Renewable Energy Research Institute

Las Palmas de Gran Canaria, Spain

Renewable Energy Research Institute

Las Palmas de Gran Canaria, Spain
Time filter
Source Type

Fernandez-Torrijos M.,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
Solar Energy | Year: 2017

Thermal ratcheting is a critical phenomenon associated with the cyclic operation of dual-media thermocline tanks in solar energy applications. To study this phenomenon, it is necessary to develop a comprehensive model of a thermocline tank that includes both the heterogeneous filler region and the composite tank wall. Because CFD models require a high computational cost to simulate a thermocline tank considering transient state operation, a simplified dual-phase model that includes the unsteady heat transfer through a multiple layer wall has been developed. The filler region consists of a rock bed with interstitial molten salt, and the tank wall is composed of a steel shell with two layers of insulation (firebrick and ceramic). In this simplified model, the fluid flow inside the tank is considered to be one-dimensional along the tank axis direction, whereas the heat conduction in the composite wall is considered to be two-dimensional. Therefore, a convective heat transfer coefficient from the bed to the wall is necessary to couple the molten salt flow with the heat transfer in the tank shell. In this work, the effects of both convective heat transfer from the bed to the wall and molten salt flow rate on the time-dependent thermal response of both the steel shell and molten salt have been analyzed. The simplified model is able to predict the temperatures of the molten salt, filler material and layer wall as well as the mechanical stress in the tank shell. © 2017 Elsevier Ltd

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.

Sanchez-Delgado S.,Charles III University of Madrid | Almendros-Ibanez J.A.,University of Castilla - La Mancha | Almendros-Ibanez J.A.,Renewable Energy Research Institute | Garcia-Hernando N.,Charles III University of Madrid | Santana D.,Charles III University of Madrid
Powder Technology | Year: 2011

In the present study, a new correlation for the determination of the minimum fluidization velocity in 2D fluidized beds was developed. The proposed correlation was based on the experimental results obtained in 2D fluidized beds with different particle sizes, bed thicknesses and bed heights. Thus, the proposed correlation depends only on the nondimensional variable t/dp, where t is the bed thickness and dp is the particle size. The proposed correlation was compared with other experimental results that can be found in the literature, and two different trends were observed. Namely, one set of experimental results was in accordance with the proposed correlation, while the other set deviated from the theoretical results. In particular, the minimum fluidization velocities of the experimental results were greater than the predicted values of the proposed correlation. In view of the differences in the experimental conditions, the observed discrepancies may be attributed to the effects of electrostatic charge and particle shape. In addition, the experimental fluidization-defluidization curves were compared to the theoretical results of Jackson's model, and the parameters were fitted to the experimental data. However, Jackson's model is based on a 1D bed; thus, general parameters could not be obtained for a bed with a fixed particle size and thickness due to the two dimensional voidage distribution in the bed and bed cohesion effects, which are a result of electrostatic forces and are not considered in Jackson's model. © 2010 Elsevier B.V.

Vigueras-Rodriguez A.,Renewable Energy Research Institute | Sorensen P.,Technical University of Denmark | Cutululis N.A.,Technical University of Denmark | Viedma A.,Technical University of Cartagena | Donovan M.H.,DONG Energy
Wind Energy | Year: 2010

This paper investigates the correlation between the frequency components of the wind speed Power Spectral Density. The results extend an already existing power fluctuation model that can simulate power fluctuations of wind power on areas up to several kilometers and for time scales up to a couple of hours, taking into account the spectral correlation between different wind turbines. The modelling is supported by measurements from two large wind farms, namely Nysted and Horns Rev. Measurements from individual wind turbines and meteorological masts are used. Finally, the models are integrated into an aggregated model which is used for estimating some electrical parameters as power ramps and reserves requirements, showing a quite good agreement between simulations and measurement. The comparison with measurements generally show that the inclusion of the correlation between low frequency components is an improvement, but the effect is relatively small. The effect of including the low frequency components in the model is much more signifi cant. Therefore, that aggregated model is useful in the power system planning and operation, e.g. regarding load following and regulation. © 2009 John Wiley & Sons, Ltd.

Soria-Verdugo A.,Charles III University of Madrid | 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-Rivas U.,Charles III University of Madrid
Chemical Engineering and Processing: Process Intensification | Year: 2011

This work studies the effect of a low-frequency rotating distributor on the motion of a large object immersed in a bubbling fluidized bed. The object size and density differ from those of the inert solids that conform the bed. Examples of objects moving in a bubbling fluidized bed include passive particles, catalysts and reactants. The rotation modifies the bed dynamics in the surroundings of the distributor and affects the motion of the object within the bed. A set of experiments was carried out in a lab-scale cylindrical bed, equipped with a perforated plate distributor that can rotate at around 1. Hz, for different bed aspect ratios, gas velocities, and object characteristics. Sizes were far larger than that of the solids of the dense phase and densities ranged from half the bed density to values around it. The experiments were video recorded, capturing the surface of the bed from above. As have often been noted, objects might remain in stagnant regions near the distributor and be "lost" or precluded to circulate. This can be avoided in most practical cases forcing the distributor to rotate. Also, the effect of rotation on the circulation time of the objects is presented, showing a general reduction of large circulation times. © 2011 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.

