Direct Energy Balance Research Articles

Transition of the Sun to a Regime of High Activity: Implications for the Earth Climate and Role of Atmospheric Chemistry

AbstractIt was recently suggested that the Sun could switch to a high‐activity regime which would lead to a rise of ultraviolet radiation with an amplitude of about four times larger than the amplitude of an average solar activity cycle and a simultaneous drop in total solar irradiance. Here, we applied the SOCOLv3‐MPIOM model with an interacting ocean to simulate the response of chemistry, dynamics, and temperature of Earth's atmosphere to such a change in solar irradiance. We studied the effect of high activity regime on the atmosphere investigating the influence of the chemical and radiative processes on the climate, and chemistry of NOx, HOx, and O3. We find a climate cooling by up to 1K and a substantial increase in stratospheric ozone (up to 14%) and total ozone (up to 8%). To understand the role of the different processes we performed simulations with two sets of forcing accounting separately for the influence on chemical processes and for direct radiation energy balance. Our calculations show that the stratospheric O3 response is almost fully driven by the chemical processes. We also found that the direct radiation processes lead to near‐surface cooling that results in the suppression of the Brewer‐Dobson circulation. This, in turn, leads to the reduction of H2O influx from the low layers of the troposphere and to less intensive transport of ozone from the tropics to the middle latitudes. The surface climate response is dominated by direct radiation influence with only a small contribution from chemical processes.

Application of direct energy balance control in Supercritical Unit Coordinated Control System

The direct energy balance control strategy is analyzed and combined with the characteristics of supercritical units. The direct energy balance strategy is reconstructed. The reconstructed direct energy balance signal is applied to the boiler master control feedforward in the coordinated control, taking a power plant’s coordinated control system as an example, t. Thus, it is applied to the control of supercritical units, which improves the regulation quality of the control system in a steady and dynamic state, and effectively solves the problems of large fluctuation of mainstream pressure, locked increase, and decrease of load command, frequent withdrawal of coordinated control and unqualified AGC index.

Optimization control of a 330 MW drum boiler unit based on DMC algorithm and DEB strategy

This paper firstly established a nonlinear dynamic model of a 330 MW drum boiler unit, which has a clear physical meaning and is conducive to the design of coordinated control system (CCS). On this basis, the boiler energy storage coefficient is calculated, and its relationship with that in direct energy balance (DEB) strategy is analyzed. Secondly, this paper proposes a CCS based on dynamic matrix control (DMC) algorithm and DEB strategy, in which: (1) energy demand and status signals of DEB are taken as the setting point and process value of DMC controller, so as to overcome the internal disturbance of the boiler; (2) a compensation signal is constructed as the feedforward input of DMC controller, so as to overcome the external disturbance of the turbine and realize the timely utilization and supplement of boiler energy. Simulation shows: (1) there is a certain deviation in main steam pressure when the energy demand and status signals are taken as the setting point and process value, but this is an illusion of insufficient energy, and in fact the energy status can already meet the demand; (2) compared with the main steam valve, the constructed compensation signal takes into account more operating parameters of the unit, which greatly improves the accuracy of the compensation. Practical application shows: the grid dispatch order can be well tracked by the unit, the maximum deviation of main steam pressure is ±0.6 MPa in constant pressure mode, and that is ±1.2 MPa in sliding pressure mode, thereby improving the control performance of the unit under high, medium, low, and overall load conditions.

Physics-informed energy-balanced modeling and active disturbance rejection control for circulating fluidized bed units

Circulating fluidized bed (CFB) units, with many advantages such as low emission and wide fuel adaptability, have been extensively applied to the power and thermal supply. To analyze their dynamic characteristics and design a suitable control strategy, a physics-informed model is derived, which is suitable for the direct energy balance (DEB) structure. By dividing the unit into three modules, a 9th-order nonlinear dynamic model is obtained based on energy conservation and mass conservation principles. The model accuracy is verified by the field running data. The nonlinearity, coupling property and high-order dynamics of the coordinated control system (CCS) are analyzed, and then the advantages of the DEB structure for CFB units are derived theoretically. Finally, the control performance of the DEB structure with proportional–integral–derivative control and active disturbance rejection control (ADRC) is compared based on the field running data. This physics-informed model can offer a solid foundation for a control strategy design under the DEB structure. The advantages of ADRC are validated quantitatively, which shows a promising future in power units.

