JP6896255B2 - Heat siphon - Google Patents

Description

The present invention relates mainly to thermal siphons used for geothermal extraction.

A heat siphon is also called a heat pipe, in which a condensable fluid such as water or alcohol is sealed as a working fluid inside a vacuum degassed closed pipe, and the closed pipe is erected in the vertical direction. The lower end side of the closed tube is an endothermic part that gives heat from the outside, and the upper end side is a heat dissipation part that takes heat away.

In the thermal siphon, the working fluid evaporates due to the heat given to the endothermic portion, and the vapor rises to the heat radiating portion and then dissipates and condenses to transport heat as latent heat of the working fluid.

In this way, heat siphons can transport heat from bottom to top without using power, so they are used in devices that extract high-temperature geothermal heat to the ground, and the heat is used for heating and cooling and melting snow. ing.

Japanese Unexamined Patent Publication No. 8-327259

In the above-mentioned thermal siphon, a closed pipe is inserted into an excavation hole dug 100 m to 300 m underground toward the tropics, and the working fluid heated and evaporated by geothermal heat at the lower end moves to the end on the ground side, where Geothermal heat is extracted by radiating heat (see Patent Document 1).

However, in the conventional heat siphon, if the closed tube is long or the temperature of the heat source is low, the steam of the working fluid does not rise sufficiently, so that there is a problem that the heat transport efficiency is lowered. ..

The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a thermal siphon capable of realizing high heat transfer efficiency by adding a simple configuration.

In order to achieve the above object, the thermal siphon according to the present invention
A bottomed main pipe that is installed in a substantially vertical direction with the lower end inserted in the heat source, and in which a condensable working fluid and a non-condensable gas are sealed in the hollow part.
A heat exchanger connected to the upper end of the main pipe so as to cover the opening of the main pipe.
A liquid return pipe that returns the working fluid condensed and liquefied by the heat exchanger to the inside of the pipe through a hole formed below the main pipe.
It was installed in the hollow portion of the main pipe at a distance from the main pipe, the upper end was opened in the heat exchanger, and the lower end was held at a position not in contact with the working fluid accumulated in the bottom of the main pipe. It is characterized by having an inner tube.

Here, a heat storage device and a pump are connected to the heat exchanger via a tube, and the heat transferred from the heat source by the working fluid by the refrigerant circulated in the tube by the pump is used in the heat storage device. It is preferable to store in.

Further, it is preferable to use water as the working fluid and air as the non-condensable gas.

Further, as the heat exchanger, it is preferable to use a shell & tube type heat exchanger in which a spiral tube is housed in a hollow portion of a cylindrical housing.

Further, the upper end portion of the main pipe projects upward from the connecting portion with the housing of the heat exchanger, a groove is formed between the main pipe and the housing, and the working fluid accumulated in the groove is the said. It is preferable that the fluid is injected into the liquid return pipe from an injection port formed at the bottom of the housing.

The thermal siphon according to the present invention adopts a double circular tube structure for the evaporation tube and encloses a non-condensable gas (air or an inert gas) in the tube in addition to the working fluid. And by circulating the non-condensable gas in the pipe, it promotes the rise of the vapor of the working fluid, and as a result, the heat transport efficiency at the time of extracting geothermal heat is greatly improved as compared with the conventional thermal siphon. There is.

It is a figure which shows the whole structure of the thermal siphon which concerns on embodiment of this invention. It is schematic cross-sectional view explaining the flow of working fluid and steam in a thermal siphon which adopted the single circular tube structure or the double circular tube structure in the heat insulating part. It is a graph which shows the experimental result of the heat transport efficiency of the heat siphon which adopted the single circular tube structure or the double circular tube structure in the heat insulating part.

Hereinafter, the thermal siphon according to the embodiment of the present invention will be described with reference to the drawings.

<Structure and function of thermal siphon>
FIG. 1 shows the basic configuration of the thermal siphon according to the present embodiment. The heat siphon 1 is composed of an evaporation pipe 2, a heat exchanger 3, and a liquid return pipe 4, and a heat storage device 5 and a pump 6 are connected to the heat exchanger 3. In the figure, the evaporation tube 2 and the heat exchanger 3 show a cross section cut in a plane passing through the axis of the tube.

