WO2012140324A1

WO2012140324A1 - Apparatus for implementing a ground source heat system and method for exploiting the same

Info

Publication number: WO2012140324A1
Application number: PCT/FI2012/050361
Authority: WO
Inventors: Juha Karvinen
Original assignee: Runtech Systems Oy
Priority date: 2011-04-14
Filing date: 2012-04-13
Publication date: 2012-10-18
Also published as: FI20115359A0

Abstract

The invention relates to an apparatus for implementing a ground source heat system and further to a method utilizing the same. The invention is characterized in that the system comprises small-diameter and shallow boreholes (8) or equivalent screwably-sunken metal pipes, such as screw piles or heat pipes inserted in the soil. The pipes (13) are inserted in the boreholes (8) for transferring heat by convection and the pipes (13) are connected to a liquid-filled ground source heat circulation (7) placed in a short trench (6), wherefrom heat is further transferred along an insulated pipe (2) to the heating/cooling system of a building (1) or, vice versa, heat is transferred therefrom into the ground.

Description

APPARATUS FOR IMPLEMENTING A GROUND SOURCE HEAT SYSTEM AND METHOD FOR EXPLOITING THE SAME

The invention relates to an apparatus according to the preamble of claim 1 for implementing a ground source heat system. The invention also relates to a method in accordance with claim 8.

Ground source heat is well known and over years the exploitation of energy extractable therewith has been attempted by means of different kinds of technical arrangements. Depending on the installation depth and type of soil, extraction of ground source energy is possible at a level of 50 - 100 kWh/year from one cubic meter of soil at a temperature of about 4 -10 °C. A ground source heat system comprises fluid-filled underground piping network, wherein the fluid is circulated to transfer heat from the soil to the heat exchanger of the heat pump system. The fluid cools down in the heat exchanger and is reheated while returning to the ground loop. Between the heat pump and the heat exchanger is circulated a refrigerant, whose temper- ature is elevated by means of a compressor to a desired level and the heat content thereof is transferred by another heat exchanger to the consumer.

The liquid-filled ground source heat loop is typically made from polyethylene or the like cost-effective polymer. For higher efficiency the looped piping is advantageously placed in a maximally moist or warm location. Typically, the piping is submerged in a waterway/lake, dug in ground or sunk into a 100 to 250 m deep well drilled in bedrock by conventional drilling techniques.

Ground source heat systems implemented using state-of-the-art methods are based on a few typical techniques for absorbing thermal energy from the ground.

One conventional technique is to gather heat close to the soil surface. In this arrangement a trench is dug close to the building to a depth below the frost line. The length of the trench is dictated by the soil properties and heat capacity need of the building, typically being about 200 - 800 m. For heat absorption, to the bottom of the trench is placed a plastic pipe for the inflow/return flow of a nonfreezing thermal transfer liquid.

An alternative technique is to drill a borehole of about 150 mm diameter to a depth of about 100 - 250 m typical, depending on the required heating capacity. Into the borehole are inserted two plastic pipes serving as the circulation loop. To the bottom of borehole is adapted a specific fitting for joining the two pipes. In addition to those described above, a new technique has become commercially available, namely a method using only a single pipe, wherein heat is transferred utilizing the so-called heat pipe principle.

The term heat pipe refers to a so-called two-phase thermosiphon comprising a sealed pipe containing a small amount of a liquid that evaporates at the higher temperature of the pipe bottom and condenses at the lower temperature of pipe top. As the heat pipe is closed at both ends, heat must be transferred to the thermal transfer liquid of the ground source heat circulation or directly to the refrigerant circulation of the heat pump circuit using an interface material of high thermal conductivity such a copper.

Disadvantages hampering present techniques are particularly related to the high cost of borehole drilling and piping trench excavation. Additionally, the borehole causes environmental risks due to, e.g., altered groundwater flows. Major earthworks may also be turn out complicated to perform in the yard of an existing house.

