DITF - TADS - Artículos de Revistas

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Disconnected, yet in the spotlight: Emergency research on extreme energy poverty in the Cañada Real informal settlement, Spain
(Elsevier, 2023-08) Ruiz-Rivas Hernando, Ulpiano; Tirado-Herrero, Sergio; Castaño-Rosa, Raúl; Martínez Crespo, Jorge
Cañada Real is a 15-km informal settlement located in Madrid, Spain. With over 8000 inhabitants most dwellers live below the poverty line in informal, low-quality housing. Due to the impossibility to have legal supply contracts with utility providers, Cañada Real settlers have relied on irregular connections to nearby electricity and water distribution networks for decades. However, in October 2020, technical changes implemented by the distribution system operator left some 4000 people without access to power, and more than two years later a large share of them remain in those conditions. Emergency research has been conducted to document the change in living conditions experienced by Cañada Real residents. Census data have been analysed together with primary data from a 39-household survey, data retrieved from electricity service continuity sensors and direct measurements of indoor thermal comfort in 12 households. This set of data provides unique evidence on the impact of a collective disconnection event of an unprecedented magnitude in an EU context. Results give evidence of a case of ‘extreme energy poverty" that existing datasets and indicators fail to capture. The collective adaptation response displayed by a group of residents, who agreed on an intermittent, predictable disconnection schedule, highlights social fabric, self-organization and local capacities as resilience factors that provide temporary relief. Still, collective reconnection appears as a necessary first step to secure a minimum level of material living conditions. Political action is needed to modify the existing framework that marginalizes vulnerable dwellers as non-compliant customers, without any provisions against supply disconnections.
Modeling and simulation of a hybrid compression/absorption chiller driven by Stirling engine and solar dish collector
(MDPI, 2020-12-02) Romage, Guerlin; Jimenez, Cuauhtemoc; Jesús Reyes, José de; Zacarias, Alejandro; Carvajal, Ignacio; Jiménez, José Alfredo; Pineda, Jorge; Venegas Bernal, María Carmen
In this paper, an evaluation of the performance and operating parameters of a hybrid compression/absorption chiller coupled with a low-capacity solar concentrator is presented. The study was carried out using energy and mass balances applied to each component of each system. The variables evaluated in the hybrid chiller were the cooling power, the supply power, the Coeficient of Performance (COP) of both cooling systems and the ratio between heat and power. The diameter and temperature of the hot spot as well as the performance of the dish collector were evaluated. The changed parameters were the heat removed by each refrigeration system, the condenser temperature, the evaporator temperature, the concentration ratio and the irradiance. Results have shown that the compression system can produce up to 53% more cooling power than the heat supplied to the hybrid system. Meanwhile, the absorption system produces approximately 20% less cooling power than the supplied heat. It has also been found that, for the cooling power produced by the hybrid cooler to be always greater than the heat supplied, the cooling power provided by the absorption system should preferably be between 20% and 60% of the total, with a Stirling engine eficiency between 0.2 and 0.3 and a condensation temperature from 28 to 37 ºC. Likewise, it has been found that the compression system can produce cooling power up to 3 times higher than the heat of the Stirling engine hot source, with Th = 200 ºC and ns = 0.3. Finally, it has been found that, in a low-capacity solar concentrator, on a typical day in Mexico City, temperatures in the hot spot between 200 and 400 ºC can be reached with measured irradiance values from 200 to 1200W/m2.
