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.

1 . A tubular radiation absorbing device ( 1 ) for solar thermal applications, especially for a parabolic trough collector in a solar power plant, said radiation absorbing device comprising 
 a central tube ( 3 ) comprising steel, said steel including chromium;    a tubular jacket ( 2 ) comprising glass and surrounding the central tube so as to form a ring-shaped space ( 6 ) between the tubular jacket and the central tube; and    a barrier coating ( 4 ) on at least an interior side of the central tube ( 3 ), wherein said barrier coating ( 4 ) is substantially impermeable to hydrogen and contains chromium oxide.    
   
   
       2 . The tubular radiation absorbing device as defined in  claim 1 , wherein said steel comprises a stainless steel.  
   
   
       3 . The tubular radiation absorbing device as defined in  claim 1 , wherein said barrier coating ( 4 ) has a thickness of 0.5 μm to 10 μm.  
   
   
       4 . The tubular radiation absorbing device as defined in  claim 1 , wherein said barrier coating ( 4 ) contains from 20 wt. % to 60 wt. % of said chromium oxide.  
   
   
       5 . The tubular radiation absorbing device as defined in  claim 1 , further comprising an outer coating ( 5 ) on an outer side of said central tube ( 3 ), and wherein said outer coating comprises said chromium oxide.  
   
   
       6 . The tubular radiation absorbing device as defined in  claim 4 , wherein said outer coating ( 5 ) has a thickness that is smaller than a thickness of said barrier layer.  
   
   
       7 . The tubular radiation absorbing device as defined in  claim 5 , wherein said thickness of said outer coating ( 5 ) is less than or equal to 0.1 μm.  
   
   
       8 . A process for making a central tube ( 3 ) of a tubular radiation absorbing device ( 1 ) for solar thermal applications, said process comprising the steps of: 
 a) prefabricating a central tube ( 3 ) made of chromium-containing steel; and    b) treating at least an interior side of the central tube ( 3 ) with free-hydrogen-containing steam at a temperature of from 500° C. to 700° C. in order to provide at least said interior side of said central tube with a barrier coating ( 4 ), wherein said free-hydrogen-containing steam comprises water and free hydrogen and said barrier coating ( 4 ) contains chromium oxide and is substantially impermeable to said free hydrogen.    
   
   
       9 . The process as defined in  claim 8 , wherein said chromium-containing steel comprises stainless steel.  
   
   
       10 . The process as defined in  claim 8 , wherein an outer side of the central tube ( 3 ) is treated with another free-hydrogen-containing steam containing said water and said free hydrogen in a ratio (V A ) of said free hydrogen to said water that is greater than a ratio (V 1 ) of said free hydrogen to said water in said free-hydrogen-containing steam that treats said interior side of said central tube ( 3 ).  
   
   
       11 . The process as defined in  claim 10 , wherein said ratio (V A ) of said free hydrogen to said water in said another free-hydrogen-containing steam is from 10 to 1000, said ratio (V 1 ) of said free hydrogen to said water in said free-hydrogen-containing steam is from 1 to 100, and said ratio (V A ) of said free hydrogen to said water in said another free-hydrogen-containing steam is greater than or equal to ten times said ratio (V 1 ) of said free hydrogen to said water in said free-hydrogen-containing steam.  
   
   
       12 . The process as defined in  claim 8 , further comprising working said central tube ( 3 ) on an outer side of the central tube so that surfaces of said outer side have a surface roughness (R A ) less than 0.3 prior to treating with said steam.  
   
   
       13 . The process as defined in  claim 12 , wherein said working comprises polishing.  
   
   
       14 . The process as defined in  claim 8 , further comprising working said central tube ( 3 ) on an outer side of the central tube so that surfaces of said outer side have a surface roughness (R A ) less than 0.25 prior to treating with said steam.  
   
   
       15 . The process as defined in  claim 14 , wherein said working comprises polishing.