AU2010346227A1

AU2010346227A1 - Geothermal power generation apparatus and method for using ultrahigh-pressure hot water in geothermal power generation

Info

Publication number: AU2010346227A1
Application number: AU2010346227A
Authority: AU
Inventors: Kiyotaka Fujii; Yoshiyuki Yokohama
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 2010-02-18
Filing date: 2010-02-18
Publication date: 2012-03-29
Anticipated expiration: 2030-02-18
Also published as: AU2010346227B2; WO2011101964A1

Abstract

Provided are a geothermal power generation apparatus using ultrahigh-pressure hot water and a method for using ultrahigh-pressure hot water in geothermal power generation, wherein ultrahigh-pressure ground water such as fossil seawater can be used, and a general-purpose pump can be used as a return pump for returning the water that has been used for power generation into the ground. The geothermal power generation apparatus has a production well for retrieving geothermal water from the earth, power-generating facilities for retrieving electric energy from the geothermal water by driving a turbine using the thermal energy of the geothermal water, a return pump for increasing the pressure of the geothermal water from which the thermal energy has been removed, and a return well for returning the geothermal water having a pressure increased by the return pump into the earth. A power retrieving turbine for retrieving energy of the geothermal water and reducing the pressure of the geothermal water is provided on a passage of the geothermal water, said passage extending from the production well to the power generation facilities, so that the pressure of the geothermal water is reduced to 7 MPa or less by the power retrieving turbine, and the energy retrieved by the power retrieving turbine is used as a part of driving power for driving the return pump.

Description

DESCRIPTION GEOTHERMAL POWER PLANT AND METHOD OF USING ULTRAHIGH-PRESSURE HOT WATER IN GEOTHERMAL POWER GENERATION TECHNICAL FIELD [0001] This invention relates to a geothermal power plant and, in particular, to a geothermal power plant in which high-pressure geothermal hot water having a pressure exceeding 7 MPa is suitably used. BACKGROUND ART [0002] Recently, renewable energy has become considered potential and effective measures to cope with the finite nature of exhaustible energy and to reduce the global warming, and thus the use of renewable energy has been increased. One of known ways of using the renewable energy is geothermal power generation in which electric power is generated with use of geothermal heat. [0003] A conventional typical geothermal power plant, which is configured to directly use hot water taken from a geothermal production well, will be described with reference to FIG. 7. FIG. 7 is a block diagram illustrating a conventional geothermal power plant.