Insights into the co-pyrolysis of olive stone, waste polyvinyl chloride and Spirulina microalgae blends through thermogravimetric analysis
(Elsevier, 2022-03-01) Rasam, Sajjad; Azizi, Kolsoom; Moraveji, Mostafa Keshavarz; Akbari, Ali; Soria Verdugo, Antonio
Co-pyrolysis of binary and ternary blends composed of different biomass samples was analyzed based on thermogravimetric analysis (TGA). The study was carried out considering biomass samples from very different origins and uses, including Olive Stone (OS) as a representative of lignocellulosic biomass, waste Polyvinyl Chloride (PVC) as an example of a widely generated residue, and Spirulina microalgae (MA) as an archetype of a highly valuable biomass. The synergetic effects of mixing and combining these dissimilar biomass samples were evaluated. The obtained results were evaluated by four iso-conversional methods, namely Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), Starink (STK), and Modified-Friedman (M-F) to determine the activation energy and the pre-exponential factor of the process, whereas the order of reaction was calculated by Coats-Redfern model. By analyzing the correlation coefficient, the FWO model was found to better describe the thermogravimetric results for single, binary and ternary blends. The lowest and highest values of calculated activation energy correspond to OS and MA samples, obtaining values of 80.8 kJ/mol and 158.7 kJ/mol, respectively. Additionally, blending of OS, MA and PVC with a composition of 50%:25%:25% resulted in lower activation energy for ternary mixtures. The synergistic and inhibitive effects of the addition of the second and third components were also investigated according to the comparison of the experimental and theoretical mass evolutions. The results confirmed the existence of synergistic effect for the interaction between PVC-OS in the complete temperature range during the process, while the synergistic interaction between MA-OS and PVC-MA were observed above conversion values of 0.15 and 0.8, respectively. Furthermore, for all mass ratios of ternary blends, the interaction effect was inhibitive for low temperatures and then turned into synergistic for increasing temperatures.
Bimodal particle distributions with increased thermal conductivity for solid particles as heat transfer media and storage materials
(ELSEVIER BV, 2022-03) Christen, Chase E.; Gómez Hernández, Jesús; Otanicar, Todd P.
Solid particles are being considered in several high temperature thermal energy storage systems and as heat transfer media in a variety of advanced power generation systems, particularly in concentrated solar power plants. The downside of such an approach is the low overall heat transfer coefficients caused by the inherently low thermal conductivity values of the low-cost solid media when coupled to heat exchanger for the power cycle working fluid. Choosing the right particle size distribution, emittance, and material of the solid media can all make a substantial difference in packed bed thermal conductivity. Current research though exclusively focuses on continuous unimodal distributions of particles. Here, we propose the use of a binary particle system with a bimodal size distribution to significantly increase packed bed thermal conductivity by reducing packed bed porosity. This is the first study related to ceramic solid particle heat transfer that has considered the thermal conductivity of non-unimodal size distributions at room and elevated temperatures. The following study found that for the binary particle system using Carbo particles CP 16/30 - CP 70/140 where the large particle volume fraction was 50% there was an 17-47% increase in packed bed thermal conductivity when compared with a nearly unimodal particle size distribution of CP 16/30 between 50 and 300 °C. Two different porosity and effective thermal conductivity models were studied, with one providing better prediction of porosity but both effective thermal conductivity models providing less predictive capacity. Importantly this approach can have a substantial impact of thermal performance, with little to no impact on the particle cost.
Influence of longitudinal clips in thermal stresses and deflection in solar tubular receivers
(Elsevier, 2020-03-01) Montoya Sancha, Andrés; Rodríguez Sánchez, María de los Reyes; López Puente, Jorge; Santana Santana, Domingo José; Ministerio de Ciencia e Innovación (España)
Mechanical boundary conditions in tubular receivers of solar power tower plants have a main role in the thermal stress distribution and tube deflection. Longitudinal supports, particularly, has an strong influence on stress and displacements, since they prevent the tube bending. In this work, the influence of longitudinal supports, on tube deflection and stress has been studied in external-cylindrical receivers, using an analytical methodology, which it is able to take into account the tube geometry in the deflection calculation. Therefore, real tube geometry with elbows can be considered. Results for two aiming strategies, one equatorial and another that flattens the heat flux, have been compared for different clips distances, from 1 to 9 meters. The analytical methodology developed in Matlab provides lower computational cost than the numerical model developed in Abaqus. Results show that clip distribution has a significant impact on thermal stress. For clips distance of 2 meters or lower, the generalised plane strain solution provides the stress distribution along the tube accurately, with a tube deflection lower than 1 millimetrer. When clips distance increases, the longitudinal stress distribution differs from the plane strain case, and the deflection increases to non-desirable values. Deflection is greater at tube ends, and aiming strategies that flatten the heat flux increases the displacement in that regions.