In an arrangement based on digging a long trench in the soil frequently is hampered by the limited area of the parcel. Moreover, also yards are destroyed under extensive digging operations, whereupon they must be restored. This is because a typical length of piping and its trench is up to hundreds of meters, typically about 200 - 800 m.

Ground heat extraction by the borehole method is problematic in planned areas of large suburban districts that may have an existing or developing plan for the use of bedrock. Frequently, permissions to make borehole wells in these areas are in short supply, particularly due to restrictions written in underground planning. Additional problems arise from the environmental risk to groundwater and high cost of drilling a long borehole.

The use of a heat pipe is well known in the art in conjunction with a ground source heat system and installed in a large-diameter borehole thereof. However, this arrangement involves the same problems as any deep ground heat boreholes to be drilled in areas regulated by a district plan.

Another further application of heat pipe known in the art relates to the transfer of heat with a large number of heat pipes installed in relatively shallow boreholes. This arrangement is typically used for keeping pedestrian walkways free from ice during wintertime. The concept is hampered by the relatively high cost of long heat pipes.

The problems of the above-described techniques can be avoided almost entirely when an arrangement according to the present invention is employed having the ground source heat circulation, i.e., the underground heat- extracting piping, replaced by a vastly more efficient one. The ground source heat circulation arrangement according to the invention uses an entirely new technique of heat extraction and implementation.

The invention relates to an entirely novel, replacing product and its technical implementation for recovering ground source heat. The embodiments of the present invention can minimize the problems hampering prior-art methods. Moreover, ground source heat systems may now be implemented at a substantially lower cost than what is possible conventionally. Resultingly, a ground source heat system can now be used in buildings smaller than those today having such an installation.

The arrangement according to the invention allows the design of implementations of substantially wider applications than those of the prior art. The essential features of the invention are characteristic to the embodiment defined in the claims. The invention offers plural significant benefits, while simultaneously it overcomes the problems hampering prior-art techniques. More specifically, the invention is characterized by what is stated in appended claims. In the following the invention is described by making reference to the following drawings, in which Figs. 1 - 5 show a few embodiments of the arrangement according to the invention.

In Figs. 1 - 5 are illustrated arrangements according to the invention together with the method and apparatus implementing the same. As shown in Fig. 1 , the invention is based on using relatively shallow small-diameter boreholes 8, wherefrom the ground source heat is transferred to a liquid-filled ground loop 7 placed in a short trench 6, either by improving the inherent thermal conductivity of the soil or by arranging heat transfer through convection. By definition, convection means heat transfer in an arrangement capable transferring heat along with fluidic flows induced by heat. Thus, when thermal transfer is based on a temperature difference, warm fluid rises upward and cooled fluid sinks downward.

An essential feature of the present embodiment relates to its basic concept which is to use boreholes 8 of relatively shallow depth of about 1 to 20 meters drilled using a small bore diameter of approx. 10 - 75 mm. The heat- extraction techniques disclosed in the invention can also be implemented using a boreless method. The common idea is employ use parallel- connected, small-diameter metallic pipes as convectors and/or use heat conductors.

In the arrangement according to the invention, the liquid-filled ground source heat circulation loop 7 is typically a piping circuit made of corrugated metal pipe or plastic tube with a typical length of 5-100 m, whereto heat is transferred from shallow ground source heat pits 8. For a new building, the piping 7 can be installed around the building 1 into an installation pit 6 dug thereabout, into a sewage trench under frost insulation 5 or into a pit 6 dug separately for the project. For an existing building, a trench 6 about 2 to 100 meter long is dug on the parcel for the piping 7. Heat-extraction boreholes 8 are drilled along the piping at a spacing of 0.2 - 10 m, advantageously spaced less than 5 m apart from each other. In Fig. 1 is illustrated the functional principle of a system implemented according to the invention. Herein, the construction of the ground source heat piping 7 is substantially more cost-effective than in the prior art, since piping 7, 13 is not needed in great lengths and in a new house being erected it can be installed in the foundation/sewage excavations 6 that anyhow are necessary in construction. Moreover, it is essential to appreciate that the boreholes 8 to be drilled may have a substantially smaller diameter than required in the art, whereby the amount of material to be removed is a fraction of that needed to be removed today. This is benefitted in faster drilling as well as cost savings due to the possibility of using appreciably lighter-duty drilling equipment.

