Description Title of the Invention: GEOTHERMAL HEAT EXCHANGER AND GEOTHERMAL POWER GENERATION EQUIPMENT Technical Field [0001] The present invention relates to a geothermal heat exchanger which conducts heat exchange by using a geothermal region as heat source as it is without extracting natural steam present in the geothermal region. The present invention also relates to geothermal power generation equipment in which the geothermal heat exchanger is used to generate electric power. Background Art [0002] A process for utilizing geothermal energy such as geothermal power generation is to use a high-temperature magma layer of earth as a heat source and is able to utilize semi-permanent thermal energy. The above described process does not produce a greenhouse effect gas in the course of electric power generation, and has thus captured attention as an alternative means of electric power which depends upon fossil fuel. Further, in view of the nuclear power plant accident, Japan's energy policy which until that time was heavily dependent on nuclear power generation has been forced to undergo a fundamental review. In this respect as well, there has been strong demands made for a means of obtaining 1 energy without adversely impacting natural environments. [0003] In conventional geothermal power generation, boring is conducted at a geothermal region to extract natural steam present in a geothermal region by utilizing natural pressure, thereby using the steam by water separation. The thus extracted steam contains a large amount of sulfur and other impurities unique to a geothermal region. The impurities adhere to a thermal well, piping and blades of a turbine, etc., as scale. Upon adhesion of scale, a power plant is decreased in output with the lapse of time, thus resulting in difficulty in prolonged use. For the purpose of solving the problem resulting from scale, a technology that adopts a process in which water is fed from the ground, heated and extracted has been disclosed in Patent Document 1, Patent Document 2, Patent Document 3, and Patent Document 4. The invention made by the present inventor is also disclosed in Patent Document 5. Prior Art Documents Patent Literature [0004] Patent Document 1: Japanese Published Unexamined Patent Application No. H9-112407 Patent Document 2: Japanese Published Unexamined Utility Model Application No. S57-12571 Patent Document 3: Japanese Published Unexamined Patent 2 Application No. 2000-161198 Patent Document 4: Japanese Published Unexamined Patent Application No. S49-103122 Patent Document 5: Japanese Published Unexamined Patent Application No. 2011-52621 Summary of the Invention Problems to be Solved by the Invention [0005] The invention described in Patent Document 1 is to extract from a rear end of a heat exchange portion a fluid at any given temperature introduced from a front edge of the heat exchange portion in a state of mixture of steam and hot water inside the heat exchange portion placed underground, that is, in a state of gas-liquid two-phase flow by geothermal heat. [0006] Further, the invention described in Patent Document 2 is characterized in that water is introduced into pipes inserted underground, changing into steam underground by geothermal heat and the steam is separated from a liquid by using a gas liquid separator to transport only the steam into a turbine and a condenser. [0007] However, neither Patent Document 1 nor Patent Document 2 describes any pressurization unit for pressurizing water to be fed underground. Further, although a pressurization unit has been described in Patent Document 3 3 and Patent Document 4, a water-supply pressurizing device described in Patent Document 3 and Patent Document 4 is installed for obtaining a driving force in order to introduce water from the ground into the underground, and extraction of steam from underground is clearly described. Therefore, in any of the above described processes, an extracted substance is in a state of a gas-liquid two-phase flow which contains hot water and steam. Thus, water separation is required for extracting only steam by water separation. [0008] Further, in Patent Document 5 which was filed by the present inventor, there is described geothermal power generation equipment having such a structure that a pressurized water injection pipe and a hot water extraction pipe are provided, the pressurized water injection pipe is disposed inside the hot water extraction pipe, and treated water which has been heated passes through an open lower end of the pressurized water injection pipe and moves to the hot water extraction pipe. However, according to the above described structure, the pressurized water injection pipe is disposed inside the hot water extraction pipe, thereby posing such a problem that the pressurized water injection pipe is less likely to receive heat from a geothermal region. Thereafter, the present inventor conducted studies and 4 solved problems found in a conventional process for extracting underground hot water and a process for generating steam inside a geothermal heat exchanger, extracting the steam as a gas-liquid two-phase flow and introducing it into a turbine via a water separator. Thus, the inventor has achieved inventions of a geothermal heat exchanger and geothermal power generation equipment which are able to obtain energy with higher efficiency. [0009] The present invention has been made for solving the above described problems, an object of which is to provide a geothermal heat exchanger and geothermal power generation equipment which are able to conduct heat exchange great in capacity and high in heat efficiency, because impurities coming from steam to be used do not adhere to the equipment and the steam can be obtained from high-temperature and high-pressure water extracted from underground, and which do not adversely impact environments in the vicinity of a geothermal region. Means for Solving the Problems [0010] In order to solve the above described problems, the geothermal heat exchanger of the present invention is a geothermal heat exchanger which is provided with a pressurized water injection pipe to which treated water which has been pressurized by a high-pressure water supply pump is supplied 5 and a hot water extraction pipe in which heat is supplied from a geothermal region to the treated water coming down through the pressurized water injection pipe to the geothermal region to generate hot water and the hot water rises in a state free of steam, in which the hot water extracted from the hot water extraction pipe is fed to a steam generator and extracted as steam only inside the steam generator, and the geothermal heat exchanger is structured in such a manner that the