Large Time Constants Research Articles

Leveraging supercapacitors to mitigate limitations and enhance the performance of battery energy storage systems: A simulation and experimental validation

The importance of supercapacitors has grown significantly in recent times due to several key features. These include their superior power density, faster charging and discharging capabilities, eco-friendly nature, and extended lifespans. Battery Energy Storage Systems (BESS), on the other hand, have become a well-established and essential technology in the field of energy storage. However, BESS still faces challenges such as lower power density, limited cycle stability, and potential chemical hazards. Additionally, batteries perform poorly during transient events (sudden changes in power) due to their larger time constants, which can also shorten their lifespan. By strategically combining supercapacitors with BESS, their combined strengths can optimize battery performance. This study explores the role of supercapacitors in enhancing battery performance through both simulation and experimentation. The results obtained experimentally matched with simulated results.

The Trade‐Off between Transconductance and Speed for Vertical Organic Electrochemical Transistors

AbstractThe high transconductance gm of organic electrochemical transistors (OECTs) has received widespread attention and made OECTs a valid candidate for wearable sensor systems. However, the large transconductance is often accompanied by large switching time constants, τ, making the transconductance an imperfect benchmark. For a fair assessment, the ratio of transconductance to switching time constant has to be considered instead of any single parameter in isolation. One approach put forward to optimize OECTs is a vertical design, in which the channel length can be scaled into the sub‐micrometer regime. Here, a new vertical device geometry is proposed, in which the active volume of the mixed conductor is confined to a small cavity between source and drain, yielding excellent performance and reproducibility. It is shown that this approach yields optimized ratios instead of maximized transconductance only. However, this scaling is effective only in a small range of device dimensions and requires careful optimization of the device to not be limited by parasitic effects such as excess volume of the mixed conductor or parasitic series resistances. Overall, the design considerations discussed here provide new guidelines to optimize OECTs, not only for high transconductance but for operation at high frequencies as well.

Dynamic compensation by coupled triple-thermocouples for temperature measurement error of high-temperature gas flow

The gas flow temperature signal simulation system (ATSSS) plays a crucial role in the hardware-in-the-loop simulation of hypersonic aircraft. The accuracy of gas flow temperature measurements is of paramount importance for the ATSSS. This system comprises a high-temperature plasma heater, a gas flow mixing chamber, and thermocouples. The ATSSS exhibits specific characteristics such as high temperatures, rapid temperature changes, and fast flow rates. However, due to the large measurement time constant of high-temperature thermocouples, significant dynamic errors are introduced into the measurement results. To address this issue and enhance the dynamic measurement accuracy of gas flow temperature, this study establishes a thermocouple heat transfer model for the ATSSS. It also analyzes the mechanism behind the generation of dynamic errors in the ATSSS and proposes a three-thermocouple coupled temperature measurement dynamic compensation method (TTCTMDC). This method compensates for errors caused by thermal convection, thermal radiation, and thermal conduction under high-temperature conditions. Simulation results demonstrate that the proposed method can reduce errors by 50% to 80%. In addition, an experimental platform for the ATSSS is constructed, and the TTCTMDC method is employed to compensate for measurement errors. The results indicate that the dynamic measurement error can be reduced to 1/4 to 1/2 of the original value using the TTCTMDC method. This research lays a foundation for the successful development of the ATSSS and provides a novel approach to dynamically measuring high-temperature gas flow, thereby advancing scientific research in the field of measurement.

Emulation of IEEE STD 421.5/Industrial Excitation Systems Using a Micro-Alternator's Exciter

Micro-alternators, having similar parameters as turbo-alternators, play a significant role in the experimental evaluation of power system dynamics. Early thyristor excitation systems of micro-alternators have limited re-configurable options. Three possible control structures, two closed-loop control structures proposed in the literature and one open-loop control structure proposed in this paper, including a time constant regulator to mimic large machine time constants, are evaluated to emulate any IEEE 421.5 standard excitation system model or interface an industrial excitation system controller hardware in the loop (CHIL). It is shown that the communication delay introduced in CHIL does not impact the closed-loop stability within the bandwidth requirements of the excitation systems. One of the closed-loop structures, which is more generalized, and the open-loop structure performances are compared for emulating IEEE AC4C, DC1C, and ST1C excitation systems on a laboratory micro-machine based single machine infinite bus test system with buck exciter and H-bridge chopper (HBC) exciter using EMTP simulations under small and large disturbances. HBC exciter is used in the experimental validation with both the closed-loop and the open-loop control structures. The proposed open-loop structure is easy to implement and performs better than the closed-loop structure.

