Session: 17-01: Poster Presentations

Paper Number: 164842

164842 - Making Small-Volume Heat Pump Water Heaters Larger: A Design Framework for Integrated Phase Change Material Heat Exchangers 

Abstract: 

In recent years, the buildings sector has seen major pushes towards decarbonization through innovations that promote deep electrification. 20% of an average household’s energy use comes from water heating, and in the US, over half of all households still use gas water heaters. Of those that use electricity for water heating, the majority use resistive elements rather than heat pump water heaters (HPWHs), the latter of which use 60–70% less energy than the former. One major barrier to wider HPWH adoption is the added equipment required, which prevents current 50–80-gallon tanks on the market from fitting into smaller utility closets sized for 30-40 gallons, such as those found in manufactured housing. Additionally, these smaller HPWHs tend to underperform relative to their larger counterparts. One solution that addresses both space and performance concerns is thermal energy storage, and in particular, phase change materials (PCMs). PCMs have been studied extensively in building envelope and HVAC systems, but remain a nascent technology in residential water heating. While water itself has a uniquely high energy storage capacity, PCMs have an even higher energy storage density, thus providing the potential to elevate the performance of 40-gallon HPWHs to that of 50-gallon or larger tanks. This research is part of a larger project that seeks to utilize thermal energy storage to enable decarbonized water heating in low-income communities.

In this study, we outline the design process used to produce novel PCM heat exchangers for use in small-volume HPWH tanks, including the identification of design constraints and performance targets relevant to real-world applications. In order to ensure optimal PCM utilization and tank storage capacity, we focus here on co-maximizing surface area and PCM volume in the heat exchangers; therefore, this research targets triply periodic minimal surface (TPMS) lattices. TPMS lattices boast enhanced heat transfer capabilities compared to traditional heat exchanger geometries and offer highly tailorable designs; thus, they pair well with the growing field of additive manufacturing, or 3D printing. Starting with a suite of TPMS lattices, we demonstrate a systematic approach for narrowing down feasible designs that comply with identified constraints while meeting PCM performance objectives. Our current results indicate that tuning lattice properties can effectively produce geometries that provide enough energy storage to create a 50-gallon capacity out of a 40-gallon HPWH, even when placing the PCM heat exchanger inside the tank. Our work here also includes examples of the optimized heat exchangers successfully fabricated via additive manufacturing, as well as an outline of upcoming experimental testing. Ultimately, this work will help push forward decarbonization goals by enabling widespread deployment of HPWHs in all housing types. Specifically, the technology will help residents in manufactured homes to have access to highly efficient water heating technology.

Presenting Author: Erin Blackley Colorado School of Mines

Presenting Author Biography: Erin Blackley is a third-year PhD student and IBUILD fellow at Colorado School of Mines in the inter-disciplinary Advanced Energy Systems program, a partnership with the National Renewable Energy Laboratory (NREL). She holds a bachelor’s degree in chemical engineering with a concentration in sustainable energy systems from Northeastern University.

Erin’s research focuses on expanding the accessibility of electrified systems in low-income housing by enabling the integration of thermal energy storage into heat pump water heaters. Her work involves the design, optimization, and testing of additively manufactured heat exchangers containing phase change materials for use in small-volume units.