REFEREED ARTICLES

1. H. ZHANG, J. DEGREVE, J. BAEYENS, AND R. DEWIL. Wall-to-Bed Heat Transfer at Minimum Gas-Solid Fluidization. J. OF POWDER TECH., Vol. 2014, Article ID 163469, dx.doi.org/10.1155/2014/163469 (2014)
   Corresponding author : ,
   Abstract
   The heat transfer from a fluidized bed to the cooling jacket of the vessel has been studied for various powders at minimum fluidization conditions, by both convection and conduction approaches. These heat transfer characteristics are important as the point of transition between packed and fluidized bed operations and are needed in designing heat transfer operations where bubble flow is not permitted. The effective thermal conductivity of the emulsion moreover determines the contact resistance at the heating or cooling surface, as used in packet renewal models to predict the wall-to-bed heat transfer. In expressing the overall heat transfer phenomenon as a convective heat transfer coefficient, it was found that the results could be fitted by Numf,𝑗 = 0.01Ar^0.42.
   
   
2. B. Boissière, R. Ansart, D. Gauthier, G. Flamant, M. Hemati. “Experimental Hydrodynamic Study of Dense Gas-Particle Suspension Upward Flow for Application as New Heat Transfer and Storage Fluid“. Canadian J. of Chem. Engrg, Vol. 93, pp. 317-330, doi10.1002/cjce.22087 (2015)
   Corresponding author : ,
   Abstract
   This paper focuses on a new concept of Heat Transfer Fluid -HTF- for Concentrating Solar Plants –CSP applications through fluidised bed. CSP plants with very high concentration (like solar tower plant technology) offer good efficiencies because of high operating temperatures. CSP efficiency could be greatly increased through highest operating temperatures of their HTF, currently restricted by their nature.
   Dense Particle Suspension -DPS- fluidised with air (approximately 40% of solid) are proposed to replace classical HTF. This innovation patented by Flamant and Hemati in 2010 [1] consists in creating a dense suspension upward flow of fine particles in vertical absorbing tubes. In this technological breakthrough, the concentrated solar energy is collected, carried and stored directly by particles flowing upward or downward, with a suspension void fraction close to that of a dense fluidised bed. This void fraction offers a good contact area between the wall and the particles, contrary to circulating fluidized bed “risers”. This new HTF eliminates most of the used HTF (molten salts, mineral oils, water and air) drawbacks: limited range of operating temperature, corrosiveness, pressurisation, storage capacity and toxicity. This new HTF has a volume heat capacity similar to that of liquid HTF and is only limited by the maximal working temperature of the receiver tubes (1,100 K), thus opening new opportunities for high efficiency thermodynamic cycles.
   The work presented here deals with the upward flow of such dense suspensions at ambient temperature. The “cold” study of this technology is justified by the important hydrodynamic and thermal coupling: ambient flows had to be understood and controlled first. The construction of a “cold” mock-up showed the ability to handle a regular solid upward flow, with various solid flow rates and suspension void fractions. This mock-up is a 2-pass exchanger, each pass composed of two vertical parallel tubes. Pressure drop, solid weight and helium volume fraction measurements demonstrated the hydrodynamic feasibility of this exchanger technology: it can generate steady flow of dense suspension with an even distribution between the tubes for solid feeding flow rate ranging from 20 to 130 kg.h-1. Moreover, the governing parameters of this flow were established as: the solid feeding flow rate, the fluidisation velocity, the solid holdup, the freeboard pressure and the aeration velocity. The secondary air injection, also called “aeration”, is the most important parameter for the stability and the even distribution of the total solid flow rate in the tubes.
   
   
3. J. MARTI, A. HASELBACHER, A. STEINFELD, « A numerical investigation of gas-particle suspensions as heat transfer media for high-temperature concentrated solar power » INT. J. HEAT & MASS TRANSFER, Vol. 90, pp. 1056-1070, dx.doi.org/10.1016/j.ijheatmasstransfer.2015.07.033 (2015)
   Corresponding author : ,
   Abstract
   This paper investigates the detailed heat-transfer mechanisms in dense gas-particle suspensions used as heat transfer media for high-temperature concentrated solar power applications. A two-phase Euler– Euler model for dense gas-particle systems is built on the open-source code OpenFOAM. The model is capable of predicting the complex hydrodynamic behavior of bubble formation, coalescence, and breakup together with conduction, convection, and radiation heat transfer. At each time step, the model calculates the effective radiative properties as a function of the local solid volume fraction. Therefore, the model captures radiation penetrating through gas bubbles near the riser wall and radiation being absorbed within a few millimeters by the dense gas-particle suspension. Comparisons with on-sun experimental results indicate that the model accurately predicts coupled hydrodynamics and heat transfer in dense gas-particle systems. The model is used to investigate the heat-transfer mechanisms in a slowly rising, dense gas-particle suspension located in a directly irradiated riser tube. The majority of the heat transfer takes place within a distance of a few particle diameters from the heated riser wall. In this region, the particles are heated by solid conduction and heat is then transferred by solid convection to the colder flow in the center of the riser. It is shown that with a moderate riser wall temperature of 581 K and a particle diameter of 64 lm, solid conduction accounts for about 97% of the wall-to-suspension heat flux. Increasing the wall temperature to 981 K together with a particle diameter of 400 µm leads to an increase of the radiation heat-flux contribution up to about 10% of the total wall-to-suspension heat flux.
   
   
4. H. BENOIT, PEREZ LOPEZ I., GAUTHIER D., SANS J.-L., FLAMANT G., « On-sun demonstration of a 750°C heat transfer fluid for concentrating solar systems: dense particle suspension in tube » SOLAR ENERGY, Vol. 108, pp 622-633, dx.doi.org/10.1016/j.solener.2015.06.007 (2015)
   Corresponding author : ,
   Abstract
   Dense particle suspensions were tested in a high temperature single tube on-sun solar receiver at the CNRS solar facility. Powder outlet temperature as high as 750°C was measured thus proving the capacity of this heat transfer fluid concept to reach high operating temperatures. Wall-to-suspension heat transfer coefficient was calculated on the basis of detailed temperature measurements and accounting for the strong recirculation phenomena occurring at the tube wall. They range from 420 W/m².K to 1100 W/m².K for a solid mass flux of 10 kg/m².s and 45 kg/m².s, respectively. The temperature increase positively influences the heat transfer coefficient; with a 30% increase measured between ~200 °C and 600 °C powder average temperatures. Finally, a correlation is proposed to predict the dependency of heat transfer coefficient and solid mass flux.
   