Improving the CCS performance of coal-fired drum boiler units base on PEB and DEB strategies

This paper proposes a coordinated control system (CCS) for a coal-fired drum boiler unit based on precise energy balance (PEB) and direct energy balance (DEB) strategies. Firstly, considering the boiler energy storage is extremely complex, this paper proposes a drum boiler model with clear physical meaning. Based on the model, the boiler energy storage is further tested and analyzed. Secondly, considering several key parameters, a PEB strategy for feedforward control is proposed. Combined with the DEB strategy and load command (LDC) reconstruction, the CCS of the unit is designed. Simulation results show that the PEB strategy can achieve precise feedforward control, the LDC reconstruction can greatly improve the evaluation indicators of the unit load, and the CCS can adapt well to changes in coal quality and boiler energy storage. Application results show that the unit load can track its set point well under small and large ranges of load swing. The ramp rate is up to 13.2MW/min, the load accuracy is less than 1.2MW/min, the response time is less than 22s, and the comprehensive indicator is up to 4.35. Therefore, the CCS can be highly adapted to the dispatching demand of power grid. In addition, the boiler combustion is stable, the max deviations of steam pressure and temperature are 0.7MPa and 6.4°C, respectively.

Reduction in Energy Requirement and CO2 Emission for Microalgae Oil Production Using Wastewater

A comparative evaluation of energy requirement and CO2 emission was performed for native polyculture microalgae oil production in a wastewater treatment plant (WWTP). The wastewater provided nutrients for algae growth. Datasets of microalgae oil production and their details were collected from the Minamisoma pilot plant. Environmental impact estimation from direct energy and material balance was analyzed using SimaPro® v8.0.4. in two scenarios: existing and algal scenarios. In the existing scenario, CO2 emission sources were from wastewater treatment, sludge treatment, and import of crude oil. In the algal scenario, CO2 emission with microalgae production was considered using wastewater treatment, CO2 absorption from growing algae, and hydrothermal liquefaction (HTL) for extraction, along with the exclusion of exhausted CO2 emission for growing algae and use of discharged heat for HTL. In these two scenarios, 1 m3 of wastewater was treated, and 2.17 MJ higher heating value (HHV) output was obtained. Consequently, 2.76 kg-CO2 eq/m3-wastewater in the existing scenario and 1.59 kg-CO2 eq/m3-wastewater in the algal scenario were calculated. In the HTL process, 21.5 MJ/m3-wastewater of the discharged heat energy was required in the algal scenario. Hence, the efficiency of the biocrude production system will surpass those of the WWTP and imported crude oil.

Control-oriented modelling and investigation on quick load change control of subcritical circulating fluidized bed unit

The flexibility enhancement of circulating fluidized bed (CFB) power plant contributes to both power grid stability and renewable energy accommodation. The considerable energy storage makes it possible for CFB units to perform quick load change, while there are many CFB units cannot meet the basic requirement of load change speed from power grid. The study aims at realizing the quick load change control of subcritical CFB unit. With reasonable assumptions, a mechanism-based boiler-turbine system model of subcritical CFB unit is established to design control strategy. Through the analysis on energy storage utilization, a control strategy based on dynamic matrix control, which can overcome the large delay and inertia of CFB unit, is designed to realize the quick load change control. The simulation results show that the proposed control strategy can shorten the response time of the main steam pressure and accelerate the load change speed, while the traditional direct energy balance control strategy is unsuitable for quick load change control. Finally, a case study on the 300 MW subcritical CFB unit reveals that the load change speed can be raised to 3% of rated power output per minute and it almost takes no impact on operational safety.

Modified DEBA for determining size dependent shear fracture energy of laminates

It has been argued that fracture energy of composite laminates depends on their thickness and number of layers. In this paper a modified direct energy balance approach (DEBA) has been developed to evaluate the mode-II shear fracture energy for E-glass/Epoxy laminates from finite element model at an arbitrary thickness. This approach considers friction and damage/plasticity deformations using cohesive zone modeling (CZM) and nonlinear finite element modeling. The presence of compressive stress and resulting friction was argued to be a possible cause for the thickness dependency of fracture energy. In the finite element modeling, CZM formulation has been developed with bilinear cohesive constitutive law combined with friction consideration. Also ply element have been developed with shear plastic damage model. Modified direct energy balance approach has been proposed for estimation of mode-II shear fracture energy. Experiments were performed on laminates of glass epoxy specimens for characterization of material parameters and determination of mode-II fracture energies for different thicknesses. Effect of laminate thickness on fracture energy of transverse crack tension (TCT) and end notched flexure (ENF) specimens has been numerically studied and comparison with experimental results has been made. It is shown that the developed numerical approach is capable of estimating increase in fracture energy due to size effect.

