Page 134 - A Comprehensive Guide to Solar Energy Systems

Chapter 7 • Concentrating Solar Thermal Power 133

solar field piping to keep the oil in a liquid phase when it is at its maximum working tem-
perature. These properties, together with its affordable price, are the reasons why no other
thermal oil is nowadays used in CSTP plants. However, the main problem with this oil is
that it is an environmental hazard (if leaks occur) and can be harmful to workers. The ther-
mal limit of 398°C is another constraint of this thermal oil, because the overall CSTP plant
efficiency depends on the temperature of the superheated steam delivered to the power
block, and such temperature is limited by the temperature of the oil used to generate it.
The thermal limit of this oil and its environmental hazards are the reasons why alter-
native working fluids are being investigated. The three main alternative fluids are: (1) liq-
uid water/steam (direct steam generation), (2) molten salt mixtures, and (3) compressed
gases. The advantages and disadvantages of these working fluids are analyzed in Ref. [10].
As silicone-based oils have higher thermal limits and are less harmful to the environ-
ment and to humans, they are also being considered an interesting option to replace the
current thermal oils in CSTP plants. However, long-term experience with silicone oils
working under real solar conditions is lacking. Trials are ongoing and if successful, the first
commercial CSTP plants using silicone oil could be in operation in China in 2018–19.

7.2.3 Main Applications of PTC
PTC has two main applications:

(a) Electricity generation, and
(b) Industrial process heat (IPH) applications

Electricity generation is the most widely used application nowadays, and such systems
using PTC are called CSTP or STE plants.
Fig. 7.4 shows the simplified scheme of a typical CSTP plant with PTC and thermal oil as
working fluid. The plant is composed of four main subsystems: (1) solar field, (2) thermal
energy storage (TES), (3) steam generator, and (4) power block (also called power conver-
sion system, PCS). The thermal oil is heated in the solar field from 295°C up to about 395°C.
Once heated, the hot oil is sent to the steam generator or/and to the TES, depending on
the decision taken by the plant operator. The hot oil sent to the steam generator is used
to produce the superheated steam required to drive the steam turbine of the PCS, which
is mechanically coupled to the electricity generator. The PCS of this type of CSTP plant
is based on a Rankine cycle with steam reheating because the steam leaving the high-
pressure stage of the turbine is reheated before entering the low-pressure stage. A small
fraction of the hot oil delivered by the solar field is used for this steam reheating. Because
of the thermal stability limitation of current thermal oils (398°C), the maximum steam
temperature that can be achieved in the steam generator is about 385°C, thus limiting the
PCS efficiency, and therefore the overall plant efficiency.
The most common TES system used nowadays in CSTP plants with PTC is based on mol-
ten salts (Fig. 7.4), so that the thermal energy is stored as sensible heat (i.e., temperature in-
crease) of a binary mixture of molten salts, the so-called “Solar Salt” (i.e., 40% of potassium