Abstract
The effect of a variety of metal-chlorides additions on the melting behavior and thermal stability of commercially available salts was investigated. Ternary salts comprised of KNO3, NaNO2, and NaNO3 were produced with additions of a variety of chlorides (KCl, LiCl, CaCl2, ZnCl2, NaCl and MgCl2). Thermogravimetric analysis and weight loss experiments showed that the quaternary salt containing a 5 wt% addition of LiCl and KCl led to an increase in short term thermal stability compared to the ternary control salts. These additions allowed the salts to remain stable up to a temperature of 630 °C. Long term weight loss experiments showed an upper stability increase of 50 °C. A 5 wt% LiCl addition resulted in a weight loss of only 25% after 30 hours in comparison to a 61% loss for control ternary salts. Calorimetry showed that LiCl additions allow partial melting at 80 °C, in comparison to the 142 °C of ternary salts. This drop in melting point, combined with increased stability, provided a molten working range increase of almost 100 °C in total, in comparison to the control ternary salts. XRD analysis showed the oxidation effect of decomposing salts and the additional phase created with LiCl additions to allow melting point changes to occur.
Highlights
The potential of solar energy as a sustainable and affordable power source has resulted in a great deal of worldwide interest
It was seen that the additions of Magnesium Chloride (MgCl2) and Zinc Chloride (ZnCl2) both result in a reduction in the heat flow comparted to the ternary salt, as seen by the significant decrease in intensity
The melting point of pure LiCl is over 600 °C whilst LiNO3 is over 250 °C, so it can be assumed a new phase is being formed with the current eutectic
Summary
The potential of solar energy as a sustainable and affordable power source has resulted in a great deal of worldwide interest. Within Concentrated Solar Power (CSP) applications molten salts can be used as Heat Transfer Fluids (HTF) and/or as for Thermal Energy Storage (TES). HITECs upper operating temperatures have been reduced from the Solar Salts 600 °C to 454 °C for long term (and 538 °C for short term behaviour), giving a long term working range of 312 °C Additions to this ternary mixture have been used to improve the stability somewhat, but little stability above 500 °C has been observed. Materials such as Lithium Nitrate (LiNO3), NaNO3 and KNO3 have eutectic temperature of 120 °C and below[5] These eutectics and the result of other works[1] show that the melting temperatures of quaternary salts with cations of Ca, K, Li and Na may be reduced to below 100 °C, allowing improved salt performance. Lower melting temperatures may allow an increased temperature differential, allowing a greater operating range and increasing the energy stored per kg[7]
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