The evaporation pipe 2 has a long, bottomed main pipe 21 installed in a substantially vertical direction with the lower end inserted into a heat source in the ground, and a hollow portion of the main pipe 21 so that the axis of the cylinder coincides with the center of the cylinder. It is composed of a housed inner pipe 22. A working fluid WF is sealed in the hollow portion of the main pipe 21. The main pipe 21 is basically installed in the vertical direction, but even if it is slightly tilted, the function of the thermal siphon is not hindered.

The main pipe 21 is formed by connecting a plurality of pipes in the vertical direction, and a flange 24 is formed at the joint portion. The inner pipe 22 is supported at the upper end of the main pipe 21 by a jig (not shown) in a state where the axis coincides with the main pipe 21. Further, the jig is provided with an opening so that steam can pass through.

In the present embodiment, an aluminum pipe is used as the main pipe 21 from the viewpoint of heat resistance and strength, and further because it is lightweight, and a polycarbonate pipe is used as the inner pipe 22 from the viewpoint of heat insulation. Further, water is used as the working fluid WF. As the main pipe 21, a stainless steel pipe may be used, although it is slightly disadvantageous in terms of weight. Further, although a polycarbonate pipe is used as the inner pipe 22, a pipe made of another material having excellent heat insulating properties may be used.

As described above, in the normal use state, the heat siphon 1 digs a vertical hole having a length that reaches the heat source in the ground, and accommodates the evaporation pipe 2 in the vertical hole. In the present embodiment, in order to confirm the performance of the thermal siphon 1, a heater 7 is arranged around the bottom of the main pipe 21 as a heat source, and the heater 7 heats and evaporates water as a working fluid.

A heat exchanger 3 is attached to the opening at the upper end of the main pipe 21. In the present embodiment, a shell & tube type heat exchanger is used as the heat exchanger 3, and the spiral copper tube 33 is housed in the hollow portion of the cylindrical housing 31. As shown in the figure, the hollow portion of the heat exchanger 3 is connected to the hollow portion of the evaporation pipe 2 and is sealed by the housing 31 and the lid 32.

In the conventional heat siphon, the working fluid is sealed in the space surrounded by the main pipe 21, the housing 31 of the heat exchanger 3, and the lid 32 after vacuum degassing the non-condensable gas. In this form, air, which is a non-condensable gas, is sealed together with water, which is a working fluid WF. When the evaporation tube 2 is not heated by the heater 7, the air pressure is substantially equal to the atmospheric pressure.

The tube 33 of the heat exchanger 3 is connected to the tube 51 connected to the heat storage device 5 through a hole formed in the lid 32, and the refrigerant flowing through the tubes 33 and 51 is driven by the driving force of the pump 6. It circulates between the heat exchanger 3 and the heat storage device 5.

In the present embodiment, pure water is used as the refrigerant, and the pure water flowing through the tube 33 of the heat exchanger 3 is mainly heated by the steam rising in the hollow portion of the inner tube 22, and then inside the tube 51. It flows and is carried to the heat storage device 5, and stores heat in the heat storage device 5.

A liquid return pipe 4 is attached between the bottom surface of the housing 31 of the heat exchanger 3 and the lower side surface of the main pipe 21, and the working fluid WF cooled by the heat exchanger 3 and condensed / liquefied is an injection port. It is injected from 41, returned to the main pipe 21 from the discharge port 44 through the liquid return pipe 4, and collects at the bottom of the main pipe 21.

The control valve 42 provided on the upper part of the liquid return pipe 4 is a valve that regulates the flow rate of the working fluid WF returned to the main pipe 21. Further, the tank 43 adjusts the amount of the working fluid accommodated in the evaporation pipe 2, and when it is desired to reduce the amount, the working fluid is discharged from the tank 43 to the outside.

In the configuration of the thermal siphon 1, the lower part of the evaporation pipe 2 surrounded by the heater 7 is generally called an "evaporation part", and the heater 7 heats and evaporates the working fluid WF to generate steam.