pressurized water injection pipe is disposed on the side of an outer circumference of the hot water extraction pipe and the hot water moves to the hot water extraction pipe via a lower part of the pressurized water injection pipe. [0011] The treated water pressurized by the high-pressure water supply pump is supplied to the pressurized water injection pipe and the treated water comes down through the pressurized water injection pipe and reaches a geothermal region. Thereby, heat is supplied to the treated water from the geothermal region to generate hot water, and the hot water is extracted on the ground and changed into steam by using a steam generator. Therefore, the steam is free of impurities and, as found in direct use of natural steam present in a geothermal region, no scale adheres to a turbine, piping etc. Thus, it is not necessary to remove scale and easy to conduct 6 maintenance. It is also possible to prevent fouling of steam caused inside a pipe by a gas-liquid two-phase flow and vibration due to unstable fluidity. Therefore, the geothermal heat exchanger is advantageous in view of safety during operation. [0012] Further, the hot water rises from the hot water extraction pipe to the surface of earth in a state free of steam until being fed into a steam generator disposed on the ground and is subjected to extraction. Therefore, higher energy efficacy can be achieved than in a case where steam is extracted in a gas liquid mixture of two-phase flow which is a mixed state of hot water and steam. Still further, the pressurized water injection pipe is disposed on the side of an outer circumference of the hot water extraction pipe. Thus, the pressurized water injection pipe is able to easily receive heat from a geothermal region and water filled into the pressurized water injection pipe can be efficiently maintained in a high-temperature state. [0013] In the geothermal heat exchanger of the present invention, it is preferable that in a zone from the surface of earth to a site on its way to a geothermal region, an intermediate layer is installed between the pressurized water injection pipe and the hot water extraction pipe, thereby 7 providing a triple pipe structure which consists of the pressurized water injection pipe, the intermediate layer and the hot water extraction pipe in order closer to the geothermal region, and the intermediate layer is a gas layer or a heat insulation material packed layer. [0014] Since the above described intermediate layer is installed, it is possible to prevent thermal conduction from the hot water extraction pipe in which high-temperature hot water rises to the pressurized water injection pipe. Therefore, the hot water inside the hot water extraction pipe can be extracted, while maintaining a hot-temperature state. In particular, in a zone from the surface of earth to a site on its way to the geothermal region, there is a great difference in temperature between the hot water extraction pipe and the pressurized water injection pipe. Therefore, the intermediate layer is installed in the zone to provide heat insulation, thereby greatly contributing to prevention of heat loss. [0015] In the geothermal heat exchanger of the present invention, it is preferable that the hot water extraction pipe and the pressurized water injection pipe are formed in such a manner that the hot water extraction pipe is smaller in cross sectional area than the pressurized water injection pipe. 8 Here, cross sectional area means a cross sectional area in which the hot water extraction pipe and the pressurized water injection pipe are cut in a direction perpendicular to a main flow. Thereby, hot water which has moved from the pressurized water injection pipe to the hot water extraction pipe is allowed to rise inside the hot water extraction pipe at a greater flow velocity. As a result, such effects are expected that heat loss is prevented to extract hot water inside the hot water extraction pipe in a high-temperature state. [0016] In the geothermal heat exchanger of the present invention, it is preferable that the pressurized water injection pipe is formed with a material high in thermal conductivity and a middle pipe which constitutes the intermediate layer and the hot water extraction pipe are formed with a material high in heat insulation. Thereby, heat from a geothermal region is effectively conducted to water coming down through the pressurized water injection pipe, and also thermal conduction from hot water rising through the hot water extraction pipe is suppressed. It is, therefore, possible to extract the hot water inside the hot water extraction pipe in a high-temperature state. [0017] In the geothermal heat exchanger of the present 9 invention, it is preferable that in a zone which is structured so as to give a double pipe where the pressurized water injection pipe is formed directly outside the hot water extraction pipe, a plurality of inlet holes are made in an outer circumference of the hot water extraction pipe, and hot water present in the vicinity of the lower part of the pressurized water injection pipe is introduced into the hot water extraction pipe through the inlet holes. Thereby, staying time of a fluid at the lowest part region which is expected to be the highest temperature part is increased, allowing water inside the pressurized water injection pipe which becomes higher in temperature due to supply of heat from a geothermal region to move to the hot water extraction pipe smoothly. [0018] In the geothermal heat exchanger of the present invention, there is provided a triple pipe structure in which the intermediate layer is installed along an entire length of the hot water extraction pipe outside the pipe. The geothermal heat exchanger is to be structured so as to introduce hot water from the lowest part of the hot water extraction pipe. Since the intermediate layer is installed along an entire length of the hot water extraction pipe, it is possible to enhance heat insulation effects between the pressurized water 10 injection pipe and the hot water extraction pipe by the intermediate layer. [0019] In the geothermal heat exchanger of the present invention, it is preferable that an intermediate lid portion for preventing natural hot water or steam present underground from rising through a geothermal well is installed at the side of an outer circumference of the pressurized water injection pipe. Thereby, it is possible to prevent the natural hot water or steam which is present underground from rising up to the ground, resulting in loss of thermal energy retained by a heat source. It is also possible to prevent destruction of natural environments due to loss of geothermal water and steam in nature. [0020] In