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2020, Artículos Científicos

Experimental Devices to Investigate the Long-Term Stability of Phase Change Materials under Application Conditions

Experimental Devices to Investigate the Long-Term Stability of Phase Change Materials under Application Conditions, C. Rathgeber, S. Hiebler, R. Bayón, L. F. Cabeza, G. Zsembinszki, G. Englmair, M. Dannemand, G. Diarce, O. Fellmann, R. Ravotti, D. Groulx, A. C. Kheirabadi, S. Gschwander, S. Höhlein, A. König-Haagen, N. Beaupere, L. Zalewski, Applied Sciences, 10, 7968, 2020, Online version, https://doi.org/10.3390/app10227968

Abstract

An important prerequisite to select a reliable phase change material (PCM) for thermal energy storage applications is to test it under application conditions. In the case of solid–liquid PCM, a large amount of thermal energy can be stored and released in a small temperature range around the solid–liquid phase transition. Therefore, to test the long-term stability of solid–liquid PCM, they are subjected to melting and solidification processes taking into account the conditions of the intended application. In this work, 18 experimental devices to investigate the long-term stability of PCM are presented. The experiments can be divided into thermal cycling stability tests, tests on PCM with stable supercooling, and tests on the stability of phase change slurries (PCS). In addition to these experiments, appropriate methods to investigate a possible degradation of the PCM are introduced. Considering the diversity of the investigated devices and the wide range of experimental parameters, further work toward a standardization of PCM stability testing is recommended.
2020, Artículos Científicos

Comparative Analysis of Energy and Exergy Performance of Hydrogen Production Methods

Comparative Analysis of Energy and Exergy Performance of Hydrogen Production Methods, A. Martínez-Rodríguez, A. Abánades, Entropy, 22, 1286, 2020, Online version,  https://www.mdpi.com/1099-4300/22/11/1286

Abstract

The study of the viability of hydrogen production as a sustainable energy source is a current challenge, to satisfy the great world energy demand. There are several techniques to produce hydrogen, either mature or under development. The election of the hydrogen production method will have a high impact on practical sustainability of the hydrogen economy. An important profile for the viability of a process is the calculation of energy and exergy efficiencies, as well as their overall integration into the circular economy. To carry out theoretical energy and exergy analyses we have estimated proposed hydrogen production using different software (DWSIM and MATLAB) and reference conditions. The analysis consolidates methane reforming or auto-thermal reforming as the viable technologies at the present state of the art, with reasonable energy and exergy efficiencies, but pending on the impact of environmental constraints as CO2 emission countermeasures. However, natural gas or electrolysis show very promising results, and should be advanced in their technological and maturity scaling. Electrolysis shows a very good exergy efficiency due to the fact that electricity itself is a high exergy source. Pyrolysis exergy loses are mostly in the form of solid carbon material, which has a very high integration potential into the hydrogen economy.
2020, Artículos Científicos

Drift analysis in tilt-roll heliostats

Drift analysis in tilt-roll heliostats, A. Martínez-Hernández, I. B. Gonzalo, M. Romero, J. González-Aguilar, Solar Energy 211, 1170-1183, 2020, Online version, https://doi.org/10.1016/j.solener.2020.10.057

Abstract

Drift is a relevant issue in concentrated solar tower facilities since this time-dependent pointing error severely affects their performance. This work deals with the optical analysis of drift for tilt-roll heliostats in terms of the geometrical parameters associated to the heliostat mechanical structure and the local time. For each drift source or error, flux maps have been obtained by means of Monte Carlo ray-tracing calculations as a function of the source magnitude and daily time in winter and summer solstices. Then the corresponding pointing errors and the drift curves are determined from the displacement of the center of gravity of the flux maps. As a result, drift related to each source is identified and characterized. Additionally, drift sources are compared in terms of the standard deviation and the mean of their daily pointing errors. The analysis evidences that main drift sources are the misalignments of the pedestal like the one of an inclination with respect to the zenith.
2020, Eventos: Jornadas, foros, ferias

