高效低污染超临界发电技术

本文介绍了超临界压力火电机组的一些概念以及形成超临界压力火电技术的理论基础,对超临界与亚临界火电机组在许多方面做了相应的比较;同时对超临界压力火电机组的技术特点进行了分析与总结。     

Optimized rapid thermal process for high efficiency silicon solar cells

Rapid thermal processing is opening new possibilities for a low-cost and environmentally safe silicon solar cell production, keeping the process time at high temperature in the order of 1 min, due to enhanced diffusion and oxidation mechanisms. Controlling the surface concentration of the junction is one of the major parameters, in order to obtain suitable front surface recombination velocities. Simultaneous diffusion of phosphorus and aluminum is used to realize emitter and back surface field in a single high-temperature step, with optimized gettering effect. Controlling the mentioned parameters on industrial 1 Ω cm Cz material lead in 17.5% efficient solar cells on a surface of 25 cm². All results are discussed in terms of process temperature, dopant source concentration and effective process time, below 1 min including high heating and cooling rates.     

Extended mechanical efficiency theorems for engines and heat pumps

Prior work by the author established basic theorems relating the mechanical efficiency of an engine to its thermodynamic cycle, external pressure, and the effectiveness of its mechanism. That work treated the elementary single‐workspace reciprocating piston heat engine. This paper extends the analysis to cover more complex engine types and heat pumps, including double‐acting and split‐workspace devices. Theorems are derived which allow best‐possible estimates and broad comparisons of the overall performance of a large variety of thermomechanical machines. Examples from the field of Stirling engines illustrate the application of the main results. Copyright © 2000 John Wiley & Sons, Ltd.     

The efficiency of endoreversible heat engines with heat leak

Finite-time thermodynamics are used for studying the performance of endoreversible heat engines with heat leak. A comprehensive formulation and a general solution methodology, valid for any mode of heat supply or release, are presented. Detailed analyses are conducted for several heat transfer modes and universal analytical and numerical results for the efficiency at maximum power are generated. Many established laws and major conclusions derived in several references are shown to represent very special cases of the new formulation. Furthermore, the nature of the leakage power law is found to deeply affect the efficiency at maximum power. Finally, for no leakage situations, if the heat to the engine is supplied and released via similar heat transfer modes, then the lowest efficiency at maximum power, when the only thermal resistance is between the working fluid and the hot reservoir, is found to be given by 1/n, n being the power of the heat transfer law.     

Optimal regenerator performance in Stirling engines

The key component of a Stirling engine is its regenerative heat exchanger. This device is subject to losses due to dissipation arising from the flow through the regenerator as well as due to imperfect heat transfer between the regenerator material and the gas. The magnitudes of these losses are characterized by the Stanton number St and the Fanning friction factor f, respectively. Using available data for the ratio St/f, results are found for the Carnot efficiency and the power output of the regenerator. They depend on the conductance and on the ratio of pressures at the two sides of the regenerator. Optimum results for efficiency and power output of the regenerator are derived in the limit of zero Mach number. The results are applied to the Stirling engine. The efficiency and the power output of the engine are found for given amplitude of the compression piston. Optimization with respect to regenerator conductance and piston phase angle leads to a maximum possible value of the power output. Under optimal conditions, the Carnot efficiency just below this maximum is close to 100%. Copyright © 2009 John Wiley & Sons, Ltd.     

Solar thermal heat engines for water pumping: An update

Solar thermal-driven heat engines for water pumping have been previously reviewed for some authors in the past century. However, some devices have not been treated as metal hydride-based systems or the pumping subsystems of solar thermal-driven reverse osmosis desalination systems. Following the typical classification given in the previous literature, in this work an update of the solar heat engines for water pumping based in thermodynamic methods (conventional and unconventional) is presented. Besides small remarks about systems previously quoted by other authors, new designs found in the literature are described. In general, the main characteristics of these systems is their low efficiency, low power output and, in the case of unconventional designs, its simplicity. This work in conjunction with previous review papers make up reference point for the knowledge of the use of solar thermal energy for liquid pumping purpose.     

Perspectives of Stirling engines use for distributed generation in Brazil

This work presents an evaluation of the development of Stirling engines and the advantages and the main obstacles against their widespread introduction in energy-generation practices. It also shows how the economic, technical and environmental characteristics presented by these engines support their insertion in the energy sector. An economic and environmental evaluation of this technology aiming at introducing it in the Brazilian energy scenario is also presented. Changes in legislation, financing and technology within the next few years must encourage the implementation of alternative generation technologies that present lower environmental impacts. Also, tendencies and economical studies are presented, trying to find the optimal condition for this technology to be feasible. The option regarding the trading of carbon credits when biomass is used as fuel is analyzed as well.     

