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Fluidized beds are particularly suitable for integration into Concentrated Solar Power (CSP) plants with beam-down reflectors. Due to the high mixing rates and heat diffusion typical of fluidized beds, they enable the highly concentrated solar flux from the beam-down reflector to impinge directly on the particles. Depending on size, density, surface roughness, optical and mechanical properties, some granular materials are more appropriate than others to store thermal energy at high-temperatures in a fluidized bed. In this line, this work compares the experimental performance of three different granular materials: sand, carbo Accucast ID50 and SiC, considering different airflow rates, and two fluidization technologies: bubbling and spouted bed. Experimental tests were carried out in a facility specifically tailored for these type of tests that permitted different flow arrangements to apply different fluidization techniques using air as the working fluid, including a beam-down reflector with a 4 kW e lamp to simulate the concentrated solar radiation (peak radiation flux of 115 kW/m2). The temperature evolution and storage/recovery efficiencies for the different materials were compared using the two fluidization technologies, and varying the radiation level and airflow rate. Because of these tests, the influence of the airflow rate and radiation levels on the thermal efficiency of the beds was evaluated. The experimental results showed that SiC was the best candidate, as it exhibited the best thermal performance, reaching a peak storage efficiency of η C = 0.95 , obtained with an airflow rate of 2.5 U mf . Maximum temperatures of 250 ° C were reached in a cylindrical bed with an aspect ratio of L/D ≈ 1.

This paper presents two simple numerical models to analyze the behavior of two different fluidization technologies with concentrated solar energy on the top of the bed. A bubbling fluidized bed, which operates with low particle size (Geldart A-B particles with d p = 0.1 - 0.5 mm) and a spouted bed, which uses particles with larger sizes (Geldart D with d p ≳ 1 mm). Two cases were studied with the same mass of particles (150 kg), considering the same incident solar radiation concentrated on the top of the bed (50 kW). The results obtained indicate that the airflow rate in the bubbling fluidized bed has a notable effect on the maximum temperature reached in the bed. Increasing the airflow rate (from 1.5 to 2.5 times the minimum fluidization velocity), the maximum temperature reached in the bed is reduced from 950 to 750 K. In the spouted bed, higher temperatures are reached (over 1000 K), although it takes almost 4 times longer for the temperature to be reached than in the bubbling bed. Consequently, the charging efficiency, defined as the energy stored in the particles divided by the solar energy concentrated on the bed, are similar for both cases.

This paper presents the results of a 3-D numerical simulation of a freely bubbling fluidized bed, based on the Eulerian¿Lagrangian approach, using the software Barracuda (CPFD-Barracuda). The main results obtained were assessed in terms of frequency analysis, bubble pierced length, bubble size, bubble passage frequency and bubble velocity. The results obtained were also compared with experimental data obtained in a 3-D fluidized bed using pressure and optical probes, and with the numerical results using the more common Eulerian-Eulerian approach, implemented in the commercial software Fluent (TFM-Fluent). The results show that CPFD-Barracuda satisfactorily predicts the global behaviour of bubbling beds with a low computational cost, although it computes smaller bubble sizes and lower bubble velocities than TFM-Fluent and experiments. Additionally, the spectra of pressure and particle volume fraction obtained with CPFD-Barracuda resemble those from the experiments and the TFM-Fluent simulations, but with a larger contribution of lower frequencies. The peaks of the pressure spectra from CPFD-Barracuda are close to those from the experiments and the TFM-Fluent simulations, whereas those in the solid volume spectra seem to be underestimated by CPFD-Barracuda. The results also indicate that the particle fraction threshold value chosen to distinguish bubbles contours notably influences the results of the bubble characteristics, especially for TFM-Fluent, whereas CPFD-Barracuda is less sensitive to this threshold value. This work was partially funded by the Ministerio de Economía y Competitividad (Projects ENE2016-78908-R and RTI2018-096664- B-C21 (MICINN, FEDER/UE)) of the Spanish Government, the Regional Government of Castilla-La Mancha (project SBPLY/17/180501/000412) and the Ministerio de Ciencia, Innovación y Universidades - Agencia Estatal de Investigación (AEI) (RED2018-102431-T).

This review summarizes different solar thermal energy storage techniques from a particle technology perspective, including sensible, latent and thermochemical techniques for low- and high-temperature applications that use particles as the storage medium in the thermal energy storage system. The focus is on applications, experimental results, modeling and future trends. This review describes two different particle technologies used to store thermal energy: packed and fluidized beds. The advantages and disadvantages of both technologies are reviewed throughout different studies found in the literature for various thermal energy storage systems. Packed beds have the main advantage of thermal stratification, which increases the efficiency of solar collectors in low-temperature sensible energy storage systems and augments the exergy content in the bed. Moreover, they have been proven to be suitable as dual-media thermocline storage systems for CSP plants. In contrast, the high mixing rates of fluidized beds makes them suitable for the rapid distribution of concentrated solar energy in particle receiver CSP systems. In addition, their high heat and mass transfer rates, compared with those of packed beds, make them the preferred particle technology for thermochemical energy storage applications. This review also notes that it is important to find new materials with an appropriate size and density that can be properly used in a fluidized bed. Additionally, more specific research efforts are necessary to improve the understanding of the behavior of these materials during the fluidization process and over a high number of charging/discharging cycles.

