field gradients, spatial locations of incidence, ion masses, and ion energies.

Electrical transport properties in zirconia/alumina functionally graded materials

Functionally graded materials (FGMs) are promising candidates for the fabrication of technological components, not only as structural devices, but also in electrochemical ones, such as solid oxide fuel cells (SOFC), or high-efficiency hybrid direct energy conversion systems. In the present work FGMs were prepared by the sequential slip casting technique, starting with an yttria tetragonal zirconia polycrystalline layer and increasing subsequently the amount of Al 2O 3 in the following layers. Electrochemical impedance spectroscopy (EIS) analysis was used to evaluate the electrical characteristics of these materials and to compare with those of the monolithic compacts. In general, it was observed that the FGM conductivity is ruled by the conductivity of the layer which contains the highest amount of alumina blocking particles. By EIS no electrical interfaces between adjoining layers were detected and, accordingly, no specific electric ohmic losses were observed. The conductivity of the FGMs is close to that calculated using the normalized thicknesses and the alumina volume fractions of the layers after measuring the conductivity of the monolithic materials with the same composition to what correspond to that of the final layer in the FGM. These results suggest that the gradient structure can be used to control the oxygen vacancy motion, and then applied in electrochemical devices.

Ｎｉ／Ａｌ２Ｏ３系対称型傾斜機能材料の設計と作製

In order to meet various power requirements in the next century, thermoelectric (TE) power generation system has been focussed on a candidate of direct energy conversion systems with a nuclear heat source. An important thing in the TE power generation system is to use efficiently the heat source by means of high density induction of thermal energy for the TE module. The present work investigated a compliant pad which can served as both electrode and conductible coupling between TE module and heating or cooling duct. This pad demands (1) a high flux, direct conduction path to heat source and heat sink, (2) the structural flexibility to protect the TE module from high stress due to thermal expansion, (3) electrical insulation of the electrode from heating or cooling duct, (4) an extended durability by a simple FGM structure. In this paper, optimization for FGM was performed by using thermal stress analysis and thermal conduction analysis, densification of nickel and alumina powder, and process of manufacturing nickel/alumina/nickel symmetric FGM were investigated.

97/01560 Modeling and simulation of energy conversion in combined gas-steam power plant integrated with coal gasification

The generation of electric power through the conventional conversion systems may no longer be sufficient to keep pace with the increasing electricity usage and demands. Therefore, an enhancement to these systems can be made by incorporating a topping unit to the existing power plants (thermal, hydroelectric), a direct energy conversion system. Among such is the Magnetohydrodynamics (MHD) system. In an MHD conversion system, electricity is generated when heat energy is converted into electrical energy without the use of mechanical rotating parts. Additionally, the heat energy (ionised gas) required inside the MHD system usually comes from the combustion of fossil fuels. However, this system is still under development in developing countries like South Africa. The current study aims to investigate through simulations and theoretical modelling two existing Open-Cycle Gas Turbines (OCGT) in South Africa (Ankerlig and Gourikwa), which can be converted into Combined-Cycle MHD Gas-Steam Turbines (CCMGST). The processes involved by incorporating the MHD system with these conventional turbines have been further discussed. With setting up a CCMGST in South Africa, the obtained modelling results project an additional generation of electric power which will further improve the existing power generation.

Thermal Energy Conversion Systems Overview for Fusion Reactors

The author describes various thermal energy conversion systems reviewed in order to select an efficient power cycle that is compatible with fusion reactor requirements. The most suitable power cycles were selected for a toroidal confinement system with deuterium-tritium and deuterium-deuterium fuel cycles and for a tandem mirror reactor (TMR) with a D-/sup 3/He fuel cycle. The steam cycle was found to be most suitable for the reversed-field pinch and tokamak reactors, whereas a combination of a direct energy conversion system and a steam cycle was found to be most suitable for a D-/sup 3/He TMR. It is anticipated that the energy conversion systems of all fusion reactors fall under these two categories.

Energy Conversion Systems Design for Fusion Reactors

Various energy conversion systems have been reviewed in order to select an efficient power cycle to be compatible with the fusion reactor requirements. The power cycles were selected for a toroidal confinement system with D-T and D-D fuel cycles and a tandem mirror reactor (TMR) with a D-/sup 3/He fuel cycle. Reversed Field Pinch Reactor (RFPR) was selected as an example of a toroidal confinement system with D-T fuel cycle since there has recently been a comprehensive design study for it. Tokamak was selected as an example of a toroidal confinement system with D-D fuel cycle. Tandem mirror reactor was chosen as an example of confinement for D-/sup 3/He fuel cycle. The steam cycle was found to be most suitable for the RFP and Tokamak reactors while a combination of direct energy conversion system and steam cycle was found to be most suitable for D-/sup 3/He tandem mirror reactor.