Improving the efficiency of gas turbine systems with volumetric solar receivers
(Elsevier, 2017-10-01) Petrakopoulou, Foteini Konstantina; Sánchez Delgado, Sergio; Marugán Cruz, Carolina; Santana Santana, Domingo José; European Commission; Ministerio de Economía y Competitividad (España)
The combustion process of gas turbine systems is typically associated with the highest thermodynamic inefficiencies among the system components. A method to increase the efficiency of a combustor and, consequently that of the gas turbine, is to increase the temperature of the entering combustion air. This measure reduces the consumption of fuel and improves the environmental performance of the turbine. This paper studies the incorporation of a volumetric solar receiver into existing gas turbines in order to increase the temperature of the inlet combustion air to 800 °C and 1000 °C. For the first time, detailed thermodynamic analyses involving both energy and exergy principles of both small-scale and large-scale hybrid (solar-combined cycle) power plants including volumetric receivers are realized. The plants are based on real gas turbine systems, the base operational characteristics of which are derived and reported in detail. It is found that the indications obtained from the energy and exergy analyses differ. The addition of the solar plant achieves an increase in the exergetic efficiency when the conversion of solar radiation into thermal energy (i.e., solar plant efficiency) is not accounted for in the definition of the overall plant efficiency. On the other hand, it is seen that it does not have a significant effect on the energy efficiency. Nevertheless, when the solar efficiency is included in the definition of the overall efficiency of the plants, the addition of the solar receiver always leads to an efficiency reduction. It is found that the exergy efficiency of the combustion chamber depends on the varying air-to-fuel ratio and, in most cases, it is maximized somewhere between the applied inlet combustion air temperatures of 800 °C and 1000 °C.
Exergy recovery from solar heated particles to supercritical CO2
(Elsevier, 2019-01-05) Hernández Jiménez, Fernando; Soria Verdugo, Antonio; Acosta Iborra, Antonio; Santana Santana, Domingo José; Ministerio de Economía y Competitividad (España)
In this work, the technical feasibility of a fluidized and a fixed bed heat exchanger in a concentrating solar power (CSP) tower for heat recovery applications is analysed using Two-Fluid Model simulations. The heat recovery process analysed in this work corresponds to the discharge of sensible heat from solid particles. In the cases studied, the fluidizing agent of the bed is carbon dioxide (CO2) in supercritical conditions and the particles, which constitute the bed material, are sensible heat storage material. CO2 is gaining attention in its application as a working fluid in thermodynamic cycles for power generation, especially in transcritical and supercritical conditions due to its high density and excellent heat transfer characteristics. Currently, research is focused on exploring the CO2 capabilities when used in combination with CSP technologies, together with systems that allow the storage and recovery of the solar thermal energy. Fixed or fluidized beds work as a direct contact heat exchanger between the particles and the working fluid that percolates through the bed material. Several bed configurations are presented to derive the optimal configuration of the bed that enhances the efficiency from both the energetic and the exergetic points of view. The results indicate that a fixed bed heat exchanger produces a maximum increase of availability in the CO2 flow during longer times than a fluidized bed heat exchanger. Therefore, to maximise the exergy recovery from solar heated particles to supercritical CO2 a fixed bed heat exchanger is more suitable than a fluidized bed heat exchanger.