In the geothermal power plant 1 shown in FIG. 7, gas-liquid mixed water, that is spouted from a hot water layer 102 located in the earth by means of a geothermal production well 4, is fed into a separator 13. The separator 13 separates the gas-liquid mixed water fed from the geothermal production well 4 into vapor and water, and the vapor is fed to a turbine generator 14 while the water is fed to a pit 19. [0004] The turbine generator 14 has a vapor turbine 41 and an electric generator 42, a rotor (not shown) of the vapor turbine 41 is rotated by the vapor, and the rotation of the rotor activates the electric generator 42 connected to the rotor via a speed reducer or the like. [0005] The vapor, the temperature and pressure of which have been reduced by the rotation of the rotor of the vapor turbine 41,is guided into a condenser 16 to be cooled thereby, and the cooled vapor becomes water. This water is sent to the pit 19 by a pump 18. [0006] The pressure of the water that has been sent from the separator 13 or the pump 18 and accumulated in the pit 19 is raised by a reinjection pump 24 to a pressure higher than a pressure in the hot water layer 102, and then returned to the hot water layer 102 through a reinjection well 6. [0007] Groundwater in the hot water layer 102 to be used in the 2 conventional geothermal power plant 1 as shown in FIG. 7 usually has a pressure of around 1 MPa. An underground hot water source of such groundwater having a general pressure of around 1 MPa flows from where the groundwater is present toward a region having a lower pressure, passing through a heat source produced by magma. If there is any soft ground spot in the course of this flow, the hot water will spout out of the ground. This constitutes a so-called a natural hot spring source. In the case of geothermal power generation by the geothermal power plant as shown in FIG. 7, boring is carried out from the ground surface to the underground hot water source so that hot water is drawn up to the ground to be used as energy for power generation. This means that, roughly speaking, a general underground hot water source can be comprehended as a system that is open at some point. [0008] However, as shown in FIG. 6, it has recently been found that, in a basin 53 formed by vast sedimentary layers 54, there may be a regionwhere a groundwater aquifer 55 extends deep under the ground up to a depth of about 4 to 5 km. It has also been found that such a groundwater aquifer 55 may constitute an independent water source that is completely closed. When the groundwater aquifer 55 constitutes such an independent water source that is completely closed at deep underground, a rock body 56 located near the groundwater aquifer 55 under the ground has a temperature much higher than that of a rock body located near the ground surface. Groundwater present in the groundwater aquifer 55 is heated in the 3 closed state by the high-temperature rock body 56, and is sealed underground at a higher temperature and a higher pressure than those of the groundwater used for common geothermal power generation. The groundwater sealed underground in such high-temperature and high-pressure state is also referred to as fossil seawater. Though varying from place to place, the fossil seawater may have a pressure as high as about 35 MPa and a temperature as high as about 250*C. [0009] It is desirable in terms of efficiency of geothermal power generation to use energy of the fossil seawater for the geothermal power generation. However, in the conventional geothermal power plant shown in FIG. 7, the pressure of water accumulated in the pit 19 must be raised by the reinjection pump 24 to a pressure higher than that of the hot water layer 102. If the fossil seawater is to be used in this conventional geothermal power plant shown in FIG. 7, the pressure of the water accumulated in the pit 19 must be raised by the reinjection pump 24 to a pressure higher than the pressure of the fossil seawater that is as high as about 35 MPa. This requires the reinjection pump 24 to have an extremely high capacity. Therefore, the use of the fossil seawater is not realistic when the production cost and installation space of the reinjection pump 24 are taken into consideration. [0010] It can therefore be conceived that energy of the fossil 4 seawater is to be collected when using the fossil water for geothermal power generation. Techniques relating the collection of energy are described for example in Patent Literature 1 and Patent Literature 2. Patent Literature 1 and Patent Literature 2 disclose techniques in which a part of liquid discharged from a pump is collected, power of the collected liquid is collected by a power collecting turbine provided in a channel for the liquid, and the collected power is used as part of the energy for drive the pump. [0011] Patent Literature 1: Japanese Patent Application Publication No. H5-49833 Patent Literature 2: Japanese Patent Application Publication No. 2004-257340 [0012] However, the techniques disclosed in Patent Literature 1 and Patent Literature 2 are both for collecting power from collected liquid. In geothermal power generation, however, the collected liquid is water that is used for power generation and then returned to the underground, and the pressure of this water must be raised to a pressure equivalent to or higher than the pressure of