Artículos Científicos | Instituto IMDEA Energía

Category: Artículos Científicos

2022, Artículos Científicos

Designing a flat beam-down linear Fresnel reflector

Designing a flat beam-down linear Fresnel reflector, S. Taramona, P. Á. González-Gómez, J. V. Briongos, J. Gómez-Hernández, Renewable Energy, 187, 484-499, 2022, Online version, https://doi.org/10.1016/j.renene.2022.01.104

Abstract

A linear beam-down solar field consists of two reflections that concentrate the solar irradiation on heavy materials located on the ground. Several rows of linear Fresnel reflectors, which have the same width, aim the solar irradiation to a secondary mirror with a hyperbolic shape that redirects the solar concentration towards the ground receiver. This paper overcomes the main limitation of the previously proposed hyperbolic secondary reflector. A new secondary reflector composed by several fixed flat mirrors located at the same height is proposed. A model to calculate the optimal layout of this novel solar field, as well as the efficiency and concentration, is developed and validated against a Monte-Carlo Ray-Tracing software, obtaining relative errors lower than 15%. Two new dimensionless parameters are proposed to facilitate the design of the flat beam-down linear Fresnel reflector. The concentration, optical efficiency and receiver width can be easily obtained, without performing any simulation, as a function of the dimensionless parameters. This novel solar field can achieve concentration ratios of up to 31 and optical efficiencies of up to 60%, obtaining similar concentrations with better optical efficiency than a field using a hyperbolic reflector.

2022, Artículos Científicos

Evaluating the Corrosion Resistance of Inconel 625 Coatings, Processed by Compact Plasma Spray, for Applications in Concentrating Solar Power Plants

Evaluating the Corrosion Resistance of Inconel 625 Coatings, Processed by Compact Plasma Spray, for Applications in Concentrating Solar Power Plants, F. Rubino, D. Merino, C. Munez, P. Poza, Key Engineering Materials, 926, 1736-1745, 2022, Online version, https://doi.org/10.4028/p-d3uuc2

Abstract

Thermal energy storage (TES) systems have paramount importance in the design of Concentrating Solar Power (CSP) plants. TES systems allow storing the energy collected from solar radiation as heat energy in a thermal fluid and, in that way, extending the energy duration period of the plant and making the produced electricity dispatchable, depending on the actual demand and not only on the availability of the sun. The thermal fluids, synthetic oils, or molten salts, usually operate at temperatures from 500°C up to 800°C. The harsh operative conditions bring out issues related to the compatibility with the construction materials of CSP components, i.e., carbon and stainless steel. Coating of low-alloy structural steel with high-resistant materials has been addressed as a promising solution for mitigating the corrosion in TES system components. Compact plasma spray process was used to deposit Inconel 625 alloy onto T22 carbon steel coupons. Nitrate salts mixture, 60%NaNO₃-40KNO₃, commonly employed in CSP systems as operative and thermal storage fluid was used as corrosion medium. The tests were conducted by immersing coated and uncoated samples in molten salts at 500°C for 1, 3 7, and 14 days to assess the corrosion behavior of the In625 coatings. After 24 hours of exposition to molten nitrate salts, the T22 surface showed a pronounced oxidized layer having a thickness of approximately 20 µm. This layer is mainly composed of oxygen, iron, and chromium, which are the main constituents of carbon steel, with a few traces of sodium and potassium derived from the reaction of salts with the steel. Inconel 625, on the other hand, showed the formation of very thin scales of corrosion products localized only on the surface of the sample. Longer exposition is expected to produce a more pronounced degradation of uncoated steel, but barely affect the Inconel 625 coating.

2022, Artículos Científicos

Alternative low-power plasma-sprayed inconel 625 coatings for thermal solar receivers: Effects of high temperature exposure on adhesion and solar absorptivity

Alternative low-power plasma-sprayed inconel 625 coatings for thermal solar receivers: Effects of high temperature exposure on adhesion and solar absorptivity, D. Merino-Millan, C. J. Múnez, M. Á. Garrido-Maneiro, P. Poza, Solar Energy Materials and Solar Cells, 245, 111839, 2022, Online version, https://doi.org/10.1016/j.solmat.2022.111839

Abstract

Over recent years, renewable energy technologies have focused on increasing performance and efficiency, and on the reduction of maintenance costs. In this work, thermal-sprayed Inconel 625 coatings have been studied as an alternative for concentrated solar power plants receivers. A low-power compact plasma spray system was used to deposit coatings onto two substrates: grade 22 ferritic steel and AISI 316 L austenitic steel. This system may be used for in-situ maintenance or repair purposes. The coatings were heat-treated at two temperatures: 520 °C and 800 °C, at different exposure times. The aim of this work was to evaluate the effect of this treatment on the adherence and solar absorptivity of the Inconel 625 coatings. The results showed that, at higher temperatures and longer exposure times, better adherence and absorptivity are achieved. Adherence values above 60 MPa were obtained due to diffusion in the coating-substrate interface. Additionally, absorptivity values above 93% were measured due to oxide formation on the coating surface during heat treatment. Furthermore, the highest temperature of the oxidized treatment reported the highest values of absorptivity. These results show that the developed Inconel 625 coatings could be considered as a possible alternative to improve the performance of concentrated solar power plants.

