Particle Outlet Temperature Research Articles

Discrete Element Method Modelling of the Particle Flow in Centrifugal Solar Particle Receiver

The centrifugal solar particle receiver (CentRec) is a promising design compared to other particle receiver concepts because it allows for an active adjustment of particle residence time and particle outlet temperature by adjusting the rotational speed of the drum and particle mass flow rate. A Discrete Element Method (DEM) tool is utilized to model the particle flow in CentRec. However, the numerical modeling of the particle flow in large scale receiver is computationally infeasible because of excessive number of particles in the simulation. Thus, in this study, a scale down approach is developed and validated to be used to estimate the particle film characteristics in large scale receivers. The particle velocity profile and film thickness distribution are employed to compare different receiver sizes.

Heat transfer in a fluidized bed tubular solar receiver. On-sun experimental investigation

Using particles as heat transfer fluid in a solar receiver is an attractive way to increase the global efficiency of solar power plants. In the case of the particle-in-tube solar receiver concept, the fluidized particles circulate vertically in tubes thanks to a controlled pressure gradient and an additional air injection. Experiments are conducted with olivine particles in a one-tube mock-up at the 1 MW CNRS solar furnace (France). The tube of high aspect ratio (more than 3 m height over 48 mm internal diameter) is irradiated along a 1-m height with several solar flux configurations that correspond to realistic operation conditions of a solar thermal power plant. The novelties of this paper lie in extending the operating parameters compared to earlier studies and in linking the fluidization regimes to wall heat transfer. Slugging, turbulent fluidization and fast fluidization regimes are detected within the receiver tube. It is shown that the system can tolerate high solar flux densities, up to 800 kW/m2. The system proves to be highly controllable and flexible with respect to particle mass flux variation and transient operations. Particle temperature increase ranges from 100 to 650 °C depending on the particle mass flux. Maximum thermal efficiency and particle outlet temperature of respectively ∼ 60 % and 680 °C are obtained, proving that this technology can be combined with highly efficient thermodynamic cycles. Global wall-to-particle heat transfer coefficient varies strongly with particle mass flux. It reaches a quasi-plateau above 15 kg/(m2s) with a mean value of 1000 ± 200 W/(m2K) and peaks data up to 1500 W/(m2K). A dimensionless heat transfer index is derived to account for the effects of the particle mass flux on the experimental results. It highlights that both increasing the temperature and working in the turbulent fluidization regime result in the increase of the heat transfer between the tube walls and the particles.

Techno-economic analysis of multi-tower solar particle power plants

Solar tower systems using solid particles as heat transfer and storage medium promise to achieve, in combination with advanced power cycles, lower levelized cost of electricity (LCOE) than state-of-the-art concepts. In this study the cost potential of a multi-tower solar power plant with centrifugal particle receiver is evaluated. Most of the underlying assumptions were chosen in accordance with those specified within the US-DoE Gen3 project “G3P3”. For the power block a high-efficiency supercritical CO2 system with 100MWe was assumed. The solar subsystem is built from 12 identical solar tower modules, each with heliostat field, tower, receiver and storage. The nominal thermal power of each module is 41MWth.Since the cost assumptions for the tower and the heat exchanger (HX) are still relatively uncertain, different correlations were used representing optimistic and conservative cases. For the particle receiver two outlet temperature levels were investigated, namely 800 °C and 1000 °C.The results of the techno-economic analysis indicate a strong impact of the cost assumptions for the primary heat exchanger and the tower. With 800 °C as particle outlet temperature, the estimated LCOE range from $56.18/MWh (optimistic values for tower and HX cost) to $67.30/MWh (conservative values for tower and HX cost).Increasing the particle outlet temperature to 1000 °C results in a significant reduction of LCOE (about 12% for the optimistic cases, down to $50.46/MWh). The reason for this is the significant reduction of heat exchanger area, particle inventory and particle mass flow, resulting from the larger temperature difference in the particle system. However, operating the system at such elevated particle temperatures will require some modifications to the components that are not fully reflected in the used cost correlations yet. Nevertheless, the results propose further investigation of this option. All in all the results indicate a clear potential to achieve the SunShot goal of less than $60/MWh.