Bulk-driven CMOS linear transconductance-cell for AC amplifiers with very low cut-off frequency

This manuscript proposes a novel linearized transconductor for AC amplifiers with a very low cut-off frequency. The cell uses an alternative solution that is based on a modified source-degeneration OTA, using two bulk-driven buffers, incorporating bootstrapping techniques to a buffer and a degenerated resistor. In the buffer, the bootstrapping effect is implemented by using a pseudo-resistor and a linear capacitor properly configured, whereas in the resistor the output voltages of the buffers are used with this goal. Thus, the proposed solution allows realizing a very large time constant, avoiding the use of very large size resistors and capacitors, thus saving chip area. The technique is demonstrated in an 0.18μm CMOS technology by designing an OTA operating with a supply voltage of 0.6 V and achieving a low cut-off frequency of around 2Hz with a power consumption of 462 nW.

A study on transient overvoltage caused by HVDC tie blocking in multi-rectifier systems

In a multi-rectifier AC-DC system the blocking of a HVDC tie can cause severe transient overvoltage problems, which may trip off the renewable energy sources, endangering the safe operation of the system. This paper shows that the blocking of a HVDC tie can introduce two types of transient overvoltage. For Type I overvoltage, which is caused by the surplus reactive power injection immediately after the blocking, a simple estimation formula is proposed. Suppression strategies that can decrease the key matrix are examined using a matrix-theoretic approach. The formula is applicable for system integrated with renewable energy sources and it has reasonable accuracy for overvoltage level of interest. To study Type II overvoltage, a reduced-order mathematical model is first derived with singular perturbation method. Then it is demonstrated that this type of overvoltage is due to the increase of transient electromotive force of synchronous machines, and an analytical calculation procedure is suggested. In addition, it is found that the voltage subsystem of a multi-rectifier system is an input–output monotonic system with nice order-preserving properties. Increasing the proportional gain constant of excitation system or choosing generators with large open-circuit transient time constant can monotonically decrease this type of overvoltage. The simulations of an actual large-scale multi-rectifier system in China are described to validate the introduced results.

Study of the method of using an electric starter to stabilize the rotational speed of a gas turbine plant

The article discusses the use of a synchronous motor as an electric starter of a gas turbine plant. A gas turbine plant is used to drive an electric generator in a power plant. It is shown that the same starter can be used in the electric motor mode to stabilize the rotational speed of the gas turbine plant. This is especially important and relevant for fast and large changes in the load of a gas turbine plant. Changing the electrical load of the power plant directly affects the load of the gas turbine plant. Since the gas turbine plant has a large time constant, it is necessary to quickly supply additional power to stabilize the rotational speed of its shaft. It is proposed to use an electric starter for this. The control of the starter by a perturbing signal is studied by means of simulation. Then the scheme of the control system is modified to determine the disturbing effect by calculation. Graphs of simulation results are presented and the results obtained are discussed.

Magnetic behavior and chaining of strontium ferrite-nylon composite above the melting temperature

To better understand Magnetic Field Assisted Additive Manufacturing (MFAAM) the effect of a magnetic field on the orientation and distribution of magnetic particles in a molten magnetic composite was studied. Vibrating Sample Magnetometer (VSM) measurements were made on Sr-ferrite/PA12 fused deposition modeling filaments of different packing fraction (5 and 40 wt. %). The rotation of the sample’s magnetic moment upon application of a field perpendicular to the easy axis was monitored with a biaxial VSM above the PA12’s softening temperature. The observed magnetic moment transients depend on the temperature, the applied alignment field, the packing fraction, and the initial field-anneal procedure. Longer field-anneals result in larger time constants and seem to induce a hurdle that prevents complete alignment at low temperatures and/or for small fields. Results indicate the molten composite is a non-Newtonian fluid that can support a yielding stress. Scanning Electron microscopy (SEM) images taken on field-annealed samples at 230 °C show strong chaining with little PA-12 left between individual Sr-ferrite particles suggesting that direct particle to particle interaction is the reason for the observed non-zero yielding stress. The melt viscosity of the composite increases with the number of thermal cycles above the melting temperature (Tm). Room temperature (RT) torque magnetometry measurements show that magnetic anisotropy depends on the field annealing process through induced shape anisotropy contributions originating from magnetic particle agglomerates.