Numerical study of the effect of pressure and temperature on the fluidization of solids with air and (supercritical) CO2
(Elsevier, 2019-05) Hernández Jiménez, Fernando; García Gutiérrez, Luis Miguel; Acosta Iborra, Antonio; Soria Verdugo, Antonio; Ministerio de Economía y Competitividad (España)
This work performs numerical simulations of fluidized beds under different conditions of pressure and temperature and using air and CO2 as fluidizing agents. The conditions of high temperature and pressure tested turn the CO2 into supercritical conditions, so the differences when the fluidizing agent is at supercritical conditions are also tested. The results show that when pressure and temperature are increased, fluidization with air or CO2 shifts from the typical bubbling fluidization characteristic of ambient conditions, to a more homogeneous fluidization where not only bubbles and dense phase are present in the bed, but also a dilute phase of moderate solids concentration. The main consequence is an increase of the lateral motion of gas and solids at high pressure and temperature. A deviation from the classical Two-phase theory occurs because the gas velocity through the dense phase at high pressure and temperature is higher than the corresponding minimum fluidization velocity.
Lateral solids meso-mixing in pseudo-2D fluidized beds by means of TFM simulations
(Elsevier, 2018-07-01) Hernández Jiménez, Fernando; Sánchez Prieto, Javier; Cano Pleite, Eduardo; Soria Verdugo, Antonio
This work studies the solids mixing process in fluidized beds by means of numerical simulations using the two-fluid model (TFM) available in the MFIX code. The numerical results are compared with experiments conducted in a pseudo-2D fluidized bed. The experiments were performed by placing particles of the same diameter and density but of different colour in two vertical layers. To reproduce numerically the experimental results, three phases are defined: one for the gas phase and two for the solid phases, corresponding to the particles of different colours employed in the experiments, to make them separately traceable. To improve the simulation prediction, a friction model that accounts for the effect of the front and rear walls on the continuum solid phases was introduced in the TFM. Mixing times of the same order of magnitude are obtained from the simulations and the experiments when the mixing process is analysed macroscopically. Furthermore, the simulations are employed to study the solids mixing in the fluidized bed based on a more detailed mixing index. This new mixing index is determined from information of the three phases involved and it is used to predict the mixing behaviours beyond the capabilities of the experimental facility.
Improvement of the simulation of fuel particles motion in a fluidized bed by considering wall friction
(Elsevier, 2017-08-01) García Gutiérrez, Luis Miguel; Hernández Jiménez, Fernando; Cano Pleite, Eduardo; Soria Verdugo, Antonio
The mixing of fuel particles is a key issue on the performance of fluidized bed reactors. In this work, the motion of a non-reactive fuel particle in a pseudo-2D bubbling fluidized bed operated at ambient conditions is simulated employing a hybrid-model and introducing a new friction term that accounts for the effect of the bed vessel front and rear walls. The hybrid-model, implemented in the code MFIX, simulates the dense and gas phases using a Two-Fluid Model (TFM) whereas the fuel particles are modeled using a Discrete Element Method (DEM). The importance of the present hybrid-model is that the interaction of the continuum phases with the fuel particles behavior is fully coupled. To improve the accuracy of the simulated fuel particle motion in a bubbling fluidized bed, a model accounting for the effect of the bed front and rear walls on the continuum solid phase is combined with the hybrid-model. The rising and sinking velocity of the fuel particles, the circulation time and statistical parameters associated to the location of the fuel particle in the bed were obtained from the simulations and compared with experimental measurements. According to the results, the prediction of these parameters is clearly improved when the friction term is included in the simulation.
Development of an empirical wall-friction model for 2D simulations of pseudo-2D bubbling fluidized beds
(Elsevier, 2016-03) Hernández Jiménez, Fernando; Sánchez Prieto, Javier; Cano Pleite, Eduardo; García Gutiérrez, Luis Miguel; Acosta Iborra, Antonio; Comunidad de Madrid; Ministerio de Ciencia e Innovación (España)
Pseudo-2D fluidized beds have been crucial for the understanding of the dynamics of gas-particle systems. In these systems the distance between the front and back walls is narrow, which restricts and creates a resistance to the solids motion, leading to a different flow behaviour compared to fully 3D systems. This interaction of the particle motion with the walls can be significant and should not be neglected in numerical simulations. The present work develops a new model to easily account for the friction effect between the walls and the particles in a pseudo-2D bed. The model is based on experimental results combined with simplifications of the shear force on a wall provided by the kinetic theory of granular flows. The dependence on the particle diameter and bed thickness is directly introduced in the model through the use of a straightforward expression that is easy to code and does not lead to numerical divergence. To test the model two beds of different thickness were simulated, and the resulting time-averaged solids concentration and velocity as well as bubble properties were compared with experiments. It is shown that the numerical results with the new wall-friction model improve the prediction of the standard 2D-simulations.