2022, Artículos Científicos

An optimisation method for the cold-spray process: On the nozzle geometry

An optimisation method for the cold-spray process: On the nozzle geometry, L. Alonso, M. A. Garrido, P. Poza, Materials & Design, 214, 110387, 2022, Online version, https://doi.org/10.1016/j.matdes.2022.110387

Abstract

Currently, the cold-spray process, or simply cold spray, is an extensively used technique in coating applications. The low temperature of the deposition process is the distinctive feature that makes it suitable for many additive manufacturing activities such as repair and restoration of damaged components. The reliability of the coatings is strongly dependent on the velocity of the powder during its impact on the target surface. Spraying conditions such as the pressure and temperature of the carrier gas and the geometry of the nozzle control the acceleration of the powder particles. Consequently, there is an increasing interest in the optimisation of nozzle geometry so as to maximise the acceleration of the particles through the nozzle path that they follow. In contrast with various extant approaches to achieve this aim (finite element modelling, experimental approach, and analytical methods), an alternative model based on the one-dimensional isentropic theory that accounts for the dynamics of the dilute two-phase flow was developed in this study. First, an analysis of the common hypotheses used to obtain the equation of motion of the particle was carried out. Subsequently, with the new insights revealed from the previous analysis, a new theoretical model for the optimisation of the divergent part of the nozzle was performed considering a geometric angle restriction. This model is based on the numerical integration of the equation of motion of the particle, ensuring the maximisation of the particle drag force by means of the Lagrange multiplier method. Once the analytical model is formulated, a set of curves describing the optimal geometric parameters for different conditions is obtained. Moreover, some optimal geometries are presented demonstrating the low influence of the angle restriction. Additionally, the inversely proportional relationship between stagnation pressure and temperature is revealed.

2022, Artículos Científicos

Development of stable porous silica-coated Ca(OH)2/γ-Al2O3 pellets for dehydration/hydration cycles with application in thermochemical heat storage

Development of stable porous silica-coated Ca(OH)₂/γ-Al₂O3 pellets for dehydration/hydration cycles with application in thermochemical heat storage, L. Briones, C. M. Valverde-Pizarro, I. Barras-García, C. Tajuelo, E. S. Sanz-Pérez, R. Sanz, J.M. Escola, J. González-Aguilar, M. Romero, Journal of Energy Storage, 51, 104548, 2022, Online version, https://doi.org/10.1016/j.est.2022.104548

Abstract

Thermochemical heat storage based on the CaO + H₂O ↔ Ca(OH)₂ system is extremely promising in CSP plants that can reach medium to high temperatures, such as those equipped with tower and heliostats. However, the attrition of pure CaO pellets is a major drawback that hampers an actual commercial development. This work proposes the dip-coating of mixed Ca(OH)₂/γ-Al₂O₃ spherical and cylindrical pellets with dense silica and Al-MCM-41 (mesoporous silica) gels. The original hardness of pure Ca(OH)₂ pellets (<2 N) can be increased up to 31 N using 40 wt% alumina as binder and applying a silica coating. Both gels formed a hard calcium silicate layer upon calcination that helped keeping the structural integrity of the samples after dehydration/hydration cycles. The samples were tested in 10 consecutive cycles at dehydration and hydration temperatures of 600 °C and 250–425 °C, respectively. Cylindrical pieces displayed higher hardness values and hydration yields compared to the spherical counterparts. Interestingly, porous silica-coated cylindrical pellets achieved a remarkable hydration yield of 85% and presented a hardness value of 8 N after cycling. This was due to its porous nature and the composition of the coating, formed by thin sheets and small grains, which allowed preserving the outer porous structure of the pellet.

