Variable RC Research Articles

Multichannel Electrochemical Impedance Spectroscopy and Equivalentcircuit Synthesis of a Kw-Scale Vanadium Redox Flow Battery

An original multichannel Electrochemical Impedance Spectroscopy (EIS) system operating at high bias current and suitable for kW-class Vanadium Redox Flow Batteries (VRFBs) is presented. Power and signal connections, whose stray parameters affected measurements, required a careful optimization and calibration in the implementation of the measurement chain. In any cases, electromagnetic interferences set an upper limit to the applied stimulus frequency that limited the set of measurement data at each bias point. The work provides early results obtained with this EIS system, which were used to identify an equivalent circuit of the whole stack in which each cell is represented with a dynamic Thévenin equivalent, i.e. the series of a voltage source, a variable resistor and a variable RC loop. The dependence of the equivalent parameters on the operating conditions has been experimentally analyzed, confirming that only the RC-loop resistance is strongly affected by mass transport. In addition, more complex equivalent circuits have been proposed, i.e. a Randles cell modified with CPE, a Randles cell modified with CPE and Warburg element and a two-loop model of ZARC elements. The inclusion of the constant phase element constitutes a substantial improvement in the fit, Fig. 1. Conversely, the addition of the Warburg element, which aims to model the mass transfer in the electrochemical process, does not produce significant effects for the frequencies at which we have experimental data. To our knowledge, this is the first dynamic equivalent circuit of a whole stack that has been validated against EIS measurements taken on a real industrial-scale VRFB. Building on the measurement methodology here presented, advanced online state of health of industrial flow batteries can be developed and implemented.

Multichannel Electrochemical Impedance Spectroscopy and equivalent circuit synthesis of a large-scale vanadium redox flow battery

An original multichannel Electrochemical Impedance Spectroscopy (EIS) system operating at high bias current and suitable for kW-class Vanadium Redox Flow Batteries (VRFBs) is presented. Power and signal connections, whose stray parameters affected measurements, required a careful optimization and calibration in the implementation of the measurement chain. In any cases, electromagnetic interferences set an upper limit to the applied stimulus frequency that limited the set of measurement data at each bias point. The paper provides early results obtained with this EIS system, which were used to identify an equivalent circuit of the whole stack in which each cell is represented with a dynamic Thévenin equivalent, i.e. the series of a voltage source, a variable resistor and a variable RC loop. The dependence of the equivalent parameters on the operating conditions has been experimentally analyzed, confirming that only the RC-loop resistance is strongly affected by mass transport. To our knowledge, this is the first dynamic equivalent circuit of a whole stack that has been validated against EIS measurements taken on a real industrial-scale VRFB. Building on the measurement methodology here presented, advanced online state of health of industrial flow batteries can be developed and implemented.

On the Dynamics of Evaporative Losses from Penman-Monteith with Fixed and Variable Canopy Resistance During Partial and Complete Maize Canopy

