An arrangement and method for storing cold in the ground

Abstract

The invention relates to arrangement (1) for storing cold in the ground. The arrangement comprises a circuit (2) containing a working fluid. The circuit has a plurality of pipes (3) which pipes are distributed throughout a volume of the ground forming a cold storage, and each pipe (3) is arranged in a channel (4) in the ground for heat exchange between the working fluid and surrounding ground. The volume of the ground forming the cold storage is water-saturated ground frozen to artificial permafrost by circulating the working fluid in the circuit (2).

Description

[0001] An arrangement and method for storing cold in the ground

[0002] TECHNICAL FIELD

[0003] The invention relates to an arrangement and a method for storing cold in the ground.

[0004] BACKGROUND

[0005] There is a need for storing cold for cooling purposes. Weather and seasonal changes cause variations in both production and consumption of cold, heat and electricity. Today there are some methods available for storing energy. For example, hot water can be stored in pits, rock shelters and drill holes, and electricity can be stored in batteries or as potential energy by pumping water into reservoirs.

[0006] All these storing methods have one or more drawbacks, such as having low efficiency and high costs, being unsuitable for longer storage periods, providing too low temperature when the energy is to be retrieved and used, etc.

[0007] Further, there is still a need for efficient storage of cold, particularly for long term storage of cold in areas where cooling is needed during at least part of the seasons.

[0008] SUMMARY

[0009] An objective of the invention is to provide an arrangement for storing cold in the ground, by which arrangement cold can be stored in an effective way.

[0010] The objective is achieved by an arrangement for storing cold in the ground, wherein the arrangement comprises a circuit containing a working fluid, and the circuit has a plurality of pipes which pipes are distributed throughout a volume of the ground forming a cold storage, and each pipe is arranged in a channel in the ground for heat exchange between the working fluid and surrounding ground, wherein the volume of the ground forming the cold storage is water-saturated ground frozen to artificial permafrost by circulating the working fluid in the circuit.

[0011] The invention is based on the insight that by such an arrangement, cold can be stored for long periods at low temperatures in the ground in an efficient way. When freezing the water-saturated ground to artificial permafrost or ice-cement, a stable ground with higher thermalconductivity is achieved. A large-scale underground cold storage system utilizing the inherent geological insulation, thermal capacity and stability, can be created. Such a cold storage has potential to reduce the energy losses as compared to above ground based cold storage facilities.

[0012] Cold can be stored over the seasons and when cooling is needed, cold can be taken out from the cold storage. By circulating working fluid in the circuit, where the working fluid flowing into the cold storage has a higher temperature than the frozen ground, the temperature of the working fluid can be decreased. The cooled working fluid can then be used for cooling purposes. The capability to provide various predetermined sub-zero operating temperatures in the cold storage enables a wide range of technical and industrial applications requiring different temperatures for various reasons associated with technical aspects and standards. As an example only, the cold storage can provide cooling to thermal power plants (nuclear, coal, gas, biomass), particularly during summer or other warm periods when natural cooling is less efficient.

[0013] Although, theoretically, when storing cold, the ground can be cooled to temperatures down to -210°C, as an example only, a typical temperature for the cold storage can be in the range -100°C to -5°C, and preferably in the range -70°C to -10°C but temperatures down to -200°C to -100°C can also be applied.

[0014] Before the working fluid is circulated in the circuit to store cold, the requisite temperature of the working fluid to be used for storing cold in the ground can be obtained by different methods known to the skilled person. For example, working fluid with the desired temperature can be provided by a refrigeration cycle using off-peak electricity, or by using waste cold recovered from an LNG (Liquefied Natural Gas) regasification process, etc.

