Design optimization and structural assessment of a header and coil steam generator for load-following solar tower plants

The aim of this work is to explore the capabilities, from heat transfer and structural point of view, of a novel header and coil steam generator for a 100 MWe solar tower plant using molten-salt as heat transfer fluid. A methodology for the design and economic optimization of the header and coil steam generator is presented, paying special attention to the structural assessment which considers complex phenomena such creep-fatigue and stress relaxation due to the high working temperatures of solar tower plants. The results showed that header and coil steam generators provide economically effective overall heat transfer coefficients with lower pressure drops on the shell side compared to conventional shell-and-tube steam generators, leading to a reduction in the annual pumping costs of around 3.6 times. The structural assessment reveals that the critical points are located in the headers of superheater, reheater and evaporator. Different redesign actions have been performed to increase the lifetime in the critical points without affecting to the optimum thermo-economic solutions. Finally, the results showed that the header and coil steam generator is able to operate with fast daily startups at 6.1 K/min, a ramp-up 2.4 times higher than conventional shell-and-tube steam generators.This research is partially funded by the Madrid Government (Comunidad de Madrid) under the project ZEROGASPAIN-CM-UC3M (2020/00033/002) belonging to the program of Multiannual Agreement with UC3M in the line of "Fostering Young Doctors Research" and in the context of the V PRICIT (Regional Programme of Research and Technological Innovation, the Spanish government under the project RTI2018-096664-B-C21 (MICINN/FEDER, UE) and the scholarship "Ayudas para la formación del profesorado universitario" (FPU-02361) awarded by the Spanish Ministerio de Educación, Cultura y Deporte (MECD)

Non-conventional tube shapes for lifetime extend of solar external receivers

In this work, several novel tube shapes of solar tubular receivers that differ from the classical circular shape are analysed aiming to reduce the stresses of the receiver tubes, without penalizing its thermal efficiency. The analysis is performed using analytical thermal and mechanical models of the literature adapted for their use with non-circular tube shapes, verifying the assumptions made with FEM simulations due to the lack of experimental data available. Among the geometries studied, the results show that oval cross-section tubes improve the thermal efficiency of the receiver at the expense of increasing the stresses considerably. Ovoidal tubes show worse thermal and mechanical behaviour when the frontal part becomes peakier. Semicircle tubes reduce the stress by 10.9%, while keeping constant or even slightly improving the thermal performance. The last ones, increase the lifetime of the receiver and reduces the receiver costs if the manufacturing of the new geometries does no overpass 3.5 times the present price of production of the circular tubes. Therefore, the use of asymmetric cross-section tubes with low rear-front surface ratios, and smooth front surfaces can be considered a good alternative for substituting traditional circular cross-section tubes in central receivers.This research is partially funded by the Spanish government under the project RTI2018-096664-B-C21 (MICINN/FEDER, UE) and by the Madrid Government (Comunidad de Madrid) under the Multiannual Agreement with UC3M in the line of "Fostering Young Doctors Research" (RETOrenovable-CM-UC3M), and in the context of the V PRICIT (Regional Programme of Research and Technological Innovation)

