Developing novel high-performance polyethylene-embedded phase change emulsions with minimal supercooling for efficient thermal energy storage

Abstract

Phase change emulsions are promising thermal fluids, but have been frustrated by high degrees of supercooling. The existing methods for supercooling reduction involve the addition of various nucleating agents (NAs) but have achieved limited success due to the lack of commonality and the negative impact on emulsion stability. This study explored a strategy by integrating interface-induced and NA-induced nucleation. A diblock copolymer was positioned at the interface and a new NA, polyethylene, was efficiently embedded into the droplets assisted by ultrasonication to minimize supercooling and extend stability. Firstly, hexadecane emulsion was chosen for assessment of the chemical compositions and ultrasonication conditions. A higher ultrasound intensity facilitated the polyethylene embedment, providing sufficient nucleation sites within the droplets with a low mass content (1 %). The functionality of polyethylene was found to be size-dependent, resulting in a single melting/freezing peak and a low supercooling degree (ΔT) of 3.7 ℃ for hexadecane droplets of 256.3 nm average diameter. Subsequently, the effectiveness of polyethylene was evaluated on other seven paraffin emulsions with higher melting points (Tm), achieving minimal supercooling (ΔT = 0.2–2.6 ℃). Furthermore, dynamic stability was evaluated by applying an emulsion template (Tm ≈ 54 ℃) to a rig for long period and repeated thermal cycles (6 months and 9000 cycles). The average diameter increased marginally from 374.1 to 393.4 nm, while the ΔT remained stable at 2.5 ℃, and the latent heat slightly decreased. No obvious sign of emulsion breakdown was observed during an additional six-month storage period. An effective strategy has been developed for formulating a range of novel phase change emulsions with extended service life and minimal supercooling which are desirable for thermal energy storage applications.