In this paper, numerical simulations are carried out to analyse the influence of geometrical parameters on the heat transfer and flow patterns for six spirally corrugated tubes at turbulent flow ( R e = 25 × 10 3 ). Some previous studies are restricted to 2-D analyse or 3-D numerical simulations considering a reduced section length of the tubes as the computational domain in order to reduce the computation cost. The novelty of this paper is to perform a high cost numerical simulation using a computational domain with a length of the tube ( L = 2 m ) and dimensions similar to those used in commercial applications. In addition, a new non-dimensional parameter, the corrugation shape factor ( C S F ) is proposed, which takes into account the width of the corrugation, a geometrical dimension obviated in other studies. A 3-D unstructured tetrahedral mesh was created, and a grid independence analysis of the numerical solution was performed. The Realizable k − e turbulence model with enhanced wall treatment was proposed to model the turbulence. Simulations were validated with experimental data available in the open literature. Regarding the ratio between height and diameter ( H / D ), the Nusselt number augmentation seems to augment as the H / D increases for low values of H / D ( H / D 0.05 ), whereas it decreases for values of H / D > 0.05 . Considering the cases with the same diameter, in all the analysed cases, the pressure drop and Nusselt number augmentation decrease linearly as the P / D increases as well as both parameters increase linearly as the H / D , S I and C S F increase. The numerical results indicate that the augmentation of the Nusselt number and the friction factor (compared with the smooth tube) increases approximately linearly with the C S F in the range tested in this work ( 4 C S F 18 ), regardless of the tube diameter.

This work presents a model of Artificial Neural Networks (ANNs) capable of accurately predicting the heat transfer rate and pressure drop in a triple concentric-tube heat exchanger (TTHX) with corrugated and non-corrugated inner tubes and involving a fluid typically used in the food industry. Pitch and depth are varied in the case of corrugated tubes. The ANN model was developed and validated using a huge databank including 181 experimental datasets. The best training algorithm is the Bayesian regulation. A back-propagation algorithm, which is considered to be the most common learning method for ANNs, is used in the training and testing of the network. Different network configurations were tested, and the optimum ANN configuration consisted of a network with two hidden layers with 15 and 21 nodes in the first and second layer, respectively. The ANNs results were found to be in good agreement with the experimental data, with the absolute average relative deviation (AARD) being below 1.91% for the heat transfer coefficient and below 3.82% for the pressure drop, respectively. The simplicity of the developed ANN model and its low levels of error for a huge experimental databank are some of the key features of the model.

This paper reports the simulation results of two different solar-assisted heat pump (SAHP) systems able to cover a significant part of the space heating demand of a single-family house located in Madrid (Spain) using solar energy. Most SAHP systems reported in the literature serve large buildings located in cold climates using long-term (seasonal) storage tanks, a solution that leads to high investment, operation and maintenance costs that greatly limit their applicability to milder climates and/or small-scale buildings. To explore this potential application in more detail, this study examines the integration of a water-to-water heat pump in two solar house-heating systems equipped with different short-term (diurnal) stores: water, and latent (containing a phase-change material -PCM) tanks for the weather conditions of Madrid. The SAHP system’s energy performance is assessed using the TRNSYS program, implementing in the simulation scheme an experimentally validated slab-like macro-encapsulated PCM tank model (as a TRNSYS component) for the simulations including the PCM tank. Simulation results indicated that the integration of the intermediate PCM tank in the SAHP system may perform unexpectedly, generating degradation in the system performance if only conventional thermostatic control strategies are considered, diminishing the solar energy transferred to the heat pump up to approximately a 30%, and lowering the heating availability of the system from 98.9% to 72.8%. An analysis of this thermal performance degradation led to the conclusion that more advanced control strategies based on accurate knowledge of forecasts for the following days’ heating requirements, PCM temperatures and associated states of charge of the tank from the history of the fluid inlet/outlet temperatures are required.

This paper analyses the effect of spirally corrugation in a simple tube on the heat transferred and friction factor using numerical simulations. The simulations have been validated with experimental data available in the literature. The study compares the behaviour of both smooth and spirally corrugated tubes considering turbulent flow at four Reynolds numbers (15- 40 × 10 3 ) and two Prandtl numbers (2.9 and 4.3). The main novelty of this paper is to perform a 3-D simulation because some previous studies using similar geometry were restricted to a 2-D analysis. For the smooth and corrugated tubes, stainless steel tubes with an inner diameter of 18 mm, a length of 6 m and a wall thickness of 1 mm were used. The corrugated tube has a corrugation depth of 0.43 mm and a helical pitch of 15.86 mm. The meshing process was performed using ANSYS Workbench (v.17.0) with an unstructured grid with a refined mesh near the wall to ensure that the laminar viscous sub-layer was captured. Hence, a k-epsilon ( k − e ) turbulence model with a near-wall treatment was used in the proposed simulations. Two grids were used to perform a grid sensitivity analysis. The results for the corrugated tube indicate that the numerical model predicts an average Nusselt number within a maximum relative error of 17% compared with the experimental data, and the differences in the Fanning factor are lower than 9%.

This paper presents the experimental results obtained in directly irradiated conical spouted and spout-fluid beds. A conical bed with a minimum and maximum inner diameter of 80 and 314.76 mm was filled with SiC particles and directly irradiated from the top with a beam-down reflector with a 2 kW Xe-lamp. The study examines the influence of the bed height and airflow rate on the temperature distribution over the top surface of the bed (where the concentrated irradiation impinged), measured with an infrared camera. In addition, two type of beds were compared: conventional spouted and spout-fluid beds. The heat exchange of the concentrated radiative flux over the fluidized particles in motion under the action of fountain bursting was assessed by characterizing the temperature of the particles (Probability Density Function, mean value and standard deviation). The results of this work show that the spouted bed can reduce the maximum temperature of the particles, especially in the case of shallow beds. The behavior in the two cases is different, with the exception of the deep bed, where the spouted bed presents slightly lower homogeneous temperatures than the spout-fluid bed (reducing the appearance of hot spots) and with an associated reduction of 52.28 % in the pumping costs. Spout-fluid temperatures strongly depend on the air velocity and the height of the bed of particles, exhibiting a wider temperature range than the spouted case due to the different fluidization regimes observed.

