Geothermal Probe

Abstract

The invention relates to a geothermal probe for the exploitation of geothermal heat, comprising a cluster of several closed heat tubes, which for transporting the heat are filled with a two-phase working medium that can be evaporated by means of the geothermal heat and can be condensed in a heat discharging zone. In this case, the respective heat tubes are subdivided over their length into at least a heat receiving zone, a heat transporting zone and a heat discharging zone, at least two heat tubes in the heat receiving zone and/or in the heat transporting zone being of different lengths.

Description

The invention relates to a geothermal probe for using geothermal heat, having a bundle comprising a number of closed heat pipes that are filled for heat transport with a two-phase working medium that can be evaporated by means of geothermal heat and can be condensed in a heat output zone, the respective heat pipes being subdivided over their length into at least a heat absorbing zone, heat transport zone and heat output zone.

Geothermal heat for heating streets is proposed in DE 35 32 542 A1. The planned application intended was an automobile test track. In order to heat this large surface area during a test program, very large heat absorbing spaces have to be opened up in the ground for very long bores. In order to limit this, the heat was collected and stored in storage containers and output when required to the street to be heated by a controller guided by climate sensors. Consequently, there is provided in the case of this heating a suitably designed system that outputs heat only upon overshooting or undershooting of climate specific limit values such as atmospheric humidity, air temperature, wind speed, surface temperature and surface humidity, and thus spares the heat stored in the ground.

Publication DE 30 37721 A1 describes a heat pipe for utilizing the heat capacity of the ground and/or groundwater, for example in order to prevent points from freezing. The probe pipe is introduced into the ground in the form of a drilling core such that the surrounding ground acts as heat capacity. The heat pipe is here a closed pipe having a liquid/gaseous working medium and surface enlarging inserts for an improved transfer of heat at the heat source of the heat in the ground to the working medium. A heat absorbing surface enlarged by satellite pipes or ribs is mentioned here as a particular feature. The aim of these measures is to maximize the quantity of the heat flowing in. In fact, the quantity of heat flowing in is limited largely by the conductivity of the surrounding ground, and so an increase in the quantity of heat that can be absorbed is chiefly achieved by enlarging the diameter of the cylindrical absorbing surface. In order to utilize the heat more rationally, switches are built into the heat pipe to control the heat transport according to requirements by interrupting the return of condensate or by separating condensation and evaporation parts, or by means of a switching liquid. A water/alcohol mixture, HCFC or HFC is used here as working liquid.

Moreover, an implemented prototype system with propane as working medium that uses a similar method is known.

Publication EP 1529880 and WO 002005045134 relates to a geothermal probe that heats traffic installations directly and whose heat flux is led via at least one heat pipe from the heat source over the transport zone and is distributed over a number of heat distribution pipes before the heat is output in the region of a heat sink.

Patent EP 1194723 B1 represents a design in which the liquid medium flows downward in a spirally guided pipe that winds around the pipe with the rising medium. A similar proposal is made in U.S. Pat. No. 5,816,314.

Patent DE 4240082 C1 describes a probe in which the advance of the gas is separated from the return of the condensate of the working medium by perforated plates. This is performed in order in the case of a capillary tube to separate the gas bubbles from the liquid as early as on the transport path, and to feed them to the gas flow.

DE29824676U1, DE20320409U1, DE20210841U1 are known as publications concerning applications of CO₂ as medium in geothermal probes.

DE20210841U1 describes a probe whose condensate flow is separated from the gas flow by virtue of the fact that the gas flow rises in the middle in a separate, central, perforated pipe, and the condensate flow flows downward along the pipe outer wall.

DE 29824676U1 describes a probe that can be provided with ribs in order to improve the heat transfer.

