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.

Description

This application claims the benefit under 35 U.S.C. 119(e) of U.S. provisional application Ser. No. 62/693,939, filed Jul. 4, 2018.

FIELD OF THE INVENTION

The present invention relates to the recovery of heat from a geothermal system including a well in the ground from which heat is extracted, and more particularly the present invention relates to a geothermal system which is seasonal varied between a heat recovery mode in which heat is drawn out of the geothermal well and a heat storage mode in which heat is stored in the geothermal well.

BACKGROUND

Below the Earth's surface, temperature increases with depth. The rate of temperature increase, or geothermal gradient typically ranges between 2 and 3° C. per 100 metres of depth. However, in some regions, the gradient can be significantly higher with very hot temperatures found closer to the Earth's surface. Current technologies are exploiting some of these near surface hot spots for electrical power generation and heating.

There are two commonly used heat recovery methods for deeper geothermal systems. The most common practice is where boreholes tap in to underground hot zones either by drilling a pair of boreholes or a single borehole that recover heated water from hot rock material at depth. In the first case (two well method) one borehole is for injecting cooler water and the other borehole is for producing the heated water. The wells are connected at depth usually by an aquifer or by fracturing the rock material between the wells. Water is pumped down the injection well, through the permeable zone between the wells then pumped back to the surface in the second well. The other, single well method, is where an aquifer has sufficient hot water recharge that it can produce heated water without the need of a second injection well. These technologies are used in many countries where subterranean hot zones occur at shallower than normal depths. These existing systems have several drawbacks. The cost of drilling the wells is high, and the success rate of properly connecting the two wells at depth can be relatively low. Further, the connected zone between the well pairings is difficult to control as there are fluid losses in to the rock formations. In this case a significant amount of pumping is required to counteract the fluid loss. The greatest drawback of the single aquifer pumping well method is when there is less than adequate water recharge from the aquifer. Both single and double well methods require large pumps and power to lift the water from deep underground. Furthermore, the fluid returned to the surface may contain high levels of noxious contaminants which have to be disposed of safely. Formational water can also be corrosive which reduces the life expectancy of the pipes and pumps.

A third, less commonly used method was developed whereby two pipes are contained within the other in a single well. This coaxial borehole is sealed at the base which prevents any interaction between the borehole and formational fluids and gases. The two pipes act as a long, linear heat exchanger. Water is pumped in to the well between the two pipes and is heated from the adjacent rock formation as the water descends. At the bottom of the borehole the heated water is then redirected to surface through an insulated inner pipe. Once the heated water reaches the surface, the heat is extracted and the resultant cooled water is then re-reinjected in to the same well to complete the water circuit. One of the advantages over the heated water recovery process is there is no production of hazardous fluids and gases. Another advantage is, since the closed loop circuit is sealed, the pumping demand is significantly reduced because there is no hydraulic head to overcome. The disadvantage of this heat exchanger technology is the significantly reduced amount of heat recovered. Because the descending cool water and the ascending heated water have to share the same borehole, the volumes of water are significantly reduced. Technically it is difficult to achieve sufficient water residence time for boreholes less than 178 mmm diameter. Further, the higher the flow velocity of the injected water reduces the residence time which reduces the amount of heat recovered.

There are two key factors that limit the economics of the coaxial well technology. The first factor is the high cost and the space restrictions of the insulated inner pipe. The second factor is the restricted heat recovery due to the shortened residence time of the fluid during the heat transfer process. Although the heat exchange technology has significant environmental benefits, low heat productivity and high capital costs have prevented this technology from flourishing.

Several patents or patent applications relevant to deep single or multiple well geothermal energy recovery include the following: (i) Horton, U.S. Pat. No. 5,203,173 for a Device for utilization of geothermal energy; and (ii) Montgomery, U.S. Pat. No. 8,708,046 for a Closed Loop Energy Production from Geothermal Reservoirs. The prior patents or patent applications noted above focus mainly on recovering high temperature geothermal energy for electrical power generation. Therefore, the technology is designed to recover fluids or steam hot enough to drive turbines for power generation. In order to do so, higher temperature heat sources are required. The heat recovery happens in a zone near the bottom of the well where temperatures are highest. Piping systems above the targeted heat zones are insulated in order to convey the superheated fluid to the surface so it can be converted to steam for turbine power. Pipes are also coated to prevent deterioration by corrosive fluids. The Horton and Montgomery patents take a different approach and call for the heat exchange within a coaxial, closed loop system and within a naturally occurring geothermal gradient.