Direct energy balance based active disturbance rejection control for coal-fired power plant

The conventional direct energy balance (DEB) based PI control can fulfill the fundamental tracking requirements of the coal-fired power plant. However, it is challenging to deal with the cases when the coal quality variation is present. To this end, this paper introduces the active disturbance rejection control (ADRC) to the DEB structure, where the coal quality variation is deemed as a kind of unknown disturbance that can be estimated and mitigated promptly. Firstly, the nonlinearity of a recent power plant model is analyzed based on the gap metric, which provides guidance on how to set the pressure set-point in line with the power demand. Secondly, the approximate decoupling effect of the DEB structure is analyzed based on the relative gain analysis in frequency domain. Finally, the synthesis of the DEB based ADRC control system is carried out based on multi-objective optimization. The optimized ADRC results show that the integrated absolute error (IAE) indices of the tracking performances in both loops can be simultaneously improved, in comparison with the DEB based PI control and H∞ control system. The regulation performance in the presence of the coal quality variation is significantly improved under the ADRC control scheme. Moreover, the robustness of the proposed strategy is shown comparable with the H∞ control.

Control-oriented modeling and analysis of direct energy balance in coal-fired boiler-turbine unit

Direct energy balance (DEB) coordinated control scheme is widely used by field engineers in coal-fired power plants while attracting little attention in the academic community. This paper aims to derive a mathematical model that is suitable for DEB research. To balance the model’s fidelity and simplicity, the power plant is divided into three transformation modules and, using conservation laws, a dynamic model is developed to describe each module. Within reasonable assumptions, numerous module equations are combined to yield a 6th-order nonlinear model. Time constants of the model are identified based on Pareto optimization. Model accuracy is confirmed using field measurements from a 300 MW coal-fired power plant. Based on the linearized model, the merits of the DEB control structure are analyzed. It is confirmed that the DEB control is sufficient to fulfill the fundamental goals of power plant regulation. An illustration of performance improvement is given by introducing gain scheduling techniques to the DEB structure. The proposed model can provide groundwork for future development of advanced control algorithms under the DEB structure.

Stochastic response of a parametrically excited vibro-impact system with a nonzero offset constraint

In this paper, a recently proposed solution procedure is extended in a straightforward manner to obtain the probability density function (PDF) of the stochastic response of a vibro-impact Duffing system with a nonzero offset constraint. The Duffing system is simultaneously excited by external and parametric Gaussian white noises and it undergoes repetitive instantaneous impacts against the constraint. First, in terms of the Zhuravlev non-smooth coordinate transformation, the original equation of motion and the impact condition are combined into a new equation without any constraints by adding an additional impulsive damping term. Second, the PDF of the new system is governed by the associated Fokker–Planck equation which is solved by the exponential-polynomial closure method. Last, the PDF of the original vibro-impact Duffing system is formulated in terms of the methodology on seeking the PDF distribution of a function of random variables. In order to evaluate the effectiveness of the proposed solution procedure, four illustrative examples are studied considering different values of the parameters, namely nonzero offset, nonlinear stiffness and parametric excitation intensity. A comparison with the direct energy balance method is also presented. Comparison with the simulation result shows that the proposed solution procedure can provide a satisfactory PDF solution, even in the tail region for the examined examples. In addition, the nonzero offset constraint significantly affects the PDF distribution of the response.

Direct Energy Balance of Anaerobic Digestion (ad) Toward Sustainability

Currently, the biogas production from organic waste (OW) refuses through AD has been and continues to be one of the most widely used processes for energy production. Usually, AD is carried out in thermophilic (TC) or mesophilic conditions (MC); some authors claim that it is far better to work in TC than MC, others explain that there are no major differences, while some others affirm that the MC has highest performance. Even if each in of single case, the statement is true, it is necessary nowadays to have a valid and objective criterion in order to compare different studies and mainly to assess the energetic sustainability of AD. To this purpose, in this paper, an Energy Sustainability Index (ESI) parameter is candidate. The ESI is the ratio between the energy obtained under form of H2 and/or CH4, and that spent as direct energy to heat the fermenting broth at the working temperature and when present, the energy spent as heat in the broth pretreatment. In Author's opinion this fact is the first check toward sustainability of AD technology as mean to produce energy. Only in the case of ESI > 1, it is possible to consider others energy spent to produce the energy, in fact, for ESI 1. The ESI here proposed represents the first step of a more detailed energy sustainability evaluation procedure, performed using a LCA (life cycle assessment) approach