Conceptual design of a fusion engineering test facility based on mirror confinement (FEF)

The results are presented from the feasibility study on a fusion engineering test facility based on mirror confinement (FEF) and are intended to clarify the critical issues both in the physics and engineering. The objectives of FEF are (i) to test many fusion engineering components associated with 14 MeV neutrons, such as material tests, tritium test, thermo-hydraulic test as well as the interactive and integrated tests, and (ii) to study the useful applications of fusion produced neutrons for other purposes, such as fission fuel production, radio isotope production and transmutation of nuclear waste. The design philosophy of the FEF is to build a compact and linear fusion plasma neutron source in an axisymmetric magnetic field configuration which could be built in near future. The characteristics of FEF are (i) usage of radio frequency power for heating and stabilization of the MHD mode of the plasma, (ii) usage of water for the shielding of the device, and (iii) usage of a direct energy conversion system for the end region of the mirror plasma. The basic specifications of FEF as the irradiation facility are: 14 MeV neutron wall leading of 1–2 MW/m 2 , and irradiation area of 1 m 2 . The major critical issues which are found through this work are: (i) the physics aspects, needs of the data bases for RF ponderomotive stabilization of the MHD mode and for pellet injection and (ii) the engineering issues, bombardment of high energy particles on the rf antenna and on the vacuum chamber and damage of the insulators.

Variance Analysis of Wind Characteristics for Energy Conversion

Variance analysis of hourly wind data is utilized to assist in wind characteristic assessment for direct energy conversion systems. Feasibility of large-scale wind energy utilization, as well as sitting, sizing and operating policy, depend on the variability of the wind. Several Midwestern sites are used to illustrate a temporal study of variance sources that leads to the evaluation of equivalent independent hours. For the sites considered these vary from two to three per day. This information is utilized to determine duration of survey requirements for stated confidence levels and desired accuracy. Reasonably reliable and accurate estimates of the mean seasonal wind can be obtained in one or two years for the sites analyzed. Autocorrelation and cross-correlation analysis confirms the existence of significant correlation in the wind at a single site for a period of 8–12 h and between sites for similar time lags and separations up to 100 km or more. Meaningful prediction of variation from the mean appears possible in these cases. The diurnal cycle effect is seen to depend significantly on season and elevation.

Commercial application of direct energy conversion (DEC systems vs. batteries)

The general design requirements, or criteria, which must be met by Direct Energy Conversion (i.e., Thermoelectric, Thermionic, and Fuel Cells) Systems if they are to replace battery powered systems in commercial applications are considered. Both long term and short term unattended site applications are investigated. Since the use of DEC systems in place of batteries will, in general, be determined principally, on a total system annual cost basis, the design requirements which influence cost are considered. The design requirements are placed in perspective by comparison with DEC state-of-the-art estimates and projections.

Hermetically-Sealed Nickel-Cadmium and Silver-Cadmium Storage Batteries

The galvanic battery is the most highly developed means of storing electrical energy today. In satellite and space-vehicle applications where auxiliary electrical power is required for extended periods of time, it has been found that the most feasible power supply currently available consists of silicon solar cells used in conjunction with sealed nickel-cadmium storage batteries. They are capable of thousands of repeated cycles and have an energy output of about 12 watt-hours per pound. Sealed silver-cadmium cells are of interest because of their higher output, 24 watt-hours per pound, but they are not as far developed, nor do they as yet have the cycle life of nickel-cadmium cells. In the development of hermetically-sealed nickel-cadmium batteries, the following areas have received considerable attention: 1) Hermetic seal, 2) Continuous overcharge capability and low internal cell pressure, 3) Separator, and 4) Cycle life. In the development of hermetically-sealed silver-cadmium cells, four technical problems have received considerable attention: 1) Separator, 2) Improved silver electrode capable of delivering energy at the Ag <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+II</sup> potential, 3) Continuous overcharge capability, and 4) Cycle life. Although other direct-energy conversion systems appear to be promising and are being intensively developed, it is expected that the nickel-cadmium and silver-cadmium storage batteries will continue to be of importance for many years to come.

The role of industry in energy-conversion research

It is apparent that some means of alleviating the burden on our supply of fossil fuel is a necessity. The extensive research effort required and some of the major problems involved in the development of practicable direct energy-conversion systems are described.