groundwater. Accordingly, it is impossible to collect power from this water. Thus, there is little possibility to apply the techniques disclosed in Patent Literature 1 and Patent Literature 2 directly to the geothermal power plant. DISCLOSURE OF THE INVENTION 5 [0013] In view of these problems of the conventional techniques, this invention has an object to provide a geothermal power plant using ultrahigh-pressure hot water, in which ultrahigh-pressure groundwater such as fossil seawater can be used, and a general-purpose pump can be used as a reinjection pump for returning water that has been used for power generation, and a method of using ultrahigh-pressure hot water in the geothermal power generation. [0014) In order to solve the aforementioned problems, this invention provides a geothermal power plant having a geothermal production well for drawing geothermal hot water from the earth; a power generation facility for collecting thermal energy of the geothermal hot water as electrical energy by driving a turbine with use of the thermal energy of the geothermal hot water; a reinjection pump for raising pressure of the geothermal hot water from which the thermal energy has been taken; and a reinjection well for returning the geothermal hot water, the pressure of which has been raised by the reinjection pump, into the earth. The geothermal power plant of the invention is characterized in that the geothermal hot water is high-pressure geothermal hot water having a pressure higher than about 7 MPa; a power collecting turbine for collecting energy from and reducing the pressure of the geothermal hot water is provided on a geothermal hot water channel extending from the production well to the power generation facility; the pressure of the geothermal hot water is reduced to about 7 MPa or less by the power 6 collecting turbine; and the energy collected by the power collecting turbine is used as part of drive power for the reinjection pump. [0015] According to the invention, even if the geothermal hot water has a high pressure of about 35 MPa like fossil seawater, for example, the pressure is reduced to about 7 MPa by the power collecting turbine collecting part of energy of the geothermal hot water. Therefore, the pressure resistance of a channel extending from the power collecting turbine, to the power generation facility, and to the reinjection pump can be designed to be equal to or lower than the reduced pressure, resulting in reduction of the cost of materials for the facility. [0016) The reduction of pressure by collecting part of the energy of the geothermal hot water by means of the power collecting turbine makes it possible to reduce the inlet pressure of the reinjection pump, and thus to reduce the design inlet pressure of reinjection pump. Since most of commonly used general-purpose pumps have an inlet pressure of 7 MPa or lower, the reduction of pressure of the geothermal hot water to 7 MPa or lower by means of the power collecting turbine enables use of a general-purpose pump as the reinjection pump. [0017] Further, since the energy collected by the power collecting turbine is used as part of the drive power for the reinjection pump, 7 the energy that the geothermal hot water has can be used without waste. [00181 The geothermal power plant may further have an electric motor driven by electrical energy, and a rotating shaft of the electric motor is mechanically connected to a rotating shaft of the reinjection pump, and the rotating shaft of the electric motor is mechanically connected also to a rotating shaft of the power collecting turbine. This makes it possible to use the energy collected by the power collecting turbine as part of drive power for the reinjection pump, with a simple configuration. When taking into consideration the mechanical loss and pressure loss of the geothermal hot water, the energy collected by the power collecting turbine alone is insufficient as the drive power for the reinjection pump. Therefore, electrical energy must be externally supplied to drive the electric motor in order to compensate the shortage. [0019] A pressure gauge may be provided on the output side of the reinjection pump. A value detected by the pressure gauge is input to an electric motor control unit which controls the drive of the electric motor, and the electric motor control unit controls the drive of the electric motor so that the value detected by the pressure gauge becomes higher than a predetermined pressure that is set to be higher than the pressure of the geothermal hot water drawn from the earth. 8 This makes it possible to reliably keep the pressure of the output side of the reinjection pump higher than the pressure of the geothermal hot water. [0020] The geothermal power plant may further include an electric generator for collecting the energy collected by the power collecting turbine as electrical energy, so that electrical energy collected by the electric generator is used as part of drive power for the reinjection pump. This eliminates the restriction on layout since the power collecting turbine and the reinjection pump are mechanically independent from each other. Furthermore, since the power collecting turbine collects part of the energy of the geothermal hot water as electrical energy, a backup can be provided only for the reinjection pump, which enables stable operation of the geothermal power plant