Design methodology for phase change material heat exchangers in heat pump water heaters to improve thermal capacity

Abstract

Water heating represents a significant component of household energy consumption in the U.S., but high-efficiency heat pump water heaters (HPWHs) provide energy-saving opportunities. However, adoption of this technology is hindered in space-constrained housing (e.g., multifamily and manufactured housing) due to installer-recommended system up-sizing and the small utility closets present in these homes. To address this gap, this work outlines a novel design methodology for phase change material (PCM)-integrated heat exchangers based on 3D-printed triply periodic minimal surface (TPMS) lattices. This methodology outlines a systematic down-selection process for a large-scale TPMS heat exchanger integrated with a 120 V, 151-l (40-gal) HPWH based on practical design constraints such as system weight and dimensions, manufacturing considerations, and PCM discharge rate. Starting with a suite of over 1000 heat exchanger geometries, this study demonstrates how a series of parametric analyses and numerical models can effectively and efficiently identify optimal designs. Notably, this work targets consumer-facing water heater performance metrics such as first hour rating (FHR) to benchmark designs against an up-sized 189-l (50-gal) HPWH, ultimately demonstrating that intentional, application-specific heat exchanger design has the potential to eliminate HPWH up-sizing concerns. Over a range of convective coefficients in the tank, multiple designs improve the FHR of the baseline tank by between 36 and 47%, exceeding the 25% increase in FHR seen when simply up-sizing the tank. The methodology and results outlined in this study demonstrate the importance of considering practical, system-level constraints and performance metrics when designing thermal energy storage devices.