This paper presents a 2-D numerical simulation of a freely bubbling fluidized bed with immersed surfaces, using the Computational Particle Fluid Dynamics ( C P F D ) model implemented in the Barracuda commercial software. The heat transfer coefficients obtained are compared with an experimental study available in the open literature and numerical simulations based on the two-fluid model approach performed by other authors. Two different immersed surfaces, representing spherical and cylindrical geometries were studied. The simulations results show different heat transfer mechanisms, depending on the angular position in the two immersed surface geometries studied. The time average heat transfer coefficient around the whole heat transfer surface were 25 % and 38 % lower than the experimental study, for the cylindrical and spherical surfaces, respectively. These differences are lower than the results obtained with the two-fluid model approach reported in the open literature. The numerical results indicate that CPFD-Barracuda is able to properly simulate the heat transfer and the dynamics of the bed in defluidized regions, such as on the top of an immersed surface, where the two-fluid model fails and overpredicts the heat transfer rate.

The main limitation of Concentrated Solar Power (CSP) is its maximum operating temperature, since the main fluid used as heat transfer fluid (HTF) and thermal storage media (TES) is molten salts, which degrade around 565 °C. Therefore, there is great interest in developing HTFs that allow the maximum working temperature to be increased to more than 565 °C. This increase in the maximum operating temperature of the HTFs will, in turn, allow CSP plant efficiency to be enhanced. Different studies have proposed using particles as a medium to capture and store solar energy, but very little is known about the effect on the particles after resisting several cycles of cooling and heating and what happens to the particles’ physical properties when they are fluidized. It is true that a variety of solid particles permit high performance to be achieved at high temperatures and with low material costs, but not all particles are valid for this purpose. This work focuses on different materials considered promising candidates for use as alternative HTF and storage materials in CSP plants, which were characterized and tested: sand, carbo Accucast ID50 and silicon carbide. The properties of the particles before and after being fluidized (assessing their resistance to abrasion) and before and after high temperature (900 °C) thermal treatments (aging treatment) are the main finding of this paper. This work highlights the considerable advantages of SiC and carbo over sand. This study also determines that the combination of high temperature and several hours of fluidization does not affect the diameter of the particles: However, the fluidized samples vary considerably in diameter. In this way, SEM and DSC techniques highlight the problem of the fine appearance associated with sand during fluidization.

This paper presents the numerical results obtained in a directly irradiated fluidized bed. The numerical model was implemented using CPFD-Barracuda software coupled with a P-1 radiation model. The bed consists of a cylindrical geometry containing SiC particles, with a diameter of 7.62 cm and a fixed bed height of 8 cm. The irradiation on the top of the bed was implemented by including a high temperature surface that directly irradiated on the top of the bed, with a uniform radiation flux of 3 × 1 0 4 kW ∕ m 2 . The work studies the influence of the airflow rate and the thermal behavior of the irradiated bed. The results show that increases in the airflow rate (up to 2.5 times the minimum fluidization conditions) notably enhance the mixing rate and the agitation levels on the top of the bed, reducing the appearance of hot spots on the top surface, with the bed exhibiting a more uniform temperature in its upper half. At higher airflow rates, the maximum temperature of the directly irradiated particles is reduced from 454 K to 384 K when the airflow rate is increased from 1.5 to 2.5 times the minimum fluidization airflow rate. In contrast, the mean temperature of the whole bed of particles rises (from 315.9 K to 327.5 K), increasing the storage efficiency.

The organic Rankine cycle (ORC) is widely accepted to produce electricity from low-grade thermal heat sources. In fact, it is a developed technology for waste-heat to electricity conversions. In this paper, an ORC made up of super-heater, turbine, regenerator, condenser, pump, economizer and evaporator is considered. An optimization model to obtain the maximum performance of such ORC, depending on the super-heater pressure, is proposed and assessed, in order to find possible new working fluids that are less pollutant with similar behavior to those traditionally used. The different super-heater pressures under analysis lie in between the condenser pressure and 80% of the critical pressure of each working fluid, taking 100 values uniformly distributed. The system and optimization algorithm have been simulated in Matlab with the CoolProp library. Results show that the twelve working fluids can be categorized into four main groups, depending on the saturation pressure at ambient conditions (condenser pressure), observing that the fluids belonging to Group 1, which corresponds with the lower condensing pressure (around 100 kPa), provide the highest thermal efficiency, with values around η=23−25%. Moreover, it is also seen that R123 can be a good candidate to substitute R141B and R11; R114 can replace R236EA and R245FA; and both R1234ZE and R1234YF have similar behavior to R134A.

