Scientific productions

  1. Experiments support an improved model for particle transport in fluidized beds (KU LEUVEN, EPPT, CNRS)

Sci Rep 7, 10178 (2017).

The upwards flow of particles in an Upflow Bubbling Fluidized Bed (UBFB) is studied experimentally and modelled from pressure drop considerations and energy loss equations. For Geldart group A powders tested, the upward solid flux, G s , in the tube can be expressed in terms of the applied superficial gas velocity, the free fall (terminal) velocity of the particles during their hindered settling, KU t , the pressure exerted at the base of the conveyor tube, and the tube length. The model expression 𝐺𝑠=𝚫𝑃(𝑈𝑔−𝐾𝑈𝑡)+𝐾2𝑔𝐿(𝑈𝑔−𝐾𝑈𝑡) can be used for design purposes, with K, the correction factor for hindered settling of the particles, approximately equal to 0.1 at high G s -values, but a function of the solids fraction in the upward conveying. The energy efficiency of the system increases with increasing U and Gs. The model equation was tentatively applied to predict the effects of particle size, tube length and operation in Circulating Fluidized Bed mode. It is demonstrated that the UBFB is an efficient and flexible way of transporting particles upwards, with limited particle attrition or tube erosion due to the low gas velocity applied.

Figure: 3D view of the pilot multi-tube solar receiver (Credit: PROMES-CNRS), with solar cavity, refractory lined; and 16 parallel UBFB receiver tubes.

3D view of the pilot multi-tube solar receiver (Credit: PROMES-CNRS), with solar cavity, refractory lined; and 16 parallel UBFB receiver tubes.

2. Bubbling and Slugging of Geldart Group A Powders in Small Diameter Columns (EPPT, CNRS. Not in Open Access)

Ind. Eng. Chem. Res. 2017, 56, 14, 4136–4144.

Bubbling and slugging were investigated in a 0.05 m internal diameter bed of Geldart group A powders. The slugging characteristics were assessed in terms of the operating parameters. Neither bed height nor particle properties affect the excess gas velocities at the onset of slugging, as predicted by the Stewart criterion. Slugging characteristics (length and frequency) and fluidized bed properties were determined. The bottom zone of the bed is freely bubbling, and the bubble sizes can be predicted by dB = 0.047H0.37, where dB is the frontal diameter of the bubble and H is the fluidized bed height. Mostly, wall slugs occur higher in the bed when dB exceeds about 50% of the column diameter. The slug frequency varies with the height in the bed but reaches an equilibrium value of 1 Hz for bed heights exceeding ∼1 m. Three distinct zones appear in a deep bed of group A powders. The design and scale-up should combine the bubbling and slugging modes within their corresponding regions in the bed.

3. High-efficiency solar power towers using particle suspensions as heat carrier in the receiver and in the thermal energy storage (CNRS, EPPT. Not in Open Access)

Renewable Energy 111 (2017) 438-446.

Solar thermal electricity generated by concentrated solar power (CSP) plants is increasingly implemented. CSP plants can supply electricity on a fully matched supply-demand basis if equipped with a thermal energy storage. To increase the efficiency and reduce both capital and operating costs, a next generation of CSP concepts is required. Particle suspensions can be applied to meet these targets and can be used throughout the CSP conversion process, as high temperature heat transfer medium in the receiver, for heat storage, and in the power block of the plant. This work presents the novel concept of using particle suspensions as heat carriers, mostly further to initial testing at the CNRS 1 MW solar furnace of Odeillo Font-Romeu (F). Values of the heat transfer coefficient up to 1100 W/m2K (bare tubes) and 2200 W/m2K (finned tubes) were obtained for operation at low superficial gas velocities of 0.04–0.19 m/s, thus limiting heat losses by the exhaust air. Despite additional costs for particle handling and for an appropriate boiler, the required overall investment and operating costs are expected to be significantly lower than for common equivalent molten salt CSPs, leading to a reduction in Levelized Cost of Electricity (LCOE) from approximately 125 €/MWh to below 100 €/MWh.

4. Energy analysis of a particle suspension solar combined cycle power plant (KU LEUVEN. Not in Open Access)

Energy Conversion and Management 163 (2018) 292-303.

