Page 135 - A Comprehensive Guide to Solar Energy Systems

134 A COmPREHEnSIVE GuIdE TO SOlAR EnERGy SySTEmS

FIGURE 7.4 Simplified scheme of a typical CSTP plant with parabolic-trough collectors.

nitrate + 60% of sodium nitrate). This type of TES system had been implemented in 22 STE
plants with PTC (as of the end of 2016). With a specific investment cost of about €35–40
−1
(kW h) of capacity, TES systems using molten salts are cost-effective as they can meet the
electricity demand not only during sun-light hours but also overnight.
When the hot oil is sent to the TES (TES charging process), it circulates through a
heat exchanger where its thermal energy is transferred to the molten salt at 290°C that is
pumped from the “cold tank” to the “hot tank,” where it is stored at 385°C. Once cooled
down in the TES heat exchanger, the oil is used to feed the solar field again.
When the plant operator wants to recover the thermal energy stored in the TES, cold oil
is circulated through the TES heat exchanger while the hot salt at 385°C is pumped from
the hot tank to the cold tank through the same heat exchanger, thus transferring thermal
energy from the salt to the oil, which then goes to the steam generator to produce super-
heated steam for the turbine, which keeps the electricity generator running. This hot oil
is coming from the TES is cooled down at 295°C in the steam generator and it is then sent
to the TES heat exchanger again to continue with the discharging process, which can con-
tinue until all the molten salt is in the cold tank and the hot tank is empty.
The first CSTP plants with PTC installed in the world were designed by the company
LUZ International and built in the Mohave dessert (California, USA) between 1985 and
1991. These plants were named SEGS (solar electricity generating system) and numbered
from I to IX [11]. The SEGS plants were the precursor of the modern CSTP plants installed
worldwide a few decades later with PTC.
A special design of a CSTP plant with PTC is called an Integrated Solar Combined Cycle
System (ISCCS) plant. This type of plant is basically a natural gas combined-cycle plant
with a small PTC solar field coupled to the Rankine cycle, so that the thermal energy