Nanofluids for Heat Transfer and Thermal Energy Storage in Concentrated Solar Power (CSP) Applications

D. Singh
Argonne, US

Keywords: concentrated, solar

Summary:

This presentation will cover the use of nanoparticles to : (a) improve thermal properties of heat transfer fluids (HTFs) and (b) enhance thermal energy storage (TES) capacity of salts used for CSP. Liquids  in which nanoscale particles are uniformly dispersed, have the potential for enhanced thermal performance. Copper nanoparticles were synthesized and dispersed in a commercial solar heat transfer fluid at various particle loadings up to 2 vol.%.  Characterizations such as thermal conductivity, viscosity, specific heat, and fluid stability were performed. Thermal conductivity enhancements over the base fluids were as high as ~20% at 2 vol.% nanoparticle loading. Enhnaced thermal properties of the fluid on the overall efficiency of nanofluids and its implications for CSP application will be presented. Core/shell nanoparticles of a phase change material (PCM) were investigated to increase the volumetric TES capacity of a typical eutectic salt used for CSP.  Commercial zinc nanoparticles (heat of fusion of ~110 J/g) were used as the PCM. To contain the melted zinc and protect it from interacting with the salt, zinc nanoparticles were coated with a high temperature organic.  As-received and coated zinc nanoparticles were extensively characterized for particle size, phase purity, structure, and heat of fusion. Subsequently, the coated nanoparticles were dispersed in a eutectic chloride salt mixture.  The volume loadings of the coated zinc nanoparticles ranged from 8-12 vol.% in the composite eutectic mixture.  Differential scanning calorimetry (DSC) was performed on the base salt and the composite eutectic salt mixtures to determine the increased thermal energy storage capacity of the composites. The composite eutectic salt with 10 vol.% zinc core/shell nanoparticle loading showed enhanced volumetric TES capacity (~30%) as compared to the base salt.  Extensive characterizations were carried out to evaluate the durability of the core/shell nanoparticles over multiple melting/freezing cycles as the salt was exposed to temperatures ranging from 300°C to 500°C. Implications of increased TES of eutectic choloride salt are additional energy storage for the same volume of the storage tanks, or smaller storage tanks for future CSP plants which will reduce the capital costs.  *Work supported by the Solar Energy Technology Program (ARRA funding) of the U.S. Department of Energy under contract number DE-AC02-06CH11357 at Argonne National Laboratory, managed by the University of Chicago Argonne LLC.