as a whole at low cost and easily. [0021] A pressure gauge may be provided on the output side of the power collecting turbine, so that a value detected by the pressure gauge is input to an electric generator control unit which controls the drive of the electric generator. The electric generator control unit controls the drive of the electric generator so that the value detected by the pressure gauge becomes higher than a predetermined pressure that is preset. This enables reliable control of the pressure of the output side of the power collecting turbine so as to be kept at or higher 9 than a predetermined pressure, and thus the pressure of the output side of the reinjection pump can be reliably kept higher than the pressure of the geothermal hot water. [0022] Another aspect of the invention for solving the problems provides a method of utilizing hot water in geothermal power generation in which geothermal hot water is drawn from the earth through a geothermal production well, a turbine is driven with use of thermal energy of the geothermal hot water to collect the thermal energy as electrical energy, the geothermal hot water from which the thermal energy has been taken is raised in pressure and returned to the earth through the reinjection well. The method is characterized in that the geothermal hot water is high-pressure geothermal hot water having a pressure higher than about 7 MPa, the pressure of the geothermal hot water is reduced to about 7 MPa or lower by a power collecting turbine provided on a channel for the geothermal hot water drawn from the geothermal production well, and the energy of the geothermal hot water the pressure of which has been reduced is used to drive the turbine, and energy collected by the power collecting turbine is used as part of power for the raising of the pressure of the geothermal hot water. [0023] This invention is able to provide a geothermal power plant using ultrahigh-pressure hot water, in which ultrahigh-pressure groundwater such as fossil seawater can be used, and a general-purpose pump can be used as a reinjection pump for returning 10 underground water that has been used for power generation, and a method of using ultrahigh-pressure hot water in the geothermal power generation. BRIEF DESCRIPTION OF THE DRAWINGS [0024] FIG. 1 is a block diagram illustrating a geothermal power plant according to a first embodiment; FIG. 2 is a block diagram illustrating a periphery of a reinjection pump when the reinjection pump is provided with a backup in the geothermal power plant according to the first embodiment; FIG. 3 is a block diagram illustrating a geothermal power plant according to a second embodiment; FIG. 4 is a block diagram illustrating a geothermal power plant according to a first comparative example; FIG. 5 is a block diagram illustrating a geothermal power plant according to a second comparative example; FIG. 6 is a schematic diagram illustrating a geological formation in which fossil seawater is produced; and FIG. 7 is a block diagram illustrating a conventional geothermal power plant. BEST MODE FOR CARRYING OUT THE INVENTION [0025] Preferred embodiments of the invention will be described in detail by way of example with reference to the drawings. However, 11 unless specifically stated otherwise, the dimensions, materials, shapes, and relative arrangement of components and the temperature, pressure and so on of geothermal hot water described in these embodiments are not intended to limit the scope of the invention, but are merely to provide exemplary illustrations. An object of this invention is to realize effective and efficient use of ultrahigh-pressure hot water. Therefore, it is not intended to limit the mode of power generation on the secondary side of a heat exchanger, and the description here is illustrative only. Example [0026] (First Embodiment) FIG. 1 is a block diagram illustrating a geothermal power plant according to a first embodiment. In FIG. 1, reference numeral 2 indicates an ultrahigh-pressure high-temperature groundwater aquifer, that is a groundwater aquifer of fossil seawater having a pressure of about 35 MPa, exceeding 7 MPa. [0027] Firstly, a configuration of a geothermal power plant 1 according to the first embodiment will be described with reference to FIG. 1. There are provided two wells extending from the ground surface to the ultrahigh-pressure high-temperature groundwater aquifer 2. One of the wells is a geothermal production well 4. The geothermal production well 4 is provided for drawing groundwater (fossil 12 seawater) from the ultrahigh-pressure high-temperature groundwater aquifer 2. The other of the wells is a reinjection well 6. The reinjection well 6 is provided for returning the fossil seawater that has been heat-exchanged by a heat exchanger 10 which forms a power generation facility 8 to be described later, to the ultrahigh-pressure high-temperature groundwater aquifer 2. The geothermal production well 4 and the reinjection well 6 are connected to the heat exchanger 10 via a channel 5 and a channel 7, respectively. [0028] A power collecting turbine 22 is provided on the channel 5 extending from the geothermal production well 4 to the heat exchanger 10, so that the power collecting turbine 22 is driven by the fossil seawater fed from the geothermal production well 4 to the heat exchanger 10 and collects energy therefrom. A