Modified Materials for Thermochemical Energy Storage

Kurzfassung

Thermal energy storage plays an important role for utilizing Concentrated Solar Power (CSP) plants as dispatchable renewable electricity source. Thereby, thermochemical energy storages can make a decisive contribution due to their high energy density in combination with low material cost. A suitable reaction system in appropriate temperature range (~500°C) is the reversible gas-solid reaction of calcium oxide with water vapor to calcium hydroxide: CaO(s) + H2O(g) ⇌ Ca(OH)2(s) + ΔHR For application in CSP plants two main aspects are focused. On the one hand the separation of power and capacity is reasonable, if a constant power over a long period (e.g. night cycle) is required. In this case, the material which is stored in reservoirs, defining the storage capacity, has to be moved through a reactor providing the desired power level. To permit the flow of the cohesive material agglomeration effects have to be prevented and flow behavior needs to be improved. A systematical analysis of the improvement of powder bed properties by addition of nanomaterial is presented. It will be shown that agglomeration can be already prevented by using only small amounts of nanoparticles and the flow behavior can be clearly improved. On the other hand the working temperatures of current CSP plants range from ~250°C to ~550°C, whereas the minimum temperature of the CaO/Ca(OH)2 reaction system is ~400°C. In an approach to decrease the equilibrium temperature of hydration-/dehydration reaction several modifications on the material, as for example doping with other metal oxides and addition of catalysts were investigated to probe the possibility and range of an adjustment. A survey was made to proof if the characteristic features of the system CaO / Ca(OH)2 are also assignable to other alkaline earth metal oxides / hydroxides.