159
4 Additives to Molten Salts
Different methods have been utilised to improve the performance of molten salts in
the thermal energy storage system. For example, heat transfer characteristics of a
binary nitrate mixture were substantially enhanced by incorporating it into a skeletal structure consisting of metal foams [32, 33]. The melting temperature of a ternary LiNaK carbonate mixture was reduced by dispersing 10 wt.% hydroxides [34].
Compared with macroscale particulates, there is an upsurge of interest in singlecomponent nanomaterials of carbon (nanotubes, nanofibres, graphenes), metals
(magnesium, copper), metal oxides (SiO 2 , Al 2 O 3 , CuO), or other predictable
compounds as additives in the molten salts for effective enhancement of the specific
heat capacity. For example, a maximum increase of 15% was obtained by doping
1 wt.% of SiO 2 nanoparticles in the eutectic mixture of Li 2 CO 3 –K 2 CO 3
(62.0–38.0 mol.%) [20], whilst addition of 1.5 wt.% graphene in the same eutectic
salt led to an enhancement up to 18.57% [21]. Besides, composite nanomaterials
have also been investigated. Tiznobaik et al. [29] obtained an enhancement of 57%
in the solid phase and 22% in liquid phase by dispersing 1.0 wt.% of SiO 2 –Al 2 O 3 in
the eutectic mixture NaNO 3 –KNO 3 (40–60 wt.%). The specific heat capacity is not
only affected by the type of nanomaterials but also other factors such as the
concentration and particle size. For composite nanomaterials, more factors such as
elemental composition, morphology, surface effect and density [3] could impact on,
and bring more variations to the result.
5 Specific Heat Capacity of Molten Salts with Nanomaterials
In contrast with water/organic liquid based nanofluids for which the specific heat
capacity is lower than that of pure water and decreases gradually with the increasing
volume concentration of nanoparticles [35, 36], nanomaterial-doped molten salts
perform in a distinct way. Table 2 summarises the measured specific heat capacity
of some typical eutectic mixtures of salts with nanomaterials compared to their base
salts in solid and liquid phases. It can be seen from Table 2 that the addition of
certain nanomaterials has a positive influence on the specific heat capacity of
eutectic salts in both solid and liquid phases, albeit with a few exceptions.
In the situation of an identified composition of the eutectic mixture of salts, different nanomaterials exhibited varied enhancement capabilities. In addition, discrepancies in the enhancement of specific heat capacity were found between
different eutectic salts with the same type of nanomaterial.
By analogy with many physical properties, a conventional model based on the
thermal equilibrium has been originally proposed to predict the specific heat capacity of molten salts with nanomaterials which is expressed as Eq. (3) [37],
Nanomaterials Enhanced Heat Storage in Molten Salts
4 Additives to Molten Salts
Different methods have been utilised to improve the performance of molten salts in
the thermal energy storage system. For example, heat transfer characteristics of a
binary nitrate mixture were substantially enhanced by incorporating it into a skeletal structure consisting of metal foams [32, 33]. The melting temperature of a ternary LiNaK carbonate mixture was reduced by dispersing 10 wt.% hydroxides [34].
Compared with macroscale particulates, there is an upsurge of interest in singlecomponent nanomaterials of carbon (nanotubes, nanofibres, graphenes), metals
(magnesium, copper), metal oxides (SiO 2 , Al 2 O 3 , CuO), or other predictable
compounds as additives in the molten salts for effective enhancement of the specific
heat capacity. For example, a maximum increase of 15% was obtained by doping
1 wt.% of SiO 2 nanoparticles in the eutectic mixture of Li 2 CO 3 –K 2 CO 3
(62.0–38.0 mol.%) [20], whilst addition of 1.5 wt.% graphene in the same eutectic
salt led to an enhancement up to 18.57% [21]. Besides, composite nanomaterials
have also been investigated. Tiznobaik et al. [29] obtained an enhancement of 57%
in the solid phase and 22% in liquid phase by dispersing 1.0 wt.% of SiO 2 –Al 2 O 3 in
the eutectic mixture NaNO 3 –KNO 3 (40–60 wt.%). The specific heat capacity is not
only affected by the type of nanomaterials but also other factors such as the
concentration and particle size. For composite nanomaterials, more factors such as
elemental composition, morphology, surface effect and density [3] could impact on,
and bring more variations to the result.
5 Specific Heat Capacity of Molten Salts with Nanomaterials
In contrast with water/organic liquid based nanofluids for which the specific heat
capacity is lower than that of pure water and decreases gradually with the increasing
volume concentration of nanoparticles [35, 36], nanomaterial-doped molten salts
perform in a distinct way. Table 2 summarises the measured specific heat capacity
of some typical eutectic mixtures of salts with nanomaterials compared to their base
salts in solid and liquid phases. It can be seen from Table 2 that the addition of
certain nanomaterials has a positive influence on the specific heat capacity of
eutectic salts in both solid and liquid phases, albeit with a few exceptions.
In the situation of an identified composition of the eutectic mixture of salts, different nanomaterials exhibited varied enhancement capabilities. In addition, discrepancies in the enhancement of specific heat capacity were found between
different eutectic salts with the same type of nanomaterial.
By analogy with many physical properties, a conventional model based on the
thermal equilibrium has been originally proposed to predict the specific heat capacity of molten salts with nanomaterials which is expressed as Eq. (3) [37],
Nanomaterials Enhanced Heat Storage in Molten Salts