US Patent Application Publication No. 2011/067399 by Rogers and U.S. Pat. No. 9,121,393 by Schwarck disclose systems which can be operated by pumping fluid down a centre tube so that the fluid rises in heat exchanging relationship with the surrounding wellbore in the annulus. Neither system includes various operating modes according to different seasonal temperatures with the goal of improving the overall efficiency of heat recovery from a wellbore throughout various seasons without the need of supplemental heat from other sources.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided 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.

According to a second aspect of the present invention there is provided a geothermal system for extracting heat from a ground source, 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 a part of the outer pipe and so as to extract heat from the heat exchanger fluid at the piping.

Preferably the inner pipe is partially to totally insulated along its length to minimize heat losses from the ascending heat exchanger fluid.

The heat storage mode described herein works together with the heat recovery mode so that the overall efficiency of heat recovery throughout the thermal storage and recovery process is improved. The method can recover heat at a temperature adequate for space heating and other uses without the need of supplemental heat from other sources.

The invention uses the previously developed single well coaxial pipe heat exchanger technology. The inventive nature of this Application is to include several equipment and operational modifications to significantly increase the amount of recoverable heat, and also increase the recoverable temperature at surface. These equipment and operational changes can also significantly reduce capital and operating costs.

Much of the heat energy from deeper geothermal wells is used for space heating during colder months especially in higher latitude countries. Therefore, the heat demand only occurs over the colder season, leaving the well dormant when heat demand is limited. The concept behind this invention is to continue to recover and store the heat during the period of normal well dormancy, then release the heat for use when needed. A key to success for this option is the stored heat has to be readily available when needed.

In the second mode, the heat exchanger fluid may be circulated for 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. A variable rate pump may be used to vary a flow rate of the heat exchanger fluid between a high heat demand and a low heat demand in the second mode.

In the first mode, heat loss from the piping at the surface is minimized, using insulation and/or by diverting flow to minimize the length of the flow path between the inner and outer pipes.

In the second mode, the heat exchanger fluid may be circulated for 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.

The method may further include (i) providing a heat exchanger in communication with the piping, (ii) 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 (iii) bypassing the heat exchanger in the first mode.

The method may further include 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.

Heat may be transferred from the heat exchanger fluid to the ground source along an entirety of the outer pipe in the first mode.

The geothermal system may be operated only in the first mode for an entire season, and operated only in the second mode at varying flow rates for an entire season.

The flow rate of the heat exchanger fluid through the closed loop may be controllably varied so as to maintain a temperature of the heat exchanger fluid at the top of the borehole at a substantially constant set point temperature.

The heat exchanger fluid may be pumped through the closed loop in a common direction in both the first mode and the second mode, for example the heat exchanger fluid may be pumped downwardly through the outer passage and upwardly through the inner passage in both the first mode and the second mode.

An inner surface of the inner pipe may be smoother than an outer surface of the inner pipe.

A lower portion of the inner pipe may be uninsulated, or alternatively an entirety of the inner pipe may be insulated.

A cross-sectional area of the inner passage is preferably smaller than a cross-sectional area of the outer passage.

In the first mode, heat may be transferred from a solar heat collector into the heat exchanger fluid at the piping.

The method may further include 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.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention will now be described in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic representation of the geothermal system operating in a first heat storage mode; and

FIG. 2 is a schematic representation of the geothermal system operating in a second heat recovery mode.

In the drawings like characters of reference indicate corresponding parts in the different figures.

DETAILED DESCRIPTION

Referring to the accompanying figures there is illustrated a geothermal system 10 installed in the ground such that the ground is used as a heat source and/or heat sink. In the illustrated embodiment, the geothermal system is installed within a reclaimed hydrocarbon wellbore in which the existing well casing of the wellbore defines an outer pipe 12. The well casing is plugged at a location above a production zone of the wellbore to define a bottom end of the outer pipe 12 such that the outer pipe extends longitudinally downward from a top end at a surface of the ground heat source to the bottom end which is closed. Alternatively, any borehole formed in the ground and lined with an outer pipe may be used.

