Influence of the stop/start system on CO2 emissions of a diesel vehicle in urban traffic

This paper presents measurements of CO2 emission and efficiency of stop/start technology on a diesel vehicle in urban traffic. Two four-wheel-drive diesel vehicles with on-board exhaust emission and vehicle activity measurement systems were tested in two urban driving circuits representative of downtown Madrid. The vehicles had similar turbocharged and intercooled diesel engines fulfilling the same Euro 4 emissions regulation; but one had an improved engine incorporating stop/start technology. CO2 emission reduction of more than 20% for the car equipped with the stop/start system was obtained. Regardless of the variability in driving style, the grade and type of streets, traffic congestion, and the engine operating temperature, the car equipped with the stop/start system has intrinsically a lower CO2 emission factor

On existence of trends applicable to thermoeconomic optimisation of combined cycle gas turbine power plants

This paper aims at the influence of the nominal power on the design configuration of combined cycle gas turbine (CCGT) power plants. This is achieved by means of a thermoeconomic model aimed at the minimisation of the power plant cost. The present work starts with the establishment of trends in existing commercial gas turbines. Based on these, other trends are found for the design of the whole CCGT, leading to the assessment of the most suitable heat recovery steam generator type and the optimal design parameters. Finally, an analysis of the influence of fuel price on the design configuration is carried out. This serves two purposes: to observe the dependence of economic results with fuel prices and to determine whether fuel price variations might influence the previously established trends in the CCGT design

Solar multiple optimization for a solar-only thermal power plant, using oil as heat transfer fluid in the parabolic trough collectors

Usual size of parabolic trough solar thermal plants being built at present is approximately 50 M We. Most of these plants do not have a thermal storage system for maintaining the power block performance at nominal conditions during long non-insolation periods. Because of that, a proper solar field size, with respect to the electric nominal power, is a fundamental choice. A too large field will be partially useless under high solar irradiance values whereas a small field will mainly make the power block to work at part-load conditions. This paper presents an economic optimization of the solar multiple for a solar-only parabolic trough plant, using neither hybridization nor thermal storage. Five parabolic trough plants have been considered, with the same parameters in the power block but different solar field sizes. Thermal performance for each solar power plant has been featured, both at nominal and part-load conditions. This characterization has been applied to perform a simulation in order to calculate the annual electricity produced by each of these plants. Once annual electric energy generation is known, levelized cost of energy (LCOE) for each plant is calculated, yielding a minimum LCOE value for a certain solar multiple value within the range considered

Rendimiento térmico de diferentes tipos de turbinas de gas en aplicaciones de pequeña potencia

El objetivo de este trabajo es estudiar si la pérdida de rendimiento que tiene lugar a bajos números de Reynolds (Re) puede representar un problema en turbinas de gas de pequeña potencia y, en caso afirmativo, si el empleo de ciclos alternativos como los Joule-Brayton inversos o los supercríticos con CO2 (sCO2) pueden considerarse una solución a ese problema. En primer lugar, se explican y se comparan dos procedimientos diferentes para el cálculo de la pérdida de rendimiento por bajos Re en turbomáquinas. A continuación, se utilizan los parámetros de diseño de un cierto número de turbinas de gas comerciales para demostrar que esa pérdida de rendimiento es significativa para potencias inferiores a 1,5 MW aproximadamente. Por último, se calculan los rendimientos y números de Reynolds de los otros tipos mencionados de turbinas de gas. El sCO2 se muestra como una alternativa prometedora, ya que, con ese ciclo, Re experimenta un considerable aumento y el rendimiento puede considerarse bueno. El ciclo inverso, por el contrario, es una opción menos interesante por su bajo rendimiento y porque no conlleva mejoras en el Re.Ministerio de Economía y Competitivida

Herramientas de cálculo para el modelado de receptores centrales de energía solar de concentración