   
5. H. ZHANG, J. DEGREVE, R. DEWIL, J. BAEYENS. Wall-to-Suspension Transfer in a CFB downcomer.J. OF POWDER TECH., Vol. 2015, Article ID 293165, (2015) http://dx.doi.org/10.1155/2015/293165
   Corresponding author : ,
   Abstract
   With the development of circulating fluidized beds (CFB) and dense upflow bubbling fluidized beds (UBFB) as chemical reactors, or in the capture and storage of solar or waste heat, the associated downcomer has been proposed as an additional heat transfer system. Whereas fundamental and applied research towards hydrodynamics has been carried out, few results have been reported on heat transfer in downcomers, even though it is an important element in their design and application. The wall-to-suspension heat transfer coefficient (HTC) was measured in the downcomer. The HTC increases linearly with the solids flux, till values of about 150 kg/m2 s. The increasing HTC with increasing solid circulation rate is reflected through a faster surface renewal by the downflow of the particle-gas suspension at the wall. The model predictions and experimental data are in very fair agreement, and the model expression can predict the influence of the dominant parameters of heat transfer geometry, solids circulation flow, and particle characteristics.
   
   
6. H. ZHANG, J. DEGREVE, J. BAEYENS, S. WU. Powder attrition in gas fluidized beds.POWDER TECH., Vol. 287, pp 1-11 (2016) http://dx.doi.org/10.1016/j.powtec.2015.08.052
   Corresponding author : ,
   Abstract
   New developments of fluidized beds are focusing on their use in powder circulation systems for thermal energy capture, storage and re-use. Although fast particle motion and associated high degree of mixing favor the high rate of heat transfer in fluidized beds, they however cause inter-particle collision and bed-to-wall impacts, both leading to particle attrition. Experimental work on attrition was carried out in batch gas fluidized beds of 6.62 cm and 10 cm I.D. The rate of attrition was determined both for bubbling fluidized bed conditions, and when adding jet-orifices above the distributor. The attrition was determined from the variation in composition of bed material and collected carryover. A literature survey determined the dominant fluidization parameters, including operating superficial gas velocity, orifice velocity and size, bed weight and diameter, and particle characteristics. Analysis of the experimental findings resulted in a correlation that encompasses all relevant operating characteristics. As a result, the attrition rate can be correlated as the sum of the bubble-induced and jet-induced contribution: Rt = K₁[γ(U−U_mf ).W/D] + K₂[n_or . d_or² . U_or²] with K₁ and K₂, the intrinsic attrition rate constants, being mainly a function of particle characteristics. K1 is ~10⁻⁵ for soft, ~10⁻⁶ for hard and ~10⁻⁷ for very hard particles, respectively. K₂ is ~10⁻⁵ for silica sand. In general, attrition is negligible at low superficial gas velocities, but significantly increases through the jet-induced contribution if the orifice velocity exceeds ~30 m/s.
   
   
7. H. BENOIT, SPREAFICO L., GAUTHIER D., FLAMANT G., « Review of heat transfer fluids in tube-receivers used in concentrating solar thermal systems: Properties and heat transfer coefficients », RENEWABLE & SUSTAINABLE ENERGY REVIEWS Vol. 55, pp 298-315, dx.doi.org/10.1016/j.rser.2015.10.059 (2016)
   Corresponding author : ,
   Abstract
   The Heat transfer fluid (HTF) is a key component of solar thermal power plant because it significantly impacts the receiver efficiency, determines the type of thermodynamic cycle and the performance it can achieve, and determines the thermal energy storage technology that must be used. This paper reviews current and future liquid, gas, supercritical, two-phase and particulate HTFs. Thermophysical properties are presented as well as correlations to determine the receiver tube-HTF heat transfer coefficients. Variations of convective heat transfer coefficients as a function of temperature are illustrated for all selected HTFs in their stable operation temperature ranges. Finally, recent developments on new HTFs working at 700°C and beyond are discussed.
   
   
8. H.L. ZHANG, H. BENOIT, D. GAUTHIER, J. DEGREVE, J. BAEYENS, I. PÉREZ -LÓPEZ, M. HEMATI, G.FLAMANT “Particle circulation loops in solar energy capture and storage: gas-solid flow and heat transfer considerations” APPLIED ENERGY, Vol. 161C, pp 206-224.  doi: 10.1016/j.apenergy.2015.10.005 (2016)
   Corresponding author : ,
   Abstract
   A novel application of powders relies on their use as heat transfer medium for heat capture, conveying and storage. The use of powders as heat transfer fluid in concentrated solar systems is discussed with respect to current technologies. The specific application reported upon is the use of powder loops in Solar Power Tower plants. In the proposed receiver technology, SiC powder is conveyed as a dense particle suspension through a multi-tube solar receiver in a bubbling fluidization mode, the upwards flow being established by pressurizing the powder feed. Tests were conducted with a single-tube receiver unit at the 1 MW solar furnace of CNRS (Odeillo Font-Romeu, F). The measured wall-to-suspension heat transfer coefficient is a function of operating temperature, applied air velocity and imposed solid circulation flux: values increased with increasing solids flux from ~ 430 to 1120 W/m²K. Empirical approaches and a heat transfer model were applied to compare experimental and predicted values of the heat transfer coefficient, with a fair agreement obtained. The research moreover provides initial data concerning the overall economy of the system. The high temperature of the circulating powder leads to an increased power cycle efficiency, an increased storage density, reduced thermal power requirements, reduced heliostat field size, reduced parasitic power consumption and increased plant capacity factor.
   
   
9. H.L. ZHANG, J. BAEYENS, G. CACERES, J. DEGREVE, Y.Q. LU “Thermal energy storage: recent developments and practical aspects” PROGRESS IN ENERGY AND COMB. SCI., Vol. 53, pp. 1-40  doi: 10.1016/j.pecs.2015.10.003 (2015)
   Corresponding author : ,
   Abstract
   Thermal Energy Storage (TES) transfers heat to storage media during the charging period, and releases it at a later stage during the discharging step. It can be usefully applied in solar plants, or in industrial processes, such as metallurgical transformations. Sensible, latent and thermo-chemical media store heat in materials which change temperature, phase or chemical composition, respectively. Sensible heat storage is well-documented. Latent heat storage, using Phase Change Materials (PCMs), mainly using liquid-solid transition to store latent heat, allow a more compact, efficient and therefore economical system to operate. Thermo-chemical heat storage (TCS) is still at an early stage of laboratory and pilot research despite its attractive application for long term energy storage. The present review will assess previous research, while also adding novel treatments of the subject. TES systems are of growing importance within the energy awareness: TES can reduce the LCOE (levelized cost of electricity) of renewable energy processes, with the temperature of the storage medium being the most important parameter. Sensible heat storage is well documented in literature and applied at large scale, hence limited in the content of the present review paper. Latent heat storage using PCMs is dealt with, specifically towards high temperature applications, where inorganic substances offer a high potential. Finally, the use of energy storage through reversible chemical reactions (Thermo-chemical storage, TCS) is assessed. Since PCM and TCS storage media need to be contained in a capsule (sphere, tube, sandwich plates) of appropriate materials, potential containment materials are examined. A heat transfer fluid (HTF) is required to convey the heat from capture, to storage and ultimate re-use. Particle suspensions offer a valid alternative to common HTF, and a preliminary assessment confirms the advantages of the upflow bubbling fluidized bed and demonstrates that particulate suspensions enable major savings in investment and operating costs. Novel treatments of the TES subject in the review involve the required encapsulation of the latent and chemical storage media, the novel development of powder circulation loops as heat transfer media, the conductivity enhancement of PCMs, the use of Lithium salts, among others.
   
