Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 463

Figure 14.5.3 Power block scheme for an indirect cycle.
In general, a simpler layout can also be adopted, with fewer regenerative bleedings or
without RH. However, the higher the number of regenerators, the higher the power
cycle efficiency since all the heat collected in the solar field is used for steam evaporation
and superheating, rather than economization. Reheating is fundamental to achieving
high conversion efficiencies, as well as a high vapour fraction at the turbine outlet.
HTF thermal power is transferred to the power cycle in four different heat-
exchangers: an evaporator, an economizer, and a superheater and a reheater. Different
heat exchanger configurations can be adopted: the RH section can be in parallel with
the SH only, or with the whole boiler, as in SEGS VI plant (Patnode, 2006). Optimal
configuration depends on heat transfer and steam temperatures, and is usually selected
to minimize HTF boiler outlet temperature and heat transfer irreversibilities inside
the boiler.
The steam turbine is usually divided into a high pressure (HP) section and a low
pressure (LP) section; between the two sections the steam is sent to the boiler for
reheating. The maximum steam temperature that occurs at the turbine inlet, and after
reheating, is in the range of 371 ◦ C when synthetic oil is used as the heat transfer
fluid; this temperature occurs in all existing plant with indirect cycle configuration
because of the maximum working temperature of the synthetic oil (400 ◦ C) - a con-
servative maximum temperature in the solar field of 391 ◦ C is usually taken. If molten
salts are adopted as the heat transfer fluid, steam temperature can be increased up to
500-540 ◦ C. For example, the Archimede plant in Sicily (Italy) works with a steam
temperature of 525 ◦ C at the turbine inlet. The drawbacks of this HTF have already
been discussed in the previous section. Because of the moderate maximum temperature