Of the configuration of the thermal siphon 1, the intermediate portion of the evaporation pipe 2 is generally called a "heat insulating portion", and by covering the outer peripheral portion with the heat insulating material 23, the heat transported by steam is released to the outside of the evaporation pipe 2. It is prevented from flowing. As will be described in detail later, the vapor of the working fluid WF heated and evaporated by the heater 7 in the evaporation portion rises in the heat insulating portion mainly through the hollow portion of the inner pipe 22 and reaches the heat exchanger 3.

Of the configuration of the heat siphon 1, the heat exchanger 3 is generally called a "condensing part", and transfers the heat of steam to pure water flowing through the tube 32 and transports it to the heat storage device 5. In the following description, the above-mentioned "evaporation part", "heat insulation part" and "condensation part" are used as necessary.

The steam of the working fluid that has reached the hollow portion of the heat exchanger 3 is condensed and liquefied by coming into contact with the tube 33 cooled by pure water, and is accumulated at the bottom of the housing 31. As shown in FIG. 1, the upper end portion 21a of the main pipe 21 protrudes into the housing 31, and a ring-shaped groove 34 is formed between the main pipe 21 and the housing 31. The working fluid WF condensed and liquefied in the heat exchanger 3 collects in the groove 34.

As shown in FIG. 1, since the injection port 41 of the liquid return pipe 4 is formed at the bottom of the groove 34, the working fluid WF accumulated in the groove 34 of the housing 31 passes through the liquid return pipe 4. It is carried downward and returned into the main pipe 21 from the discharge port 44.

A part of the vapor of the working fluid that is heated and evaporated by the heater 7 and rises in the hollow portion of the inner pipe 22 is cooled by coming into contact with the inner peripheral surface of the inner pipe 22, condensed and liquefied, and then gravity. It becomes a liquid film flow and is returned to the bottom of the main pipe 21.

<Flow of working fluid>
Next, with reference to FIG. 2, the working fluid and the flow of steam thereof in the thermal siphon of the present invention will be described in comparison with the conventional thermal siphon.

FIG. 2A is a schematic cross-sectional view showing the flow of working fluid in a conventional thermal siphon in which a single circular tube structure is adopted for the heat insulating portion, and FIG. 2B is a double circular tube structure adopted for the heat insulating portion. It is the schematic cross-sectional view which shows the flow of the working fluid in the thermal siphon which concerns on this invention. In the figure, black arrows indicate the flow of the working fluid WF, and white arrows indicate the flow of vapor and air of the working fluid.

In the figure, the tube in the heat exchanger 3 is omitted in order to avoid complication. In addition, the vertical length of the evaporation pipe is displayed extremely short so that the flow of working fluid and steam can be easily seen.

The thermal siphon 1 according to the present invention shown in FIG. 2 (b) adopts a single circular tube structure for the heat insulating portion in that it adopts a double circular tube structure for the heat insulating portion. It is different from the conventional thermal siphon 1A. On the other hand, the hollow portions of the thermal siphons 1 and 1A are both filled with air, which is a non-condensable gas, and water, which is a working fluid.

First, with reference to FIG. 2A, the working fluid and the flow of its vapor in the conventional thermal siphon 1A adopting a single circular tube structure for the heat insulating portion will be described. In the conventional heat siphon 1A, the upper end of the bottomed evaporation tube 2A is covered with the housing 31 of the heat exchanger 3, and air and water are sealed in the hollow portion.

The water vapor heated and evaporated by the heater 7 rises in the hollow portion of the evaporation tube 2A and reaches the heat exchanger 3. Then, it comes into contact with the tube 33 of the heat exchanger 3, is cooled by the pure water flowing in the tube, and is condensed and liquefied. The liquefied water is stored in the groove 34 at the bottom of the housing 31, then returned to the bottom of the evaporation pipe 2A through the liquid return pipe 4, and is heated again by the heater 7.

A part of the water vapor comes into contact with the inner peripheral surface of the evaporation tube 2A while ascending through the hollow portion of the evaporation tube 2A to be cooled, and after condensing and liquefying and returning to water, it becomes a liquid film flow and becomes an evaporation tube 2A. Return to the bottom of.

