Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 456

heat can be completely recovered by the reverse reaction. Usually, catalysts are required
to control the reverse reaction. The main advantages of thermal storage with ther-
mochemical reversible reactions are the high density of storage and the possibility of
maintaining the energy stored at a temperature close to that of the environment. Major
issues are the complexity and the costs of the system itself and of materials, the effec-
tive control of the kinetics of reactions with different operating conditions and aspects
related to the properties of the components involved in the reactions (thermodynamic
properties, toxicity, flammability, etc.). This type of storage will be the object of study
and experimentation in the near future but at present it can be considered far from
commercialization.
14.4.2.2 Integration schemes between storage and power plant
There are different ways in which TES systems can be integrated into a solar plant.
These depend on the storage medium, heat transfer fluid in the solar field and the type
of process coupled to the solar plant. For simplicity, in the following we refer to the
case in which the process is a power plant producing electricity.
The integration of TES can use a direct or indirect system. In a direct system, the
heat transfer fluid is also used as the storage medium, while in an indirect system a
second medium is used for storing the heat.
A direct storage system is composed of a plant configuration with two tanks where
the HTF produced by the solar field is directly stored in a hot tank. The cooled HTF is
pumped to the other, cold tank where it remains until the solar field starts to operate.
The adoption of the very same fluid in the solar field and in storage tanks implies that
it must have, at the same time, the characteristics of a good HTF and good storage
medium. The use of molten salts as an HTF and storage medium allows the solar
field to be operated at temperatures higher than current heat transfer fluids such as
synthetic oil. This configuration also allows a substantial reduction in the costs of
TES systems, owing to the elimination of expensive heat exchangers, improving the
performance of the plant (no temperature difference due to the heat exchange) and
reducing the LCOE. Moreover it is worth noting that the maximum temperature of
HTF can be increased with respect to synthetic oil thanks to the higher thermal stability
of salts. Some complications, in the case of molten salts, are due to the high freezing
point (from 120 ◦ Cto240 ◦ C depending on the type of salt used, see also Table 14.4.1)
and this means that special care has to be taken to avoid the salt freezing in the solar
field. Hence, freeze protection operations must be undertaken, increasing O&M costs.
Figure 14.4.1 describes the layout of the Solar Tres plant, a solar tower plant which
uses molten salts (NaNO 3 and KNO 3 ) as HTF (Gil et al., 2010).
In an indirect storage system , a second fluid is used for storing the thermal power
produced by the solar field. Within this configuration, both the two-tank and single-
tank systems (the thermocline system, as stated earlier) are the adoptable solutions.
The main disadvantages of this configuration relate to the presence of heat exchangers,
meaning additional cost, heat transfer irreversibility and the temperature limitation of
one of the fluids, usually the one circulating in the solar field.
Figure 14.4.2 depicts a power plant based on the indirect two-tank storage system ,
in which the heat transfer fluid which circulates in the solar field is different from the
storage medium. Here, the energy is stored by another medium in the TES (generally