Publication DE20320409U1 describes a probe made from stainless steel in the form of a corrugated pipe, which is designed as a single pipe or double pipe. The aim of this is to enable even relatively long pipes made from non stainless steel of relatively large diameter to be introduced into the borehole from a roll in one piece. A further goal in this case is for the condensate film flowing down not to be impeded by the rising gas flow in the case of relatively large borehole depths, a large pipe diameter and, therefore, a large moving volume of the working medium. Given the pipe diameter or volume of the probe pipe envisaged here, this component is subject to relatively strict provisions of the GPSG (German equipment and product safety act).

In the known designs of geothermal probes, copper pipes, protected against corrosion by PE film coating, are also used with the maximum possible diameter that still permits a slight deformability such that the pipes can be unwound from a roll, shaped to be straight and lowered into the borehole. The greatest possible pipe diameter is prescribed by the minimum deformability required for installation. Use is made here of a number of identical-length pipes in a bore. These designs have already been installed in the case of numerous building heating systems that use geothermal heat. It is likewise prior art to use a press material in the borehole with additives that improve the thermal conduction. In all known cases, the heat absorption takes place uniformly over the entire borehole depth, at least from below the neutral zone—a depth where there are no temperature fluctuations over the course of the year.

In a predominant proportion of the known probes, the heat is collected either in pipes of the largest possible diameter and fed to just one use, or it is collected in a number of pipes of smaller diameter and fed to just one use, for example a heat exchanger with a downstream heat pump.

It is the object of the invention to develop a device for collecting geothermal heat that is adapted to the specific requirements of an intended use and in this case constitutes a more efficient and more economic solution.

The invention is reflected in the features of claim 1. The further back-referred claims relate to advantageous refinements and developments of the invention.

The invention includes the technical teaching with reference to a geothermal probe for using geothermal heat, having a bundle comprising a number of closed heat pipes that are filled for heat transport with a two-phase working medium that can be evaporated by means of geothermal heat and can be condensed in a heat output zone. In this case, the respective heat pipes are subdivided over their length into at least a heat absorbing zone, heat transport zone and heat output zone, at least two heat pipes having different lengths in the heat absorbing zone and/or in the heat transport zone.

The invention proceeds in this case from the consideration that, in order to collect geothermal heat by means of heat pipes, in the case of thermal probes a liquid/gaseous working medium is used whose condensation temperature is somewhat below the temperature of the heat source used. The latter preferably serves as a heat source for keeping traffic routes and transportation facilities such as railroad facilities free from frost. This is chiefly done by using a slight temperature gradient between thermal heat and/or the heat contained in the groundwater and/or waste water by comparison with the temperatures required for the buildings or facilities to be heated, at least by comparison with the freezing point of water. There is no need in this case for any additional pumps driven by extraneous energy, or for other moving parts. In this context, geothermal heat is to be understood as any low temperature heat source available below the Earth's surface, for example including waste heat from sewers.

The invention proposed here operates on the basis of what are known technically as heat pipes. A heat pipe is a pipe that is closed in a gastight fashion and installed in a way largely vertical or greatly sloping and in which the working medium evaporates at the heat source, rises in the heat sink, condenses there and flows down again to the heat source in the same pipe.

The mass flow is the essential influencing variable for transmissible power. The maximum transmissible power results from the chain composed of the productiveness of the ground in the heat absorbing region, the thermal conductivity of the ground in the surroundings of the bore and of the press material around the bore, the thermal conductivity of the pipe material and the type, alignment and size of the heat absorbing surface. In this case, the productiveness and conductivity as well as the size and alignment of the absorbing surface are the main criteria. This shows that the thermal conduction of the probe material is of lesser significance.

Since there is a requirement for the possibility of transmitting a number of small, in parts identical powers to in each case exactly the same number of heat sinks, and not of guiding as great as possible a power from one or more pipes onto a heat exchanger, it is proposed not to feed a number of heat sinks from a probe pipe, but to assign each heat sink exactly one heat pipe from a pipe bundle of a probe. In this case, there is extracted from the ground over the extent of the respective heat absorbing surface of a heat pipe only as much heat as is required for the respective heat sink.