The key to achieve an economically more attractive concentrated solar power plant is to work at higher operating temperatures, allowing both higher power conversion efficiencies resulting in a smaller heliostat field for a given energy output, and higher temperature ranges in the storage tanks, with increased energy storage density and smaller size, hence less expensive. This fostered the development of using particle suspensions as heat transfer media. This paper presents a theoretical framework for the energy analysis of a particle-in-tube solar power plant, hybridized, with topping air-Brayton cycle turbine, and bottoming steam block. From studying the effects of essential design parameters on the energy efficiency, the heat transfer efficiency of the turbine air preheater is of paramount importance to increase the solar contribution within the hybrid concept, while the energy efficiency moreover increases by an optimum air-Brayton cycle turbine operation (mostly through the pressure ratio, less by the operating temperature). The overall efficiency of the concept varies from about 40% when using combined low and high pressure Brayton cycle turbines only, to over 48% in a fully combined air-steam concept. Energy efficiency findings are in agreement with the literature data.

5. Particles in a circulation loop for solar energy capture and storage (KU LEUVEN, CNRS, EPPT. Not in Open Access)     

Particuology 43 (2019) 149-156.

This study defines and assesses the selection criteria for suitable particulate materials to be used in an upflow bubbling fluidized bed (UBFB) or dense up-flow powder circulation system for solar energy capture and storage. The main criteria identified are based on the thermophysical and thermomechanical properties, attrition behavior, and the considerations of health and environmental hazards of the candidate powders. Finally, a cost comparison and tentative ranking of the different candidate powders is presented in addition to a weighted scoring. Significant scoring differences can be observed between the various materials. Olivine possesses the most favorable characteristics and appears to be the particulate material of choice for solid/gas suspension heat transfer fluids.

6. Experiments support simulations by the NEPTUNE_CFD code in an Upflow Bubbling Fluidized Bed reactor (CNRS, INPT. Not in Open Access)

Chemical Engineering Journal (2021) 385, 123568.

Long tubes with small internal diameter find increasing applications in indirect concentrated solar receivers using an Upflow Bubbling Fluidized Bed of Geldart-A powders as heat carrier. Although successfully demonstrated for tubes of 0.5 to 1 m length, longer tubes are required to increase the solar energy capture efficiency and capacity. The fluidization phenomena differ with the tube length, and freely bubbling fluidization will be transformed into slugging, thus hampering the heat transfer. The behavior of Geldart-A powders in tall tubes of small I.D. has not been extensively studied. The research experimentally investigated the different fluidization modes in a 4 m long tube, and demonstrated the occurrence of freely bubbling at the bottom section of the bed, and slugging from a bed depth in excess of about 1.25 m. Slug characteristics (frequency, length, velocity) were measured and correlated. The results were used to validate 3D numerical simulations based on an Euler-Euler approach in the NEPTUNE_CFD code applied to a fine mesh of 15,000,000 cells. A positive match between experimental and simulation results concerning frequency and velocity of large bubble structures was obtained. The effect of mesh refinement on the slugging behavior prediction was discussed.

7. Thermal analysis of fluidized particle flows in a finned tube solar receiver (CNRS)

Solar Energy 191 (2019) 19–33.

This paper addresses experimental results on fluidized particle-in-tube solar receiver using a finned tube in order to increase wall-to-particle heat transfer. On-sun tests of a single finned tube solar receiver were performed at the focus of the 1 MW solar furnace of Odeillo. Several solar flux densities and distributions (mean values 236–485 kW/m2) and particle mass flux densities (G = 20–110 kg/m2·s) were tested. A detailed analysis of tube wall and particle temperature distributions and temperature measurement accuracy is proposed. The power extracted by the particle suspension ranges between 17.8 kW and 32 kW and the typical thermal efficiency of this lab-scale solar receiver is about 75%. The mean global wall-to-fluidized particle heat transfer coefficient is calculated as 1200 ± 400 W/m2·K for G in the range 30–110 kg/m2·s. The main uncertainty on the heat transfer coefficient is due to uncertainty on wall temperature measurement during on-sun experiments. The range of this uncertainty is estimated by comparing infra-red camera measurements and wall-welded thermocouple data.

