THERMAL ENERGY STORAGE WITH ENCAPSULATED PHASE CHANGE MATERIALS FOR CONCENTRATING SOLAR PLANT APPLICATIONS

摘要

The objective of this research was to develop a storage technology for thermal energy, applicable for use in concentrating solar plants. Latent heat storage, utilizing the heat of solid-liquid phase change to complement sensible heat storage was the approach chosen. In this research, phase change materials (PCMs) with melting temperatures above 300℃ and sufficiently large latent heats of fusion were selected as the storage medium. The PCM was then encapsulated into capsules with optimized dimensions to achieve desirable heat transfer rates. A pilot-scale thermocline with multiple PCM capsules was built and tested, in order to determine its performance in energy storage and retrieval during multiple thermal cycles. A simulation model was also developed for the thermocline, to compare experimental performance with theoretical expectations. The thermocline was first qualified using solid copper capsules (no phase change) to verify the experimental methodology. Results showed satisfaction of energy balance within 98%, and agreement between experimental measurements and model predictions for stored energy within 97%, as illustrated in figure below. The thermocline was then reloaded with capsules containing NaNO_3 as the PCM, with its phase change temperature of 308℃ and latent heat of 176 kJ/kg. Repeated thermal cycles between ambient and 440℃ temperatures demonstrated the ability of the encapsulated PCM to store and discharge thermal energy with no perceptible deterioration over multiple cycles. Agreement within 8% was obtained between model predictions and experimental performance. Based on these results, it was determined that cyclical operation of these EPCMs with a temperature swing between 250℃ and 350℃ would achieve a storage capacity of 184 kJ/kg of the PCM, with 41 % of this amount contributed by latent heat of phase change.