Ohmic Load Research Articles

The Performance of SOFC Model Under Different Three‐phase Load Conditions

AbstractIn this paper, the dynamic performance of a modified thermal based dynamic model of a solid oxide fuel cell (SOFC) is presented under different three‐phase load conditions. The modified thermal based fuel cell model contains ohmic, activation and concentration voltage losses, thermal dynamics, methanol reformer and fuel utilization factor limiting stage. SOFC model and the power conditioning unit (PCU), which consists of a DC‐DC boost converter, a DC‐AC inverter, their controller, transformer and filter, are developed on Matlab/Simulink environment. The simulation results show that the real and reactive power management of the inverter is performed successfully in an AC power system with the proposed thermal based SOFC model under three‐phase load conditions such as ohmic loads, switched ohmic−inductive loads and a three‐phase induction motor. Finally, the three‐phase induction motor is performed both no load and load conditions. The simulation results show that the modified thermal based fuel cell model provides an accurate representation of the dynamic and steady state behavior of the fuel cell under different three‐phase load conditions.

Stability of a boost converter fed from photovoltaic source

The use of photovoltaic panels has become very attractive in distributed power generation systems as they provide a clean and cheap form of energy. There are various converter topologies that are employed in order to connect these sources to the grid but almost always the main component is a DC–DC converter. Most readily available DC–DC converters are designed to work under a (nearly) constant voltage source and therefore their behaviors may not be as expected when connected to a variable current source like a photovoltaic panel. In fact, as it is reported in this paper, the behavior of the overall system (PV panel/DC–DC converter) can be drastically different from the desired one which may have a detrimental effect on the grid. As a case study, this paper explores the dynamics and stability of a boost converter that is fed from a photovoltaic panel under an ohmic load. All major control methods (peak/average current mode control, voltage mode control) are considered. We show with numerical, experimental and analytical results that the converter can behave unpredictably (or chaotically) when the output of the PV varies in response to the variation in solar radiation, and we report for the first time how the domain of stability in the parameter-space and the mechanisms of instability are affected by the characteristics of the photovoltaic source. The dynamical features are explored from circuit theory and nonlinear dynamics points of view. This knowledge will help in compensating for the aforementioned uncertainty and can be used to design converters that remain stable throughout the range of incident solar radiation and load values. The results have been experimentally validated.

A pacemaker powered by an implantable biofuel cell operating under conditions mimicking the human blood circulatory system – battery not included

Biocatalytic electrodes made of buckypaper were modified with PQQ-dependent glucose dehydrogenase on the anode and with laccase on the cathode and were assembled in a flow biofuel cell filled with serum solution mimicking the human blood circulatory system. The biofuel cell generated an open circuitry voltage, Voc, of ca. 470 mV and a short circuitry current, Isc, of ca. 5 mA (a current density of 0.83 mA cm(-2)). The power generated by the implantable biofuel cell was used to activate a pacemaker connected to the cell via a charge pump and a DC-DC converter interface circuit to adjust the voltage produced by the biofuel cell to the value required by the pacemaker. The voltage-current dependencies were analyzed for the biofuel cell connected to an Ohmic load and to the electronic loads composed of the interface circuit, or the power converter, and the pacemaker to study their operation. The correct pacemaker operation was confirmed using a medical device - an implantable loop recorder. Sustainable operation of the pacemaker was achieved with the system closely mimicking human physiological conditions using a single biofuel cell. This first demonstration of the pacemaker activated by the physiologically produced electrical energy shows promise for future electronic implantable medical devices powered by electricity harvested from the human body.

Mathematical modeling and steady-state analysis of a proton-conducting solid oxide fuel cell

This paper presents a study of mathematical modeling and steady-state analysis of a proton-conducting solid oxide fuel cell (SOFC). The SOFC has a SrCe0.95Yb0.05O3−α (SCY) electrolyte and two platinum electrodes. A mathematical model of the SOFC is first developed. The model captures electrochemical processes as well as the transport phenomena. The existence of steady-state multiplicity in the cell under three modes of constant ohmic load, potentiostatic and galvanostatic operations is studied. Simulation results show that a multiple steady-states region exists at low inlet fuel and air temperatures under constant ohmic load and potentiostatic operations. The occurrence of ignition and extinction in the cell solid (electrolyte, anode and cathode) temperature is reported. This result is in agreement with those for oxygen ion-conducting solid oxide fuel cells in which the existence of steady-state multiplicity has been attributed to the dependence of the electrolyte oxygen-ion conductivity on temperature. This work shows that concentration and temperature multiplicities coexist.

