Multicriteria Synthesis of Trigeneration Systems Assisted With Renewable Energy Sources and Thermal Energy Storage

The increasing world energy demand as a result of society development brings forth a growing environmental concern. The use of high-efficiency alternative systems is becoming progressively more interesting due to economic reasons and regional incentives. The issue of finding the best configuration that minimizes total annual cost is not enough anymore, as the environmental concern has become one of the objectives in the synthesis of energy systems. The minimization of costs is often contradictory to the minimization of environmental impact. Multi-objective optimization tackles the conflicting objectives issue by providing a set of trade-off solutions, or Pareto solutions, that can be examined by the decision maker in order to choose the best configuration for the given scenario. The present work proposes a mixed integer linear programming model for the synthesis of a trigeneration system that must attend the electricity, heat, and cooling demands of a multifamily building complex in Zaragoza, Spain. The objective functions to be minimized are the overall annual costs and the overall annual CO2 emissions, considering investment, maintenance and operation costs. As a first approach, the single-objective configurations for each objective function are evaluated. Then, the Pareto frontier is obtained for the minimization of total annual costs and total annual CO2 emissions, allowing to obtain the best trade-off configuration, which brings results close to the optimal single objectives. It is worth mentioning that the treatment of the energy prices was simplified in order to keep on the same level of detail as energy CO2 emissions, which are given only on an annual basis. On the other hand, the optimization model developed can be further complicated in order to consider more complex situations.

Experimental and Numerical Study of Cold Gas-Solid Flow Regimes in a Fluidized Bed Gasifier

The objective of this work is to analyze the dynamics and regimes of cold gas-solid flow in a biomass gasifier that is built at North Carolina Agricultural and Technical State University and to identify its corresponding ranges of operating conditions. The value of the minimum fluidization velocity Umf ≈ 8 cm/s has been found experimentally in a series of measurements of a pressure drop in the fluidized bed filled with Gledart type-B silica sand for the range of superficial gas velocities between 0 and 40 cm/s. To complement the experimental results, a set of three-dimensional numerical simulations of the isothermal gas-solid flow based on Eulerian-Eulerian approach has been performed. The analysis of the fluidization characteristics such as axial void fraction distributions has allowed us to evaluate the dependence of the bed expansion ratios from the flow superficial velocity. Good agreement between experimental and numerical results for the considered operating conditions of the gasifier has been observed.

Techno-Economic Assessment of Utilizing Wind Energy for Hydrogen Production Through Electrolysis

Hydrogen as a clean alternative energy carrier for the future is required to be produced through environmentally friendly approaches. Use of renewables such as wind energy for hydrogen production is an appealing way to securely sustain the worldwide trade energy systems. In this approach, wind turbines provide the electricity required for the electrolysis process to split the water into hydrogen and oxygen. The generated hydrogen can then be stored and utilized later for electricity generation via either a fuel cell or an internal combustion engine that turn a generator. In this study, techno-economic evaluation of hydrogen production by electrolysis using wind power investigated in a windy location, named Binaloud, located in north-east of Iran. Development of different large scale wind turbines with different rated capacity is evaluated in all selected locations. Moreover, different capacities of electrolytic for large scale hydrogen production is evaluated. Hydrogen production through wind energy can reduce the usage of unsustainable, financially unstable, and polluting fossil fuels that are becoming a major issue in large cities of Iran.

Exergoenvironmental Evaluation for a Coal-Fired Power Plant of Near-Zero Air Pollutant Emission

Owing to the growing environmental concerns, super-critical and ultra-supercritical coal-fired power plants dominate the electricity generation with the demand of near-zero air pollutant emission in China. Therefore, it is highly expected to assess the environmental impact and optimize the design at global and local levels. Exergoenvironmental analysis is a valid approach to investigate the formation of environmental impacts (EIs) associated with energy conversion systems at the component level. It generates information crucial for designing systems with a lower overall environmental impact, based on life cycle assessment (LCA) and exergy analysis. A 600 MW supercritical coal-fired system with and without dust, SO2 and NOx mitigation controls was analyzed. Heat transfer in the boiler, condenser (CND), low pressure cylinder (LP), air preheater (APH) show high potential to decrease the environmental impact due to high exergy destructions. The deaerator (DEA), induced draft fan (IDF), forced draft fan (FDF) should be focussed on construction design and manufacturing optimization. Purification units reveal high benefit for reducing EI produced by coal combustion, but there is a large space for the EI saving for it. The specific EI of electricity in China is much greater than European.

A Modified Master Cycle Off-Design Performance and Heat Rate Improvement Optimization

Coal-fired power generation will continue to be the cornerstone of China’s energy sources in the coming decades and advanced ultra-supercritical technology is the future of coal-fired power generation. This paper selects double reheat cycle design for study and incorporates back pressure extraction steam turbine (BEST) into current cycle design, which used to drive boiler feed water pump and feed regenerative heaters. This design prevailed in US in 1960s and gradually was replaced by condensing turbine due to less efficiency benefits at subcritical steam condition. Reinvention of BEST design in current double reheat cycle is an evitable choice, because the efficiency advantage is improved at USC steam condition. BEST configuration incorporated into current double reheat cycle and advanced cycle is developed to compare with other two conventional systems in this study. Thermodynamic simulation at design and off-design condition shows that BEST configuration has an obvious efficiency advantage at design load, but the advantage decreases at partial load. BEST expansion line and reheat pressure is integrated in cycle heat rate optimization. Genetic algorithm is chosen to implement the optimization and exergy analysis method is utilized to evaluate BEST expansion line optimization results. Finally, BEST design limitation and future work is practically concluded.

