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Almendros-Ibanez J.A.,University of Castilla - La Mancha | Almendros-Ibanez J.A.,Renewable Energy Research Institute | Pallares D.,Chalmers University of Technology | Johnsson F.,Chalmers University of Technology | Santana D.,Charles III University of Madrid
Powder Technology | Year: 2010

In this work, a new method for measuring void fraction distribution around endogenous bubbles in a 2D fluidized bed is presented. The technique is based on illuminating a transparent-wall 2-dimensional bed with diffuse light from the rear and recording the distribution of light that penetrates the bed. The recording is made with a high speed video-camera, which gives frames with grey level corresponding to the light penetration and from which the voidage distribution around the bubbles can be determined. In this way, voidage distribution in the region very close to the bubble contour (r/Rb ≲ 1.2) is obtained, which was not possible in previous studies due to limitations in spatial resolution. A correlation is proposed for the voidage at the contour of the bubble, with the voidage depending on the radial position and the polar angle ε(r, θ). In addition, the effect of the voidage distribution on the throughflow crossing the bubbles was studied and an increase of 20% was determined for the average bubble geometry of the more than 100 bubbles analysed. Crown Copyright © 2009.

Izquierdo-Barrientos M.A.,Charles III University of Madrid | Belmonte J.F.,Renewable Energy Research Institute | Belmonte J.F.,University of Castilla - La Mancha | Rodriguez-Sanchez D.,Renewable Energy Research Institute | And 4 more authors.
Applied Thermal Engineering | Year: 2012

Phase Change Materials (PCMs) have been receiving increased attention, due to their capacity to store large amounts of thermal energy in narrow temperature ranges. This property makes them ideal for passive heat storage in the envelopes of buildings. To study the influence of PCMs in external building walls, a one-dimensional transient heat transfer model has been developed and solved numerically using a finite difference technique. Different external building wall configurations were analyzed for a typical building wall by varying the location of the PCM layer, the orientation of the wall, the ambient conditions and the phase transition temperature of the PCM. The integration of a PCM layer into a building wall diminished the amplitude of the instantaneous heat flux through the wall when the melting temperature of the PCM was properly selected according to the season and wall orientation. Conversely, the results of the work show that there is no significant reduction in the total heat lost during winter regardless of the wall orientation or PCM transition temperature. Higher differences were observed in the heat gained during the summer period, due to the elevated solar radiation fluxes. The high thermal inertia of the wall implies that the inclusion of a PCM layer increases the thermal load during the day while decreasing the thermal load during the night. © 2012 Elsevier Ltd. All rights reserved.

Garcia-Hernando N.,Charles III University of Madrid | Almendros-Ibanez J.A.,University of Castilla - La Mancha | Almendros-Ibanez J.A.,Renewable Energy Research Institute | Ruiz G.,Tecnicas Reunidas | De Vega M.,Charles III University of Madrid
Energy Conversion and Management | Year: 2011

The influence of the pressure drop in Plate Heat Exchangers (PHE) in the boiling temperature of LiBr-H2O and NH3-H2O solutions is studied. For the NH3-H2O solution, the pressure drop-temperature saturation relationship estates that high pressure drops can be allowed in the solution with negligible changes in the saturation temperature, and in the PHE performance. Besides, in the case of the LiBr-H 2O solution, as the working pressure is usually very low, the analysis of the pressure drop must be taken as a main limiting parameter for the use of Plate Heat Exchangers as vapour generators. In this case, the pressure drop may considerably change the boiling temperature of the solution entering the heat exchanger and therefore a higher heating fluid temperature may be required. A guideline to design these systems is proposed. © 2010 Elsevier Ltd. All rights reserved.

Izquierdo-Barrientos M.A.,Charles III University of Madrid | Sobrino C.,Charles III University of Madrid | Almendros-Ibanez J.A.,University of Castilla - La Mancha | Almendros-Ibanez J.A.,Renewable Energy Research Institute
Chemical Engineering Journal | Year: 2013

The objective of the present work was to research the storage behavior of a fluidized bed filled with a granular phase change material (PCM) with a small particle diameter (dp-=0.54mm). The performance of the fluidized bed was compared to that of well-known storage methods such as fluidized beds with sand and packed beds based of sand and PCM. For this purpose, heating experiments were conducted in a cylindrical bed with air as the working fluid.The influence of the bed height and flow rate on the storage and recovery efficiencies of the fluidized bed of PCM was analyzed. Additionally, the stability of the PCM during various charging-discharging cycles was studied.The results indicate that this PCM is an alternative material that can be used in fluidized bed systems to increase the efficiency of storing thermal energy in the form of latent heat. Under the experimental conditions tested in this study, higher charging efficiencies were observed for fixed and fluidized beds based on PCM than those of sand. High gas velocity and low bed height shorten the charging time but also reduce the charging efficiency. The cycling test shows that the PCM is stable under bubbling conditions up to 15 cycles, which corresponds to approximately 75. h of continuous operation. © 2013 Elsevier B.V.

Almendros-Ibanez J.A.,University of Castilla - La Mancha | Almendros-Ibanez J.A.,Renewable Energy Research Institute | Soria-Verdugo A.,Charles III University of Madrid | Ruiz-Rivas U.,Charles III University of Madrid | Santana D.,Charles III University of Madrid
Applied Thermal Engineering | Year: 2011

This work presents a theoretical study of the energetic performance of a moving bed heat exchanger (MBHE), which consists of a flow of solid particles moving down that recovers heat from a gas flow percolating the solids in cross-flow. In order to define the solid conduction effects, two solutions for the MBHE energy equations have been studied: an analytical solution considering only convection heat transfer (and neglecting solid conduction) and a numerical solution with the solid conductivity retained in the equations. In a second part, the power requirements of a MBHE (to pump the gas and to raise the down-flowing particles) are confronted with the heat transferred considering the variation of design parameters, such as gas and solids' velocities, solids particle diameter or MBHE dimensions. The numerical results show that solid conductivity reduces the global efficiency of the heat exchanger. Therefore, a selection criterion for the solids can be established, in which their thermal conductivity should be minimized to avoid conduction through the solid phase, but to a limit in order to ensure that temperature differences inside an individual solid particle remain small. Regarding the other energy interactions involved in the system, these are at least one order of magnitude lower than the heat exchanged. Nevertheless, for a proper analysis of the system the efficiency of the devices used to pump the gas and to raise the particles and the relative costs of the different energy forms present in the system should be taken into account. © 2010 Elsevier Ltd. All rights reserved.

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