Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 418

which reflect the solar energy at the focus of the parabola; the parabolic shape is made
by a rigid metallic structure. Discrete concentration is achieved by several mirrors
which move independently in order to collect the solar energy at the same focus point.
Most of the existing solar thermal plants (2050 MW vs. 2120 MW data from 2012
(“NREL Database,'' n.d.)) are based on a linear focus technology, but the point focus
is seen as the most attractive because of its potential for cost reduction.
Before discussing the technology in detail, we need to define solar multiple (SM)
and introduce the general approach to CSP design. SM is the ratio between the power
delivered by the solar field at design conditions (

.
Q SF , design ) and the nominal thermal

.
Q PB , design . input of the power cycle

· Q SF , design
· Q PB , design

SM = (14.3.1)

A solar multiple equal to one corresponds to a solar field aperture area which delivers
at nominal conditions the design thermal energy input of the power cycle. The SM
coefficient is one of the optimization parameters of a CSP plant. It is usually above one
in order to: (i) mitigate solar transient and fluctuation in irradiance; and, if available,
(ii) store part of the collected thermal energy in a Thermal Energy Storage. The advan-
tage of a SM above one, even without thermal storage, is to increase plant operating
hours. One drawback, during high radiation days, is that part of the solar field might
be defocused in order to respect the thermal input to the power block, thus wasting
potential solar radiation.
Existing plants are usually designed with SM higher than one and with a TES for
the above-mentioned advantages.
Moving to the energy conversion process, all concentrating solar technologies
convert solar energy into thermal energy supplied to a fluid (i.e. the fluid increases its
temperature in the solar field). Thus, solar field efficiency can be introduced as the
ratio between the thermal power absorbed by the fluid and the direct normal solar
radiation available on the collector area. Solar field efficiency (

η

SF ) is defined as:

· Q FLUID
· Q SUN

· Q FLUID
G

η SF = = (14.3.2)

· A

where · Q FLUID [kW] is the thermal power transferred to the fluid and · Q SUN [kW] is
the solar energy coming from the sun. Q SUN can be expressed as a product of the
direct beam radiation, G (W/m 2 ) and the collector surface area A [m 2 ]. Solar field
efficiency can be further divided into two different efficiencies: optical and thermal. In
order to better explain this concept, a schematic of a concentrating system with main
conversion steps 4 is represented in Figure 14.3.1.

4 A more rigorous representation would also consider other phenomena such as absorbance of
the receiver.