TECHNICAL FIELD 
       [0001]    This invention relates to a marine power generation system and a marine power generating method. 
       BACKGROUND ART 
       [0002]    With the rapid increase of the energy demands on the global scale, various research and development are actively promoted for utilizing a natural energy source without the fear of scarcity and pollution from the new energy source or the alternative energy source instead of the traditional fossil fuels such as oil and liquefied natural gas (LNG), and nuclear energy on which we now depend. 
         [0003]    As the power generation method for utilizing the natural energy source, the wind power generation for utilizing the wind power and the marine power generating for utilizing the tidal current source are researched and developed. Regarding the wind power generation, the generated power amount is unstable because of the fluctuation of the wind amount. However, with the marine power generation, the generated power amount is stable because the tidal current is relatively stable. Therefore, the marine power generation system is expected to be a more stable power generation system compared with the wind power generation system. 
         [0004]    NISHIOKA Toshihisa, the inventor of this invention, as the professor of the marine engineering field at the Georgia Institute of Technology and the Faculty of Maritime Sciences, KOBE University Graduate School of Maritime Sciences, has promoted the power generation utilizing the marine power, in which tidal current energy is huge in amount but without enough speed for power generation. NISHIOKA Toshihisa has studied the possibility of high efficient and stable power generation system utilizing tidal current energy. 
         [0005]    In order to obtain power generation by driving the massive power generating turbine rotationally, a certain speed and pressure of the medium running through the massive power generating turbine are required. However, the marine tidal current is not enough for driving the massive power generating turbine rotationally. 
         [0006]    One of the places in the world where the tidal current speed is relatively large is the Straits of Dover. The tidal current of the Straits of Dover is about 6 knots, 12 kilometers per hour at maximum. 
         [0007]    As the technology of the marine power generation utilizing the tidal current of the Straits of Dover, “Straits of Dover tidal current power generation system” (Prior art 1: JP 2009-270491) is known. This prior art discloses the following. The power generating facility is constructed by digging plural tunnels for embedding the power generation system. The waterwheels are installed onto the Straits of Dover floor at the location of the above of the embedded power generation system and connected to the power generation system. The motion energy of the tidal current is converted into rotation energy by the blades of waterwheel, and the power generation system is driven by the rotation energy. The depth of the Straits of Dover is about 60 meters, so the waterwheel should be installed below the depth of 40 meters because the large vessels have has about 35 meters of waterline in general. 
         [0008]    The prior hydroelectric power generation obtains the electromotive force by rotating the large propeller or the large turbine rapidly by utilizing the water pressure. For example, the hydroelectric power generation utilizing the Pelton turbine requires the high speed water flow to rotate the Pelton turbine rapidly by directing the water flow to its blades. For example, the hydroelectric power generation utilizing the Francis turbine requires the high pressure water flow to rotate the Francis turbine rapidly by applying the water pressure to its blades and discharge to the atmospheric pressure. 
         [0009]    If the tidal current is utilized as the power source of the prior hydroelectric power generation, the total amount of the energy included in the tidal current is great enough, but there is not enough speed for rotating the propeller or turbine applied in prior power generation installed in the tidal current and there is not enough pressure between the water pass in front and behind the propeller or the turbine for rotating them normally.  FIG. 11  is a schematic view showing the problem in the prior marine power generation system. As shown in  FIG. 11 , the speed of the tidal current is relatively show, and it is difficult for rotating the water wheel or turbine in the required rotation speed by introducing the tidal current to the power generator. Therefore the tidal current is not suitable as the power source for the prior power generation system. The improvement is required for the waterwheel and the turbine in the generator for securing the required rotation speed of the waterwheel and the turbine utilizing the tidal current as the power source. 
         [0010]    The following prior art is known as the marine power generation system which can generate electricity high efficiently and stably by utilizing the tidal current even if the speed of the tidal current is not enough for the generator. 
         [0011]    For example, the marine power generation system disclosed in the prior art 2 (JP 2011-208531) comprises the first propeller and the second propeller, whose rotation direction is the same, driven by the tidal current. Both the first propeller and the second propeller are combined and connected via the acceleration mechanism. The acceleration mechanism is disclosed as the planetary gear mechanism that is the combined mechanism of the inner gear, the planetary gear and the sun gear. This prior art tries to accelerate the rotation speed by utilizing the acceleration mechanism.  FIG. 12  is a schematic view showing the marine power generation system disclosed in the prior art 2. 
         [0012]    For example, the marine power generation system disclosed in the prior art 3 (JP Tokkai-Hei07-259064) comprises a submarine facility for the marine power generation moored in the marine where the tidal current is stable in its speed and direction.  FIG. 13  is a schematic view showing the submarine facility for the marine power generation disclosed in the prior art 3. The floating body B comprising the guide pipe A and the generator C is moored at the predetermined depth in the tidal current, with its head facing to the tidal current, by the mooring ropes  9  and the anchor  10 . The tidal current energy running through the guide pipe A is converted into the electric energy by the propeller  6  and the generator C, and the generated electricity is transmitted to the ground facility by the transmission cable. The guide pipe is equipped with a neck part at the center in a flowing direction in order to accelerate the water flow at the installation portion of the propeller  6 , and is equipped with a taper from the center to the terminal inlet and the terminal outlet to enlarge the cross-sectional area gradually. 
