Experimental study of fixed and fluidized beds of PCM with an internal heat exchanger

This work presents the results of experiments performed in a thermal energy storage tank filled with particles, which was heated using hot air and then discharged with a cold water stream circulating inside a heat exchanger immersed in the bed. Both fixed- and fluidized-bed configurations were studied. The materials used were sand, which is a material commonly employed in sensible heat storage, and a granular phase change material (PCM), in which the phase change occurs over the temperature range of 4050 degrees C. Three different heat exchangers with different helical coil geometries were tested by measuring the temperatures in the bed and at the water inlet and outlet. Higher heat transfer coefficients between the bed and the water flow and higher heat exchanger effectiveness were observed for the heat exchanger with the greatest distance between coils, as it allows better contact between the bed particles and the heat exchanger surface when the particles are fluidized. (C) 2016 Elsevier Ltd. All rights reserved.This work was partially funded by the Spanish Government (Project ENE2010-15403), the regional Government of Castilla-La Mancha (Project PPIC10-0055-4054) and Castilla-La Mancha University (Project GE20101662)

The water cost effect of hybrid-parallel condensing systems in the thermo-economical performance of solar tower plants

The importance of considering the water price in the analysis of the impact of dry versus hybrid condensing systems in the thermo economical performance of solar tower plants was demonstrated in this work. The dry condensing system consists of several induced-draft air-cooled condenser cells (ACCs) and the hybrid system consists of a parallel system where the condensing steam is split between the ACCs and a surface steam condenser where circulating water is cooled in a wet mechanical-draft cooling tower. The influence of the operating parameters of either the dry or wet cooling systems on the cooling load and fan power consumption were studied. Then, for a given condensing system (a system with a defined number of installed ACCs units and cooling tower units) and given the dry-air and wet-bulb air temperatures, the operating parameters were optimized to maximize the revenues of the power plant. This optimization depends on the water-to-electricity price ratio , showing that at low ambient temperature when this ratio increases it is not profitable to turn on the cooling towers since the water cost is not counterbalanced by the higher cycle efficiency obtained with the lower condensation temperature. Finally, the annual operation and the LCOE and NPV of the CSP plant located in Dunhuang were analyzed for both dry and hybrid condensing systems with different number of ACCs and wet towers, showing that the most cost-effective configuration is the 16 ACCs with 3 wet cooling towers for water-to-electricity price ratio = 4 ($/m3)/($/kWhe) and = 5 ($/m3)/($/kWhe), but for = 10($/m3)/($/kWhe), the best option is with only 2 wet towers.This research is partially funded by the Spanish government under the project RTI2018-096664-B-C21 (MICINN/FEDER, UE)

Development and validation of the ADAS scale and prediction of attitudes toward affective-sexual diversity among spanish secondary students

Violence against non-heterosexual adolescents in educational contexts remains a worrying reality, but no adequate attitudes towards affective-sexual diversity (AtASD) measure exists for Spanish adolescent students. We developed a 27-item scale including cognitive, affective and behavioral aspects, which was completed by 696 secondary school students from the Madrid area. Factor analyses suggested a unidimensional model, Cronbach’s alpha indicated excellent scale scores reliability, and item calibration under the Item Response Theory framework showed that the scale is especially informative for homophobic attitudes. A hierarchical multiple regression analysis showed that variables traditionally related to AtASD (gender, age, religion, nationality, perceived parental/peer attitudes, direct contact with LGB people) also were so in our sample. Moreover, interest in sexuality topics and perceived center’s efforts to provide AtASD education were related to better AtASD. Our scale was reliable and valid, and it may also prove useful in efforts to detect those students with homophobic attitudes and to guide intervention

Economic and thermo-mechanical design of tubular sCO2 central-receivers

Supercritical CO2 central-receivers must withstand high temperatures and pressures combined with cyclic operation, which makes the solar receiver susceptible to creep-fatigue failure. In this work, a creep-fatigue analysis of a sCO2 Inconel 740H tubular receiver of a 2 MWe solar tower plant has been accomplished to study the influence of the tube size on the receiver and solar field design. A 2D numerical model of the tubular receiver that accounts for the thermal conduction in both radial and circumferential directions was developed to determine the sCO2 and wall temperature profile, which is crucial for the creep-fatigue calculations. The receiver flux distribution, which is an input to the model, was obtained with SolarPILOT, while a conventional recompression model was used to calculate the cycle efficiency and inlet temperature to the receiver. Comparison of the results of the 2D model with those of a 1D model showed that the 1D model overestimates the creep fatigue rupture time by two orders of magnitude. Furthermore, the efficiency and costs of the heliostat field and receiver were calculated for different receiver tube sizes. Smaller tubes allowed a higher maximum heat flux leading to smaller receiver and heliostat field designs, which resulted in higher overall efficiency of the power plant and lower material costs. For a design ensuring 25 year receiver lifetime the minimum sCO2 solar receiver cost, 345 €/kWth, was obtained for the smallest pipe diameter.This research is partially funded by the Spanish government under the projects RTI2018-096664-B-C21 (MICINN/FEDER, UE) and RED2018-102431-T (AEI, MICINN) and the fellowship “Programa de apoyo a la realizaci on de proyectos interdisciplinares de I+D para j ovenes investigadores de la Universidad Carlos III de Madrid 2019e2020” under the project ZEROGASPAIN-CM-UC3M (2020/00033/001), funded on the frame of “Convenio Plurianual Comunidad de Madrid-Universidad Carlos III de Madrid 2019e202”