An inner pipe 14 is installed within the outer pipe 12 in which the inner pipe has an outer diameter which is less than the inner diameter of the outer pipe so as to define an annular space between the inner and outer pipes extending along the length of the pipes. The inner pipe 14 extends longitudinally downward from a top end adjacent to the top end of the outer pipe to a bottom end of the inner pipe which is spaced above the bottom end of the outer pipe while remaining in close proximity to the bottom end of the outer pipe relative to the overall length of the pipes.

The inner pipe 14 defines an inner passage 16 therein which extends longitudinally through the inner pipe between the top and bottom ends of the inner pipe. Similarly, an outer passage 18 is defined within the annular space between the inner and outer pipes to extend longitudinally along the full length of the pipes between the top and bottom ends thereof. The inner pipe 14 is typically insulated along the length thereof so as to provide a heat insulating barrier between the inner passage 16 and the outer passage 18 along the length thereof.

The bottom end of the inner pipe 14 remains open so as to be in open communication with the bottom end of the surrounding outer pipe.

The top ends of both the inner pipe and the outer pipe are closed off other than to communicate with suitable piping 20 interconnected between the top end of the outer passage and the top end of the inner passage such that the inner passage, the outer passage, and the piping collectively define a closed loop containing heat exchanger fluid therein which is circulated through the closed loop as described in further detail below.

The piping 20 includes an inlet portion 22 communicating with the top end of the outer passage 18 for subsequent connection to a T-junction which joins the inlet portion 22 to a first branch portion 24 from which it receives fluid in a first mode and a second branch portion 26 from which it receives fluid in a second mode. Similarly, an outlet portion 28 of the piping is in open communication with the top end of the inner passage such that the other ends of the first and second branch portion 24 and 26 are connected to the outlet portion 28 with another T-junction such that the first and second branch portions are in parallel flow relative to one another between the inlet portion 22 and the outlet portion 28. The outlet portion 28 receives fluid exiting the inner passage 16. In this manner, the first branch 24 defines a first loop of fluid between the outer passage and the inner passage for use in a first heat storage mode, while the second branch portion 26 defines a second loop of fluid between the outer passage and the inner passage for use in a second heat recovery or heat extraction mode.

A suitable pump 30 is connected in series with the outlet portion 28 of the piping that is oriented for drawing fluid upwardly through the inner passage of the inner pipe 16 to be pumped into either one of the first or second branch portions 24 or 26 such that fluid returns through one of the first or second branch portions 24 or 26 before descending downwardly through the outer passage 18 between the inner and outer pipes.

A heat exchanger 32 is connected in series with the second branch portion 26 to receive the heat exchanger fluid of the geothermal system circulated there through in the second heat recovery mode and to permit heat ‘q’ to be extracted from the heat exchanger fluid at the heat exchanger for producing useful work such as generating electrical energy or for heating other devices for example during some modes of operation.

One or more first valves 36 are connected in series with the first branch portion 24. In the illustrated embodiment, a single first valve 36 may be provided in the first branch portion 24 if the first branch portion is short in length.

Optionally, a solar heat collector 38 (shown in broken line in FIG. 1) may be connected in series with the first branch line 24. In this instance a pair of first valves 36 are preferably connected in series with the first branch line at opposing ends of the first branch line so that the solar heat collector 38 communicates with the first branch line between the two first valves 36. The solar heat collector 38 is adapted to collect heat and transfer the heat ‘q’ to the heat exchanger fluid.

A set of second valves 34 are similarly connected in series with the second branch line 26 at opposing ends of the second branch line 26 to receive the heat exchanger between the second set of valves 34.

A controller 40 is provided in operative connection to the pump 30 and the valves 34 and 36 for operating the pump and the valves in various modes and according to various demands for heat, as well as varying amounts of available heat from the ground heat source.

When there is at least some demand for heat to be extracted from the heat exchanger fluid at the heat exchanger 32, the controller operates the system in the second mode to recover heat from the ground source. In this instance the second valves 34 are opened and the first valves 36 are closed such that the heat exchanger fluid is circulated through the heat exchanger 32 in the second branch portion 24 of the piping as it is pumped through the closed loop by the pump 30. In this mode, heat is transferred from the surrounding ground source into the heat exchanger fluid along a lower portion of the outer pipe adjacent to the bottom end thereof. The heat exchanger fluid then carries the heat upwardly along the inner passage for subsequent circulation through the heat exchanger 32 where heat ‘q’ can be extracted from the heat exchanger fluid.

