157
• Nitrates/nitrites
The so-called solar salt is the binary nitrate eutectic of NaNO 3 –KNO 3
(60–40 wt.%) [9–12]. It has been commercially utilised as the TES medium in
the exiting CSP plants. The melting point (m.p.) of ‘solar salt’ is approximately
220 °C, and the practical operating temperature range is from 288 to 565 °C for
sensible heat storage. Apart from the ‘solar salt’, other salt mixtures have been
well applied, such as HITEC (7 wt.% NaNO 3 –53 wt.% KNO 3 –40 wt.% NaNO 2 ,
m.p. = 120 °C) and HITEC XL (48 wt.% Ca(NO 3 ) 2 –7 wt.% NaNO 3 –45 wt.%
KNO 3 , m.p. = 142 °C) with a temperature limit of over 500 °C [13, 14]. In addition, studies have been carried out to decrease the freezing temperature by adding LiNO 3 in the nitrate/nitrite salt systems such as quaternary eutectic mixtures
of LiNO 3 –NaNO 3 –KNO 3 –Ca(NO 3 ) 2 [15] and LiNO 3 –NaNO 3 –KNO 3 –NaNO 2
[16] with a melting temperature of 132 °C and 94.3 °C, and maximum operating
temperature of 553 °C and 425 °C, respectively.
• Carbonates
Carbonate salts are also considered good candidates for TES media due to their
relatively higher specific heat capacity and thermal stability. Since the melting
points of pure carbonate salts are much higher than those of nitrate/nitrite salts,
the eutectic carbonates are more likely to be utilised in medium to high temperature TES systems. Binary eutectic carbonates of different composition ratios
have been investigated by researchers [17–19]. A eutectic of Li 2 CO 3 –K 2 CO 3
(62–38 mol.%) was intensively reported in literature [18, 20–22] with the melting point of approximately 500 °C. The investigation of ternary eutectic carbonates has been focused on the LiNaK carbonate system [19, 23, 24], which has a
lower melting point of approximately 400 °C. It has been reported that the LiNaK
carbonates are thermally stable below 800 °C in the Ar atmosphere [25] and up
to 1000 °C at the atmosphere of CO 2 [26].
• Chlorides
Chloride salts also have good potentials in serving as TES media, although they
have not received much attention at present. There are several chlorides which
may be considered as TES media including chlorides of alkali metals, alkaline
earth metals, transition metals, p-block metals and rare earth metals.
Thermophysical properties and estimated costs of a variety of eutectic chlorides
have been reported [27].
It is worth noting that some ternary chloride mixtures exhibit multiple eutectic
(invariant) compositions which show large discrepancies in melting (invariant)
point and heat of fusion, depending mainly on the different cations selected, but
less on the actual compositions in the respective eutectics. For example, the three
eutectic mixtures of NaCl–KCl–ZnCl 2 of different compositions melt completely
at temperatures below 250 °C, whilst the three eutectic mixtures of NaCl–KCl–
MgCl 2 of different compositions melt completely before 450 °C [28].
Nanomaterials Enhanced Heat Storage in Molten Salts
• Nitrates/nitrites
The so-called solar salt is the binary nitrate eutectic of NaNO 3 –KNO 3
(60–40 wt.%) [9–12]. It has been commercially utilised as the TES medium in
the exiting CSP plants. The melting point (m.p.) of ‘solar salt’ is approximately
220 °C, and the practical operating temperature range is from 288 to 565 °C for
sensible heat storage. Apart from the ‘solar salt’, other salt mixtures have been
well applied, such as HITEC (7 wt.% NaNO 3 –53 wt.% KNO 3 –40 wt.% NaNO 2 ,
m.p. = 120 °C) and HITEC XL (48 wt.% Ca(NO 3 ) 2 –7 wt.% NaNO 3 –45 wt.%
KNO 3 , m.p. = 142 °C) with a temperature limit of over 500 °C [13, 14]. In addition, studies have been carried out to decrease the freezing temperature by adding LiNO 3 in the nitrate/nitrite salt systems such as quaternary eutectic mixtures
of LiNO 3 –NaNO 3 –KNO 3 –Ca(NO 3 ) 2 [15] and LiNO 3 –NaNO 3 –KNO 3 –NaNO 2
[16] with a melting temperature of 132 °C and 94.3 °C, and maximum operating
temperature of 553 °C and 425 °C, respectively.
• Carbonates
Carbonate salts are also considered good candidates for TES media due to their
relatively higher specific heat capacity and thermal stability. Since the melting
points of pure carbonate salts are much higher than those of nitrate/nitrite salts,
the eutectic carbonates are more likely to be utilised in medium to high temperature TES systems. Binary eutectic carbonates of different composition ratios
have been investigated by researchers [17–19]. A eutectic of Li 2 CO 3 –K 2 CO 3
(62–38 mol.%) was intensively reported in literature [18, 20–22] with the melting point of approximately 500 °C. The investigation of ternary eutectic carbonates has been focused on the LiNaK carbonate system [19, 23, 24], which has a
lower melting point of approximately 400 °C. It has been reported that the LiNaK
carbonates are thermally stable below 800 °C in the Ar atmosphere [25] and up
to 1000 °C at the atmosphere of CO 2 [26].
• Chlorides
Chloride salts also have good potentials in serving as TES media, although they
have not received much attention at present. There are several chlorides which
may be considered as TES media including chlorides of alkali metals, alkaline
earth metals, transition metals, p-block metals and rare earth metals.
Thermophysical properties and estimated costs of a variety of eutectic chlorides
have been reported [27].
It is worth noting that some ternary chloride mixtures exhibit multiple eutectic
(invariant) compositions which show large discrepancies in melting (invariant)
point and heat of fusion, depending mainly on the different cations selected, but
less on the actual compositions in the respective eutectics. For example, the three
eutectic mixtures of NaCl–KCl–ZnCl 2 of different compositions melt completely
at temperatures below 250 °C, whilst the three eutectic mixtures of NaCl–KCl–
MgCl 2 of different compositions melt completely before 450 °C [28].
Nanomaterials Enhanced Heat Storage in Molten Salts