In Figs. 2 - 4 is shown an advantageous method for heat transfer from a borehole 8, wherein the convective technique, i.e., convection, is used from heat transfer from borehole 8 to the liquid-filled ground source heat circulation 7.

In the embodiment according to the invention into the small-diameter boreholes 8 is inserted a pipe 13 that may be bent into such a small radius that the looped pipe 13 can be inserted into the borehole, whereby no joint is necessary at the pipe bottom. In an alternative arrangement two separate pipes 13 can be used connected to each other at their ends. An essential detail is to make the pipe from a noncorroding material. In general, a metallic material is required to secure good heat transfer even though the surface area of the pipe 13 is reduced. The pipe loop 13 is mated with the actual liquid-filled ground source heat circulation 7 so that a portion of the loop circulation enters the boreholes 8.

In Fig. 2 is illustrated the arrangement for passing the partial flows into the inflow pipe 13 using the conventional ejector principle by way of having the ends of pipes 13 cut obliquely 10 in the inflow 10 and outflow 10 directions. The oblique ends 10 provide a pressure differential facilitating the flows.

In Fig. 3 is shown the side-flow diversion of the inflow 3 from one circulation pipe 7 and return thereof to another circulation pipe 7, where the pressure is lower. The pipes 13 are separated from each other by small plastic double brackets or, alternatively, the circulation pipe is insulated by 1/3 of the length thereof to minimize heat transfer between the pipes in the pipe loop 13. The outflow pipe portion is warmer than the inflow pipe portion thus bringing warmer fluid from the ground to the heat pump 24.

This heat transfer method offers the possibility of transferring heat out from building 1 during summertime to boreholes 8. Herein, the liquid of ground source heat circulation 7 passes through a heat exchanger 27 located in the air-conditioning duct 26 of the A/C equipment cooling the inlet air of the building. Alternatively, heat can be transferred into the ground from, e.g., solar heat collectors with the help of separate heat exchangers.

A second alternative convection technique is to arrange convection to take place by means of separate liquid or gas fluid circulations, whereby a geothermal convection pillar 19 is employed having no circulation of the heat transfer liquid from the liquid-filled ground source heat circulation pipes 7.

Accordingly, as shown in Fig. 4, heat is transferred from boreholes 8 by convection so that into the borehole 8 is inserted a pipe loop 19 filled with a liquid or gas. The other portion of the pipe loop 19 is either provided with insulation 20 or has a smaller diameter, whereby the temperature difference thus created induces natural convection between pipes 19, 20. The operating principle is illustrated in Fig. 4, wherein pipe loop 19 is depicted. The arrangement can also be implemented so that the pipes are located coaxially. Then, the heat from convection pipes 19 is transferred to the ground source heat circulation piping with the help of a material 18 of high thermal conductivity adapted to make contact with both circulation pipes 7, 19.

A third alternative convection method can be implemented with the help of a screw pile. In this embodiment, a hollow pile is screwed into the soil and thereupon one of the above-described convection pillar arrangements is adapted therein. This system works so that the screw pile is sunk into the soil either mechanically or manually, whereupon the above-described pipe loop is inserted therein or, alternatively, the circulations illustrated in Fig. 2 or 3 are adapted therein. Further alternatively, the screwably-sunken screw pile can function as a convection pillar without the need for separate piping. The essential feature is this embodiment is that it utilizes the thermal conductivity of the metallic screw piles.