[0015] By the expression artificial permafrost is meant that the temperature of the volume of ground constituting the cold storage is decreased by heat exchange with the working fluid to a temperature below zero such that water in the ground is frozen and the ground is ice-cemented, which ground otherwise would not be frozen due to a relatively high natural temperature in ground and above ground. This artificial permafrost is maintained over periods, at least several weeks, preferably months, and often years.According to one embodiment of the arrangement, said plurality of pipes are flexible corrugated pipes, preferably metal pipes. Hereby, any thermal contraction or shrink of the pipe can be absorbed by minor flexing in each bend of the pipe. The flexible corrugated pipe is also less sensitive to any movement of the surrounding rock and / or sediment volume due to thermal contraction. Such a flexible corrugated pipe can be provided in lengths which are sufficient for use in the cold storage without the need of welding or screwing pipe portions together.

[0016] According to a further embodiment of the arrangement, a space of each channel not occupied by the pipe is filled with a granular filler material, preferably sand. Hereby, good heat transfer between the pipe and the surrounding ground is achieved. A granular material also allows some movement of the pipe, which may reduce stress introduced during thermal contraction and expansion cycles. The grain size distribution of the granular material can be designed to minimize the pore space, i.e. to minimize the effect of water to ice expansion.

[0017] According to a further embodiment of the arrangement, a temperature of the ground of the cold storage is below -5°C, preferably below -10°C, more preferably below -20°C, often below -40°C, and a temperature of the ground of the cold storage can be below -100°C.

[0018] The cold storage temperature is preferably maintained below zero with a margin of at least a few degrees for minimizing the risk of temperatures above zero in the cold storage when extracting cold from the cold storage. For example, a cold storage temperature in the range from -10°C to -5°C can be used.

[0019] By a cold storage temperature below -10°C (and above -20°C), the cold storage provides capabilities beyond standard chilled storages, enabling rapid pre-cooling or initial phase of freezing of foodstuffs and other material, and extracted cold can be used for products requiring conditions colder than standard chilling but warmer than typical deep freezing.

[0020] By a cold storage temperature below -20°C (and above -40°C), the cold storage will address requirements according to standards for the frozen food industry. Such a cold storage is also suitable for providing cold for laboratory reagents, enzymes and biological samples.

[0021] By a cold storage temperature below -40°C (and above -100°C), the cold storage provides advanced applications such as very rapid freezing processes (“flash freezing”), and high-power cooling discharge applications to meet peak cooling demands during heat waves, for instance.

[0022] Further, such a cold storage temperature in the range -70°C to -40°C providing a relatively high thermal driving force for heat exchange during extraction of cold from the cold storage, is often advantageous in that a techno-economic optimum can be achieved, where the energy required for charging the cold storage, the cold storage capacity and the available discharge power are balanced relative to each other.

[0023] By a cold storage temperature below -57°C (T=-56.6°C and P=5.18 bar represent the triple point of carbon dioxide, CO2), preferably below -60°C, the cold storage can provide cold for facilitating liquefaction of CO2, and storage and transport of liquid CO2, at relatively low pressures, which in turn can reduce costs, improve safety margins and simplify the management of CO2 in many applications.

[0024] According to a further embodiment of the arrangement, a freezing point of the working fluid is below -5°C, preferably below -10°C, more preferably below -20°C, and often below -40°C, and the freezing point can be below -100°C. Theoretically, under ideal conditions, the freezing point of the working fluid can be just slightly lower than the temperature in the cold storage, but in practice a significant lower freezing point of the working fluid than the cold storage temperature will be required.

[0025] The invention also relates to a method for storing cold in the ground by using an arrangement comprising a circuit containing a working fluid, wherein the circuit has a plurality of pipes which pipes are distributed throughout a volume of the ground forming a cold storage, and each pipe is arranged in a channel in the ground for heat exchange between the working fluid and surrounding ground, and wherein the method comprises circulating the working fluid in the circuit such that the volume of the ground which is water-saturated and forms the cold storage is frozen to artificial permafrost.

[0026] The invention is based on the insight that by such a method, cold can be stored for long periods at low temperatures in the ground in an efficient way. When freezing the water-saturated ground to artificial permafrost or ice-cement, a stable ground with higher thermal conductivity is achieved. A large-scale underground cold storage system utilizing the inherent geological insulation, thermal capacity and stability, can be created. Such a coldstorage has potential to reduce the energy losses as compared to above ground based cold storage facilities.