Microalgae pyrolysis under isothermal and non-isothermal conditions
(Elsevier, 2020-10) Cano Pleite, Eduardo; Rubio Rubio, Mariano; García Hernando, Néstor; Soria Verdugo, Antonio; Comunidad de Madrid
The present work analyzes and compares the non-isothermal and isothermal pyrolysis processes of two different kinds of microalgae: Spirulina platensis and Chlorella pyrenoidosa. The non-isothermal pyrolysis process is carried out in a Thermogravimetric Analyzer (TGA), whereas a macro-TGA bubbling fluidized bed reactor is used for the isothermal pyrolysis reaction. The simplified Distributed Activation Energy Model (DAEM) is employed to derive the kinetic parameters of the reaction in the TGA, i.e., the pre-exponential factor and the activation energy. A consecutive reaction, first-order pyrolysis model with three competitive pseudo-components for the released pyrolysis vapors is proposed to evaluate the isothermal pyrolysis experiments conducted in a macro-TGA bubbling fluidized bed. The fluidized bed experimental setup allows measuring the real-time mass evolution during the microalgae pyrolysis process, which could be used to obtain the kinetic parameters of the different reactions of which the consecutive reaction model is comprised. The activation energies of the reactions using this isothermal pyrolysis model are in good agreement, both qualitatively and quantitatively, with those obtained from the non-isothermal pyrolysis measurements run in the TGA. A quantitative comparison reveals that the TGA measurements could be employed to provide a useful first estimation of the activation energy of the isothermal pyrolysis process in the bubbling fluidized bed. Qualitatively, the Differential Thermogravimetric (DTG) curves obtained from the thermogravimetric analysis revealed the presence of three peaks in the Chlorella pyrenoidosa DTG curve and two peaks in the Spirulina platensis DTG curve, which is in agreement with the number of pseudo-component reactions in the isothermal pyrolysis model. This indicates the capability of the non-isothermal pyrolysis tests in a TGA to predict the number of reactions required to characterize the isothermal pyrolysis process.
Experimental performance comparison of three flat sheet membranes operating in an adiabatic microchannel absorber
(Elsevier, 2019-04) García Hernando, Néstor; Venegas Bernal, María Carmen; Vega Blázquez, Mercedes de; Ministerio de Economía y Competitividad (España)
A microchannel absorber working adiabatically with the H2O-LiBr pair was tested experimentally using three different nanofibrous flat membranes separating the vapour from the solution. Pore diameters of the membranes were 1 and 0.45 mu m, and thicknesses vary from 25 to 175 mu m. The experimental absorption rates varied from 1.5.10(-3) to 2.6.10(-3) kg/m(2) s varying linearly with the solution mass flow rate circulating through the channels. The reduction in pore diameter from 1 mu m to 0.45 mu m induced the need for higher pressure potential or solution mass flow rate to obtain similar performance. Relationships between changes in diameter pore and membrane thickness from previous models were used to quantify the effect of these membranes characteristics on the absorption ratio. The analytical results compared well with our experiments. In the present design, the solution film thickness was 150 mu m and the solution mass transfer resistance dominated the process. The experimental overall resistances, compared with calculated values from correlations used in previous models, showed differences below 30%.
