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.

Partial cycle operation of latent heat storage with finned tubes

“… [23] aim to maximize the performance of latent heat TES using time dependent fin shape optimization. Previous experimental investigations with our test-rig showed good results for a combination of transversal and longitudinal fins [24] . Non-finned tubes are applied in a study from Tehrani et al.…”

“…Due to the low thermal conductivity of sodium nitrate low power rates are expected for higher fraction of PCM during discharging. In order to evaluate the efficiency of the novel fins introduced in [24] for partial load states the following experimental investigations had been carried out within this work:…”

The Impact of Heat Exchangers’ Constructions on the Melting and Solidification Time of Phase Change Materials

“…They concluded that using longitudinal fins in horizontally oriented HX is more advisable than using circular fins because after 8 h of charging the PCM melted completely only in the HX with longitudinal fins. However, Scharinger-Urschitz et al [88] reported that when the longitudinal and circular fins were used simultaneously, the melting was faster by 32% compared to HX with only longitudinal fins. Nevertheless, the application of longitudinal fins can reduce the melting time of PCM by 50% compared to the non-finned HX [89].…”

“…Aly et al [95] found that when the straight longitudinal fins were replaced by corrugated fins, the solidification time of PCM decreased by 38%. Furthermore, Scharinger-Urschitz et al [88], Zhang et Besides the arrangement of fins, numerous researchers investigated the fins of various shapes. Aly et al [95] found that when the straight longitudinal fins were replaced by corrugated fins, the solidification time of PCM decreased by 38%.…”

“…Aly et al [95] found that when the straight longitudinal fins were replaced by corrugated fins, the solidification time of PCM decreased by 38%. Furthermore, Scharinger-Urschitz et al [88], Zhang et al [96], and Luo and Liao [97] proposed using fractal Y-shaped (tree-shaped, dendritic) fins. Zhang et al [96] found that using Y-shaped fins resulted in reduced melting and solidification time by 4.4% and 66.2%, respectively, compared to straight fins.…”