[0027] Cold can be stored over the seasons and when cooling is needed, cold can be taken out from the cold storage. By circulating working fluid in the circuit, where the working fluid flowing into the cold storage has a higher temperature than the frozen ground, the temperature of the working fluid can be decreased. The cooled working fluid can then be used for cooling purposes. The capability to provide various predetermined sub-zero operating temperatures in the cold storage enables a wide range of technical and industrial applications requiring different temperatures for various reasons associated with technical aspects and standards. As an example only, the cold storage can provide cooling to thermal power plants (nuclear, coal, gas, biomass), particularly during summer or other warm periods when natural cooling is less efficient.

[0028] As it regards temperatures and further advantages of the method, these are similar to what is described hereinabove when discussing the temperatures and advantages related to the arrangement.

[0029] According to a further aspect, the invention also relates to a method for creating an arrangement for storing cold in the ground, wherein the method comprises pre-freezing of water-saturated ground, drilling of at least one borehole in the pre-frozen ground, arranging one of said plurality of pipes in said at least one borehole, circulating the working fluid in the pipe for pre-freezing of water-saturated ground around the pipe, drilling of at least one further borehole in the pre-frozen ground around the pipe, and arranging a further pipe of said plurality of pipes in the further borehole, and circulating the working fluid in the further pipe for pre-freezing of water-saturated ground around the further pipe.

[0030] By this method, drilling of boreholes for providing channels for the pipes can be facilitated since drilling can be performed in stabilized frozen ground instead of trying to force unfrozen ground that is more difficult to drill. By repeating the process, the desired number of boreholes of the arrangement can be provided while always drilling in stabilized frozen ground. Of course, when the ground around a pipe has been frozen, it is possible to drill several or a lot of boreholes around the pipe, and repeat the process with pipes and circulation of the working fluid for these boreholes and pipes. In this way, the time for setting up the arrangement can be significantly reduced.According to one embodiment, the method comprises adding water to the ground for making the ground around said at least one borehole water-saturated before circulating the working fluid in the pipe for pre-freezing of the water-saturated ground around the pipe, and preferably adding the water to the ground via said at least one borehole. Hereby, it can be ensured that the ground to be frozen has a water content sufficient to be ice-cemented when circulating the working fluid in the circuit.

[0031] According to a further embodiment, the method comprises adding a filler material to a space of said at least one borehole not occupied by the pipe after arranging the pipe in said at least one borehole and before circulating the working fluid in the pipe for pre-freezing of water-saturated ground around the pipe. Hereby, good thermal transfer between the pipe and the surrounding ground is achieved. A granular material also allows some movement of the pipe, which may reduce stress introduced during thermal contraction and expansion cycles. The grain size distribution of the granular material can be designed to minimize the pore space, i.e. to minimize the effect of water to ice expansion.

[0032] Further advantages and advantageous features of the invention are disclosed in the following description and in the claims.

[0033] BRIEF DESCRIPTION OF THE DRAWINGS

[0034] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

[0035] In the drawings:

[0036] Fig. 1 is a schematic view showing an arrangement for storing cold,

[0037] Fig. 2A shows a cut view along A-A in Fig. 1 , illustrating a channel accommodating a pipe in the form of a U-pipe heat exchanger used in the arrangement,

[0038] Fig. 2B shows a variant of the pipe in Fig. 2A designed as a coaxial pipe heat exchanger,Fig. 3 is a cut view along B-B in Fig. 1 , illustrating pipes which are fluidly connected to each other in series, wherein each pipe is arranged in the respective channel,

[0039] Fig. 4A is a schematic flow chart for illustration of a method for creating an arrangement for storing cold, and

[0040] Fig. 4B is a sectional view showing the ground with a central borehole in which a pipe has been arranged, and further boreholes being drilled, when creating the arrangement for storing cold.

