Capital investment reduction, exergetic efficiency improvement and material compatibility issues have been identified as the primary techno-economic challenges associated, with the near-term development and deployment of thermal energy storage (TES) in commercial-scale concentrating solar power plants. Three TES techniques have gained attention in the solar energy research community as possible candidates to reduce the cost of solar-generated electricity, namely (1) sensible heat storage, (2) latent heat (tank filled with phase change materials (PCMs) or encapsulated PCMs packed in a vessel) and (3) thermochemical storage. Among these the PCM macro-encapsulation approach seems to be one of the most-promising methods because of its potential to develop more effective energy exchange, reduce the cost associated with the tank and increase the exergetic efficiency. However, the technological barriers to this approach arise from the encapsulation techniques used to create a durable capsule, as well as an assessment of the fundamental thermal energy transport mechanisms during the phase change. A comprehensive study of the energy exchange interactions and induced fluid flow during melting and solidification of a confined storage medium is reported in this investigation from a theoretical perspective. Emphasis has been placed on the thermal characterization of a single constituent storage module rather than an entire storage system, in order to, precisely capture the energy exchange contributions of all the fundamental heat transfer mechanisms during the phase change processes. Two-dimensional, axisymmetric, transient equations for mass, momentum and energy conservation have been solved numerically by the finite volume scheme. Initially, the interaction between conduction and natural convection energy transport modes, in the absence of thermal radiation, is investigated for solar power applications at temperatures (300 - 400°). Later, participating thermal radiation within the storage medium has been included in order to extend the conventional natural convection-dominated model and to analyze its influence on the melting and freezing dynamics at elevated temperatures (800 - 850°). A parametric analysis has been performed in order to ascertain the effects of the controlling parameters on the melting/freezing rates and the total and radiative heat transfer rates at the inner surface of the shell. The results show that the presence of thermal radiation enhances the melting and solidification processes. Finally, a simplified model of the packed bed heat exchanger with multiple spherical capsules filled with the storage medium and positioned in a vertical array inside a cylindrical container is analyzed and numerically solved. The influence of the inlet mass flow rate, inner shell surface emissivity and PCM attenuation coefficient on the melting dynamics of the PCM has been analyzed and quantified.

Thermal energy storage (TES) is one of the most attractive and cost effective solutions to the intermittent generation systems like solar, wind and other renewable sources, compared to alternatives. It creates a bridge between the power supply and demand during peak hours or at times of emergency to ensure the continuous supply of energy. Among all the TES systems, latent heat thermal energy storage (LHTES) draws lots of interests as it has high energy density and can store or retrieve energy isothermally. Two major technical challenges associated with the LHTES are low thermal conductivity of the phase change materials (PCMs), and corrosion tendency of the containment vessel with the PCMs. Macro-encapsulation of the PCM is one of the techniques to encounter the low thermal conductivity issue as it will maximize the heat transfer area for the given volume of the PCM and restrict the PCMs to get in contact with the containment vessel. However, finding a suitable encapsulation technique that can address the volumetric expansion problem and compatible shell material are significant barriers of this approach. In the present work, an innovative technique to encapsulate PCMs that melt in the 100-350 oC temperature range was developed for industrial and private applications. This technique did not require a sacrificial layer to accommodate the volumetric expansion of the PCMs on melting. The encapsulation consisted of coating a non-reactive polymer over the PCM pellet followed by deposition of a metal layer by a novel non-vacuum metal deposition technique. The fabricated spherical capsules were tested in different heat transfer fluid (HTF) environments like air, oil and molten salt (solar salt). Thermophysical properties of the PCMs were investigated by DSC/TGA, IR and weight change analysis before and after the thermal cycling. Also, the constrained melting and solidification of sodium nitrate PCM inside the spherical capsules of different sizes were compared to explore the charging and discharging time. To accomplish this, three thermocouples were installed vertically inside the capsule at three equidistant positions. Low-density graphene was dispersed in the PCM to increase its conductivity and compared with pure PCM capsules. A laboratory scale packed-bed LHTES system was designed and built to investigate the performance of the capsules. Sodium nitrate (m.p. 306oC) was used as the PCM and air was used as the heat transfer fluid (HTF). The storage system was operated between 286oC and 326oC and the volumetric flow rate of the HTF was varied from 110 m3/h to 151 m3/h. The temperature distribution along the bed (radially and axially) and inside the capsules was monitored continuously during charging and discharging of the system. The effect of the HTF mass flow rate on the charging and discharging time and on the pressure drop across the bed was evaluated. Also, the energy and exergy efficiencies were calculated for three different flow rates. Finally, a step-by-step trial manufacturing process was proposed to produce large number of spherical capsules.