Experimental characterisation of a novel adiabatic membrane-based micro-absorber using H2O-LiBr
(Elsevier, 2019-02) García Hernando, Néstor; Vega Blázquez, Mercedes de; Venegas Bernal, María Carmen; Ministerio de Economía y Competitividad (España)
In the interest of reducing the size of absorption chillers, a novel adiabatic membrane-based micro-absorber prototype is experimentally studied. Water-lithium bromide solution is used as the working fluid flowing through 50 rectangular microchannels of 0.15mm height, 3 mm width and 58 mm length. In the present study, a laminated microporous PTFE membrane of 0.45 µm pore diameter, separating the solution from the vapour, is tested. It incorporates a supporting layer of polypropylene. Different operating parameters were tested, including the inlet solution mass flow rate, temperature and concentration and the pressure potential for absorption. The measured concentration and temperature of the solution at the absorber outlet are used to evaluate the mass transfer characteristics of the micro-absorber. It is demonstrated that the process is controlled by the solution mass transfer resistance. Calculated results of the absorption rate and the absorption ratio show the advantages of the proposed design considering its compactness. The cooling power of a hypothetical chiller equipped with the tested micro-absorber of 73.7 cm3 effective volume, for the range of variables considered in this study, is 41 W. The modular configuration of the absorber allows to easily scale-up the cooling capacity.
Modeling and performance analysis of an absorption chiller with a microchannel membrane-based absorber using LiBr-H2O, LiCl-H2O, and LiNO3-NH3
(Wiley, 2018-09) Vega Blázquez, Mercedes de; Venegas Bernal, María Carmen; García Hernando, Néstor; Ministerio de Economía y Competitividad (España)
In order to develop compact absorption refrigeration cycles driven by low heat sources, the simulated performance of a microchannel absorber of 5-cm length and 9.5cm(3) in volume provided with a porous membrane is presented for 3 different solution-refrigerant pairs: LiBr-H2O, LiCl-H2O, and LiNO3-NH3. The high absorption rates calculated for the 3 solutions lead to large cooling effect to absorber volume ratios: 625kW/m(3) for the LiNO3-NH3, 552kW/m(3) for the LiBr-H2O, and 318kW/m(3) for the LiCl-H2O solutions given the studied geometry. The performance of a complete absorption system is also analyzed varying the solution concentration, condensation temperature, and desorption temperature. The LiNO3-NH3 and the LiBr-H2O solutions provide the largest cooling effects. The LiNO3-NH3 can work at a lower temperature of the heating source, in comparison with the one needed in a LiBr-H2O system. The lowest cooling effect and coefficient of performance are found for the LiCl-H2O solution, but this mixture allows the use of lower temperature heating sources (below 70 degrees C). These results can be used for the selection of the most suitable solution for a given cooling duty, depending on the available heat source and condensation temperature.
Adiabatic vs non-adiabatic membrane-based rectangular micro-absorbers for H2O-LiBr absorption chillers
(Elsevier, 2017-09-01) Venegas Bernal, María Carmen; Vega Blázquez, Mercedes de; García Hernando, Néstor; Ruiz-Rivas Hernando, Ulpiano; Ministerio de Economía y Competitividad (España)
Simplified model of a membrane-based rectangular micro-desorber for absorption chillers
(Elsevier, 2016-11) Venegas Bernal, María Carmen; Vega Blázquez, Mercedes de; García Hernando, Néstor; Ruiz-Rivas Hernando, Ulpiano; Ministerio de Economía y Competitividad (España)
The simulation of the heat and mass transfer in a H2O-LiBr microchannel desorber endowed with a microporous membrane is presented. The heat and mass transfer processes are modelled by means of selected correlations gathered from the open literature. The simulation provides the evolution along the channels of the parameters involved in the process: heat and mass transfer coefficients, solution concentration, temperatures of the working fluids and pressure potential. The calculated values of the desorption rate are compared to experimental data gathered from the open literature. For the case considered in this study, the maximum ratio between cooling capacity of the chiller and desorber volume is 2312 kW m(-3). This value is more than one order of magnitude higher than the ones found in conventional small-size absorption cooling chillers.
