| Popis: |
This paper studies the thermal performance of a silicon based latent heat thermal energy storage (LHTES) system, operating at ultra-high temperatures (>1100 K), during charge, discharge and storage period. During charging, the system stores energy in the form of latent heat. During discharging, the stored energy is released from the vessel bottom and is converted into electricity. Owing to the system’s operation at such unprecedented temperatures, there is a high necessity to deal with several technical issues, including high heat losses through the vessel sidewalls. Thus, a validated transient computational fluid dynamics model is applied in ANSYS Fluent v17.0 platform, combining the volume of fluid method with the enthalpy porosity approach, and proper user-defined functions to consider lateral heat losses. Initially, a parametric study is conducted by varying the insulating materials thermal resistance (R) within the range 0.1-4.8 m2K/W. Furthermore, different vessel geometries (truncated cone, cylinder) with tapering ratios (TR) –ratio of bottom to upper surface- equal to 0.225, 0.45 and 1, and volumes ranging from 4V to V/4 (V=8.33e-04 m3) are investigated. During solidification/melting, it is observed that the average heat losses can be an order of magnitude higher for an insulation with R≈0.1 m2K/W than that with R≈2 m2K/W. Another important conclusion drawn is that during discharging the excessive lateral heat losses might cause an undesirable solidification of the PCM near the vessel sidewalls and upper part. Owing to this, there is a high risk of entrapping the molten PCM inside the solid one, and therefore increase the pressure inside the system. Considering the vessel geometry, a moderate effect of the TR on the lateral losses is noticed, especially for insulating materials with R≈2 m2K/W. Finally, the crucible volume plays an important role on the system losses, especially during storage period that the material should maintain its temperature close to its melting point. |