DEB-oriented Modelling and Control of Coal-Fired Power Plant

Direct-energy-balance (DEB) based coordinated control strategy is critical in achieving good load following and steam pressure stability in modern power plant. Dealing with the adverse impact of perturbation in coal quality, however, is still a challenge to be resolved. In this paper, a 300MW drum-boiler unit model is first established and verified by experimental data, for the purpose of DEB and calorific value flexibility study. Unlike conventional measurement correction methods, the heat value variation is considered here as a part of the internal disturbance, which is estimated and compensated in an improved active disturbance rejection control (ADRC) structure. The proposed control strategy brings a novel design concept for a critical problem of heat variation in any coal-fired power plant. The simulation results demonstrate that the dynamic performance of main steam pressure can be significantly improved.

EU Emission Trading Related CO2 Monitoring in Power Plants

European Commission has set new regulations for the third Emission Trading System period (2013-2020) that increased requirements for risk assessment, uncertainty estimation and continuous accuracy surveillance for CO2 monitoring system. The objective of this paper is to describe and discuss three independent methods to determine CO2 emissions in power plants that fulfill the new requirements. The presented methods are standard method, direct measurement method and energy balance method. The methods themselves, the required measurements and their properties are discussed. The methods are demonstrated in a 500 MWth pulverized coal fired CHP power plant and the results are discussed.

Validation of mid-infrared spectrometry in milk for predicting body energy status in Holstein-Friesian cows

Cow energy balance is known to be associated with cow health and fertility; therefore, routine access to data on energy balance can be useful in both management and breeding decisions to improve cow performance. The objective of this study was to determine if individual cow milk mid-infrared spectra (MIR) could be useful to predict cow energy balance across contrasting production systems. Direct energy balance was calculated as the differential between energy intake and energy output in milk and maintenance (maintenance was predicted using body weight). Body energy content was calculated from (change in) body weight and body condition score. Following editing, 2,992 morning, 2,742 midday, and 2,989 evening milk MIR records from 564 lactations on 337 Scottish cows, managed in a confinement system on 1 of 2 diets, were available. An additional 844 morning and 820 evening milk spectral records from 338 lactations on 244 Irish cows offered a predominantly grazed grass diet were also available. Equations were developed to predict body energy status using the milk spectral data and milk yield as predictor variables. Several different approaches were used to test the robustness of the equations calibrated in one data set and validated in another. The analyses clearly showed that the variation in the validation data set must be represented in the calibration data set. The accuracy (i.e., square root of the coefficient of multiple determinations) of predicting, from MIR, direct energy balance, body energy content, and energy intake was 0.47 to 0.69, 0.51 to 0.56, and 0.76 to 0.80, respectively. This highlights the ability of milk MIR to predict body energy balance, energy content, and energy intake with reasonable accuracy. Very high accuracy, however, was not expected, given the likely random errors in the calculation of these energy status traits using field data.

Influences of Friction, Geometric Nonlinearities, and Fixture Compliance on Experimentally Observed Toughnesses from Three and Four-point Bend End-notched Flexure Tests

Three and four-point bend end-notched flexure tests, comprising a number of different test geometries, are performed on two different graphite/epoxy composites, and the toughnesses are obtained by a compliance calibration method of data reduction. The coefficient of friction along the crack plane and the flexural modulus of each material are then determined experimentally and used with nonlinear finite-element analyses to simulate these same test configurations. By using the mean experimentally observed critical load, these simulations are used to obtain the materials' toughnesses by three different methods. The first uses a previously developed `direct energy balance approach,' which is assumed to produce the `true' toughness. The second is by a simulated compliance calibration procedure, which is used to obtain the perceived toughness for an infinitely stiff fixture. In the third approach, experimentally determined fixture compliances, as a function of the test geometry, specimen, and crack length, are used along with the simulated compliance calibration procedure to obtain a perceived toughness that accurately accounts for the effects of friction, geometric nonlinearities, and fixture compliance. The perceived toughnesses as obtained by these simulations are shown to accurately recreate the perceived values of toughness that are obtained from the physical tests. Moreover, the finite-element simulations indicate that the true toughness values are essentially constant for a given material, and that the three-point bend end-notched flexure test will provide perceived toughnesses that correspond quite closely to the true value, whereas the four-point bend test often will not. For these reasons, the three-point bend configuration was found to be the more preferable of the two tests.

