US20110036112A1

US20110036112A1 - Turbulence Inducing Heat Exchanger

Info

Publication number: US20110036112A1
Application number: US12/858,287
Authority: US
Inventors: Andy Livingston; Stephen R. Lafaille
Original assignee: AMERICAN ECOTHERMAL Inc
Current assignee: AMERICAN ECOTHERMAL Inc
Priority date: 2009-08-17
Filing date: 2010-08-17
Publication date: 2011-02-17
Also published as: WO2011022406A2; WO2011022406A3

Abstract

A heat exchanger includes an elastically flexible liner formed as an elongated hose having an open end and a closed end. The elongated hose defines an inner surface. The heat exchanger also includes a plurality of longitudinal sleeves substantially disposed within the elongated hose. Each of the plurality of longitudinal sleeves has a proximate end defining an inlet opening and a distal end defining an outlet opening. When a fluid is pumped from the inlet openings of the plurality of longitudinal sleeves to the outlet openings of the plurality of longitudinal sleeves, the fluid passes from the outlet openings into the closed end of the elongated hose, along the inner surface of the elongated hose, and out through the open end of the elongated hose.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This Patent Application claims the benefit of U.S. Provisional Patent Application No. 61/234,416 filed on Aug. 17, 2009, entitled, “TURBULENT GROUND-SOURCE HEAT EXCHANGER”, the contents and teachings of which are hereby incorporated by reference in their entirety.

BACKGROUND

Vertical ground-source or ground-coupled heat exchangers are widely used in geothermal applications because they allow one to utilize the near constant temperature of the earth to increase the efficiency of the vapor-compression refrigeration cycle which is taking place inside the heat pump. They also have advantages over other geo-exchange arrangements because they do not require a large amount of space to install.
The industry standard for vertical ground-source heat exchangers for use specifically in geothermal or geo-exchange applications includes rigid tubing usually between 0.75 and 1.5 inches in diameter. The tubing is placed in a borehole in the ground in what is known as a U-tube configuration. The rigid pipe is typically inserted all the way to the bottom of the borehole. At the end of this pipe a u-bend fitting is attached and another piece of pipe is connected which spans the depth of the hole, extending back to the surface. This allows a complete loop to take place in the borehole which is known as the ground-loop. Heat exchange fluid is circulated down one side of the U-tube and then back up the other side, with the ultimate goal being to have the fluid entering the ground-loop either reject or absorb heat from the earth.
The efficiency at which heat is rejected or absorbed by the earth is determined by the thermal resistance of the borehole. The overall thermal resistance of the borehole should be as low as possible to ensure good heat transfer to the earth to reject and absorb heat as efficiently as possible. The thermal resistance of the borehole is affected by several factors. The first is the thermal resistance of the tubing itself. The second would be the thermal resistance of grout that surrounds the space between the U-tube and the surrounding earth. The third factor is the thermal resistance of the surrounding earth. However, the thermal resistance of the earth is a limiting factor for heat transfer as it is a constant and cannot be improved. The last factor that affects heat transfer rates within the ground loop is the flow through the ground loop. The flow rate through the ground loop as well as the characteristics of the flow can affect heat transfer.
An improved ground-source heat exchanger specifically aimed at geo-exchange applications is incorporated into the Kelix™ heat transfer system (produced by Kelix Heat Transfer Systems, LLC that is headquartered in Tulsa, Okla.). This system uses a 3.5 inch diameter rigid carbon fiber tube as the heat exchanger in place of the traditional U-tube. Inside this larger tube is another tube which forms a coaxial heat exchanger. The heat transfer fluid flows down the inner tube and then back up to the top and through the heat pump by flowing along the outside of the inner tube. The diameter and inner wall roughness of the inner tube is designed such as to promote laminar flow in hope to utilize the boundary layer as a thermal barrier between the incoming fluid and outgoing fluid. The outer wall of the inner tube is designed to promote turbulent flow which increases heat transfer by breaking up the boundary layer.