[0041] DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0042] Fig. 1 is a schematic view showing an arrangement 1 for storing cold in the ground. The arrangement 1 is illustrated in a top view looking at the ground from above.

[0043] The arrangement 1 is a so called underground cold storage. The arrangement comprises a circuit 2 containing a working fluid. The working fluid is circulated in the circuit 2. The circuit 2 is at least partly arranged in the ground, preferably most of or substantially the entire circuit 2 is arranged underground, though for illustration purposes the circuit is shown in Fig. 1 . The cold storage utilizes the soil, sediment and / or bedrock, which is found naturally in the ground below the surface of the earth, for storing cold within the ground which surrounds the circuit 2.

[0044] The circuit has a plurality of pipes 3 which pipes 3 are distributed throughout a volume of the ground forming a cold storage. In Fig. 1 , such a pipe 3 is schematically indicated by two small circles. Each pipe 3 is arranged in a channel 4 in the ground for heat exchange between the working fluid and the surrounding ground. Such a pipe 3 arranged in such a channel 4 forms a working fluid collector well. Each channel 4 can be a borehole, preferably a substantially vertical borehole, drilled in the ground for receiving the vertically arranged pipe 3.

[0045] The volume of the ground forming the cold storage is water-saturated ground frozen to artificial permafrost by circulating the working fluid in the circuit 2. This means that the ground in the cold storage is ice-cemented and has a temperature below 0°C.The working fluid can flow in such a pipe 3 of the circuit 2 from a position close to the ground surface, to the bottom of the channel 4 and back up to a position close to the ground surface. In general, there are two main options of creating the desired circuit 2 or circuits for the pipe configuration used.

[0046] According to a first option, only the pipes which are included in a predetermined circuit where the working fluid is to be circulated are fluidly connected to each other, such as in series or in parallel to each other. According to another option, more pipes, or all pipes of the arrangement, are fluidly connected to each other and by means of valves and control equipment the flow can be open or closed between the pipes. Thus, the pipes to be included in a certain circuit can be selected by controlling the valves such that the working fluid is circulated through these selected pipes, which pipes can be connected in series or in parallel to each other. Both these options can of course be combined and used in the same cold storage.

[0047] In the example embodiment illustrated in Fig. 1 , two circuits 2, 2’ are schematically shown. In addition to the circuit 2 for storing cold in the cold storage, the arrangement comprises a further circuit 2’ for extraction of cold from the cold storage. Both the circuit 2 and the further circuit 2’ are schematically shown and comprise only some of the pipes 3 for illustration purposes. However, depending on the current application, the number of such circuits used in the arrangement and the number of pipes comprised in the circuit can be varied. All pipes of the arrangement are suitably used for storing and / or extraction of cold in / from the cold storage. Optionally, one and the same circuit can be used for both storing cold in the cold storage and extraction of cold from the cold storage.

[0048] As schematically shown in Fig. 1 , the arrangement 1 comprises a cooling plant 6 providing the working fluid for storing cold in the cold storage. The arrangement 1 can comprise a pump 7 that is used for pumping the working fluid in the circuit 2. The working fluid is preferably circulated from the cooling plant 6 to the centre of the cold storage, and from the centre of the cold storage via the pipes 3 to the periphery of the cold storage and back to the cooling plant 6. In this way, the temperature will be lowest in the centre of the cold storage. As an example only, the temperature of the working fluid flowing out from the cooling plant 6 to the pipes can be -35°C to -40°C, and the temperature of the working fluid flowing from the pipes 3 back to the cooling plant 6 can be approximately zero or somewhatlower. The cooling plant can comprise standard components for cooling the working fluid by a refrigeration cycle process, for instance.