An approximate line attractor in the hypothalamus encodes an aggressive state

The hypothalamus regulates innate social behaviors, including mating and aggression. These behaviors can be evoked by optogenetic stimulation of specific neuronal subpopulations within MPOA and VMHvl, respectively. Here, we perform dynamical systems modeling of population neuronal activity in these nuclei during social behaviors. In VMHvl, unsupervised analysis identified a dominant dimension of neural activity with a large time constant (>50 s), generating an approximate line attractor in neural state space. Progression of the neural trajectory along this attractor was correlated with an escalation of agonistic behavior, suggesting that it may encode a scalable state of aggressiveness. Consistent with this, individual differences in the magnitude of the integration dimension time constant were strongly correlated with differences in aggressiveness. In contrast, approximate line attractors were not observed in MPOA during mating; instead, neurons with fast dynamics were tuned to specific actions. Thus, different hypothalamic nuclei employ distinct neural population codes to represent similar social behaviors.

Deep reinforcement learning control for non-stationary building energy management

Developing an optimal supervisory control policy for building energy management is a complex problem because the system exhibits non-stationary behaviors, and the target policy needs to evolve with changes in the state transition and reward functions. Non-stationary real-world problems often present a set of challenges: non-stationary changes are difficult to detect; and thermodynamic systems with larger time-constants can create sample-inefficiency problems for learning algorithms. In addition, the system may have to satisfy safety–critical constraints, and, therefore, the policy must be learned offline unless actuation rules are correctly designed.To address these challenges, we propose a data-driven deep reinforcement learning framework. A reinforcement learning supervisory controller is firstly developed and deployed on the building for the heating, ventilation and air conditioning (HVAC) system and monitored for performance degradation by tracking an aggregate metric. When degradation is detected, a relearning loop is triggered. Then, a set of data-driven models of the building behavior is updated with the latest real data. Subsequently, the deployed controller is re-tuned by letting it interact with the model and is then redeployed on the system.Our proposed approach is demonstrated extensively on the standard ASHRAE 5-zone testbed and a real building. It is benchmarked against state-of-the-art algorithms in building supervisory control: Guideline-36, Proximal Policy Optimization, Deep Deterministic Policy Gradient, and Model Predictive Path Integral control. Our approach performs significantly better than the previously mentioned supervisory control strategies and highlights the need for a condition-based offline relearning framework in dynamic systems.

Transactive control of air-conditioning systems in buildings for participation in Singapore’s Demand Response market through load curtailment

Air-conditioning and mechanical ventilation (ACMV) systems account for significant electrical demand within buildings. These systems have large thermal time constants and can be utilized as thermal energy storage. This paper is focused on participation of commercial buildings in Singapore’s Demand Response (DR) programme through curtailment in electricity demand of ACMV systems for financial incentive and power grid benefits. A novel Transactive Energy-based virtual market is proposed for a building where multiple air-conditioning zone agents transact to purchase conditioned-air from central ACMV system. These zone agents consider forecasts of occupancy and outdoor temperatures to submit bids for conditioned-air corresponding to increasing electricity market prices, using a novel dynamic programming approach to utilize the temporal flexibility in cooling demand through pre-cooling. ACMV electricity demand bid is calculated based on net conditioned-air bids by using existing accurate building models in whole-building energy modeling (BEM) platforms corresponding to increasing electricity prices for submission to DR market. Case studies are performed in MATLAB–EnergyPlus co-simulation environment on a benchmark five-zone building model representing a small office building. Results show that the proposed control is effective in providing curtailment of ACMV electricity demand for DR according to real Singapore market price signals, while respecting occupants’ thermal comfort preferences and up to 56% reduction in ACMV electrical power demand is possible for 1 h. Further, revenue analysis shows that DR market participation incentive from 1 h participation would be higher than energy costs for ACMV operation for the entire day, which makes the DR market participation commercially lucrative.