Exergy analysis of solar central receivers

A high-resolution method to analyse the exergy of the SPT external tubular receivers is presented, examining the different heat transfer process involved individually. This sheds light on the role that each irreversibility source plays in the outcome, aiding in the receiver design and the facility location selection. The exergy efficiency is around 32% in the base configuration. Besides the exergy loss in the heliostat field, over 40%, it is found that the biggest exergy destruction cause are the radiation emissions and absorptions in the tube outer surface, around 17%. From the remaining ones, the greatest are the exergy destructed in the HTF and the one escaping to the ambient (over 4% each). Then, the exergy balance for a variety of strategies and ambient conditions is performed: optical properties of the tubes coating, peak and flat aiming strategies, DNI and ambient temperature. The heliostat field exergy loss rate only varies when changing the aiming. However, the emission and absorption losses and the ones in the HTF suffer the greater modifications with all the parameters studied. The impact of the optical properties degradation, 1% descent in the efficiency per 5% degradation, would advise repainting works in order to avoid greater exergy destruction. The surroundings temperature modification impacts considerably the exergy efficiency, showing the suitability of locations with low ambient temperature and a moderate DNI: descends of over 0.35% occur every 5 °C increase of the temperature for a fixed DNI.This research is partially funded by the scholarship "Ayudas para la formación del profesorado universitario"; (FPU-02361) awarded by the Spanish Ministerio de Educación, Cultura y Deporte (MECD), the fellowship"Ayuda a la investigación en energía y medio ambiente" of the Iberdrola España Foundation, the Spanish government under the project RTI2018-096664-B-C21 (MICINN/FEDER, UE) and the call "Programa de apoyo a la realización de proyectos interdisciplinares de I+D para jóvenes investigadores de la Universidad Carlos III de Madrid 2019-2020", under the frame of the Convenio Plurianual Comunidad de Madrid- Universidad Carlos III de Madrid

Design and evaluation of a prototype of an ergonomic handle for minimally invasive surgery

Grado en Ingeniería en Tecnologías Industriale

Elliptical tubes receivers efficiency analysis in solar power towers

Proceeding of: SolarPACES 2018: International Conference on Concentrating Solar Power and Chemical Energy Systems, 2-5 October 2018, Casablanca, MoroccoThe high costs of the heliostat field, as well as the issues related with the receiver operation in solar power towers are some of the main reasons to study different tube geometry shapes for the receiver. The new geometries intend to improve its thermal efficiency and increase its lifetime, such as the elliptical tubes presented in this research. A thermal model considering circumferential and longitudinal divisions of the tubes has been used to study the thermal efficiency of elliptical tubes receiver. For this analysis, the tube's geometry varies according to three different considerations when its major axis increases, and the resulting efficiencies are compared with the ones obtained for circular tubes of a fixed diameter. The results show that there is an improvement in the efficiency with the new tubes, because of the radiative exchange with the environment using the elliptical tubes, resulting in lesser radiative heat losses.This study has been supported and financed by the Iberdrola España foundation under the program "Ayudas a la investigación en energía y medio ambiente" and by the Spanish Economy and Competitiveness Ministry under the project ENE2015-69486-R (MINECO/FEDER, UE)

Deflection and Stresses in Solar Central Receivers

The aim of the design of central solar receivers is to withstand the high non-uniform solar-heat-flux and temperature during the solar-power-plant lifetime. This high non-uniform tube temperature causes high thermal stress, producing creep and fatigue damage. Therefore, is necessary to obtain an accurate estimation of the tube stresses during the receiver operation. In the same way, to ensure the panel integrity, the frontal and lateral tube deflections must be obtained to avoid excessive panel bowing and warpage, respectively. The huge number of simulations needed to perform the creep-fatigue analysis precludes the use of high time-consuming CFD-FEM simulations. To resolve this drawback, a reliable, accurate and fast procedure to obtain the tube stresses, using analytical stress estimation, is proposed. The procedure considers the temperature dependence of the thermo-mechanical properties. The temperature-dependent hoop stress is estimated using the solution for constant mechanical properties whereas the radial stress is estimated taking constant the Young modulus only. The temperature-dependent axial-bending stress is obtained using the non-homogeneous beam equation subjected to the movement restriction produced by tube clips. When the tube displacement is restricted by tube clips, the equivalent stress difference is less than 2% taking temperature-dependent properties and slightly higher than 10% for constant properties. The proposed stress estimation is enough accurate to perform a reliable fatigue-creep analysis and two order of magnitude faster than the CFD-FEM simulations. Finally, the tube deflection and displacement, restricted by tube clips, are derived straightforward using the temperature-dependent tube curvature and the beam theory