8. Annual performance of subcritical Rankine cycle coupled to an innovative particle receiver solar power plant (IMDEA)

Renewable Energy 130 (2019) 786-795.

Concentrated solar power plants using molten salts as heat transfer and storage fluid have emerged as the preferred commercial solution for solar thermal electricity in central receiver technology. Despite their ability to store large amounts of thermal energy and efficient receiver designs, further efficiency improvements are constrained by tight temperature restrictions when using molten salts (290 °C–565 °C). In this work, a novel heat transfer fluid based on a dense particle suspension (DPS) is used due to its excellent thermophysical properties that extend the operating temperature of solar receiver and allow its coupling with higher-efficiency power cycles. In this paper, the design of a DPS solar receiver working at 650 °C has been optimized for two commercial sizes (50 MWth and 290 MWth) coupled to an optimized subcritical Rankine cycle. The results showed that a five-extraction reheated Rankine cycle operating at 610 °C and 180 bar maximizes power plant efficiency when coupled with a DPS central receiver, giving 41% power block efficiency and 23% sun-to-electricity efficiency. For optimization purposes at design point conditions, in-house code programmed into MATLAB platform was used while TRNSYS software was employed for annual plant performance analysis.

9. Flexible electricity dispatch for CSP plant using un-fired closed air Brayton cycle with particles based thermal energy storage system (IMDEA)

Energy 173 (2019) 971-984.

This paper presents a novel power block concept forflexible electricity dispatch in a Concentrating SolarPower (CSP) plant. The power block is based on intercooledeunfired regenerative closed air Braytoncycle that is connected to a pressurized solar air receiver. The Closed Brayton cycle uses a massflowregulation system centered on the pressure regulation (auxiliary compressor and bleed valve) in order tocontrol the Turbine Inlet Temperature (TIT). Doing so, the system is able to modulate turbine electricityproduction according to variations in the solar resource and changes in power electric demand. It hasbeen found that the proposed power block is able to fully cover the electricity demand curve for thosedays with high solar resource. In case of integrating particles-based high temperature Thermal EnergyStorage (TES) system, the power block can extend its production till the next day following the electricitycurve demand during summer period. During winter period, the power plant can extend its productionfor a few hours due to the lower solar resource and the higher electric curve demand load.

10. Solids Flow in a “Particle-in-Tube” Concentrated Solar Heat Absorber (EPPT, CNRS. Not in Open Access)

Ind. Eng. Chem. Res. 2019, 58, 11, 4598–4608.

Fluidized particle-in-tube solar absorbers are increasingly investigated both as receivers in a solar power concept or as a receiver/reactor for the thermal decomposition of minerals or for gas–solid reactions such as CO2 looping concepts and thermo-chemical energy storage. Such applications require a high heat transfer rate from the tube wall to the upflowing suspension of particles and a strict control of the particle residence time. Conversion and heat transfer depend upon the particle mixing, its residence time (RT), and residence time distribution (RTD). Both parameters are hence important in fluidized bed applications. The present research experimentally investigated the use of an Upflow Bubbling Fluidized Bed (UBFB) as a particle-in-tube concentrated solar receiver and/or reactor. The RTD was determined by tracer response and compared with predictions from different models. Both a cascade of stirred tank reactors and a plug flow with dispersion model provide a good fitting of the experimental results. The former is however preferred, with a number of cascade mixing cells between 3 and 4, slightly dependent on both the operating air velocity and the particle circulation rate. The commonly applied ratio of the superficial fluidization velocity and the minimum fluidization velocity of the particles is between 3 and 20, whereas solid circulation fluxes up to 100 kg/m2 s are used. Design equations are derived. The results are moreover used in test cases of a concentrated solar power absorber and in the use of a UBFB as solar limestone calcination furnace. The approach can also be applied to processes of, for example, heat absorption or drying provided kinetic data are known.

11. Computation of canting errors in heliostats by flux map fitting: experimental assessment (CNRS)

Optics Express Vol. 28, Issue 26, pp. 39868- 39889 (2020).