   
10. D. VALDESUEIRO, P. GARCIA-TRIÑANES, G.M.H. MEESTERS, M.T. KREUTZER, J. GARGIULI, T. LEADBEATER, D.J. PARKER, J.P.K. SEVILLE, J.R. VAN OMMEN « Enhancing the activation of silicon carbide tracer particles for PEPT applications using gas-phase deposition of alumina at room temperature and atmospheric pressure » NUCLEAR INSTRUM. & METHODS IN PHYSICS RES. A Vol. 807, pp. 108-113, dx.doi.org/10.1016/j.nima.2015.10.111 (2016)
    Corresponding author : ,
    Abstract
    We have enhanced the radio-activation efficiency of SiC (silicon carbide) particles, which by nature have a poor affinity towards 18F ions, to be employed as tracers in studies using PEPT (Positron Emission Particle Tracking). The resulting SiC-Al2O3 core-shell structure shows a good labelling efficiency, comparable to -Al2O3 tracer particles, which are commonly used in PEPT. The coating of the SiC particles was carried at 27 ± 3 °C and 1 bar in a fluidized bed reactor, using trimethyl aluminium and water as precursors, by a gas phase technique similar to atomic layer deposition. The thickness of the alumina films, which ranged from 5 to 500 nm, was measured by elemental analysis and confirmed with FIB-TEM (focus ion beam – transmission electron microscope), obtaining consistent results from both techniques. By depositing such a thin film of alumina, properties that influence the hydrodynamic behaviour of the SiC particles, such as size, shape and density, are hardly altered, ensuring that the tracer particle shows the same flow behaviour as the other particles. The paper describes a general method to improve the activation efficiency of materials, which can be applied for the production of tracer particles for many other applications too.
    
    
11. REYES URRUTIA A., BENOIT H., ZAMBON M., GAUTHIER D., FLAMANT G. and MAZZA G. “Simulation of the behavior of a dense sic particle suspension as an energy transporting vector using computational fluid dynamics (CFD)” CHEM. ENGRG RES. & DESIGN, Vol. 106, pp. 141-154, doi 10.1016 j.cherd.2015.12.008 (2015)
    Corresponding author : ,
    Abstract
    In the search for greater efficiency and storage capacity improvements in solar energy concentration plants, a new concept for fluid transfer was proposed. This concept consists of a dense suspension of SiC particles (dp= 6.4 10-5 m) that is air fluidized, which allows operation at higher temperatures than the fluids currently used, such as molten salts, water, oils and air. The suspension, as a fluid, also provides energy storage. The upward flow of the SiC-air suspension inside a steel tube is achieved using a circulating fluidized bed dense regime. Concentrated solar radiation impinges the walls of the tube, increasing the temperature of the granular material up to 200-250°C.The system in this study is part of a prototype in the PROMES laboratory in France. Maintaining low fluidization velocities guarantees high solid fractions throughout the tube (0.28-0.45). This study simulates heat transfer between the wall and the suspension using computational fluid dynamics (CFD) (ANSYS-Fluent 14.5) for different operating conditions. The Euler-Euler model is used as the multi-phase model. The average experimental temperature in the emulsion at the exit of the heat transfer zone compares well with the temperature obtained in the CFD simulation. The global heat transfer coefficients obtained in the simulation are in good agreement with those obtained experimentally for all operating conditions. These results show that the developed simulation approach is a good representation of the real process and provides relevant information related to the movement of particles in the tube and its relation to heat transfer in the prototype
    
    
12. I. PEREZ LOPEZ, H. BENOIT, D. GAUTHIER, J.L. SANS, E. GUILLOT, G. MAZZA, G. FLAMANT « On-sun operation of a 150 kWth pilot solar receiver using dense particle suspension as heat transfer fluid » SOLAR ENERGY, Vol. 137, pp 463-476, dx.doi.org/10.1016/j.solener.2016.08.034 (2016)
    Corresponding author : ,
    Abstract
    Previous studies proved the Dense Particle Suspension (DPS) - also called Upward Bubbling Fluidized Bed (UBFB) - could be used as Heat Transfer Fluid (HTF) in a single-tube solar receiver. This article describes the experiments conducted on a 16-tube, 150 kWth solar receiver using a dense gas-particle suspension (around 30 % solid volume fraction) flowing upward as HTF. The receiver was part of a whole pilot setup that allowed the continuous closed-loop circulation of the SiC particles used as HTF. One hundred hours of on-sun tests were performed at the CNRS 1 MW solar furnace in Odeillo. The pilot was tested under various ranges of operating parameters: solid mass flow rate (660-1760 kg/h), input solar power (60-142 kW), and particle temperature before entering the solar receiver (40-180 °C). Steady states were reached during the experiments, with continuous circulation and constant particle temperatures. For the hottest case, the mean particle temperature reached 430 °C in the collector fluidized bed, at the receiver outlet, and it went up to 700 °C at the outlet of the hottest tube, during steady operation. A temperature difference between tubes is observed that is mainly due to the incident solar flux distribution heterogeneity. The thermal efficiency of the receiver, defined as the ratio of power transmitted to the DPS in the form of heat over solar power entering the receiver cavity, was calculated in the range 50-90 % for all the experimental cases. The system transient responses to variations of the solar irradiation and of the solid mass flow rate are also reported.
    