Dynamic simulation and parametric influence analysis on thermal performance of a quartz tube solid particle receiver in solar tower power plants

Due to a higher operating temperature (≥800 °C), Solar Particle Receiver (SPR) which uses particles as the working medium is considered as one of best candidates to improve the thermoelectric conversion efficiency of concentrating solar power plants. In this paper, a quartz tube solid particle receiver model is fully developed by using the discretized lumped parameter method, in which the calculation process of particle temperature and thermal loss is clearly given. In order to improve the manipulation level of particle receiver during the operation, the dynamic characteristics of the quartz tube particle receiver are comprehensively studied by the disturbance test of selected input parameters. Besides, in order to grasp the influence rule of key parameters on the thermal performance of particle receiver, the key parameters’ sensitivity analysis is also deeply studied. The results show that the particle outlet temperature can reach as high as 810 °C under a relatively small value of solar flux 600 kW/m2, but the receiver efficiency is only about 75%; Besides, the receiver efficiency shows a variation tendency that it rises first falls afterwards with the increase of incident solar flux. The validity of proposed model is verified by a heating experimental system with a single quartz tube, and the relative error is not more than 7.9%. The research results are beneficial for understanding the dynamic characteristics and designing the particle receiver.

Design Evaluation of a Next-Generation High-Temperature Particle Receiver for Concentrating Solar Thermal Applications

High-temperature particle receivers are being developed to achieve temperatures in excess of 700 °C for advanced power cycles and solar thermochemical processes. This paper describes designs and features of a falling particle receiver system that has been evaluated and tested at the National Solar Thermal Test Facility at Sandia National Laboratories. These advanced designs are intended to reduce heat losses and increase the thermal efficiency. Novel features include aperture covers, active air flow, particle flow obstructions, and optimized receiver shapes that minimize advective heat losses, increase particle curtain opacity and uniformity, and reduce cavity wall temperatures. Control systems are implemented in recent on-sun tests to maintain a desired particle outlet temperature using an automated closed-loop proportional–integral–derivative controller. These tests demonstrate the ability to achieve and maintain particle outlet temperatures approaching 800 °C with efficiencies between 60 and 90%, depending on incident power, mass flow, and environmental conditions. Lessons learned regarding the testing of design features and overall receiver operation are also presented.

Numerical investigation of the air-particles heat transfer characteristics of moving bed - effect of particle size distribution

Particle vertical moving beds (PVMB) are widely used in industry, usually accompanied by gas-solid heat exchange or chemical reaction. It is worth noting that the actual industrial particles usually have a wide particle size distribution (PSD), which is an important property that can influence the hydrodynamics and heat conversion in moving bed system. Therefore, understanding the multiphase flow and thermal behavior of PVMB with multi-size particles is essential for process designing and optimizing. Based on the CFD-DEM coupling method, a mathematical model of gas-solid heat transfer for PVMB with different PSDs is established- all with the same mean particle diameter. Furthermore, the effects of 3 influential factors on the exergy transfer in a small PVMB were analyzed. Results show that an increased PSD range leads to an increased specific surface area, but this does not imply a favorable heat transfer between air and particles in PVMB, which is contrary to the findings related to the fixed bed. Increasing particle mass flow rate or particle inlet temperature will cause the cooling air and particle outlet temperature to increase approximately linearly. More interestingly, as inlet air mass flux increases, exergy recovery efficiency decreases, however with the further increase of inlet air mass flux, exergy recovery efficiency reaches a minimum and then gradually increases. Exergy recovery efficiency decreases and tends to be flat with increasing particle mass flow rate. This method is an essential procedure for modeling the heat transfer of discrete particles in industrial-scale equipment.

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

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.