Moreover, besides the above-described convection methods, the invention also relates to the improvement of heat transfer by means of enhancing thermal conductivity. Basically, the thermal conductivity of a metal is vastly higher than that of soil. When a metallic pipe 7 is used in the liquid-filled ground source heat circulation, the amount of thermal energy thereto is dictated by the thermal conductivity of soil that is inferior to that of a metal. Hence, the thermal conductivity of soil needs to be improved, e.g., by inserting into the boreholes a material of high thermal conductivity. To this end, metal rods or pipes may be used. Alternatively, the boreholes may be filled with metallic powder/grains or, instead of a metal, some other equivalent material of high thermal conductivity. As to other aspects of the embodiment, heat is transferred to the liquid-filled circulation 7 in the same fashion as described above with regard to the convection-based embodiments.

In the boreless embodiment, the thermal conductivity of soil can be improved by inserting metallic heat conductors in the soil to a depth of about 0.2 - 20 m directed in different directions from the trench 6 of the liquid-filled ground source heat circulation pipe to a depth dictated by the penetrability of the soil. Alternatively, the method may utilize metal rods inserted in the soil as screwably-sunken screw piles.

In Fig. 5 is shown a heating system according to the invention having a building equipped with inlet air preheating/cooling and ground heat transfer implemented with a heat conductor 7.

In its most advantageous embodiment, a heating system according to the invention functions so that the same liquid-filled ground source heat circulation 7 is used either for heating or, alternatively, for cooling a building. With the help a pump 25 the fluid flowing in the liquid-filled circulation 7 is transferred to a heat exchanger 27 that serves to preheat the inlet air of the building and is located poststream of the A/C equipment 28. In this fashion, energy extracted from ground heat can be exploited with the help of a liquid circulation in accordance with the invention for heating a building. In summertime, the same circulation is utilized in a reverse manner for cooling the building by way of passing heat to the soil and boreholes. The heat thus stored may be later extracted back during the heating period. Similarly, the same liquid-filled circulation 7 can be used to store other kind of energy such as solar energy provided by sun in the summertime. Heat transfer is extremely efficient owing to the use of the metallic pipes 7, 13.

To the same liquid-filled circulation 7 is also adapted another circulation pump 23 connected to a heat pump 24. With the help of the heat pump 24, the internal temperature of the building is elevated to a desired level. In Fig. 5 is shown an embodiment suited for passing heat into a building via a heat exchanger 29 located downstream of A/C equipment 28. The heat may also be passed to elevating the temperature of central-heating circulating water or heated household water. The preheating heat exchanger 27 offers the benefit of obtaining a lower temperature of the heat pump return circulation in the wintertime. This is because heat is transferred to the cold inlet air instead of wasting heat to the return circulation of the heat pump. Return flow from the heat exchanger 27 has a lower temperature than that from heat pump 24.

In this situation, inflow to circulating pipe 7 toward the ground at a lower temperature. Hence, heat transfer from the ground to the liquid-filled ground source heat circulation 7, 13 is enhanced due to the higher temperature difference. Alternatively, the flow rates of circulating pumps 25 and 23 can be reduced thus achieving energy savings.

The installation techniques of the ground source heat field may be varied within certain limits, even permitting construction during wintertime. In winter, the boreholes 8 are drilled through the frost layer while simultaneously installing the heat transfer pipes 13; the remaining excavation jobs are completed during the summertime. This is a significant benefit inasmuch as it allows installation operations the year around. The heating system may also be accomplished using oblique drilling as shown in Fig. 1 . The liquid-filled circulation 7 can be adapted to a single central pit or a short trench 6, whereby any of the heat transfer methods according to the invention may be exploited therewith.

The heat extraction system in accordance with the invention is prepared as shown in Fig. 1 by first digging a trench or pit 6 of a width suitable for mount- ing the circulating pipe 7 therein. The trench can be dug relatively shallow, because the circulating pipe 7 is not necessarily a ground source heat extracting borehole pipe. Transmission of frost from ground surface to the ground source heat circulation 7 is prevented by frost insulation 5. Owing to the short length of trench 6, the cost of frost insulation 5 becomes relatively low. Using a rock drill, e.g. such as that conventionally employed in rock blasting, to the bottom of trench 6 are drilled boreholes 8 to the depth of 1 - 20 m at suitable spacings of about 0.2 - 10 m. Alternatively, the boreholes can be drilled in a fanned fashion from the same central pit 6 in different directions obliquely to the ground. Using any of the arrangements according to the present invention, heat is transferred from the boreholes 8 to the liquid- filled ground source heat circulation 7 located in the pit or a short trench 6.