Next, with reference to FIG. 2B, the working fluid and the flow of steam thereof in the thermal siphon 1 according to the present invention in which the double circular tube structure is adopted for the heat insulating portion will be described. As described with reference to FIG. 1, in the thermal siphon 1 according to the present invention, a double circular pipe structure in which the inner pipe 22 is arranged in the main pipe 21 is adopted as the heat insulating portion of the evaporation pipe 2, and the evaporation pipe 2 is used. Water, which is a working fluid WF, and air, which is a non-condensable gas, are sealed therein.

Further, the upper end of the inner pipe 22 is located above the upper end of the main pipe 21 and is opened into the heat exchanger 4, and the lower end is positioned so as not to come into contact with the working fluid (water) WF accumulated in the bottom of the main pipe 21. It is held.

Most of the water vapor heated and evaporated by the heater 3 reaches the heat exchanger 3 through the hollow portion of the inner pipe 22. Then, it comes into contact with the tube 33 of the heat exchanger 3 (see FIG. 1), is cooled by the pure water flowing in the tube, and is condensed and liquefied. The liquefied working fluid (water) WF is stored in the ring-shaped groove 34 formed at the bottom of the housing 31, then returned to the bottom of the main pipe 21 through the liquid return pipe 4, and is heated again by the heater. Will be done.

The air in the housing 31 of the heat exchanger 3 is pushed by the rising steam in the inner pipe 22, and as shown by the white arrow, descends through the gap between the main pipe 21 and the inner pipe 22, and then descends. It rises in the inner pipe 22 together with the steam of the working fluid and circulates between the main pipe 21 and the inner pipe 22.

As described above, in the thermal siphon 1 according to the present invention, the air layer circulates in the hollow portion of the evaporation pipe 2 to promote the rise of steam, and further, the air layer recirculates in the gap between the main pipe 21 and the inner pipe 22. Insulation effect can be obtained. On the other hand, the conventional thermal siphon 1A does not have the effect of promoting the rise of steam due to the circulation of air. As a result, the heat siphon 1 according to the present invention improves the heat transport efficiency of geothermal extraction as compared with the conventional heat siphon 1A.

<Comparison of heat transport efficiency>
Experiments were conducted on the conventional thermal siphon 1A shown in FIG. 2 (a) and the thermal siphon 1 according to the present invention shown in FIG. 2 (b), and the heat transport efficiency of each thermal siphon was calculated. The result is shown in the graph of FIG.

In the figure, the horizontal axis shows the amount of heat input, and the vertical axis shows the heat transfer efficiency. In the figure, the solid line shows the heat transport efficiency of the heat siphon 1 according to the present invention, and the broken line shows the heat transport efficiency of the conventional heat siphon 1A. Hereinafter, a method for calculating the heat transport efficiency will be described.

The heat transport efficiency η _all of the heat siphon is expressed by the following equation (1).

ここで、Ｖはヒータの入力電圧（Ｖ）、Ｒはヒータの抵抗（Ω）、ｑ_lossはヒータの単位時間当たりの放熱量（Ｗ）を示す。 In the formula (1), q _in represents the amount of heat input (W) per unit time in the evaporation part of the evaporation tube, and is calculated using the following formula (2).

Here, V is the input voltage (V) of the heater, R is the resistance (Ω) _{of the heater, and q loss} is the heat dissipation amount (W) per unit time of the heater.

ここで、λは、図示しないが、ヒータを取り巻くように配置された断熱材の熱伝導率（Ｗ/（ｍ・Ｋ））、Ｔ₁およびＴ₂は、当該断熱材の内周側および外周側に設置された２つの熱電対で測定した温度（Ｋ）で、Ｔ₁は内周側、Ｔ₂は外周側の値を示す。またΔｘは熱電対間の距離（ｍ）、Ａはヒータの外周面の表面積（ｍ²）を示す。 Then, the heat dissipation amount q _loss per unit time is calculated using the following equation (3).

Here, although λ is not shown, the thermal conductivity (W / (m · K)) of the heat insulating material arranged so as to surround the heater, and T ₁ and T ₂ are the inner peripheral side and the outer peripheral side of the heat insulating material. The temperature (K) measured by two thermocouples installed on the side, T ₁ indicates the value on the inner peripheral side, and T ₂ indicates the value on the outer peripheral side. Further, Δx indicates the distance between thermocouples (m), and A indicates the surface area (m ² ) of the outer peripheral surface of the heater.