SUMMARY

Unfortunately there are deficiencies to the above-described conventional heat exchangers. For example, conventional heat exchangers have poor utilization of the borehole volume. The typical borehole diameter is six inches due to the size of the common drill bit used by industry professionals. The heat exchanger diameter of the Kelix™ system is only about 58% of what could fit inside the typical borehole. The remaining space is typically filled with grout. If the volume of the borehole were better utilized, there would be the potential to increase the heat transfer rate by using more heat transfer fluid per unit of vertical depth. As the heat transfer rate is increased, the required depth to be drilled can be decreased. The goal to increase heat transfer rates and decrease drilling depth is financially desirable because a large percentage of the cost of installing a geothermal heating and cooling system is the cost of drilling.
Another deficiency to the above-described conventional heat exchangers is that although the Kelix™ system aims to make use of turbulent and laminar flow, the invention still uses a significant amount of grout between the heat exchanger and the earth which significantly increases the thermal resistance of the borehole. They also add additional cost to their heat exchanger by using a manufacturing process to create turbulence inducing fins when the earth itself can better provide this turbulent effect.
In contrast to the above-identified conventional U-tube and Kelix™ system heat exchangers, an improved heat exchanger involves using a system and heat transfer column that increases heat transfer of fluid with the borehole yet decreases overall cost. Such a system significantly reduces the thermal resistance of the bore hole by utilizing the entire volume of the drilled borehole. It also eliminates the need for expensive grouts which add thermal resistance to the borehole therefore decreasing heat transfer. Due to the fact that it utilizes the entire volume of the drilled borehole the heat exchanger can hold 66% more heat transfer fluid than the Kelix™, and 93% more than the traditional U-tube configuration. The larger diameter heat exchanger provides more surface area with more intimate contact with the earth as well as a much larger volume of fluid. The significant increase in heat transfer fluid allows the temperature of the exiting water to be closer to the earth temperature as the overall temperature in the heat exchanger will not be as affected by the entering water. The proposed heat exchanger is made from an elastic, flexible, tear-proof, thin-walled material. The diameter of the heat exchanger is equal to the diameter of the corresponding borehole. This means once filled with heat transfer fluid, the heat exchanger takes the shape of borehole and the pressure of the fluid causes it to seal against the rock that makes up the borehole. This means the overall bore resistance is now only dependent on the thermal resistance of the thin walled material, the flow inside the heat exchanger, and the earth. This heat exchanger entirely removes the thermal resistance that the grout typically added to the overall borehole resistance. The inside of the borehole is rarely a smooth surface, and the proposed heat exchanger takes advantage of all the voids, nooks, and crannies that are a result of the drilling process. The flexible material conforms to fill these voids and imperfections along the inner surface of the borehole. These voids create random eddies, vortices, and other various flow fluctuations that aid in creating a turbulent flow regime which increase heat transfer to the surrounding earth.
One embodiment is directed to a heat exchanger for a geothermal heat pump system. The heat exchanger includes an elastically flexible liner formed as an elongated hose having an open end and a closed end. The elongated hose defines an inner surface. The heat exchanger also includes a plurality of longitudinal sleeves substantially disposed within the elongated hose. Each of the plurality of longitudinal sleeves has a proximate end defining an inlet opening and a distal end defining an outlet opening. When a fluid is pumped from the inlet openings of the plurality of longitudinal sleeves to the outlet openings of the plurality of longitudinal sleeves, the fluid passes from the outlet openings into the closed end of the elongated hose, along the inner surface of the elongated hose, and out through the open end of the elongated hose.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the invention.

FIG. 1 is a side view of a geothermal heat pump system having a heat exchanger.

FIG. 2 is a side cross section view of the heat exchanger of FIG. 1.

FIG. 3 is an overhead cross section view of the heat exchanger of FIG. 1.

FIG. 4 a is a side view of the heat exchanger of FIG. 1 prior to insertion in a borehole.

FIG. 4 b is a side view of the heat exchanger of FIG. 1 inserted into the borehole, wherein the heat exchanger is not pressurized with fluid.

FIG. 4 c is a side view of the heat exchanger of FIG. 1 inserted into the borehole, wherein the heat exchanger is pressurized with fluid.

FIG. 5 is a flowchart of a method of exchanging heat in the heat exchanger of FIG. 1.