When there is low demand for heat, excess heat carried by the heat exchanger fluid can be transferred back into the ground heat source along the upper portion of the outer pipe adjacent to the top end thereof in addition to withdrawing heat at the heat exchanger.

Alternatively, when under high demand for heat at the heat exchanger, in which more heat is withdrawn from the heat exchanger fluid at the heat exchanger, the heat exchanger fluid may withdraw heat from the surrounding ground heat source along both the lower and upper portions of the outer pipe such that heat is withdrawn from the ground source along the full length of the outer pipe 12.

Temperature sensors may be provided in the outer passage adjacent to the outer boundary formed by the outer pipe at the top end thereof to monitor temperature of the heat exchanger fluid and/or to monitor the temperature of the ground source at the top end of the outer pipe. In this instance, the controller may vary the flow rate of the heat exchanger fluid being circulated by the pump in order to maintain a substantially constant temperature at the sensor location.

A reduced flow rate allows more heat to be collected from the ground source at the lower portion of the well which can result in transfer of heat back into the surrounding ground source along the upper portion of the outer pipe at certain times during the operation of the system. Alternatively, an increased flow rate allows less heat to be collected from the ground source at the lower portion.

The rate of heat withdrawal from the heat exchanger fluid at the heat exchanger can be controlled by varying the flow rate of a second heat exchanger fluid that exchanges heat with the primary heat exchanger fluid of the system at the heat exchanger. Withdrawing more heat from the primary heat exchanger fluid in response to increased heat demands lowers the temperature of the heat exchanger fluid being returned along the outer passage which may then result in heat being withdrawn from the surrounding ground source along both the lower and upper portions of the outer pipe in order to reach the target temperature for the heat exchanger fluid.

When there is no demand for heat to be withdrawn from the heat exchanger fluid at the heat exchanger 32, the system may be operated in the first heat storage mode for storing more heat across a greater portion of the ground surrounding the outer pipe according to FIG. 1. In this instance the first valves 36 are opened and the second valves 34 are closed so that the heat exchanger fluid circulated by the pump passes through the first branch portion 24 of the piping so as to bypass the heat exchanger 32. In this instance, heat is continued to be transferred into the heat exchanger fluid from the ground heat source along the lower portion of the outer pipe; however, the increased temperature of the heat exchanger fluid results in the excess heat being transferred back into the ground heat source along the upper portion of the outer pipe. To store further heat in the ground, the optional solar heat collector 38 can be used to collect solar heat and transfer the heat ‘q’ into the heat exchanger fluid along the first branch portion 24 of the piping.

In some instances, sufficient heat may be collected by the solar heat collector to create a sufficient temperature differential that heat may be transferred from the heat exchanger fluid back into the ground heat source along the lower portion of the wellbore as well. Alternatively, heated water from the solar collectors and/or from a geothermal source can be stored on surface using an insulated storage tank.

As described above, FIGS. 1 and 2 are visual descriptions of the coaxial pipe configurations. The illustrated example uses, but is not limited to, a pre-existing oil and gas well. These wells typically have a steel production pipe installed and cemented to the enclosed rock formations for the total length of the pipe. The purpose of the cemented pipe is to stabilize and isolate the adjacent rock material from the borehole. Once the borehole ceases hydrocarbon production, the base of the borehole is sealed above the previous production zone. This is done by placing a permanent plug then covering it with a thick protective layer of cement. This is to prevent any inflow of fluids or gases in to the cased well.

This steel production casing becomes the outer pipe of the geothermal heat recovery system. A second smaller diameter pipe is inserted inside the outer pipe. The inner pipe can either be supported at the surface using existing well hangers or supported at the base by a bottom hole assembly that designed as to not impede water flow between the pipes.

The inside surface of the inner pipe should be smooth, whereas the outside surface of the inner pipe can benefit from having a rougher surface. This rougher exterior will cause more fluid turbulence which improves heat transfer. For shallower, lower temperature wells the inner tube can be plastic or fibreglass in composition. For deeper wells with higher temperatures and heavier pipe weight, a steel string is the preferred option.

The apparatus is designed to utilize available boreholes that are no longer in use. Repurposing existing boreholes can significantly reduce the capital costs of recovering geothermal energy. Oil and gas wells normally have three different sizes of production pipe diameters. These are 178, 149 and 118 mm diameter. For each converted well the selected inner pipe size has to leave sufficient room in the outer pipe for the proper flow conditions as described.