Oxygen Adsorption and Desorption Kinetics in CuO Nanowire Bundle Networks: Implications for MOx-Based Gas Sensors

Copper oxide (CuO) nanostructures have demonstrated a good chemiresistive response for the sensing of various gases. The precise sensing mechanisms of this material and, in particular, the kinetics of the response to gases and the sensor recovery remain much less investigated. For metal oxide (MOx) chemiresistive gas sensors, oxygen adsorption and desorption reactions on a material surface play a major role in the sensor kinetics, as most other gases react with oxygen on the sensor surface in order to provide the sensing signal, and nanoscale materials are known to achieve faster response. Here we investigate the oxygen adsorption and desorption kinetics on CuO nanowire bundle (NWB) networks by analyzing the electrical response and recovery curves of sensing devices exposed to oxygen. The observation of multiple time constants for the response and recovery curves led us to discuss the various potential mechanisms related to surface reaction kinetics and diffusion of oxygen species. By comparing the reaction rate constants and diffusion constants extracted from the obtained time constants, we conclude that the diffusion of oxygen defects into the material might be the main reason for the observed large time constants. Surprisingly, we observe that the sensor devices based on as-deposited materials show 4–16 times faster response and recovery as compared to devices after being annealed, depending on the temperature. By comparing this result with detailed characterization using scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy, we discuss the role of the junctions between nanostructures in the network and of the hydroxylated CuO surface to explain this result. Our investigation on the kinetics of adsorption and desorption of oxygen of CuO NWB provides a direct understanding of the general response of the sensor and will be useful in order to further understand and optimize these materials for the sensing of various gases.

A numerical tool to optimize voltage waveforms for plasma breakdown and early ramp-up in the presence of constraints

Plasma initiation is an important phase in a tokamak discharge. The main objective of magnetic control in this phase is to first obtain sufficiently high electric field and connection length (implying a low poloidal magnetic field) in the breakdown region, and then, once plasma is formed, to force the plasma current to increase, guaranteeing force balance equilibrium. The problem is dominated by the presence of significant eddy currents induced in the passive structures and by the many constraints due to the coil system and power supplies. The present paper shows how, with minor simplifying hypotheses, the magnetic control design problem can be converted into a Quadratic Programming problem with both linear and quadratic constraints that can be solved with an ad hoc iterative algorithm. On the other hand, the full design problem, including the choice of possible Switching Network resistors, becomes nonlinear and an evolutionary Genetic Algorithm solver is proposed. The algorithms are tested on DEMO breakdown and early ramp-up design which is a quite challenging problem because of the large distance between active coils and vacuum chamber, the presence of shielding passive structures with large time constants, and the active constraints on the CS/PF coil system.

Simple and efficient PID algorithm design method for controlling first order systems with a time delay

Many processes in industry are characterized by delay time or by slow aperiodic dynamics called lag behavior. In addition, many plants in the industry are described mathematically by higher order systems that are approximated with the lower order systems, most frequently such processes and systems are described mathematically as first-order-systems-with time-delay (FOSTD), also called first-order-plus-dead-time (FOPDT). The presence of time delays causes degradation and limitation of achieving desired performance, moreover, it can induce instability. In such cases, design of feedback control algorithm becomes difficult and tedious task. The present work suggests an efficient, simple, linear, and easy to apply design expressions for designing continuous PID (proportional-integral-derivative) control algorithm modes to control the behavior of FOPDT systems. The design expressions are intended to overcome negative effects of time delay presence, as well as, to simplify the control algorithm design process and help designer, in easy and simple way, to get system under control with acceptable system stability, medium fastness of response and without or with minimum possible overshoot, oscillation and error. For testing and evaluating the correctness, applicability and efficiency of the derived expressions, MATLAB/Simulink software was applied to develop refined software simulation model that simulates real life values and returns maximum needed numerical and graphical data for assessment process. In addition, various FOSTD systems’ types and forms were used in the simulation model, in particular, systems with small, medium and large time constants, DC gains, and time delay, unstable systems, systems with variable delay. Furthermore, to assess the efficiency of suggested design expressions, the resulted overall system response were compared with resulted responses when two design methods were applied; worldwide known Ziegler Nichols method and MATLAB/Simulink auto-tuned PID block. Analysis of numerical and graphical testing results, show that, The designed control algorithm applying the suggested expressions can, not only simplify the design process, but also, efficient for successful in getting system under control and improving controlling system performance, speeding up response, reduce overshoot, and minimize error, but also stabilize an unstable plants.