    
13. P. GARCIA-TRIÑANES, J.P.K. SEVILLE, B. BOISSIERE, R. ANSART, T.W. LEADBEATER, D.J. PARKER « Hydrodynamics and particle motion in upward flowing dense particle suspensions: Application in solar receivers  » CHEM. ENGRG. SCI., Vol. 146, pp 346-356 (2016) dx.doi.org/10.1016/j.ces.2016.03.006
    Corresponding author : ,
    Abstract
    Dense gas-solid suspensions have the potential to be applied as heat transfer fluids (HTF) for energy collection and storage in concentrated solar power plants. At the heart of these systems is the solar receiver, composed of a bundle of tubes, which contain the solid suspension used as the thermal energy carrier. In the design investigated here, the particles form a dense upward-flowing suspension. The density of the suspension of these particles and their movement each has a strong influence on the heat transfer. An apparatus was designed to replicate at ambient temperature the hydrodynamic and particle motion in the real solar energy plant. The governing parameters of the flow were established as the solid feeding flow rate, the fluidisation velocity, the solids holdup, the freeboard pressure and the secondary air injection (aeration) velocity. In the case studied, aeration was applied, with air introduced into the uplift transport tube some way up its length. This study finds that the amount of this secondary air injection is the most important parameter for the stability and the uniform distribution of the solids flow in the tubes. Solids motion was measured using the non-invasive positron emission particle tracking (PEPT) technique to follow the movement of a 60 μm tracer particle, onto which was adsorbed the positron emitting 18F radioisotope. Analysis of the resulting three-dimensional trajectories provides information on solids flow pattern, solids velocity and mass transfer rate. Results show the overall behaviour of the bulk material in detail: small step-wise movements associated with bubble motion superimposed on a general trend of upward flow in the centre and downward flow close to the walls. These findings suggest that this particular type of flow is ideal for transporting energy from the walls of the solar receiver tubes.
    
    

PRESENTATIONS

14. H. BENOIT, I. PEREZ LOPEZ, D. GAUTHIER, G. FLAMANT, J.-L. SANS, « Récepteur à suspension dense de particules : une nouvelle option pour la conversion de l’énergie solaire à haute température » Journées Nationales sur l’énergie solaire (JNES 2014), Campus université de Perpignan, Perpignan, France, 8-10 juillet (2014)
    Corresponding author : ,
    Abstract
    Après avoir démontré dans un précédente publication la capacité des suspension denses de particules solides (« Dense Particle Suspension », DPS) à être utilisées comme fluide de transfert thermique (« Heat Transfer Fluid », HTF) pour les récepteur des centrales solaires à tour, de nouveaux résultats obtenus à haute température sont présentés ici. La DPS est obtenue en fluidisant des particules fines de carbure de silicium (SiC), d’un diamètre moyen de 63,9 µm, à des vitesses d’air très faibles, de l’ordre du centimètre par seconde. Le principe du récepteur solaire à suspension dense de particules (DSPR) est de mettre en circulation ascendante la DPS à partir d’un lit fluidisé émetteur, avec une fraction volumique de solide comprise entre 25 et 35%, dans un faisceau de tubes absorbeurs verticaux exposés au rayonnement solaire concentré. La DPS se comporte comme un HTF avec une capacité thermique comparable aux HTF liquides, mais sans dangerosité et ayant comme seule limite supérieure de température celle des tubes récepteurs. Les résultats ont été obtenus sur un pilote mono-tubulaire, modifié par rapport aux travaux précédents afin de permettre le préchauffage du solide dans le lit fluide émetteur. Les nouveaux résultats montrent une importante augmentation du coefficient de transfert thermique h entre la paroi du tube et la DPS avec la température moyenne des particules. Les essais sont toujours en cours mais une analyse préliminaire montre déjà que pour une fraction volumique de solide de 30 % et une vitesse moyenne des particules de 0,02 m/s, la valeur de h, qui était proche de 350 W/m².K à une température moyenne de la DPS de 250 °C dans le tube récepteur, dépasse 700 W/m².K lorsque cette température atteint 480 °C. Des valeurs de h approchant 1000 W/m².K sont ainsi attendues pour des valeurs supérieures de la fraction volumique de solide, de la vitesse moyenne de particule et de la température des particules. Au cours de ces derniers essais, des températures de solide de 650°C ont été atteintes. Ce niveau de température ouvre la voie vers les cycles thermodynamiques supercritiques à haut rendement.
    
    
15. Zhang H.L., Flamant G., Gauthier D., Ansart R., Hemati M., Baeyens J., Boissière B. “The use of dense particle suspensions as heat transfer carrier in solar thermal plants“ International Conference on Solar Heating and Cooling, Gleisdorf, Austria, June 25-27 (2014)
    Corresponding author : ,
    Abstract
    Concentrated solar systems efficiently produce high temperature heat and subsequent power thanks to heat capture/storage and hybridization. Among available technologies, solar towers (central receiver systems) enable to reach temperatures in excess of 500 °C, as needed to power efficient Rankine steam cycles. Current heat carriers are mostly molten salts with drawbacks of temperature limitation (< 600 °C), required heat tracing of the circuits, and high pumping power requirements. The use of particle suspensions as heat carrier to transfer solar power from the receiver to the energy conversion process can overcome the drawbacks of molten salts. The concept of using particle suspension carriers was tested at the solar receiver of Font Romeu (F) for solar flux densities of 200-250 kW/m². The selected particles were 64 µm silicon carbide, conveyed in a single tube at air velocities between 0.03 and 0.22 m/s, achieving powder circulation fluxes between 8 and 25 kg/m²s. The tube wall temperature reached values up to ~370 °C. Throughout its conveying within the tube, the SiC powder increased its temperature by 50 to 150 °C. The wall-to-suspension heat transfer coefficient was determined, and ranged from 140 to ~ 530 W/m²K, as function both of applied air velocity and imposed solid circulation flux. The results of this sun-on proof of concept will be presented in the paper, together with a heat transfer model to predict the heat transfer coefficient for different operating conditions, with air flow rate and solids circulation flux as dominant parameters. Model predictions and experimental results are in very fair agreement (within 10 %). A higher heat transfer coefficient is expected at higher temperatures, where both the increased thermal conductivity of the air, and the contribution of radiation heat transfer will enhance the heat transfer. Since the particle suspension has a heat capacity similar to that of molten salts, without temperature limitation except for the maximum allowable wall temperature of the receiver tube, suspension temperatures of up to 750 °C can be tolerated for refractory steel tubes (even higher when using ceramic tubes), thus offering new opportunities for highly efficient thermodynamic cycles such as obtained when using supercritical steam or CO₂.
    