Heat generation depth and temperature distribution in solar receiver tubes subjected to induction

Induction heating is commonly used in laboratory-scale facilities to replicate the heating conditions of the receiver tubes of concentrated solar power plants. This work aims at shedding light at the induction heating characteristics for such applications through the development of a multiphysics numerical model capable of replicating the experimental conditions of a molten salt loop locally heated by an induction heater. In the experiments, a stainless steel pipe is heated on its external surface by the induction heater, which is switched on and off during the experimental data acquisition while molten salts are continuously circulating in its interior. These conditions are replicated, for the first time, in a two-dimensional numerical domain fully coupling the electromagnetic and thermal physics, including thermally dependent material properties of the heated pipe. Once validated against the experiments, the numerical results revealed that the volumetric nature of the induction heating shall be considered for an accurate representation of the temperature profile inside the tube. As a novelty, different equivalent surface boundary conditions are presented and, despite the Gaussian-like behavior of the induction heating on the surface of the tube, the results indicate that there exists no equivalent wall boundary condition to fully replicate the temperature profile obtained with the induction heater. The effect of independently varying experimental parameters such as the geometry of the pipe (i.e., diameter and thickness) and its distance to the induction heating system is also evaluated. Using large diameters of the tube reduces the difference between the angular temperature profile obtained using induction heating and a simplified wall boundary condition. For small wall thicknesses, the induction heating is capable of penetrating along the whole thickness of the tube, the total heat generated in the volume of the tube being exposed to the counteracting effects of the volumetric generation and the enhancement of the heat dissipation by the molten salt, as both of them increase for small thicknesses. The distance of the inductor to the pipe wall appears to maintain the volumetric characteristics of the heating and only affects the induction heating magnitude and efficiency.This work has been funded by Programa de Atracción de Talento (Modalidad 2) de la Comunidad de Madrid (Spain) 2019-T2/AMB-15938 and the project RTI2018-096664-B-C21 (MICINN, FEDER/UE). Eduardo Cano-Pleite acknowledges support from the CONEX-Plus programme funded by Universidad Carlos III de Madrid and the European Union's Horizon 2020 programme under the Marie Sklodowska-Curie grant agreement No. 801538

A review of solar thermal energy storage in beds of particles: Packed and fluidized beds

This review summarizes different solar thermal energy storage techniques from a particle technology perspective, including sensible, latent and thermochemical techniques for low- and high-temperature applications that use particles as the storage medium in the thermal energy storage system. The focus is on applications, experimental results, modeling and future trends. This review describes two different particle technologies used to store thermal energy: packed and fluidized beds. The advantages and disadvantages of both technologies are reviewed throughout different studies found in the literature for various thermal energy storage systems. Packed beds have the main advantage of thermal stratification, which increases the efficiency of solar collectors in low-temperature sensible energy storage systems and augments the exergy content in the bed. Moreover, they have been proven to be suitable as dual-media thermocline storage systems for CSP plants. In contrast, the high mixing rates of fluidized beds makes them suitable for the rapid distribution of concentrated solar energy in particle receiver CSP systems. In addition, their high heat and mass transfer rates, compared with those of packed beds, make them the preferred particle technology for thermochemical energy storage applications. This review also notes that it is important to find new materials with an appropriate size and density that can be properly used in a fluidized bed. Additionally, more specific research efforts are necessary to improve the understanding of the behavior of these materials during the fluidization process and over a high number of charging/discharging cycles

Experimental study of the preheating process of tubular external molten salt receivers