DETAILED DESCRIPTION

A heat exchanger for geothermal heat pump systems utilizes an elastically flexible liner to contour to the surface of a borehole. By mimicking the surface of the borehole, the heat exchanger utilizes almost all of the volume of the borehole for pumping heat transfer fluid, which eliminates the need for grout and increases the heat transfer rate. Additionally, by using the contour of the borehole itself to create turbulent flow, expensive additional structures such as fins are not needed to produce turbulence.
FIG. 1 shows a geothermal heat pump system 100 having a heat exchanger 102, a well head 108, a fluid-line-in 110, and a fluid-line-out 112. The heat exchanger 102 is disposed vertically into the ground 114.
As shown in FIG. 1, heat exchange fluid enters the fluid-line-in 110, passes through the well head 108, and into the heat exchanger 102. While in the heat exchanger 102, the fluid either expels or receives heat from the ground 114. For example, if the ground 114 is warmer than the fluid, then the fluid receives heat, but if the ground 114 is cooler than the fluid, then the fluid expels heat. After the fluid passes through the heat exchanger 102, the fluid passes through the well head 108 and out the fluid-line-out 112. Many different heat exchange fluids may be used in the geothermal heat pump system. Examples of such fluids that may be used alone or in combination include water, propylene glycol, denatured alcohol, methanol, etc.
FIGS. 2-3 show the heat exchanger 102 that includes an elastically flexible liner 104, and a plurality of longitudinal tubes or sleeves 106 (i.e., one or more longitudinal sleeves 106). In particular, four longitudinal sleeves 106 a, 106 b, 106 c, and 106 d are shown, however this amount is shown only by way of example. The elastically flexible liner 104 is formed as an elongated hose having an open end 122 and a closed end 124. The plurality of longitudinal sleeves 106 are substantially disposed within the elongated hose and each define an inlet opening 118 and an outlet opening 120.
The plurality of longitudinal sleeves 106 are arranged along the outer circumference of the liner 104. Other positions of the sleeves 106 may be used as well. The sleeves 106 may be rigidly attached to the liner 104, although this is not required.
In use, fluid enters the inlet openings 118 of the plurality of longitudinal sleeves 106 from the fluid-line-in 110 via the well head 108. Inside the wellhead 108 is a manifold to distribute the incoming fluid from the fluid-line-in 110 to each of the tubes 106. The fluid passes from the inlet openings 118, through the sleeves 106 and out the outlet openings 120. The flow of the fluid through the sleeves 106 is substantially laminar.
Upon exiting the outlet openings 120, the fluid dumps into the closed end 124 of the liner 104 hose that forms a mixing area 126. The dumping of the incoming fluid into the mixing area at the very bottom of the elongated hose disrupts fluid flow to induce a turbulent zone at the bottom of the heat exchanger. The turbulent fluid flow then travels upward along the middle portion while exchanging heat with the earth 114 as shown by the arrow 128. Finally, the fluid exits the heat exchanger at the open end 122 to pass through the well head 108 and out the fluid-line-out 112.
FIG. 4 shows how the elastically flexible nature of the liner 104 enhances the turbulent flow of the fluid as it rises from the mixing area 126 to the open end 122 of the elongated hose.
As seen in FIG. 4 a, a borehole 116 is drilled into the ground 114. The borehole 116 has a borehole surface 132. In many cases, the borehole surface 132 is not uniformly smooth and may have a very coarse texture (this is shown in FIG. 4 as borehole surface 132 being uneven lines).
In FIG. 4 b, the heat exchanger 102 is inserted into the borehole 116. The liner 102 is sized to have substantially the same circumference as the borehole 116 in which it is inserted. The liner 104 of the heat exchanger 102, when not being pumped with fluid, is of a uniform shape (this is shown in FIG. 4 b as the edges of the heat exchanger 102 being straight lines). This concept is similar to an uninflated elongated balloon having a uniform contour.
In FIG. 4 c, fluid is pumped through the heat exchanger. The pressure that the fluid puts on an inner surface 136 of the liner 104 results in the liner 104 expanding and contouring to the shape of the borehole 116. An outer surface 130 of the liner 104 hugs the borehole surface 132. This concept is similar to the elongated balloon discussed above, after it is inserted into a soda bottle and subsequently inflated. The newly inflated balloon takes on the shape of its surroundings, and in particular the soda bottle.
