Thermal Storage
As described by Gil et al [6] there are three types of Thermal Energy Storage (TES) systems, depending on whether they use sensible, latent or chemical heat.
Sensible heat thermal storage is achieved by heating the storage medium (liquid sodium, molten salt or pressurised water) and increasing its energy content but not changing state during accumulation. Energy is released and absorbed by the medium as its temperature reduces and increases respectively. Sensible heat can be stored in either solid media (in packed beds, e.g. concrete, requiring a fluid to exchange heat) or in liquid media such as molten salt or pressurised water.
On the other hand, latent heat is associated with changes of phase. Energy required during charging is used to convert a solid material in a liquid material (such as paraffin wax), or a liquid material in to a gas. Phase change materials have the benefit of high thermal capacity but have the drawback of degrading performance after a number of freeze-melt cycles. In order to use latent heat storage, the storage material should have a melting temperature within the range of the charging and discharging temperatures of the Heat Transfer Fluid (HTF). As thermal storage has not been previously investigated for an industrial application, this limits the availability of suitable latent heat storage systems.
Similarly, endothermic chemical reactions require a specific temperature at which a chemical product is dissociated in a reversible chemical reaction and heat is retrieved when the synthesis reaction takes place. The development of such reactions is already at a very early stage and as the reaction temperature should lie within the charging and discharging temperature of the HTF, therefore the use of such technology needs to be case specific.
Hence, out of the options considered, it was most viable to use sensible heat storage, such as pressurised water and molten salt.
Pressurised Water
In the case where steam is being used as a working fluid, excess steam can be stored in a steam accumulator as pressurised water. Direct storage of steam gives the benefit of fast reaction times and high discharge rates. [7] Excess steam is stored in a pressurised vessel with a mass of water inside (shown in Figure 1), the capacity of which is limited by the volume of the pressure vessel [8].
During charging, the temperature of liquid water increases, which increases the overall pressure of the accumulator and condenses the superheated steam introduced to the vessel [9]. The pressure will keep on increasing (reducing the injector’s capacity) until it equalises with the boiler’s pressure [10].
During discharging, saturated steam is produced by lowering the pressure of the saturated water and evaporating it adiabatically [8].
Medrano et al [7], states that steam accumulators provide saturated steam on discharge. If superheated steam is needed, a second storage system must be connected to the exit of the steam accumulator (shown in Figure 2). This concept of a hybrid storage system is of direct steam storage connected with an indirect storage using heat exchangers. Most indicated concepts suitable for a secondary storage system are concrete and molten salt. Concrete requires a higher temperature to store energy (min. 200oC) [6] in comparison to molten salt.
Figure 1. Pressure vessel for storage [7]
Molten Salt
Molten salt can be used as a secondary storage, using heat exchangers for charging and discharging the medium, while using steam as the HTF. According to Gil et al [6], molten salt has the benefits of high volume specific thermal capacity, is readily available and is relatively cheap but it has the drawback of a high freezing temperature (120-220oC). This means that special care must be taken to ensure that the salt does not freeze (solidifies). Routine freeze protection operation increases maintenance and operational costs.
Extensive research is being done on finding different combinations of salt mixtures that have low melting temperatures. Researchers at Sandia Laboratories [11] in France have developed a salt mixture with the lowest melting temperature of 72oC. It is formulated to contain approx. 30 mol. % of Lithium ion, 50 mol. % of Potassium ion and 20 mol. % of Sodium ion with a 0.56 ratio of nitrate/nitrite. Although this salt has the benefit of lower melting temperature, it becomes thermally unstable at higher temperatures as the nitrite anions begin to oxidise.
Another research by the Sandia team has shown promising results with using HitecXL which is a salt mixture composed of 45 wt% calcium nitrate, 11 wt% sodium nitrate, and 44 wt% potassium nitrate [12]. Experimental results have shown it to have a freeze temperature of 120oC, however comprehensive studies are required before recommending it for commercial use.
Figure 2. Additional hybrid storage system [7]