(567g) Design and Demonstration of a High Energy Density Hydrogel-Based Thermal Energy Storage Device

Thermal energy storage (TES) technologies are integral components in the shift towards more sustainable and efficient energy systems, playing a pivotal role in managing the supply and demand of energy, particularly from renewable sources. Among the various TES methods, such as sensible heat, latent heat, and thermochemical energy storage, the latter stands out due to its superior energy storage density and the ability to store heat for extended periods without noticeable losses. Thermochemical energy storage (TCES) devices harness chemical reactions to absorb or release thermal energy, offering a groundbreaking solution to the intermittent challenges of renewable energy sources. Conventional materials used in TCES systems, including metal hydrides, zeolites, and salt hydrates, are chosen for their favorable reaction enthalpies, which significantly influence the system's energy density. These materials can store and release large amounts of heat through reversible chemical reactions, making TCES systems highly efficient and compact compared to other TES technologies. The energy density of TCES systems is higher than that of sensible and latent heat storage methods, allowing for more energy to be stored in a smaller volume i.e., significantly higher energy densities, thereby enhancing the feasibility and efficiency of renewable energy systems. This unique advantage positions thermochemical energy storage as a key technology in the development of future energy storage solutions, offering a sustainable path towards meeting global energy demands. Despite having many advantages, TCES systems face several challenges and shortcomings that result in impeding their adoption of residential and commercial applications. These shortcomings not only encompass material and system-level challenges but also directly influence the energy input required for recharging and the overall round-trip efficiency of the systems. Addressing these issues is crucial for advancing TCES technologies and ensuring their viability in the broader energy market.

To overcome these shortcomings, we have developed and optimized a novel thermal storage material with tunable performance i.e., uptake and kinetics. The proposed TES device utilizes a specially engineered hydrogel material that exhibits a tunable phase change temperature, desorption temperature, water uptake, and significant latent heat storage capacity. Hydrogels, with their unique network structure capable of holding large amounts of water, have been modified to undergo a solid-liquid phase transition at a predetermined temperature, suitable for TES applications. This phase change is associated with the absorption or release of substantial thermal energy, enabling efficient storage and release of heat. The hydrogel's chemical composition and physical properties have been optimized to maximize energy storage density and thermal conductivity while ensuring stability and durability over repeated thermal cycling (> 200 cycles). We overcome the limitations of current solutions using a novel hydrogel/salt composite as the adsorbent, reaching higher energy density with lower discharging temperature ranging from 50-80˚C, while offering the benefits of low cost and scalable fabrication. The hydrogel is superior to most other storage media except for metal organic frameworks (MOFs) that can typically be very expensive and require high regeneration/desorption. After incorporating hygroscopic salts into a hydrogel matrix, the matrix can prevent the hydrated salts from deliquescence, which maintains the outstanding moisture absorbing capability of salts, over >1.8 gram of water per gram of polymer at 30%RH, and stable cyclability with no hysteresis. The adoption of hydrogel for TES combines the benefits of high energy density and low discharging temperature, both of which are critical to reducing the cooling cost in residential and commercial buildings. Using a PAM-LiCl (4 g LiCl per gram of polymer) resulted in a system with record high energy density > 200 kWh/L. The findings and innovations detailed in this work have the potential to revolutionize the energy sector, fostering the transition to a more sustainable and resilient energy future.