Numerical evaluation of the effects of friction and geometric nonlinearities on the energy release rate in three- and four-point bend end-notched flexure tests

Nonlinear finite element analyses are used to examine the effects of friction and geometric nonlinearities on the energy release rate in three- and four-point bend end-notched flexure tests. Energy release rates are first determined by a recently developed direct energy balance approach. It is shown that the finite diameter loading rollers that are typically used in practical test set-ups cause both tests to be inherently nonlinear. The effect of these nonlinearities on the energy release rate is shown to be larger in the four point than the three point test and to increase with increasing roller diameter, increasing coefficient of friction along the crack plane, and decreasing supporting span length. For the four point test, the effect of these nonlinearities is also shown to increase with increasing ratio of inner to outer span length. Next, energy release rates at the onset of crack advance are determined by a simulated compliance calibration technique. This “perceived toughness” is compared with predictions of the “true toughness” given by the direct energy balance approach at the same load. It is shown that perceived toughnesses from this simulated compliance calibration procedure are larger than previously reported results that were obtained in a similar fashion using linear theory. In addition, the perceived toughness is shown to strongly depend upon the range used for fitting the load versus deflection data to obtain compliance. These findings are used to make some general recommendations regarding use of the two test methods and their associated data reduction techniques.

A Direct Energy Balance Approach for Determining Energy Release Rates in Three and Four Point Bend End Notched Flexure Tests

A direct energy balance approach has been developed and used to determine energy release rates in three and four point bend end notched flexure tests. This study was performed in the context of the larger goal of understanding the wide variation in mode II toughnesses that have been obtained by the two tests when used on the same material. The primary motivation for developing the direct energy balance approach was to fully account for the effects of friction, large deformations, and other geometric nonlinearities that occur during these tests. The direct energy balance approach simulates crack advance as it occurs in physical testing. Most significantly, this approach accounts for frictional dissipation that occurs during crack advance, which is an effect that has been neglected in previous analyses of these tests. The direct energy balance approach is used to show that, for most cases of practical interest, the virtual crack closure technique is quite accurate, and predictions by this latter approach are only in error when moderately large geometric nonlinearities occur prior to crack advance. Based on these results, a “cut-off value,” expressed in terms of the maximum slope in the specimen as predicted by classical beam theory, is suggested for the upper limit of applicability of the virtual crack closure technique.

Effects of Friction, Geometry, and Fixture Compliance on the Perceived Toughness from Three-and Four-Point Bend End-Notched Flexure Tests

Linear and nonlinear finite element analyses are used to examine the effects of friction, test geometry, and fixture compliance on the perceived toughness as obtained from threeand four-point bend end-notched flexure tests. To this end, a newly developed ‘direct energy balance approach’ is used to obtain the ‘true’ energy release rate for any given specimen, test geometry, and coefficient of friction. Finite element analyses are also used in a simulated compliance calibration technique, which is combined with experimental results from fixture compliance tests to obtain a perceived toughness, i.e., the value that would be obtained by experiment. By varying the different parameters, the individual and combined effects of friction, test geometry, and fixture compliance on the ratio of the perceived to true toughness is obtained. The approach is applied to two graphite/epoxy materials for which toughnesses by the three(3ENF) and four-point bend end-notched flexure (4ENF) tests are obtained experimentally for a range of geometries. These experiments produced larger perceived mode II toughnesses, GIIc, by the 4ENF than the 3ENF test, and GIIc values from the 4ENF test were observed to decrease with increasing outer span length. The finite element simulations were shown to accurately recreate the perceived values of GIIc obtained from these tests. Moreover, the finite element simulations indicate that the true toughness values are essentially constant for a given material. These findings are used to make some general recommendations for choosing 3ENF and 4ENF specimen and test geometries, as well as to discuss the relative advantages and disadvantages of the 3ENF and 4ENF test methods.

Nonlinear Coordinated Control of Drum Boiler Power Unit Based on Feedback Linearization

This work analyzes the feasibility of applying feedback linearization technique to the nonlinear control of superheater steam pressure and power output of a boiler-turbine generating unit. A nonlinear coordinated controller has been designed and evaluated through computer simulation using a well-established and validated nonlinear boiler-turbine model. The simulation results are presented to illustrate the performance of a nonlinear coordinated control system and compare the proposed strategy with the conventional direct energy balance control strategy. The simulation results show that improvement can be achieved in control with a nonlinear control system. The real benefits obtained include improved dispatch rate, tighter regulation of steam pressure, and reduced maintenance over a wide range of operating conditions, and these improvements in control can be maintained for large changes in operating conditions.