Each year quadrillions of Btus of thermal energy are consumed in buildings in the United States. As such, effective thermal energy utilization is recognized as essential to reduce and more effectively use this thermal energy by providing demand flexibility and time-shifting of the load. Thermochemical energy storage is promising due to its advantages of high-energy density and low-self discharging. One promising reaction material is the hydration and dehydration of hygroscopic salts such as calcium chloride (CaCl2), magnesium sulfate (MgSO4), and aluminum sulfate (Al2(SO4)3). However, pure salts have poor cycle stability and low thermal conductivity. As such, impregnating a porous host, such as expanded graphite, zeolites, and activated carbon, with salt has been investigated as a method to improve stability and conductivity. These hosts improve stability but often reduce the energy density. The present work investigates the development of new composite material for thermochemical storage by impregnating a framework of crystalline nanocellulose (CNC) with salt. Calcium chloride (CaCl2) salt was impregnated by CNCs, derived from softwood. Various weight ratios of CNC:salt were prepared by dispersing the salt and CNC in deionized water and using mechanical stirring and ultrasonication processes. The CNC-salt solution was dried at ambient temperature to obtain CNC-impregnated salt formulations. The attachment of the salt to the CNC was determined by SEM and FTIR analyses. The enthalpy of dehydration and water uptake of the composite material and pure salt were measured with digital scanning calorimetry and thermogravimetry, respectively. The stability of the composite material was determined by subjecting the CNC-salt material and control samples of pure CaCl2 to multiple hydration and thermal cycles. The samples were hydrated at room temperature in a flow of moist air and dehydrated by heating to 105 °C in a flow of dry nitrogen. The results show that the nanocellulose binds to the salt crystals and thus provides a nano-scale architecture to stabilize the salt. The CNC-salt material shows improved energy density and stability compared to pure salt. For example, the specific enthalpy of dehydration of a sample with a 1:1 weight ratio of CNC to salt is ~200 J/g while for the pure CaCl2 the enthalpy of dehydration is 154 J/g. Thus, the CNC-salt material shows &nbsp;~1.3-x improvement in energy density. Additionally, over the course of multiple cycles, that caused the pure CaCl2 to lose energy density and ultimately form a deliquescent, the CNC-salt material retained its structural integrity and its energy density. Therefore, the results demonstrate a new, promising, and stable composite material for thermochemical energy storage.

A Novel Composite Material of Hygroscopic Salt Stabilized by Nanocellulose for Thermochemical Energy Storage

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

ASME 2021 Energy Sustainability (ES2021)

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