[0049] As also schematically shown in Fig. 1 , the arrangement 1 can comprise a heat exchanger 8. The further circuit 2’ is fluidly connected to the heat exchanger 8 for extraction of cold from the cold storage. By the heat exchanger 8, heat can be transferred from an energy carrier 9 to the working fluid of the circuit 2’. The energy carrier 9 can then be used for cooling purposes. The arrangement 1 suitably comprises a pump 10 for circulating the working fluid in the further circuit 2’. The working fluid is preferably circulated from the heat exchanger 8 and the periphery of the cold storage via the pipes 3 to the centre of the cold storage, and from the centre of the cold storage back to the heat exchanger 8. In this way, the temperature of the working fluid will be decreased gradually and will have the lowest temperature just before reaching the heat exchanger 8. As an example only, the temperature of the working fluid flowing from the heat exchanger 8 to the pipes 3 can be approximately zero or somewhat lower, and the temperature of the working fluid flowing from the pipes 3 back to the heat exchanger 8 can be -30°C to -35°C.

[0050] Further, in some applications, the channels are not vertically arranged. For example, by using a tunnel, shaft or rock shelter, the channels can be drilled in various directions in the ground to obtain the cold storge. The pipes can be installed in channels with any orientation. This is important since it allows installation around tunnels and shafts. The pipes can also be installed in curved channels that could help to reduce the surface footprint if channels are all drilled from substantially the same position at the surface and curved to distribute the channels over a larger volume underground.

[0051] In the example embodiment illustrated in Fig. 1 , the volume of the ground forming the cold storage is defined by the surface area 5 over which the pipes 3 are distributed and by the length of these pipes 3, where the depth of each channel 4 accommodating such a pipe 3 is suitably the same as the length of the pipe 3 or slightly longer.

[0052] The number of pipes 3 can be 10-10000, preferably 50-5000, and often the number of pipes 3 is in the range 100-1000. Different types of pipe 3 can be used. Some examples are shown more in detail in Figs. 2A and 2B where cut views of a channel 4 accommodating a pipe 3 and the surrounding ground are illustrated.The size of the channel 4 is suitably in the range 30-250 mm, preferably 50-200, and often 100-150 mm in diameter. The diameter of the pipe 3 can be varied depending on the application and the size of the channel 4. For example, the diameter of the pipe 3 can be in the interval 30-80 mm.

[0053] The distance between the channels 4 can be selected based on the temperature of the working fluid when storing cold in the ground, the temperature in the cold storage, etc. For example, the distance between two adjacent channels 4 (pipes 3) can be in the range 0.5-15 meters, preferably 1-10 meters, and often the distance is in the range 2-5 meters.

[0054] The length of the pipes 3 can be 1-1000 meters, preferably 10-500 meters, and often 50-300 meters.

[0055] As an example only, this means that the cold storage can have a size that covers a ground surface area in the size of 10 000 m2and that the approximate volume of ground forming the cold storage can be in the size of 1 500000 m3.

[0056] Fig. 2A shows a pipe 3 that is a U-pipe forming a U-pipe heat exchanger. The channel 4 is a vertical borehole in the ground. The U-pipe is part of the circuit 2 such that the working fluid can be circulated in the U-pipe. At the same time, heat can be exchanged between the working fluid and the surrounding ground. The U-pipe has a first pipe portion 11 and a second pipe portion 12. The first pipe portion 11 and the second pipe portion 12 extend in the longitudinal direction of the channel 4 such that the working fluid can be circulated while flowing in the first pipe portion 11 in a direction 13 into said channel 4 and in the second pipe portion 12 in a direction 14 out from said channel 4. Of course, the arrangement can be designed such that the flow direction of the working fluid can be reversed for any pipe configuration.

[0057] The pipe 3 is suitably a flexible corrugated metal pipe. The pipe 3 can be made of stainless steel for instance, though also flexible corrugated plastic pipes can be used, at least for relative high temperatures above approximately -40°C. When using a flexible corrugated pipe, a minor part of the pipe, such as the U-part connecting the first pipe portion and the second pipe portion, can be made from a non-corrugated pipe portion, i.e. from a usual solid pipe portion.The arrangement 1 has suitably spacers 15 arranged between the first pipe portion 11 and the second pipe portion 12. The spacer 15 is arranged to keep the first pipe portion 11 and the second pipe portion 12 apart from each other. The spacer 15 can also be arranged to position the first pipe portion 11 and the second pipe portion 12 adjacent to or in contact with an inner surface of the channel 4, i.e. the first pipe portion 11 and the second pipe portion 12 may abut against the inner surface of the channel 4.

