Molten Salt Power Towers Operating at 600-650 degrees C: Salt Selection and Cost Benefits

This analysis examines the potential benefit of adopting the supercritical carbon dioxide (sCO₂) Brayton cycle at 600–650 °C compared to the current state-of-the-art power tower operating a steam-Rankine cycle with solar salt at approximately 574 °C. The analysis compares a molten-salt power tower configuration using direct storage of solar salt (60:40 wt% sodium nitrate: potassium nitrate) or single-component nitrate salts at 600 °C or alternative carbonate- or chloride-based salts at 650 °C. The increase in power cycle efficiency offered by the sCO₂ Brayton cycle is expected to reduce the size and cost of the solar field required for a given thermal energy input. Power cycle capital cost is expected to decrease compared to the superheated steam-Rankine cycle, based on projections from sCO₂ cycle developers. Maximizing the ΔT of the storage system is required for viable deployment of sensible-salt TES. In this regard, the partial-cooling sCO₂ cycle is noted as a better option than the recompression sCO₂ cycle. In the current analysis it is assumed that a ΔT = 180 K can be achieved with the partial-cooling cycle. Even with ΔT = 180 K, the potential benefits of the sCO₂ Brayton cycle are partially or completely offset by increased thermal storage cost, albeit for reasons that differ for the different salts. An approximate 5% reduction in levelized cost of energy (LCOE) is achieved with either solar salt at 600 °C or ternary magnesium chloride salt at 650 °C. The potential of using pure sodium nitrate or potassium nitrate is considered because the cold tank temperature for the sCO₂ power cycle is estimated at 420 °C, which would allow use of a salt with a higher melting point than solar salt. Sodium nitrate is the most cost effective, resulting in an overall LCOE reduction of 8.5%; however, sodium nitrate is known to have lower thermal stability than potassium nitrate. The strong influence of salt cost and hot-tank cost on overall economics led to the analysis of single-tank thermocline options. The thermocline design significantly reduces salt inventory (by 50% or more) and in many cases also reduces the tank size versus the hot salt tank of the 2-tank system. It is speculated that integration of encapsulated phase-change material (PCM) in the thermocline could further increase the thermal-storage energy density and reduce storage tank volume. The thermocline cases led to three scenarios with relative LCOE reductions of approximately 10%; however, this must be tempered by possible operational inefficiencies of the thermocline temperature profile.