the geothermal heat exchanger of the present invention, it is preferable that a heat transfer area increasing unit for increasing a heat transfer area to facilitate heat transfer from a geothermal region is installed on the pressurized water injection pipe. Since the heat transfer area increasing unit is installed, it is possible to easily transfer heat of the geothermal region to the pressurized water injection pipe and also to heat pressurized water flowing through the pressurized water 11 injection pipe efficiently. [0021] In the geothermal heat exchanger of the present invention, it is preferable that a support base is installed at a bottom of the pressurized water injection pipe. The support base is able to receive in a dispersed manner loads of a double pipe structure body which consists of the pressurized water injection pipe and the hot water extraction pipe or loads of a triple pipe structure body which consists of the pressurized water injection pipe, the hot water extraction pipe and the intermediate layer. Therefore, the support base can be installed in a geothermal well more reliably than a suspension type support base. [0022] In the geothermal heat exchanger of the present invention, it is preferable that a reinforcement portion for preventing vibration is installed at the deepest part of the triple pipe structure and at any given site on its way thereto. The part of the triple pipe structure is more likely to cause vibration, in particular, lateral vibration. However, the thus installed reinforcement portion is able to prevent vibration at this part. [0023] In the geothermal heat exchanger of the present invention, one or a plurality of insertion pipes, each of which is formed in combination with at least the one hot water 12 extraction pipe and the one pressurized water injection pipe, are inserted into one or a plurality of geothermal wells, and the geothermal heat exchanger can be arranged so that the insertion pipe is in combination with the high-pressure water supply pump and the steam generator which are disposed on the ground. [0024] Such usage is possible that one insertion pipe is inserted into one geothermal well. However, both temperatures and pressures are different depending on a site which is subjected to boring, and upon generation of electric power, electric power generated at each geothermal well is different in output. Thus, regarding a plurality of geothermal wells, outlets of the hot water extraction pipes of the insertion pipes are connected in parallel to collect hot water obtained from each of the geothermal wells in an aggregate manner. Thereby, a steam generator, a turbine, a condenser, a generator, a transformer etc., can be designed to be larger in capacity, which is advantageous in increasing the efficiency of a power plant as a whole. [0025] In the geothermal heat exchanger of the present invention, the geothermal well is to be attached to existing facilities. The insertion pipe which is arranged in combination with 13 the hot water extraction pipe and the pressurized water injection pipe is inserted and used in an empty geothermal well or a geothermal well out of operation which is attached to existing facilities. It is, thus, possible to extract energy from hot water without any boring. [0026] The geothermal power generation equipment of the present invention is to generate electric power by using the above described geothermal heat exchanger. Electric power is generated by using the geothermal heat exchanger of the present invention, thus making it possible to generate electric power at high energy efficiency without adversely impacting natural environments. Effects of the Invention [0027] According to the present invention, it is possible to provide a geothermal heat exchanger and geothermal power generation equipment which are able to conduct heat exchange great in capacity and excellent in heat efficiency, because impurities coming from steam to be used will not adhere to the equipment and steam can be obtained from high-pressure high-temperature water extracted from underground, and which do not adversely impact environments in the vicinity of a geothermal region. Brief Description of the Drawings 14 [0028] Fig. 1 is a drawing which shows a configuration of a geothermal heat exchanger according to an embodiment of the present invention. Fig. 2 is a drawing which shows an arrangement of power generator units in generator room. Fig. 3 is a drawing which shows a structure of the geothermal heat exchanger in which an intermediate layer is installed along an entire length of a hot water extraction pipe outside the pipe. Fig. 4 is a drawing which shows a structure of the geothermal heat exchanger in which a heat transfer area increasing unit is installed on the side of an outer circumference of a pressurized water injection pipe. Fig. 5 is a drawing which shows a structure of the geothermal heat exchanger in which an intermediate lid portion is installed on the side of the outer circumference of the pressurized water injection pipe. Fig. 6 is a phase diagram of water. Fig. 7 is a drawing which shows a change in heat transfer mode at a saturated boiling region. Fig. 8 is a drawing which shows a change in inlet pressure due to a difference in piping material (Thermal conductivity) upon extraction as a single phase flow which consists 15 exclusively of hot water. Fig. 9 is a drawing which shows a change in void fraction upon extraction as a single phase flow which consists exclusively of hot water. Fig. 10 is a drawing which shows a change in liquid phase temperature upon extraction as a single phase flow which consists exclusively of hot water. Fig. 11 is a drawing which shows thermal output at a flow rate of 0 . 