Steady-state multiplicity in a solid oxide fuel cell: Practical considerations

Steady-state multiplicity in a solid oxide fuel cell (SOFC) in three modes of operation, constant ohmic external load, potentiostatic and galvanostatic, is studied using a detailed first-principles lumped model. The SOFC model is derived by accounting for heat and mass transfer as well as electrochemical processes taking place inside the fuel cell. Conditions under which the fuel cell exhibits steady state multiplicity are determined. The effects of operating conditions such as convection heat transfer coefficient and inlet fuel and air temperatures and velocities on the steady state multiplicity regions are studied. Depending on the operating conditions, the cell exhibits one or three steady states. For example, it has three steady states: (a) at low external load resistance values in constant ohmic external load operation and (b) at low cell voltage in potentiostatic operation.

Switching frequency determination of a bidirectional AC–DC converter to improve both power factor and efficiency

This paper investigates the possibility to optimize both the efficiency and the power factor of energy conversion systems which include power electronic converters. The values of these important power quantities depend on different parameters, such as the switching technique and the switching frequency of the power electronic elements. Comparing the already known and used switching techniques with each other we can conclude that the sPWM method offers the highest values of the efficiency and the power factor simultaneously. However, when using this technique the basic current harmonic lags as to the voltage in case of ohmic inductive loads resulting in the decrease in the power factor. In the present work a new sPWM switching technique is suggested in order to achieve higher power factor and efficiency compared to the traditional sPWM method. The aforementioned switching technique is a modified sPWM method based on an appropriate shifting of the input current to the grid voltage. But, the following question is raised: Is there a switching frequency value in a given converter topology by which both the efficiency and the power factor gain optimal value? Thus, the main aim of the present paper is determine this specific switching frequency value. This is achieved by using a simple method based on a criterion suggested in this work. In order to demonstrate how this specific frequency value can be found, a bidirectional AC–DC converter composed of a single phase bridge and two switching elements at the DC side is used. This proposed converter topology excels in that the total number of the switching elements is minimal. The study of this subject is carried out through Matlab/Simulink simulation and experimentation.

Highly Uniform Switching of Tantalum Embedded Amorphous Oxide Using Self-Compliance Bipolar Resistive Switching

A new approach of self-compliance bipolar switching of tantalum embedded amorphous oxide for highly reliable and uniform switching was investigated. Based on analytic results, the formation of a metallic tantalum embedded amorphous oxide film was confirmed. Robust characteristics of over cycles with no change at both resistance states under voltage pulses were achieved due to the self-compliance function, which originated from the limitation of current by metallic ohmic load resistance. In addition, an oxygen plasma pulse method for interface oxidation was demonstrated.

Characteristics of field line resonance type geomagnetic pulsations and variations of plasmaspheric plasma density distribution

The period of field line resonance (FLR) type geomagnetic pulsations depends on the length of the field line and on the plasma density in the inner magnetosphere (plasmasphere), where field lines are closed. Here as FLR period, the period belonging to the maximum occurrence frequency of the occurrence frequency spectrum (equivalent resonance curve) of pulsations has been considered. The resonance system may be replaced by an equivalent resonant circuit. The plasma density would correspond to the ohmic load. The plasma in the plasmasphere originates from the ionosphere, thus FLR period, occurrence frequency are also affected by the maximum electron density in the ionosphere. The FLR period has shown an enhancement with increasing F region electron density, while the occurrence frequency indicated diminishing trend (possible damping effect). Thus, the increased plasma density may be the cause of the decreased occurrence of FLR type pulsations in the winter months of solar activity maximum years (winter anomaly).

Axisymmetric multistage cavity resonators

We study theoretically multistage cavity resonators as well as the gyrotrons manufactured on their basis. Coupled cavities with mode conversion and echelette cavities are particular cases of such devices. The possibilities of reducing the diffraction Q-factor of the operating mode of a cavity, with the preservation of its selective properties and significant increase in the gyrotron radiation power, and provision of high efficiency and acceptable heat ohmic load of the cavity at the first harmonic of the gyrofrequency are considered. Achievable parameters of the gyrotrons at the second and third harmonics with typical electron guns and cryomagnetic systems are analyzed.