The Penman-Monteith (PM) model is based on a one-dimensional aerodynamic and energy balance approach and provides powerful information and assessment regarding the interrelationships between vegetation surface properties, evaporative losses from the surface, and the microclimatic conditions of the boundary layer. However, some challenges still exist in terms of the practical applicability of the model to estimate actual evapotranspiration (ETc) rates from different vegetation surfaces using a one-step approach. Using field measurements of ETc, leaf area index (LAI), canopy height (h), leaf stomatal resistance (rL), net radiation (Rn), soil heat flux (G), and other microclimatic and plant physiological variables, we made an attempt to delve into and tackle the following topics related to the operational characteristics of the PM model: (1) estimating ETc using the one-step approach of the PM for a non-stressed maize (Zea mays L.) canopy from measured and scaled up canopy resistance (rc) values; (2) comparing PM one-step ETc values (ETc-PM) with variable rc and those estimated using the ASCE PM (ETc-ASCE-PM) two-step approach [i.e., reference ET adjusted with crop coefficients (ETref Kc) with fixed rc] with the Bowen ratio energy balance system-measured ETc (ETc-BREBS) and assessing the diurnal, daily, and seasonal pattern of all ETc values during partial and complete canopy conditions; and (3) investigating the data source impact [i.e., solving the PM using the near-reference weather station-measured microclimatic data (ETc-PM-WS) over the grass surface and using the BREBS-measured microclimatic data (ETc-PM-BREBS) over the maize canopy] on the performance and dynamics of the PM model. The seasonal average minimum canopy resistance (rc-min) was measured as 56 s m-1, whereas the seasonal average rc-max was 109 s m-1. There was a strong correlation [r2 = 0.88, root mean squared difference (RMSD) = 0.11 mm h-1, n = 768] between the ETc-PM-WS and ETc-BREBS. Overall, the ETc-PM-WS overestimated ETc-BREBS by 9%. Overestimation was larger at higher ETc rates (>0.8 mm h-1). The ETc-ASCE-PM with a fixed rc value (50 s m-1) performed similarly to the ETc-PM-WS (r2 = 0.88, RMSD = 0.10 mm h-1). When we solved the PM model using the BREBS-measured climatic data (including measured Rn and G) over the maize canopy (ETc-PM-BREBS), the performance of the PM improved significantly. The estimations were within 2% of the ETc-BREBS with a higher r2 (0.93) and lower RMSD (0.08 mm h-1) than the ETc-PM-WS and ETc-ASCE-PM. We found that when the weather station-measured climatic data were used to solve the PM model, a 7% error can be introduced by using estimated Rn and G relative to using measured Rn and G. Overall, the ETc-PM-WS and ETc-PM-BREBS daily estimates were close to those of ETc-BREBS at a wide range of LAI, with ETc-PM-BREBS performing better than the ETc-PM-WS in most cases. The ETc-PM-WS and ETc-PM-BREBS underestimated during early stages of plant development. The ETc-PM-WS usually overestimated towards the end of the season, with the magnitude of overestimations being greater from the maximum LAI (5.30) to the end of the season (LAI = 3.70). The ETc-ASCE-PM overestimated by approximately 10% in the 1.2 < LAI < 2.7 range and then consistently underestimated by 10% to 20% until LAI reached 4.6. We observed that some of the largest underestimation by the ETc-PM-BREBS occurred in the early season during partial canopy when 1.20 < LAI < 2.00, with underestimations reaching up to 30%. Unlike with the ETc-PM-WS and ETc-ASCE-PM, we did not observe a distinct pattern of over or underestimation by the ETc-PM-BREBS for the LAI range of 1.50 < LAI < 5.30. The PM model successfully tracked the measured ETc throughout the season for a wide range of LAI using scaled up variable rc.

Validation of laser Doppler anemometer measurements of power dissipation in an arbitrary termination

The two-microphone technique is commonly used to measure power dissipation in arbitrary acoustic terminations. This method is well established and provides reliable results when care is taken to avoid problematic microphone configurations (e.g., a pressure node at a microphone location or the microphone spacing being too small in comparison to the wavelength). A laser Doppler anemometer (LDA) technique of measuring power dissipation is discussed in this paper. The LDA method has several disadvantages over microphone methods, including cost of the equipment, the need to use periodic test signals, the need for tracer particles, and the need for a transparent pipe. However, rather than being limited to fixed microphone locations and working under the plane wave assumption, the LDA method allows measurement of multiple components of the velocity field at numerous locations with arbitrary spacing in three dimensions. The purpose of this paper is to validate the LDA method by measuring the power dissipated in a variable RC load connected to one end of the pipe. [Fusco and Ward, J. Acoust. Soc. Am. 91, 2229-2235 (1992)]. The measured result agrees to within 0.8% for the power dissipated in the RC load. [Work supported by the Penn State Graduate Program in Acoustics.]

A simple parameterization of bulk canopy resistance from climatic variables for estimating hourly evapotranspiration

AbstractThis paper examines a model for estimating canopy resistance rc and reference evapotranspiration ETo on an hourly basis. The experimental data refer to grass at two sites in Spain with semiarid and windy conditions in a typical Mediterranean climate. Measured hourly ETo values were obtained over grass during a 4 year period between 1997 and 2000 using a weighing lysimeter (Zaragoza, northeastern Spain) and an eddy covariance system (Córdoba, southern Spain). The present model is based on the Penman–Monteith (PM) approach, but incorporates a variable canopy resistance rc as an empirical function of the square root of a climatic resistance r* that depends on climatic variables. Values for the variable rc were also computed according to two other approaches: with the rc variable as a straight‐line function of r* (Katerji and Perrier, 1983, Agronomie 3(6): 513–521) and as a mechanistic function of weather variables as proposed by Todorovic (1999, Journal of Irrigation and Drainage Engineering, ASCE 125(5): 235–245).In the proposed model, the results show that rc/ra (where ra is the aerodynamic resistance) presents a dependence on the square root of r*/ra, as the best approach with empirically derived global parameters. When estimating hourly ETo values, we compared the performance of the PM equation using those estimated variable rc values with the PM equation as proposed by the Food and Agriculture Organization, with a constant rc = 70 s m−1. The results confirmed the relative robustness of the PM method with constant rc, but also revealed a tendency to underestimate the measured values when ETo is high. Under the semiarid conditions of the two experimental sites, slightly better estimates of ETo were obtained when an estimated variable rc was used. Although the improvement was limited, the best estimates were provided by the Todorovic and the proposed methods. The proposed approach for rc as a function of the square root of r* may be considered as an alternative for modelling rc, since the results suggest that the global coefficients of this locally calibrated relationship might be generalized to other climatic regions. It may also be useful to incorporate the effects of variable canopy resistances into other climatic and hydrological models. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Daily reference evapotranspiration estimates by the Penman-Monteith equation in Southern Italy. Constant vs. variable canopy resistance