Improving thermal efficiency by reducing cooling losses in hydrogen combustion engines

Hydrogen can be readily used in spark ignition engines as a clean alternative to fossil fuels. However, the higher burning velocity and shorter quenching distance of hydrogen compared with hydrocarbons cause a larger heat transfer from the burning gas to the combustion chamber walls. Because of this cooling loss, the thermal efficiency of hydrogen-fueled engines is sometimes lower than that of conventionally fueled engines. Therefore, reducing the cooling loss is a crucial element in improving the thermal efficiency of hydrogen combustion engines. Previous research by the author and others has proposed the direct injection stratified charge as a technique for reducing the cooling loss in hydrogen combustion and shown its effect in reducing cooling loss through experiments in a constant volume combustion vessel. However, it is known that a reduction in cooling loss does not always improve thermal efficiency due to a simultaneous increase in the exhaust heat loss. This paper explains the relation between cooling loss reduction and thermal efficiency improvements by the direct injection stratified charge in hydrogen combustion engines.     

不同热漏对热机功率效率特性的影响

建立了一个包含多种不可逆性的不可逆热机模型,并将热漏分为外热漏和内热漏两种方式。在此基础上求得存在热阻、热漏和内不可逆损失的定常态流不可逆卡诺热机的功率、效率关系。分析了两种热漏方式对热机最优性能的影响,发现内热漏对热机功率效率特性的影响不同于外热漏,而且与摩擦、涡流和非平衡等不可逆效应也不同;内热漏不能归结于外热漏作为整个热机的热漏或合并为除热阻和热漏外的其他不可逆性。分析表明,当有内热漏存在时,一定温比下热机的最佳功率和最佳效率工作状态分别对应不同的面积比。所得结果对热机设计具有一定指导意义。     

Latest concepts for combustion and waste heat recovery systems being considered for hydrogen engines

A more sustainable transportation calls for the use of alternative and renewable fuels, a further increase of the fuel energy conversion efficiency of internal combustion engines as well as the reduction of the thermal engine energy supply by recovering the braking energy. The paper presents two concepts being developed to improve the fuel conversion efficiency of internal combustion engines for transport applications. The first concept works on the combustion evolution to increase the amount of fuel energy transformed in piston work within the cylinder. The second concept works on the waste exhaust and coolant energies to be recovered through a power turbine downstream of the turbocharger turbine on the exhaust line and a steam turbine feed with the steam produced by a boiler/super heater made of the coolant passages and a heat exchanger on the exhaust line. The concepts work with hydrogen (and in this case a water injector is also necessary) as well as lower alkanes (methane, propane, butane). Preliminary simulations show improvement of top fuel conversion efficiencies to above 50% in the high power density operation. The waste heat recovery system also permits faster warm-up during cold start driving cycles.     

Optimal design of plate-and-frame heat exchangers for efficient heat recovery in process industries

The developments in design theory of plate heat exchangers, as a tool to increase heat recovery and efficiency of energy usage, are discussed. The optimal design of a multi-pass plate-and-frame heat exchanger with mixed grouping of plates is considered. The optimizing variables include the number of passes for both streams, the numbers of plates with different corrugation geometries in each pass, and the plate type and size. To estimate the value of the objective function in a space of optimizing variables the mathematical model of a plate heat exchanger is developed. To account for the multi-pass arrangement, the heat exchanger is presented as a number of plate packs with co- and counter-current directions of streams, for which the system of algebraic equations in matrix form is readily obtainable. To account for the thermal and hydraulic performance of channels between plates with different geometrical forms of corrugations, the exponents and coefficients in formulas to calculate the heat transfer coefficients and friction factors are used as model parameters. These parameters are reported for a number of industrially manufactured plates. The described approach is implemented in software for plate heat exchangers calculation.     

Application of the multi-objective optimization method for designing a powered Stirling heat engine: Design with maximized power,thermal efficiency and minimized pressure loss

In the recent years, numerous studies have been done on Stirling cycle and Stirling engine which have been resulted in different output power and engine thermal efficiency analyses. Finite speed thermodynamic analysis is one of the most prominent ways which considers external irreversibilities. In the present study, output power and engine thermal efficiency are optimized and total pressure losses are minimized using NSGA algorithm and finite speed thermodynamic analysis. The results are successfully verified against experimental data.     

Effect of high compression ratio on improving thermal efficiency and NOx formation in jet plume controlled direct-injection near-zero emission hydrogen engines

The Plume Ignition and Combustion Concept (PCC) developed by the authors significantly reduced nitrogen oxide (NOx) emissions in a direct-injection hydrogen engine under high-load operation. With PCC, a rich fuel plume is ignited immediately after completion of injection in the latter half of the compression stroke to reduce NOx formation. Simultaneously, high thermal efficiency was also achieved by mitigating cooling losses through optimization of the jet configuration in the combustion chamber. This basic combustion concept was applied to burn lean mixture in combination with the optimized hydrogen jet configuration and the application of supercharging to recover the power output decline due to the use of a diluted mixture. As a result, a near-zero-emission-level engine has been achieved that simultaneously provides high thermal efficiency, high power output and low NOx emissions at a single-digit ppm level [1]. In this study, a high compression ratio was applied to improve thermal efficiency further by taking advantage of the characteristics of hydrogen fuel, especially its diluted mixture with a high anti-knock property. As a result, NOx emissions at a single-digit ppm level and gross indicated thermal efficiency of 52.5% were achieved while suppressing knocking at a compression ratio of 20:1 by optimizing the excess air ratio and injection timing, and increasing power output by supercharging.     

