(698a) Characteristics of Phase Changing Material Nanoemulsions

Phase change material (PCM) is an energy storage alternative that consists of heat storage performance due to latent heat capacity. In buildings, PCMs can be utilized to save energy in heating and cooling systems implementing it into the walls of the structure. However, an alternative is circulating the PCM throughout the building by firstly turning them into emulsions. The motive is to create nanoemulsions of these PCMs with the lowest possible viscosity to ensure pumpability while increasing the amount of PCM by mass to enhance larger latent heat storage capacity which can reduce energy cost. The concern with nanoemulsion PCMs is choosing the correct emulsifiers as well as phase changing material based on applicability, stability, and economical costs. Knowing the finer details of these substances can be a stepping stone to choosing the correct materials.

This study aims to analyze the thermal performance and molecular structures of these PCM nanoemulsions through using various surfactants, thermal cycling, and the effect of aging for a size distribution ranging from 100 to 400 nm acquired by Dynamics Light Scattering (Malvern Zetasizer ZS90). Through rheological properties and molecular structures by NMR spectroscopy on these PCM nanoemulsions we may see how these nanoemulsions evolve and how this can create differences in the heat transfer coefficients obtained in forced convection through a heated circular tube. Octadecane is used as the PCM in the dispersed phase, with the utilization of different surfactants: stearic acid and the Tween & Span families, within the continuous phase, water. The preliminary data acquired of octadecane at a volume fraction of 30% with Span 60 and Tween 60 emulsifiers (3 wt%) showed shear thinning effects with the presence of minute yield stresses observed at all temperatures (10-35^oC) indicating a viscoelastic region; evident by the exponentially decreasing slope of the viscosity profile with an increase in shear rate (0.01-1000 S^-1). The temperature dependency is undoubtedly visible as the viscosity decreases with increase temperature from measurements performed on 50mm parallel plate geometry with a peltier heating plate for the higher volume fractions of oil (octadecane), as well as through a cup-and-bob geometry utilizing the ARES G2 Tensiometer (TA instruments). Solution-state ¹H and ¹³C NMR measurements reveal molecular-level information on the local environments, compositions, and dynamics of the oil and surfactant species, which significantly depend on the thermodynamic state, age, and size distribution of the dispersed phase. Differential Scanning Calorimetry (TGA Q1500) measurements show the PCM nanoemulsion melting and freezing temperatures and how they change with thermal cycling and aging. A test section consisting of a 304 stainless steel tube heated uniformly and thermocouples installed at the inlet, outlet, and several on the tube wall to measure the temperature along the tube, the heat transfer coefficient can be acquired. This can indicate the possibility of using the PCM nanoemulsions as a secondary refrigerant for thermal energy storage and transport.