Solar energy conversion with thermal cycles - Solar Energy Sciences and Engineering Applications - page 426

Figure 14.3.6 Commercial collectors efficiency as function of fluid temperature (DNI = 800W/m 2 )
(Manzolini et al., 2011a). In this case, Schott PTR 70 performances do not correspond
to the latest version.

Table 14.3.1 Features of commercial absorbers (Price et al., 2002; Forristall, 2003).

Luz Black Luz Solel Solel Schott Archimede Solar

Chrome Cermet UVAC UVAC 2008 PTR 70* Energy

Absorption 0.94 0.92 0.955 0.97 0.955 0.945

Transmittance 0.935 0.935 0.965 0.97 0.965 0.97

Absorber's Length [m] 4.06 4.06 4.06 4.06 4.06 4.06

D in absorber [mm] 64 64 64 64 64 64

D out absorber [mm] 70 70 70 70 70 70

D in glass envelope [mm] 109 109 109 109 119 109

D out glass envelope [mm] 115 115 115 115 125 115

than 70% at temperatures below 400 ◦ C. Moreover, recent coatings can withstand tube
temperatures of about 550 ◦ C, with significant thermodynamic advantages. The main
properties of HCE are summarized in Table 14.3.1.
Because the absorber tube works at high temperature, a glass tube surrounding
the absorber tube is also adopted. A schematic of a heat collection element with all the
fundamental components is shown in Figure 14.3.4.
The glass tube, usually made by Pyrex®, makes a vacuum annulus between the
glass and the tube which prevents coating oxidation and reduces heat loss; pressure
within the annulus is typically about 1 10 − 2 Pa (Price et al., 2002). The glass tube

has an antireflective coating on both surfaces to maximize solar transmittance while

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