Experimental and numerical investigation of bimodal particle distributions for enhanced thermal conductivity in particle based concentrating solar power applications

Abstract

Ceramic-based solid particles have recently attracted substantial interest as a thermal transport medium in high-temperature energy storage and thermal energy conversion systems due to their ability to operate at high temperatures (>1000 °C). This is especially useful in the concentrating solar power (CSP) industry where solid particles are utilized as heat transfer and energy storage media. Thermal conductivity of particles in CSP is critically important to the overall heat transfer that occurs within a heat exchanger. A low-cost and effective method to increase the thermal conductivity of a particle bed is by reducing its porosity by employing 2 differently sized particles. This is done in this work by examining several large particle volume fractions for the binary particle mixture of HSP 16/30–40/70. The thermal conductivity can be increased further by applying a load to the particles. At lower temperatures (20 to 300 °C), previous work has demonstrated a binary particle distribution has superior thermal conductivity. In this paper, the thermal conductivities of binary particle distributions under pressure loading are explored at ambient temperatures revealing enhanced thermal conductivity. Furthermore, high temperature (from 300 to 700 °C) analysis of binary particle distributions are also explored with results being that monodispersed distributions yield higher thermal conductivities due to enhanced surface radiation in larger particles. A bimodal distribution increases packed bed thermal conductivity only up to around ∼ 375 °C at which monodisperse distributions with larger particles yield the highest thermal conductivities. These results are compared to existing models (such as the widely used ZBS model) to demonstrate that the current models don’t adequately predict the impact of porosity on thermal conductivity, particularly at higher temperatures where radiation becomes the dominant heat transfer mechanism.