Design and manufacturing of the ITER ECRH upper launcher mirrors

Four of the 16 ITER upper port plugs will be devoted to electron cyclotron resonance heating (ECRH) in order to control magnetohydrodynamic (MHD) instabilities [1,2]. In order to achieve the stabilisation of the neoclassical tearing modes (NTM) and sawtooth oscillation, a deposition of a very localized and peaked current density profile over a broad poloidal steering range is required. In the present optical configuration eight 2 MW mm-wave beams enter each of the four upper launchers (UL) through waveguides into the vacuum vessel. Each beam line comprises consecutive corrugated waveguide sections with two mitre bends, orientating the poloidal and toroidal directions and three sections of quasi-optical transmission [3]. The beam waist locations and beam shaping properties in free space propagation are defined by two additional mirrors, the first being a static focusing mirror and the second a plane poloidally steerable mirror. Each mirror reflects a group of 4 mm-wave beams. The three types of UL mirrors (mitre bend, focusing and steering) absorb heat generated essentially by three sources: the ohmic loss of the RF beam reflected at the mirror surfaces and the nuclear and thermal radiation coming from the plasma. While the average heat load is within reasonable engineering limits, three elements condition the actual mirror design, the peak ohmic heat load (Gaussian or Bessel type heat deposition profiles), the electromagnetic forces generated in vertical disruption events (VDE), and the ITER cooling water requirements. This paper provides an overview of the different upper port-plug mirror designs and cooling schemes and an outlook on the prototype manufacturing activities and the future test program. The optimized mm-wave layout within the ECH port plugs is also presented.

Numerical simulation of dynamic processes in gyrotrons with low-Q cavities

We study theoretically models of gyrotrons having fixed and self-consistent axial structures of the high-frequency field in the cavity with an actual profile. The limits of applicability of the model with a given field structure are determined during the study of competition of the operating and parasitic modes. It is shown that for the regimes which are optimal in terms of efficiency, the influence of the nonfixed nature of the high-frequency field structure on the starting current, the stability of stationary generation of the operating mode, the efficiency, the output power, and the thermal ohmic load of the cavity is insignificant. As the beam current and the mismatch between the cutoff frequencies of the operating and parasitic modes increase, significant qualitative differences between the models with given and self-consistent axial structures of the high-frequency field are observed.

Theoretical investigation of steady state multiplicities in solid oxide fuel cells*

The nonlinear steady state behaviour of solid oxide fuel cells (SOFCs) is investigated. It is found that the temperature dependence of the electrolyte conductivity has a very strong influence on the occurrence of multiple steady states, instabilities and the formation of hot spots. Two correlations from the literature for the electrolyte conductivity are studied in a lumped model and in a 1D spatially distributed model of a SOFC. The cases of galvanostatic operation, potentiostatic operation, and operation under a constant ohmic load are considered. The lumped model possesses a unique steady state under galvanostatic operation and up to three steady states under potentiostatic operation or under constant load. In the distributed model, three steady states may coexist under galvanostatic operation and up to five under potentiostatic operation.

Development of a Prototype of a 1-MW 105-156-GHz Multifrequency Gyrotron

We present the results of studying the possibility of numerical and experimental optimization of the helical electron beam (HEB) formation system and the processes of mode interaction in the electron beam – cavity system for a prototype of a 1-MW 105–156-GHz step-tunable gyrotron with various operating modes. The system parameters are optimized to achieve the maximum efficiency of the gyrotron with an acceptable ohmic load in the cavity. We also analyze the influence of mode competition and depression of the electron-beam potential as well as the possibility of energy recovery of the collector electron beam. The possibility of optimizing the mode converter system for various operating modes is demonstrated.

A bidirectional, sinusoidal, high-frequency inverter design

A new method for the design of a bidirectional inverter based on the sinusoidal pulse-width modulation principle and the use of a low-cost and lightweight ferrite-core transformer is presented. The inverter is designed for either ohmic or inductive loads. In the case of inductive loads the reactive power is transferred back to the DC input power source using a new active rectifier design. The inverter is controlled by two minimum-time feedback loops, providing relatively low output voltage distortion (less than 2% for DC input higher than 24V) and good load regulation (better than 2%), while the inverter efficiency remains relatively constant (from 80 to 85%) over a wide output power range (75 to 200W) and DC input voltage range (23 to 28V). Theoretical results are experimentally verified using a laboratory prototype.