ここで、ｔは熱交換時間（ｓ）、ｍは冷却水の流量（ｇ/ｓ）、ｃ_pは冷却水の比熱（Ｊ/（ｇ・Ｋ））、Ｔ₁は冷却水の入口温度（Ｋ）、Ｔ₂は冷却水の出口温度（Ｋ）を示す。 Further, in the equation (1), q _out represents the amount of heat recovered (W) per unit time in the heat exchanger, and is calculated using the following equation (4).

Here, t is the heat exchange time (s), m is the flow rate of the cooling water (g / s), c _p is the specific heat of the cooling water (J / (g · K)), and T ₁ is the inlet temperature of the cooling water ( K) and T ₂ indicate the outlet temperature (K) of the cooling water.

In this embodiment, an experiment was conducted using an aluminum main pipe having a total length of 1760 mm, a diameter of 52 mm, and a thickness of 2 mm, including a heat exchanger having a length of 400 mm. The flow rate of the cooling water at that time was about 15 mL, and the temperature of the initial cooling water was 20 ° C.

As is clear from the graph of FIG. 3, in the conventional heat siphon 1A having a single circular tube structure shown by a broken line, the heat transport efficiency decreases as the amount of heat input decreases, and the heat transport efficiency also decreases. Shown. On the other hand, the heat siphon 1 having a double circular tube structure according to the present invention shown by a solid line generally has high heat transfer efficiency and shows a stable value with respect to the amount of heat input. From this result, it can be seen that the heat siphon 1 according to the present invention has significantly improved heat transport efficiency of geothermal extraction as compared with the conventional heat siphon 1A.

The graph shown in FIG. 3 uses a small heat siphon having a total length of about 1.8 m and a heater as a heat source. The heat siphon according to the present invention adopts a double circular tube structure for the heat insulating portion. Is clearly superior in heat transport efficiency compared to the conventional one that uses a single circular tube structure for the heat insulating part, so it is sufficient even when extracting geothermal heat from the geothermal 100m to 300m underground. The effect can be expected.

In the present embodiment, the evaporation tube is filled with atmospheric pressure air as the non-condensable gas described above, but the same effect can be obtained by using an inert gas such as nitrogen or argon. Further, when the gas pressure is too low, the effect of filling the gas is reduced, but if the gas having an appropriate pressure is filled, the same circulation effect as when the atmospheric pressure air is filled can be obtained.

1 Heat siphon 2 Evaporation pipe 3 Heat exchanger 4 Liquid return pipe 5 Heat storage machine 6 Pump 7 Heater 21 Main pipe 22 Inner pipe 23 Insulation material 24 Flange 31 Housing 32 Lid 33, 51 Tube 34 Groove 41 Injection port 42 Control valve 43 Tank 44 Discharge port WF working fluid

Claims (5)

A bottomed main pipe that is installed in a substantially vertical direction with the lower end inserted in the heat source, and in which a condensable working fluid and a non-condensable gas are sealed in the hollow part.
A heat exchanger connected to the upper end of the main pipe so as to cover the opening of the main pipe.
A liquid return pipe that returns the working fluid condensed and liquefied by the heat exchanger to the inside of the pipe through a hole formed below the main pipe.
It was installed in the hollow portion of the main pipe at a distance from the main pipe, the upper end was opened in the heat exchanger, and the lower end was held at a position not in contact with the working fluid accumulated in the bottom of the main pipe. A heat siphon characterized by having an inner tube. A heat storage device and a pump are connected to the heat exchanger via a tube.
The heat siphon according to claim 1, wherein the heat transferred from the heat source by the working fluid is stored in the heat storage device by the refrigerant circulating in the tube by the pump. The thermal siphon according to claim 1 or 2, wherein water is used as the working fluid and air is used as the non-condensable gas. The heat siphon according to any one of claims 1 to 3, wherein as the heat exchanger, a shell & tube type heat exchanger in which a spiral tube is housed in a hollow portion of a cylindrical housing is used. The upper end of the main pipe projects upward from the connecting portion of the heat exchanger with the housing, and a groove is formed between the main pipe and the housing.
The thermal siphon according to claim 4, wherein the working fluid accumulated in the groove is injected into the liquid return pipe from an injection port formed at the bottom of the housing.

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