Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 451

the Rankine cycle efficiency with respect to synthetic oil plants. In addition to this,
molten salts have excellent heat transfer properties, are non-flammable, non-toxic,
environmentally friendly and cheaper than other HTFs. Furthermore, using molten
salt in both the solar field and thermal energy storage system eliminates the need for
expensive heat exchangers and also allows for a substantial reduction in the cost of the
storage system. In fact, with respect to synthetic oil plants, the HTF temperature varia-
tion in the solar field can increase up to a factor of 2.5, reducing the physical size of the
storage system for a given capacity. The first example of a parabolic trough plant using
a molten salt mixture as HTF and thermal storage fluid is “Progetto Archimede'' in
Italy, based on ENEA technology collectors reaching 550 ◦ C (Manzolini et al., 2011a).
Regarding other CSP technologies, molten salts have also been employed in central
receiver plants with storage systems (Gil et al., 2010).
The main drawbacks of molten salts are corrosivity at high temperatures and,
more importantly, high freezing points (120 ◦ C-220 ◦ C depending on salt composition).
Corrosion issues can be easily solved by adopting stainless steel (AISI 316 or 321)
for tubes, piping and storage systems, together with other devices such as a layer of
nitrogen at atmospheric pressure in the upper part of storage tanks.
As far as high solidification is concerned, this should be avoided at all costs because
it can disrupt circuits and cause mechanical failures in the reverse process of liquefac-
tion. In fact, the specific volume of the mixture increases by about 5% when changing
from solid to liquid state. Therefore freeze protection methods should be adopted along
with specific operational and maintenance requirements.
Molten salt technology was first developed in the United States for central receiver
systems, thanks to the operation of the 10 MWe “Solar Two'' plant in Barstow,
California. In that plant, before filling the boiler with salt each morning, the receiver
was heated to approximately 290 ◦ C to reduce thermal stresses and to ensure that solid-
ification of salts did not take place inside the tubes. This pre-heating was achieved by
focusing a selected subset of the heliostat field onto the receiver so as to reach a uniform
temperature distribution both vertically and circumferentially.
For parabolic trough plants the technical issues are more challenging. During the
first start-up of the power plant a boiler or heater is necessary to obtain melting of
the HTF; at the same time pre-heating of the tubes and piping in the whole solar field
should prevent thermal stresses during plant filling. The solution is to employ extensive
heat-tracing equipment on piping and collector receivers (heating is obtained through
resistive Joule effect). The same operation can be used in instances of a restart of one
loop following a failure or for routine loop maintenance that requires HTF removal.
On the other hand, during operation the solar field cannot be drained and solidi-
fication must therefore be avoided. Freeze protection during night-time is achieved by
means of a low-flow circulation of hot salt in the solar field: a fossil-fuelled boiler can be
used for heating the salt or, alternatively, molten salts can be taken from thermal storage
tanks (from a cold tank in case of a two-tank storage system - see the following para-
graph). In this way, critical thermal gradients during start-up are prevented. Assuming
overnight heat losses of approximately 25 W/m 2 , a storage capacity of 1 hour is suit-
able for freeze protection operation, according to an annual performance calculation
(Kearney et al., 2003).
A direct comparison among the various types of molten salts suggests that solar
salt has the highest operating temperature limit: it can be used for temperatures up