As described above, the essential feature of the present invention is the use of a distributed small-bore drilling system and combination of novel, effective heat conductors and/or metallic heat collectors into an integrated system. Through the use metal tubes, the invention achieves a cost-effective and simultaneously technically more effective method than those of the prior art for extracting heat from the ground and, vice versa, storing heat into the ground.

Accordingly, the invention relates to a novel method and apparatus for implementing a ground source heat system. The system comprises a plurality of small-diameter and shallow boreholes or, equivalent to them, screwably sinkable metal pipes such as screw piles or heat pipes sinkable in the ground. Into the holes thus provided are installed pipes for heat transfer by convection and the pipes are connected to a liquid-filled ground source heat circulation mounted in a short trench. Heat is transferred along an insulated pipe to the heating/cooling system of a building or, vice versa; off from system to the ground.

In addition to those items already mentioned, the building's heating/cooling system includes - mounted to operate in conjunction with the liquid-filled ground source heat circulation - a circulation pump of a preheating/precool- ing unit, a heat exchanger of preheating/precooling, an A/C equipment as well as a circulation pump of the heat pump system, a heat pump and a heat exchanger of a postheating/postcooling unit; all serving to heat/cool the inlet air entering the building.

The invention offers significant benefits by way of combining small-diameter drilling of boreholes and heat conductors collecting heat from a large volume. Thereby the invention facilitates optimised heat transfer from ground to a pipe of acid-proof or similar material. In contrast, prior-art embodiments do not provide a cost-effective method for extracting heat from ground by metallic pipes, since the heat extraction capability of such pipes surpasses the heat-releasing capability of ground.

Conventional embodiments fail to extract heat in a proficient manner and yet the costs thereof are substantially higher. Now, in addition to the benefits described above, cost savings are achieved inasmuch as extensive landscaping operations are avoided. Installation can be performed quickly and even round the year as the connecting pipes are readily mountable. The pumping costs of the circulating fluid are essentially lower than in "prior-art systems. In summary, the present arrangement offers a cost-effecting method for heating/cooling a building at extremely low energy consumption.

To a person skilled in the art it is obvious that the invention is not limited by the above-described exemplary embodiments, but rather may be varied within the inventive spirit and scope of the appended claims.

Claims

1 . An apparatus for implementing a ground source heat system, characterized in that the system comprises small-diameter and shallow boreholes (8) or equivalent screwably-sunken metal pipes, such as screw piles or heat conductors inserted in the soil, while pipes (13) are inserted in the boreholes (8) for transferring heat by convection and the pipes (13) are connected to a liquid-filled ground source heat circulation (7) placed in a short trench (6), wherefrom heat is transferred along an insulated pipe (2) to the heating/cooling system of a building (1 ) or, vice versa, heat is transferred therefrom into the ground.

2. The apparatus of claim 1 , characterized in that the heating/cooling system of the building (1 ) includes, mounted to operate in conjunction with the liquid-filled ground source heat circulation (7), a circulation pump (25) of a preheating/precooling unit, a heat exchanger (27) of preheating/precooling, an A/C equipment (28) as well as a circulation pump (23) of the heat pump system, a heat pump (24) and a heat exchanger (29) of a postheating/post- cooling unit; all serving to heat/cool the inlet air (26) entering the building (1 ).

3. The apparatus of claim 1 , characterized in that the boreholes (8) have a diameter of about 10 - 75 mm and a depth of about 10 - 20 m, whereby they are drilled along the piping of the liquid-filled ground source heat circulation (7) at a spacing of 0.2 - 10 m, advantageously spaced less than 5 m apart from each other, depending on the energy need, whereby the drilled holes may be arranged in a fanned fashion extending obliquely in different directions.