SFERA-III/ACES2030 Joint Workshop (ONLINE) on MST-Materials for Solar Thermochemistry

The energy transition to a climate neutral economy by year 2050, based upon the massive deployment of renewable energy and the circular economy, makes compulsory to promote new technological solutions for decarbonization of transport and key industrial sectors like chemical industry that are currently not on track to achieve the objectives for 2030 and 2050. Solar chemistry by using concentrating solar systems and thermodynamic cycles based on metal oxides for the water splitting (alone or combined with CO2) or for thermochemical storage may have a relevant role in this context. However, the associated technology challenges, such as the ultra-high irradiance and temperature of operation in the solar reactor and the preservation of high material performance after numerous cycles, are key subjects for its eventual implementation.

The EU project Sfera-III and the Comunidad de Madrid project ACES2030 are partnering in the organization of this joint workshop that addresses the most recent advances on materials R&D for solar thermochemistry.

The participation is free of charge and requires registration. To register for the event, please fill in the online registration form.

Download the full programme here.

For additional information, email us at [email protected]

2020, Artículos Científicos

Coating of Ca(OH)2 / γ-Al2O3 pellets with mesoporous Al2O3 and its application in thermochemical heat storage for CSP plants

Coating of Ca(OH)2 / γ-Al2O3 pellets with mesoporous Al2O3 and its application in thermochemical heat storage for CSP plants, C. M. Valverde-Pizarro, L. Briones, E. Sanz, J. M. Escola, R. Sanz, J. González-Aguilar, M. Romero, Renewable Energy, 162, 587-595, 2020, Online version, https://doi.org/10.1016/j.renene.2020.08.095

Abstract

Thermochemical heat storage using the Ca(OH)2 dehydration/hydration reaction for CSP plants requires the development of advanced materials that avoids the breakage shown by conventional CaO pellets. In this work, spherical cores made up of pelletized 60 wt % Ca(OH)2 and 40% g-Al2O3 powders were coated with different mesoporous alumina coverings by means of a dip-coating route, in order to enhance its structural integrity for dehydration/hydration cycles. Three strategies of preparation were devised using as coatings alumina gels containing cetyltrimethylammonium bromide as surfactant in neutral (A2 CS) and acid pH (A4 CS) and without surfactant in neutral medium (A3 CS). In both A2 CS and A4 CS materials, denser alumina coatings were attained originating higher crushing strengths, pointing out the positive role played by the surfactant in the gel makeup. SEM images indicated that A2 presented small alumina grains (0.3-3 µm) highly dispersed over their surface, while over A4 CS, long sheets were appreciated. All the materials exhibited high hydration capacities after 10 cycles (>80%, >800 kJ/kg) at 250 °C, but only A2 CS hold its structural integrity that was preserved even after 20 cycles without loss of its hydration capacity.
2020, Artículos Científicos

Optimization of a New Design of Molten Salt-to-CO2 Heat Exchanger Using Exergy Destruction Minimization

Optimization of a New Design of Molten Salt-to-CO2 Heat Exchanger Using Exergy Destruction Minimization, M. J. Montes, J. I. Linares, R. Barbero, B. Y. Moratilla, Entropy, 22, 883, 2020, Online version, https://doi.org/10.3390/e22080883

Abstract

One of the ways to make cost-competitive electricity, from concentrated solar thermal energy, is increasing the thermoelectric conversion efficiency. To achieve this objective, the most promising scheme is a molten salt central receiver, coupled to a supercritical carbon dioxide cycle. A key element to be developed in this scheme is the molten salt-to-CO2 heat exchanger. This paper presents a heat exchanger design that avoids the molten salt plugging and the mechanical stress due to the high pressure of the CO2, while improving the heat transfer of the supercritical phase, due to its compactness with a high heat transfer area. This design is based on a honeycomb-like configuration, in which a thermal unit consists of a circular channel for the molten salt surrounded by six smaller trapezoidal ducts for the CO2. Further, an optimization based on the exergy destruction minimization has been accomplished, obtained the best working conditions of this heat exchanger: a temperature approach of 50 °C between both streams and a CO2 pressure drop of 2.7 bar.
2020, Artículos Científicos