reinjection pump 24 is provided on the channel 7 extending from the heat exchanger 10 to the reinjection well 6. The reinjection pump 24 raises the pressure of the fossil seawater that has been heat-exchanged by the heat exchanger 10 to a pressure higher than the hydraulic pressure in the ultrahigh-pressure high-temperature groundwater aquifer 2 in order to return the fossil seawater to the ultrahigh-pressure high-temperature groundwater aquifer 2. (0029] A rotating shaft of the reinjection pump 24 and a rotating shaft of an electric motor 26 capable of driving the reinjection 13 pump 24 are mechanically connected to each other concentrically. Also, the rotating shaft of the electric motor 26 and a rotating shaft of the power collecting turbine 22 are mechanically connected to each other concentrically. This means that the rotating shaft of the reinjection pump 24 and the rotating shaft of the power collecting turbine 22 are mechanically connected to each other via the rotating shaft of the electric motor 26. [0030] A pressure gauge 28 is provided on a channel extending from the heat exchanger 10 to the reinjection well 6 and on the downstream side of the reinjection pump 24. A value detected by the pressure gauge 28 is input to a controller 29 for controlling the drive of the electric motor 26. [0031] Reference numeral 8 indicates a power generation facility, which is a so-called binary-type power generation facility. The binary-type power generation facility is configured to drive the turbine generator 14 with use of a heat carrier that has been heat-exchanged by the heat exchanger 10 with high-temperature high-pressure groundwater led from the channel 5. Since no groundwater (fossil seawater) flows into the secondary side of the heat exchanger (the heat carrier side), this power generation facility 8 is able to operate in a clean state. Further, since there occurs no phase-change process on the primary side of the heat exchanger (on the groundwater (fossil seawater) side), the power generation facility 8 is able to use energy without waste. The 14 power generation facility 8 includes the heat exchanger 10, a flasher 12 which reduces the pressure of the heat carrier, which has been heated by the heat exchange in the heat exchanger 10, rapidly down to a pressure equal to or lower than saturation pressure and thereby generates vapor, the turbine generator 14 which is driven by the vapor generated by the flasher and has a turbine 41 and an electric generator 42, a condenser 16 which cools the heat carrier used in the turbine generator 14 so as to return the heat carrier to liquid, and pumps 18 and 20 for sending the liquid heat carriers generated by the condenser 16 and the flasher 12 to the heat exchanger. [0032] Next, operation of the geothermal power plant 1 having the aforementioned configuration according the first embodiment will be described. In FIG. 1, gas-air mixed fossil seawater having a high temperature of about 250*C and a high pressure of about 35 MPa is spouted from the ultrahigh-pressure high-temperature groundwater aquifer 2 located deep in the earth through the geothermal production well 4, and this seawater is led to the power collecting turbine 22. The power of this fossil seawater is collected by the power collecting turbine 22 and the pressure thereof is reduced to about 7 MPa. [0033] 15 The fossil seawater the power of which has been collected by the power collecting turbine 22 and the pressure of which has been reduced to about 7 MPa is led to the heat exchanger 10 forming the power generation facility 8 through the channel 5 and is cooled by exchanging heat with a heat carrier to be described later. The fossil seawater is cooled down by the heat exchange in the heat exchanger 10 to such a temperature that hardness component in the fossil seawater will not precipitate, namely to a temperature of about 140 to 150*C. It should be noted that a safe temperature at which the hardness component will not precipitate must be finally determined for each well by analyzing the composition of the water. [0034] The fossil seawater coming out of the heat exchanger 10 is increased inpressureby the reinjectionpump24 to apressure higher than that in the ultrahigh-pressure high-temperature groundwater aquifer 2, and then returned to the ultrahigh-pressure high-temperature groundwater aquifer 2. [0035] The power collected by the power collecting turbine 22 is transmitted to the reinjection pump 24, while a mechanical loss occurring in the transmission from the power collecting turbine 22 to the reinjection pump 24 and a pressure loss occurring in the transmission of the fossil seawater from the power collecting turbine 22 to the heat exchanger 10 and to the reinjection pump 24 are compensated by the electric motor 26, whereby the reinjection 16 pump 24 is driven to increase the pressure of the fossil seawater to a pressure higher than that in the ultrahigh-pressure high-temperature groundwater aquifer 2. Specifically, the output of the electric motor 26 is controlled by the controller 29 such that the pressure of the pressure gauge 28 provided on the output side of the reinjection pump 24 is equal to or higher than a reference pressure that is preset to be higher than the pressure in the ultrahigh-pressure high-temperature groundwater aquifer 2. [0036] On the other hand, the heat carrier heated by heat