There are two separate, distinct modes of operation. These are referred to as the Heat Storage Mode (FIG. 1) and the Heat Recovery Mode (FIG. 2). The Heat Storage Mode commences after the existing well has been sanitized, the equipment installed and system tested. The purpose of this initial stage is to develop the heat envelope around the well bore. Unheated fresh water is pumped in to the well via the annular space. As the water descends it will start to collect heat from the surrounding rock once it reaches the depth where the surrounding rock is warmer than the descending water. When it reaches the well bottom the heated water is redirected back to surface via the inner pipe. Some heat will be lost as the water ascends but it will be re-absorbed by the descending water within the inner pipe. Once the ascending water with the remaining absorbed heat reaches the surface, it is redirected back down the well to complete the circuit. Each cycle of the water circuit will slightly increase the temperature of the water and the resultant heat will be transferred in to cooler surrounding rock. This stage, which can continue for a period of up to 7 months, effectively transfers the heat from the lower hot section to the upper cold section of the well. As time progresses heat will continue to be pushed into the rock formation causing the heat envelope to expand laterally.

Shortly before the start of the heating season the valves are opened to the connecting surface heat exchanger system. This is the start of the Heat Recovery Mode of operation. The surface pump is connected to the heat exchanger by a hot and a cold pipe assembly (FIG. 2). After the heat is recovered in the surface heat exchanger the resultant cooled water is re-injected through the annulus and transported to the well bottom to recover the maximum amount of additional heat from the higher temperature zone in the well. As the water ascends any additional required heat will be recovered from the stored heat envelope developed during the previous Heat Storage Mode. In so doing, the maximum amount of heat is recovered from the hotter, lower zone and the upper zone is only required for topping up the water temperature if insufficient heat was recovered from the lower zone. Flow direction in the well can be reversed if deemed more efficient for certain well conditions.

The Invention calls for the use of an electrically powered variable frequency drive pumps (VFD) for controlling the amount of heat transfer both underground and at the surface. When the heating season starts, the demand for heat is low. For example, the heating season normally starts in Autumn and may need some heat overnight but not during daylight hours. The VFD pump would operate at a very low flow rate and only a small amount of heat would be transferred. The remaining heat in the water would then be returned to the well. As the season progresses the demand for heat increases hitting peak demand in mid Winter After that, demand starts to decrease again until late Spring. When demand increases the VFD pump increases the flow volume in order to accommodate the need for more heat. As well, as the heat demand increases the exit temperature of the return cooled water drops which creates the demand for more heat recovery from the well. The preferred source for the increased heat demand is at the bottom of the well where the heat pool is most sustainable. The goal is to complete the heating season by maintaining a constant maximum temperature of the casing wall at surface. The design also calls for excess heat in the upper section so that colder than normal weather conditions can be accommodated by the variable speed pump and the extra heat stored “in the Bank”.

Once the heating season is over, the well system is returned to Heat Storage Mode in preparation for the next heating season.

This developed heat envelope has two key benefits. With the envelope heat recovery occurs over the total length of the well. Without the heat envelope, a coaxial geothermal well can only recover heat from near the bottom of the borehole. The Invention can provide greater heat recovery and a higher well head temperature. The second benefit is, as the heat envelope develops, the surrounding rock expands and seals the natural or drilling induced fissuring and fracturing. This helps prevent upward migration of light hydrocarbons (such as methane) or other harmful fluids and gases. Fugitive methane gas is a Greenhouse Gas (GHG) and is considered a major contributor to climate change.

If possible, the volume of the inner pipe should be less than the volume of the space between the inner and outer pipe.

The outer pipe is sealed at the base of the borehole. The inner pipe is open at its base.

The bottom of the inner pipe is placed sufficiently above the base of the borehole to prevent erosion of the borehole bottom.

The transport fluid is fresh water, and may have minor amounts of corrosion inhibitors, antifreeze and anti-friction materials if warranted.

The inner and outer pipes will be connected at surface by a variable speed pump and two sets of valves. One set of 2 valves control the water flow from the well to the surface heat exchanger and back. The third valve controls the water flow through the bypass pipe as shown in FIG. 1.

During Heat Storage Mode the 2 valves to and from the surface heat exchanger are closed. The valve connecting the pump to the space between the inner and outer pipes via the bypass pipe is opened as shown in FIG. 1.