Hydrogen production by water splitting with Mn3-xCoxO4 mixed oxides thermochemical cycles: A thermodynamic analysis

Hydrogen production by water splitting with Mn3-xCoxO4 mixed oxides thermochemical cycles: A thermodynamic analysis, M. Orfila, M. Linares, R. Molina, J. Marugán, J. A. Botas, R. Sanz, Energy Conversion and Management, 216, 112945-112955, 2020, Online version,  https://www.sciencedirect.com/science/article/pii/S0196890420304830?via%3Dihub

Abstract

The high temperature required for hydrogen production by solar driven thermochemical cycles is a critical factor hindering full-scale applications. The thermochemical cycle based on Mn3O4/MnO redox pair is one of the most studied despite the high operating temperature required for complete de reduction step (1623–1723 K). The combination of Mn3O4 with Co3O4, a metal oxide with lower reduction temperature than Mn3O4 that cannot be used for hydrogen production due to thermodynamic limitations, is presented as a way to decrease significantly the energy demand of the cycle. In this work, a complete thermodynamic study of thermochemical cycles with different Mn/Co mixed oxides (Mn3-xCoxO4, 0.9 < x < 2.7) for hydrogen production has been performed. The study of the variation of Gibbs energy with temperature allowed to determine that the thermal reduction of the metal oxide (Mn3-xCoxO4) takes place at temperatures between 1048 and 1173 K, which can be achieved by conventional solar concentration technologies. Unfortunately, the oxidation of the reduced metal oxide (Mn3-xCoxO3) with water to produce H2 is not feasible from a thermodynamically point of view, so a stronger oxidizing agent, as sodium hydroxide, is required. The optimum temperature for the oxidation with NaOH was found to be 1373 K, meaning that this reaction takes place at higher temperatures than those actually required for the reduction, something uncommon in thermochemical cycles for water splitting. On the other hand, after the study of the variables affecting the equilibrium like the inert gas/solid ratio, it can be concluded that in both reactions, thermal requirements can be reduced by operating at lower temperatures by means of a higher inert gas/Mn3-xCoxO4 ratio. Finally, energy and exergy analysis of the system based on the solar absorption efficiency and the energy requirements predicts a solar-to-fuel efficiency and exergy efficiency of 40% and 23%, respectively. These values are comparable or even higher than those found in literature for other metal oxides thermochemical cycles for water splitting, thus with the advantage of working at a considerable lower temperature (1373 K).
2020, Artículos Científicos

Thermodynamic analysis of multi-heating cycles working around the critical point

Thermodynamic analysis of multi-heating cycles working around the critical point, L.F. González-Portillo, J. Muñoz-Antón, J. M. Martínez-Val, Applied Thermal Engineering, 174, 115292, 2020, Online version,  https://doi.org/10.1016/j.applthermaleng.2020.115292

Abstract

Multi-heating cycles reduce regenerator irreversibility, resulting in an increase of exergy efficiency. While a single heat source supplies the thermal power in simple regenerative cycles, in multi-heating cycles additional heat sources at lower temperatures supply part of this power along the regeneration. The greater the number of these additional heats, the higher the exergy efficiency is. An exergy analysis compares multi-heating and simple regenerative cycles working in the surroundings of the critical point. The main purpose is to identify the thermodynamic region in which there is a greater benefit of multi-heating, trying to abstract from the fluid as much as possible. The effect of compressor inlet temperature and limiting pressures is studied by means of parametric analysis. Cycles with the compressor working at pressures above the discontinuity line (set of coexistence line and Widom line) have small precooler irreversibilities and large regenerator irreversibilities. Multi-heating cycles take advantage of these conditions since they can reduce the regenerator irreversibilities and maintain the precooler irreversibilities small, leading to high exergy efficiencies. The results show that the exergy efficiency of multi-heating cycles may be up to 24% greater than in simple cycles.
2020, Artículos Científicos

