Heat transfer in a directly irradiated solar receiver/reactor for solid–gas reactions

Particle laden solar receivers can be used at high temperatures for efficient heat transfer and fuel generation via chemical reactions. A theoretical analysis of a directly irradiated, particle laden, solar receiver is presented here and compared with experiments. The radiation characteristics of the particles are approximated using a method, which adapts Mie theory to certain cases where a solar receiver is used with seeded particles of variable sizes and shapes. Based on this model carbon black particles whose effective radius, rp, is less than 100 nm are inefficient in absorbing solar energy and the most suitable particle sizes is in the same range as the wavelengths of the radiation (100 nm < rp < 1000 nm). The heat transfer coefficient between the particles and the gas was calculated using a refined limiting sphere model developed for the transition regime between molecular and continuum transfer. Previous models assume that there are no gas molecule collisions in the energy transfer layer and the mean free path of the gas molecules is equal to the thickness of this layer. The present model accounts for molecule collisions in the energy transfer layer and therefore enables the thickness of this layer to be larger than one mean free path length. The model was extended to estimate the Nusselt number for gases with several atoms as well as for monatomic gas. A code to simulate the flow and heat transfer in the receiver was developed, utilizing the models for heat transfer from sunlight to the particles and from the particles to the gas. The receiver simulations show good agreement with the wall temperature distribution measured in experiments, but the gas exit temperature in the model was significantly lower than the measured value. This discrepancy could be due to limitations of the simulation code and the particle heat transfer models. The simulation suggests that changing the Nusselt number and particle radius have a small influence on the receiver wall and gas temperatures. Increasing the particle cloud concentration improves the receiver heat transfer up to a threshold value; further increase of the particles concentration has only a marginal influence on the receiver’s heat transfer. This result from the receiver modeling was in a good agreement with solar experiments. 2007 Elsevier Ltd. All rights reserved.