RC Filter Design for Wireless Power Transfer: A Fourier Series Approach

In this letter, we study the impact of the low-pass resistor-capacitor (RC) filter on radio frequency (RF) wireless power transfer (WPT). The RC filter influences both the RF bandwidth by removing the harmonics as well as the ripple voltage at the output of the rectifier. In particular, a large (small) RC time constant, reduces (increases) the ripple but decreases (enhances) the direct-current (DC) component. By following a Fourier series approach, we obtain closed-form expressions for the rectifier's output voltage, the RC filter's output as well as the DC voltage. Our analytical framework provides a complete characterization of the RC filter's impact on the WPT performance. We show that this complete and tractable analytical framework is suitable for the proper design of the RC filter in WPT systems.

Transient simulation of a control strategy for solar receivers based on mass flow valves adjustments and heliostats aiming

Solar central receivers have inherently large time constants and time delays caused by their thermal inertia are due to the transport of the heat transfer fluid alongside the panels. This problem is even accentuated if these systems are continuously exposed to direct normal irradiance (DNI) fluctuations. Ensuring an adequate operation of this kind of process implies the adaptation of control strategies. This paper proposes a strategy that couples the temperature control of the outlet fluid while considering the heliostats' aiming points location to maintain the receiver operating below its design limit and simulates its performance under several scenarios. These tests are carried out on a non-steady-state solar plant model that considers its optical and thermal characteristics. The structure of the proposed control system is based on using intermediate flow valves along the heating flow path to reduce time delay. Hence, it renders the control system more responsive to either a DNI variation affecting the solar field or moving its operation from one state to another. The control strategy structure contains feedback and feedforward loops to determine the near-future process response to DNI measurements. It allows adding more information to the loop used to maintain the system's desired steady-state operation. The overall strategy can take the process to a steady-state operation regardless of the hour or day of operation. It produces a low variation of the outlet temperature, with a standard deviation of 3 °C, while the receiver is under a transient DNI and maintains the heliostats aimed toward the receiver without exceeding its heat flux limits. Results indicate that the proposed strategy achieves over 80% improvement based on the integral absolute error (IAE) performance index compared to conventional single-loop temperature control. Besides the simulated numerical results, this research also presents the advantages of considering intermediate mass-flow valves for control purposes of the receiver. This work should be considered a step towards improving multivariable control algorithms for the safe operation of solar central receivers.

Origin of the Kink Effect in AlInN/GaN High Electron-Mobility Transistor

Kink effect is observed in Al0.83In0.17N/GaN high electron mobility transistor by measuring ID-VDS characteristics at a low sweep rate. It is inferred that the kink is induced due to the trapping/detrapping of charge carriers at deep levels present in the GaN buffer in the gate–drain access region. The detrapping of charge carriers from the deep levels is by the hot-electron-assisted mechanism. Two types of traps with activation energies, 0.29 eV (donor-like) and 0.57 eV (acceptor-like) were extracted by temperature-dependent transient drain current analysis. It is concluded that the deep-acceptor-like trap with a large emission time constant is responsible for the kink effect.

A Novel Approach for the Design of Fast-Settling Amplifiers for Biosignal Detection

The most common method used to pick up biomedical signals is through metallic electrodes coupled to the input of high-gain, low-noise amplifiers. Unfortunately, electrodes, amongst other effects, introduce an undesired contact resistance and a contact potential. The contact potential needs to be rejected since it would otherwise cause the saturation of the input stage of the amplifiers, and this is almost always obtained by inserting a simple RC high-pass filter in the input signal path. The contact resistance needs to be estimated to ensure that it does not impair correct measurements. Methods exist for estimating the contact resistance by dynamically modifying the input network configuration, but because of the presence of the input RC filter, long transients are induced any time a switch occurs between different input configurations, so that the measurement time may become unacceptably long. In this paper, we propose a new topology for a DC removal network at the input of the differential signal amplifier that results in an AC filter whose time constant can be continuously changed by means of a control voltage. As such, we can speed up the recovery from transients by setting very short time constants (during the input resistance estimation process) while maintaining the ability to obtain very low cut-in frequencies by setting a much larger time constant during actual measurements. A prototype of the system was built and tested in order to demonstrate the advantage of the approach we propose in terms of reduced measurement time.