Dynamic modeling of a particle/supercritical CO2 heat exchanger for transient analysis and control

A dynamic model of a moving packed-bed particle-to-sCO2 heat exchanger and control system for concentrating solar power (CSP) applications is presented. The shell-and-plate heat-exchanger model allows for numerically investigating the transient operation and control of the heat addition to the power cycle in a particle-based CSP plant. The aim of the particle-to-sCO2 heat exchanger is to raise the sCO2 temperature to 700 °C at a pressure of 20 MPa. The control system adjusts both the particle and sCO2 mass flow rates as well as an sCO2 bypass to obtain the desired sCO2 turbine inlet and particle outlet temperatures for a prescribed thermal duty. The control system is demonstrated for disturbances in particle and sCO2 inlet temperatures as well as changes in thermal duty for part-load operation. A feed-forward control strategy that adjusts the sCO2 and particle mass-flow rates as functions of measured inlet temperatures and a steady-state model solution was able to return the heat exchanger to the desired operating condition, but not without experiencing significant deviations in the sCO2 turbine inlet and particle outlet temperature (>40 °C) during the transient. To reduce both sCO2 and particle temperature deviations, a feedback control strategy was investigated, where sCO2 and particle mass-flow rates based on the steady-state model solution were corrected based on measured outlet temperature deviations. The feedback control strategy maintains sCO2 turbine inlet and particle outlet temperature to within 16 °C of the set points with a three-minute settling time for step changes in inlet conditions and thermal duty. This finding demonstrates the possibility of dynamically dispatching next-generation particle-based CSP plants driving sCO2 power cycles.

Solar electricity via an Air Brayton cycle with an integrated two-step thermochemical cycle for heat storage based on Co3O4/CoO redox reactions III: Solar thermochemical reactor design and modeling

A two-step solar thermochemical cycle based on Co3O4/CoO redox reactions integrated into an Air Brayton cycle is considered for thermochemical heat storage. The two-step cycle encompasses (1) the thermal reduction of Co3O4 to CoO and O2 driven by concentrated solar irradiation and (2) the re-oxidation of CoO with O2 to Co3O4, releasing heat and completing the cycle. An evacuated horizontal solar thermochemical reactor is proposed with an inclined slope and quartz window for promoting direct irradiation of dense, granular Co3O4/CoO flows. Mechanical analysis of flat and spherical quartz window designs for a 5kWth scale prototype was performed to ensure window stability. Detailed mass and heat transfer analysis for a 5kWth scale prototype was performed coupling Monte Carlo ray tracing for radiative heat exchange to the energy balances for the bed and the reactor. A parametric study of the reactor design was performed with varying cavity depth, particle inlet temperature, and solar concentration ratio. The optimal solar reactor design maximized conversion of Co3O4 to CoO and particle outlet temperature while preventing particle overheating and achieved a Co3O4 to CoO conversion of 0.91, particle outlet temperature of 1385K, maximum flow temperature of 1572K, and absorption efficiency of 0.76.

Modeling of a concentrated-solar, falling-particle receiver for ceria reduction

This paper presents a steady-state model coupling three-dimensional, spectral radiation heat-transfer to a quasi-one-dimensional particle flow model for assessing performance of a directly-irradiated, falling-particle solar receiver with a closed cavity for thermochemical reduction of ceria. A semi-empirical gas-phase flow entrainment model captures the heat and mass exchange between the surrounding gas and the reducing ceria particles. With bulk and surface thermochemistry for the oxide-ion transport and surface reduction kinetics, model results indicate that for particle diameters under 500μm, surface chemistry controls the rate of ceria reduction in the receiver-reactor. For the range of particle inlet temperatures, flow rates, and diameters studied, the degree of ceria reduction correlates well with particle outlet temperature and increases to as high as 6% at maximum outlet temperatures of 1900K. However, with the relatively low absorptivity of ceria, higher outlet temperatures lower the fraction of solar energy absorbed by the particles from approximately 30% at outlet temperatures of 1500K to just above 10% at outlets of 1900K. Most of the heat recovered in the ceria is due to sensible heating, and re-radiation losses can account for as much as 60–75% of the solar energy input due to the required high reduction temperatures and radiation properties of the ceria particles. The predicted low efficiencies show that ceria particle reduction must utilize significant heat recovery and alternative receiver optics other than simple direct radiation to improve the feasibility of solar-driven ceria redox cycles based on falling particle receivers.