2021, Artículos Científicos

A New Thermal-Solar Field Configuration: The Rotatory Fresnel Collector or Sundial

A New Thermal-Solar Field Configuration: The Rotatory Fresnel Collector or Sundial, J. Cano-Nogueras, J. Muñoz-Antón, J. M. Martinez-Val, Energies, 14 (14), 4139, 2021, Online version, https://doi.org/10.3390/en14144139

Abstract

A new type of Fresnel array has been devised and constructed as an answer to the need to reduce the investment costs of solar thermal collectors, without jeopardizing their efficiency in capturing solar radiation at high temperatures. The array of mirror bands is fixed onto a horizontal platform, which rotates around a virtual vertical axis, so that the sun is in the extrapolated vertical plane of symmetry of the array. The receptor central line is also placed in said plane, and it is physically made of at least one tube at each side of the plane. The geometrical relation between the mirrors and the receptor is therefore fixed. The platform rotates with the same speed as that of the sunlight’s azimuthal component. On the contrary, the angle of incidence of the sunlight on the mirrors changes as the sun rises and declines in its daily apparent motion, but this effect does not disturb the radiation concentration kinematics, although it induces a shift along the receptor. This is a new configuration based on the use of simple and cheap flat mirrors to obtain circular cylindrical mirrors. These mirrors are made of originally flat mirrors that are bent by applying an inexpensive and simple bending technique patented by our research group. The radius of curvature of each mirror is tuned to the distance from the mirror to the receiver central line. The integration of different scientific domains (such as structural analysis) and elementary technologies (such as 3D printing) in this innovative solar radiation concentrator and receiver can lead to a large reduction in costs. Nevertheless, the first experimental campaign has shown additional problems in the receiver configuration, which should be addressed in a next stage of research. This paper explains the methodology used and procedures in the development of the first prototype of the Sundial.

2022, Artículos Científicos

Air solid packed-beds for high temperature thermal storage: practical recommendations for predicting their thermal behaviour

Air solid packed-beds for high temperature thermal storage: practical recommendations for predicting their thermal behaviour, E. Alonso, E. Rojas, Applied Thermal Engineering, 202, 117835, 2022, Online version, https://doi.org/10.1016/j.applthermaleng.2021.117835

Abstract

An advantageous solution for thermal energy storage is an air solid packed bed that consists in a tank filled with a solid material through which an air stream passes transferring heat to the filler in charge and collecting it in discharge. Effective tools to predict the thermal behaviour are required to optimize efforts towards the technology consolidation. To contribute to it, an experimental and numerical study on an air solid packed bed is presented here including some novel assumptions. The model is based on two energy balances applied to the solid and air phase separately. It also accounts for the thermal capacity of the tank walls by means of a correction of the solid density. The effective conductivity of the solid was evaluated through two components: one for the conduction through the solid and another for the radiative heat transfer. The last one gains relevance as the system average temperature rises, although it can be neglected below 500 °C. Convective heat transfer coefficient was introduced according to published correlations that were adequate for the operation conditions. The model was validated with experimental results from an own facility. The most critical parameter in the system behaviour is the thermal capacity of the filler. This fact points out the need of an accurate measurement of the specific heat. Finally, the possibility of simulating this kind of systems assuming an effective medium and solving an only heat balance was proposed if the convective heat transfer coefficient between solid and air is above a limit established in 100 W/m²K. These conclusions serve as reference to optimize efforts in predicting the thermal behaviour of air solid packed beds systems.

2022, Artículos Científicos

Hydrogen production by thermochemical water splitting with La0.8Al0.2MeO3-δ (Me= Fe, Co, Ni and Cu) perovskites prepared under controlled pH

Hydrogen production by thermochemical water splitting with La0.8Al0.2MeO3-δ (Me= Fe, Co, Ni and Cu) perovskites prepared under controlled pH, A. Pérez, M. Orfila, M. Linares, R. Sanz, J. Marugán, R. Molina, J. A. Botas, Catalysis Today, 390, 22-33, 2022, Online version, https://doi.org/10.1016/j.cattod.2021.12.014

Abstract

Aluminum based perovskites, La0.8Al0.2MeO3-δ (Me = Co, Ni, Fe, Cu), were synthesized following the Pechini method at different pHs and evaluated for hydrogen production by water splitting in a two-step thermochemical cycle. The pH of the synthesis medium showed a critical influence in the redox properties of the aluminum-based perovskites during the thermochemical cycle. Both the thermal reduction and the hydrolysis step produce irreversible changes in the crystalline structure of La0.8Al0.2MeO3-δ perovskites prepared at acid pH, avoiding the cyclability of the material and a stable production of hydrogen during consecutive cycles. However, these changes are not observed when the perovskites were synthesized at basic pH and operated at isothermal conditions at 800 ºC. In this case, the materials keep the crystalline structure leading to stable hydrogen production during consecutive cycles. Among all the studied materials, the nickel-based La0.8Al0.2NiO3-δ perovskites exhibited the best hydrogen productivity per cycle, 4.4 cm3 STP /gmaterial·cycle (Standard Temperature and Pressure conditions), which is a remarkable result considering that the thermochemical water splitting is conducted under isothermal conditions at just 800 ºC. Moreover, this result is accompanied with a good performance of the perovskite in terms of solar to fuel efficiency (being 0.46 the ratio between the potential energy recovery from the produced hydrogen and the solar heat required for its production), comparable or even higher than values reported in the literature for other metal oxides. These results confirm the La0.8Al0.2NiO3-δ perovskite as an auspicious material for a full-scale H2 production from water splitting by solar-driven thermochemical cycles at low temperatures.

