Specific heat enhancement of nano- engineered eutectic nitrate salts for thermal energy storage

Abstract

In the pursuit of affordable, reliable, environmentally-friendly, renewable electricity, concentrated solar power (CSP) plants with integrated thermal energy storage (TES) have shown great potential. Compared to other solar technologies, the TES component represents THE key that enables: i) generation to better match demand profiles, and ii) maximum revenue by selling at price peaks. Nitrate molten salts represent the dominant thermal storage medium to date, but their low specific heat value is a major disadvantage. Thus, one of the most direct ways to increase the competitiveness of CSP is to increase the specific heat of molten salts. This can enable a sizable reduction in storage volume (lower costs), or, with the same storage volume, enable more energy storage (better demand/price matching). Researchers have shown that the specific heat of molten salts can be altered through the addition of nanoparticles (termed nanosalts), but the magnitude, and even direction of the alteration, has been debated in the literature. In fact, experimental studies have regularly shown changes of +/-30% in the specific heat of nanosalts. This thesis investigates the influence of various nanoparticle morphologies and preparation methods on the specific heat of nanosalts, critical initial conditions that have not been explored or explained in prior studies. Experimental results confirmed that specific heat enhancement of 17% are possible for well-prepared nanosalts, while improperly prepared samples showed similar degradations as observed in the literature. These results were found to agree will with the compressed phase theory which states than salts undergo a phase change near the particles, adding to the change predicted by effective medium theories. After studying nanosalts at the nano/micro-scale, this thesis also conducts a macro-scale economic sensitivity analysis to identify the potential viability of pursuing these specific heat enhancements. It was shown that in order for nanosalts to be economically feasible, the premium paid to produce the nanosalt (with ~30% enhancement) cannot exceed US$1.4/kg (compared to base salt price of US$1/kg). Ultimately, this study adds to the fundamental-level understanding of nanoparticle-based specific heat enhancements and determines the system-level constraints which, if met, could lead their uptake.