Increasing Ramp Rate Research Articles

Costs versus revenues of flexibility enhancement techniques for thermal power units in electricity-carbon joint markets

Thermal power units (TPUs) are required to enhance operational flexibility to ensure power grids accommodate the increasing penetration of intermittent renewables. The flexibility enhancement includes increasing peak shaving depth, increasing ramp rate, or both. In electricity-carbon joint markets, increasing peak shaving depth means TPUs can generate less electricity during low-tariff periods while increasing ramp rate means TPUs can respond faster to changes in electricity tariffs. Flexibility enhancement brings additional revenues, but also additional investments. The tradeoff between costs and revenues is important for the construction of sustainable power systems mainly based on renewables, but limited literature models them. To address this gap, variable costs, fixed costs, revenues, and an optimization model for maximum profits are proposed and modeled. Then the economic applicability of different flexibility enhancement aspects is evaluated by comparing the maximum profit increase for a day before and after the flexibility enhancement of TPUs. Furthermore, the profitability of applying different flexibility technologies under the same investments is analyzed, obtaining the optimal flexibility enhancement route. The proposed method is applied to TPUs with different live steam parameters, installed capacities, and reheat stages. Numerical simulations provide detailed economic analysis and optimal flexibility enhancement routes for TPUs in electricity-carbon joint markets.

Propagation-Based Network Partitioning Strategies for Parallel Power System Restoration With Variable Renewable Generation Resources

Grid resiliency is defined as the ability to prepare for and adapt to changing conditions and withstand and recover rapidly from disruptions. Following large-scale disruptions of the power grid such as a complete blackout, parallel power system restoration accelerates the recovery process by allowing for simultaneous restoration in each island through network partitioning, thus enhancing grid resiliency. However, existing network partitioning strategies have not sufficiently considered variable renewable resources yet, and may not be flexible or computationally efficient to accommodate additional requirements from the evolving bulk power grid. To bridge these gaps, this study proposes a propagation-based optimization approach for power grid network partitioning. The requirements of blackstart resources, power balancing, synchrocheck relays for ties lines, and increased ramp rate requirements due to variable renewable resources are modeled in the constraints. Through propagation on the physical connectivity of grid assets, the network partitioning issue is formulated as a mixed integer linear programming (MILP) problem. The linearity ensures highly computational performance and optimality of the solutions from the proposed approach to identify the cutset edge possibilities for bulk power grid applications. Graph reduction strategies are proposed to further improve the computational performance for online or nearly online applications. Case studies based on two IEEE benchmark systems show that the proposed MILP model is able to determine the optimal cutset for network partitioning in approximately 0.28 seconds, and the proposed network reduction strategy is able to further improve the computational performance by approximately 9%.

