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5/16/2017  |   3:55 PM - 4:20 PM   |  2

Thermal conductivity measurements of powdery thermochemical heat storage materials

Thermochemical energy storage (TCES), based on reversible solid-gas reactions, offers the possibility to store heat over long time periods and to increase energy density. Thermal transport of mainly powdery thermochemical materials (TCM) has an important implication on the performance of a TCES. This study has chosen calcium oxalate monohydrate (CaC2O4.H2O) as one of several precursors which dehydrates to calcium oxalate (CaC2O4) after thermal treatment. Preliminary thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) tests were carried out to identify reaction free temperature intervals and specific heat capacity c_p(T). Laser Flash Analysis (LFA) was used to measure the thermal diffusivity a(T). Effective thermal conductivity was calculated with k_eff(T)=a(T).cp(T).D(T) on the compacted CaC2O4.H2O samples (D(T=25°C)=1638 kg/m³). In addition direct k_eff(T) measurements in the packed bed (D(T=25°C)=386 kg/m³) were conducted by using the Transient Hot Bridge (THB) method. Preliminary STA and DSC results indicated a solid-solid phase transition of CaC2O4.H2O prior the dehydration reaction and defined the LFA and THB experiment temperature limits. The compacted CaC2O4.H2O LFA samples showed shiny surfaces and a translucent behavior to the LFA laser, which was eliminated by a graphite coating. At T=25°C, indirect calculation for k_eff of the compacted sample results in 0.415 W/(m.K) with a combined standard uncertainty of u_c=0.033 W/(m.K). The direct THB packed bed measurement indicated k_eff=0.055 W/(m.K) with u=0.001 W/(m.K). Our findings show the strong dependency of k_eff(T) and powder density. Additional research on ZrO2 particles is planned to fully understand the influence of different gases, particle size and also thermal expansion on k_eff(T).

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Daniel Lager (POC,Primary Presenter,Author), AIT Austrian Institute of Technology GmbH,;
Andreas Weinberger (Co-Author), Institute of Applied Synthetic Chemistry - Vienna University of Technology,;
Christian Knoll (Co-Author), Institute of Applied Synthetic Chemistry - Vienna University of Technology,;
Wolfgang Hohenauer (Co-Author), AIT Austrian Institute of Technology GmbH,;

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