MnO2-supported catalytic bodies for selective reduction of NO with NH3: Influence of NO2 and H2O

MnO2-supported catalytic bodies for selective reduction of NO with NH3: Influence of NO2 and H2O, A. Serrano-Lotina, A. Iglesias-Juez, M. Monte, P. Ávila, Molecular Catalysis, 491, 111004, 2020, Online version, https://doi.org/10.1016/j.mcat.2020.111004 

Abstract

Catalytic bodies based on MnO2, TiO2 and sepiolite showed an improved behavior for selective reduction of NOx with NH3 at low temperatures. In this work, the influence of Mn %wt and NO2 and/or water vapor presence was assessed. The materials were prepared by wet equilibrium impregnation of the pre-shaped support. All the catalysts exhibited the characteristic diffraction peaks of TiO2-anatase, hexagonal MnO2and anhydrous sepiolite. According to SEM-EDX, MnO2 was preferentially deposited over TiO2.

The optimum nominal content of Mn was 7.5 %wt, what may be related to an appropriate particle dispersion that confers the suitable properties: lower acid strength, and better redox properties with improved reducibility. This catalyst was then evaluated under NO2 and/or H2O presence. NO2 improved the catalytic response and the best result was obtained with 1:1 NO2:NO ratio showing 90 % NOx conversion and 97 % N2 selectivity at very low temperature, 110 °C. A higher amount of NH4NO3 seems to be formed with the increase in the NO2/NO ratio causing a modification of N2 selectivity. Although water presence affected the performance, mainly at low temperatures, the catalyst showed good resistance to water vapor at temperatures above 180 °C maintaining good activity (90 % of NO conversion and 96 % of N2 selectivity). When NO2 was co-added to the feed, a beneficial effect was again observed with 91 % of NOx and 95 % of N2 selectivity converted at 143 °C.

At low temperatures, deactivation has to do with a competitive effect of H2O with NOx towards the adsorption centers, which decreases the concentration of nitrate species adsorbed on the surface. However, the adsorption of ammonia was less affected.

7.5 Mn catalyst is a highly efficient system to work in real conditions, where there is water in gas stream. In this situation, co-feeding NO2 improves activity.

2020, Artículos Científicos

Improving durability of silicone-based paint coatings used in solar power plants by controlling consolidation procedures

Improving durability of silicone-based paint coatings used in solar power plants by controlling consolidation procedures, N. Martínez; A. Rico, C. J. Múnez, C. Prieto, P. Poza, Solar Energy, 199, 585-595, 2020, Online version, https://doi.org/10.1016/j.solener.2020.02.049

Abstract

Pyromark is a silicone-based black paint that is commonly used in concentrated solar power plants because of its relatively high absorptivity. However, it has very low durability, which decreases the optical performance of the surfaces coated with this paint over time, thus reducing the thermal efficiencies of these plants. Recent studies have tried to develop promising materials to circumvent this problem; however, there is no commercial substitute for Pyromark as yet. Therefore, improving the durability of this paint is necessary to enhance the power plants operation. In this work, we study the durability of Pyromark through the micro-wear performance of the coatings assessed by scratch tests. Curing and vitrification procedures control the physical and mechanical properties of the manufactured coatings. For the first time, we demonstrate that the time-dependent behavior and related properties such as the material relaxation times are key parameters controlling the durability of this type of paint. The ability of the material to store elastic energy also explains why some consolidation procedures are better to increase its durability.
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