It is also possible in this way to fulfil the requirement for various temperatures for the respective heat sinks. Since the ground also becomes warmer at greater depth, the individual pipes can be designed in various lengths and can then supply different temperatures for their limited heat absorbing zone from the respective layers. The heat absorbing surface is limited in that the pipe is thermally insulated thereabove, and so a heat absorbing region that is precisely defined in terms of temperature and area is present.

In cases where the absorbing zone of a probe reaches through a number of geologic layers each having a different heat capacity, the quantity of heat required at the heat sink is supplied by adapting the length of the respective heat absorbing zone.

It is clear from the discussion that a high gas throughput in a pipe leads to high flow rates. The pipe diameter limits the mass throughput given a prescribed maximum speed of the medium. This means that the uniform flow of the condensate film at the pipe wall downward to the probe foot is prevented by the rising gas flow as soon as the flow rate and/or the mass flow exceed(s) the critical magnitude. This is prevented here by virtue of the fact that the heat absorbing surface, the quantity of the fluid in the pipe, and thus the maximum power throughput are limited. The heat absorbing surface is delimited by virtue of the fact that the pipe is thermally insulated thereabove and a heat absorbing region precisely defined in terms of temperature and area is therefore present.

The particular advantage persists in that the inventive geothermal probe has an efficiency with reference to heat use, material use and economy that exceeds the current level. The efficiency with reference to heat use results from the optimized absorption of the heat led under control from the ground and flowing in via a temperature gradient.

It follows that in the case of a probe there is an application specific subdivision of the heat available in the ground over the working medium in the framework of a number of small heat sinks having precisely defined quantities of heat. Subdivision of the heat flux into a pipe bundle with a number of heat exchangers improves the operational reliability and functionality via a multiplicity of subsystems operating independently of one another. Consequently, a source of error in the adaptation of the heat fluxes that supply the quantity of heat to the heat exchangers is just as much avoided as a potential source of damage, specifically leakiness of the probe pipe at least three additional connecting points per subdivision.

Different performance requirements of a number of heat exchangers can be fulfilled in terms of the concept by means of a single probe. In this case, the material outlay can likewise be minimized, particularly through the different lengths of the probe pipes.

Such geothermal probes with novel components for obtaining heat are suitable for heating buildings and facilities and particularly for transportation facilities, for example in railroad engineering for keeping points free from snow and ice, platforms and grade crossings and other traffic areas including outside railroad engineering.

The heat requirement referred to the application can be fulfilled with heat pipes of smaller diameter than previously customary. The use of a relatively small pipe diameter permits substantially thinner walls in conjunction with the same bursting strength. The following further advantages result therefrom: material savings, better workability and handling, and greater flexibility in selection of material. Moreover, a further material saving of up to 45% can be realized owing to the different lengths of probe pipes.

At least two heat pipes in the heat absorbing zone and/or in the heat transport zone can advantageously be of different diameter. Alternatively, or in combination with this, different gas pressures can prevail in at least two heat pipes. Furthermore, different working media can be introduced into at least two heat pipes. All these embodiments open up the possibility of designing individual heat pipes with different thermal performances, in accordance with requirement.

In a preferred refinement of the invention, the heat pipes in the heat transport zone can be thermally insulated. In order further to improve the obtaining of heat, each pipe can be thermally insulated in the section above the heat absorbing zone, specifically also against the parallel running probe pipes with heat absorbing zones lying further below. As a result, only a negligibly small uncontrolled heat exchange with the surroundings takes place in the heat transport zone of each pipe.

In a preferred embodiment, the heat absorbing zone is located at the lower end of the respective heat pipe in the pipe bundle, in order to ensure in each case that no liquid working medium that would be withdrawn from the thermal circulation can collect at the pipe end. In order to achieve a more effective use of the heat source, the various heat absorbing zones of a pipe bundle are thus arranged at various depths.

A number of heat absorbing zones and heat transport zones can advantageously be arranged on a heat pipe.

