OPTIMUM OPERATION FOR DIFFERENT DESIGN CONDITIONS OF A SINGLE PRESSURE HEAT RECOVERY STEAM GENERATOR (HRSG) FOR MINIMUM ENTROPY GENERATION
Heat Recovery Steam Generator (HRSG) is the critical link between the “topping” gas and “bottoming” steam power cycle of a combined power plant. Depending on the mass flow and property of exhaust flue gas from the gas cycle, optimum design and operation, for minimum entropy generation in a single pressure HRSG generating superheated steam is reported in this paper. It shows that an optimum pressure of steam generation exists for minimum entropy generation. The interdependence between the physical size (and hence cost) of different heat transfer components of HRSG with the thermodynamic optimum operation for minimum entropy generation is presented. Optimum pressure of steam generation increases with greater size of super heater, economizer and evaporator up to certain limit, beyond which the increase in optimum saturation pressure is marginal with increase in size of the heat transfer units. However, increasing evaporator size and degree of superheat have the adverse effect of lesser steam generation for a given economizer and super heater for minimum entropy generation.
Second law analysis of a waste heat recovery based power generation system
In the present paper the performance of a waste heat recovery power generation system based on second law analysis is investigated for various operating conditions. The temperature profiles across the heat recovery steam generator (HRSG), network output, second law efficiency and entropy generation number are simulated for various operating conditions. The variation in specific heat with exhaust gas composition and temperature are accounted in the analysis and results. The effect of pinch point on the performance of HRSG and on entropy generation rate and second law efficiency are also investigated. The second law efficiency of the HRSG and power generation system decreases with increasing pinch point. The first and second law efficiency of the power generation system varies with exhaust gas composition and with oxygen content in the gas. Approximating the exhaust gas as air, and the air standard analysis leads to either underestimation or overestimation of power plant performance on both first law and second law point of view. Actual gas composition and specific heat should be used to accurately predict the second law performance. The present results contribute further information on the role of gas composition, specific heat and pinch point influence on the performance of a waste heat recovery based power generation system based on first and second law of thermodynamics.
Unidirectional hexagonal rare-earth disilicide nanowires on vicinal Si(100)-2×1
Rare-earth disilicide nanowires grown on vicinal Si(100) with a miscut of 2–2.5° toward the [110] azimuth at 600 °C were studied by scanning tunneling microscopy and compared with those grown on flat Si(001). In contrast to rare-earth disilicide nanowires grown on flat Si(100) surfaces, the nanowires grow unidirectionally along the [01̄1] direction of the vicinal Si(100) surface. Rare-earth disilicide nanowires form bundles composed of single nanowire units on both flat and vicinal surfaces. Yet, on the vicinal surface, the bundle width is comparable to the width of the terrace. The average nanowire length on the vicinal substrate is longer than that on the flat substrate. Scanning tunneling spectroscopy shows that the rare-earth disilicide nanowires have metallic properties.