Heat Transfer in Latent High-Temperature Thermal Energy Storage Systems—Experimental Investigation

Abstract

Thermal energy storage systems with phase-change materials promise a high energy density for applications where heat is to be stored in a narrow temperature range. The advantage of higher capacities comes along with some challenges in terms of behavior prediction. The heat transfer into such a storage is highly transient and depends on the phase state, which is either liquid or solid in the present investigation. The aim is to quantify the heat transfer into the storage and to compare two different fin geometries. The novel geometry is supposed to accelerate the melting process. For this purpose, a single tube test rig was designed, built, and equipped with aluminum fins. The phase-change material temperature as well as the heat-transfer fluid temperature at the inlet and outlet were measured for charging and discharging cycles. Sodium nitrate is used as phase-change material, and the storage is operated ±30 °C around the melting point of sodium nitrate, which is 306 °C. An enthalpy function for sodium nitrate is proposed and the methodology for determining the apparent heat-transfer rate is provided. The phase-change material temperature trends are shown and analyzed; different melting in radial and axial directions and in the individual geometry sections occurs. With the enthalpy function for sodium nitrate, the energy balance is determined over the melting range. Values for the apparent heat-transfer coefficient are derived, which allow capacity and power estimations for industrial-scale latent heat thermal energy systems.