In solar tower plants, thousands of heliostats reflect sunlight into a central receiver. Heliostats consist of a subset of mirrors called facets that must be perfectly oriented (i.e., canted) to concentrate as much solar radiation as possible. This study presents and validates the so-called flux map fitting technique to detect and correct canting errors. The computed distributions were matched to a series of images through an optimization algorithm. According to the sensitivity analysis, three images spread along a single day provide sufficient information for the algorithm to succeed. Using this methodology, four heliostats at the THEMIS research facility were recanted, thereby substantially increasing the optical quality in three of them. The procedure to infer the heliostat aimpoint was assessed.

12.  Shaping High Efficiency, High Temperature Cavity Tubular Solar Central Receivers (CNRS)

Energies 2020, 13(18), 4803.

High temperature solar receivers are developed in the context of the Gen3 solar thermal power plants, in order to power high efficiency heat-to-electricity cycles. Since particle technology collects and stores high temperature solar heat, CNRS (French National Center for Scientific Research) develops an original technology using fluidized particles as HTF (heat transfer fluid). The targeted particle temperature is around 750 °C, and the walls of the receiver tubes, reach high working temperatures, which impose the design of a cavity receiver to limit the radiative losses. Therefore, the objective of this work is to explore the cavity shape effect on the absorber performances. Geometrical parameters are defined to parametrize the design. The size and shape of the cavity, the aperture-to-absorber distance and its tilt angle. A thermal model of a 50 MW hemi-cylindrical tubular receiver, closed by refractory panels, is developed, which accounts for radiation and convection losses. Parameter ranges that reach a thermal efficiency of at least 85% are explored. This sensitivity analysis allows the definition of cavity shape and dimensions to reach the targeted efficiency. For an aperture-to-absorber distance of 9 m, the 85% efficiency is obtained for aperture areas equal or less than 20 m2 and 25 m2 for high, and low convection losses, respectively.

13. Design and performance of a modular combined cycle solar power plant using the fluidized particle solar receiver technology (CNRS. Not in Open Access)

Energy Conversion and Management 220 (2020) 113108.

The design and the performance of a medium-scale modular solar power plant (~20 MW) integrating a gas turbine combined cycle with a fluidized particle-in-tube receiver and direct thermal storage are investigated in this paper. A practical technique is used to design each part of the solar power plant. The complete design starts with the solar gas turbine (SGT) since it defines the necessary power to run it; then, the other parts are designed upstream. Three different cases are investigated under different operation strategies corresponding to two particle temperatures 750 °C and 880 °C, and hybrid and solar-only operation modes respectively. The results show that the nominal efficiency of the components including the heliostat field, the solar receiver, the gas turbine, and the steam turbine can reach 67%, 80%, 32%, and 34.5% respectively. As a result, the nominal thermal efficiency and the annual capacity factor of the complete solar power plant achieve 46% and 33.47% respectively. The overall nominal efficiency (solar-to-electric efficiency) of the plant for hybrid operation mode is 25.80%. It varies from 21.16% to 24.7% for the solar-only operation mode. Special interest is shown to the part-load operations.

14. Aiming Strategy on a Prototype-Scale Solar Receiver: Coupling of Tabu Search, Ray-Tracing and Thermal Models (CNRS)

Sustainability 2021, 13(7), 3920.

An aiming point strategy applied to a prototype-scale power tower is analyzed in this paper to define the operation conditions and to preserve the lifetime of the solar receiver developed in the framework of the Next-commercial solar power (CSP) H2020 project. This innovative solar receiver involves the fluidized particle-in-tube concept. The aiming solution is compared to the case without the aiming strategy. Due to the complex tubular geometry of the receiver, results of the Tabu search for the aiming point strategy are combined with a ray-tracing software, and these results are then coupled with a simplified thermal model of the receiver to evaluate its performance. Daily and hourly aiming strategies are compared, and different objective normalized flux distributions are applied to quantify their influence on the receiver wall temperature distribution, thermal efficiency and particle outlet temperature. A gradual increase in the solar incident power on the receiver is analyzed in order to keep a uniform outlet particle temperature during the start-up. Results show that a tradeoff must be respected between wall temperature and particle outlet temperature.