A high percentage of the heat that is supplied to thermoelectric power plants is discarded to ambient and must be handled by an external cooling system. This cooling system typically consists of wet cooling towers because of the excellent thermo-physical properties of water. However, the amount of water consumed for power production has reached alarming levels in developed countries. Air-cooled heat exchangers (ACHXs) appear to be the most adequate technology to substitute for wet cooling towers, but the use of this technology has some limitations. The most important limitation is the higher condenser pressures in the cycle, which produce backpressures at the condensing turbine's exit, increases in heat rejection and losses in the net plant efficiency. This paper presents a concept for the use of ACHXs in the cooling systems of power plants using an absorption refrigeration system (ARS) as an intermediary. Heat from the steam condenser is handled by the evaporator of the ARS and “lifted” to a higher temperature level, where the ACHXs are fitted to work. The generator of the ARS is fed by the power plant itself, extracting (bleeding off) some of the steam that flows through the steam turbine at the correct pressure and temperature.

This work studies the integration of a fluidized bed energy storage unit containing Phase-Change Materials (PCMs) into solar air-based systems for heating of buildings. The use of a PCM fluidized bed energy storage unit in air systems offers an interesting alternative to the usual energy storage media based on packed beds of rocks or pebbles because it enables faster charging (or discharging) of the thermal energy storage (TES), associated with high energy densities provided by the latent heat of the PCM. The airflow rate that passes through the solar collectors is sufficiently high to provide satisfactory fluidization of the bed, achieving high heat transfer coefficients. The results of the simulation study conducted on the proposed air system show that the system can successfully supply a significant part of the heating requirements of a single-family house in locations with mild winter conditions, such as Barcelona and Madrid, where a TES containing between 1000 and 2000 kg of PCM achieves solar contributions of about 50%. For locations with more severe winter conditions, such as Zurich and Stockholm, very large storage capacities (up to 5000 kg of PCM) combined with large collector areas (20 m 2 ) are required to meet between 20 and 25% of the house heating needs.

This paper presents the experimental results obtained in a directly irradiated fluidized bed. A bed with a diameter of 314.76 mm was filled with SiC particles and directly irradiated from the top with a beam-down reflector with a 2 kW Xe-lamp. The study examines the influence of the bed height and airflow rate on the temperature distribution over the top surface of the bed (where the concentrated irradiation impinged), measured with an infrared camera. In addition, two fluidization patterns were compared: even and uneven fluidization. In the first case, the air is uniformly distributed through the cross-sectional area of the bed, while in the second case, a higher flow is deliberately introduced through the center of the bed, with minimum fluidization conditions being maintained in the rest of the bed. The heat exchange of the concentrated radiative flux over the fluidized particles in motion under the action of bubble bursting was assessed by characterizing the temperature of the particles (Probability Density Function, mean value and standard deviation). The results of this work show how the uneven fluidization can reduce the maximum temperature of the particles, especially in the case of shallow beds. In any event, uneven fluidization exhibits a similar behavior to even fluidization but with lower pumping costs, as the airflow rate is reduced by up to 26.4%, compared to even fluidization. The effects of the uneven fluidization are more important for shallow beds especially at high air velocities, although under the experimental conditions of this work, no clear heat diffusion improvements were found in the uneven fluidization tests when the air velocity in the center of the bed exceeded two times the minimum fluidization velocity.

In this study, 3-D numerical simulations were carried out to analyse the influence of geometrical parameters for eight spirally inner corrugated tubes at turbulent flow ( R e = 25 × 10 3 ) in a double pipe heat exchanger. As a novelty, different combinations of pitch and height in a 3-D inward corrugated tube numerical model were analysed and validated with an experimental setup. This had not previously been conducted in a double pipe heat exchanger. Furthermore, the numerical model included the entire geometry of the heat exchanger, with dimensions of the computational domain similar to those used in actual commercial applications. Grid independence analysis of the numerical solution was performed based on a 3-D unstructured tetrahedral mesh scheme, considering the Realizable k − e turbulence model. Case 8, with the highest corrugation height ( H / D =0.05) and the lowest helical pitch ( P / D =0.682) presented the highest pressure drops in both inner and annular tubes, being 4.15 and 1.27 times higher in the inner tube and in the annulus side than in the smooth tube, respectively. Regarding heat transfer, Case 9, with the smallest helical pitch and an intermediate corrugation height ( H / D =0.041) obtained the highest number of transfer units ( N T U ) value, which, under the experimental conditions of this work, resulted in an increase of 29% compared with the smooth tube. In Cases 7 and 9, the inner tubes showed optimal results when considering the combined influence of the enhanced heat transfer performance and pressure drop using the performance evaluation criteria ( P E C ).

The aim of this paper was to study the influence on thermo-hydraulic performance of different configurations of regularly spaced twisted tape elements (TTEs) inserted into a smooth double tube heat exchanger (DTHX). In order to emulate the behavior of high-viscosity food-industry fluids, a 60° brix solution of water and sugar was used as the Heat Transfer Fluid (HTF). A total of 320 experimental tests were carried out, varying the flow velocity (Reynolds number) and the TTE configuration. We report the results from nine different configurations, including different twisted tape element pitches and free-space lengths. Finally, the results from each case were correlated for Nusselt number and friction factor using the minimum variance methodology. The agreement between the results obtained from the experimental data and those obtained from the proposed correlations is reasonable, with an absolute average relative derivation of less than 9.5% for the heat transfer rate and 15% for the friction factor.

