Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 452

to 600 ◦ C. Beyond this temperature solar salt degrades and nitrite formation occurs,
creating solid precipitates. In addition, it is one of the lowest-cost nitrate salts (about
0.6 /kg (Manzolini et al., 2011b)). Mechanical integrity studies for parabolic troughs

(Hasuike et al., 2006; Yang et al., 2010) have shown that, with salt temperatures of
600 ◦ C and heat flux on the receiver up to 800 kW/m 2 , a maximum surface temperature
of 700 ◦ C is ensured for every geometry of the receiver tube, demonstrating the technical
feasibility of high-temperature collectors. However, a major disadvantage of solar salt
is its relatively high freezing point of 220 ◦ C. Hitec salt offers a lower freezing point
(about 140 ◦ C) but at higher cost. Finally, HitecXL is a calcium nitrate salt mixture
with a lower freezing point of about 120 ◦ C. The density, viscosity and heat capacity
properties are comparable for all nitrate salts, as shown in Table 14.4.1.
Research into HTFs has led to experiments with a new category of fluids that
exhibit very low freezing points: the so-called room temperature ionic liquids (RTIL)
(Moens et al., 2003). They are essentially salt-like materials, usually in the classes of
quaternary ammonium compounds, that are composed of organic cations combined
with organic or inorganic anions, and which are liquid at or near ambient temperature.

a

14.4.2 Storage
Thermal energy storage systems allow efficient storing of solar energy as heat. Thermal
energy storage can avoid the effects of variation of the solar source, which by its nature
is highly variable, thereby making the system more flexible and meeting the needs of
productive processes. In this way, in CSP plants, the thermal input to the power section
can be more constant and in general electricity production can be independent from
the collection of solar energy. Alternatively, it is possible to integrate it with fossil
fuels or renewable fuels, such as oil, natural gas or biomass, obtaining a so called
“hybrid'' plant.
With regard to CSP plants, the sizing of thermal storage can be carried out
according to different design philosophies:

• Buffering . Here, thermal storage is designed to cancel out the effect of clouds

transiting over the power block. The quick variation in steam flow and quality
could cause severe dynamic variations of the turbine's working conditions. In
particular, load variations faster than a few MW/minute can be unacceptable for
the engine, bringing about a degradation or affecting the lifetime of the turbine
itself. The required storage capacity for “buffering'' purposes is relatively small
(typically delivering up to 1 hour of power at full load).

• Displacement of the production period . In this case, storage allows a decoupling

of the electricity production from sunny periods, when energy demand or prices
on the grid can be higher. The displacement of production generally involves the
use of a medium-high storage capacity (typically delivering between 3 and 6 hours
of power at full load), and does not necessarily require an increase in the surface
of the solar field.

• Extension of the production period . This type of storage is aimed at extending the

operating hours of the plant beyond the insolation period. This solution requires
a proper sizing of the storage together with an increase in the solar field surface