AN ADDITIVELY MANUFACTURED, SALT HYDRATE-BASED LATENT HEAT THERMAL ENERGY STORAGE FOR PEAK LOAD SHIFTING IN BUILDING EQUIPMENT

Abstract

As renewable energy usage increases, imbalances between electricity demand and supply grow, raising the risk of grid failures and blackouts. These imbalances become even more severe during peak load hours. Peak load shifting involves managing demand and supply to reduce the grid load during peak times, helping to match supply levels. A latent heat thermal energy storage (LHTES) device using solid-liquid phase change material (PCM) can be integrated into buildings’ heating and cooling equipment systems, offering energy storage capacity for peak load shifting of thermal loads. An LHTES must be capable of delivering significant power during peak loads, despite the internal thermal resistance caused by the low thermal conductivity of the PCM material. To boost power, conductivity can be enhanced with additives, but this can potentially increase costs and reduce energy storage density (ESD). Alternatively, power density (PD) can be improved by increasing the heat transfer area and reducing heat transfer pathway length without the need for conductivity-enhancing additives. A better thermal design can thus improve PD without increasing the cost, supporting commercialization. This dissertation examines the thermal performance of a novel cross-media thermal energy storage device (CMTES), a polymer-metal structure made via additive manufacturing, integrated with a low-cost, salt hydrate PCM. The CMTES core structure employs low-cost polymers as a barrier between the heat transfer fluid (HTF) and the PCM with embedded metal wires for high thermal conductance, transferring heat efficiently between the HTF and PCM sides. For this investigation, an in-house salt-hydrate composite PCM was developed using Glauber’s salt with small doses of economical additives to achieve desired thermal characteristics, enabling its use in Heating Ventilation and Air Conditioning (HVAC) systems. This composition contains 9 wt.% NaCl for melting point reduction, borax to reduce supercooling, and sodium polyacrylate as a thickener. Thermal characterization of the PCM was conducted using a modified T-history method, providing detailed insights into its phase change characteristics. The results indicated a melting range of 18–22 °C, and the latent heat measured at 171 kJ/kg. The PCM also exhibits a high volumetric ESD compared to organic types with similar melting points. The cyclic stability was also investigated, and it was found that the PCM was stable for 300 cycles, equivalent to a year-long TES operation. An experimental investigation was conducted to assess the impact of rehydration time on the PCM’s latent heat of fusion. Findings revealed that the latent heat could change by 70% as the rehydration period increased from three hours to 20 hours. The CMTES integrated with the composite PCM was experimentally tested. The CMTES demonstrated a volumetric PD of 190-450 kW/m³ during discharging and 238–300 kW/m³ during charging, despite the metal wires occupying only 3% of the total TES volume. A numerical modeling approach was used to compare the CMTES with a competitive low-cost PCM slab design, showing that the developed concept boosts PD, equivalent to a 20-fold increase in thermal conductivity for the same PCM in the slab design. A figure of merit traditionally used for comparing heat exchangers, which combines thermal performance with compactness, was adopted to compare the performance of the CMTES with other enhanced TES designs reported in the literature. Based on the figure of merit introduced in this work, the results indicated that the CMTES performed 1.6–16 times better than conventionally manufactured enhanced TES designs and comparable to state-of-the-art additively manufactured metal fin designs. The findings of this study provide significant and novel contributions by demonstrating the potential of salt-hydrate composites as a low-cost, high-performance phase change material (PCM) solution for latent heat thermal energy storage (LHTES) applications. By leveraging a cross-media approach, the study shows how these composites effectively combine economic feasibility with enhanced thermal performance. When integrated into the cross-media thermal energy storage system (CMTES), the proposed salt-hydrate composite serves as a cost-effective alternative to traditional microencapsulation techniques, while achieving internal thermal conductance levels comparable to state-of-the-art enhanced TES designs. Additionally, the study addresses key practical challenges associated with salt hydrate PCMs—specifically, the effects of rehydration time on latent heat recovery and the implications for long-term cyclic stability—thereby laying the groundwork for future efforts aimed at improving energy storage density (ESD).