Preparation & Characterization of Sodium Sulfide Hydrates for Application in Thermochemical Storage Systems

Bottlenecks for realizing a commercial system for thermochemical heat storage (TCS) with hygroscopic salts are the chemical, physical and mechanical stability of the salt under operation conditions. Hence, improved knowledge of thermochemical materials (TCMs) is critical to spur progress in TCS system development. Sodium sulfide hydrates (Na2S.nH2O, n=0-9) are highly interesting as TCMs because they exhibit a high energy density under operation conditions and are potentially readily available and affordable. Preparation methods for well-defined nonahydrate and pentahydrate crystals of Na2S were developed and the resulting samples were subjected to cycling experiments under conditions representative for TCS. Before and after cycling, crystal samples were taken and characterized using techniques like SEM/TEM, XRD. Mechanical strength was evaluated using a salt bed stability test. Based on the extensive characterization of sodium sulfide hydrate salts, a method has been proposed to improve the stability of the salt by blending it with cellulose. First trials on these composites yielded promising results with respect to improved material stability.