exchange with the fossil seawater in the heat exchanger 10 is decreased in pressure rapidly to the saturation pressure or lower by the flasher 12 and vapor is generated. The vapor generated by the flasher drives the turbine 41 in the turbine generator 14, and electric power is generated in the electric generator 42 by the drive of the turbine 41. The heat carrier that has been used in the turbine generator 14 is cooled by the condenser 16 and returned to liquid. The liquid heat carrier produced by the condenser 16 and the flasher 12 are sent again to the heat exchanger 10 by the pumps 18 and 20, respectively. It is preferable to use water as the heat carrier. This is because water is easily available, and water having a boiling point of 1000C under normal pressure can be sufficiently vaporized since the fossil seawater which will exchange heat with the heat carrier has a high temperature of about 250*C. 17 [0037] According to the configuration and operation described above, the power of the fossil seawater, which has spouted from the geothermal production well 4 at an ultrahigh pressure of about 35 MPa, is collected by the power collecting turbine 22, whereby the pressure of the fossil seawater is reduced to about 7 MPa. Therefore, the channels 5 and 7 extending from the power collecting turbine 22 to the heat exchanger 10, and to the reinjection pump 24 can be designed at a pressure rating that is far lower than that of the outlet of the geothermal production well 4, and hence the cost of materials or the like for the facility can be reduced. [00381 The power of the fossil seawater having an ultrahigh pressure of about 35 MPa is collected by the power collecting turbine 22, whereby the pressure thereof is reduced. Although, in this embodiment, the power collection is performed by the power collecting turbine 22 such that the pressure of the fossil seawater is reduced to about 7 MPa, this is not limited to 7 MPa but can be any value equal to or lower than 7 MPa. The reduction of pressure of the fossil seawater by the power collecting turbine 22 collecting power therefrom makes it possible to reduce the pressure at the inlet of the reinjection pump 24. This makes it possible to reduce the design pressure at the inlet of the reinjection pump 24. Most of the general-purpose pumps have an inlet pressure of 7 MPa or lower. Therefore, the power collecting turbine 22 performs power collection such that the pressure of the fossil seawater is reduced 18 to 7 MPa or lower, and a general-purpose pump is used as the reinjection pump 24, so that the cost relating to the reinjection pump 24 can be reduced. [0039] Further, the power collected by the power collecting turbine 22 is used as drive power for the reinjection pump 24, which makes it possible to use the energy of the fossil seawater efficiently without any waste. [0040] Although in the first embodiment the power collecting turbine 22, the electric motor 26, and the reinjection pump 24 are coaxially connected to each other, a speed increasing/decreasing device or a clutch for matching the number of rotations or the direction of rotation can be interposed between the power collecting turbine 22 and the electric motor 26 or between the reinjection pump 24 and the electric motor 26. [0041] Further, a backup for the reinjection pump 24 can be provided to ensure stable operation of the geothermal heat power generation facility 1 as a whole. FIG. 2 is a block diagram illustrating a periphery of the reinjection pump 24 in the geothermal power plant according to the first embodiment when the reinjection pump 24 is provided with a backup. A power collecting turbine 22b having a capacity equivalent to that of the power collecting turbine 22 is provided in parallel with the power collecting turbine 22, and a reinjection pump 24b 19 having a capacity equivalent to that of the reinjection pump 24 is provided in parallel with the reinjection pump 24. The turbine 22b and reinjection pump 24b are mechanically connected to each other via a electric motor 26b in the same manner as the turbine 22 and reinjection pump 24. In this manner, a backup can be provided by a set of a power collecting turbine and a reinjection pump. [0042] (Second Embodiment) FIG. 3 is a block diagram illustrating a geothermal power plant according to a second embodiment. In FIG. 3, like reference numerals to those in FIG. 1 indicate components having like functions and effects to those in FIG. 1, and description thereof will be omitted. [0043] In the second embodiment, an electric generator 27 is mechanically connected to the power collecting turbine 22 directly or via a clutch or a speed increasing/decreasing device, so that the electric generator 27 collects the power obtained by reducing the pressure of the fossil seawater by the power collecting turbine 22, as electrical energy. The energy collected by the electric generator is used as power for driving the reinjection pump 24. [0044] A pressure gauge 30 is provided at the outlet of the power collecting turbine, and the electrical energy collected by the 20 electric generator 27 is controlled by the controller 32 such that a constant value is detected by the pressure gauge 30. This makes it possible to keep constant the pressure on the outlet side of the power collecting turbine 22, and hence to keep constant the inlet pressure of the reinjection pump 