Thermal properties enforcement of carbonate ternary via lithium fluoride: A heat transfer fluid for concentrating solar power systems

- 15 wt% LiF is introduced to improve properties of Li2CO3-Na2CO3-K2CO3 based HTFs.
- Melting point of 368 °C and upper temperature of 753 °C allow wide working range.
- Specific heat of liquid LiF-Li2CO3-Na2CO3-K2CO3 is approximately 1.917 J/(g·°C).
- LiF-Li2CO3-Na2CO3-K2CO3 has thermal conductivity of 1.021 W/(m·°C) at 320 °C.
- XRD indicates physical mixture and crystal doping appearance in the fabrication.

A novel eutectic of Li2CO3-Na2CO3-K2CO3 improved by LiF, when employed as a heat transfer fluid in concentrating solar power systems, is prepared to eliminate the disadvantage of limited operating temperature range and low specific heat as well as thermal conductivity. Using the static melting method, Li2CO3-Na2CO3-K2CO3 with 15.0 wt.% LiF which is simply physical mixture determined by X-ray diffraction, is chosen as the optimized candidate. Results indicate that, at 10 °C/min of heating rate, LiF-Li2CO3-Na2CO3-K2CO3 reached melting point of 368 °C and upper temperature limit of 753 °C, allowing it to be effectively employed in thermal storage and heat transfer in high temperature CSP systems. During temperature range from 400 to 600 °C, the liquid quaternary exhibited notably mean specific heat of 1.917 J/(g·°C), which is larger than that of the original ternary eutectic. Thermal conductivity of solid LiF-Li2CO3-Na2CO3-K2CO3 reached the stable level of 1.021 W/(m·°C) at 320 °C. Density of improved eutectic tested by the archimedean principle is approximately 1.95 g/cm3. In conclusion, LiF-Li2CO3-Na2CO3-K2CO3 can be employed as the admirable heat transfer fluid for thermal storage and heat transfer in high-temperature CSP plants. This investigation will provide useful information for further improvement of molten salt based heat transfer fluids.

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