001m 3 /s upon extraction as a single phase flow which consists exclusively of hot water. Fig. 12 is a drawing which shows a change in thermal output due to a change in flow rate upon extraction as a single phase flow which consists exclusively of hot water. Fig. 13 is a drawing which shows a change in inlet pressure due to a difference in piping material (thermal conduction rate) upon extraction as a gas-liquid two-phase flow. Fig. 14 is a drawing which shows thermal output at a flow rate of 0.001m 3 /s upon extraction as a gas-liquid two-phase flow. Fig. 15 is a drawing which shows a change in thermal output due to a change in flow rate upon extraction as a gas-liquid two-phase flow. Fig. 16 is a drawing which shows a change in void fraction 16 upon extraction as a gas-liquid two-phase flow. Best Mode for Carrying Out the Invention [0029] Hereinafter, a description will be given of the geothermal heat exchanger and the geothermal power generation equipment of the present invention based on their embodiments. Fig. 1 is a drawing which shows a configuration of the geothermal heat exchanger according to the embodiment of the present invention. Fig. 1(a) is a drawing which shows an overview of the geothermal heat exchanger, Fig. 1(b) is a cross sectional view taken along the line of A to A in Fig. 1(a), and Fig. 1(c) is a cross sectional view taken along the line of B to B in Fig. 1(a). [0030] As shown in Fig. 1, a geothermal heat exchanger 1 of the embodiment of the present invention is inserted into a geothermal well 2 and provided with a pressurized water injection pipe 11 and a hot water extraction pipe 12 in which the pressurized water injection pipe 11 is disposed on the side of an outer circumference of the hot water extraction pipe 12. The pressurized water injection pipe 11 and the hot water extraction pipe 12 are both buried in the ground, and the pressurized water injection pipe 11 and the hot water extraction pipe 12 are set for a depth so that a prescribed zone closer to a lower part of the pressurized water injection 17 pipe 11 and the hot water extraction pipe 12 are in contact with a geothermal region 10 present at an underground deep part. Due to the above described system, pressurized hot water generated by heating the geothermal region 10 as a heat source moves to the hot water extraction pipe 12 via the lower part of the pressurized water injection pipe 11. [0031] The geothermal heat exchanger 1 is provided with an intermediate layer 13 between the pressurized water injection pipe 11 and the hot water extraction pipe 12 in a zone from the surface of earth 3 to a site on its way to the geothermal region 10. That is, in this zone, there is provided a triple pipe structure which consists of the pressurized water injection pipe 11, the intermediate layer 13 and the hot water extraction pipe 12 in order from the side closer to the geothermal region 10. The intermediate layer 13 is given as a gas layer or a heat insulation material packed layer so as to have insulation effects. As one example of the gas layer, the intermediate layer 13 is in a vacuum or hollow in a state of reduced pressure. It is also acceptable that the intermediate layer 13 itself is formed with a material high in thermal insulation or formed so as to be closed in place of being hollow. [0032] The pressurized water injection pipe 11 which is 18 in contact with the geothermal region 10 is formed with a material high in thermal conductivity and great in strength such as a ceramic-based composite material or a carbon-based composite material in order to improve efficient heat supply from the geothermal region 10 to injected water coming down through the pressurized water injection pipe 11. On the other hand, a middle pipe 14 which constitutes the intermediate layer 13 and the hot water extraction pipe 12 are formed with a material high in thermal insulation in order to keep the hot water flowing through the hot water extraction pipe 12 at high temperatures. As one example, an ordinary metal material to which insulation coating is performed may be used. Pump pressure coming from a high-pressure water supply pump 17 and geothermal pressure coming from the geothermal region 10 are applied to a part of the triple pipe structure. Therefore, the pressurized water injection pipe 11 which is an outer pipe is decreased in pipe thickness so that heat in order to improve thermal conductivity from the geothermal region 10, while keeping the strength, with consideration given to balance with the pressures. [0033] Below the above described zone of the triple pipe structure, there is provided a double pipe structure in which the pressurized water injection pipe 11 is formed directly 19 outside the hot water extraction pipe 12. In this zone, a plurality of inlet holes 15 is made in an outer circumference of the hot water extraction pipe 12. Hot water present in the vicinity of the lower part of the pressurized water injection pipe 11 is introduced into the hot water extraction pipe 12 through the inlet holes 15. A lower end lb of the pressurized water injection pipe 11 is structured so as to be thick for securing the strength, which is a structure for supporting a lower end 12b of the hot water extraction pipe 12. [0034] At the deepest part of the triple pipe structure and at any given number of sites on its way thereto, a reinforcement portion 16 is installed for preventing lateral vibration. It is, therefore, possible to adopt a supporting frame structure as a specific structure of the reinforcement portion 16. A pressure adjusting portion 18 is installed at an upper part of the middle pipe 14 on the surface of earth 3 and the pressure adjusting portion 18 is used to adjust the pressure of the middle pipe 14. Further, a lid 19 is installed on the geothermal well 2 on the surface of earth 3, by which natural hot-spring water is prevented from flowing out and environmental destruction is also prevented. The part of the triple pipe structure is inserted underground in such a manner that a triple pipe with a certain length is manufactured at 20 a plant and jointed at a construction site. [0035] As shown in Fig. 1(b) and Fig. 1(c), the hot water extraction pipe 12 which is an inner pipe is formed so as to be smaller in cross sectional area than the pressurized water injection pipe 11 which is an outer pipe. Thereby, hot water which rises inside the hot water extraction pipe 12 can be increased in flow velocity and is expected to prevent heat loss effectively. [0036] High-purity treated water which has been pressurized by the high-pressure water supply pump 17 and from which impurities have been removed is supplied to the pressurized water injection pipe 11 from the side of the upper end 11a. The treated water comes down through the pressurized water injection pipe 11, as indicated with a white arrow, and reaches the vicinity of the lower end lb. In the vicinity of the lower end lb, as indicated by a black arrow, the treated water is heated by heat supplied from the geothermal region 10, and the treated water which has been heated flows through the inlet holes 15, moves to the hot water extraction pipe 12, and rises through the hot water extraction pipe 12, as indicated by a white arrow, with a pressurized state and a high temperature being maintained. Moreover, the treated water reaches the upper end 12a in a state of consisting exclusively 21 of hot water and free of steam and is extracted. [0037] As apparent from the above description, the high-pressure water supply pump 17 imparts a pressure necessary for extracting the treated water in a state which consists exclusively of hot water and free of steam to the treated water which is filled into the pressurized water injection pipe 11. A detail description will be later given of specific feasibility of the pressurization. [0038] Hot water extracted from the hot water extraction pipe 12 is fed to a steam generator 21 and reduced in pressure. In the steam generator 21, a high-pressure state is maintained at a pressure lower than a pressure applied to the treated water which is filled into the pressurized water injection pipe 11, thus making it possible to obtain steam high in temperature and pressure. Generation of the high-temperature high-pressure steam allows movement of a great thermal energy. According to the above described process, water is not boiled by using a geothermal heat exchanger, piping etc., installed underground but the high-pressure water supply pump 17 is used to apply pressure to extract only hot water on the ground. Thereby, the water is boiled under reduced pressure only within the steam generator 21 to extract the high-temperature high-pressure steam. 