High-Temperature Liquid-Fluoride-Salt Closed-Brayton-Cycle Solar Power Towers

Abstract

Liquid-fluoride-salt heat transfer fluids are proposed to raise the heat-to-electricity efficiencies of solar power towers to about 50%. The liquid salt would deliver heat from the solar furnace at temperatures between 700°C and 850°C to a closed multireheat Brayton power cycle using nitrogen or helium as the working fluid. During the daytime, hot salt may also be used to heat graphite, which would then be used as a heat storage medium to make night-time operations possible. Graphite is a low-cost high-heat-capacity solid that is chemically compatible with liquid fluoride salts at high temperatures. About half the cost of a solar power tower is associated with the mirrors that focus light on the receiver, and less than one-third is associated with the power cycle and heat storage. Consequently, increasing the efficiency by 20–30% has the potential for major reductions in the cost of electricity. Peak temperatures and efficiencies of current designs of power towers are restricted by (1) the use of liquid nitrate salts that decompose at high temperatures and (2) steam cycles in which corrosion limits peak temperature. The liquid-fluoride-salt technology and closed Brayton power cycles are being developed for high-temperature nuclear reactors. These developments may provide the technology and industrial basis for an advanced solar power tower.

Thermodynamic investigation of the (LiF+NaF+CaF2+LaF3) system

“…The MSR belongs to the Generation IV Initiative and several fuel concepts based on fluoride systems exist for this reactor type. Moreover, it was also proposed to use fluoride salts for the energy storage and transfer in solar power towers [1].…”

Materials corrosion in molten LiF–NaF–KF salt

“…Molten fluoride salts have been proposed for use as primary reactor coolants, media for transfer of high temperature process heat from nuclear reactors to hydrogen production facilities [1], for solar thermal energy storage [2], and as a means to raise heat-toelectricity conversion efficiencies of solar power towers to about 50% [3]. These salts are characterized by high thermal conductivities, high specific heats, low viscosities, and high boiling points.…”

The potential role of concentrated solar power (CSP) in Africa and Europe—A dynamic assessment of technology development, cost development and life cycle inventories until 2050

“…Those technologies are selected which seem to be the most cost effective ones in the different scenarios. Nevertheless, we acknowledge that there are some other new and promising developments (for example the utilisation of molten salts as primary fluid, Forsberg et al, 2007) that could lead to technologies capable to supplement or possibly outperform the technologies selected here.…”