15. Design and off-design performance comparison of supercritical carbon dioxide Brayton cycles for particle-based high temperature concentrating solar power plants (IMDEA)

Energy Conversion and Management 232 (2021) 113870.

Concentrated solar power (CSP) plants using dense particle suspension as heat transfer fluid and particles as the storage medium are considered as a promising solution to provide the high temperature required for the supercritical carbon dioxide (S-CO2) Brayton cycle. During plant operation, variations in the heat transfer fluid temperature and ambient temperature would significantly affect system performance. Determining the suitable S-CO2 Brayton cycle configuration for this particle-based CSP plant requires accurate prediction and comprehensive comparison on the system performance both at design and off-design conditions. This study presents a common methodology to homogeneously assess the plant performance for six 10 MW S-CO2 Brayton cycles (i.e. simple regeneration, recompression, precompression, intercooling, partial cooling and split expansion) integrated with a hot particles thermal energy storage and a dry cooling system. This methodology includes both design and off-design detailed models based on the characteristic curves of all components. The optimal design for each thermodynamic cycle has been determined under the same boundary design constrains by a genetic algorithm. Then, their off-design performances have been quantitatively compared under varying particle inlet temperature and ambient temperature, in terms of cycle efficiency, net power output and specific work. Results show that the variation in ambient temperature contributes to a greater influence on the cycle off-design performance than typical variations of the heat transfer fluid temperature. Cycles with higher complexity have larger performance deterioration when the ambient temperature increases, though they could present higher peak efficiency and specific work at design-point. In particular, the cycle with maximum efficiency or specific work presents significant changes in different ranges of ambient temperature. This means that for the selection of the best configuration, the typical off-design operation conditions should be considered as well. For integrating with high-temperature CSP plants and dry cooling systems, the simple regeneration and the recompression cycles are the most suitable S-CO2 Brayton cycle configurations due to their fewer performance degradations at ambient temperatures above 30 °C, which is a frequent environmental condition in sunny areas of the world.

16. Dense Upflow Fluidized Bed (DUFB) Solar Receivers of High Aspect Ratio: different fluidization modes through inserting Bubble Rupture Promoters (CNRS, EPPT, INPT. Not in Open Access)

Chemical Engineering Journal (2021) 418, 129376.

A fluidized bed of Geldart-A particles is promoted as heat transfer fluid in the tubular solar receivers of solar towers. A pressure-driven upward particle flow affects the hydrodynamic flow structure and properties of the fluidized bed. Experiments involved a tube of 0.05 m internal diameter but of very high height/diameter ratio (>120), representative of the future solar receiver and of numerous chemical reactors. Solid circulation fluxes and aeration velocities were varied. Configurations of a bare tube and a tube with bubble rupture promoters were compared. In the bare tube, freely bubbling is transformed into axi-symmetric slugging at a bed level of ~1 m. With bubble rupture promoters, freely bubbling prevails to about a bed level of 3 m, and a turbulent fluidization mode develops higher up the tube (a more chaotic two-phase system with elongated and unstable “gas voids” and “dense solid clusters”), without axi-symmetric slugging detected. Experimental results for both tube configurations were assessed and compared with CFD predictions by the Euler n-fluid code, NEPTUNE_CFD. A good agreement of bed properties was obtained for slug/void frequencies and solids volume fraction in both tube configurations. BRPs moreover enhance the bubble through flow of the fluidizing gas, thus limiting the visible bubble flow rate and bubble sizes while increasing the gas/particle contact, and hence important in designing multi-tube chemical reactors. Whereas slugging limits the heat transfer from the tube wall to the suspension at ~200 W/m2K, the presence of BRPs maintains a heat transfer coefficient in excess of 600 W/m2K.


 10th International Conference on Environment Science and Engineering

SolarPACES 2020 – Solar Power & Chemical Energy Systems

  • The Conference Proceedings for the 2020 edition of SolarPACE have not been published yet. However, Next-CSP presented 5 papers as well as a poster. More information here.

SolarPACES 2019 – Solar Power & Chemical Energy Systems

SolarPACES 2018

 SolarPACES 2017