Session: 11-03-01: Heat Transfer in Batteries and Energy Storage Technologies

Paper Number: 95246

95246 - Suppressing Subcooling in an Inorganic Phase Change Material Based Thermal Energy Storage System Using Self-Seeding Technique 

Salt-hydrate based phase change materials (PCM) have gained interest in recent decades in thermal energy storage (TES) for a variety of applications due to their high energy density and low cost. However, one key barrier to its adoption is the phenomenon of subcooling which prevents refreezing and subsequent release of thermal energy during normal operation. Furthermore, identifying a stable nucleating agent to significantly suppress subcooling without incurring a penalty on capacity, stability or material compatibility has been proven to be extremely difficult. The current study explores a simpler technique - self-seeding or cold finger method - that do not require any additive as an effective alternative to this problem. This study experimentally investigates a TES design that comprises an aluminium microchannel heat exchanger filled with Lithium Nitrate Trihydrate (LiNO₃.3H₂O, h_sf = 282kJ/kg, T_m = 30°C) PCM between the fins, that is subjected to specific operating condition to implement self-seeding. LiNO₃.3H₂O is a PCM that needs high subcooling (∆T_s > 20°C), but is otherwise a prime candidate in the 25 – 35°C range due to its high energy density, stability and compatibility with aluminium. By controlling and limiting the melt fraction during TES charging (melting) cycle to 90%, the TES was able to preserve a cold spot where the solid crystals serve as nucleation sites during refreezing. Subsequently, during the discharge (freezing) cycle the TES was able to refreeze the PCM immediately without subcooling. With an additional 10% capacity and simple operation control, the TES was able to charge and discharge itself without losing capacity or undergoing subcooling during the 100 cycle test. The results indicate a very low subcooling of ∆T_s < 1°C compared to using nucleating agents as reported in the literature (∆T_s > 5°C, Zn₃(OH)₄(NO₃)₂). This can significantly reduce the chances of operational failure or the need for extra subcooling for a TES in a typical operating environment. The study further illustrates methods to monitor melt fraction through T-history measurements to prevent complete charging (melting). Additionally, finer details critical to TES operation such as performance characteristics of the heat exchanger under different water inlet temperatures and flow rates are also discussed. The above two factors are critical in optimizing the operational parameters during thermal cycling to maintain a cold spot (solid crystals) which enable the implementation of the self-seeding technique. This study thus illustrates the operation of a prototypical TES system, which in spite of using a high subcooling PCM without a nucleating agent, can avoid subcooling and operate within practical temperature operating conditions using the self-seeding technique.

Presenting Author: Sarath Kannan University of Cincinnati

Presenting Author Biography: Sarath Kannan is pursuing his PhD in Mechanical Engineering from the University of Cincinnati in the field of thermal energy storage using phase change materials under Dr Raj Manglik. <br/><br/>After obtaining his bachelor&#39;s degree from the National Institute of Technology Calicut India in 2014 he worked at Thermoking as a mechanical engineer for 2 years. Following this, he got admitted to the University of Cincinnati for an MS degree in Mechanical Engineering, where he did his research on Thermal Energy Storage Materials for a powerplant cooling project funded by NSF. After graduation, he worked in Viking Cold Solutions, Houston TX for a year, which provided thermal energy storage solutions to the cold storage industry to reduce electricity consumption. His main responsibilities were to design phase change materials, build research and test capabilities and work as a thermal engineer for various projects.<br/><br/>In 2020 he returned to the University of Cincinnati to pursue his PhD in the field of thermal energy storage.

Authors:

Sarath Kannan University of Cincinnati
Milind Jog University of Cincinnati
Raj Manglik University of Cincinnati