During Heat Storage Mode unheated water is pumped into the borehole via the outer passage between the inner and outer pipes.

As it descends heat will be transferred to the water once it reaches a depth where the surrounding temperature is higher than the descending water temperature. As the water continues to descend the rate of heat recovery will increase as the temperature differential between the water and the adjacent rock increases.

When the heated water reaches the base of the borehole it will be redirected back to the surface via the inner passage of the inner pipe.

During this process the transfer of heat stays within the water loop, and progressively redistributes the heat into the cooler upper rock zone over time.

With each water circuit cycle the well head temperature increases slightly until the ascending water temperature at surface stabilizes.

Continued pumping will push more heat in to the surrounding rock as the radius of maximum temperature increases away from the borehole.

Upon completion of the Heat Storage Mode, the Heat Recovery Mode starts. The valves connecting the borehole to the surface heat exchanger pipes are opened and the bypass pipe valve closed as shown in FIG. 2. The heated water is then redirected through the surface heat exchanger.

The resultant cooled water from the surface heat exchanger is returned to the borehole to recover more heat, and to complete the heat recovery circuit.

The equipment configuration is designed to work with a summer solar heating assembly. The resultant additional heat can then then be introduced in to the Heat Storage Mode and used to top up the temperature within the heat envelope.

The present invention described herein provides a method of operating a coaxial borehole whereby the rock material surrounding the borehole is pre-heated prior to recovering heat at the surface. The apparatus typically comprises two pipes where the smaller pipe is inserted inside the larger pipe and fluid is pumped downwardly between the two pipes and returned upwardly through the inner pipe. The inner pipe, comprising of a smaller area than the area between the inner and outer pipe, can provide a higher flow velocity in one direction and a slower flow velocity in the opposite direction.

The inside wall of the inner pipe, consisting of a smooth surface to minimize flow turbulence, can reduce heat loss.

The outside wall of the inner pipe can consist of a rougher surface to enhance turbulence and heat exchange.

The two pipes at surface are connected together by a variable speed pump in a closed loop water circuit.

The pipe and pump are sealed from the outside environment to improve pump efficiency and eliminate environmental risks.

The borehole pipe system is connected to a two-pipe system at the surface to transport the recovered heat to a surface heat exchanger.

The fluid flow from the downhole piping system to the surface piping system is controlled by valves on the two connected surface pipes. This is to change the configuration from a Heat Storage Mode to a Heat Recovery Mode.

In the Heat Storage Mode, the water pumped into the well collects heat from the lower section of the well and is transferred and stored in the surrounding rock in the upper, cooler section of the well. Heat that is not transferred in to the surrounding rock is recycled within the well and will eventually be transferred to the surrounding rock over successive water cycles.

At the completion of the Heat Storage Mode, the valves are opened and the configuration becomes the Heat Recovery Mode whereby the heated water is redirected to the user of the heat.

The inner pipe attributes allow for rapid ascent of the water to minimize both heat loss from the upper section of the well. This allows for a greater temperature differential of the water and the surrounding rock which increases heat recovery in the higher temperature zone at its base.

The larger exterior space and the rougher exterior wall of the inner pipe in allow for slower fluid velocity and higher flow turbulence. These attributes increase heat recovery from the surrounding rock.

The improved thermal conductivity in the lower section of the well reduces the heat demand as the transporting fluid ascends in to the previously developed heat envelope in the upper, cooler section of the well. Reduced heat demand in the upper section of the well allows for a higher more sustainable water temperature at the surface.

The higher recovered water temperature (i) reduces the need for upgrading the heat to match users' heating needs, and (ii) allows the inclusion of more wells that are too shallow or too small in diameter to be considered economically viable without the Invention.

The heat recovered only requires energy in the form of electricity to operate the pump. The only source of Greenhouse Gas (GHG) emissions is limited to the amount of carbon based electrical power generation.

The heat envelope developed in the surrounding rock causes the heated rock to expand. The expansion of the rock leads to closing of the cracks and fissures which reduces or eliminates any potential conduits outside the pipe. This is beneficial because seepage of hydrocarbon and noxious fluids and gases into aquifers and to the surface have a harmful effect on the atmosphere and surface waters.

Since various modifications can be made in the invention as herein above described, and many apparently widely different embodiments of same made, it is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.

Claims

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.