BIMODAL PARTICLE MIXTURES FOR ENHANCED THERMAL CONDUCTIVITY IN PACKED BED ENERGY STORAGE APPLICATIONS

Solid particles are of substantial interest as a thermal transport medium in high temperature energy storage and thermal energy conversion systems. This has resulted in numerous studies into fundamental properties of particles at high temperature, particularly for the concentrating solar power (CSP) industry. One of the downsides of solid particles as a thermal transport medium is the inherently low thermal conductivity of a packed bed of particles for materials relevant for low cost energy storage applications, such as sand, and ceramic proppants. This limits the performance of current flowing packed bed heat exchangers which suffer from low overall heat transfer coefficients as a result of low packed bed thermal conductivity as well as the low near wall effective thermal conductivity of most solid particle media. Here a flowing packed bed heat exchanger model is developed in conjunction with thermal conductivity models to explore the potential of binary particle systems followed by experimental testing of particles at relevant temperatures. Modeling indicates a substantial increase in effective packed bed thermal conductivity and overall heat transfer coefficient in a heat exchanger when a binary mixture is utilized. Experimental testing using a transient plane source technique, reveals that increases in thermal conductivity are not observed at temperatures beyond 400°C. We hypothesize that this is a result of increasing radiative transport in the particle bed. As temperatures increase to 700°C the mixture of bimodal particles results in a decreased effective thermal conductivity relative to a unimodal bed of large particles. Comparisons to common existing correlations for effective packed bed thermal conductivity (such as the ZBS) reveal poor agreement with experimental results, particularly as the temperature is increased. Additionally, testing of bulk porosity does not indicate substantial variation from pour to pour of particles but this is observed with thermal conductivity, likely indicating a modification of near wall packing that disrupts the measured signal.