This paper reviews the extensive literature about the different experimental correlations for effective thermal conductivity in packed beds. The review covers correlations for stagnant thermal conductivity (with a motionless fluid) k eff 0 , including thermal radiation heat transfer, k eff 0 , rad . In addition, correlations for effective conductivity with the fluid in motion for both axial, k eff a , and radial, k eff r , directions are reviewed together with expressions for effective conductivity in packed beds filled with encapsulated phase change materials k eff pcm . High discrepancies of one order of magnitude in the correlations available in the literature for axial effective thermal conductivity, k eff a , are observed. Nevertheless, a sensitivity analysis concludes that the influence of k eff a on a packed bed performance is negligible for most cases because the conduction characteristic time is usually much larger than that for convection. Only for very low flow rates (as in thermocline tanks) can it influence the charging and discharging times. The review concludes that more research and new correlations are necessary for thermocline tanks and for beds filled with encapsulated PCMs. In addition, new correlations for the axial effective thermal conductivity obtained when the fluid flows in the same direction as the heat flux are necessary. The numerical models available in the literature used correlations that were not obtained for the working conditions of these energy storage systems, which may result in large discrepancies in the results of these models.

Inverse heat transfer problems typically rely on temperature measurements for estimating unknown boundary heat flux, such as that in the water tubes of steam boilers or central receivers in solar tower power plants. In this work, an experimental facility consisting of a molten salt loop that simulates a tube of a solar tower receiver is presented to obtain the outer tube surface temperatures under solar tower power plant operating conditions. The external surface f the pipe in the test section is heated in a controlled manner with an induction heater, which provides a very high nonuniform heat flux. An inverse thermal method has been applied to obtain the incident heat flux onto the receiver tube from the outer surface temperature measurements. To solve the inverse problem, a transient two-dimensional numerical model of a circular pipe flowing molten nitrate salt and subjected to a nonhomogeneous circumferential heat flux has been developed. The heat flux calculation with the inverse method is in accordance with the heat flux estimation based on the calibration of the induction heater. A good agreement between the experimental and calculated temperatures is observed. Furthermore, the deflection of the tube caused by the nonhomogeneous heat flux is measured and is compared to the deflection calculated from the radial temperature profile from the inverse problem solution, and a good agreement between both results is observed. This work was partially funded by the Ministerio de Eduación, Cultura y Deporte para la Formación de Profesorado Universitario (FPU-04941), the Ministerio de Ciencia, Innovacion y Universidades (Project RTI2018-096664-B-C21 (MICINN/FEDER, UE)) of the Spanish Government and the Project SBPLY/17/180501/000412 of the Regional Government of Castilla-La Mancha.

This work presents a simple method for modeling the performance of a thermal energy storage (TES) system that contains a phase-change material (PCM). The developed approach is similar in concept to the bypass factor method, widely used in air conditioning applications to estimate the outlet air conditions from a coil. The proposed method provides reasonably accurate results while requiring only the most relevant input parameters (a total of 10) commonly available during the early stages of TES system design, wherein the impact of decision making is greatest and not all aspects of the TES system may be fully defined. The method is particularly suitable for application during preliminary stages. The results exhibit a good agreement (maximum discrepancies in the cumulative energy of below 13%), with the experimental results consisting of a total of 21 charging and discharging cycles of two TES systems. Each system has a different heat exchanger and contains a different PCM: one system contains an inorganic material, and the other contains an organic material. The method is generally applicable to any TES system with a PCM, thereby simplifying the performance characterization and implementation of such TES systems in energy analysis tools such as TRNSYS ® .

This paper presents an analysis of extended surfaces with constant cross-sectional area and a prescribed temperature at the tip. The expressions of the fin efficiency and effectiveness for extended surfaces with a constant cross-sectional area can be found in the literature for a boundary condition of an insulated or convective tip but not for a prescribed temperature at the tip. In this case, there are two different expressions for efficiency and effectiveness depending on the value of the non-dimensional parameter mL with respect to the critical value mL∗. The results indicate that for excess temperature ratios θ L / θ b ≳ 0.25 , the fin efficiency decreases for certain values of mL. This region should be avoided for proper fin design.

The integration of phase change materials (PCMs) for cooling applications in buildings reduces the amplitude of indoor air temperature fluctuations due to solar and internal gains, enabling passive solar, HVAC system downsizing or off-peak cooling designs. This work studies the use of radiant systems for discharging heat stored in a PCM; this approach differs from the typical practice of accomplishing this task by either night cooling ventilation or embedding an active heat exchanger into the PCM of the wall. In this study, a PCM is incorporated into the floor, and a hydronic radiant ceiling system is used as the energy discharge system. The advantages and disadvantages of this configuration in terms of cooling energy demands and thermal comforts of occupants are analyzed using the simulation software TRNSYS®. Five design parameters are optimized using the software GENOPT®. The simulation reveals that when accompanied by an air-to-air heat recovery system, this configuration can significantly reduce the cooling energy demand (more than 50% compared to the cooling energy demand of the same building without PCM) and can thus significantly reduce the energy consumption. However, the degrees of occupant comfort will inevitably vary (i.e., the predicted percent dissatisfied (PPD) increases by 2–5%).

Over the last years, the development of PCMs has opened new ways to increase the storage capacity and the energy stored due to the high latent heat and high storage density of these materials. The aim of this work is to model the charging process of a fluidized bed with PCM material operating as an energy storage device. The temperature in the bed during the charging process of the fluidized bed has been modeled using the two-phase theory of fluidization. The dense phase is taken to be perfectly mixed and the bubble phase is taken to be in plug flow. The numerical model presented takes into account the fact that the phase change process of the bed material occurs over a temperature range. The energy equation of the dense phase is numerically solved in enthalpy form considering the dependence of enthalpy on temperature for phase change occurring over a range of temperatures. The model validity is verified against experimental data for two granular materials: sand, a typical material used in fluidized beds, and a granular phase change material with a mean particle diameter of 0.54 mm and a phase change temperature around 50 °C. For both materials the temperature profiles obtained numerically agree with the ones measured experimentally.

