Session: 08-09-01: Thermal Energy Storage

Paper Number: 94810

94810 - Thermal Performance of Phase Change Material Based Heat Exchangers 

One of the important challenges in utilizing solar energy is that its availability is intermittent. Hence, designing an efficient energy storage system is critical to make solar energy a reliable source. A latent heat energy storage system (LHESS) is one such technology where a large amount of thermal energy can be stored in the form of latent heat. Phase change materials (PCM) used in such a system will undergo phase change by storing and discharging thermal energy for several hours for later use. The heat exchanger considered in this study is one of the key components of the solar-powered membrane distillation system designed by the present authors.

            Most phase-changing materials are poor thermal conductors. This presents a challenge to effectively transfer heat within the storage systems. There are several types of latent heat energy storage systems, one of which is to use a heat exchanger to transport thermal energy from Heat transfer fluid (HTF) to PCMs. Effective design of these heat exchangers is critical to improve the heat transfer as well as optimize the charging or discharging cycle times.

Transient heat transfer numerical simulations will be carried out in ANSYS Fluent that includes flow, thermal and phase change analysis. Two heat exchangers designs are considered for these simulations, helical coil heat exchanger and fins-based heat exchanger. The helical coil has the advantage of proper mixing of heat transfer fluid and thereby decreasing the thermal gradient within the coil. While fin-based heat exchanger significantly increases the contact surface area with PCM. Both designs will be evaluated for their thermal performance by keeping the volume of PCM (+/-5%) constant. Parameters considered for this evaluation are by comparing the energy charging and discharging rates, time to melt and solidify PCM, and the liquid fraction with time. Computational studies are carried out for both laminar and turbulent flow regimes.

PCM considered for this study is Erythritol. This PCM is considered due to its high latent heat value of 340kJ/kg and is suitable for applications in the membrane desalination process. The heat transfer fluid considered for this study is thermal oil, which can withstand temperatures up to 623^oK.

During the charging cycle, heat transfer fluid enters at a uniform temperature of 495^oK, the initial temperature of PCM considered is about 385^oK.  Velocity boundary conditions are considered at the inlet and a zero-gauge pressure boundary condition is applied at the outlet. Assuming the system is well insulated, an adiabatic boundary condition is applied on the external walls of PCM and heat exchanger pipes. Convection within the PCM is not considered for this study. During discharging cycle, inlet temperature will be set at 295^oK.

Turbulent model SST K-Omega will be employed for this study. It is a hybrid model which uses K-Omega near the wall region and K-Epsilon model in the free stream region.  Ansys Fluent employs the Enthalpy-Porosity technique to model the solidification and melting process. This technique considers PCM to convert from solid to liquid between lower and upper limit temperatures called solidus and liquidus temperature. Solidus and liquidus temperatures consider for this study are 390^oK and 392^oK, respectively.

Presenting Author: Abhinay Soanker Lehigh University

Presenting Author Biography: Abhinay Soanker is a Ph.D. student in the Mechanical Engineering Department at Lehigh University. His research area includes Renewable and Sustainable Energy, Solar Energy, Heat and Mass Transfer, Computational Fluid Dynamics.

Authors:

Abhinay Soanker Lehigh University
Alparslan Oztekin Lehigh University