Calculating molten-salt central-receiver lifetime under creep-fatigue damage

One of the major uncertainties in the design of molten-salt solar receivers is the estimation of the lifetime because solar receivers have to withstand high temperature, thermal stress and corrosive-media in addition to daily star-up and shutdown. The first step to estimate the receiver damage and therefore, to monitor and manage the receiver life cycle, is to obtain properly the tube stresses under daily operating conditions. Whit this aim, we propose a straightforward low-computational cost procedure to determine the stresses under elastic-plastic regime. In addition, the stress relaxation due to visco-elastic behavior of the material, is also calculated. The analytical thermo-elastic, elastic-plastic and relaxation stress models for the high-nickel alloy Haynes 230 were verified against finite element simulations showing a good accuracy with a low-computational cost. For exemplification purposes, a reference operation day of a molten-salt solar receiver like Gemasolar is investigated. A receiver creep-fatigue damage assessment was developed for a flat aiming strategy. The results showed that the fatigue damage is 0.03% of the total damage. The average-field-receiver efficiency was 42.18% with a predicted lifetime higher than the expected. On the contrary, the receiver lifetime increases unnecessarily and the field-receiver efficiency drops to 26.25% when the receiver is operated limiting the stress as one third of the ultimate tensile strength (UTS/3). This limit results in a loss of revenues of around 250,000Euros per month. Hence, the excessive level of conservatism produced by stress limit methods are not suitable for solar receivers due to the resulting economic penalties.This research is partially funded by the Spanish government under the project RTI2018-096664-B-C21 (MICINN/FEDER, UE), the fellowship “Ayuda a la investigación en energía y medio ambiente” of the Iberdrola España Foundation, the scholarship “Ayudas para la formación del profesorado universitario” (FPU-02361) awarded by the Spanish Ministerio de Educación, Cultura y Deporte (MECD), and the fellowship “Programa de apoyo a la realización de proyectos interdisciplinares de I + D para jóvenes investigadores de la Universidad Carlos III de Madrid 2019-2020” under the projects RETOrenovalbe-CM-UC3M (2020/00034/001) and ZEROGASPAIN-CM-UC3M (2020/00033/001), funded on the frame of “Convenio Plurianual Comunidad de Madrid-Universidad Carlos III de Madrid 2019-2022”

A procedure to predict solar receiver damage during transient conditions

The successful deployment of solar-power-towers lies in the reliable design of solar receivers, assuring the lifetime under high heat flux and cloud passages. Two main damage mechanisms appear during central-receiver operation: creep, during hold times at high temperatures and stresses, and fatigue, caused by transient operation. This work aims to determine the extent of each damage on a Haynes 230 receiver. It is analyzed using transient DNI of a whole year, controlling the operation to guarantee the receiver preheat and minimize the start-ups. The molten-salt flow-rate during cloud passages or hazy days remains as the clear-sky scheduled one, to avoid tube overheat, preventing salt degradation and stress reset, when the clouds cleared. The results show that creep dominates, with a minimum share of 73.8% for the transient case. High creep is motivated by the long operating (6 h per day on average), mostly at high DNI, while the low number of average start-ups per day (1.4) do not increase much the fatigue, barely affected by small transients. The use of clear-sky, instead of the transient DNI, constitutes a conservative estimation, with the creep greatly increasing, showing a minimum share of 82%. Consequently, the predicted lifetime is lower, going from 45 years to 27.8. Intermediate mass-flow control modes fall in-between them. Clustering the days according to their DNI features shows that the high-energy days, with high creep, are the most common, highlighting the creep interest and that the fatigue can be estimated taking only the strain range produced by the start-up.This research is partially funded by the scholarship "Ayudas para la formación del profesorado universitario" (FPU-02361) awarded by the Spanish Ministerio de Educación, Cultura y Deporte (MECD), the Spanish government under the project RTI2018-096664-B-C21 (MICINN/FEDER, UE) and the call "Programa de apoyo a la realización de proyectos interdisciplinares de I + D para jóvenes investigadores de la Universidad Carlos III de Madrid 2019-2020", under the projects RETOrenovable-CM-UC3M and ZEROGASPAIN-CM-UC3M, funded on the frame of the Convenio Plurianual Comunidad de Madrid- Universidad Carlos III de Madrid