24. Accordingly, when the reinjection pump 24 is driven at a constant speed, the outlet pressure of the reinjection pump 24 also can be kept constant. [0045] The reinjection pump 24 is supplied with the electrical energy collected by the electric generator 27, and is driven while a mechanical loss occurring between the power collecting turbine 22 and the reinjection pump 24 and a pressure loss occurring when the fossil seawater passes from the power collecting turbine 22 to the heat exchanger 10 and to the reinjection pump 24 are compensated by a power supply unit 36 such as an electric motor. The reinjection pump 24 thus raises the pressure of the fossil seawater to a pressure higher than the pressure in the ultrahigh-pressure high-temperature groundwater aquifer 2. Specifically, the output of the power supply unit 36 is controlled by a controller 38 such that the pressure of the pressure gauge 40 provided at the outlet side of the reinjection pump 24 becomes a pressure equal to or higher than a reference pressure that is preset to be higher than the pressure in the ultrahigh-pressure high-temperature groundwater aquifer 2. While both of the control by the controller 32 and the control by the controller 38 can be used as shown in FIG. 3, it is also 21 possible to omit either one of them or the both of them. [0046] Like the first embodiment, the second embodiment is also able to reduce the inlet pressure of the reinjection pump 24, and thus to reduce the design inlet pressure of reinjection pump 24. The power collected by the power collecting turbine 22 is used as power for driving the reinjection pump 24, which makes it possible to use the energy of the fossil seawater efficiently without any waste. [0047] Further, the second embodiment provides its unique effect of eliminating the restriction on layout since the power collecting turbine 22 and the reinjection pump 24 are mechanically independent from each other. Since part of the energy of the fossil seawater is collected by the power collecting turbine as electrical energy, a backup 25 can be provided only for the reinjection pump, which enables stable operation of the geothermal power plant 1 as a whole at low cost and easily. [0048] (First Comparative Example) FIG. 4 is a block diagram illustrating a geothermal power plant according to a first comparative example. In FIG. 4, like reference numerals to those in FIG. 1 and FIG. 7 indicate components having like functions and effects to those in FIG. 1 and FIG. 7, and description thereof will be omitted. [0049] In the first comparative example, a pressure-reducing valve 22 34 is provided, in addition to the components of the conventional geothermal power plant shown in FIG. 7, on a channel extending from the geothermal production well 4 to the separator 13, so that fossil seawater in the ultrahigh-pressure high-temperature groundwater aquifer 2 is utilized to perform geothermal power generation. According to this configuration, the fossil seawater is introduced into the separator 13 after the ultrahigh pressure of the fossil seawater is reduced. Therefore, the separator can be a separator designed to have the same resistance to pressure as those of conventional separators. However, in this case, the pressure of the fossil seawater must be reduced by the pressure-reducing valve 34 to a pressure close to atmospheric pressure, and thus minerals dissolved in the fossil seawater may possibly precipitate in piping extending from the pressure-reducing valve 34 to the separator 13. Further, the pressure reduction inhibits sufficient use of the energy of the fossil seawater. Still further, the pressure must be raised by the reinjection pump 24 from a pressure close to atmospheric pressure up to a pressure higher than the pressure in the ultrahigh-pressure high-temperature groundwater aquifer 2, which requires the reinjection pump 24 to have such a high capacity that is unrealistic. [0050] (Second Comparative Example) FIG. 5 is a block diagram illustrating a geothermal power plant according to a second comparative example. In FIG. 5, like reference numerals to those in FIG. 1 and 23 FIG. 3 indicate components having like functions and effects to those in FIG. 1 and FIG. 3, and description thereof will be omitted. [0051] In the second comparative example, the binary-type power generation system 8 is employed. In this case, the fossil seawater that has spouted from the geothermal production well at a pressure of about 35 MPa flows into the reinjection pump 24 at a very high pressure that corresponds to the spouting pressure reduced only by pressure loss occurring in the piping and the heat exchanger. Therefore, the pressure at the inlet of the reinjection pump 24 exceeds 7 MPa, that is the upper limit of the inlet pressure of general-purpose pumps commonly used. This means that a general-purpose pump cannot be used as the reinjection pump 24. INDUSTRIAL APPLICABILITY [0052] This invention is applicable as a geothermal power plant using ultrahigh-pressure hot water, which enables use of ultrahigh-pressure groundwater such as fossil seawater, and furthermore enables use of a general-purpose pump as a reinjection pump for returning the water that has been used for power generation to the underground. The invention is also applicable as a method for utilization of ultrahigh-pressure hot water in geothermal power generation. 24