22 [0039] The geothermal heat exchanger 1 is arranged in such a manner that one or a plurality of insertion pipes, each of which is arranged in combination with at least the one hot water extraction pipe 12 and the one pressurized water injection pipe 11, are inserted into one or a plurality of geothermal wells 2 and the insertion pipe is combined with the high-pressure feed-water pump 17 and the steam generator 21 which are disposed on the ground. [0040] Such usage is also possible that one insertion pipe is inserted into one geothermal well. However, both temperatures and pressures are different depending on a site to be subjected to boring, and upon generation of electric power, electric power generated at each geothermal well is different in output. Thus, regarding a plurality of geothermal wells, outlets of the hot water extraction pipes of the insertion pipes are connected in parallel to collect hot water obtained from each of the geothermal wells in an aggregate manner. Thereby, a steam generator, a turbine, a condenser, a generator, a transformer etc., can be designed to be larger in capacity, which is advantageous in increasing the efficiency of a power plant as a whole. [0041] For example, where three geothermal wells are used, the thermal output of each of the geothermal wells is converted 23 to the output of a generator, which is to be 500kW for a first well, 400kW for a second well and 600kW for a third well. In this case, rather than composing an electric power generation system with three independent units, these wells are designed so as to give one unit consisting of the first well + the second well + the third well of 1500kW in an aggregate manner. Thereby, although a total output is the same, the steam generator, the turbine, the condenser, the generator and the transformer can be individually designed so as to give a greater capacity. Since electrical equipment is increased in efficiency in accordance with the capacity, a power plant is increased in total efficiency when used in generating electric power. It is also possible to significantly decrease building expenses such as construction costs. [0042] Further, the geothermal heat exchanger 1 can be used not only in a newly built geothermal well 2 but also used in a geothermal well 2 attached to existing facilities, for example, an existing geothermal power plant, that is, an empty geothermal well or a geothermal well which is out of operation by inserting an insertion pipe arranged in combination with the hot water extraction pipe 12 with the pressurized water injection pipe 11. [0043] As described in the present invention, in the 24 geothermal heat exchanger which is arranged so that the pressurized water injection pipe 11 and the hot water extraction pipe 12 are inserted into the geothermal well 2, treated water which has been pressurized is supplied to the pressurized water injection pipe 11 to extract only hot water free of steam from the hot water extraction pipe 12. Thereby, outstanding effects are obtained. [0044] For example, when extraction in a state of gas liquid mixture is compared with extraction of pressurized hot water in relation to a pipe diameter of the extraction pipe, it is necessary that the extraction pipe is made larger in pipe diameter than the pipe used in extraction of pressurized hot water in order to equalize an amount of energy obtained by extraction in a state of gas liquid mixture with an amount of energy obtained by extraction of pressurized hot water. However, a greater pipe diameter of the extraction pipe will result in a greater cross sectional area on boring to be conducted for forming the extraction pipe. Therefore, there is a disadvantage that a large burden is placed on environments such as great loss of hot spring resources in construction work and operation. Further, construction is performed on a larger scale, which requires large-scale construction facilities to result in a longer construction period and higher costs. 25 [0045] Further, a process for boiling water underground is difficult in feasibility due to instability in terms of fluid dynamics and problems involved in steam blocking. Still further, a gas-liquid two-phase flow is much greater in heat transfer rate than a single phase flow which consists exclusively of hot water, resulting in great loss of energy upon long-distant transportation of a heating medium. [0046] On the contrary, according to the present invention, water is boiled under reduced pressure on the ground, by which not only can the amount of heat (energy) obtained by a working medium through geothermal heat be converted at high efficiency but also the hot water extraction pipe can be designed to be smaller in pipe diameter due to the above described reasons. It is, thereby, possible to reduce burdens on environments and downsize the scale of construction. [0047] As described above, in aprocess for heating treated water which has been pressurized to extract only hot water, the pipe diameter can be maintained small by the present invention in which the pressurized water injection pipe 11 and the hot water extraction pipe 12 are essential components. As a result, the process is able to generate electric power in a great capacity and at a high efficiency, with consideration given to environmental burdens. The process for heating the 26 treated water which has been pressurized to extract hot water is applied to the system of the present invention, thereby providing working effects which are quite useful and unique. [0048] Fig. 2 shows an arrangement of a generator room 20. Steam generated in the steam generator 21 is further heated by a steam superheater 22 and fed to the turbine 23 as high-temperature and