La energía solar está viendo actualmente importantes avances en las tecnologías desarrolladas para su aprovechamiento a nivel industrial, ya sea calor de proceso o producción de energía eléctrica. Una parte importante de los esfuerzos en este ámbito se dirigen al receptor de la radiación solar concentrada, elemento en el que se persigue transmitir la energía reflejada por el sistema concentrador al fluido calorífero con las menores pérdidas térmicas posibles. La consecución de un diseño adecuado o la mejora de uno ya establecido debe llevarse a cabo a través de un análisis exhaustivo de la física que se desarrolla en el interior del receptor, para lo que existen diversas herramientas. Las más comunes pasan por el ámbito computacional y tratan de simular la física que aparece en estos elementos, pudiendo realizarse la implementación de la física y la geometría del receptor de forma manual o adquiriendo la licencia de algún programa comercial que tenga implementada la física y quede a cargo del usuario implementar la geometría. El uso de una herramienta debe estar fundamentado por el objetivo que se persigue, por lo que no se debe tomar a la ligera. El objetivo de este trabajo es realizar una revisión las principales tipologías de herramientas de software utilizadas en el estudio de receptores de torre central (los asociados a un mayor flujo de potencia junto con los discos parabólicos), discutiendo en qué caso es más recomendable una u otra herramienta e indicando recomendaciones sobre en qué ocasiones puede ser útil el desarrollo de una herramienta propia. Se muestra, además, el ejemplo de una herramienta generada por los autores, útil cuando se persigue el diseño conceptual (no de detalle) de una tecnología de este tipo.Los autores agradecen las discusiones técnicas con el resto de integrantes del Grupo de Investigaciones Termoenergéticas de la Universidad Politécnica de Madrid. Este trabajo ha sido parcialmente financiado por ayudas del plan nacional Spanish Grant ‘VI Plan Nacional’ ENE2012- 37950-C02-01 y ENE2012-37950-C02-02

Reynolds-number-dependent efficiency characterization of a micro-scale centrifugal compressor using non-conventional working fluids

The selection of working fluids other than air is a key issue in improving the efficiency of new thermodynamic cycles intended for low-to-moderate temperature small power plants. The aim of this paper is to study whether the low efficiency typical of small turbomachinery is still a problem when using alternative fluids. Based on a new design of power cycles named balanced hybrid Rankine-Brayton cycles, five different fluids were selected as potential working fluids: carbon dioxide, propane, isobutane, pentafluoroethane and sulfur hexafluoride. Dimensional analysis was used to compare the performances of a micro-scale centrifugal compressor working in homologous points where the efficiency variation depends only on the Reynolds-number (Re). The influence of Re on efficiency was calculated by means of four different methods for comparative purposes. Numerical simulations were also carried out in order to validate the methodological approach proposed. The results show the efficiency variations as a function of Re for increasing fluid densities. All the non-conventional fluids studied provide better performance in terms of efficiency than air. Particularly, isobutane and propane have been identified as potential working fluids candidates for the aforementioned innovative power cycle

Effect of pressurization on tip leakage losses in micro-scale centrifugal compressors

Turbomachinery miniaturization has been an emerging research field during the last decades and, especially, micro-scale radial compressors are becoming increasingly relevant due to the development of high-speed electric motors. They can be found in a wide range of applications such as micro gas turbines, aerial unmanned vehicles propulsion, fuel cells and even medical ventilators. Furthermore, the use of micro-scale centrifugal compressors has been recently proposed for heat pump applications using working fluids different than air which need to be pressurized. Since this down-scaling implies low inlet Reynolds-number, the use of these pressurized fluids entails beneficial effects on the internal efficiency of the machine due to the increase in Reynolds-number within a laminar-to-turbulent regime. Nevertheless, one of the main challenges in micro-turbomachinery is the large relative tip clearances due to assembly and manufacturing tolerances. These large clearance ratios lead to increased tip leakage losses that limit drastically the efficiency in the considered reduced-scale compressors. Besides, pressurized working fluids used in the aforementioned heat pump applications result in an increase in the pressure difference between the pressure and suction sides of the blade. This is precisely why the main goal of this work is to quantify the effect of inlet pressure on the tip leakage losses in micro-scale centrifugal compressors. The present paper proposes the assessment of the tip leakage losses by means of both analytical and numerical approaches. First, new operating conditions when using different working fluids are calculated considering the corresponding corrections due to Reynolds-number-dependent losses. Then, one-dimensional methods are applied to estimate the expected reduction in internal efficiency and the clearance ratio. On the other hand, a numerical model of the impeller-diffuser reference micro-compressor stage is developed using ANSYS CFX 19 in order to characterize the complex three-dimensional interactions within the tip leakage phenomena. Results show the effect of pressurization in four selected working fluids (air, carbon dioxide, isobutane and propane) in the range of desired inlet pressures for heat pump applications. A comparison is presented between analytical and numerical results. While slight differences in clearance ratio are found, expected reduction in efficiency provided by analytical methods are not aligned to the numerical results. However, both methods agree in the fact that inlet pressure has low influence on the internal efficiency for all working fluids simulated