Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 460

Figure 14.5.1 Power production (solid lines) vs. solar radiation (dashed lines) for two different solar
field and storage sizes during a summer day (left side) and mid-season day (right side).

Solar dish, which does not have a storage system, is more affected by the variation
in solar radiation during the day.
The efficiency of power section is defined as follows:

Net Power Output
Thermal input

W net

η PS = = (14.5.1)

· m HTF ·

( h HTF , in − h HTF , out )

where W net is the net power output generated in the thermodynamic cycle [W], · m HTF
is the HTF mass flow rate [kg/s] and h HTF are the enthalpies of the heat transfer fluid
at the inlet and outlet of the power section. W net depends on the thermodynamic cycle
adopted for thermal conversion.
Another classification of solar plants is between stand-alone plant and hybrid
plant. In stand-alone plant the entire power output of the plant is related to the input
of solar power. Another configuration consists of the integration of the solar field
in a solid fuel-based power plant. Solid fuel can be either fossil (coal, natural gas)
or renewable (e.g. biomass). Integrated solar combined cycle (ISCC) plants, which
integrate the solar field in a combined cycle, are already commercially available, as in
Indiantown (Florida, USA). Usually, hybrid power plants have a limited solar energy
share of total electricity production. The plant mentioned above, for example, has a
net power output from the solar plant of 75 MW out of 3.7 GW of the entire plant
(Martin Next Generation Solar Snergy Center). Hybrid plants have a higher flexibility
than stand-alone, since fossil fuel can be regulated to maintain a constant thermal input
to the power section by balancing solar input fluctuation. Moreover, this configuration
exploits an economic advantage as a consequence of the power section having a higher
number of operating hours, hence reducing its cost per kWh produced.
Since the power section is fundamental for the success of CSP technology and, with
actual technologies, has a significant impact on overall plant costs (about 40-45%),
research activity has been focusing on other options with the aim of cost reduction and
faster start-up, as well as higher conversion efficiency. One of the most investigated