Experimental study of screening polyols and their binary eutectic phase change materials for long-term thermal energy storage

Abstract

Polyols are appealing long-term thermal energy storage materials due to their severe supercooling properties and high energy density. However, spontaneous crystallization during storage and slow crystallization during crystallization hinder their reliability and applicability. In this article, two single polyols (xylitol and sorbitol) and nine binary eutectic mixtures, prepared by a method based on Gibbs excess energy, were screened as long-term thermal energy storage phase change materials. The results conclusively demonstrated that xylitol and its binary eutectic mixtures (xylitol + inositol, xylitol + mannitol, and xylitol + dulitol, with a 150 g bulk scale) could be stored at room temperature in isolation from the air for more than 300 days without any crystallization. However, other samples either transformed into a glassy state (xylitol + sorbitol and sorbitol + erythritol) or crystallized (sorbitol, xylitol + erythritol, sorbitol + inositol, sorbitol + mannitol, and sorbitol + dulcitol). The exothermic performance test showed that three additives (xylitol particles, ethanol, and acetone) could effectively trigger the crystallization of xylitol and its binary eutectic mixtures after reheating to an appropriate temperature before or after storage. This process excludes sorbitol and its binary eutectic mixtures. Simultaneously, the exothermic performance test also indicated that xylitol, xylitol + erythritol, xylitol + inositol, xylitol + mannitol, and xylitol + dulcitol had attractive crystallization heat (164.71, 165.56, 130.58, 145.56, and 147.89 kJ/kg), high maximum crystallization temperatures (84.94, 73.75, 83.85, 83.52, and 83.13 °C), short times to reach maximum crystallization temperatures (186, 477, 204, 198, and 183 s), and satisfactory energy efficiency (57.77%, 56.68%, 51.01%, 54.52%, and 54.84%). After 300 days of storage, the crystallization heat of the supercooled samples decreased by merely 4.377%, 3.358%, 4.511%, 4.314%, and 6.309%. Finally, 30 cyclic tests were conducted on newly prepared xylitol and xylitol + erythritol. The pair exhibited relatively stable crystallization heat (145.60–175.64 kJ/kg and 134.52–169.97 kJ/kg, respectively) and high maximum crystallization temperature (82.8–88.1 °C and 72.3–76.2 °C, respectively).