Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 450

on the fluid). Advanced HTFs as ionic liquids have been already studied and proposed
(Moens et al., 2003), with the aim of reducing the freezing point below even room
temperature.
Alternatively, liquid metals like sodium can be used in CSP plants as HTF. Sodium's
low melting point (97.7 ◦ C) and high boiling point (873 ◦ C) allows a much larger range
of operational temperatures and the use of advanced cycles such as combined Bray-
ton/Rankine cycles in central receiver systems. Nevertheless, the use of sodium poses
many technological issues due to its high flammability. Another option is the adoption
of gases as HTFs, however their application involves high pressure drops and low heat
transfer coefficients.
In the following the commonly used and most promising HTFs are described.

14.4.1.1 Synthetic oil
Synthetic oil is by far the most common solution adopted in solar plants. Commer-
cially adopted synthetic oil is typically a diphenyl/biphenyl oxide. Dowtherm A and
Solutia Therminol VP-1 are industrial products that have been used in SEGS plants
in the United States or in Andasol plants in Spain. On the other hand, mineral oils
are not used in CSP because of their temperature limitations to 300 ◦ C, in spite of
their low cost (San Diego Regional Renewable Energy Study Group, 2005). The field
of exploitable temperature with synthetic oil (see Table 14.4.1) varies between 13 ◦ C,
where solidification takes place, and a maximum of 400 ◦ C, beyond which the phe-
nomenon of thermal cracking occurs. The maximum working temperature in the solar
field is therefore typically limited to about 390 ◦ C, while the operating pressure of oil is
about 12-15 bar in order to keep it in a liquid state and avoid evaporation at normal
working temperatures (Giostri et al., 2012). The oil is generally expensive (about 5 /kg

(Manzolini et al., 2011b)), flammable and highly toxic for life and the environment,
so that spills and leakages should be avoided. Although collector design has advanced
to excellent levels of performance and reliability, occasional spills of HTF may occur,
primarily because of piping or equipment failure. Existing plants have reduced HTF
spills to very low levels (Cohen and Kearney, 1999); good maintenance practices and
the use of ball-joint assemblies rather than flexible hoses between trough collectors are
the major contributors to this improvement. In case of any spill or release, the affected
collector loop is immediately separated from the rest of the circuit and shut down.
An appropriately equipped crew will repair the damage and remove any hazardous
wastes, moving it to an on-site bioremediation facility which employs indigenous bac-
teria to digest the hydrocarbon contamination, and in two to three months restore the
soil to a normal condition. Following these operations and maintenance (O&M) best
practices, the average fluid losses for SEGS plants at Kramer Junction (USA) between
1996 and 2002 was 2-3% of the site inventory per year (Cohen and Kearney, 1999).

a

14.4.1.2 Molten salts

Molten nitrate salts are, typically, a mixture of NaNO 3 and KNO 3 of variable compo-
sition, even if the most commonly used is a mixture known as “solar salt'' (respectively
60% NaNO 3 and 40% KNO 3 ). The use of molten salt HTF in a trough plant has sev-
eral advantages, as has already been pointed out. Depending on the fluid, the solar field
output temperature can be raised up to 550 ◦ C (see Table 14.4.1), thereby increasing