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systems, the storage and the HTF both operate as sensible heat systems, meaning there is no phase change. In water systems, most of the transferred energy is transferred as latent heat during condensation and boiling. Thus, the key operating temperatures of the storage device are characterized by the saturation temperature of the HTF during charging (condensation) and discharging (boiling). Increasing the temperature change between the charging and discharging modes is known to increase efficiency but requires pressure changes between the operating modes. By using two sets of pipes, the charging and discharging piping sets would be able to be maintained at separate and consistent pressures and a different HTF can be utilized for the charge and discharge cycles of the system. A disadvantage that arises out of using two sets of pipes is that now heat deposited during charging must be conducted through the concrete structure, which has a low thermal conductivity, in order to reject heat to the discharging fluid. A single-pipe model and a two-pipe model have been produced. 2.1 Model Development The concrete media model is a TRANSFORM-compliant solid media model, meaning that there are equations of state for density, thermal conductivity, and heat capacity, as shown in equations (1)–(3), respectively. The values for these properties were created by notating published physical value results for HEATCRETE®, the specialized concrete formula espoused by EnergyNest, a European concrete battery company. This media model is presently used in both configurations. Replacing the media with proprietary data would be a straightforward change in future applications. Low-flow and no-flow conditions can be highly cumbersome to implicit non-linear solvers due mostly to the momentum equation. The concrete model used here is simplified from a fully developed flow model in order to avoid the computational restrictions at low flow and no flow by assuming a spatially equal pressure and spatially equal mass flow across the concrete structure. This assumption replaces the conservation of mass and the conservation of momentum equations, leaving the system to be described by the conservation of energy. The model equation for mass flow rate is an incompressible flow assumption: The pressure is taken to be the pressure at the cold end of the TES model, also known as the discharging inlet or the charging outlet. The pressure is taken there to enforce the most conservative assumption during either operational mode. During charging, the outlet pressure is used to apply conservatism in storage effectiveness as a lower pressure would correspond to a lower saturation temperature and, thus, lower energy transfer into the concrete. Conversely, the discharging inlet would have a higher pressure leading to a higher saturation temperature and similarly lower the energy transfer out of the concrete. The design and operation of concrete TES centers on the ability to successfully deposit and remove energy using a material that is typically a thermal insulator. The characteristic temperature of the system will be dictated either by the inlet charging and discharging conditions from a sensible heat fluid (such as gas, thermal oil, or molten salt) or by the saturation temperatures from a fluid that experiences phase change such as water. In a system using water or another similar boiling and condensing fluid, the characteristic temperatures will be dominated by the saturation temperatures of the fluid rather than just the inlet conditions. Because concrete is a sensible heat storage system, the gap in the saturation 3PDF Image | Thermal Energy Storage Model Development
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