Physico-chemical assessment of torrefied Eurasian pinecones

BackgroundBiomass pre-treatment is gaining attention as a standalone process to improve the qualitative aspect of the lignocellulosic material. It has been gaining ground in the power station by replacing the coal with the pre-treated biomass. In this context, this paper enlightens the operating condition of carrying out the torrefaction so that the process can be made relatively more effective. The influence of physico-chemical characteristics on the heat of reaction of pyrolysis reactions, mass loss and temperature regimes are evaluated by thermogravimetry of the pre-treated samples of the pinecone; whereas, the structural transformation in the basic constituents is determined via knowing the fractional change in cellulose, hemicellulose and acid-insoluble lignin contents of the pine cone. The thermogravimetric (TGA) and differential thermal analysis (DTA) were performed to determine the physical as well as the thermal behaviour of the thermally processed biomass. The samples had undergone thermal decomposition at heating rates of 5 °C min−1, 10 °C min−1 and 15 °C min−1. Nitrogen gas was used as a purge gas for the pyrolysis of the pre-treated samples. The volumetric rate of 200 ml min−1 was pre-set for the thermal decomposition of the samples at 600 °C; whereas, the selected torrefaction temperature range varied from 210 to 250 °C.ResultsThe heat of reaction for the pre-treated samples was found to vary from 1.04 to 1.52 MJ kg−1; whereas, it was 0.91–1.54 MJ kg−1 for the raw samples. The total annual production cost of processing 3.6 Mg of fuel in a year at a pilot scale was $ 36.72; whereas, the fiscal burden per kilogram of fuel during thermal degradation of the processed fuel was reduced by 0.08–1.5ȼ. The entropy of the system decreased with an increasing ramp rate. The exergetic gain in the system increased by 1–2%. The loss of energy during the energy-intensive processing of the pre-treated fuel was relatively low at a heating rate of 5 °C min−1.ConclusionBy the physico-chemical assessment, it was determined that pinecones required the highest torrefaction temperature and time to provide the upgraded pinecones. It was concluded that the duration of the torrefaction should be at least 15 min for a temperature of 250 °C so that the chemical exergy of the system, energy yield and the energy density of the processed material are qualitatively improved. The volatile and ash contents were noticed to decrease during the torrefaction process. The least fractional change in the volatile content was estimated at 210 °C for a torrefaction time of 15 min; whereas, the ash content was minimum at 210 °C for a torrefaction time of 5 min.

Effects of physiological parameter evolution on the dynamics of tonic-clonic seizures.

The temporal and spectral characteristics of tonic-clonic seizures are investigated using a neural field model of the corticothalamic system in the presence of a temporally varying connection strength between the cerebral cortex and thalamus. Increasing connection strength drives the system into ∼ 10 Hz seizure oscillations once a threshold is passed and a subcritical Hopf bifurcation occurs. In this study, the spectral and temporal characteristics of tonic-clonic seizures are explored as functions of the relevant properties of physiological connection strengths, such as maximum strength, time above threshold, and the ramp rate at which the strength increases or decreases. Analysis shows that the seizure onset time decreases with the maximum connection strength and time above threshold, but increases with the ramp rate. Seizure duration and offset time increase with maximum connection strength, time above threshold, and rate of change. Spectral analysis reveals that the power of nonlinear harmonics and the duration of the oscillations increase as the maximum connection strength and the time above threshold increase. A secondary limit cycle at ∼ 18 Hz, termed a saddle-cycle, is also seen during seizure onset and becomes more prominent and robust with increasing ramp rate. If the time above the threshold is too small, the system does not reach the 10 Hz limit cycle, and only exhibits 18 Hz saddle-cycle oscillations. It is also seen that the time to reach the saturated large amplitude limit-cycle seizure oscillation from both the instability threshold and from the end of the saddle-cycle oscillations is inversely proportional to the square root of the ramp rate.

Integration of compressed air energy storage and gas turbine to improve the ramp rate

Manufacturers are trying to increase ramp rates to improve the operational flexibility of gas turbines. However, higher ramp rates lead to rapid variation in the combustion gas temperature and shorten the life of the turbine. To increase the rate without reducing the life, this study considers the use of a compressed air energy storage (CAES). Injecting pressurized air that is stored in CAES into the combustor of a gas turbine increases the power output of the gas turbine without increasing fuel supply. Therefore, the ramp rate of the turbine can be increased while suppressing the temperature change of the combustion gas. The injection schedule was optimized through a dynamic simulation to increase the ramp rate. A genetic algorithm was used in the optimization of the schedule. The optimized schedule turned out to be a simple form consisting of a linear increase and decrease in the injection flow rate. Furthermore, despite the increased ramp rate, the fluctuation of turbine inlet temperature could be maintained under a safe level due to the optimized compressed air injection.

