Charge State Sensor for Thermal Energy Storages Based on Phase Change Slurries
Abstract
The present paper describes different techniques that can be used to determine the charge state of a phase change slurry. The functional fluid used is a surfactant stabilised paraffin-in-water dispersion containing different amounts of phase change material. Density, nuclear magnetic resonance (NMR) relaxometric properties and ultrasonic velocity are measured and evaluated. Results from heating the dispersion at a constant gradient of 0.25 Kmin-1 from 5 °C to 35 °C corresponds to the peak melting rate identified by differential scanning calorimetry. The employed sensors are part of a test rig made from standard components and data is acquired from three experimental runs. Thermal expansion of the paraffin is derived from measured data and is linked to its state of aggregation. The T2 NMR relaxation spectrum of protons within the fluid is used to estimate the amount of solid-, melting- and liquid material inside the wax droplets. Though theoretically suitable, the speed of ultrasound can hardly be determined with the used standard equipment. Its application is limited to dilute dispersions.