high-pressure steam, and electric power is generated by a generator 24. Steam inside the turbine 23 is fed to a condenser 25 and condensed water generated by the condenser 25 is mixed with highly treated water, fed to a high-pressure water supply pump and fed back to a geothermal well. [0049] Fig. 2 shows an example in which the geothermal heat exchanger of the present invention is used for generating geothermal power. The geothermal heat exchanger of the present invention shall not be applied only thereto. There is available, for example, such a system that steam obtained by the geothermal heat exchanger of the present invention is directly used for air conditioning. The geothermal heat exchanger is applicable also to other applications. [0050] Fig. 3 shows a structure of the geothermal heat exchanger in which an intermediate layer is installed along an entire length of a hot water extraction pipe outside the 27 pipe. As shown in Fig. 3, the geothermal heat exchanger is structured so as to give a triple pipe in which the intermediate layer 13 is installed along an entire length of the hot water extraction pipe 12 and a hot water introduction port 26 is installed at the lowest part of the hot water extraction pipe 12. Hot water heated while coming down through the pressurized water injection pipe 11 is introduced into the hot water extraction pipe 12 from the hot water introduction port 26. According to the above described system, the intermediate layer 13 is installed along an entire length of the hot water extraction pipe 12, by which the intermediate layer 13 is able to enhance heat insulation effects between the pressurized water injection pipe 11 and the hot water extraction pipe 12. [0051] Fig. 4 shows a structure of the geothermal heat exchanger in which an intermediate lid portion is installed on the side of an outer circumference of the pressurized water injection pipe. Fig. 4(a) is a drawing which shows an overview of the geothermal heat exchanger and Fig. 4(b) is a cross sectional view taken along the line of C to C in Fig. 4(a). As shown in Fig. 4, an intermediate lid portion 27 is installed on the side of the outer circumference of the pressurized water injection pipe 11 at an intermediate position 28 in a depth direction of the pressurized water injection pipe 11. The intermediate lid portion 27 is installed so as to block the geothermal well 2 and protrude in a radial direction along the outer circumference of the pressurized water injection pipe 11, and the geothermal well 2 is structured so as to be partitioned in a vertical direction by the intermediate lid portion 27. [0052] As described above, installation of the intermediate lid portion 27 makes it possible to prevent natural hot water present underground from rising up to the ground. Further, natural steam is condensed by the intermediate lid portion 27 and returns underground. It is, therefore, possible to prevent loss of thermal energy retained by the heat source. It is also possible to prevent loss of natural geothermal water and steam, resulting in destruction of natural environments. [0053] A support base 28 is attached at the bottom of the pressurized water injection pipe 11 and disposed in such a manner that a lower end face of the support base 28 is in contact with the bottom of the geothermal well 2. The support base 28 is installed, by which loads of a double pipe structure body consisting of the pressurized water injection pipe 11 and the hot water extraction pipe 12 or loads of a triple pipe structure 29 body consisting of the pressurized water injection pipe 11, the hot water extraction pipe 12 and the intermediate layer 13 can be received in a dispersed manner by the support base 28. It is possible to install the support base 28 on the geothermal well 2 more reliably than a suspension type support base. The number of the support bases 28 may be changed depending on the situation, whenever necessary. [0054] Fig. 5 shows a structure of the geothermal heat exchanger in which a heat transfer area increasing unit is installed on the side of the outer circumference of the pressurized water injection pipe. Fig. 5(a) is a drawing which shows an overview of the geothermal heat exchanger, and Fig. 5(b) is a cross sectional view taken along the line of D to D in Fig. 5(a). As shown in Fig. 5, a plurality of side wall fins 29 which function as a heat transfer area increasing unit for increasing a heat transfer area to facilitate heat transfer from a geothermal region are installed on the side of the outer circumference of the pressurized water injection pipe 11. The side wall fin 29 can be formed in a disc shape so as to protrude in a radial direction along the outer circumference of the pressurized water injection pipe 11 and may be changed in the shape and the number depending on the situation, whenever 30 necessary. [0055] Further, a bottom fin 30 which is structured so as to protrude in a downward direction is installed at the bottom of the pressurized water injection pipe 11. The bottom fin 30 also functions as a heat transfer area increasing unit. In Fig. 5, the bottom fin 30 which is formed in a pin shape is shown but may be changed in the shape and the number depending on the situation whenever necessary. It is noted that the intermediate lid portion 27, the support base 28, the side wall fins 29 and the bottom fin 30 which have been described above can be attached to the structure shown in Fig. 3 in a similar manner. Further, the reinforcement portion 16 may be installed at the lowest part of the triple pipe structure on the structure shown in Fig. 3 in a similar manner as that shown in Fig. 1. [0056] The present invention has a main characteristic that water which has been pressurized on the ground by the high-pressure water supply pump 17 is extracted on the ground in a state free of gas, that is, a single phase flow, without generating steam inside a geothermal heat exchanger installed underground, the water is thereafter boiled under reduced pressure by using a steam generator 21 to extract geothermal power as steam. Hereinafter, a detailed description will be 31 given of the feasibility thereof. [0057] Fig. 6 is a phase drawing of water. When a phase is changed from liquid to gas, that is, a temperature when a line of T to C in Fig. 6 crosses from left to right denotes a boiling point, the boiling point can be raised with an increase in pressure. Water supply and pressurization of the present invention are conducted for the purpose of suppressing the phase change inside a underground geothermal heat exchanger (geothermal heat exchanger which is of single-phase flow, mono-axial and triple pipe-type), and water is extracted by design as a single-phase