Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 420

material optical properties with the incidence angle, together with the cosine effect. The
cosine effect consists of the reduction of radiation by the cosine of the angle between
solar radiation and a surface normal. The cosine effect occurs for both single-axis
and double-axis tracking systems being more important for the single-axis technology
(about 60% on a yearly base) than in the double-axis (about 80%). This is because
the former can reduce to zero only the azimuth angle.
Variation in material properties is usually expressed in the incidence angle mod-
ifier (IAM), which includes τ and α variation with . The best way to predict optical

efficiency at different solar positions is by means of laboratory experiments, owing to
the difficulty of splitting the contribution of τ and α .
The contribution of the K ( θ ) is usually predominant over other terms, making
the average yearly optical efficiency of single axis-tracking systems much lower than
double-axis tracking ones.
Nominal optical efficiency is defined at peak conditions with an incidence angle ( θ )
equal to zero. It can be seen as representative of the property of the collector materials
(i.e. reflector, absorber, etc.) under perpendicular solar radiation and is defined as:

θ

η opt − peak = ρ · γ · τ · α | θ = 0 (14.3.5)

where ρ is the mirror reflectivity, γ the intercept factor, τ the vacuum glass
transmissivity and the receiver absorptivity.

After this short general introduction on concentration systems, the description will
be divided between adopted tracking systems.

α

14.3.1 Linear focus
Linear focus concentration is based on parabolically curved mirrors or segmented
mirrors, according to the Fresnel principle, which concentrates solar energy onto a
receiver pipe. This configuration allows single-axis tracking. A collector field comprises
many collectors (usually named trough) in parallel rows aligned with the tracking axis
orientation.
The fluid is heated in the receiver and then sent to the power block to convert
the thermal energy into electricity. The distribution of the fluid along the field is made
through pipes called headers. The header that brings the fluid from the power block to
the trough is named “cold header'', since it is at lower temperature, while the header
that collects the fluid is named “hot header'' because of the higher temperature. The
particularity of these pipes is in the variation in diameter along with the variation
in fluid flow: the target is to keep an almost constant fluid velocity. The connection
between the header and the power cycle is made by connecting pipes which transfer
the fluid from the solar field to the power cycle.
An example of a solar field layout is given in Figure 14.3.2. In particular, the two
typical configurations are shown: on the left side the “H'' configuration is reported. On
the same side of the power block there are two different sections of the solar field, with
two cold and two hot headers. This configuration is adopted in Andasol project (Her-
rmann and Geyer, 2002), SEGS VIII and IX. In the “I'' configuration (used in SEGS VI,
(Patnode, 2006)), reported on the right side, there is just one section. In both the “H''
and “I'' configurations one loop is composed of two solar collector rows, connected by