2022, Artículos Científicos

A new design of multi-tube receiver for Fresnel technology to increase the thermal performance

A new design of multi-tube receiver for Fresnel technology to increase the thermal performance, M. J. Montes, R. Abbas, R. Barbero, A. Rovira, Applied Thermal Engineering, 204, 117970, 2022, Online version, https://doi.org/10.1016/j.applthermaleng.2021.117970

Abstract

Solar heat for industrial processes is a promising way to meet the high thermal demand required by the industry, while this application becomes an important niche market for solar technology. In this research line, it is proposed a novelty concept based on a rotary Fresnel solar collector to supply heat above 150 °C. This work is focused on the multi-tube receiver for this Fresnel collector, proposing a thermal design based on three criteria that can be generalized for any multi-tube receiver: the fluid flow layout is arranged to meet the symmetry of the solar flux map; the fluid circulates from the lower to the higher flux density zone; and the fluid velocity is modified by modifying the tube diameter, to optimize the heat transfer. Following these criteria, the final configuration of the receiver is chosen based on an exergy optimization, in which both heat loss and pressure drop must be quantified. It has been also accomplished a generalization of the optimization methodology for Fresnel collectors providing heat at different temperatures, showing that, in these cases, the configuration that maximizes the exergy efficiency does not correspond to the one with the highest energy efficiency. This thermal design method can be applied to multi-tube receivers working at higher temperatures in longer Fresnel loops, in which case the optimization will result in more marked differences between the optimal values and the standard ones.

2022, Artículos Científicos

Thermo-economic analysis of a particle-based multi-tower solar power plant using unfired combined cycle for evening peak power generation

ES Thermo-economic analysis of a particle-based multi-tower solar power plant using unfired combined cycle for evening peak power generation, F. Rovense, M. Á. Reyes-Belmonte, M. Romero, J. González-Aguilar, Energy, 240, 122798, 2022, Online version, https://doi.org/10.1016/j.energy.2021.122798

Abstract

This work analyses a 150 MWe multi-tower solar-only combined cycle power plant (nominal efficiency ∼50%) for evening peak operation. Olivine particles are used as heat transfer fluid and thermal energy storage medium based on their suitable thermo-physical properties for high temperature operation. Technical constraints to handle hot particles lead to an integration of the power block and thermal storage system with an array of heliostat fields (with a solar receiver per field). Unitary 53.0 MWth solar tower was designed to satisfy these constrains. Two electricity dispatch strategies covering the evening peak power have been analyzed. Number of solar fields and storage capacity have been optimized from thermo-economic optimization. It is concluded that the best layouts have seven solar towers and storage capacities of 2.0 GWh for the first dispatch scenario (with an electricity generation from 17:00 to 22:00) and eight solar towers with 2.5 GWh for the second one (from 17:00 to 24:00). Solar multiple is between 1.1 and 1.25. These two configurations cover 56.2% and 55.8% of the total energy demand at full power with LCOE of 14.6 c€ kWh−1 and 13.2 c€ kWh−1. A sensitivity analysis on the components costs indicates that 11.0 c€ kWh−1 could be achieved.

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Designing a flat beam-down linear Fresnel reflector

Abstract

Evaluating the Corrosion Resistance of Inconel 625 Coatings, Processed by Compact Plasma Spray, for Applications in Concentrating Solar Power Plants

Abstract

Alternative low-power plasma-sprayed inconel 625 coatings for thermal solar receivers: Effects of high temperature exposure on adhesion and solar absorptivity

Abstract

An optimisation method for the cold-spray process: On the nozzle geometry

Abstract

Development of stable porous silica-coated Ca(OH)2/γ-Al2O3 pellets for dehydration/hydration cycles with application in thermochemical heat storage

Abstract

A New Thermal-Solar Field Configuration: The Rotatory Fresnel Collector or Sundial

Abstract

Air solid packed-beds for high temperature thermal storage: practical recommendations for predicting their thermal behaviour

Abstract

Hydrogen production by thermochemical water splitting with La0.8Al0.2MeO3-δ (Me= Fe, Co, Ni and Cu) perovskites prepared under controlled pH

Abstract

A new design of multi-tube receiver for Fresnel technology to increase the thermal performance

Abstract

Thermo-economic analysis of a particle-based multi-tower solar power plant using unfired combined cycle for evening peak power generation

Abstract

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