Once the liner 104 expands to the shape of the borehole surface 132, the inner surface 136 of the liner 104 mimics the coarse texture of the borehole surface. Fluid passing from the mixing area 126 to the open end 122 of the elongated hose passes over this coarse inner surface 136 of the liner 104 which induces more turbulent flow in the fluid.
In certain boreholes 116, the texture of the borehole surface 132 would not have the desired roughness. One example is a borehole 116 drilled into a ground 114 with sandy soil conditions. The sandy nature of this type of ground 114 may lead to a much smoother borehole surface 132 than rockier conditions would have yielded. In these applications, the interior surface 136 of the liner 104 may be made with a coarse texture to provide the desired roughness to produce turbulent flow in pumped fluids.
Turbulent fluid flow in the geothermal heat exchanger 102 transfers more heat or cooling than laminar flow. Without structure to disrupt the fluid flow, the flow is laminar (e.g., the flow of fluid along the surfaces of the longitudinal sleeves 106 is laminar due to the smoothness of these surfaces). Since heat transfer across a boundary layer of a laminar flow is poor, turbulent flow is desired for more efficient heat transfer. Because of the different heat transfer properties between laminar and turbulent flow, the enhanced heat transfer experienced between the mixing area 126 and the open end 122 of the elongated hose due to turbulence is not substantially affected by the laminar flow of fluid in the plurality of longitudinal sleeves 106.
Traditional heat exchangers use rigid U-tubes inside the borehole. The U-tubes are smooth and therefore promote laminar instead of turbulent flow. To produce turbulent flow, additional structure (e.g., fins) must be provide such as that provided by the Kelix™ heat exchangers. This would mean extra expense that is not needed in the heat exchanger 102. The rigid U-tubes of the prior art also requires grout between the tube and the borehole. Typically, grout has poor heat transfer qualities and is an added cost that is not needed by the heat exchanger 102.
The liner 104 may be provided rolled up and may have a substantially rigid cap at the end 124. The liner 104 can be deployed inside a borehole 116 by mechanically pushing the end out with an elongated tool, by applying fluid pressure to the inside, or both. Once the liner is deployed within the borehole 116, pressure can be applied to the inside so that intimate contact with the rough surface 132 of the borehole 116 may be established. In this way, the inside 136 of the liner 104 takes on the rough surface texture of the borehole in the same way a latex glove takes on the texture of the wearer's fingers. Additionally, an adhesive 134 may be applied to at least a portion the outside 130 of the liner 104 to stick the outside 130 of the liner 104 to the borehole 116, depending on local conditions.
The heat exchanger eliminates the need for expensive grout and dedicated turbulence-inducing structures inside the heat exchanger. Also, the entire borehole volume can be used for heat exchange. This increases its efficiency and reduces the cost of a geothermal heat exchanger system allowing for boreholes that do not need to be drilled as deeply.
FIG. 5 shows a method 200 for exchanging heat in a heat exchanger 102 of a geothermal heat pump system 100.
Step 202 is to pump a fluid into the plurality of flexible sleeves 106, each of the plurality of flexible sleeves 106 having a proximate end defining an inlet opening 118 and a distal end defining an outlet opening 120, wherein the fluid moves from the inlet opening 118 to the outlet opening 120;
Step 204 is to pump the fluid from the outlet opening 120 into a mixing area 126 of the heat exchanger 102, the mixing area 126 formed by the closed end 124 of the elongated hose and inducing turbulent flow in the fluid.
Step 206 is to pump the fluid from the mixing area 126 to the open end 122 of the elongated hose.
Although the preferred embodiments of the present invention have been described herein, the above description is merely illustrative. Further modification of the invention herein disclosed will occur to those skilled in the respective arts and all such modifications are deemed to be within the scope of the invention as defined by the appended claim.

