Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 424

reliability and durability. These general targets can be translated into the following
sub-tasks:

• High optical and tracking accuracy;

• Low heat losses;

• Manufacturing simplicity;

• Reduced number of parts and field erection costs;

• Increased aperture area.

Hence, R&D activities aim at fulfilling these targets in order to make the parabolic
trough technology competitive to commercial power generation technologies from an
economic point of view.
At present, the solar collector assemblies have an aperture area ranging from
5.77 m for Siemens (Siemens Sunfield LP) to 6 m for Skyfuel (Skytrough Brochure) 5 .
The length for elements is up to 12 m while the total trough length, which is the union
of elements, reaches a total length of about 100-120 m (“Skytrough brochure,'' n.d.).
The absorber tube diameter is usually 70 mm (SCHOTT PTR® 70 Brochure;
Archimede Solar Energy), making the concentration ratio (CR) of parabolic trough
collectors in the range of 80. The concentration ratio is defined as follows 6 :

aperture width
adsorber diameter =

W
D abs

CR = (14.3.6)

After this short and general introduction, a detailed description of parabolic trough
technology is now presented, in which the three main components - reflectors, heat
collection element and structure - are discussed.

14.3.3 Reflectors
Reflectors must reflect and concentrate solar direct beam radiation onto the linear
receiver located at the focus of the parabola, called the heat collection element (HCE).
Aside from pure performances whilst new and in clean conditions, the reflec-
tors must maintain constant reflectivity over the years. Since solar plants are usually
installed in places with high solar radiation, often in desert locations, the ambient is
abrasive because of sand and dust transported by the wind. Receivers must be resis-
tant to these conditions, making the endurance test really challenging. For this reason,
protective coatings are deposited on the reflector surface to keep high reflectivity and
reduce wear effects.
In addition, the reflectors must guarantee high reflectivity at all solar incidence
angles: reflectivity

τ

is usually defined at zero incidence angle where it has the maximum
values. Usually, it is known that reflectivity varies as a function of solar incidence
angle. For higher incidence angles, mirror reflectivity lowers, affecting optical and
overall plant efficiency. Variation in performance with the incidence angle is usually

5 It should be noted that Skyfuel is testing a solar collector with an aperture area of about 8 m.
6 There is discussion on how to define the concentration ratio. Some authors prefer to adopt the
receiver diameter instead of the circumference as in this work.