¶The performance of the Penman-Monteith (PM) equation to estimate daily reference evapotranspiration (ETO) was investigated by attributing three distinct features to the canopy resistance (rc): (i) rc constant at 70 s m−1 (Allen et al., 1998; FAO Irrigation and Drainage Paper n. 56), (ii) rc variable as linear function of a critical resistance rc, depending on weather variables and empirical parameters relating rc to r* (Katerji and Perrier, 1983; Agronomie, 3[6]: 513–521) and (iii) rc variable as a mechanistic function of weather variables only (Todorovic, 1999; J. Irrig. Drainage Eng., ASCE, 125[5]: 235–245). Daily weather and grass lysimeter data, measured for a period of seven years at Policoro (Southern Italy), were used. The results confirmed the relative robustness of the PM method with constant rc while better estimates were obtained only when variable rc was used. The mechanistic approach of Todorovic (1999) provided the best estimates, while the approach of Katerji and Perrier (1983), with empirically derived parameters, has shown to be not conservative enough to be extended to different locations without calibration.

Single-Layer Evapotranspiration Model with Variable Canopy Resistance

A new approach to modeling canopy resistance is presented as an alternative to the Food and Agricultural Organization of the United Nations Penman-Monteith method with the constant canopy resistance. The evapotranspiration (ET) model is based on the “big-leaf” approach and a variable canopy resistance. The model's input requires standard meteorological data as in the Penman-Monteith combination approach. The model was validated using weather and grass lysimeter data measured on an hourly basis at Davis, Calif., and on a daily basis at Policoro, Southern Italy. ET estimates from the model were compared with the results of ET values obtained by the Food and Agricultural Organization of the United Nations Penman-Monteith approach using the constant canopy resistance rc = 70 s m−1. The results showed a very convincing performance of the model for estimating reference ET on both an hourly and daily basis. This work confirms that the canopy resistance depends on climate, and that a variable rc is recommended for ET models. The proposed model does not introduce any empirical parameter, does not require calibration for the two sites tested or for different time scales, and it is simple enough for direct practical application.

Transient supply clamp with a variable RC time constant

This paper summarizes several basic supply clamping techniques and presents an innovative transient clamp circuit that makes use of some properties of a variable voltage clamp. This circuit varies its RC time constant dependent upon whether or not the integrated circuit (IC) is mounted on a printed circuit (PC) board. If the IC is mounted the time constant of the transient clamp is set very short so it will not interfere in the normal operation of the device. However, if the IC is free standing the RC time constant is set much longer to insure the clamp will stay on long enough to discharge the entire ESD pulse should the need arise. This clamp circuit was used as a component in the electrostatic discharge (ESD) protection network for an analog intermediate frequency (IF) Limiter IC. Human Body Model (HBM) ESD levels increased from less than 300 V to greater than 2000 V due to the addition of this protection network.

Efficient, steady state solution of a time variable RC network, for building thermal analysis

In buildings, variable influences such as time dependent ventilation lead to thermal systems where the time-constant of the building becomes a function of time. This paper introduces an efficient method for obtaining the steady state, periodic thermal response of a building with arbitrary, time dependent ventilation. The method is applicable to a single time-constant thermal model but can be extended to higher order models. A simple, elementary numerical method for integrating the governing differential equation is proposed which does not need Fourier analysis, convolution or even evaluating an exponential, as required by most other methods. The initial value is also obtained explicity. Hence, the usual initial period of integration—to get rid of transients—is not required. By direct comparison of the numerical method with an exact analystical solution in a special case, it is proved that the method is sufficiently accurate, provided the sampling interval is not too large compared to the thermal time-constant of the building. The method is further demonstrated by calculating the interior temperature of a building subjected to forced night cooling.

GHz-Band Monolithic Modem IC's

GHz-band monolithic modem IC's, such as a double balanced mixer IC, a 90° phase shifter IC, and a carrier switch IC, for high-speed QPSK modems have been developed using monolithic lumped constant circuit techniques and advanced silicon bipolar process technology and are discussed in this paper. A double balanced mixer with unbalance-balance converters and a common-mode feedback circuit has achieved an amplitude error of less than 0.2 dB/sub p-p/ and a phase error of Iew than 1.7° p-p at 1 GHz local oscillator frequency. A precise monolithic 90° phase shifter for the 1-GHz band was constructed by using variable RC phase stiifters. An ON/OFF ratio of more than 65 dB for frequencies below 1 GHz has been achieved by employing a carrier switch consisting of a differential stage with a cascode circuit configuration. These IC's are applicable to QPSK modems transmitting baseband signals to the extent of 400 Mb/s at 1 GHz local oscillator frequency.

