Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 461

candidates is the Brayton cycle, which is cheaper than Rankine and it has faster start-
up; however, this technology to be competitive requires temperatures in the solar field
above 700 ◦ C and/or the adoption of fluid in the solar field in gas phase. These two
requirements put Brayton cycle still far from the commercialization phase.
14.5.1 Rankine cycle
The power section (which converts thermal power into electricity) in all operating CSP
plants with a net power output above 1 MW is based on water Rankine cycle. Water
Rankine cycles are based on a closed loop where heat is supplied externally (i.e. by the
solar field). The power output is obtained by the expansion of water in the gaseous
phase in a steam turbine. Compression is performed in the water liquid phase, hence
requiring less power.
The same cycle is applied in coal-based plant (which accounts for half of the world's
power production (Key World Energy, 2012)) and nuclear plant. Even if the concept
is the same, thermodynamic conditions and performance of the Rankine cycle in CSP
plant are significantly different than other applications. For example, modern coal-
based plant - usually called advanced supercritical or ultra supercritical pulverized
coal plant - works with live steam pressure at turbine inlet of between 250 and 300
bar, and temperatures of 600-620 ◦ C (Manzolini et al., 2011). This is because the
temperature of coal combustion is about 1500-2000 ◦ C.
The temperature in the solar field ranges between 400 ◦ C (linear focus technologies)
and 800 ◦ C (point focus technologies), hence limiting the maximum temperature of the
steam. It must be noted that the only operating solar tower plant works with saturated
steam at a temperature of 250 ◦ C (ref. PS-10).
In general, the higher is the evaporation pressure and the steam temperature,
the higher is the conversion efficiency of the Rankine cycle. For this reason, current
research activity is focusing on the development of innovative HCE (see Section 14.3)
and the adoption of innovative heat transfer fluids (see Chapter 14.4) which can with-
stand temperatures of up to 550-600 ◦ C for linear collectors and 1000 ◦ C for point
focus concentrators.
Two different types of power cycle configurations can be adopted: the indirect cycle
and the direct cycle. A schematic of the two configurations is shown in Figure 14.5.2.
The indirect cycle configuration is characterized by the adoption of different work-
ing fluids between the solar field and the power cycle. In detail, the heat collected in
the solar field is transferred to a heat transfer fluid which is sent to the power section.
In the power section, the HTF is cooled in heat exchangers by evaporating and super-
heating steam. This configuration allows a degree of freedom since the mass flow rate
and the pressure in the solar field differ from the power cycle; however, it does require
the adoption of expensive heat exchangers.
The direct cycle configuration is based on direct steam generation inside the solar
field. In this configuration the adoption of the HTF and all its associated components,
in particular the boiler, can be avoided. The maximum temperature in the solar field
coincides with the steam temperature at the turbine inlet, with advantages from an
efficiency point of view. Moreover, lower circulating pump consumption occurs in the
solar field. Obviously there also some drawbacks, namely: (i) higher fluid pressure
inside the solar field (about 100 bar vs. about 35 bar of synthetic oil), with penalties