Session: 06-04: CSP Receivers and Reactors III

Paper Number: 156921

156921 - On the Modeling of Carbon Particle Oxidation and Its Effects on Temperature Distribution in a Small Particle Heat Exchange Receiver 

Abstract: 

This research is an advancement in the understanding of the Small Particle Heat Exchange Receiver (SPHER) in which sunlight is concentrated into a chamber into which a gas seeded with submicron-sized particles flows. The micro-particles absorb sunlight which, in turn, raises the temperature of the gas flow carrying them. The high temperature gas can then be used as a source of energy for many processes to make useful fuels and chemicals, or to produce electricity.  In this research it is assumed that the SPHER will use an air/carbon particle mixture suitable for driving a combined Brayton/Rankine thermodynamic cycle.  It is important to note that combined cycle plants are the state-of-the-art for fossil fuel generation due to the high efficiency they achieve by using two thermodynamic cycles in series.  However, although stand-alone Rankine steam cycle plants have been built to use solar power, to date no solar-driven combined cycle plant has been constructed.  The primary reason for this is that no solar receiver has yet been developed that can achieve the high temperatures needed for the gas turbine inlet.  The SPHER is one of the only receiver concepts that may attain this high-temperature goal.

Carbon particle oxidation within a small particle heat exchanger receiver is modeled and added to the already existing coupled FLUENT-Monte Carlo Ray Trace model developed by previous researchers associated with SDSU’s Combustion & Solar Energy Laboratory. Built in FLUENT particle tracking algorithms are used alongside custom user-defined functions to find the spatial and size distribution of particles within the receiver. Fortran functions are then used to calculate the absorption, scattering, and scattering phase function of the carbon particle laden flow. These variables determine the radiative function of the SPHER. Energy sources from a Fortran Monte Carlo ray tracing program are returned to FLUENT, thus completing the computational loop. 

The effect of carbon particle oxidation is the primary focus of this work. Previous iterations of SPHER modeling assumed constant mass loading, only affected by density changes of the surrounding carrier fluid (air). The inlet to the SPHER is an annular slit through which carbon laden air enters the domain with a swirled flow. A central domed window allows focused sunlight form a heliostat field to enter the SPHER and oxidize the carbon particles due to the particles strong interaction with solar wavelengths. Particle trajectories, gas temperatures, and receiver efficiency is determined for various inlet flow rates. At the design point the receiver  is upwards of 93% efficient in converting the input solar energy to fluid enthalpy rise while keeping the walls of the domain sufficiently cool to allow for the use of common engineering materials. Such a feat is realized with 300 nm particles and modest flow rates of carbon of 4-5 g/s carried in air flows of 4 – 5 kg/s, respectively for a total receiver input power of a 3.67 MW. 

Presenting Author: Fletcher Miller San Diego State University

Presenting Author Biography: Fletcher Miller is a Professor of Mechanical Engineering at San Diego State University. He has a background in high-temperature solar particle receivers and high temperature thermal storage systems.