Investigation of new control strategies for acid gas absorber columns to improve the response rates using dynamic process simulation

Increasing penetration of intermittent renewable sources of electrical energy such as wind or solar into the energy mix places an increasing demand on the flexibility of other power plant, in particular fossil fuel-based units.The suppliers of natural gas combined cycle equipment have been particularly successful in designing plants with increased ramp rates. In a recent review (Todd et al., 2014) EPRI has analysed the potential for using such equipment to improve the ramp rates of IGCC power plant. The review concluded that this was certainly possible, and identified the limiting equipment to be the acid gas removal system. Experience with such units in chemical applications has shown that at faster ramping rates than about 3%/min, the design sulfur specification cannot be maintained and a short-term sulfur breakthrough occurs. In that review it was postulated that this could be attributed to reduced solvent flow in the lower part of the column, while the hold-up required for higher load operation was being built up.Follow up work has been performed at the Technical University of Darmstadt to verify this hypothesis and propose mitigation measures. A typical tray column using a physical solvent was modeled, initially in ASPEN Plus and then in ASPEN Plus Dynamics. The dynamic model was calibrated against typical performance of industrial plant. The initial hypothesis could be verified and further refined. A number of proposals for mitigation measures were investigated and evaluated. The understanding gained by this work will be applicable also to packed columns and chemical solvents, though in the latter case the model will need to be extended to include kinetic effects.This paper describes the simulation work and a control strategy to improve the ramp rate of an acid gas removal system.

Effects of temperature ramp rate during the primary drying process on the properties of amorphous-based lyophilized cake, Part 1: Cake characterization, collapse temperature and drying behavior

In the lyophilization process for injectable pharmaceuticals, the shelf of the lyophilizer is heated to the target temperature at a constant ramp rate during the primary drying process. Although the shelf temperature (Ts) and chamber pressure (Pc) have mainly been investigated to optimize the primary drying process, the impact of the ramp rate on the lyophilized cake properties is poorly understood. We evaluated the relationship between the ramp rate and cake properties using a model formulation containing 10% trehalose as a bulking agent. Elegant lyophilized cakes were obtained when lyophilization was conducted at a fast ramp rate. In contrast, the lyophilized cakes collapsed at slow ramp rate cycles. To identify the cause of collapse, the impact of the ramp rate on the collapse temperature (Tc) was evaluated by light transmission freeze-dry microscopy. The Tc decreased with the decrease in the ramp rate. Lower Tc and higher resistance of the dried matrix in the low sublimation state were hypothesized as the cause of the collapse. Numerous lyophilization runs were executed at different Ts and ramp rates. We confirmed that an increased ramp rate led to successful lyophilization at a higher Ts and that the drying time was significantly reduced.

Gas assisted doubly fed induction generator with ramp rate controller

All the sources are required to supply the scheduled power to the system as directed by the system operator, within a time frame of operation. The fluctuating nature of wind makes supplying the scheduled power a challenging task. To address this, gas turbines are proposed to augment the wind generator to work as buffer owing to their high ramping capability. However, the cost of gas turbine increases with increased ramp rates, whereas, operating efficiency and the life time decreases. Hence, to relieve the buffering gas turbines from higher ramp rate requirements, emulation of inertia of the wind generator is proposed in this paper. The controller is designed to operate in inertia emulation mode whenever there is a retardation of rotor of the wind turbine and the proposed ramp controller will be in action only for first few seconds after the fall in wind speed. While emulating the inertia, the performance of the proposed control philosophy with different ramp rates is investigated in this paper. Guide lines for the selection of the ramp duration and ramp rate for seamless transition from inertia emulation mode to maximum power extraction mode are also suggested in this paper.

A parametric approach for the valuation of power plant flexibility options

Conventional generation units encounter a changing role in modern societies’ energy supply. With increased need for flexible operation, engineers and project managers have to evaluate the benefits of technical improvements. For this purpose, a valuation tool has been developed, comparing economical cornerstones and technical constraints of generation units to European Energy Exchange prices for PHELIX 2014. It enables the user to relate a change in technical parameters to an economic effect and possible revenues. Four different types of conventional power plants are investigated in scenarios with increasing CO2 and fuel prices to determine the impact of different flexibility options. Results show that an increased ramp rate has not the same magnitude of positive economic impact as reduced minimum operation load, based on an observation on a price signal with resolution of fifteen minutes.