    
16. H. ZHANG, G. FLAMANT, D. GAUTHIER, R. ANSART, M. HEMATI, J. BAEYENS J., B. BOISSIERE. The use of dense particle suspensions as heat transfer carrier in solar thermal plants World Renewable Energy Congress WREC XIII, Univ. of Kingston, London, UK, 3-8 August (2014)
    Corresponding author : ,
    Abstract
    Concentrated solar systems efficiently produce high temperature heat and subsequent power thanks to heat capture/storage and hybridization. Among available technologies, solar towers (central receiver systems) enable to reach temperatures in excess of 500 °C, as needed to power efficient Rankine steam cycles. Current heat carriers are mostly molten salts with drawbacks of temperature limitation (< 600 °C), required heat tracing of the circuits, and high pumping power requirements. The use of particle suspensions as heat carrier to transfer solar power from the receiver to the energy conversion process can overcome the drawbacks of molten salts. The concept of using particle suspension carriers was tested at the solar receiver of Font Romeu (F) for solar flux densities of 200-250 kW/m². The selected particles were 64 µm silicon carbide, conveyed in a single tube at air velocities between 0.03 and 0.22 m/s, achieving powder circulation fluxes between 8 and 25 kg/m²s. The tube wall temperature reached values up to ~370 °C. Throughout its conveying within the tube, the SiC powder increased its temperature by 50 to 150 °C. The wall-to-suspension heat transfer coefficient was determined, and ranged from 140 to ~ 530 W/m²K, as function both of applied air velocity and imposed solid circulation flux. The results of this sun-on proof of concept will be presented in the paper, together with a heat transfer model to predict the heat transfer coefficient for different operating conditions, with air flow rate and solids circulation flux as dominant parameters. Model predictions and experimental results are in very fair agreement (within 10 %). A higher heat transfer coefficient is expected at higher temperatures, where both the increased thermal conductivity of the air and the contribution of radiation heat transfer will enhance the heat transfer. Since the particle suspension has a heat capacity similar to that of molten salts, without temperature limitation except for the maximum allowable wall temperature of the receiver tube, suspension temperatures of up to 750 °C can be tolerated for refractory steel tubes (even higher when using ceramic tubes), thus offering new opportunities for highly efficient thermodynamic cycles such as obtained when using supercritical steam or CO₂.
    
    
17. H. ZHANG., J. BAEYENS, J. DEGRÈVE. Phase change materials (PCMs) in novel heat capture and storage systems. World Renewable Energy Congress WREC XIII, Univ. of Kingston, London, UK, 3-8 August (2014)
    Corresponding author : ,
    Abstract
    Heat capture and storage is important in both solar energy projects and in the recovery of waste heat from industrial processes. Whereas heat capture mostly relies on the use of a heat carrier, the high efficiency heat storage cannot solely rely upon sensible heat. Latent heat storage with phase change materials (PCMs) provides a high energy density storage due to the phase change by solidifying/melting at a constant temperature. In comparison with sensible heat storage, PCMs require a smaller volume of material for a given amount of captured/stored energy, and maintain a high and constant temperature difference between the heat carrier and the PCMs during the phase change. PCMs have been initially investigated for e.g. heating/cooling applications in buildings, or towards thermal storage systems for heat pumps. Research into their use in solar engineering and industrial waste heat capture and recovery is more recent. The present paper will review (i) the required properties of PCMs, with their respective advantages and disadvantages; (ii) the opportunities of using PCMs in thermal energy recovery; (iii) the current state of development and manufacturing; (iv) the development of PCM applications, mostly in view of overcoming their slow charging/discharging rates, due to their low thermal conductivity, and including their incorporation into heat exchangers, the insertion of a metal matrix into the PCM, and the use of PCM dispersed with high conductivity particles; (v) the illustration of PCM uses through some case-studies. The present paper will finally illustrate first results obtained when encapsulating e.g. NaNO₃/KNO₃-PCM in an AISI 321 tube, as example of a storage application using a multi-tubular exchanger filled with PCM, with and without inserts. Results of the achieved discharging (cooling) rates will be interpreted by solving the unsteady state conduction equation, and by using Comsol Multiphysics.
    
    
18. R. ANSART, B. BOISSIERE, M. HEMATI, H. BENOIT, D. GAUTHIER, G. FLAMANT, « Experimental study and 3D numerical simulation of Dense Gas-Particle Suspension Upward Flow set in a hot solar receiver » 21st International Congress of Chemical and Process Engineering CHISA 2014 Prague, CZECH REPUBLIC, August 23-27, 2014
    Corresponding author : ,
    Abstract
    Following Heat Transfer Fluid (HTF) have drawbacks, in particular a limited working temperature domain for salt. Moreover, it involves safety risks, which explain why there is currently no industrial application. A solution to overcome these drawbacks is using solid particles as HTF. In the frame of the call for projects of the European Commission which aims to find alternative HTF in order to extend working temperature and to decrease environmental impact of standard (HTF) used in concentrating solar power (CSP) plants, we proposed to use Dense Particle Suspensions (DPS) fluidized with air (approximately 50% of solid) in tubes as new HTF and storage media. DPS will enable operating temperature over 1,000°C which corresponds to the sintering temperatures of the solid against 560°C for the most efficient molten salts, thus increasing the plant efficiency and decreasing the cost per kWh produced, have no lower limitation of temperature and are riskless. Boissiere et al. [1] determined, on a cold mock up the required operating conditions to ensure a constant solid flow rate in the exchanger. Flamant et al. [2] have demonstrated the capacity of dense suspensions of solid particles to transfer concentrated solar power from a tubular receiver to an energy conversion process by acting as a HTF. Moreover, they measured the first value of the heat transfer coefficient on sun proof. In this paper, the heat transfer coefficient is determined on a laboratory hot mock up with thermal operating conditions controlled by ovens. The hot mock up is composed of a single opaque metallic tube of 27 mm internal diameter located in ovens. The inlet of the tube exchanger is set in a dense fluidized bed where the freeboard pressure is controlled. The thermal flux generated by ovens is equal to 50 kW/m2 and suspension mass flow rate varies from 20 to 130 kg/h. According to the multiphase flow code NEPTUNE_CFD, 3D numerical simulations coupling hydrodynamics and heat transfer are realized. As the particle are A type, the mesh is expensive with more 1,6 millions of cells. The pressure and temperature measurements are compared to the numerical results.
    