Revised receiver efficiency of molten-salt power towers
(Elsevier, 2015-12) Rodríguez Sánchez, María de los Reyes; Sánchez González, Alberto; Santana Santana, Domingo José; Ministerio de Economía y Competitividad (España)
The demonstration power plant Solar Two was the pioneer design of a molten-salt power tower. In the report "Final Test and Evaluation Results from the Solar Two Project" (Pacheco, 2002, [151) the efficiencies of the three main subsystems: heliostats, receiver and power block were measured or estimated. The efficiency of the global plant and the power block could be obtained with confidence, whereas the efficiencies of the heliostat field and the receiver could only be estimated because the solar flux reflected by the heliostats and intercepted by the receiver cannot be measured. The receiver efficiency was estimated using the Power-On Method. The authors themselves highlighted that this method contain an important assumption: the temperature distribution on the receiver surface is independent of the incident power level. This assumption is equivalent to have a Blot number much smaller than one for the solar receivers operation, fixed inlet and outlet salt temperature. For Solar Two reported data the hot number is of order unity and then the external tube temperature depends on the receiver load; and the thermal losses vary linearly with the incident solar flux rather than constant. Besides, our results show that receiver efficiency is around 76% for full load and 69% for half load instead of 87% and 80% reported when external tube temperature was assumed to be independent on the incident power.
Dynamic performance and stress analysis of the steam generator of parabolic trough solar power plants
(Elsevier, 2019-01-25) González Gómez, Pedro Ángel; Gómez Hernández, Jesús; Ferruzza, D.; Haglind, F.; Santana Santana, Domingo José; Ministerio de Economía y Competitividad (España)
The thermal stress on thick-walled components, such as tubesheets and steam drums, limits both the temperature ramp-up rates and the temperature differences between outer and inner walls. The cyclic operation of concentrating solar power plants may lead to fatigue damage. For these reasons, a stress analysis of the steam generator is required to assure its lifetime. A methodology is presented for the thermo-mechanical analysis of the steam generator for a parabolic trough power plant. This methodology consists of coupling transient thermodynamic and stress models of the heat exchangers in order to calculate the stress. Besides the heat exchanger models, a transient model for a TEMA H heat exchanger is proposed. Finite element simulations are carried out to calculate the deviations of the simplified analytical models. In this way, a powerful tool that allows the analysis and optimization of the steam generator operation is proposed. The results suggest that U-tube tubesheets are exposed to high thermal stresses on the no-tube-lane zone, especially in the reheater. The steam generator start-up can be accomplished in around 45 min using 36.4 MWh(th). Furthermore, the TEMA X evaporator presents a thermal stress reduction of 35% compared to the kettle evaporator.
Maximizing the power block efficiency of solar tower plants: Dual-pressure level steam generator
(Elsevier, 2018-11-05) Gómez Hernández, Jesús; González Gómez, Pedro Ángel; Villa Briongos, Javier; Santana Santana, Domingo José; Ministerio de Economía y Competitividad (España)
Solar tower plants (STP) are one of the most promising renewable technologies to substitute conventional power plants. To further increase its penetration in electricity markets, it is necessary to maximize its efficiency. This work addresses the challenge of increasing the inlet pressure up to 165 bar at the high pressure turbine (HPT) to improve the power block efficiency. Nowadays, these plants operate Rankine cycle with reheating using steam at 126 bar at the inlet of the HPT. This pressure is limited in current STP by the working temperatures of the molten salt, which range from 285 degrees C and 565 degrees C, the pinch point temperature difference in the evaporator and the current steam generator (SG) layout. A new steam generator design with dual-pressure level evaporation is proposed. The heat exchangers that form the SG are designed thermomechanically and the power cycle performance is analyzed using the first and second laws of thermodynamics. The results show that the novel dual-pressure level SG layout increases the power block efficiency from 44.14% to 44.64%. Assuming a market pricing scenario of two-tier tariff and a power purchase agreement price of 16.3c is an element of/kWh(e), the new SG layout yields an extra economic benefit of 623 k is an element of per year due to an increase of energy produced of 5.71 GWh(e),/year.

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