Damping circuit design for ferroresonance in floating power systems

AbstractPractical countermeasures to prevent the occurrence of ferroresonance in floating systems are compared. Most attention is paid to the design of practical damping circuits. Existing criteria to determine the necessary ohmic load are evaluated and a new mathematical tool based on harmonic balance is presented. As the damping resistor can cause thermal overload of the voltage transformers, the use of a non‐linear damping circuit is advocated in some cases. Therefore, the proposed calculation method is extended to include the effect of non‐linear damping.

Numerical Study of Processes in the Cavity of the 170 GHz Gyrotron for ITER Operating at the TE25.10 Mode

The opportunity of use of a TE25.10 operating mode for CW 170 GHz/1 MW gyrotron for ITER is estimated. Parameters are optimized to achieve maximum efficiency of the gyrotron with an acceptable Ohmic load on the cavity. The influence of unwanted mode conversion at the output of the resonator, mode competition, electron beam potential depression, ion compensation of the space charge and beam energy recovery is investigated.

Voltage monitoring device for characterization of semiconducting and superconducting circuits

The characterization of semiconducting and superconducting devices under extreme temperature or radiation conditions imposes certain restrictions on the connection of the sample to the tester, which may involve elements with electrical properties that are not well controlled. Specifically, insufficient characterization of the potential drop along long cables, contact resistances, or interconnects may render the voltage biasing of the device extremely difficult. In this article, we present a measurement setup that complements commercial equipment and solves the problem for a wide range of ohmic loads, for several types and any length of cable, and for typical cryogenic conditions down to liquid helium temperature. As examples, we test a low-noise GaAs MESFET preamplifier at frequencies of 0–100 kHz and temperatures of 4–300 K and discuss the capability of the device to characterize high Tc superconducting strips.

Comparison of small inverters for grid-independent photovoltaic systems

A test facility was developed and successfully put into operation, enabling the complete and accurate electrical characterization of stand-alone inverters. This paper presents a description of the facility and the measurement method, as well as results from thirty new products (available in Switzerland) in the range 100 to 1000 W(AC) output power and input voltages of either 12 or 24 VDC. With the exception of seven test pieces, all generate a simple rectangular or trapezoidal output voltage, useful for insensitive ohmic loads. The exceptions are the latest sinusoidal inverters, producing grid-quality power, suitable for all applications. Maximum efficiencies in the range of 85 to 95%, for non-sinusoidal types, and 89 to 95% for sinusoidal, were measured, indicating large improvement in performance during the last 15 years. Compared to former times, the specifications have become more precise, but in some cases still promise too much. However, in a few cases the efficiency was even higher than specified. The output voltage at nominal output power nevertheless often was too low. Except for five inverters, the standby losses were fairly high, reducing unnecessarily the battery's operating time. The question as to whether pulsating current withdrawal from the battery storage, caused by sinusoidal inverters, has a negative impact on the battery's lifetime cannot be answered from the tests performed so far. The specific prices of the inverters tested were from 1 to 2 dollars per Watt output power, with a cost reduction of up to a factor of two for sinusoidal inverters over the last 15 years. The results of randomly chosen inverter tests were approved by the Swiss Federal Inspectorate of Heavy Current.

A -90-dB THD rail-to-rail input opamp using a new local charge pump in CMOS

This paper describes the principle and design of a CMOS rail-to-rail input operational amplifier with THD performance of -90 dB which is suited for high-quality audio systems. A new output stage has been used featuring an output suing that extends to either supply rail and is capable of driving a low ohmic load (32 /spl Omega/). The opamp, which is realized in a 0.5-/spl mu/m 3.3-V digital CMOS process, uses a standard two-stage Miller configuration. The rail-to-rail input functionality is achieved with a new area-efficient on-chip charge pump which provides the local supply voltage for the input differential pair. THD levels below -90 dB have not yet been shown with existing rail-to-rail techniques. This rail-to-rail input configuration also behaves independently of the common mode level with respect to transconductance and slewing characteristics.

Numerical simulation of mode interaction in 170 GHz/1 MW gyrotrons for iter

The paper presents an advanced method for and results of calculating main parameters of CW 170 GHz/1 MW gyrotrons operating at the TE28.7 and TE31.8 modes for ITER. Parameters are optimized to achieve maximum efficiency of the gyrotron with an acceptable Ohmic load on the cavity. Numerical modeling of starting up a gyrotron with an optimized cavity and processes of mode interaction are discussed.