Each year, buildings in the United States consume quadrillions of Btus of thermal energy. As such, effective thermal energy storage is recognized as necessary to reduce and more effectively use this energy. Thermal energy storage based on thermochemical reactions, wherein the thermal energy is storage in chemical bonds, is promising due to its potential high energy density and low self-discharging. One thermochemical reaction that could be used to storage energy for buildings is the hydration and dehydration of hygroscopic salts such as MgSO4, CaCl2, and MgCl2. However, pure salts have cycle stability issues and low thermal conductivity. Additionally, reactors utilizing salts often have a reactor-bed structure consisting of a packed-bed of particles. These reactor-bed structures must balance the requirements of high energy density, low pressure drop, and heat and mass transfer limitations. But traditional manufacturing processes of particle-beds provide limited control over key variables such as bed porosity. The present work investigates using a molding technique to fabricate stable salt structures. The hypothesis being that the binder selection and heat treatment processes associated with molding will stabilize the resulting salt structure. Two type of salts were investigated: pure MgSO4 and a blend of 53%, by weight, CaCl2 with 47% MgSO4. These salts were mixed with two common binders and hot pressed. Various post-hot-pressing conditions were considered including the debinding temperature, whether or not the sample was sintered, and, if it was sintered, the sintering temperature. The samples were subjected to ten hydration and thermal cycles. The samples were hydrated at room temperature in a flow with a relative humidity of 75%, and then dehydrated at 140 °C in a flow of dry nitrogen. The hydration reaction was monitored by measuring the relative humidity during the process. The samples were visibly inspected for changes between each half cycle. The results indicate that molding can result in stable structures. All the samples of 53wt%CaCl2+47%wtMgSO4 and one sample of pure MgSO4 retained their integrity through the course of cycling. Of the samples that did not retain their integrity through cycling, the results show that fabrication parameters can be used to improve the cycle stability of the molded sample. The hydration data shows that, for the samples that retained their structure, stable hydration rates are achieved. For example, for one of the samples consisting of CaCl2+MgSO4, the hydration rates decreased from cycle 1 to cycle 5 but then stabilized for the remaining cycles. This indicates that the microstructure has stabilized for thermochemical cycling. These results indicate the feasibility of using molding or similar manufacturing techniques to fabricate a stable structure of hygroscopic salts for thermochemical-based, thermal energy storage. Eventually, the modeling process could be used to construct optimized reactor-bed structures that maximize the energy density while minimizing the pressure drop and maximizing the power output.

Using Molding to Fabricate Stable Salt Structures for Thermochemical Energy Storage

Paper Proceedings: https://asmedigitalcollection.asme.org/ES/ES2021/volume/84881

ASME 2021 Energy Sustainability (ES2021)

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