Tubular radiation absorbing device for a solar power plant with reduced heat losses

Abstract

The tubular radiation absorbing device (1) for solar thermal applications includes a central tube (3) made of chromium steel, particularly stainless steel; a glass tubular jacket (2) surrounding the central tube so as to form a ring-shaped space (6); and a barrier coating (4) on at least an interior side of the central tube (3), which is substantially impermeable to hydrogen and contains chromium oxide. The barrier coating (4) is provided by a process in which the central tube (3) is treated with steam containing free hydrogen at a temperature of 500° C. to 700° C.

Description

CROSS-REFERENCE [0001] The invention described and claimed hereinbelow is also described in German Patent Application 10 2005 057 277.4-15, filed Nov. 25, 2005 in Germany, which provides the basis for a claim of priority under 35 U.S.C. 119. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates to a tubular radiation absorbing device for solar thermal applications, especially for a parabolic trough collector in a solar power plant, which comprises a central tube made from a chromium steel, especially stainless steel, and a glass tubular jacket surrounding the central tube so as to form a ring-shaped space between the tubular jacket and the central tube. [0004] 2. Related Art [0005] Tubular radiation absorbing devices or absorber pipes are used in parabolic trough collectors to utilize solar radiation. The solar radiation is concentrated by a tracking mirror on a tubular radiation absorbing device and converted into heat. The heat is conduc...

AI Technical Summary

Benefits of technology

[0015] It has been surprisingly found that coatings containing chromium oxide largely prevent passage of hydrogen.

[0021] However it is preferred that the thickness of the outer coating is less than the thickness of the barrier coating. This coating merely serves as an adherent layer for a subsequently applied selective thin layer. The thickness of the outer layer amounts to preferably less than 0.1 μm. It has been shown that a spinel layer, which has a rough surface and is porous, is formed on the upper surface of the chromium oxide coating with a layer thickness of greater than 0.1 μm. This spinel layer is not suitable to support a subsequently applied smooth selective thin layer. The spinel layer does not interfere with the interior barrier coating, so that greater thickness is possible.

[0023] Preferably the ratio VA=H2 / H2O of the steam for treating the outer side of the central tube is greater than the ratio V1=H2 / H2O of the steam for treating the inner side of the central tube. The formation of the spinel layer on the outer side is avoided by these process steps.

[0025] According to another embodiment the coating thickness on the outer side can be reduced so that the central tube is worked or process on its outside prior to the steam treatment, so that it has a roughness Ra less than 0.3. Preferably the roughness Ra is less than 0.25.

Problems solved by technology

This pressure increase leads to increased heat losses and to a reduced efficiency of the tubular radiation absorbing device.

This arrangement has the disadvantage that the strip is in a region, which can be exposed to direct radiation.

Because the getter material with a predetermined loading degree has a temperature dependent equilibrium pressure (equilibrium between gas desorption and adsorption), temperature fluctuations of the getter material lead to undesirable pressure fluctuations.

The temperature of the tubular jacket greatly increases after consumption of the getter material and the absorber tube becomes unusable.

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