In a preferred refinement of the invention, the heat absorbing zones of the individual heat pipes can be arranged in a multistage fashion. Consequently, a number of heat pipes with heat absorbing zones can be arranged at each stage around the circumference of the pipe bundle. The number is governed by the fact that every point at which heat is extracted in the ground is not substantially influenced by the adjacent heat extraction zones. For example, approximately six heat pipes with heat absorbing zones are thus arranged in each stage around the circumference.

It is advantageously possible that the heat pipes have at least sectionally axially running protuberances, or there are arranged in the interior guide plates whose influence targets the transport of the returning condensate. In this case, the returning condensate is fed into the probe pipe, into a region partially separated from the gas by uninterrupted protuberances, and also flows therein down to the transport zone as far as into the heat absorbing zone. This raises the effectiveness, because the rising gas makes scarcely any contact with the returning condensate, and so the flow losses and pressure losses in the pipes are minimized.

Alternatively, the heat pipe can advantageously have within its walls a spirally running embossment. As a pipe turned in on itself and having a spiral embossment, the heat pipe improves the flow conditions, and thus the efficiency. The effect of this is that a swirl is imparted to the rising gas, and likewise to the condensate flowing down. Owing to the different densities of the two aggregate states of the working medium, and to the different centrifugal forces resulting therefrom, the gas is very largely separated from the condensate, the condensate flowing down at the outer wall.

It is advantageously possible that the heat absorbing surfaces of the heat pipes are enlarged by spiral guidance of the heat pipes in the heat absorbing zone around the borehole axis and/or by additional outer ribs. The smaller diameter of the probe pipes, correspondingly decreases the circumference, and thus the absorbing surface, by comparison with a pipe of large diameter. In order, nevertheless, to utilize optimally thermal capacity of the ground surrounding the heat absorbing zone, the pipe can, for example, be guided in a spiral fashion around the pipe bundle in the zone respectively absorbing heat, the result being to multiply the absorbing surface.

In a preferred refinement of the invention, it is possible that in the heat absorbing zone the heat pipes are designed at least sectionally as a panel heat exchanger that wholly or partly grips the inwardly lying heat pipes of the bundle. Relatively large proportions of geothermal heat are thereby efficiently collected in a simple way over the entire circumference, or else only a portion of the pipe bundle and are, for example, made available to selected heat pipes.

It is advantageously possible that the inner wall of the heat pipes is rough at least in the region of the heat absorbing zone. Consequently, alongside the increased heat transferring surface it is additionally possible also to distribute the returning fluid over the entire surface in the pipe interior.

It is likewise possible that at least one heat pipe is widened at its pipe end in the region of the heat absorbing zone. Consequently, there is an increased heat input, in particular, at the lowermost collecting point of the returning liquid working medium.

It is possible in a preferred refinement of the invention that the heat pipes are twisted about their own axis in the region of the respective heat absorbing zones. An improvement in the principle of the heat absorption is achieved by arranging the pipes in such a way that only the outer ones absorb heat, and it can additionally be improved by virtue of the fact that the heat absorbing surface is enlarged in the absorbing zone by being spirally guided around the insulated heat pipes, which here form a type of insulated core. This can also be performed in such a way that a number of pipes are guided in a quasi-parallel fashion like a multistart thread with a relatively large pitch. If this results in a flat pitch of the spiral of the heat absorbing pipe section, and the condensate flows only at the pipe bottom and is not distributed uniformly on the pipe wall, it is proposed that the pipe be rotated into itself in the heat absorbing region. As a result, the condensate is thrown back again onto the pipe inner wall at least partially with each rotation of the pipe.

The heat pipe can advantageously be made from aluminum or steel.