Electromagnetic modeling of REBCO high field coils by the H-formulation

In this paper, we employ the anisotropic bulk approximation to successfully implement the electromagnetic modeling of superconducting coils wound with rare-earth-barium-copper-oxide (REBCO) tapes based on the H-formulation, in which the field-dependent critical current density and highly nonlinear characteristic are considered. The total number of turns in the stacks of REBCO pancake coils is up to several thousand. We validate the anisotropic bulk model by comparing the ac loss of a small four-pancake coil between the bulk model and the original model which takes the actual thickness of the superconducting layer into account. Then, the anisotropic bulk model is used to investigate the self-field problem of the REBCO prototype coils of the National High Magnetic Field Laboratory all-superconducting magnet. The field and current density distributions are obtained, and an obvious shielding effect is observed at the top and bottom of the coils. The ac losses in the first and second cycles are calculated. The former is crucial to the design of the cooling system and the latter relates to the routine consumption of the liquid helium. It is found that the ac loss in the first cycle is 2.6 times as large as that in the second cycle. We also study the ac loss dependences on some key parameters (the critical current, n-value and ramp rate of the applied current). It is found that both in the first and second cycles, the ac loss increases with decreasing critical current. Moreover, the influence of the n-value on the ac loss is negligible. In addition, the ac loss decreases logarithmically with increasing ramp rate. However, the average power loss increases linearly with increasing ramp rate. We also compare some analytical estimates with the simulation result for the ac loss of the dual prototype coils. It is found that the results of Bean’s slab model are closer to the simulation result. The presented model is a useful tool to help us understand electromagnetic behavior and ac losses in REBCO high field coils. It also provides a basis to analyze the mechanical characteristics in the coils in the future.

Transient wall pressures in an overexpanded and large area ratio nozzle

Surveys of the fluctuating wall pressure were conducted on a sub-scale parabolic-contour rocket nozzle to infer an understanding of the flow and shock structure pattern during fixed and transient operations of the nozzle. During start-up, the nozzle is highly overexpanded, which results in unsteady wall pressure signatures driven by shock foot unsteadiness. Wall pressure data are first analyzed using spatial Fourier transformations to extract the azimuthal modes during various operating states. A time-frequency analysis of the temporal azimuthal mode coefficients is then used to characterize the time-dependent spectral behavior of the wall pressure signatures during start-up. For both fixed and transient operations of the nozzle, the axisymmetric breathing mode (m = 0) comprises most of the resolved energy. As for the transient operations alone, slight deviations in ramp rate are shown to considerably influence the amount of unsteadiness that the nozzle wall is exposed to, even though the general spectral and temporal features remain similar. In particular, increased ramp rates result in increased wall pressure intensity. Secondly, three major low-frequency events (f ≲ 400 Hz) were observed during start-up and are attributed to: (1) \(\hbox{FSS} \rightarrow \hbox{RSS}\) transition, (2) the passing of the reattachment line from the first separation bubble, and (3) the ‘end-effects regime’. The last of these refers to a condition where a trapped separation bubble opens intermittently to ambient at the nozzle lip.

Trailing-Edge Flap Control of Dynamic Pitching Moment

The effects of upward ramp rate, actuation start time, and duration of a moveable trailing-edge flap on the critical aerodynamic values of an oscillating wing were investigated. The largest improvement in the peak negative pitching moment was obtained with a fast ramp rate and a start time near the mean angle. The largest value of net work coefficient, however, was generated with a delayed start time and a slower ramp rate. The peak lift coefficient decreased with increasing ramp rate and decreasing start time. The inclusion of a short steady-state portion in the flap motion was beneficial. An upward flap deflection initiated slightly after the mean angle during pitch-up, moved to its maximum deflection at a moderate speed, remained steady for a rather short period, and then returned to its initial position, which spanned half the oscillation cycle, was found to provide a best compromise between the various aerodynamic requirements.

