Numerical Analysis of Total Energy Storage of Nanofluidized Heat Transfer Fluid in Thermocline Thermal Energy Storage System

Abstract

Thermal energy storage (TES), when combined with a concentrating solar power (CSP) plant has potential to produce electricity at a cost-competitive rate to traditional sources of electricity production. In single tank TES system both the hot fluid and cold fluid settle in the same tank. The region of contact of these two fluids is called thermocline. Preservation of this thermocline region in the cylindrical tank during charging and discharging cycles depends on the uniformity of the velocity profile at any horizontal plane. So to maintain this thermocline region, a pipe flow distributor was placed near the inlet and outlet of the cylindrical tank. To optimize the efficiency of this single tank TES system is to increase the thermo-physical properties of heat transfer fluid. This addition will result in harnessing solar energy by increasing thermal efficiency of the thermodynamic cycles. Adding of nanoparticles, in the heat transfer fluid give rise of this thermo-physical properties i.e. thermal conductivity (k) and specific heat capacity (Cp). Hitec® molten salt is used as the base-fluid and synthesized with five different types of nanoparticles (SiO2, Al2O3, Fe3O4, ZnO and Ag) with different concentrations. The values of effective k and Cp are calculated for the new Hitec® nanofluid. The doping of nano-particles results in higher k and Cp when compared to the base fluid. Higher Cp is expected to improve the thermal storage capacity but higher value of k is expected to increase the thermal diffusivity, thereby affecting the performance of the thermocline. The diffusivity depends on the ratio of k to Cp and density of the effective properties. So there is a need to balance the effective properties to improve thermal storage performance. The total energy storage capacity is then checked by finite volume based computational fluid dynamics software. The simulation shows how the performance of the nanofluid changes at different concentrations in a single tank TES system during its charging-discharging cycle.