Single resistance controlled variable frequency RC notch filter

The use of a bridged RC ladder as a single resistance controlled variable frequency notch filter is discussed.

Variable frequency RC oscillator with single resistance control

The Wien bridge oscillator circuit is generalized through the addition of two resistors, and it is shown that an appropriate choice of these leads to single resistance control of the frequency of oscillation over a, theoretically, wide range. The limitations of the scheme are discussed and the range of frequencies obtainable in practice is indicated.

A sine wave generator using a frequency-dependent negative conductance

A realization of a variable frequency RC oscillator, using the concept of frequency-dependent negative conductance, is proposed, The frequency variation is obtained by controlling a single resistance in the network.

High-selectivity variable null RC networks†

Abstract It is shown that, by using two inputs, it is possible to realize high-selectivity variable null RC networks; the null itself can be controlled by varying tile amplitude ratio of the two inputs.

Model for visual luminance discrimination and flicker detection.

A model for vision is proposed. Its basic units are RC stages whose time constants—in three instances—are parametrically controlled. The requirements of compressing the dynamic range of the input and of fitting luminance pulse-detection data suffice to determine the arrangement and parameters of the components. This model accurately predicts the psychophysical results of flicker detection (DeLange characteristics at above 10 Hz), the Ferry–Porter and Weber laws in the ranges where they apply, the effects of light adaptation, and it accounts for individual differences. By considering the variable RC stage as an approximate analog of a synaptic excitatory process which is controlled by inhibition, significant correspondences are observed between the internal connectivity of the model and the neural connectivity of the retina.

Energy-Decay-Rate Timer

A device is described for the measurement and automatic printout of the decay rate of reverberant sound energy or the damped vibration of structures. The design provides for a choice of decay ranges from 10 to 100 dB (in 10-dB steps), the point in the decay at which the timing starts (in 2-dB steps), and the number (up to 99) of decays to be measured and accumulated before printing. The signal to be measured is rectified and smoothed by a variable RC network, and then actuates a Schmidt trigger that starts and stops an electronic counter. The maximum measurable decay rate is better than 2000 dB/sec.

Volume Conductivity of Borosilicate Glass

Through use of circuitry of variable RC time constants it has been shown that the slow exponential rise and decline of the charging current of the glass condenser between metal plates that do not exchange ions with the glass, in the past assumed to be characteristic of the glass, is an instrumental RC-conditioned time constant. The previously unexplained linear decline, sometimes called the ``anomalous'' charging current, in consequence represents the migration of the mobile alkali ions to create a cathode potential fall establishing the steady-state conduction current. While temperature variation of resistance follows the previously assumed pattern, the steady-state current is limited by the rate of recombination of the alkali ions in the space-charge sheath close to the cathode with O− ions created by electrons from the metal electrode. Near the anode, loss of alkali ions through accumulation of O− ion space-charge fields provides the electron current to the anode. From the alkali atom density, measurement of the time constant of the linear decline, measurement of the steady-state current density, and measurement of the activation energy, one can estimate the dissociation energy, the depth of the ionic potential well, the alkali-ion density, and the ionic mobility. The mobility is in fortuitously good agreement with that calculated from logically assumed basic constants. The data yield an ``effective'' ion-electron surface recombination coefficient responsible for steady-state current flow. The theory of the linear current indicates limits to the superposition principle. Current passage, at room temperatures at high current density, on the order of hundreds of hours reduces the alkali-ion concentration near the anode and changes the resistance. Thus, there is achieved a unified self-consistent theory for the bulk conductivity, the constants of which are directly determinable from the measurements, with minimum assumptions of arbitrary nature.

A New Method for the Measurement of Cathode Interface Impedance

A new simple and sensitive bridge method for the measurement of cathode interface impedance is reported in this paper. A tube with an interface impedance under test in this method forms a bridge together with a reference tube with no interface layer and with an external variable RC circuit in its cathode return. The bridge is excited by a set of three sinusoidal signals of 200 kc, 1.1 and 5 Mc. The unknown interface impedance is then measured by simultaneously balancing the bridge for these frequencies. The detector consists of amplifiers tuned to each frequency and a cathode-ray oscilloscope that displays each signal separately. A small wobbling of the transconductance in the reference tube is also provided for simplifying the measurement and overcoming drift of tube characteristics. This bridge has a sensitivity of the order of 0.002 rad as a minimum detectable phase shift, and allows a measurement of interface resistances as low as a few ohms with time constants of 0.01 μsec or more, even in ordinary tubes.