Numerical Simulation of a Centrifugal Particle Receiver for High-Temperature Concentrating Solar Applications

A three-dimensional, steady-state finite-element model (FEM) model of a centrifugal particle receiver for high-temperature concentrating solar applications has been developed. The model is built according to an experimentally examined 15 kW prototype in order to investigate the thermal receiver performance and the governing heat transfer mechanisms of the proposed receiver concept. Comparison to the experimental data reveals good agreement in terms of particle outlet temperature and absorbed heat. For a target particle outlet temperature of 900°C and a design input flux of 1 MW/m², a receiver efficiency of >85% is predicted, indicating promising perspectives for the proposed receiver concept.

Proof of Concept Test of a Centrifugal Particle Receiver

As a proof of concept a Centrifugal Particle Receiver (CentRec) prototype in laboratory scale (∼10 kWth) was designed, built and tested in the High-Flux Solar Simulator in Cologne, Germany. Since they can be heated up to 1000°C without sintering in the storage, small bauxite particles produced for the oil and gas industry are used as the absorber, the heat transfer and storage media at the same time. A defined rotation is used to form a thin direct absorbing particle film on the inner surface of the cylindrical receiver. Depending on the incoming heat flux the particle residence time can be adjusted by controlling rotation speed and mass flow. A constant particle outlet temperature for all load conditions can be therefore ensured. First experiments were conducted with various power levels and mass flows at two different receiver inclinations. Particle outlet temperatures of up to 900°C were demonstrated and the expected simple control capability of the receiver concept was verified.

Computational Fluid Dynamics Modeling of Gas-Particle Flow Within a Solid-Particle Solar Receiver

A detailed three-dimensional computational fluid dynamics (CFD) analysis on gas-particle flow and heat transfer inside a solid-particle solar receiver, which utilizes free-falling particles for direct absorption of concentrated solar radiation, is presented. The two-way coupled Euler-Lagrange method is implemented and includes the exchange of heat and momentum between the gas phase and solid particles. A two-band discrete ordinate method is included to investigate radiation heat transfer within the particle cloud and between the cloud and the internal surfaces of the receiver. The direct illumination energy source that results from incident solar radiation was predicted by a solar load model using a solar ray-tracing algorithm. Two kinds of solid-particle receivers, each having a different exit condition for the solid particles, are modeled to evaluate the thermal performance of the receiver. Parametric studies, where the particle size and mass flow rate are varied, are made to determine the optimal operating conditions. The results also include detailed information for the gas velocity, temperature, particle solid volume fraction, particle outlet temperature, and cavity efficiency.

Development of gas–solid direct contact heat exchanger by use of axial flow cyclone

A heat exchanger between particulate or granular materials and gas is developed. It makes use of a swirling gas flow similar to the usual cyclone separators but the difference from them is that the swirl making gas is issued into the cyclone chamber with downward axial velocity component. After it turns the flow direction near the bottom of the chamber, the low temperature gas receives heat from high temperature particles supplied from above at the chamber's center. Through this configuration, a direct contact and quasi counter-flow heat exchange pattern is realized so that the effective recovery of heat carried by particles is achieved. A model heat exchanger was manufactured via several numerical experiments and its performances of heat exchange as well as particle recovery were examined. Attaching a small particle diffuser below the particle-feeding nozzle brought about a drastic improvement of the heat exchange performance without deteriorating the particle recovery efficiency. The outlet gas temperature much higher than the particle outlet temperature was finally obtained, which is never realized in the parallel flow heat exchanger.