High Energy Density Hygroscopic Hydrogel Thermo-Adsorptive Storage

Despite significant interest in thermal storage devices, state-of-the-art phase change material (PCM) systems lack the high energy densities, while the low temperature of the building waste heat cannot charge thermos-chemical material (TCM) systems. Here, we transform thermal energy storage (TES) by proposing a new approach with the adsorption of hydrogel/salt composites. Hydrogel adsorbents are hydrophilic dry polymer networks with hierarchical pore structures. Through swelling, it enables a much higher water uptake and significantly lower cost compared to traditional adsorbents including the commercial zeolites and MOFs, at high water vapor pressure. Salts, on the other hand, adsorb water vapor efficiently at low vapor pressure, but after deliquescence, the salt solutions are strongly corrosive. Here we incorporate hygroscopic salts into a hydrogel matrix to prevent the hydrated salts from deliquescence while greatly enhancing the hydrogel’s adsorption capacity at low relative pressure levels. As a result, a water uptake of about 1.0 g/g at the operating relative pressure level (~ 30%) with high cyclability can be achieved by a unique hydrogel/salt combination. To achieve superior kinetics, several methods were considered to tune the porosity of the hydrogel such as freeze drying and ice templating. Additionally, we fine-tune the low critical solution temperature (LSCT) of the hydrogel in such a way to minimize its regeneration temperature significantly. Finally, we leverage the high-water uptake, superior kinetics, and low desorption temperature of the hydrogel to design and demonstrate a high energy density thermal storage system. Due to the favorable properties of the hydrogel adsorbent, not only does our proposed TES unit have more than 3 higher energy density compared to state-of-the-art PCM solutions, but it is also cost-effective.