Soiling effect in solar energy conversion systems: A review, R. Conceição, J. González-Aguilar, A. A. Merrouni, M. Romero, Renewable and Sustainable Energy Reviews, 162, art. no. 112434, 2022, Online version, https://doi.org/10.1016/j.rser.2022.112434
This review provides a comprehensive, detailed description and contextualization of soiling research evolution in the solar energy field throughout time. The analysis consists of past soiling research, including important notes on notable works and main researches. The current state of the art is presented, followed by an extended literature survey covering from 1942 to 2019, facilitating the finding of primordial research concerning each of the available technologies, and enriching knowledge regarding the existing extensive research database. Moreover, soiling analysis and comments are made for several specific topics, such as cleaning techniques and environmental effects on soiling deposition. Finally, future prospects and research directions on the soiling effect are given.
Optical and thermal integration analysis of supercritical CO2 Brayton cycles with a particle-based solar thermal plant based on annual performance, R. Chen, M. Romero, J. González-Aguilar, F. Rovense, Z. Rao, S. Liao, Renewable Energy, 189, 164-179, 2022, Online version, https://doi.org/10.1016/j.renene.2022.02.059
Central receiver concentrating solar power (CSP) plants based on particles as heat transfer fluid in solar circuits and supercritical CO2 (S–CO2) Brayton cycles can fulfil the requirements for next generation CSP to improve solar-to-electric efficiency and reduce energy storage costs. However, effective incorporation of these two concepts requires an in-depth understanding of their characteristics and an appropriate approach to match them. This paper addresses the importance of the design features and annualized performances of the optical subsystem (heliostat-receiver) and the thermal-to-electricity subsystem (solar receiver-energy storage-power block) on the global optimization of any integrated CSP plant. The analysis lies in a complete model of a particle-based CSP plant, which includes detailed modeling for the solar field, a cavity solar receiver with an up bubbling fluidized bed (UBFB) tubular panel, particles storage tanks and a recompression S–CO2 Brayton cycle. The design incident irradiance on the receiver aperture (IR) and the particles temperature at the receiver outlet (Tp) are identified as key parameters determining the solar-to-electric integration procedure and affecting the overall plant design and annual performance. Regarding subsystems located upstream and downstream of the receiver, the effects of heliostat and power block characteristics on the optimal IR and Tp are also evaluated, represented by the heliostat beam quality and main compressor inlet temperature. Results show that IR around 1,200–1,500 W/m2 provides the maximum system design efficiency and annual efficiency. Improvements on heliostat beam quality and power block efficiency help to increase the optimal IR and overall system efficiency. In the optimal range of IR, increasing Tp leads to higher system design efficiency, but lower system annual efficiency and annual electricity output. The optimal combination of IR and Tp contributes to a minimum heliostat design area, representing the integration trade-off between the system optical and thermal characteristics.
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
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.
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
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.
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
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.
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
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.
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
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.
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
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.
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
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.
- Modeling of Beam-down linear Fresnel conveyor dryer for bulk solid processing
- Short overview on concentrating solar energy
- Silica-coated Ca(OH)2 – Al2O3 pellets for thermochemical energy storage in CSP plants
- Caracterização Experimental da Deposição de Poeiras e Modelação num Campo Solar em Ambiente Urbano
- Síntesis y conformado macroscópico del material LSCF 6428 (La0,6Sr0,4Co0,2Fe0,8O3±δ) para la producción de hidrógeno mediante ciclos termoquímicos