Prediction Model of Flow-Induced Noise in Large-Scale Centrifugal Pumps Based on BP Neural Network

As one kind of serious environmental problems, flow-induced noise in centrifugal pumps pollutes the working circumstance and deteriorates the performance of pumps, meanwhile, it always changes drastically under various working conditions. Consequently, it is extremely significant to predict flow-induced noise of centrifugal pumps under various working conditions with a practical mathematical model. In this paper, a three-layer back propagation (BP) neural network model is established and the number of input, hidden and output layer node is set as 3, 6 and 1, respectively. To be specific, the flow rate, rotational speed and medium temperature are chosen as input layer, and the corresponding flow-induced noise evaluated by average of total sound pressure level (A_TSPL) as output layer. Furthermore, the tansig function is used to act as transfer function between the input layer and hidden layer, and the purelin function is used between hidden layer and output layer. The trainlm function based on Levenberg-Marquardt algorithm is selected as the training function. By using a large number of sample data, the training of the network model and prediction research are accomplished. The results indicate that good correlation is established among the sample data, and the predictive values show great consistence with simulation ones, of which the average relative error of A_TSPL in process of verification is 0.52%. The precision of the model can satisfy the requirement of relevant research and engineering application.

Reliable Analysis on Fast Valving of Ultra-Supercritical Unit Under Transient Fault Conditions

Fast valving of ultra-supercritical unit has great effects on over-speed prevention, load-shedding control, transient stability analysis of electrical system and other security problems. The purpose of fast valving is to maintain the stability of power system once fault or load shedding of unit occurs in the electric power system. Therefore, it is of great significance to study the reliability of fast valving for ultra-supercritical unit. In this paper, the KU ( short shedding) logic condition of SIEMENS T3000 system is analyzed as the research object of fast valving. The unit can be avoided over speed by monitoring the unit load and fast valving under faulty grid conditions based on the KU control. A series of measures will be taken after KU is triggered, for instance the governing valving will be closed quickly and the DEH (digital electro-hydraulic) control of the steam turbine will be switched to speeding control mode. On the other hand, the unit will return to normal operation if the transient fault of power grid disappears. The key contributions of this thesis include three parts: Firstly, based on the analysis of control characteristics of ultra-supercritical unit and protective logic and triggered conditions of KU function, a novel dynamic model by coupling the fast valving of steam turbine and the transient stability of generator is established by applying the PSCAD software. Then, the dynamic response process of ultra-supercritical unit is simulated and calculated by adopting the coupling dynamic model when KU function is triggered. Also the influence factors and reliability of fast valving are analyzed under transient fault conditions. Finally, two optimized measures by increasing the time delay and the speed of quantitative judgment are put forward to reduce risks and avoid the misoperation of signal distortion which may be caused by the power transmitter under transient fault conditions. The results of this study can not only help to evaluate the reliability of fast valving function scientifically in power grid transient fault, but also guide the technicians to analyze the stability of the power grid.

A 3-D Model Simulation of High Temperature Solar Cavity Receiver

In this paper, a 3-D numerical model is proposed to investigate the capability of generating high operating temperature for a modified solar cavity receiver in large-scale dish Stirling system. The proposed model aims to evaluate the influence of radiation intensity on the cavity receiver performance. The properties of the heat transfer fluid in the pipe and heat transfer losses of the receiver are investigated by varying the direct normal irradiance from 400W/m2 to 1000W/m2. The temperature of heat transfer fluid, as well as the effect of radiation intensity on the heat transfer losses have been critically presented and discussed. The simulation results reveal that the heat transfer fluid temperature and thermal efficiency of the receiver are significantly influenced by different radiation flux. With the increase of radiation intensity, the efficiency of the receiver will firstly increase, then drops after reaching the highest point. The outlet working fluid temperature of the pipe will be increased consistently. The results of the simulations show that the designed cylindrical receiver used in dish Stirling system is capable to achieve the targeted outlet temperature and heat transfer efficiency, with an acceptable pressure drop.

Numerical Study on the Erosion Characteristics of U-Type Bend for Gas Solid Flow

In pulverized coal-fired plant, the U-type bend is commonly used in flue gas and pulverized coal pipe system to due to the constraints of outer space. And gas-solid two-phase flow exists in these pipelines. The erosion of the pipe has significant effect on the safety and reliability of pipelines. In present paper, the erosion characteristics of U-type bend were investigated through CFD (Computational Fluid Dynamics) method. The wear distribution on the pipe wall was obtained. And the particle flow characteristics in U-type bend were analyzed. The influence of inlet velocity, mass loading rate and particle size on the erosion rate was studied as well. Result suggested that the maximum erosion rate increases exponentially with the increase of inlet velocity. And maximum erosion rate increases linearly with the increasing mass loading rate. Increasing particle size can aggravate the wear on the pipe wall.

Characterization of Crack Tip Damage Zone Formation on Alloy 625 During Fatigue Crack Growth at 750°C by Transmission EBSD Method

In order to clarify the mechanism of fatigue crack growth in alloy 625, which is a candidate material for use in advanced ultra supercritical power plants, the crack tip damage zone formation after a crack growth test conducted in high temperature steam was investigated. It was observed that the oxide thickness at the crack tip tended to increase with decreasing cyclic loading frequency. The crack path was a mix of transgranular and intergranular fractures. According to the grain reference orientation deviation (GROD) maps, it was revealed that the density of geometrically necessary dislocations (GNDs) in the matrix along the crack path and ahead of crack tip increased with an increase in the fatigue crack growth rate (FCGR) due to environmental effects. It was observed that (1) mobile dislocations at the crack surface were blocked due to the thick oxide layer, resulting in an increase in the density of GNDs, and (2) an increase in the density of GNDs might induce stress concentration at the crack tip, deformation twinning, and the acceleration of FCGRs.