    
19. GARCIA TRIÑANES P, ANGUITA J, GHANDI K, RAVI P. SILVA S, SEVILLE J, LEADBEATER TW, PARKER DJ, « Indirect labelling of SiC particles by using surface modification and functionalization» 12th UK Particle Technology Forum, Manchester, UK, September 12-17, 2014 
    Corresponding author : ,
    Abstract
    Silicon carbide (SiC) is being used as the main constituent of dense gas-particle suspensions for concentrated solar power plants [1]. This fluidised particle suspension acts as an alternative heat transfer fluid, therefore the hydrodynamics of the system are key performance indicators of this process. To study the circulation and mobility of the particles in the bed, the technique known as Positron Emission Particle Tracking (PEPT) is used [2]. During a typical PEPT experiment, a single particle of SiC (60 microns) would act as a positron-emitting tracer and the 3D location is obtained by the triangulation of events associated with the detection of pairs of annihilation gamma rays. The activation is achieved by altering the SiC surface with 18F based on the transfer of fluorine ions from an aqueous phase solution prepared by direct bombardment of ultra-pure water onto the particle surface [3]. However, the chemically inert properties of SiC have been shown to hinder the activation process. Nonetheless, previous studies have shown that 18F has high affinity for γ-alumina, which is currently used as a tracer in other PEPT studies. Though the true density of γ-alumina would make it a suitable replacement for SiC, its bulk density is far too low to act as such. Thus, it is essential that the surface of SiC particles is successfully activated.
    
    
20. J. Spelling, A. Gallo, M. Romero, J. González-Aguilar. “A High-Efficiency Solar Thermal Power Plant using a Dense Particle Suspension as the Heat Transfer Fluid.” SolarPACES 2014, Energy Procedia, 69 (2015), pp 388-397, Beijing, CHINA, September 16-19 (2014)
    Corresponding author : ,
    Abstract
    A novel solar power plant concept is presented, based on the use of a dense particle suspension as the heat transfer fluid which allows receiver operation at high temperatures (above 650°C), opening the possibility of using high-efficiency power generation cycles such as supercritical Rankine cycles. A 50 MWe solar power plant was designed based on this new heat transfer fluid and compared with a conventional molten salt solar power plant. The supercritical Rankine-cycle power block increases the thermal conversion efficiency from 39.9% to 45.4%, corresponding to a 9.6% reduction in the size of the heliostat field. The operating temperature range is increased by 24.5%, which leads to a 12.5% increase in storage density and a 22.5% reduction in the total storage volume. Parasitic power consumption is also reduced due to the elimination of the need for heat tracing. Overall, the combination of increased cycle efficiency, increased storage density and reduced parasitics to leads to a predicted electricity cost reduction of 10.8%.
    
    
21. A. Gallo, J. Spelling, M. Romero, J. González-Aguilar. “Preliminary Design and Performance Analysis of a Multi-Megawatt Scale Dense Particle Suspension Receiver”. SolarPACES 2014, Energy Procedia 69 (2015), pp 1160-1170, Beijing, CHINA, September 16-19 (2014)
    Corresponding author : ,
    Abstract
    A novel receiver concept is presented, based on the use of a dense particle suspension as the heat transfer medium; this medium allows receiver operation at high temperatures (above 650°C), resulting in significant gains in power plant efficiency. A 10 MWth receiver has been designed based on the scale-up of a 150 kWth prototype currently undergoing testing. The predicted thermal efficiency is 81.3%, well above the design target of 70%. Material temperatures within the absorber tubes were maintained below 850°C throughout the receiver, below the limits of high temperature steels.
    
    
22. J. MARTI, « Gas-particle suspensions as high-temperature heat transfer media for concentrated solar power applications » PhD Thesis N° 22572 ETH Zurich, SWITZERLAND, February 27, 2015
    Corresponding author : ,
    Abstract
    A major improvement of the solar-to-electricity efficiency of conventional concentrated solar power (CSP) plants requires an increase of the operating temperature to enable the use of a more efficient thermodynamic cycle. In addition, cost-effective thermal-energy storage systems are required to reduce the cost of dispatchable electricity generation by CSP plants. Finally, this enables the replacement of fossil-fired power plants. These requirements can be achieved by using a slowly upward-moving, dense gas-particle suspension as heat transfer medium (HTM) to absorb and store the solar energy. The gas-particle suspension is heated up by moving through directly irradiated riser tubes located in the solar receiver of a solar power tower.
    This dense gas-particle suspension behaves thereby like an upward-moving bubbling fluidized bed. By using a HTM based on heat-resistant particles with a high heat capacity like silicon carbide (SiC) particles, the operating temperature of the CSP plant can be increased to about 1000 °C. Additionally, the hot particles can directly be used as thermal-energy storage media.
    To advance the development of this new HTM for CSP plants, this thesis investigates the detailed hydrodynamics and heat-transfer mechanisms in dense gas-particle suspensions. Therefore, a two-phase Euler-Euler model for dense gas-particle systems is built on the open-source code OpenFOAM.
    The model is capable of predicting the complex hydrodynamic behavior of bubble formation, coalescence, and breakup together with conduction, convection, and radiation heat transfer.
    To accurately model the radiation heat transfer, an experimental-numerical approach is developed to determine the volume-averaged radiation properties: extinction coefficient, scattering albedo, and approximated scattering phase function of SiC particle suspensions with different solid fractions.
    A spectroscopic goniometry system is used to measure the angular intensity distribution around irradiated SiC particle suspensions. In a second step, this experimental intensity distribution is compared to the numerical intensity distribution of a Monte Carlo ray-tracing model matching the experimental setup and representing the particle suspension by a participating medium. The radiation properties of the participating medium are adjusted in a fitting routine until the resulting numerical intensity distribution is in good agreement with the experimental distribution. This procedure is done for different suspension thicknesses with different solid fractions. In this way, the resulting set of radiation properties can directly be applied to solve the radiation transfer equation in gas-particle systems with changing solid fractions using existing popular methods like the spherical harmonics, discrete ordinate, or statistical Monte Carlo methods.
    At each time step, the two-phase model calculates the effective radiation properties as a function of the local solid fraction based on the determined radiation properties of the SiC particle suspensions. Therefore, the model captures radiation penetrating through gas bubbles neat the riser wall and radiation being absorbed within a few millimeters by the dense gas-particle suspension.
    In addition to separate verification and validation studies of hydrodynamics and heat transfer, comparisons with on-sun experimental results indicate that the model accurately predicts coupled hydrodynamics and heat transfer in dense gas-particle systems.
    In a parameter study, the model is used to investigate the influence of the riser-wall temperature and riser diameter on the heat-transfer coefficient, solid temperature, solid mass flow rate, and solid fraction. Increasing the riser-wall temperature leads for a constant mass flow rate to a decrease of the heat-transfer coefficient as the logarithmic mean temperature difference over the irradiated riser section strongly increases. An increase of the riser diameter from 36mm to 72mm leads for a constant gas inlet velocity and riser-wall temperature to a solid temperature reduction of about 30 %.
    A detailed investigation of the involved heat-transfer mechanisms shows that the majority of the heat transfer takes place within a distance of a few particle diameters from the heated riser wall. In this region, the particles are heated by solid conduction and heat is then transferred by solid convection to the colder flow in the center of the riser. Even at a riser-wall temperature of 1281 K, radiation heat transfer is found to be a minor contribution to the overall heat transfer in the solid phase. This results from a reduced temperature difference between the wall and the adjacent solid phase together with a high extinction coefficient that prevents radiation heat transfer between solid regions farther apart. It is shown that with a moderate riser wall temperature of 581K and a particle diameter of 64 μm, solid conduction accounts for about 97% of the wall-to-suspension heat flux.
    Increasing the wall temperature to 981K together with a particle diameter of 400 μm leads to an increase of the radiation heat-flux contribution up to about 10% of the total wall-to-suspension heat flux.
    A comparison of sequential snapshots of the solid-fraction, solid-temperature, and solid-velocity field show the complex interplay between the wall heat flux and the examined solid and gas-phase property fields. The path of the rising bubbles has thereby an essential influence on the local wall heat flux by inducing a replacement of heated solid regions at the wall with colder ones from the center of the riser.
    