Exemplary embodiments of the invention are explained in more detail with the aid of schematics, in which:

FIG. 1 shows a heat probe with heat pipes of different lengths of the heat absorbing zone H below the Earth's surface,

FIG. 2 shows a heat probe with heat pipes of different lengths of the heat transport zone N through an insulation below the Earth's surface and heat absorbing zones H arranged in multistage fashion,

FIG. 3 shows a heat probe in which the heat pipe is guided in a multistage spiral fashion in the heat absorbing zone H around the borehole axis,

FIG. 4 shows a heat probe with spiral guidance of two heat pipes in the heat absorbing zone H around the borehole axis,

FIG. 5 shows a heat probe with heat pipes that are designed in the heat absorbing zone H as a panel heat exchanger, which grips the inwardly lying heat pipes of the bundle,

FIGS. 6A to H show a cross section of various designs of panel heat exchangers at different stages of a heat probe that at least partly grip the inwardly lying heat pipes of the bundle,

FIG. 7 shows a heat pipe of a heat probe with a protuberance on the wall side,

FIG. 8 shows a cross section of a heat pipe according to FIG. 7,

FIG. 9 shows a heat pipe of a heat probe with internal guide plates, and

FIG. 10 shows a cross section of a heat pipe according to FIG. 9.

Mutually corresponding parts are provided in all the figures with the same reference symbols.

FIG. 1 shows a heat probe 1 with heat pipes 2 of different lengths of the heat absorbing zone H below the Earth's surface O. The bundle is formed from a multiplicity of heat pipes 2 running parallel in the ground and which are filled with a two-phase working medium for the purpose of heat transport. Owing to the geothermal heat, the working medium evaporates in the region of the heat absorbing zone H and is transported into a heat output zone K via the heat transport zone N. In the region of the heat transport zone N, the temperature of the vaporous working medium corresponds approximately to that of the surrounding earth layer so that no heat is exchanged here. In the heat output zone K, the working medium condenses after outputting heat and flows deep again in the heat pipe 2 owing to gravity. The heat transferred in the respective heat pipe 2 is a function of the different pipe lengths in the heat absorbing zone H.

FIG. 2 also shows a heat probe 1 with heat pipes 2 of different length in the case of the heat transport zone N. The length of the heat transport zone N is fixed by an insulation 3 below the Earth's surface O. The heat is transferred to the working medium via the exposed pipe ends of the heat pipes 2. The exposed pipe ends form heat absorbing zone H, arranged in multistage fashion, for the respective heat pipe 2.

FIG. 3 shows a heat probe 1 with multistage spiral guidance of the heat pipes 2 in the heat absorbing zone H around the borehole axis. A further embodiment is illustrated in FIG. 4. There, a heat probe 1 is shown with spiral guidance of two heat pipes 2 in the heat absorbing zone H, around the borehole axis. Common to both solutions are enormously enlarged heat absorbing surfaces owing to a spiral guidance of the heat pipes 2 in the heat absorbing zone H.

FIG. 5 shows a heat probe 1 with heat pipes 2 that are designed in the heat absorbing zone H as panel heat exchangers 7 that grip the inwardly lying heat pipes 2 of the bundle. The pipe end 8 of the lowest heat pipe 2 is conically widened in the region of the heat absorbing zone H.

FIGS. 6A to H show a cross section of different designs of panel heat exchangers 7 at different stages of a heat probe 1 that at least partly grip the inwardly lying heat pipes 2 of the bundle. In this case, there is formed at the lowest lying stage (FIG. 6A) an annular panel heat exchanger 7 that opens into the heat pipe 2, which leads centrally upward. On the first stage (FIG. 6B), three further panel heat exchangers 7 are respectively arranged around one third of the circumference about the central heat pipe 2. The heat pipes 2 opening from the panel heat exchangers 7 are, in turn, guided upward in a fashion as close as possible to the centrally running heat pipe 2 and parallel to the latter. The further stages are illustrated similarly in FIGS. 6C to 6G. FIG. 6H shows a cross section through the pipe bundle as it is to be encountered, for example, in the heat transport zone N. The inwardly running pipe lengths are screened with thermal insulation in each stage.

