Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 449

Table 14.4.1 Characteristics of the nitrate salts and most used synthetic oil (Kearney et al., 2003).

Hitec XL

Solar (Calcium LiNO 3 Therminol

Property Salt Hitec Nitrate Salt) mixture VP-1

Composition, % - - - - biphenyl/diphenyl

oxide

NaNO 3 60 7 7 - -

KNO 3 40 53 45 - -

NaNO 2 - 40 - - -

Ca(NO 3 ) 2 - - 48 - -

Freezing point, ◦ C 220 142 120 120 13

Upper temperature, ◦ C 600 535 500 550 400

Density @300 ◦ C, kg/m 3 1899 1640 1992 - 815

Viscosity @300 ◦ C, cp 3.26 3.16 6.37 - 0.2

Heat capacity@300 ◦ C, J/kgK 1495 1560 1447 - 2319

through the receiver tubes, increasing its temperature. The HTF is then used to gener-
ate high-pressure superheated steam in a steam generator coupled with a conventional
reheated steam turbine to produce electricity.
Moving to thermal energy storage, in general, solar power plants can operate as
stand-alone with TES in order to increase their operating hours, or alternatively they
can be coupled with a back-up fossil-fuelled boiler or integrated in conventional power
stations. Besides operating hours perspective, thermal energy storage is an inherent
capability of indirect configuration plants with HTF, allowing increased electricity
production and usually a decrease of its costs.

14.4.1 Heat transfer fluids
HTFs in a solar plant should satisfy many technological requirements: stability at high
temperature, low vapour pressure at high temperature, low freezing point, high boiling
point, low flammability, low corrosivity and relatively low cost. Firstly, the selection
of HTF is crucial for increasing the operating temperature of a solar thermal plant, and
hence the efficiency conversion from heat to power (see Section 14.2). Conventional
HTFs in commercial solar power plants are synthetic oil and molten salts; they allow
good cycle performance and assure trade-off between the above-mentioned issues.
Table 14.4.1 lists the operating temperatures and the main characteristics of some
HTFs used in commercial parabolic troughs and power towers.
In existing parabolic trough plants, the most common HTF is synthetic oil
(Therminol VP-1), in spite of its characteristics of flammability and toxicity, and tem-
perature limitations (up to 400 ◦ C). The use of molten salts would primarily increase the
maximum temperature of the solar field to 500-600 ◦ C, thereby increasing the cycle
efficiency of the power plant. Moreover, molten salts are non-flammable, environ-
mentally friendly and cheaper than other HTFs. The main drawback of molten salts,
which definitely limit their application, is the requirement for expensive anti-freezing
systems because of salt's high solidification temperature (about 120-220 ◦ C depending