Understanding supercooling mechanism in sodium sulfate decahydrate phase-change material

Abstract

Salt hydrate-based phase-change materials are considered promising for future heat storage applications in residential heating/cooling systems. Smooth phase transition from the liquid to solid phase and vice versa is essential for effective heat exchanger; however, supercooling in salt hydrates delays the onset of liquid–solid phase transition. We investigate the molecular level mechanism of supercooling in sodium sulfate decahydrate (SSD). SSD is a complex salt hydrate whose properties are governed by electrostatic forces that include pure Coulombic interactions as well as hydrogen bonds. Experimentally, we examine the importance of a nucleator in reducing supercooling temperatures. We investigated the effect of various mass concentrations of a borax nucleator on a decrease of supercooling temperatures. Molecular dynamics simulation techniques are used to obtain a basic understanding of supercooling in SSD. We observe that by introducing borax as a nucleator, there is a decrease in the supercooling temperature before nucleation. Our molecular dynamics simulations show that long-range electrostatics between sodium and sulfate ion pairs and that with polar water molecules is responsible for delayed nucleation in SSD that results in supercooling, and also, dynamics of charged molecules slows down. The lack of crystallization leads to amorphous structures in supercooled SSD.

Alginate–Sodium Sulfate Decahydrate Phase Change Composite with Extended Stability

“…The temperature changes of the chamber, reference, and PCM sample were measured from 15 to 40 °C and were recorded using a data acquisition (DAQ) unit (NI-9214, National Instruments, TX, USA). The melting temperature, crystallization temperature, energy storage capacity, and degree of supercooling of the sample were then analyzed from the temperature variation using a previously reported method …”

“…The melting temperature, crystallization temperature, energy storage capacity, and degree of supercooling of the sample were then analyzed from the temperature variation using a previously reported method. 44 The temperature profiles of the 6.5 wt % alginate/SSD sample during 50 heating and cooling (inset) cycles are presented in Figure 1b. It should be noted that T-history curves for samples with higher content of sodium alginate (8, 10, and 13 wt %) were not able to be measured due to temperature window limitations of the experimental setup.…”

Nanoscale Stabilization Mechanism of Sodium Sulfate Decahydrate at Polyelectrolyte Interfaces

“…For example, sodium tetraborate decahydrate (borax) was added to SSD in varying concentrations to reduce supercooling and enhance crystallization in a timely and consistent manner. 30 Despite extensive experimental research on SSD as PCM materials, the fundamental understanding of the effect of the polyelectrolyte additives on the properties of SSD remains unclear at the molecular level. Sankar Deepa and Tewari 31 investigated the phase transition behavior of SSD in the presence of NaCl using molecular dynamics (MD) simulations.…”

“…To solve this problem, several nucleating agents have been proposed to trigger the crystallization of SSD during the required period. For example, sodium tetraborate decahydrate (borax) was added to SSD in varying concentrations to reduce supercooling and enhance crystallization in a timely and consistent manner …”

Salt hydrates as phase change materials for photovoltaics thermal management

“…F I G U R E 1 0 Supercooling curves of Na 2 SO 4 •10H 2 O salt hydrates. (A), Low rate of nucleation and thermal diffusivity, (B) low rate of nucleation, but higher thermal diffusivity, and (C) restrained nucleation due to supercooling54 …”