Experimental study of cyclically stable Glauber's salt-based PCM for cold thermal energy storage

Implementing a Thermal Energy Storage (TES) system in a data center has several advantages. They reduce energy consumption, improve resiliency in emergency conditions and reduce the carbon footprint. Organic phase change materials (PCMs) are widely used in TES devices for data centers due to their ease of implementation, high cyclic stability, and low supercooling. However, organic PCMs have a low thermal conductivity of
$\sim 0.1$
W/m-K, limiting the thermal performance of the TES. On the other hand, salt hydrates, a subclass of inorganic PCMs, generally have high energy storage density and thermal conductivity and are inexpensive compared to their organic counterparts. Therefore, they are a lucrative choice of PCM materials for low-cost, compact latent heat thermal energy storage (LHTES) devices for both electronics and data center cooling applications. However, salt hydrates suffer from issues such as supercooling and phase segregation. Sodium Sulphate Decahydrate (SSD), also known as Glauber's salt, is one of the most inexpensive salt hydrates PCMs, melting at around 32°C. For applications such as peak load shifting for data center cooling, the PCM melting point requirement could be between 20–30 °C. The SSD's melting point could be tuned by adding a melting point suppression agent/s. NaCl and KCl are the most popular melting point suppression agents used with SSD in the literature. While supercooling could be solved by adding Borax (a nucleating agent), phase segregation of SSD remains a critical issue leading to degradation in latent heat with its usage, limiting its useful life. Adding thickener increases the viscosity of the PCM and is the most popular way to minimize latent heat degradation during thermal cycling. Several thickeners were explored in the literature with SSD, but often without melting point suppression agents. This study investigated the thermophysical properties and short-term cyclic stability of two different melting point-suppressed SSD PCMs with different thickeners in each PCM. The target melting point range of the PCM is 20–25 °C, which is suitable for application in TES at data centers.