Geothermal system operable between heat recovery and heat storage modes

Abstract

The geothermal system uses an outer and an inner pipe installed in a single borehole. Cool fluids are pumped down through one pipe and returned to the surface through the other pipe. Subterranean heat increases the temperature of the cool fluid and this heat is returned to the surface where the heated fluid is recovered. The fluid with the heat removed is then pumped back down the borehole to be re-heated. Extra heat recovered from the ground surrounding a lower portion of the borehole is stored in the ground and rock formation surrounding an upper portion of the borehole during warmer seasons to optimize the amount of heat stored in the ground for extraction during colder seasons.

1 . A method of extracting heat from a ground source including rock material, the method comprising:
 providing a geothermal system comprising (i) an outer pipe supported within a borehole to extend longitudinally and downwardly into the ground source from a top end to a bottom end of the outer pipe in which the bottom end of the outer pipe is closed, (ii) an inner pipe within the outer pipe to extend longitudinally from a top end of the inner pipe in proximity to the top end of the outer pipe to a bottom end in proximity to the bottom end of the outer pipe so as to define an inner passage extending longitudinally through the inner pipe and so as to define an outer passage extending longitudinally within the outer pipe within an annular space between the inner pipe and the outer pipe in which a bottom end of the inner passage is in open communication with a bottom end of the outer passage, and (iii) piping in communication between the top end of the inner pipe and the top end of the outer pipe such that the inner and outer pipes form a closed loop;   operating the geothermal system in either one of a first mode to store heat in the ground source or a second mode to recover heat from the ground source;   in the first mode, pumping the heat exchanger fluid through the closed loop of the inner passage and the outer passage so as to collect heat into the heat exchanger fluid from the ground source along a lower portion of the outer pipe that is in proximity to the bottom end of the outer pipe and so as to transfer heat from the heat exchanger fluid to the ground source along an upper portion of the outer pipe that is in proximity to the top end of the outer pipe; and   in the second mode, pumping a heat exchanger fluid through the closed loop of the inner passage and the outer passage so as to collect heat in the heat exchanger fluid from the ground source along at least a part of the outer pipe and so as to extract heat from the heat exchanger fluid at the piping.   
     
     
         2 . The method according to  claim 1  further comprising in the second mode, transferring heat from the heat exchanger fluid to the ground source along the upper portion of the outer pipe when heat demands at the piping are low and transferring heat from the ground source to the heat exchanger fluid along the upper portion of the outer pipe when heat demands at the piping are high. 
     
     
         3 . The method according to  claim 2  including using a variable rate pump to vary a flow rate of the heat exchanger fluid between a high heat demand and a low heat demand in the first mode. 
     
     
         4 . The method according to  claim 1  further comprising in the first mode, preventing heat loss from the piping. 
     
     
         5 . The method according to  claim 1  further comprising in the second mode, collecting heat in the heat exchanger fluid from the part of the outer pipe that is surrounded by a part of the ground source having a highest temperature. 
     
     
         6 . The method according to  claim 1  further comprising:
 providing a heat exchanger in communication with the piping; 
 circulating the heat exchanger fluid through the heat exchanger in the second mode for extracting heat from the heat exchanger fluid at the heat exchanger; and 
 bypassing the heat exchanger in the first mode. 
 
     
     
         7 . The method according to  claim 1  including expanding the rock material in the ground source surrounding the outer pipe by transferring heat into the ground source along the upper portion of the outer pipe for closing fissures and other permeable conduits in the rock material and for preventing upward migration of formational fluids and gasses in the ground source. 
     
     
         8 . The method according to  claim 1  including transferring heat from the heat exchanger fluid to the ground source along an entirety of the outer pipe in the first mode. 
     
     
         9 . The method according to  claim 1  including operating the geothermal system in the first mode for an entire season. 
     
     
         10 . The method according to  claim 1  including operating the geothermal system only in the second mode at varying flow rates for an entire season. 
     
     
         11 . The method according to  claim 1  including controllably varying a flow rate of the heat exchanger fluid through the closed loop so as to maintain a temperature of the heat exchanger fluid at the top of the borehole at a substantially constant set point temperature. 
     
     
         12 . The method according to  claim 1  including pumping the heat exchanger fluid through the closed loop in a common direction in both the first mode and the second mode. 
     
     
         13 . The method according to  claim 1  including pumping the heat exchanger fluid downwardly through the outer passage and upwardly through the inner passage in both the first mode and the second mode. 
     
     
         14 . The method according to  claim 13  wherein an inner surface of the inner pipe is smoother than an outer surface of the inner pipe. 
     
     
         15 . The method according to  claim 1  wherein a lower portion of the inner pipe is uninsulated. 
     
     
         16 . The method according to  claim 1  wherein an entirety of the inner pipe is insulated. 
     
     
         17 . The method according to  claim 1  wherein a cross-sectional area of the inner passage is smaller than a cross-sectional area of the outer passage. 
     
     
         18 . The method according to  claim 1  including transferring heat from a solar heat collector to the heat exchanger fluid at the piping in the first mode. 
     
     
         19 . The method according to  claim 1  including converting hydrocarbon well having a well casing in communication with a production zone into the geothermal system by using the well casing as the outer pipe, plugging the well casing above the production zone to define the bottom end of the outer pipe, and inserting a pipe string into the well casing to define the inner pipe. 
     
     
         20 . A geothermal system for extracting heat from a ground source including rock material, the system comprising:
 an outer pipe supported within a borehole to extend longitudinally and downwardly into the ground source from a top end to a bottom end of the outer pipe in which the bottom end of the outer pipe is closed;   an inner pipe within the outer pipe to extend longitudinally from a top end of the inner pipe in proximity to the top end of the outer pipe to a bottom end in proximity to the bottom end of the outer pipe so as to define an inner passage extending longitudinally through the inner piper and so as to define an outer passage extending longitudinally within the outer pipe within an annular space between the inner pipe and the outer pipe;   a bottom end of the inner passage being in open communication with a bottom end of the outer passage;   piping in communication between the top end of the inner pipe and the top end of the outer pipe such that the inner and outer pipes form a closed loop;   a pump arranged to circulate a heat exchanger fluid through the closed loop of the inner passage and the outer passage; and   a controller arranged to operate the pump in either one of a first mode to store heat in the ground source or a second mode to recover heat from the ground source, such that:
 (i) in the first mode, the heat exchanger fluid is circulated so as to collect heat into the heat exchanger fluid from the ground source along a lower portion of the outer pipe that is in proximity to the bottom end of the outer pipe and so as to transfer heat from the heat exchanger fluid to the ground source along an upper portion of the outer pipe that is in proximity to the top end of the outer pipe; and 
 (ii) in the second mode, the heat exchanger fluid is circulated so as to collect heat in the heat exchanger fluid from the ground source along at least part of the outer pipe and so as to extract heat from the heat exchanger fluid at the piping.