4. The apparatus of claim 1 , characterized in that the heat-extracting apparatus for heat extraction and cooling is based on a convective arrangement, wherein a borehole (8) has adapted therein a loop pipe (13), either as a single bent-end loop or formed by two pipes connected to each other at their ends, the loop being connected to a ground source heat circulation (7) serving to transfer the heat further to the heating/cooling system of the building ( ) or out therefrom.

5. The apparatus of claim , characterized in that the heat-extracting apparatus for heat extraction and cooling is based on a geothermal convection pillar having the pipe loop (13) that is normally installed in the borehole (8) replaced by a coaxial pipe loop (19), which is filled with a liquid or gas and has a portion of said coaxial pipe loop (19) equipped with insulation (20) or reduced diameter, whereby natural convection is induced in the coaxial pipe loop (19) whose heat transferring material (18) thus transfers heat to the ground source heat circulation (7).

6. The apparatus of claim 1 , characterized in that the heat-extracting apparatus for heat extraction and cooling is based on a convection method implemented with the help of a screw pile, whereby the thermal conductivity of soil is simultaneously enhanced through the use of the screwably sunken screw pile.

7. The apparatus of any of claims 1 - 6, characterized in that in the arrangement thermal conductivity is enhanced by driving into the soil metallic heat conductors to a depth of 0.2 - 20 meters or, alternatively, inserting into the boreholes (8) material of high thermal conductivity such as metal bars or pipes or, alternatively, into the boreholes are poured metallic powders/grains or similar thermally conductive material.

8. A method for utilizing a ground source heat system, characterized in that in the system are used small-diameter and shallow boreholes (8) or equivalent screwably-sunken metal pipes, such as screw piles or heat pipes inserted in the soil, to extract ground source heat by convection or, alternatively, to store heat in the soil from external sources via the boreholes (8), whereby the heat is transferred to a liquid-filled ground source heat circulation (7) placed in a short trench (6), wherefrom heat is transferred further along an insulated pipe (2) to the heating/cooling system of a building (1 ) or, wee versa, heat is transferred with the help of the system into the ground.

9. The method of claim 8, characterized in that in the heating/cooling system of the building (1 ) a liquid-filled ground source heat circulation (7) is used for transferring heat with the help of a circulation pump (25) of a preheating/precooling unit from a heat exchanger (27) thereof to an A/C equipment (28) as well as with the help of a circulation pump (23) of the heat pump system from the heat pump (24) to a heat exchanger (29) of a post- heating/postcooling unit for heating/cooling the inlet air (26) entering the building (1 ).

10. The method of claim 8, characterized in that the method heat extraction/cooling is based on a convective arrangement, wherein a loop pipe (13) adapted in a borehole (8) serves to transfer hea to a ground source heat circulation (7) serving to transfer the heat to the heating/cooling system of the building (1 ) or out therefrom.

11 . The method of claim 8, characterized in that in the method heat extraction/cooling is based on a geothermal convection pillar having installed in the borehole (8) a coaxial pipe loop (19), which is filled with a liquid or gas and is partially equipped with insulation (20) for transferring heat via a heat transferring material (18) to a ground source heat circulation (7) serving to further transfer the heat to the heating/cooling system of the building (1 ) or out therefrom.

12. The method of claim 8, characterized in that in the method heat extraction/cooling is based on a convection method implemented with the help of a screw pile, whereby the thermal conductivity of soil is simultaneously enhanced through the use of the screwably sunken metallic screw pile.

13. The method of claim 8, characterized in that in the method thermal conductivity is enhanced by driving into the soil metallic heat conductors to a depth of 0.2 - 20 meters or, alternatively, inserting into the boreholes (8) material of high thermal conductivity such as metal bars or pipes or, alternatively, into the boreholes are poured metallic powders/grains or similar thermally conductive material.