Session: K6-07: Thermal Energy Storage Systems - II

Paper Number: 195367

195367 - Novel Polymer Heat Exchanger With Salt-Hydrate Compatibility for Low-Cost Phase Change Material Heat Exchange 

Abstract: 

This presentation presents the design, fabrication, and evaluation of a novel prototype heat exchanger (HX) fabricated from polymer materials compatible with corrosive salt hydrate phase change materials (PCMs). The compact, finless two-pass HX uses polyethylene tubes (6.1 mm OD, 0.05 mm wall), with 195 tubes per pass and optimized tube gaps of 1.5 mm (widthwise) and 5 mm (lengthwise) to maximize surface area and minimize thermal resistance. Two units integrated within a 60-gallon tank provide ~180 ft² heat transfer area, enabling ~2-ton capacity with 90-minute thermal storage. The resulting design can help enable low cost PCM HX for compact thermal storage applications that are low enough cost to find compelling payback periods when used to reduce peak demand changers in building applications.

The system was fabricated using polyethylene polymer tubes and a potassium carbonate–based salt hydrate PCM with a phase change temperature of 8 °C (46.4 °F). The prototype finless polymer heat exchanger will be evaluated under both constant inlet temperature/flow rate conditions and constant heat rate conditions.

Under constant inlet temperature and flow rate conditions, the thermal performance of the heat exchanger will be characterized using the effectiveness–NTU (ε–NTU) method. Effectiveness is defined as the ratio of the actual heat transfer rate to the maximum possible heat transfer rate based on the temperature difference between the heat transfer fluid (HTF) inlet and the PCM phase-change temperature. The number of transfer units (NTU) is determined from the overall heat transfer coefficient, effective heat transfer area, and the minimum heat capacity rate. During phase change, the PCM is assumed to have a very large effective heat capacity rate, such that the minimum heat capacity rate is governed by the HTF. Experiments will be conducted at steady-state flow rates of 3–10 GPM, with inlet temperatures of 50–70 °F for melting and 25–40 °F for freezing (crystallization). The results will be used to establish ε–NTU relationships and to quantify the effects of flow rate and inlet temperature on heat exchanger performance.

Under constant heat rate conditions, experiments will be conducted to develop a Ragone plot for the heat exchanger and to benchmark its performance against state-of-the-art PCM heat exchanger technologies. The heat transfer rate will be determined from inlet and outlet temperature measurements of the HTF. Tests will be performed at heat rates ranging from 0.5 to 2 tons, selected to align with the total storage capacity of the system. For a prescribed temperature difference between the inlet fluid and the PCM phase-change temperature, the average specific and volumetric energy storage capacities will be quantified at each heat rate. These results will be used to evaluate the system’s average specific power density and specific energy density up to a defined cutoff time, corresponding to the completion of the phase change or a minimum useful outlet temperature. Based on the calculated average power density and energy density, a Ragone plot will be constructed to assess the trade-off between energy storage capacity and power delivery.

Presenting Author: Kyle Gluesenkamp Oak Ridge National Laboratory

Presenting Author Biography: Dr. Kyle R. Gluesenkamp is a Distinguished R&D Scientist serving as ORNL's Subprogram Manager for Thermal Energy Storage, co-director of the Stor4Build consortium, and as a PhD student advisor with the University of Tennessee Bredesen Center. He leads teams to deliver impactful technology solutions to important energy conversion challenges, specializing in HVAC, thermal storage, residential appliances, psychrometrics, and experimental prototype design and development.

Authors:

Kyle Gluesenkamp Oak Ridge National Laboratory