Irradiative heat loss

Even if many works [3] had analyzed the problem of the heat losses due to radiation and conduction, it is important to introduce a short summary. The first term to be evaluated is the net

inward radiative heat flux defined as:

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The irradiation, G, in general case can be written as:

■ ■ ■: ■ (5)

J is the radiation heat loss from the total irradiation arriving on the solar tube, is the mutual irradiation, FoniiJ is the ambient view factor, a is the Stephan-Boltzmann constant.

image236

Solving for the surface-to-ambient radiation, qr:

image224

Assuming Gm= 0 and Famb= 1, the solution has given for a single tube. A computation of./takes count of the incident angle on the surface of the solar tube, of the reflectivity/transmittance of the first borosilicate glass shell, of the optical properties of the cermet layer. In the surface of a single evacuated tube together with the parabolic concentrator, G has a value of about 160W. Table 1 represents the radiation heat losses due to different causes. The radiation is converted into heat energy from the cermet layer and transferred by conduction phenomena to the borosilicate glass and to the different layers (aluminium, air and copper) until the vector fluid.