FIG. 7 shows a heat pipe 2 of a heat probe 1 with a protuberance 4 on the wall side. Through these axially running protuberances, the returning condensate is fed into the probe pipe in a region partly separated from the gas by continuous protuberances or plates, and also flows therein through the transport zone N downward as far as into the heat absorbing zone H. The return channel 6 leads the liquid working medium directly into the protuberance 4. FIG. 8 shows the cross section of a heat pipe according to FIG. 7.

Alternatively, FIG. 9 shows a heat pipe 2 of a heat probe 1 with inner guide plates whose influence causes the returning condensate likewise to be transported in a targeted fashion. These solutions raise the effectiveness by virtue of the fact that the rising gas scarcely makes contact with the returning condensate, and so the flow losses and pressure losses in the pipes are minimized. FIG. 10 shows a cross section of a heat pipe according to FIG. 9.

LIST OF REFERENCE SYMBOLS

• 1 Geothermal probe
• 2 Heat pipe
• 3 Insulation
• 4 Protuberance
• 5 Guide plate
• 6 Return channel
• 7 Panel heat exchanger
• 8 Widened pipe end
• H Heat absorbing zone
• N Heat transport zone
• K Heat output zone
• O Earth's surface

Claims

1. A geothermal probe (1) for using geothermal heat, having a bundle comprising a number of closed heat pipes (2) that are filled for heat transport with a two-phase working medium that can be evaporated by means of geothermal heat and can be condensed in a heat output zone, the respective heat pipes being subdivided over their length into at least a heat absorbing zone (H), heat transport zone (N) and heat output zone (K), characterized in that at least two heat pipes (2) have different lengths in the heat absorbing zone (H) and/or in the heat transport zone (N).

2. The geothermal probe as claimed in claim 1, characterized in that at least two heat pipes (2) have different diameters in the heat absorbing zone (H) and/or in the heat transport zone (N).

3. The geothermal probe as claimed in claim 1, characterized in that different gas pressures prevail in at least two heat pipes (2).

4. The geothermal probe as claimed in claim 1, characterized in that different working media are introduced into at least two heat pipes (2).

5. The geothermal probe as claimed in claim 1, characterized in that the heat pipes (2) in the heat transport zone (N) are thermally insulated.

6. The geothermal probe as claimed in claim 1, characterized in that the heat absorbing zone (H) of a heat pipe (2) is located at least at the lower end of said pipe.

7. The geothermal probe as claimed in claim 1, characterized in that a number of heat absorbing zones (H) and heat transport zones (N) are arranged on a heat pipe (2).

8. The geothermal probe as claimed in claim 1, characterized in that the heat absorbing zones (H) of the individual heat pipes (2) are arranged in a multistage fashion.

9. The geothermal probe as claimed in claim 1, characterized in that the heat pipes (2) have at least sectionally axially running protuberances (4), or there are arranged in the interior guide plates (5) whose influence targets the transport of the returning condensate.

10. The geothermal probe as claimed in claim 1, characterized in that the heat pipe (2) has within its walls a spirally running embossment.

11. The geothermal probe as claimed in claim 1, characterized in that the heat absorbing surfaces of the heat pipes (2) are enlarged by spiral guidance of the heat pipes in the heat absorbing zone (H) around the borehole axis and/or by additional outer ribs.

12. The geothermal probe as claimed in claim 1, characterized in that in the heat absorbing zone (H) the heat pipes are designed at least sectionally as a panel heat exchanger (7) that wholly or partly grips the inwardly lying heat pipes (2) of the bundle.

13. The geothermal probe as claimed in claim 1, characterized in that the inner wall of the heat pipes (2) is rough at least in the region of the heat absorbing zone (H).

14. The geothermal probe as claimed in claim 1, characterized in that at least one heat pipe (2) is widened at its pipe end in the region of the heat absorbing zone (H).

15. The geothermal probe as claimed in claim 1, characterized in that the heat pipes (2) are twisted about their own axis in the region of the respective heat absorbing zones (H).

16. The geothermal probe as claimed in claim 1, characterized in that the heat pipe (2) is made from aluminum or from steel.