Evacuated-tube solar collectors are capable of improved performance relative to

Question

Evacuated-tube solar collectors are capable of improved performance relative to flat-plate collectors. The design consists of an inner tube enclosed in an outer tube that is transparent to solar radiation. The annular space between the tubes is evacuated. The outer, opaque surface of the inner tube absorbs solar radiation, and a working fluid is passed through the tube to collect the solar energy. The collector design generally consists of a row of such tubes arranged in front of a reflecting panel. Identify all heat transfer processes relevant to the performance of this device.

Explanation / Answer

As the outer and the inner tube do not touch each other(so no conduction between outer and inner tube). Heat is transferred by Radiation from Sun to inner tube.

Later the inner tube gets hot and heat is transferred to working fluid by conduction,as well as the radiation as the photons also reach there, the tubes being transparent.

The reflecting surface increases the amount of heat absorbed by striking leftover photons once again.

Convection need heating agent to heat liquid and then convection currents flow in fluid,it is not so here.The fluid is provided heat from all sides(direct as well as reflecting photons,so convection currents are less likely to happen as fluid is almost equally heated from all sides)

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