This study investigates the integration of phase change materials (PCMs) in the heat rejection loops of absorption solar cooling systems for residential applications in Spain. The feasibility of replacing the open wet tower by an alternative air-cooled system based on a combination of dry-coolers and a thermal energy storage (TES) with PCM inside (TESpcm) has been evaluated in terms of the total cooling energy produced and the system efficiency. TRNSYS® parametric simulations of two configurations have been conducted between the months of June and September. Their performance has been characterized and their advantages and disadvantages have been summarized. The results indicate that the alternative heat rejection loop with a TESpcm of 1 m3 can improve the mean overall system performance coefficient in locations with temperate and humid summers by almost one unit, although different degrees of the deterioration of the mean chiller's performance coefficient (values between 7 and 13%) and a reduction in the total cooling energy produced in the evaporator (21–38%) are inevitable. The study concludes with some suggestions for future work.

In this paper we present a prototype for a new type of solar thermal collector designed for architectural integration. In this proposal, the conventional geometry of a flat solar thermal collector is changed to a curved geometry, to improve its visual impact when mounted on a building facade or roof. The mathematical equations for the beam and diffuse solar radiation received by a collector with this geometry are developed for two different orientations, horizontal and vertical. The performance of this curved prototype, in terms of solar radiation received, is compared with a conventional tilted-surface collector for different orientations in Madrid (Spain). The comparison is made for typical clear-sky days in winter and summer as well as for an entire year. The results demonstrate that the curved collector only receives between 12% and 25% less radiation than the conventional tilted-surface collectors when oriented horizontally, depending on the azimuth of the curved surface, although these percentages are reduced to approximately 50% when the collector is oriented vertically.

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. The author would like to thank the financial support from CDTI and S2m Solutions for the project MOSARELA (Molten salt receiver lab) whose reference is IDI-20120128. The financial support of the project ENE2012-34255 is also acknowledged. Publicado

One of the problems with solar flat plate collectors for domestic water heating is that they produce more energy in the summer months, when the domestic hot water needs are lower than in winter months. This causes a significant difference between supply and demand and thus overheating during the summer. A method to avoid this problem is to design solar collector fields that offer a 100% of the water needs in the summer, but a small percentage during the winter, which is certainly not ideal. In this work, ray tracing is used to design a solar thermal collector that offers a more uniform production during the year. A novel geometry is chosen where the collector is split in two parts, a curved absorber and a mini parabolic concentrator. The concentrator is designed to concentrate the radiation during the midday hours of winter days and to not doing it in the midday hours of summer days. This increases the energy produced in winter and prevents the installation from overheating. In order to study the hours when this geometry will concentrate the solar radiation, ray tracing is used. As the solar collector has a design that allows the collector to be easily integrated into a facade, the simulations in the most useful architectural integration positions are simulated, those are horizontal positions, but vertical positions or any other position are suitable if the collector is installed on a roof. For each position, the amount of hours where the whole collector is working and the total radiation captured are calculated and compared with the solar radiation captured by an equivalent flat surface, which would corresponds to conventional flat plate collectors. Simulation results shows how for a concentrator designed to work properly in the 5 midday hours during the winter solstice it will not work during the 5 midday hours during the summer solstice, avoiding overheating.

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 (d(p) = 0.54 mm). 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. This work was founded partially by the Spanish Government (Project ENE2010-15403), the regional Government of Castilla-La Mancha (Project PPIC10-0055-4054) and Castilla-La Mancha University (Project GE20101662). Publicado

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. This work was partially funded by the Ministerio de Educación,Cultura y Deporte para la Formación de Profesorado Universitario(FPU-04941) and the Ministerio de Economía y Competitividad (Project ENE2014-54942-R) of the Spanish Government

This work presents the results of experiments performed in a thermal energy storage tank filled with particles, which was heated using hot air and then discharged with a cold water stream circulating inside a heat exchanger immersed in the bed. Both fixed- and fluidized-bed configurations were studied. The materials used were sand, which is a material commonly employed in sensible heat storage, and a granular phase change material (PCM), in which the phase change occurs over the temperature range of 4050 degrees C. Three different heat exchangers with different helical coil geometries were tested by measuring the temperatures in the bed and at the water inlet and outlet. Higher heat transfer coefficients between the bed and the water flow and higher heat exchanger effectiveness were observed for the heat exchanger with the greatest distance between coils, as it allows better contact between the bed particles and the heat exchanger surface when the particles are fluidized. (C) 2016 Elsevier Ltd. All rights reserved. This work was partially funded by the Spanish Government (Project ENE2010-15403), the regional Government of Castilla-La Mancha (Project PPIC10-0055-4054) and Castilla-La Mancha University (Project GE20101662).

The objective of this work is to model the heat transfer coefficient between an immersed surface and fixed and bubbling fluidized beds of granular phase change material (PCM). The model consists of a two-region model with two different voidages in which steady and transient conduction problems are solved for the fixed and fluidized bed cases, respectively. The model is validated with experimental data obtained under fixed and fluidized conditions for sand, a common material used in fixed and fluidized beds for sensible heat storage, and for a granular PCM with a phase change temperature of approximately 50 degrees C. The superficial gas velocity is varied to quantify its influence on the convective heat transfer coefficient for both the materials. The model proposed for the PCM properly predicts the experimental results, except for high flow rates, which cause the contact times between the surface and particles to be very small and lead the model to overpredict the results. This work was partially funded by the Spanish Government (Project No. ENE2010-15403), the regional Government of Castilla-La Mancha (Project No. PPIC10-0055-4054), and Castilla-La Mancha University (Project No. GE20101662).