Rodriguez-Sanchez M.R.,Charles III University of Madrid | Soria-Verdugo A.,Charles III University of Madrid | Almendros-Ibanez J.A.,University of Castilla - La Mancha | Almendros-Ibanez J.A.,Renewable Energy Research Institute | And 2 more authors.
Applied Thermal Engineering | Year: 2014

One of the main problems of solar power tower plants with molten salt as heat transfer fluid is the reliability of central receivers. The receiver must withstand high working temperatures, molten salt corrosion and important solar flux transients that lead to thermal stresses and fatigue. Despite these difficulties, it is necessary an estimation of the receiver thermal efficiency in order to have an accurate estimation of the investment cost of the solar plant and to assure the lifetime estimation of the receiver. A thermal, mechanical and hydrodynamic analysis of these receivers has been developing in this work, assuming constant heat flux in each axial discretized section of the tube wall but considering circumferential temperature variations in the perimeter of the tubes caused by the difference between the heat flux received by the front part of the tubes and by the rear part. The thermal analysis shows that the radiation losses are higher than in literature, and consequently the thermal efficiency is lower too. This is due to the fact that the effective tube wall temperature for radiation is higher than the mean tube wall temperature, especially if the rear temperature of the tubes is considered. Besides, it has been found that the highest temperatures and thermal stresses are sited on the eastern and western panels of the receivers. Film temperature is the most limiting parameter for the receiver design due to it is responsible for salt decomposition and tube corrosion. Therefore, once the tube material is chosen, the film temperature cannot exceed a critical value over which the corrosion ratio raises rapidly. Small tube diameters and low number of panels results in low film temperatures, although this kind of design increases the pressure drop. Therefore, a compromise between film temperature and pressure drop can lead to a receiver design that ensures its lifetime, and at the same time, optimizes the investment and operational cost of the receiver. © 2013 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
International Journal of Multiphase Flow | Year: 2016

This work presents a numerical and experimental study of the transient response of a packed bed filled with a granular phase change material (PCM). The proposed numerical model accounts for the progressive evolution of the enthalpy with temperature during the phase change rather than using a constant phase change temperature. This temperature-dependent enthalpy is included in the model as an apparent specific heat that is dependent on temperature according to the measurements obtained by differential scanning calorimetry (DSC). The model also includes the energy stored in the wall, which has been shown to have a non-negligible effect in several experimental facilities. The equations presented are non-dimensionalized, which results in the same differential equation system regardless of whether a granular PCM or sensible heat storage material is used. In this manner, the same numerical method can be used in cases with or without a granular PCM. Numerical and experimental results are obtained for a conventional granular material (sand) and two commercial granular PCMs with different phase change temperatures. The numerical and experimental heating results exhibit good agreement, and the energy stored in the wall of the bed represents between 8 and 16% of the energy stored in the granular material. © 2016 Elsevier Ltd

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.

Jimenez F.,Gamesa | Gomez-Lazaro E.,University of Castilla - La Mancha | Fuentes J.A.,Technical University of Cartagena | Molina-Garcia A.,Technical University of Cartagena | And 2 more authors.
Renewable Energy | Year: 2013

In a number of countries where large penetration of wind generation has been developed, the increased impact of wind energy generation on power grid operations has led to the development of grid codes, with the aim of integrating this source of energy without compromising the stability of their networks. In Spain this grid code is named the Procedure of Operation for fault ride-through capability of the Spanish national grid code -PO 12.3-. To verify the fulfillment of this grid code, it has also been defined a procedure for measuring and evaluating the response of wind turbines and wind farms submitted to voltage dips -Procedure for verification, validation and certification of the requirements of the PO 12.3 on the response of wind farms in the event of voltage dips (PVVC)-.In this paper, two-phase voltage dips that comply with the requirements of the PVVC, has been applied to a Gamesa G52 with a Dip Active Converter. The measured tests are compared with the simulation of a PSCAD model of the wind turbine to verify the fulfillment of the PVVC validation process and the results obtained are presented. © 2013 Elsevier Ltd.

Loading Renewable Energy Research Institute collaborators
Loading Renewable Energy Research Institute collaborators