Artículos Científicos | Instituto IMDEA Energía

Category: Artículos Científicos

2020, Artículos Científicos

Exploring the alternative MnO-Na2CO3 thermochemical cycle for water splitting

Exploring the alternative MnO-Na2CO3 thermochemical cycle for water splitting, A. Bayón, V. A. de la Peña O’Shea, D. P. Serrano, J. M. Coronado, Journal of CO₂ Utilization, 42, 101264, 2020, Online version, https://doi.org/10.1016/j.jcou.2020.101264

Abstract

Thermochemical cycles for H₂O and CO₂ splitting using CeO₂ have demonstrated high efficiency (> 5%) and fast kinetics. Alternative multistage cycles based on stoichiometric oxides, such as Mn-Na, can potentially provide much larger efficiencies under less stringent operation conditions. However, these cycles consist of three stages operating at very different temperatures (from 50 °C to 1500 °C) and, accordingly, the overall energy yield of the cycle critically depends on efficient heat recovery. Despite of this, recently the Mn₃O₄–Na₂CO₃ cycle has shown a remarkable H₂ production and high stability. To further enhance hydrogen generation rate, in this work we explore the feasibility of the novel cycle based on MnO, which can potentially increase the yield per cycle. Several samples of MnO with different physicochemical characteristics were investigated to establish the influence on the oxide physicochemical properties on hydrogen generation. A maximum conversion of 47 %, with a productivity of 20.1 μmol H₂ min−1 g−1 were obtained using the MnO sample with relatively large surface area. As expected, this productivity is notably better than that obtained with Mn₃O₄ under identical conditions. Furthermore, the results obtained here reveals that low temperature hydrolysis under CO₂ atmosphere can improve the Na extraction by more than 10 %. This enhances the cyclability of the process and it may allow coupling hydrogen production with CO₂ capture. Furthermore, the thermal reduction of the solid product recovered after the carbonation stage yields nearly pure MnO, confirming experimental the feasibility of completing this alternative cycle.

2020, Artículos Científicos

Thermo-Economic Optimization of Solar Thermal Devices by Coherent Integration of Technologies

Thermo-Economic Optimization of Solar Thermal Devices by Coherent Integration of Technologies, J. Cano Nogueras, Energy and Power Engineering, 12, 671-707, 2020, Online version, DOI: 10.4236/epe.2020.1211040

Abstract

Radiation is a form of energy where the angular variable of the direction of its photons has a primary importance, particularly for radiation concentration processes, which are essential tools to reach high temperatures from radiation beams (as the solar ones) with moderate intensities. Solar radiation cannot be used directly to feed thermodynamic cycles, and optical concentration must be applied to that goal. In general, reflection from mirrors is preferred to refraction by lenses in this case, because they have less optical aberrations. Concentration conveys very high temperatures in the receiver. However, the higher the temperature, the lower the efficiency of the solar thermal apparatus. Besides that, economy also suffers quite a lot when going to very high concentration factors, which is one of the main burdens in the development of Solar Thermal Energy. A new configuration of solar radiation concentrator is presented. It includes a salient innovation in the way the mirrors are given the right curvature by mechanical forces. Those mirrors are originally flat and do not need any special thermal treatment for this purpose. The whole device concept has been guided by the principle of thermoeconomic coherence, which requires similar efforts in all degrees of freedom that have strong influence in the performance and cost of the system. The paper shows the decision tree that has oriented the project, following the principle of equilibrium in efforts, which leads to a design window of moderate values in the main variables. The prototype of this new configuration has already been built, and the first stage of research is considered to be finished, because the prototype has shown excellent conditions to include selected (fitting) technologies at a very low cost.

2020, Artículos Científicos

Proposal of a new design of source heat exchanger for the technical feasibility of solar thermal plants coupled to supercritical power cycles

Proposal of a new design of source heat exchanger for the technical feasibility of solar thermal plants coupled to supercritical power cycles, M. J. Montes, J. I. Linares, R. Barbero, A. Rovira, Solar Energy, 211, 1027-1041, 2020, Online version, https://doi.org/10.1016/j.solener.2020.10.042

Abstract

Solar thermal power plants coupled to supercritical CO₂ cycles seems to be a way to increase the global solar-to-electric efficiency. For that, the concentrating solar technology that is best integrated is the molten salt central receiver with a thermal energy storage associated. This work is focused on one of the main challenges of this scheme: the source heat exchanger transferring the thermal energy from the molten salt in the solar field to the CO₂ in the power cycle. A new design, based on the printed circuit heat exchanger technology is proposed, that withstands the pressure difference and avoids the molten salt plugging when circulating through microchannels. The thermo-mechanic model of this heat exchanger is also calculated. This work also addresses a thermo-economic optimization of the printed circuit heat exchanger proposed. For that, it is considered the global performance of the solar thermal plant for three layouts: recompression, intercooling and partial-cooling cycles. This optimization yields to a great reduction in the investment cost of these source heat exchangers, achieving the lowest cost in the partial-cooling configuration, followed by the intercooling and finally, the recompression. This trend is also observed in the global performance of the solar plant, so the partial-cooling layout is the one with the lowest levelized cost of electricity; this value is similar to that of the intercooling layout, and both are well below from the cost in the recompression layout, which results the most expensive configuration.

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, 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 Mn_3-xCo_xO₄ 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 (Mn_3-xCo_xO₄, 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 (Mn_3-xCo_xO₄) 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 (Mn_3-xCo_xO₃) 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/Mn_3-xCo_xO₄ 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.

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Exploring the alternative MnO-Na2CO3 thermochemical cycle for water splitting

Abstract

Thermo-Economic Optimization of Solar Thermal Devices by Coherent Integration of Technologies

Abstract

Proposal of a new design of source heat exchanger for the technical feasibility of solar thermal plants coupled to supercritical power cycles

Abstract

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

Abstract

Comparative Analysis of Energy and Exergy Performance of Hydrogen Production Methods

Abstract

Drift analysis in tilt-roll heliostats

Abstract

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

Abstract

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

Abstract

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

Abstract

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

Abstract

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