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 °C 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. 306°C) was used as the PCM and air was used as the heat transfer fluid (HTF). The storage system was operated between 286°C and 326°C and the volumetric flow rate of the HTF was varied from 110 m3/h to 151 m 3/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.
|Advisor:||Goswami, D. Yogi, Mujumdar, Ajit|
|Commitee:||Dhau, Jaspreet, Malik, Abdul, Rahman, Muhammad M., Ram, Manoj K., Stefanakos, Elias|
|School:||University of South Florida|
|School Location:||United States -- Florida|
|Source:||DAI-B 76/11(E), Dissertation Abstracts International|
|Subjects:||Alternative Energy, Mechanical engineering|
|Keywords:||Latent heat, Macroencapsulation, Metal coating, Packed bed, Polymer coating, Spherical capsule|
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