[0058] In Fig. 2B, a variant of the pipe 3’ is illustrated. The pipe 3’ is a coaxial pipe forming a coaxial pipe heat exchanger. The coaxial pipe has an inner pipe portion 16 and an outer pipe portion 17. The inner pipe portion 16 and the outer pipe portion 17 extend in the longitudinal direction of said channel 4 such that the working fluid can be circulated while flowing in the outer pipe portion 17 (outside the inner pipe portion 16) in a direction 13 into said channel 4 and in the inner pipe portion 16 in a direction 14 out from said channel 4. In the same way as described with reference to Fig. 2A, also this pipe 3’ is suitably a flexible corrugated metal pipe. The pipe 3’ can be made of stainless steel for instance, though also flexible corrugated plastic pipes can be used, at least for relative high temperatures above approximately -40°C.

[0059] The direction of flow can be reversed in the pipe in that the working fluid will suitably flow downwards in the outer pipe portion and upwards in the inner pipe portion when cold is stored in the cold storage, i.e. heat is to be transferred from the ground to the working fluid, whereas the working fluid will flow downwards in the inner pipe portion and upwards in the outer pipe portion when cold is extracted from the cold storage, i.e. heat is to be transferred from the working fluid to the ground.

[0060] As an option, it is also possible to provide the coaxial pipe forming a coaxial pipe heat exchanger by a single pipe. The single pipe arranged in the channel constitutes the inner pipe portion and the walls of the channel itself constitutes the outer pipe portion. In other words, when cold is stored in the cold storage, the working fluid can be circulated while flowing outside the pipe between the pipe and the channel walls in a direction into said channel, and flowing inside the pipe in a direction out from said channel.

[0061] In all example embodiments described herein, the working fluid can be a liquid or gas. When using a liquid, the working fluid in the circuit suitably has a working temperature range froma lowest allowable temperature to a highest allowable temperature for which working temperature range the working fluid is a liquid at the operating pressure.

[0062] Depending on the application and the working fluid used, the operating pressure in the circuit can vary from atmospheric pressure, i.e. the standard sea-level pressure, one standard atmosphere, equals 760 mmHg, to several bars and all the way up to 80 bars.

[0063] The working fluid can be for example silicone fluids, synthetic hydrocarbon blends, alkaline earth formate and acetate, monopropylene glycol, and for really low temperatures cryogenic liquids, such as liquid air, liquid nitrogen, liquid CO2 or gases of the same compounds.

[0064] The freezing point of the working fluid has to be below zero at the operating pressure, and a working fluid with a freezing point adapted to the current temperature applied in the cold storage has to be used. Often the working temperature of the working fluid is suitably in the range -100°C to -5°C, and for many applications in the range -70°C to -10°C.

[0065] In addition to the freezing point, it may also be important that the working fluid will maintain a relatively low viscosity at low temperatures allowing effective pumping of the working fluid and avoiding clogging in the system.

[0066] Fig. 3 is a cut view along B-B in Fig. 1. Although Fig. 3 shows the further circuit 2’ for extraction of cold from the cold storage, the design applies for the circuit 2 for storing cold in the cold storage as well. Fig. 3 illustrates five pipes 3 which are fluidly connected to each other in series. For illustration purposes, the distances between the pipes 3 as well as the length of the pipes 3 have been heavily shortened. Each pipe 3 is arranged in a channel 4. The working fluid is circulated in the further circuit 2’ to the pipes 3 for heat exchanging between the working fluid and the ground when the working fluid flows in the vertically arranged pipes 3. As illustrated, the pipes 3 are arranged in the ground. The channels 4 accommodating the pipes 3 are suitably formed by drilling, but may be achieved by any other equipment enabling the pipe 3 to be arranged in the ground.