The ε-NTU method has advantages over other methods because it does not require assumption of an outlet temperature to determine the duty of a heat exchanger. In this work, the ε-NTU method was used to obtain a general algebraic expression of the effectiveness of PHEs in the N-Pa pass-1 pass configuration as a function of the heat capacity ratio (C*) and the number of transfer units (NTU). This expression is valid for PHEs with a high number of plates (N-p > 40). In addition, 1 pass-1 pass and 2 pass-1 pass PHEs were numerically analyzed to obtain an algebraic expression of the effectiveness of PHEs with a low number of thermal plates (N-P < 40) as a function of C*, NTU and the number of plates. Finally, the expression obtained for N-pa pass-1 pass PHEs with a high number of plates was generalized for use in any heat exchanger with series-parallel configurations, such as series-parallel arrangements of double-pipe or shell-and-tube heat exchangers.

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.

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 Publicado

This work presents a new methodology, based on the maximum entropy method, to obtain bubble characteristics in fluidized beds. The probability distributions (PDF) of bubble pierced length and velocity are obtained applying the maximum entropy principle to experimental measurements. In addition, the bubble diameter distribution has been inferred from experimental pierced length measurements. This method is applied to characterize bubbles in fluidized beds for the first time and the most general bubble geometry, a truncated spheroid, is considered. The distance between probes, s, which is the minimum pierced length that is possible to measure accurately using intrusive probes, has been introduced as a constraint in the derivation of the size distribution equation. The maximum entropy method is applied to experimental measurements of bubble characteristics carried out using optical and pressure probes in a three-dimensional fluidized bed of Geldart B particles. Results on bubble size obtained from pressure and optical probes are very similar, although optical probes provide more local information and can be used at any position in the bed. The maximum entropy principle has been found to be a simple method that offers many advantages over other methods applied before for size distribution modeling in fluidized beds. This work has been supported by the national RDI plan of the Spanish Ministry of Science and Innovation under the project number ENE 2006-01401 Publicado

This work presents a new experimental−numerical method combining PIV (particle image velocimetry), digital image analysis, and FEM (finite element method) to obtain gas and particle motion around b...

This work presents an experimental study of the ejection velocity for different mechanisms of solid ejection in fluidized beds. The experiments were carried out in a 2D fluidized bed, where the bubble eruptions were recorded with a frequency of 250 frames per second using a high speed video camera with a resolution of 1.3 Megapixels. The results show that in isolated bubble eruption, the dome velocity is significantly reduced by the effect of a group of raining particles in the form of stalactites within the bubble. Higher velocities are observed when bubble coalescence takes place. If bubbles coalesce before the leading bubble breaks, the momentumof the trailing bubble together with the increase in the throughflowaccelerate the dome of the leading bubble. In contrast, when coalescence occurs after the breakage of the leading bubble, the wake of the trailing bubble is projected into the freeboard with a very high velocity (wake spike mechanism). The last observed mechanism, the jet spike mechanism, occurs when a stream of bubbles reaches the bed surface following the path opened by the previous bubbles. A cloud of particles moving upward is observed, although their velocities are not as high as in thewake spike mechanism due to the interchange of momentum during the collisions with other particles. Finally, an explanation for some of the patterns of gas release from erupting bubbles recently observed by Hartung et al. [G. Hartung, C.R. Müller, J. Hult, J.S. Dennis, C.F. Kaminski, Laser diagnostic investigation of the bubble eruption patterns in the freeboard of fluidized beds. 1. Optimization of acetone planer laser induced fluorescence measurements. Ind. Eng. Chem. Res. 47 (2008) 5686–5697] is proposed This work has been supported by the National Energy Programme of the Spanish Ministry of Education under the project number ENE2006-01401 Publicado

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. (C) 2016 Elsevier Ltd. All rights reserved.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 work was partially funded by the Spanish Government (Project ENE2010-15403), the regional Government of Castilla-La Mancha (Project PPIC10-0055-4054) and Castilla-La Mancha University (Project GE20101662)

In recent years, the development of phase change materials (PCMs) has introduced new ways to increase the energy storage capacity of a system due to the high latent heat and high storage density of these materials. The aim of this work is to model the charging process of a fluidized bed with PCMs operating as an energy storage device. The temperature in the bed during the charging process of the fluidized bed has been modeled using the two phase theory of fluidization. The dense phase is taken to be perfectly mixed, and the bubble phase is taken to be in plug flow. The numerical model presented takes into account the fact that the phase change process of the bed material occurs over a temperature range and also estimates the energy stored in the wall of the bed and in the distributor plate. The energy equation of the dense phase is numerically solved in enthalpy form, considering the dependence of enthalpy on temperature for phase changes occurring over a range of temperatures. The model's validity is verified against experimental data for two granular materials: sand, a typical material used in fluidized beds, and a granular PCM with a mean particle diameter of 0.54 mm and a phase change temperature of approximately 50 °C. For the sand, the temperature profiles obtained numerically perfectly agree with the values measured experimentally. In the case of the granular PCM, the fitting of the curves is improved when slow and similar heating rates are selected for the experiments and for the DSC measurements used to determine the PCM enthalpy&-temperature curve. This work was partially funded by the Spanish Government (Project ENE2010-15403), the regional Government of Castilla-La Mancha (Project PPIC10-0055-4054) and Castilla-La Mancha University (Project GE20101662). Publicado

