Vacuum tubes for solar collectors with improved heat transfer

Abstract

Heat transfer between an absorber and heat carrier tubes is simplified by using heat conducting elements which are made of compressed graphite expandate in vacuum tubes for solar collectors. The heat conducting element which is made of compressed graphite expandate can, for example, be embodied in a form-locking fit as an intermediate layer between an absorber internal wall and the carrier construction which can receive the heat carrier tubes or as a heat transfer component which is fitted into the absorber tube and which receives the heat carrier tubes in a form-locking fit.

1 . A solar collector vacuum tube, comprising:
 two glass tubes including an inner glass tube and an outer glass tube placed one inside another concentrically, each of said glass tubes being closed in a hemispheric shape at a first end, and fused to each other at a second end, said two glass tubes defining a gap there-between and said gap being evacuated;   pipes functioning as heat transfer tubes disposed in an interior space of said inner glass tube and through which a heat transmission medium flows; and   a heat conduction element disposed in said interior space of said inner glass tube and made of a compressed graphite expandate, said heat conduction element having a first side forming a form-locking connection with said interior wall of said inner glass tube and a second side forming a form-locking connection with adjoining surfaces of said heat transfer tubes.   
   
   
       2 . The solar collector vacuum tube according to  claim 1 , wherein said heat transfer tubes are selected from the group consisting of U-shaped tubes, tubes through which a flow of the heat transmission medium moves from below upwards, coaxial heat transfer tubes, heat pipes and a combination thereof. 
   
   
       3 . The solar collector vacuum tube according to  claim 1 , further comprising a carrier structure housing said heat transfer tubes, and said second side of said heat conduction element forming said form-locking connection with adjoining surfaces of at least one of said heat transfer tubes and said carrier structure. 
   
   
       4 . The solar collector vacuum tube according to  claim 3 , wherein said heat conduction element is a graphite foil, which is wound about said carrier structure that receives said heat transfer tubes, and adjoins said inner wall of said inner glass tube in said form-locking connection. 
   
   
       5 . The solar collector vacuum tube according to  claim 4 , wherein said graphite foil has a thickness between 0.1 and 1 mm, and a density between 0.5 and 1.5 g/cm 3 . 
   
   
       6 . The solar collector vacuum tube according to  claim 1 , wherein said heat transfer tubes are held in said form-locking connection in said heat conduction element made of said compressed graphite expandate, said heat conduction element forming said form-locking connection with said inner wall of said inner glass tube. 
   
   
       7 . The solar collector vacuum tube according to  claim 6 , wherein said heat conduction element is a heat transfer component composed of two half-form shapes. 
   
   
       8 . The solar collector vacuum tube according to  claim 6 , wherein said compressed graphite expandate in said heat conduction element has a density between 0.02 and 0.5 g/cm 3 . 
   
   
       9 . The solar collector vacuum tube according to  claim 6 , further comprising an absorber layer disposed on said first side of said heat conduction element that faces said inner wall of said inner glass tube. 
   
   
       10 . The solar collector vacuum tube according to  claim 1 ,
 wherein said heat transfer tubes are in said form-locking connection with said heat conduction element being a heat transfer component made of said compressed graphite expandate; and   further comprising a graphite foil wound about said heat transfer component and said graphite foil adjoining said inner wall of said inner glass tube in a form-locking connection.   
   
   
       11 . A heat transfer component for a vacuum-tube solar collector having an inner glass tube with an inner wall, the heat transfer component comprising:
 a generally cylindrical shaped piece made of compressed graphite expandate for holding heat transfer tubes in a form-locking manner, and having a circumferential surface forming a form-locking connection with the inner wall of the inner glass tube.   
   
   
       12 . The heat transfer component according to  claim 11 , wherein said shaped piece has recesses formed therein for receiving the heat transfer tubes in the form-locking manner. 
   
   
       13 . The heat transfer component according to  claim 11 , wherein the heat transfer tubes are embedded in the form-locking manner in said shaped piece. 
   
   
       14 . The heat transfer component according to  claim 11 , wherein said shaped piece is formed of two half-form pieces. 
   
   
       15 . The heat transfer component according to  claim 11 , wherein said graphite expandate has a density between 0.02 and 0.5 g/cm 3 . 
   
   
       16 . The heat transfer component according to  claim 11 , further comprising an absorber layer disposed on said circumference surface that faces the inner wall of the inner glass tube. 
   
   
       17 . A method of using compressed graphite expandate for heat transmission, which comprises the step of:
 placing the compressed graphite expandate in vacuum-tube solar collectors for performing heat transmission functions.