    
23. B. BOISSIERE, « Etude hydrodynamique et thermique d’un nouveau concept de récepteur solaire à suspensions denses gaz-particules » PhD Thesis Toulouse, FRANCE, April 17, 2015 
    Corresponding author : ,
    Abstract
    Among concentrating solar power plants, solar tower technology is one of the more power efficient. Nevertheless, their efficiency and safety can be improved. A new concept of solar receiver is presented which avoids the drawbacks of the molten salts. It uses dense suspensions of gas and fine particles as heat transfer and storage fluid. The construction and the operation of a transparent cold mock-up allowed to demonstrate the hydrodynamic feasibility of this concept. The hydrodynamic characterisation of the flow allowed to define the design rules and the set points of a steady, stable and evenly distributed solid flow. The construction and the operation of a hot mock-up, in which a dense suspension flows upward inside a single tube heated by three ovens, allowed to estimate the heat transfer efficiency of this new kind of exchanger. Thanks to the control and stability of the operating parameters, their effects on the heat transfer between the tube and the dense gas-solid suspension have been accurately determined. The modeling of the suspension upward flow has been performed using 3 approaches. Two 1D approaches have been developed, the first one is based on the 1D bubble-emulsion formalism, the other one is based on the local mass and momentum balance equations. 3D simulation (CFD) has also been performed on a complete mesh of the system, so that the boundary conditions are the same as the operating parameters.
    
    
24. GARCÍA TRIÑANES P., SEVILLE J., BOISSIERE B., ANSART R., LEADBEATER T., PARKER D.J., « Dense Particle Suspensions as a new heat transfer fluid and storage medium: Hydrodynamics, wall region contact time and heat transfer» ECCE10-10th European Congress of Chemical Engineering, Nice, FRANCE, September 26-October 1 (2015)
    Corresponding author : ,
    Abstract
    A recent innovation in concentrating solar systems [1] makes use of a new heat transfer fluid (HTF) that operates in such a way that a dense particle suspension (DSP) acts as a heat transfer fluid with a heat capacity similar to the molten salts traditionally used as a HTF in concentrated solar energy. In this kind of system the wall absorbs solar radiation and transfers the heat to a flowing heat transfer medium. This innovation is currently developed in the frame of a European project FP7 EC project CSP2 (http://www.csp2-project.eu/). To investigate the 3D particle motion in the fluidisation system complementary studies of visualisation using the non-invasive technique PEPT (Positron Emission Particle Tracking) [2] were carried out by using a single positron emitting particle which yielded in situ valuable information such as the spatial movement of the particles inside the fluidised bed and particle-wall collisions [3]. The key aim of this project is to determine the time close to the wall region due to its obvious implications for the heat transfer in the tubes. The approach taken was to determine the distribution of times-at-the-wall (figure1) on the basis of a single location and a series of consecutive locations. A faster circulation should contribute to a higher particle exchange between the wall and the tube and to a higher heat transfer coefficient. A heat transfer probe was designed to measure the experimental heat transfer coefficient of a dense suspension dense suspension of SiC particles or comparison with theoretical models and other experimental results. The probe surface temperature is measured by attaching a Type K thermocouple while the bed temperature is also measured using another thermocouple situated below the heat transfer probe. The calculated heat transfer coefficient (h) of the DSP first increased then decreased (figure 2) with increasing the gas velocity, which means there is a critical gas velocity corresponds to the maximum heat transfer coefficient (hmax). The effect of the fluidization velocity and the air flow rate injected in an additional aeration nozzle are explored to keep a steady flow of solid in the tube and to maximise the efficient heat transfer.
    
    
25. GARCÍA TRIÑANES P., CHAKRAVARTY S., MORGENEYER M., LE BIHAN O., VALDESUEIRO D., VAN OMMEN R., « Attrition and dust formation of pure and core-shell SiC powders», ECCE10-10th European Congress of Chemical Engineering, Nice, FRANCE, September 26-October 1 (2015)
    Corresponding author : ,
    Abstract
    A recent innovation in concentrating A recent innovation in concentrating solar systems [1] makes use of a new heat transfer fluid (HTF) that operates in such a way that a dense suspension of SiC particles acts as a heat transfer fluid with a heat capacity similar to the molten salts traditionally used as a HTF in concentrated solar energy. In this kind of system the wall absorbs solar radiation and transfers the heat to a flowing heat transfer medium. This innovation is currently developed in the frame of a European project FP7 EC project CSP2 (http://www.csp2-project.eu/). During powder handling operations inherent to this process a number of sources of powder attrition are present and this important phenomena is key in industrial environments not only because of its impact on the containing equipment but also because of dust formation; this has serious implications with regards to health and safety so there is a clear need for research on these operations and it appears as one of the premises of HANHAZ2015. Some causes of powder attrition are “Mechanical stress” in screw feeders or rotary valves, "Kinetic stress" caused by the impact of particles due to high velocity jets, bubbling action, collisions in tubes, baffles, cyclones; "Thermal stress" due to the thermal shock produced when cold particles get in touch with hot beds or "Chemical stress" due to evolving gases or water vapour. The sources of equipment erosion are mostly a result of mechanical action of particle impacts. The hydrodynamics of the fluidised particle suspension are a key parameter in the efficiency of the energy harvesting system. We use Positron Emission Particle Tracking (PEPT) to study the circulation and mobility of the particles in the tubes. The tracer particle is prepared by an indirect labelling technique using 18F as radio-isotope. However, SiC particles have shown low affinity and low activation efficiency for this isotope surely due to the inert chemical properties of this material. Consequently, a new tracer has to be found. In this work we aim at modifying the surface of the SiC particles so they can be used as tracers, thus providing more reliable measurements. In this work a thin layer of alumina (approximately 500nm) is deposited on the surface of the SiC using gas-phase deposition technique with a layer-by-layer growth mechanism [2]. The ratio of the film thickness compared to the particle size is very small, therefore the properties such as size, shape and density will not be influenced by the alumina coating. In this study two novel setups for particle aerosol measurement and wear [3,4] are used in order to quantify the aerosol mass concentration and aerosolized wear particles. We wanted to determine whether the alumina shell would produce energy dissipation during the collisions, when compared with the collision of two uncoated SiC particles. The release of coating is found to increase with the wear energy applied on the surface of the particles.
    