Proceeding of: XI National and II International Engineering Thermodynamics Congress (11 CNIT 2019)In this work, the heat transfer process of the tubes of a central receiver of a CSP plant during the start-up is studied. With this aim, the temperature of the external surface of the pipe is measured in a molten salt test loop. The experimental installation is formed by a cylindrical molten salt tank, a pump to circulate the molten salt through a pipe in a close circuit and an induction heater to generate the heat flux, which is applied in a small rectangular region of the tube surface. The temperatures were measured with the empty tube subjected to a high heat flux of approximately 200 kW/m2 . Once the tube reached a high temperature of approximately 800 º C, Solar Salt (a mixture of potassium and sodium nitrate in a proportion of 60/40 % wt.) started to circulate inside the tube. This process simulates the start-up process of a central tower plant, when the heliostat field focuses on the receiver to preheat the tubes prior to filling the receiver with molten salt. Central tower receivers are subjected to a unilateral concentrated solar radiation that causes a high temperature difference between the tube crown at the outside surface of the tube, which receives the highest solar radiation, and the back surface of the tube, which is in shadow. This temperature difference leads to the mechanical strain of the tube. In order to study the effects of this non-homogenous heat flux, temperatures and tube deflection of the pipe were measured at different experimental conditions. Several K-type thermocouples were welded at different azimuthal and axial positions of the pipe to measure the external surface temperature. Additionally, images of the pipe were taken during the heating process to measure the deflection of the tube.The authors would like to thank the financial support from Ministry of Economy and Competitiveness (Projects ENE2012-34255 and ENE2014-54942-R) and the Ministry of Education, Culture and Sport under the program of Formación del Profesorado Universitario (FPU14/04941)

Flow and heat transfer analysis of a gas-particle fluidized dense suspension in a tube for CSP applications

This work presents a numerical study of the flow of particles in a gas–particle fluidized dense suspension for CSP applications using the Multi-Phase Particle in Cell (MP-PIC) method, implemented in CPFD-Barracuda software. The study covers two different numerical simulations. The first is a cold and isothermal model in which the fluctuations and control of the mass flow of particles ascending along the vertical tube was studied. In the second, a high-temperature boundary condition was imposed on the external surface of the tube and the energy equation was solved. In this second case, the heat transfer coefficient between the inner surface of the tube and the particles was numerically computed. The numerical results in the cold model are highly consistent with experimental data available in the literature (with values up to 150 kg/h and differences of approximately ±10 kg/h) and underline the significant impact of the pressure at the bottom of the bed and of the aeration flow rate on the mass flow of particles. The results of the non-isothermal case present heat transfer coefficients in the range of 300–400 W∕(m2 K) with transient fluctuations during the fluidization process. These fluctuations may be an influence on the mechanical damage of the tube, which is exposed to high levels of concentrated irradiation.This work was partially funded by the project PID2021-127322OB-I00, funded by the Ministerio de Ciencia e Innovación MICIN/AEI/10.13039/501100011033/ and by FEDER Una manera de hacer Europa; Project SBPLY/21/180501/000017, funded by the Regional Government of Castilla-La Mancha and by FEDER Una manera de hacer Europa; Project RED2018-102431-T, funded by the Ministerio de Ciencia, Innovación y Universidades - Agencia Estatal de Investigación (AEI) and Project 2020-GRIN-28725, funded by Universidad de Castilla-La Mancha

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)

Hybrid storage solution steam-accumulator combined to concrete-block to save energy during startups of combined cycles

This work presents a novel steam accumulator and concrete-block storage system (SACSS) to recover part of the energy lost through the steam cycle side during startups of combined cycle power plants (CCPPs). The steam accumulators are integrated with sensible-heat concrete storage to provide superheated steam resulting then to a higher efficiency and safer steam turbine operation compared with systems based only on saturated steam. An economic analysis is performed considering two different scenarios: i) a CCPP able to execute fast startups using a Benson-type heat recovery steam generator (HRSG) and ii) a CCPP operated with conventional startups which employs a typical drum-type HRSG. It is worth mentioning that the second scenario is based on measured data. The economic optimization of the SACSS is carried out focusing in four design variables: number of steam accumulator units, storage pressure, concrete-block length and outer concrete diameter. The optimum solution presents a net present value of 4.45 M€ and a payback period of 3 years for the CCPP suitable for fast startups. For the CCPP operated with conventional startups, a net present value of 2.53 M€ and a payback period of 3.4 years are obtained. The net present value grows around 60 % in both cases if the benefits from carbon credits are considered. In addition to the efficiency improvement, the SACSS could be used to preheat critical sections of the heat recovery steam generators, reducing the thermal stress and the fatigue damage during fast startups. Finally, the emissions avoided thanks to SACSS are estimated to be around 3 640 and 2 175 tons of CO2 per year, for fast and conventional startup cases, respectively.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 project ZEROGASPAIN-CM-UC3M (2020/00033/002), funded on the frame of "Convenio Plurianual Comunidad de Madrid-Universidad Carlos III de Madrid 2019-2022"