Transient Response of a Continuous Glucose Monitor

Blood glucose is indirectly measured with a subcutaneous continuous glucose monitor (CGM). Glucose within the interstitial space is converted into hydrogen peroxide via an enzymatic reaction with glucose oxidase. Electrochemical oxidation of the hydrogen peroxide produces a faradaic current proportional to the hydrogen peroxide concentration. In vitro CGM experiments during transient operation demonstrate a range of time constants in the current response. A transient model adaptation of the steady model from Gao et al. was prepared. [1] The transient responses to potential and glucose steps were investigated experimentally and numerically.The transient current response of an electrochemical system with coupled homogeneous and heterogeneous reactions yields time constants dependent on the charging of the electrode, rates of homogeneous reactions, and mass transfer of the participating species. For nonlinear homogeneous reactions involving species of varying diffusion coefficients, analytic expression of these time constants is nontrivial and more readily obtained numerically.A transient model was discretized in space and time with finite differences and the Crank-Nicolson method. Numerical solutions of the model were obtained with Newman’s BAND algorithm. [2] The model demonstrates the charging current, faradaic current, and concentration profile responses within the sensor. When the model is perturbed with a potential step, the initial response is characterized by a charging current; whereas, larger time constants are due to both the mass transfer to the electrode through diffusion and homogeneous reactions.Steady, transient, and impedance responses of the CGM were experimentally obtained. Steady-state current was measured as a function of glucose concentration which provided an estimate for the permeability of glucose in the sensor. Current response to a glucose step was obtained, and the short-time behavior was fit with a single time constant. An equivalent circuit, fabricated based on the impedance spectra, demonstrated a potential step response similar to the sensor. The influence of dissolved oxygen concentration on the transient response was also investigated.The model was fit to the experimental current responses by adjusting diffusion and partition coefficients, rate and equilibrium constants, and electrolyte resistance and capacitance. The numerical model shows the short time constant is due to charging of the sensor, and the larger time constants are due to diffusion and enzymatic reactions. The large time constants of the current response are significantly larger than those predicted by a simple Gerischer model.[3,4] The presence of both diffusing and non-diffusing species involved in the homogeneous reactions may give rise to current instabilities and time constants.References Gao, M., Harding, M., Kong, R., & Orazem, M. (2018). Mathematical Model for the Electrochemical Impedance Response of a Continuous Glucose Monitor. Electrochimica Acta, 275. 119-132.Newman, J., & Thomas-Alyea, E. (2004). Electrochemical Systems (3rd ed.). NJ: John Wiley & Sons, Hoboken.Gerischer, H. (1951). Wechselstrompolarisation Von Elektroden Mit Einem Potentialbestimenden Schritt Beim Gleichgewichtspotential. Zeitschrift fur Physikalische Chemie, 198. 286-313.Orazem, M., & Tribollet, B. (2017). Electrochemical Impedance Spectroscopy. Hoboken, NJ: John Wiley & Sons. AcknowledgementThe support of Medtronic Diabetes (Northridge, CA), Mike Miller, program monitor, is gratefully acknowledged.

Ultrafast Silicon Nanomembrane Microbolometer for Long-Wavelength Infrared Light Detection.

The microbolometer is the cornerstone device for imaging in the long-wavelength infrared range (LWIR) at room temperature. The state-of-the-art commercial microbolometers usually have a large thermal time constant (TTC) of over 10 ms, limited by their substantial device heat capacity. Moreover, the minimal pixel size of state-of-the-art bolometer is around 10 μm by 10 μm to ensure sufficient power absorption per pixel. Here, we demonstrate an ultrafast silicon nanomembrane microbolometer with a small heat capacity of around 1.9 × 10-11J/K, which allows for its operation at a speed of over 10 kHz, corresponding to a TTC of less than 16 μs. Moreover, a compact diabolo antenna is leveraged for efficient LWIR light absorption, enabling the downscaling of the active area size to 6.2 μm by 6.2 μm. Because of the complementary metal oxide semiconductor (CMOS)-compatible fabrication processes, our demonstration here may lead to a future high-resolution and high-speed LWIR imaging solution.