    
26. H. BENOIT, PEREZ LOPEZ I., GAUTHIER D., FLAMANT G., « Temperature Influence on Wall-to-Particle Suspension Heat Transfer in a Solar Tubular Receiver » SolarPACES 2015, Cape Town, SOUTH AFRICA, October 13-16 (2015)
    Corresponding author : ,
    Abstract
    Dense Particle Suspension (DPS) can be used as high temperature heat transfer fluid in solar receiver. Tests conducted with a one-tube experimental setup in real conditions of concentrated solar irradiation resulted in determining heat transfer coefficients for the DPS flowing upward in a vertical tube. They have been obtained for solid fluxes in the range 10-45 kg/m2.s and outlet temperatures up to 1020 K. The influence of solid flux, aeration and temperature is outlined in this paper. Heat transfer coefficient variations are correlated as a function of the solid flux and the temperature for given aeration conditions.
    
    
27. PEREZ LOPEZ I., H. BENOIT, GAUTHIER D., SANS J.L., GUILLOT E., CAVAILLE R., MAZZA G., FLAMANT G., « On-sun first operation of a 100 kWth pilot solar receiver using dense particle suspension as heat transfer fluid » SolarPACES 2015, Cape Town, SOUTH AFRICA, October 13-16 (2015)
    Corresponding author : ,
    Abstract
    A 50-150 kWth pilot solar receiver that uses silicon carbide dense particles suspension as the HTF (heat transfer fluid) has been tested at the 1 MW solar furnace at Odeillo Font-Romeu. This paper presents significant experimental results on particle outlet temperature, dynamic response of the system to solid mass flow rate and solar power variations, and receiver thermal efficiency (η). Mean particles’ temperature up to 585°C was reached. The higher the particle mass flow rate, the lower the temperature increase in the tubes, and the higher the receiver thermal yield.
    
    
28. FLAMANT G., DEGREVE J., BAEYENS J., MARTI J., STEINFELD A., GAUTHIER D., ROMERO M., GONZALEZ-AGUILAR J., BENOIT H., ZHANG H.L., « Solar Heat Capture and Storage in Circulating (CFB) or Dense Upflow Fluidized Beds (UBFB) » SolarPACES 2015, Cape Town, SOUTH AFRICA, October 13-16 (2015)
    Corresponding author : ,
    Abstract
    A novel application of powders relies on their use as thermal carrier in heat capture, conveying and storage. A specific application is the use of particulate suspension flow in Solar Power Tower plants, either in an upflow bubbling fluidized bed (UBFB) or in a circulating fluidized bed (CFB) mode. Wall-to-suspension heat transfer coefficients (HTC) were measured for a single tube rig in both UBFB and CFB. In both cases, the HTC is a function of mainly applied air velocity and imposed solid circulation flux, the high temperature of operation adding a radiation component. Maximum HTC values of the UBFB are more than twice the HTC of the CFB mode, and achieved at lower solids flux and superficial air fluidization velocity. Although CFBs could be interesting for large scale operations due to the higher solid flux achieved, UBFB is favored for sizes, complexity and operating costs. The high heat transfer coefficients and the operation at high temperatures (~750°C, against ~565°C for molten salts), favors very high efficiency thermodynamic cycles, leading to a reduction of the receiver and heliostat requirements. The higher temperatures of the powders moreover reduce the storage requirement by about 30 vol%. Parasitic power consumption is significantly reduced due to the reduced pumping energy, and the lack of heat tracing
    
    
29. ZHANG H.L., BAEYENS J., DEGREVE J., RODRIGUEZ J., GAUTHIER D., FLAMANT G., « Tubular Encapsulated Phase Change Materials in Novel Thermal Storage Systems » SolarPACES 2015, Cape Town, SOUTH AFRICA, October 13-16 (2015)
    Corresponding author : ,
    Abstract
    Latent heat thermal energy storage with phase change materials (PCMs) provides high energy density storage due to the phase change by solidification/melting. Relative to sensible heat storage, latent heat storage with PCMs requires a smaller weight and volume of material for a given amount of stored energy at a constant or nearly constant temperature. For high temperature purposes, phase-changing solids (including salts) have been commonly chosen as the cheapest and most efficient PCMs to store heat. Due to their low thermal conductivity, they present however slow charging and discharging rates. In order to increase the thermal conductivity of PCMs, several heat transfer enhancement techniques have been previously studied, such as integrating a porous structure (foam, sponge) or dispersion of high conductivity particles i.e. copper, silver or aluminum particles, within the PCM. Other techniques involve porous materials like graphite; the use of high conductivity, low density materials such as carbon fibers and micro-and nano- encapsulation of PCMs.
    
    
30. H. BENOIT, « Récepteur solaire tubulaire à suspension dense de particules en écoulement ascendant » PhD Thesis Univ. Perpignan, FRANCE, December 16, 2015 
    Corresponding author : ,
    Abstract
    This thesis, financed in the frame of the CSP2 European project, concerns the study of a new kind of thermal concentrating solar receiver using a dense suspension of solid particles circulating upward in vertical tubes. The suspension is obtained by fluidizing Geldart A type particles. The principle consists in creating an upward flow of the suspension in a vertical tube exposed to the concentrated solar radiation that heats the tube wall. The heat is then transmitted to the particles circulating inside that transport it to a conversion cycle for electricity production. Contrarily to usual solar heat transfer fluids, particles can reach high temperatures (> 700 _C) that permit to power high efficiency thermodynamic cycles such as Brayton or combined cycles. Moreover they can be used as a direct heat storage medium for continuous electricity production. During this thesis, a one-tube solar receiver was successfully tested at the PROMES-CNRS solar furnace in Odeillo, with particle outlet temperatures of 750 _C reached. The first values of wall-to-suspension heat transfer coefficient were calculated and a Nusselt correlation was determined. A specific flow pattern with a particle downward flux close to the wall and upward flux in the tube center was underlined. The flow hydrodynamics and the heat transfer mechanisms were studied thanks to 3D numerical simulations. A 16-tube 150 kW_th receiver was finally tested and modeled, proving the process applicability at larger scale.