Downhole heat exchanger for a geothermal heat pump

Abstract

The invention relates to a downhole heat exchanger for extracting geothermal energy from a borehole, wherein the inner surface of the exchanger tube comprises the following roughness values: a) an arithmetic mean roughness Ra according to DIN EN ISO 4287 in the range of 1 to 15 μm, b) an averaged roughness Rz according to DIN EN ISO 4287 in the range of 8 to 80 μm, and c) a maximum roughness depth Rz1 max according to DIN EN ISO 4287 in the range of 10 to 500 μm, comprising an improved precipitation film during operation, such that the entire surface of the exchanger tube is uniformly wetted.

1 . A downhole heat exchanger, comprising a heat exchanger pipe, wherein an interior surface of the heat exchanger pipe has the following roughness values:
 a) an arithmetic mean roughness Ra in accordance with DIN EN ISO 4287 in the range from 1 to 15 μm;   b) an average peak-to-valley height Rz in accordance with DIN EN ISO 4287 in the range from 8 to 80 μm; and   c) a maximum peak-to-valley height Rz1max in accordance with DIN EN ISO 4287 in the range from 10 to 500 μm,   wherein the roughness measurement is carried out in accordance with DIN EN ISO 4288.   
     
     
         2 . The downhole heat exchanger of  claim 1 , wherein:
 Ra is in the range from 2 to 12 μm;   Rz is in the range from 10 to 60 μm; and   Rz1max is in the range from 15 to 150 μm.   
     
     
         3 . The downhole heat exchanger of  claim 1 , wherein:
 Ra is in the range from 3 to 7 μm;   Rz is in the range from 15 to 40 μm; and   Rz1max is in the range from 25 to 65 μm.   
     
     
         4 . The downhole heat exchanger any of  claim 1 , wherein the heat exchanger pipe comprises a layer comprising a thermoplastic molding composition. 
     
     
         5 . The downhole heat exchanger of  claim 4 , wherein the heat exchanger pipe or an innermost layer of the heat exchanger pipe comprises a molding composition whose matrix comprises a fluoropolymer, a polyarylene ether ketone, a polyolefin, or a polyamide. 
     
     
         6 . The downhole heat exchanger of  claim 4 , wherein the heat exchanger pipe or an innermost layer of the heat exchanger pipe comprises a molding composition comprising from 0.1 to 50% by weight of at least one selected from the group consisting of reinforcing fibers and reinforcing fillers. 
     
     
         7 . The downhole heat exchanger of  claim 1 , wherein the heat exchanger pipe comprises a metal comprising a rough coating on an interior surface. 
     
     
         8 . A process for recovering geothermal energy from a borehole, the process comprising:
 vaporizing a vaporizable refrigerant in the downhole heat exchanger of calim  1 ;   compressing the refrigerant vapor which has ascended in the heat exchanger pipe in a compressor, to liquefy the vapor and remove heat obtained from the heat of condensation; and then   feeding the cooled liquid refrigerant back to the downhole heat exchanger as falling film which is conveyed downward.   
     
     
         9 . The downhole heat exchanger of  claim 1 , in the form of a direct boiling heat exchanger, which is suitable for recovering geothermal energy from a borehole. 
     
     
         10 . The downhole heat exchanger of  claim 1 , wherein the internal diameter of the heat exchanger pipe is in the range from 15 to 80 mm. 
     
     
         11 . The downhole heat exchanger of  claim 1 , wherein the internal diameter of the heat exchanger pipe is in the range from 20 to 55 mm. 
     
     
         12 . The downhole heat exchanger of  claim 1 , wherein the internal diameter of the heat exchanger pipe is in the range from 26 mm to 32 mm. 
     
     
         13 . The downhole heat exchanger of  claim 1 , having a length in a range from 60 to 200 m. 
     
     
         14 . The downhole heat exchanger of  claim 1 , having a length in a range from 80 to 120 m. 
     
     
         15 . The downhole heat exchanger of  claim 6 , wherein the molding composition comprises from 0.5 to 20% by weight of at least one selected from the group consisting of reinforcing fibers and reinforcing fillers. 
     
     
         16 . The downhole heat exchanger of  claim 6 , wherein the molding composition comprises from 3 to 10% by weight of at least one selected from the group consisting of reinforcing fibers and reinforcing fillers. 
     
     
         17 . The downhole heat exchanger of  claim 6 , wherein the molding composition further comprises at least one selected from the group consisting of an impact modifier, a plasticizer, a stabilizer, and a processing aid. 
     
     
         18 . The downhole heat exchanger of  claim 5 , wherein the heat exchanger pipe consist of a single layer comprising the molding composition. 
     
     
         19 . The downhole heat exchanger of  claim 5 , wherein the heat exchanger pipe comprises a plurality of layers. 
     
     
         20 . The downhole heat exchanger of  claim 7 , wherein the metal is selected from the group consisting of aluminum, an aluminum alloy, and steel.