(567c) Phase Change Material Heat Exchanger to Decarbonize Water Heating in Low-Income Housing: Material Engineering Considerations

In the global push towards decarbonization, the buildings sector is particularly well-positioned to contribute by offering many opportunities for deep electrification. Of existing thermal systems in residential buildings, water heating (traditionally powered by fossil fuels) currently accounts for about 20% of household energy consumption in the US. Electric water heaters have grown in popularity in recent years, comprising nearly 50% of existing US systems; however, a majority of these water heaters are electric resistance water heaters (ERWHs), which suffer from low efficiency, high energy consumption, and sometimes require an electrical panel upgrade, making electrification difficult in low-and-moderate income housing. Heat pump water heaters (HPWHs), another electrified alternative, use 60-70% less energy than ERWHs and can therefore often be installed without requiring panel upgrades. However, deployment of HPWHs has been hindered in space-constrained low-income housing such as multifamily buildings or mobile/manufactured homes, as HPWHs tend to be significantly larger than traditional water heaters and cannot fit into the small utility closets often present in these settings. Currently, 32% of all housing in the US consists of multifamily or mobile homes, and of these units, over 60% have water heaters less than 50 gallons. Therefore, there is a need to develop smaller 30 – 40-gallon HPWHs with either equivalent or improved performance compared to larger electric or gas water heaters.

This project details the fabrication of a phase change material (PCM) heat exchanger for use in small-volume HPWHs to increase thermal storage capacity and overall performance. The design of the novel heat exchanger relies on additive manufacturing (3D printing) to produce complex geometries not possible via traditional manufacturing methods. This presentation describes efforts toward material processing and performance characterization for the 3D-printed PCM heat exchanger as well as preliminary results showing the applicability of additive manufacturing for introducing thermal energy storage into a HPWH. Printing techniques such as fused deposition modeling (FDM) pair well with micro-encapsulated PCMs (MEPCMs) and take advantage of low-cost, accessible methods and materials such as polymers. Based on standards for approved polymers in potable water systems, this work uses high-density polyethylene (HDPE); however, due to the novelty of PCM heat exchangers within domestic water heaters, there is a lack of regulation guidance for PCM selection. Therefore, our PCM selection relied on available MEPCMs with low toxicity, high energy storage density, and an appropriate transition temperature for water heating applications. Based on preliminary HPWH modeling results, our work utilizes an MEPCM with a transition temperature of 55°C. The primary goal of this work is to develop an MEPCM-HDPE composite with the highest possible MEPCM content that allows for reliable 3D printing and does not compromise the heat exchanger's structural integrity or durability. A higher MEPCM content corresponds to a heat exchanger with a higher thermal energy storage density, thus providing greater benefit for small-volume HPWHs. Preliminary efforts show the production of composite filaments of at least 60% MEPCM by weight with uniform consistency and properties, as well as small-scale heat exchanger prototypes produced via FDM. This work stands as a promising a path forward towards achieving superior HPWH performance in space-constrained, low-income housing.