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Discharge Flux Variability in Stored Thermal Energy Cookstoves

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A thermal analysis and test is performed to determine flux and temperature variability for Phase change thermal energy to investigation feasibility of use in a Stored Thermal Energy Cookstove (STEC). The phase change material (PCM) NaNO_3-KNO_3 Eutectic (52:48) Solar Salt is identified for energy storage in STEC due to a melting temperature of 222℃ which is deemed appropriate for use in cooking up to temperatures of 200℃ ±20 ℃. 1-D planar and cylindrical analytical multiphase solutions are correlated with a transient non-linear ANSYS Finite Element Model (FEM). 1-D idealized models of planar and cylindrical analytical multiphase solutions show the flux stability of cylindrical solidification is twice that of planar solidification. Flux drops a linear average of 0.5%/min in the last half hour of a one hour cooking session in cylindrical solidification vs 1%/min in planar solidification under a constant temperature (dirichlet) boundary condition of 42 ℃ below the melting point of the PCM. Solidification progresses more quickly in the planar case yielding a solid PCM thickness of 3.3 cm after one hour vs 2.4 cm in the cylindrical case. A test is performed on a simplified simple STEC apparatus to investigate cooling rates of the cooking surface while boiling water. 0.5L of water is brought to boil from room temperature with a linear average cooking surface flux of 21,000 W/m2 and a cooking surface cooling rate of 3.8 ℃/min. Results show increasing the thermal conductivity of the PCM and reducing the total thickness of the solidifying PCM layer before and after discharge will reduce cooling rates, improve stability of the flux delivery device, and increase feasibility of use. Pursuing lower flux cooking and non-cooking applications may increase likelihood of adoption by reducing thermal gradients during discharge. A proposal to explore further development of STEC to aid adoption is discussed.

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