hot water, without generating steam inside the underground geothermal heat exchanger. [0058] At a geothermal power plant based on conventional technology, there is adopted a flash process in which an underground gas-liquid two-phase flow is separated into steam and water by using a water separator (a process for allowing the flow to pass through the water separator only once is called single flash, and a process for allowing the flow to pass through the water separator again for attaining a higher electric power generation efficiency is called double flash). When compared with the above described flash process, the present invention is not a process for extracting natural steam but realizes a complete closed line from pressure application 32 by using a high-pressure pump, generation of steam, a turbine and a condenser, which leads to an advantage of the present invention. Therefore, comparison was made for evaluation with a case where pressure is applied by pumping to carry out extraction as a gas liquid mixture (gas-liquid two-phase flow). [0059] Technology of generating geothermal heat and electric power which has been in general a practical application is such that natural hot water is obtained or steam is generated inside a geothermal heat exchanger and extracted on the ground as a mixture of hot water and steam, that is, a gas-liquid two-phase flow. It is known that when droplets of water (hot water and condensed water of steam) are mixed with steam, there is caused a great decrease in heat efficiency of a turbine, compared with operation in dry steam (state of only steam), which is called moisture loss. Further, droplets in steam collide with moving blades of a turbine which rotates at high speeds or against an inner wall of piping, thereby undergoing erosion (collision wear). It causes not only a greater decrease in efficiency but also mechanical damage. [0060] Therefore, the thus extracted gas-liquid two-phase flow is required to be separated into steam and water mechanically by using a water separator prior to introduction into the turbine, which contributes to an increase in cost. 33 In a boiling water reactor, a gas-liquid two-phase flow generated at a reactor core is designed to be brought closer to dry steam by installing a water separator and a steam dryer at subsequent stages. That is, when geothermal heat is given and received as a gas-liquid two-phase flow by using a geothermal heat exchanger, there is no choice but to discard heat stored in a liquid phase. [0061] Where consideration is given to heat transfer of a fluid which flows through the geothermal heat exchanger, a gas-liquid two-phase flow is in general much greater in heat transfer rate than a single phase flow. The heat transfer of a forced flow boiling system is categorized in detail depending on an aspect of boiling and complicated. As one example thereof, Fig. 7 shows a heat transfer rate of saturated boiling, that is, a flow in such a state that a liquid temperature is equal to a saturated temperature or slightly higher than the temperature, thereby generating effective steam. The heat transfer rate is a measurement which shows how easily heat is transferred from a flowing fluid to a wall in contact with the fluid. A vertical axis represents a ratio of heat transfer rate hTP in a gas-liquid two-phase flow to heat transfer rate hLZ in a single phase flow, and a horizontal axis represents a quantity which is referred to as Lockhart-Martinelli 34 parameter. This parameter is in general used in describing pressure loss and heat transfer of the gas-liquid two-phase flow. Fig. 7 clearly indicates that a heat transfer rate of the gas-liquid two-phase flow is ten times or several tens of times greater than that of the single phase flow. It is noted that Fig. 7 which shows a change in heat transfer mode at a saturated boiling region is cited from J. G. Collier, "Convective Boiling and Condensation, " McGraw-Hill, New York. (1972). [0062] In the geothermal heat exchanger, both where the water supply side is given as an inner pipe and where the water supply side is given as an outer pipe, heat exchange is conducted at the time of water intake through a wall of piping with a low-temperature portion (low-temperature underground part and low-temperature water supply in the inner pipe in the case of water intake through the outer pipe, and water supply in the outer pipe in the case of water intake through the inner pipe). Since no complete insulating material is available, heat transfer will inevitably take place from the side of water intake which is a higher temperature portion to a low-temperature portion. [0063] As described above, a gas-liquid two-phase flow is much greater in heat transfer amount than a single phase flow, 35 that is, the gas-liquid two-phase flow is easily deprived of heat. Therefore, before earth thermal which has been received at a deep site under the ground is transferred on the ground, the earth thermal is to be returned inside the geothermal heat exchanger or into the ground. Therefore, there is a possibility that heat efficiency may be decreased or no steam may be extracted. [0064] Further, a gas-liquid two-phase flow is quite complicated in fluidity mode and heat transfer mechanism. Still further, influences of buoyancy are added thereto, which makes a phenomenon more complicated and unstable. In the present invention, it is assumed that a triple-pipe heat exchanger is manufactured on the order of several hundred meters or several kilo meters. However, where underground pressure is also added to the system at a deep site, whether or not the gas-liquid two-phase flow is driven normally remains unknown. [0065] In particular, where there is used the above described triple pipe long in flow channel, vibration resulting from generation of steam poses a problem. This problem is recognized in the case of a nuclear power reactor and measures for preventing vibration have been under development. Amplification of vibration will cause mechanical damage. More 36 important is at which site steam is generated. [0066] Since a bottom-most part is highest in temperature, it is more likely that water is evaporated at the bottom-most part or steam is generated on boiling under reduced pressure in upward flow. In either case, the greatest concern is development of a phenomenon called vapor lock in which a high-pressure portion caused by the thus generated steam blocks a flow channel. The vapor lock itself is not an event which can be prevented by using a check value etc. When this event takes place, there is a possibility that mechanical damage may be caused by superheating, and this event should be prevented. [0067] Next, a description will be given of feasibility of the present invention by numerical analysis. In experimental calculation, the heat exchanger of the present invention (80 mm in diameter of outer pipe) was installed in a 1000-meter bore hole and temperature at the deepest underground site was 270'C. The heat exchanger is made small in outer diameter due to restrictions on calculation capacity and time. However, it is also assumed that a number of heat exchangers with a similar capacity are actually installed into the ground and used as module type heat exchangers. Further, based on a flow rate, heat transfer will not be greatly influenced by a dimension of the pipe diameter. 