Claims

1. A geothermal power plant comprising: a geothermal production well for drawing geothermal hot water from the earth; a power generation facility for collecting thermal energy of the geothermal hot water as electrical energy by driving a turbine with use of the thermal energy of the geothermal hot water; a reinjection pump for raising pressure of the geothermal hot water from which the thermal energy has been taken; and a reinjection well for returning the geothermal hot water, the pressure of which has been raised by the reinjection pump, into the earth, wherein: the geothermal hot water is high-pressure geothermal hot water having a pressure higher than about 7 MPa; a power collecting turbine for collecting energy from and reducing the pressure of the geothermal hot water is provided on a geothermal hot water channel extending from the production well to the power generation facility; the pressure of the geothermal hot water is reduced to about 7 MPa or less by the power collecting turbine; and the energy collected by the power collecting turbine is used as part of drive power for the reinjection pump.

2. The geothermal power plant according to claim 1, further comprising an electric motor driven by electrical energy, wherein 25 a rotating shaft of the electric motor is mechanically connected to a rotating shaft of the reinjection pump, and the rotating shaft of the electric motor is mechanically connected also to a rotating shaft of the power collecting turbine.

3. The geothermal power plant according to claim 2, wherein: a pressure gauge is provided on the output side of the reinjection pump; a value detected by the pressure gauge is input to an electric motor control unit which controls the drive of the electric motor; and the electric motor control unit controls the drive of the electric motor so that the value detected by the pressure gauge becomes equal to or higher than a predetermined pressure that is set to be higher than the pressure of the geothermal hot water drawn from the earth.

4. The geothermal power plant according to claim 1, further comprising an electric generator for collecting the energy collected by the power collecting turbine as electrical energy, wherein the electrical energy collected by the electric generator is used as part of drive power for the reinjection pump.

5. The geothermal power plant according to claim 4, wherein: a pressure gauge is provided on the output side of the power collecting turbine; 26 a value detected by the pressure gauge is input to an electric generator control unit which controls the drive of the electric generator; and the electric generator control unit controls the drive of the electric generator so that the value detected by the pressure gauge becomes higher than a predetermined pressure that is preset.

6. Amethod of utilizing hot water in geothermal power generation in which geothermal hot water is drawn from the earth through a geothermal production well, a turbine is driven with use of thermal energy of the geothermal hot water to collect the thermal energy as electrical energy, the geothermal hot water from which the thermal energy has been taken is raised in pressure and returned to the earth through the reinjection well: wherein: the geothermal hot water is high-pressure geothermal hot water having a pressure higher than about

7 MPa; the pressure of the geothermal hot water is reduced to about 7 MPa or less by a power collecting turbine provided on a channel for the geothermal hot water drawn from the geothermal production well, and energy of the geothermal hot water the pressure of which has been reduced is used to drive the turbine; and energy collected by the power collecting turbine is used as part of power for the raising of the pressure of the geothermal hot water. 27