[0067] Thus, the pipes 3 are arranged in the bedrock or sediment. In case the channels 4 are not completely filled by the pipes 3, for at least a part of the length of the channels 4, the channels 4 can be filled with a filler material 18. In other words, a space of each channel 4 not occupied by the pipe 3 can be filled with a granular filler material 18. This can be a high-conductive filler material for filling the channels and creating the heat conduction properties that are required. The high-conductive material can be for example quartz sand or magnetite sand.

[0068] In the upper part of each channel 4, a low-conductive material 19 is however suitably used for lowering the heat transferred to the cold storage from the ground closest to the surface of the earth. The low-conductive material can be for example extruded clay or foam glass. Immediately above the pipes and the bedrock there is a layer 20 of an insulating material, such as foam glass or expanded clay. Portions of the further circuit 2’ that connect the different pipes 3 to each other, are suitably arranged in this insulating layer 20. On top of the insulating layer 20 another filling material 21 , such as soil or gravel, can be arranged.

[0069] Fig. 4A is a schematic flow chart of an example embodiment of a method for creating an arrangement 1 for storing cold. As it regards the arrangement 1 and the components thereof used in this method, see previous description and figures mentioned hereinabove with respect to the arrangement 1 and method for storing cold.

[0070] The method comprises pre-freezing 100 of water-saturated ground, and drilling 200 of at least one borehole in the pre-frozen ground. The ground can be naturally water-saturated or water can be added to the ground before freezing and drilling.

[0071] The method further comprises arranging 300 one of said plurality of pipes 3 in said at least one borehole 4, circulating 400 the working fluid in the pipe 3 for pre-freezing of water-saturated ground around the pipe, and drilling 500 of at least one further borehole 4’ in the pre-frozen ground around the pipe 3.

[0072] See also the sectional view in Fig. 4B showing the ground with a central borehole 4 in which a pipe 3 has been arranged, and as an example only, the temperature of the ground closest to the borehole 4 has been decreased to -80°C (at point A) by circulating the working fluid in the circuit 2 comprising the pipe 3. During this temperature decreasing process, heat is transferred in a direction 25 from the ground to the working fluid in the pipe 3, the rock 26 or sediment around the central borehole 4 will be ice-cemented, and redundant unfrozen water (groundwater or added water) will be forced to flow in a direction 27 away from the borehole 4. The temperature gradient in the ground is such that the temperature increases gradually from the borehole 4 to a position at a distance not affected by the freezingprocess. In the example illustrated in Fig. 4B, the temperature varies from -80°C at the central borehole 4 to +8°C (at point D) in the surrounding natural rock 28 or sediment. At the intermediate points B and C, the temperature can be -30°C and -5°C, respectively.

[0073] As also shown in Fig. 4B, one or more further boreholes 4’, 4” can be drilled in the ice-cemented ground. In these boreholes 4’, 4” illustrated when being drilled, also drill equipment 29 is schematically shown. Such drilling is favourable as compared to drill in natural rock or sediment which is not ice-cemented, since the ice-cemented ground is more predictable and stable facilitating the drilling operation.

[0074] The method further comprises arranging 600 a further pipe of said plurality of pipes 3 in the further borehole 4’, and circulating 700 the working fluid in the further pipe for pre-freezing of water-saturated ground around the further pipe. Thereafter new boreholes can be drilled and the method can be repeated until the desired numbers of boreholes are drilled.

[0075] When starting the method for creating an arrangement 1 for storing cold, a first borehole can be drilled in the natural unfrozen ground for enabling arranging of a pipe for pre-freezing the ground, or an already existing channel or space can be used to accommodate the pipe. Alternatively, any other suitable equipment for locally pre-freezing the ground before drilling the first borehole can be used. Thereafter, all boreholes can be drilled in ice-cemented ground that has been pre-frozen by circulating the working fluid.