Numerical model for predicting thermodynamic cycle and thermal efficiency of a beta-type Stirling engine with rhombic-drive mechanism

This study is aimed at development of a numerical model for a beta-type Stirling engine with rhombic-drive mechanism. By taking into account the non-isothermal effects, the effectiveness of the regenerative channel, and the thermal resistance of the heating head, the energy equations for the control volumes in the expansion chamber, the compression chamber, and the regenerative channel can be derived and solved. Meanwhile, a fully developed flow velocity profile in the regenerative channel, in terms of the reciprocating velocity of the displacer and the instantaneous pressure difference between the expansion and the compression chambers, is derived for calculation of the mass flow rate through the regenerative channel. In this manner, the internal irreversibility caused by pressure difference in the two chambers and the viscous shear effects due to the motion of the reciprocating displacer on the fluid flow in the regenerative channel gap are included. Periodic variation of pressures, volumes, temperatures, masses, and heat transfers in the expansion and the compression chambers are predicted. A parametric study of the dependence of the power output and thermal efficiency on the geometrical and physical parameters, involving regenerative gap, distance between two gears, offset distance from the crank to the center of gear, and the heat source temperature, has been performed.     

壳管式相变蓄热器传热效率研究

设计了一套定量测试不同工况下壳管式相变蓄热器传热效率装置。采用壳管式相变储热,石蜡填充入壳管间,管内通入冷、热载流体,模拟吸热放热过程。测试发现:相同入口条件下,单位时间传热量随入口水温增加呈线性增加;管内载流体流量加大有助于提高传热水平,15~60 L/h流量内单位时间传热量增速随流量增加放缓;不同材质传热管单位时间传热量变化并不明显,表明管道热阻在相变蓄热器总热阻中所占份额较小;相同工况下的蓄热过程,热载流体由下向上流动传热形式明显优于由上向下管排形式;尝试在封装相变材料中添加金属网状结构,强化相变材料内部热传导速率,对比发现相同工况下相变材料中添加金属网状结构,可提高10%~15%左右传热量。     

等效热降法在660MW超超临界机组热经济性分析中的应用

基于等效热降理论,计算了某电厂660 MW超超临界机组在100%、75%、50%设计负荷工况点的热经济指标;分析了计算结果,可为热力系统优化运行方式及机组热经济性诊断提供参考。     

Evaluation method for thermal processing of phosphoric acid with waste heat recovery

The conventional two-step thermal processing of phosphoric acid wastes enormous amounts of energy, which causes serious environmental pollution in addition to bad economic performance. A two-step waste heat recovery method using a “phosphorus burning boiler” was developed to reduce energy loss. This paper presents an exergy analysis model based on two new indicators for the “process driving force” and the “energy-saving potential” for a thermal process. Based on the model, an evaluation method was established to indicate the energy utilization ratio, the economic performance and environmental effects of the thermal process. The method was used to analyze retrofitting for the waste heat recovery for thermal processing of phosphoric acid. The results demonstrate that the retrofit is feasible. Furthermore, the analysis was used to present two additional energy-saving retrofits.     

高温热管在小氮肥余热回收中的应用

将高温热管蒸汽发生器应用于小氮肥造气工艺，以取代原普通余热锅炉回收煤气工段的高温余热，解决了合成氨生产工艺中煤气降温的难题，取得了很好的经济效益和社会效益。     

Progress in high performance,low emissions,and exergy recovery in internal combustion engines

This article first gives a brief review of thermal engines designed for terrestrial transportation since the 1900s. We then outline the main developments in the state of the art and knowledge about internal combustion engines, focusing on the increasingly stringent pollution constraints imposed since the 1990s. The general concept of high‐energy performance machines is analyzed from the energy, exergy, and public health point of view and illustrated with typical examples of clean energy production and zero emissions. Whereas the energy analysis revealed high potential of waste heat recovery from both exhaust and cooling system, the exergetic analysis revealed much higher recovery potential from exhaust gases. The exergy content of exhaust gases was observed to be within the range from 10.4% to 20.2% of the fuel energy. The cooling exergy is within the range from 1.2% to 3.4% of the fuel energy. The article concludes with some perspectives for the emergence of an economic model that could be applied to land‐based transport systems in the framework of energy transition by 2030. Copyright © 2016 John Wiley & Sons, Ltd.     

Finite size thermoeconomic optimization for irreversible heat engines

An optimization analysis for an irreversible heat engine has been carried out based on a new thermoeconomic optimization criterion. The thermoeconomical objective function has been taken as the power output per unit total cost. In the analysis, the irreversibility effects due to heat transfer across finite temperature differences, the heat leak loss between the external heat reservoirs and internal dissipation of the working fluid are taken into account. The maximum of the objective function and the corresponding optimal conditions has been derived analytically. The effects of technical and economical parameters on the global and optimal performances have been investigated.