Residual stress and structure characteristics in PZT ferroelectric thin films annealed at different ramp rates

Ferroelectric Pb(Zr 0.55Ti 0.45)O 3 (PZT) thin films were deposited on Pt/Ti/SiO 2/Si(100) by radio-frequency-magnetron sputtering at room temperature and then annealed at 600 °C for 30 s with different ramp rates 1 °C/s, 5 °C/s, 10 °C/s, 20 °C/s and 30 °C/s. The crystallographic orientation and residual stress in PZT films were investigated using an X-ray diffraction technique. The preferred orientation changed from (110) to (111) as the ramp rate increased. In residual stress analysis, both ψ splitting and oscillations were observed in the d − sin 2 ψ curves. Triaxial stress analysis indicated the normal stress components to be significant tensile in all films. The normal stress components in the films increased with the increasing ramp rate before the ramp rate of 10 °C/s, but when the ramp rate rose to 20 °C/s, the normal stresses decreased. The mean grain sizes were obtained by Williamson–Hall plots. The same tendency was observed. A simple calculation indicates that the thermal stresses imposed by the substrates are the same in all films. Therefore, the changes of the residual stresses in the films are mainly due to the variation of grain size and the crystallographic orientation caused by the different ramp rates.

Reduction in secondary phases and increased transport i c of (bi,pb)2223/ag tapes sintered with rapid heating rates

In a two-step heat treatment with one intermediate rolling the final transport critical current, (I/sub c/ at 77 K, self field, 1/spl inodot//spl grave/V/cm), of 37 filament (Bi,Pb)2223/Ag tapes increases linearly with the heating ramp rate: 20 A to 40 A, (/spl sim/20,000 Acm/sup -2/), for 20/spl deg/C/hour to 400/spl deg/C/hour. The principle benefit of the rapid ramp rate is likely to come from ramping fast though 800/spl deg/C to 832/spl deg/C in the 1st heat treatment. SEM and XRD analysis on the superconductor-Ag interface show a decrease in the amount of secondary phases, Cu/sub 2/O, Bi/sub 2/(Sr,Ca)/sub 2/Cu/sub 1/O/sub x/, Bi(Sr,Ca)O/sub x/, with increasing ramp rate and critical current.

ＩＴＥＲ‐ＣＳモデル・コイルとＣＳインサート・コイルの実験結果 ＣＳモデル・コイルとＣＳインサート・コイルのパルス励磁試験結果

Pulse charge tests of an ITER Central Solenoid (CS) Model Coil and a CS Insert Coil, hereafter referred to as CSIC, have been carried out in the CS Model Coil experiment, which was conducted by Japan Atomic Energy Research Institute in 2000. Both coils were successfully operated up to 13T with the designed ramp rate of 0.4T/s. Also, ramp rate limitation (RRL) tests were performed for both coils. The RRL test results of CSIC are reported in this paper since the AC loss and critical current evaluation of this coil has been almost completed. The results indicate that CSIC quenches at lower currents than those that were expected to be the critical current, taking into account the temperature rise by the AC losses in the much faster pulse operation than the designed. Moreover, the ratio between the quench and critical currents decreases with increasing ramp rate.

Effect of ramp rate on the C49 to C54 titanium disilicide phase transformation from Ti and Ti(Ta)

The C49 to C54 TiSi2 transformation temperature is shown to be reduced by increasing the ramp rate during rapid thermal processing and this effect is more pronounced for thinner initial Ti and Ti(Ta) films. Experiments were performed on blanket wafers and on wafers that had patterned polycrystalline Si lines with Si3N4 sidewall spacers. Changing the ramp rate caused no change in the transformation temperature for 60 nm blanket Ti films. For blanket Ti films of 25 or 40 nm, however, increasing the ramp rate from 7 to 180 °C/s decreased the transformation temperature by 15 °C. Studies of patterned lines indicate that sheet resistance of narrow lines is reduced by increased ramp rates for both Ti and Ti(Ta) films, especially as the linewidths decrease below 0.4 μm. This improvement is particularly pronounced for the thinnest Ti(Ta) films, which exhibited almost no linewidth effect after being annealed with a ramp rate of 75 °C/s.

