Environmental Engineering Reference
In-Depth Information
Figure 14.3.5
Heat collection element (Flabeg Solar; Price et al., 2002).
defined by the incidence angle modifier (IAM). However, the IAM not only takes into
account reflectivity variation, but also glass and absorber tube property variations
with the angle. Examples of IAM trends will be presented later, in the parabolic trough
performance section.
Trough-shaped mirrors can be made by a glass layer with low iron concentration
and a reflective silvered film. For example, glass-based technology is developed by
Flabeg (Flabeg). Another option is multiple layers of polymer film with a layer of pure
silver to provide for high specular reflectance (Skytrough Brochure). Other companies
which are active in reflector development are Alanod (Alanod), 3M (3M). Alanod
products, for example, are based on polished aluminium which has a slightly lower
reflectivity but a higher resistance to wear and lower weight, with advantages for
structure and plant reliability.
Typical reflectivity values are 94.4% for glass-based reflectors and slightly lower
for polymer-based ones (93%) and aluminium (92%). The higher investment cost and
weight of glass-based reflectors is nowadays balanced from an economic point of view
by a higher efficiency compared to polymer and aluminium-based reflectors.
14.3.4 Heat collection element
The absorber tube, also called heat collection element (HCE), consists of a steel tube
with a selective coating layer. The coating is added in order to increase the absorption
properties within the solar spectrum while keeping emissivity low.
The development of new coatings has increased the performance of absorber tubes
thanks to higher absorptivity and lower emissivity. An example of the improvement in
this technology is given in Figure 14.3.6 which shows the efficiencies of collectors with
different selective coatings. The coatings adopted for use in SEGS plants were LUZ
Black Chrome and Luz Cermet with efficiency in the range of 60%. R&D activity
has increased absorber performance, reaching an overall collector efficiency of more
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