Session: 11-06-01 Heat Transfer in Solar and Renewable Energy Systems - Concentrated Solar Power and Thermal Storage

Paper Number: 73089

Start Time: Monday, 12:15 PM

73089 - The Encapsulation Effect on Thermal Performance of Micro-encapsulated Phase Change Materials During Energy Absorption 

Thermal energy storage (TES) system in renewable energy technologies is essential to balance energy supply and demand. TES systems at concentrating solar power (CSP) plants allow energy to be stored in the form of heat, and enable flexible electricity generation rates. Since the performance of the TES system is governed by Carnot efficiency, High temperature TES is particularly appealing for achieving high exergetic efficiency. The current state of the art heat transfer and storage medium in CSP plants molten salt, however, hinders the task of improving efficiency, due to its corrosive nature, especially at high temperature. 

An alternative is to use micro-encapsulated metallic core phase change materials (MEPCMs). MEPCMs offer several advantages over molten salt. First, it enables latent heat absorption, storage, and release at higher temperature, to increase the TES efficiency. Second, the encapsulation prevents leakage of PCM in both liquid and solid state, and consequently minimize corrosion due to direct contact. Furthermore, microscale particles are easy to transport and store at system level, which could lead to cost reduction. Additionally, the encapsulation and high thermal conductivity of the metal make it possible for MEPCMs to be used as filler material in heat transfer fluid. 

There are only limited existing studies on MEPCMs, all of which concern manufacturing and material aspects. The purpose of this work is to investigate the thermal characteristics of the MEPCMs. The analysis intends to quantify the thermal performance of MEPCMS during melting (charge) and solidification (discharge), in terms of temperature, energy storage density, and heating/cooling time.  

We took a numerical approach to study the transient heat transfer process with phase change on a homocentric spherical particle in microscale. The core of the sphere is the PCM, and the shell is the encapsulation material. The first question is the validity of the built numerical model. Model validation was conducted in two separate segments: with and without phase change. 

Without phase change, when the ratio of the internal conductive thermal resistance and external convective thermal resistance at the surface of the sphere is sufficiently small, the spatial temperature gradient is negligible within the particle. Under this circumstance, the results should and indeed agree with the analytical solution obtained from Lumped Capacitance method.  During the isothermal phase change process, the solidification time obtained from this model is compared with Levi's model, which takes liquid-solid interface kinetics and the growth of solid into consideration. The results from both models matches very well (within 1\%). The rate of melt fraction during phase change from both models agrees as well. 

The thermal characteristics of MEPCMs are quantified using a validated model in this study. Higher volumetric energy storage density, longer charging and discharge time are observed for certain metal materials but not all. The impact of shell on response time is not monotonic, indicating that an optimal shell  thickness exists for achieving high energy storage and fast charge/discharge speed.  The results are proof that MEPCMs are promising as heat transfer fluid at high temperature (exceeding 1000 degC) for TES system application. Further, the results is instructive for manufacturing process to tune process parameters in order to fabricate MEPCMs with targeted thermal characteristics. 

Presenting Author: Jingru Z. Benner Western New England University

Authors:

Jingru Z. Benner Western New England University
Rebecca C. Shannon Western New England University
Wentao Wu Tennessee State University
Austen P. Metsack Western New England University
Lu Shen Western New England University
Jingzhou Zhao Western New England University