This work presents an experimental study to determine the capacity of a phase change material (PCM) in granular form to be used in fixed and bubbling fluidized beds for thermal energy storage. The experimental measurements are focused on determination of the heat transfer coefficient between a heated surface immersed in the bed and the granular PCM. The flow rate is varied to quantify its influence on the heat transfer coefficient. The PCM used is Rubitherm GR50 with a phase change temperature of approximately 50° C. The PCM is available in two different particle sizes, 0.54 mm and 1.64 mm, of which the finer is used in the fluidized bed and the coarser is used in the fixed bed. In addition, the results obtained for the PCM are compared with the heat transfer coefficients measured for sand, a material commonly used for thermal storage. In comparing the heat transfer coefficients for fixed and fluidized beds, the heat transfer coefficients in the fluidized bed with PCM are nearly three times higher than those for the fixed bed at the same gas flow rate. This increase in the heat transfer is a result of two main factors: first, the continuous renewal of PCM particles from the heated surface when they are fluidized, and second, the large quantities of energy in latent form absorbed by the PCM. In the fixed bed there is no renovation of particles, consequently only a small percentage of particles are able to change its phase. Hence, there is no increase in the heat transfer coefficient due to this fact. This work was partially funded by the Spanish Government (Project ENE2010-15403), theRegional Government of Castilla-La Mancha (Project PPIC10-0055-4054) and Castilla-La Mancha University (Project GE20101662). Publicado

We compared chronotopographical patterns of distribution of naturally occurring neuronal death in the ganglion cell layer (GCL) and the inner nuclear layer (INL) with patterns of tangential and radial migration of microglial precursors during quail retinal development. Apoptotic cells were identified by the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling technique, and microglial precursors were identified by immunocytochemistry with an antibody recognizing quail microglial cells (QH1 antibody). Apoptotic cells were first detectable in the GCL at the seventh day of incubation (E7), were most abundant at E10, and were absent after E13. In the INL, apoptotic cells first appeared at E7, were most abundant at E12, and disappeared entirely after the third posthatching day (P3). In both retinal layers, cell death first appeared in a small central area of the retina and subsequently spread along three gradients: central-to-peripheral, temporal-to-nasal, and dorsal-to-ventral. The chronology of tangential (between E7 and E16) and radial migration (between E8 and P3) of microglial precursors was highly coincident with that of cell death in the GCL and INL. Comparison of the chronotopographical pattern of distribution of apoptotic nuclei in the GCL with the patterns of tangential and radial migration of microglial precursors neither supported nor refuted the hypothesis that ganglion cell death is the stimulus that triggers the entry and migration of microglial precursors in the developing retina. However, microglial cells in most of the retina traversed the INL only after cell death had ceased in this layer, suggesting that cell death in the INL does not attract microglial precursors migrating radially. Dead cell debris in this layer was phagocytosed by Müller cells, whereas migrating microglial cells were seen phagocytosing apoptotic bodies in the nerve fiber layer and GCL but not in the INL.

&p.1: Sheets containing the inner limiting membrane covered by a carpet of Muller cell endfeet were used to show that ameboid microglial cells migrating tangentially in the vitreal part of the developing retina of quail embryos underwent mitosis. Double labeling with anti-β-tubulin/QH1 or Hoechst 33342/QH1 revealed that some migroglial cells with morphological features typical of active migration were in early prophase. By anaphase and early telophase, microglial cells had retracted their lamellipodia and were ovoid in shape. Later in telophase, but well before completion of cytokinesis, both daughter cells again emitted lamellipodia, thus regaining the typical morphology of migrating cells. We concluded that ameboid microglial cells go through cycles in which migration and mitosis alternate, and that both mechanisms contribute to the spread of microglia throughout the developing retina. The mitotic spindle of dividing microglial cells showed different orientations, which probably influenced the course of subsequent migration. The expression of the proliferating cell nuclear antigen in the nucleus of most tangentially migrating ameboid microglial cells at E9‐E10 confirmed their proliferative capability. However, the rate of proliferation of these cells decreased during embryonic development, and was nearly zero at E14.

We used the QH1 antibody to study changes in the morphological features and distribution of microglial cells throughout development in the quail cerebellum. Few microglial precursors were present in the cerebellar anlage before the ninth incubation day (E9), whereas many precursors apparently entered the cerebellum from the meninges in the basal region of the cerebellar peduncles between E9 and E16. From this point of entry into the nervous parenchyma, they spread through the cerebellar white matter, forming a 'stream' of labeled cells that could be seen until hatching (E16). The number of microglial cells in the cerebellar cortex increased during the last days of embryonic life and first posthatching week, whereas microglial density within the white matter decreased after hatching. As a consequence, the differences in microglial cell density observed in the cerebellar cortex and the white matter during embryonic life diminished after hatching, and microglia showed a nearly homogeneous pattern of distribution in adult cerebella. Ameboid and poorly ramified microglial cells were found in developing stages, whereas only mature microglia appeared in adult cerebella. Our observations suggest that microglial precursors enter the cerebellar anlage mainly by traversing the pial surface at the basal region of the peduncles, then migrate along the white matter, and finally move radially to the different cortical layers. Differentiation occurs after the microglial cells have reached their final position. In other brain regions the development of microglia follows similar stages, suggesting that these steps are general rules of microglial development in the central nervous system.