37 [0068] Fig. 8 shows an inletpressure necessary for causing no phase change at all in an entire channel of the heat exchanger (2000 meters) and extracting hot water at an outlet of the heat exchanger. A horizontal axis represents a thermal conductivity of the heat exchanger. The changed thermal conductivity is a value which can be easily attained by using an existing material. In either case, the flow rate is 0.001m 3 /s (3.6 m 3 /h) and the inlet pressure is adjusted so that the pressure of hot water at the outlet becomes 6MPa. The above described flow rate or pressure application is a value which can be sufficiently achieved by using an existing general-use pressure pump. [0069] Further, Fig. 9 shows a temperature distribution inside the heat exchanger and a change in void fraction along the flow channel from the inlet. The void fraction is the fraction of the channel volume that is occupied by the gas phase in gas-liquid two-phase flow, indicating values in a range from 0 to 1. Zero indicates a state of only water and one indicates a state of only steam. The drawing shows only where the thermal conductivity of piping is 0.01W/mK (heat does not easily go away) and 0.1W/mK (heat easily goes away). As a matter of course, the void fraction shows zero over an entire zone. [0070] Fig. 10 shows a change in temperature of liquid 38 phase in the entire zone corresponding to the zone shown in Fig. 9. As apparent from Fig. 10, it is possible to extract at the outlet hot water which is approximately at 260'C and 6MPa. Fig. 11 is a drawing which shows a thermal output when the flow rate of 0.001m 3 /s, and as shown in Fig. 11, a great thermal output can be obtained by comparison with pump power which has been input. Further, Fig. 12 is a drawing which shows a change in thermal output with changing the flow rate, and thermal output can be increased by an increase of flow rate. [0071] It is noted that in Fig. 11 and Fig. 12, the output indicates a thermal output which is extracted from a geothermal well and expressed in terms of KW, and the output-power indicates a value which is obtained by subtracting the capacity of a high-pressure water supply pump from the thermal output, corresponding to an actual thermal output. Further, the pump power indicates the capacity of a high-pressure water supply pump. The same will be applied to Fig. 14 and Fig. 15. [0072] Next, in Fig. 13 to Fig. 16, there are shown calculation results obtained by using the same calculation code in a case where steam is generated inside a geothermal heat exchanger and extracted as a gas-liquid two-phase flow. This case is based on an assumption that the thermal output is extracted on the same scale as the previously described case 39 that steam has been extracted as a single phase flow. [0073] Fig. 13 shows a change in inlet pressure due to a difference in piping material (thermal conductivity), Fig. 14 shows a thermal output at a flow rate of 0.001m 3 /s, and Fig. 15 shows a change in thermal output due to a change in flow rate. In this case, the outlet pressure is 3.5MPa. However, the thermal output is obtained from an amount of heat as a gas-liquid two-phase flow. As described previously, only heat coming from steam is used by subsequent water separation. [0074] Further, Fig. 16 shows a change in void fraction as compared with that shown in Fig. 9. Where boiling is started at an upward flow region beyond the lowest part. The thermal conductivity of a piping material of the heat exchanger is 0. 1W/mK and a void fraction of the outlet is approximately 0. 7 (water accounts for 30% in volume percent) and approximately 0.8 in the case of 0.1W/mK. Still further, results obtained by calculation of 0.01W/mK have clearly shown that the gas-liquid two-phase flow undergoes a drastic change in state in the vicinity of the outlet. It is also clear that no consideration is given to unstable factors caused by the above described vapor lock or boiling (bubble generation) in the calculation and the calculation has been performed under extremely ideal conditions. 40 [0075] The above described results have demonstrated the feasibility, that is, characteristics of the present invention, in which water pressurized on the ground by using a high-pressure water supply pump is extracted in a state which is free of gas, that is, as a single phase flow on the ground without generation of steam inside an underground geothermal heat exchanger, and a steam generator is, thereafter, used to boil the water under reduced pressure, thereby extracting geothermal power as steam. It has been also demonstrated that a single phase flow can be used to easily obtain the same heat efficiency as that obtained by using a gas-liquid two-phase flow, while eliminating various problems involved in use of the gas-liquid two-phase flow. Industrial Applicability [0076] The present invention can be used as a geothermal heat exchanger and geothermal power generation equipment which are able to conduct heat exchange great in capacity and excellent in heat efficiency, because no impurities coming from steam to be used adhere to the equipment and steam can be obtained from high-temperature and high-pressure water extracted from underground, and which do not adversely impacts environments in the vicinity of a geothermal region. Description of reference numerals 41 [0077] 1: Geothermal heat exchanger 2: Geothermal well 3: Surface of earth 10: Geothermal region 11: Pressurized water injection pipe lla: Upper end of pressurized water injection pipe 11b: Lower end of pressurized water injection pipe 12: Hot water extraction pipe 12a: Upper end of hot water extraction pipe 12b: Lower end of hot water extraction pipe 13: Intermediate layer 14: Middle pipe 15: Inlet hole 16: Reinforcement portion 17: High-pressure water supply pump 18: Pressure adjusting portion 19: Lid 20: Generator chamber 21: Steam generator 22: Steam superheater 23: Turbine 24: Generator 42 25: Condenser 26: Hot water introduction port 27: Intermediate lid portion 28: Support base 29: Side wall fin 30: Bottom fin 43