Claims

1. A heat exchanger for a geothermal heat pump system comprising:

an elastically flexible liner formed as an elongated hose having an open end and a closed end, the elongated hose defining an inner surface; and

a plurality of longitudinal sleeves substantially disposed within the elongated hose, each of the plurality of longitudinal sleeves having a proximate end defining an inlet opening and a distal end defining an outlet opening;

wherein, when a fluid is pumped from the inlet openings of the plurality of longitudinal sleeves to the outlet openings of the plurality of longitudinal sleeves, the fluid passes from the outlet openings into the closed end of the elongated hose, along the inner surface of the elongated hose, and out through the open end of the elongated hose.

2. The heat exchanger of claim 1:

wherein the heat exchanger is constructed and arranged to insert into a borehole having a borehole surface;

wherein the elongated hose is constructed and arranged to expand and contour to the borehole surface upon receiving pressure from the pumped fluid.

3. The heat exchanger of claim 2, wherein, upon expanding and contouring to the borehole surface, the elongated hose is constructed and arranged to generate turbulent flow in the pumped fluid along the inner surface by mimicking a coarse texture of the borehole surface.

4. The heat exchanger of claim 2:

wherein the elongated hose defines an outer surface;

wherein further comprising an adhesive covering at least a portion of the elastically flexible liner on the outer surface of the elongated hose, the adhesive constructed and arranged to adhere to the borehole surface.

5. The heat exchanger of claim 1, wherein the plurality longitudinal sleeves are each affixed to the elastically flexible liner at the inner surface of the elongated hose.

6. The heat exchanger of claim 1, wherein the plurality of longitudinal sleeves are polyethylene bags.

7. The heat exchanger of claim 1, wherein the closed end of the elongated hose forms a mixing area that induces a turbulent flow in the pumped fluid that passes from the outlet openings of the plurality of longitudinal sleeves.

8. The heat exchanger of claim 1, wherein the inner surface of the elongated hose is coarsely textured to generate turbulent flow of the fluid pumped through the elongated hose.

9. A method for exchanging heat in a heat exchanger of a geothermal heat pump system, the heat exchanger having a plurality of longitudinal sleeves and an elastically flexible liner, the elastically flexible liner forming an elongated hose having an open end and a closed end, the method comprising:

pumping a fluid into the plurality of flexible sleeves, each of the plurality of flexible sleeves having a proximate end defining an inlet opening and a distal end defining an outlet opening, wherein the fluid moves from the inlet opening to the outlet opening;

pumping the fluid from the outlet opening into a mixing area of the heat exchanger, the mixing area formed by the closed end of the elongated hose and inducing turbulent flow in the fluid; and

pumping the fluid from the mixing area to the open end of the elongated hose.

10. The method of claim 9:

wherein the heat exchanger inserts into a borehole having a borehole surface;

wherein further comprising pressurizing the elongated hose with the pumped fluid to expand and contour the elongated hose to the borehole surface.

11. The method of claim 10, further comprising, upon pressurizing the elongated hose with the pumped fluid, inducing a turbulent flow by pumping the fluid along the inner surface that is mimicking a coarse texture of the borehole surface.

12. The method of claim 10:

wherein the elongated hose defines an outer surface;

wherein further comprising adhering the outer surface of the elongated hose to the borehole surface with an adhesive applied to at least a portion of the elastically flexible liner on the outer surface of the elongated hose.

13. A geothermal heat pump system comprising:

a wellhead;

a fluid-line-in connected to the wellhead;

a fluid-line-out connected to the wellhead; and

a heat exchanger connected to the wellhead, the heat exchanger having:

an elastically flexible liner formed as an elongated hose having an open end and a closed end, the elongated hose defining an inner surface, the open end of the elongated hose constructed and arranged to deliver a fluid to the fluid-line-out via the wellhead; and

a plurality of longitudinal sleeves substantially disposed within the elongated hose, each of the plurality of longitudinal sleeves having a proximate end defining an inlet opening and a distal end defining an outlet opening, each inlet opening constructed and arranged to receive the fluid from the fluid-line-in via the wellhead;

wherein, when the fluid is pumped from the inlet openings of the plurality of longitudinal sleeves to the outlet openings of the plurality of longitudinal sleeves, the fluid passes from the outlet openings into the closed end of the elongated hose, along the inner surface of the elongated hose, and out through the open end of the elongated hose.

14. The system of claim 13:

15. The system of claim 14, wherein, upon expanding and contouring to the borehole surface, the elongated hose is constructed and arranged to generate turbulent flow in the pumped fluid along the inner surface by mimicking a coarse texture of the borehole surface.

16. The system of claim 14:

wherein the elongated hose defines an outer surface;

wherein the heat exchanger further includes an adhesive covering at least a portion of the elastically flexible liner on the outer surface of the elongated hose, the adhesive constructed and arranged to adhere to the borehole surface.

17. The system of claim 13, wherein the plurality longitudinal sleeves are each affixed to the elastically flexible liner at the inner surface of the elongated hose.

18. The system of claim 13, wherein the plurality of longitudinal sleeves are polyethylene bags.

19. The system of claim 13, wherein the closed end of the elongated hose forms a mixing area that induces a turbulent flow in the pumped fluid that passes from the outlet openings of the plurality of longitudinal sleeves.

20. The system of claim 13, wherein the inner surface of the elongated hose is coarsely textured to generate turbulent flow of the fluid pumped through the elongated hose.