[0076] Optionally, the method comprises adding 800 water to the ground for making the ground around said at least one borehole 4 water-saturated before circulating the working fluid in the pipe 3 for pre-freezing of the water-saturated ground around the pipe 3. This is suitably used in areas where the ground is not naturally water-saturated. The water can be added to the ground via said at least one borehole.

[0077] The method can also comprise adding 900 a filler material to a space of said at least one borehole 4 not occupied by the pipe 3 after arranging the pipe 3 in said at least one borehole and before circulating the working fluid in the pipe 3 for pre-freezing of water-saturated ground around the pipe 3.It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.

Claims

CLAIMS1 . An arrangement (1 ) for storing cold in the ground, the arrangement comprising a circuit (2) containing a working fluid, the circuit having a plurality of pipes (3) which pipes are distributed throughout a volume of the ground forming a cold storage, each pipe (3) being arranged in a channel (4) in the ground for heat exchange between the working fluid and surrounding ground, wherein the volume of the ground forming the cold storage is water-saturated ground frozen to artificial permafrost by circulating the working fluid in the circuit (2).

2. An arrangement according to claim 1, characterized in that said plurality of pipes (3) are flexible corrugated pipes, preferably metal pipes.

3. An arrangement according to claim 1 or 2, characterized in that a space of each channel (4) not occupied by the pipe (3) is filled with a granular filler material (18).

4. An arrangement according to any preceding claim, characterized in that a temperature of the ground of the cold storage is below -5°C.

5. An arrangement according to any preceding claim, characterized in that a temperature of the ground of the cold storage is below -10°C.

6. An arrangement according to any preceding claim, characterized in that a temperature of the ground of the cold storage is below -20°C.

7. An arrangement according to any preceding claim, characterized in that a temperature of the ground of the cold storage is below -40°C.

8. An arrangement according to any preceding claim, characterized in that a temperature of the ground of the cold storage is below -100°C.

9. An arrangement according to any of claims 1-4, characterized in that a freezing point of the working fluid is below -5°C.

10. An arrangement according to any of claims 1-5, characterized in that a freezing point of the working fluid is below -10°C.

11. An arrangement according to any of claims 1-6, characterized in that a freezing point of the working fluid is below -20°C.

12. An arrangement according to any of claims 1-7, characterized in that a freezing point of the working fluid is below -40°C.

13. An arrangement according to any of claims 1-8, characterized in that a freezing point of the working fluid is below -100°C.

14. A method for storing cold in the ground by using an arrangement (1 ) comprising a circuit (2) containing a working fluid, the circuit having a plurality of pipes (3) which pipes are distributed throughout a volume of the ground forming a cold storage, each pipe (3) being arranged in a channel (4) in the ground for heat exchange between the working fluid and surrounding ground, the method comprising circulating the working fluid in the circuit (2) such that the volume of the ground which is water-saturated and forms the cold storage is frozen to artificial permafrost.

15. A method for creating an arrangement according to claim 1 , the method comprising pre-freezing (100) of water-saturated ground, drilling (200) of at least one borehole in the pre-frozen ground, arranging (300) one of said plurality of pipes in said at least one borehole, circulating (400) the working fluid in the pipe for pre-freezing of water-saturated ground around the pipe, drilling (500) of at least one further borehole in the pre-frozen ground around the pipe, and arranging (600) a further pipe of said plurality of pipes in the further borehole, and circulating (700) the working fluid in the further pipe for pre-freezing of water-saturated ground around the further pipe.

16. A method according to claim 15, characterized by adding (800) water to the ground for making the ground around said at least one borehole water-18saturated before circulating (400) the working fluid in the pipe for pre-freezing of the water-saturated ground around the pipe.

17. A method according to claim 16, characterized by adding (800) the water to the ground via said at least one borehole.

18. A method according to any of claims 15-17, characterized by adding (900) a filler material to a space of said at least one borehole not occupied by the pipe after arranging (300) the pipe in said at least one borehole and before circulating (400) the working fluid in the pipe for pre-freezing of water-saturated ground around the pipe.

Images