The effect of patterns on thermal stress during rapid thermal processing of silicon wafers

The presence of patterns can lead to temperature nonuniformity and undesirable levels of thermal stress in silicon wafers during rapid thermal processing (RTP). Plastic deformation of the wafer can lead to production problems such as photolithography overlay errors and degraded device performance. In this work, the transient temperature fields in patterned wafers are simulated using a detailed finite-element-based reactor transport model coupled with a thin film optics model for predicting the effect of patterns on the wafer radiative properties. The temperature distributions are then used to predict the stress fields in the wafer and the onset of plastic deformation. Results show that pattern-induced temperature nonuniformity can cause plastic deformation during RTP, and that the problem is exacerbated by single-side heating, increased processing temperature, and increased ramp rate. Pattern effects can be mitigated by stepping the die pattern out to the edge of the wafer or by altering the thin film stack on the wafer periphery to make the radiative properties across the wafer more uniform.

Ramp rate instability of multifilamentary superconductors due to longitudinal magnetic field

Ramp rate instability due to an external longitudinal magnetic field of Cu/NbTi multifilamentary superconductor was studied experimentally and theoretically. Quench current was measured when transport current and external magnetic field with transverse and longitudinal component were ramped up simultaneously. At increasing ramp rate, quench current of S-twisted (anticlockwise-twisted) multifilamentary superconductor was degraded in the positive longitudinal magnetic field parallel to the transport current. The calculated stability limit agreed with the experimental quench current within an error of 17%. Furthermore, we numerically estimated the dependence of stability limit on ramp rate and longitudinal magnetic field for a multifilamentary superconductor in practical large scale devices.

Breakdown and Hysteresis in Thin Ferroelectric Polymer Films

AbstractHysteresis loops obtained from sub-micron films of ferroelectric polymers show a maximum polarization which depends on the frequency of the driving field, the thickness of the film and the number of loops previously traversed (fatigue). We speculate that these effects result from changes in domain wall mobility. For films with thickness above 200nm, the hysteresis loops show no significant thickness dependence. This implies that the dimension of regions of field non-uniformity near the electrodes is less than 200 nm.In ferroelectric polymers subject to a uniformly increasing field, the variation of breakdown strength with the field ramp-rate indicates that sample heating has a significant effect on the dielectric strength. At low ramp rates (below 100 MVm−1s−1) the heating in the sample is mainly due to Joule heating and the dielectric strength increases with increased ramp rate. At higher ramp rates the predominant heating mechanism is dipolar dielectric loss and the dielectric strength decreases with increased ramp rate. The two mechanisms combine to produce a maximum in the dielectric strength vs ramp-rate plot.

Transient performance of a prereacted epoxy-impregnated Nb/sub 3/Sn racetrack coil

The superconductor coil is made up of an eight-strand rectangular cable with dimensions of 1.09*2.21 mm. The strands measured 0.57 mm in diameter before cabling, and each contains 19 subelements of 1182 filaments each. The mean filament diameter prior to reaction was 1.2>or approximately=, and the radius to the outermost filaments is 0.20 mm. A diffusion barrier separates the copper stabilizer from the subelement region. The volume fractions are 0.05, and 0.48, and 0.47 for the barrier, stabilizer, and subelement region, respectively. The coil was tested at ramp voltages from 1 to 104 V, corresponding to field ramp rates of 0.05 to 5 T/s. The test yielded a very high current density at low ramp rate and a falloff of the coil current with increasing ramp rate. The conductor, coil design and fabrication, and observed performance are described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>