Concentrated Solar Power

Concentrated solar power (CSP) technologies can be used to generate electricity by converting energy from sunlight to power a turbine, but the same basic technologies can also be used to deliver heat to a variety of industrial applications, like water desalination, enhanced oil recovery, food processing, chemical production, and mineral processing.

CSP technologies use mirrors to reflect and concentrate sunlight onto a receiver. The energy from the concentrated sunlight heats a high temperature fluid in the receiver. Conventional CSP power towers use molten salt as heat transfer fluid that flows through the system and absorbing thermal energy. In a falling-particle receiver, sand like particles are heated directly by a beam of concentrated sunlight as they fall through open air. The heated particles are then stored in an insulated bin before passing through a particle-to-working-fluid heat exchanger. The heat exchanger's working fluid will simulate a high-efficiency Brayton cycle using supercritical carbon dioxide (sCO₂) with an exit temperature of 720°C. Then the cooled particles are collected and moved back to the top of the receiver.

Research Approach

Characterization of radiative properties of solid particles

Radiation heat transfer plays dominant role in high-temperature particle flows. Knowledge of the scattering and absorption properties of individual particles is crucial for modeling the radiative heat transfer of the particle bed. In this work, a laser scatterometer is used to measure the single-particle scattering properties at a wavelength of 635 nm by using two configurations: (1) a falling particle curtain and (2) a taped particle layer. Bauxite-based ceramic particles that are strongly absorbing in the solar spectrum and silica particles that are nonabsorbing in the visible and near-infrared are investigated. The directional-hemispherical reflectance and transmittance of the taped particles are also measured to deduce the forward and backward scattering efficiency factors and the absorption efficiency factors. The scattering phase functions of all bauxite-based particles with varying sizes and compositions are very similar and can be fitted to a Henyey−Greenstein phase function with an asymmetry factor g. For the silica particles, forward scattering dominates. A Monte Carlo method is developed to model the particle scattering characteristics, and reasonable agreements between the modeling and experimental results are observed.

Characterization of temperature dependent mechanical properties of solid particles

Understanding granular flow behavior at elevated temperature is important in determining mass transfer in heat transfer modeling. Granular flow can be simulated by LIGGGHTS software using discrete element method (DEM). The particle shape and size, elastic properties, coefficient of restitution, and friction coefficients are the primary modeling inputs for DEM to capture particle-particle and particle-surface interactions. Measurement techniques for these properties at elevated temperature were developed.

High-temperature granular flow experiment

Experiment to observe granular flow behavior at elevated temperatures within parallel plate channel configuration were conducted to understand temperature dependent granular flow behaviors and compare with heat transfer models. The parallel plate channel configuration can be found in heat exchanger for delivering thermal energy from particles to working fluids. This experiment is important in designing efficient particle based heat exchangers for the next generation CSP systems. Particles were heated up to 800 °C using tube furnace Particle mass flow rates, particle velocity profiles at the outlet, particle temperature, and wall temperatures were measured. We have observed the granular flow behavior was dependent on the temperature due to temperature dependent friction of particle-particle and particle-wall interactions.

Granular flow experiment under highly concentrated solar radiation

Granular flows of sintered bauxite particles were examined along an incline, which were directly exposed to concentrated irradiation from a high-flux solar simulator (HFSS). The goal of this work was to study the effects of large temperatures gradients in the flows and further inform the design of solar particle heating receivers.

Granular flow simulation

Granular flow models were constructed using LIGGGHTS DEM software. Particle movements were tracked by calculating forces interacting between particles and particle to wall using Hertz contact model and constant direction torque model. The measured temperature dependent mechanical properties were integrated into the model to simulate the granular flows. Particle velocity and volume fractions inside the channel were obtained from the simulations. The mass flow rate and particle velocity at the outlet was compared with experimental results and showed good agreement. The simulations were conducted at different temperatures up to 800 °C. The simulated results will be integrated into the heat transfer model.

Transient heat transfer modeling

Heat transfer modeling for high-temperature granular flows in the parallel plate channel was conducted using MATLAB. The governing energy equations were converted to system of ODEs by method of line and integrated over time in MATLAB. The results will be compared with experimental results and near-wall convective coefficients will be determined.