         [0013]    For example, the marine power generation system disclosed in the prior art 4 (JP 2007-9833) also comprises a submarine facility for the marine power generation moored in the ocean where the tidal current is stable in its speed and direction.  FIG. 14  is a schematic view showing the submarine facility for the marine power generation disclosed in the prior art 4. As shown in  FIG. 14 , the submarine facility comprises a first water guide pipe which has an inlet and an outlet at the both terminal, a second water guide pipe installed in the center portion of the first water guide pipe, a propeller supported rotatably at the inside of the second water guide pipe, and a generator for generating electricity by the rotation of the propeller. The first water guide pipe and the second water guide pipe are equipped with a neck part at the center in a flowing direction in order to accelerate the water flow at the center portion, and are equipped with a taper from the center to the terminal inlet and the terminal outlet to enlarge the cross-sectional area gradually. In short, the marine power generation system disclosed in the prior art 4 has a dual pipe structure composed of the second water guide pipe in the first water guide pipe. The tidal current introduced in the first water guide pipe from the inlet is divided into the flow to the second water guide pipe (inner flow) and the flow to the gap between the first water guide pipe and the second water guide pipe (surrounding flow), and merged at the outlet portion of the second water guide pipe. The ejector effect of extracting the inner water flow with the outer water flow is obtained. 
         [0014]    Prior art 1: JP 2009-270491 
         [0015]    Prior art 2: JP2011-208531 
         [0016]    Prior art 3: JP Tokkai-Hei07-259064 
         [0017]    Prior art 4: JP 2007-9833 
       DISCLOSURE OF THE INVENTION 
     The Problems to be Solved 
       [0018]    The above conventional marine power generation systems have the following problem. 
         [0019]    The marine power generation system disclosed in the prior 1 JP 2009-270491 discloses that the ideal installation place is the Straits of Dover. The tidal current has huge amount of energy. However, the speed is slow and the pressure is not enough for rotating the propeller and the turbine of the power generator. Therefore, some improvement in the propeller and the turbine is necessary. However, there is no disclosure about improvement of the propeller and the turbine. The normal prior propeller and the turbine are disclosed. The technology disclosed in prior 1 is not enough for obtaining the electromotive force by rotating the large scale power generator efficiently. 
         [0020]    Next, according to the prior art 2 JP 2011-208530, the first propeller and the second propeller rotating in the same rotation direction are combined and accelerated by connecting them the accelerating mechanism. However, the rotating speed of the first propeller and the second propeller is very low, so the rotary gear ratio becomes too large to drive the large scale power generator at the normal rotation speed. The required torque for rotating the large scale power generator at the normal speed is generated by the torque obtained from the tidal current via the rotary gear, so the rotary gear faces a large mechanical load. The mechanical load may exceed the mechanical strength to break. 
         [0021]    Next, according to the prior art 3 JP Tokkai-Hei07-259064, the cross-sectional area of the center portion of the guide pipe A is smaller than the that of the inlet  1 , so the flow speed at the propeller G is accelerated. However, there is rotational resistance of the propeller  6  and the passage resistance of the guide pipe A, and the flow amount of the water passing through the guide pipe A becomes smaller than that of the flow surrounding of the floating body B. As a result, depending on the difference of the flow amount between the inside and the outside of the guide pipe A, large pressure proportional to the cross sectional area in the flowing direction works on the floating body B. It is difficult for the floating body  6  to be sustained at the predetermined position by the mooring rope  9 . 
         [0022]    In addition, the difference of the cross sectional area between the center portion of the guide pipe A and the inlet  1  should become larger in order to accelerate the flow of the guide pipe A up to enough for rotating the power generator in normal speed, but the size of the floating body B then becomes too large compared to the that of the working propeller. 
         [0023]    In addition, it is difficult for the floating body B to be sustained at the predetermined direction stably because the floating body B is moored at the predetermined depth in the floating state only with the mooring rope  9 . 
         [0024]    Next, according to the prior art 4 JP 2007-9833, the cross-sectional area of the center portion of the first water guide pipe is smaller than that of the inlet  1 , so the flow speed at the propeller is accelerated. However, compared with the prior art 3, the prior art 4 has the ejector effect by merging the inner flow and the outer flow of the second water guide pipe. The difference of the flow amount between the inner flow and the outer flow become smaller by the ejector effect, the floating body become stable by the mooring ropes. However, the floating body B is moored at the undersea depth only by mooring ropes  9 , so it is very difficult for the floating body to be sustained at the predetermined position by the mooring rope  9 . The problem is the same as that explained for the prior art 3. 
         [0025]    The same as the prior art 3, the prior art 4 has the same problem that the difference of the cross sectional area between the center portion of the guide pipe A and the inlet  1  should become larger in order to accelerate the flow of the guide pipe A to be enough for rotating the power generator at normal speed, but the size of the floating body B becomes too large compared to the that of the working propeller. In addition, the prior art 4 has the problem that the floating body becomes too large because the first water guide pipe becomes larger than the second water guide pipe, and the whole body in the ocean becomes larger. 
         [0026]    In short, both the prior art 3 and the prior art 4 have the problem that with the technology utilizing the floating body with the taper shape, the size of the floating body based on the size of the propeller for the power generator becomes too large to drive, and the tidal current cannot be accelerated enough. 
         [0027]    It is an object of the present invention to provide a marine power generation system that is engineered to be able carry out power generation stably and highly efficiently, utilizing the huge source of energy even in marine current having insufficient speed to create a water current, similar to a water discharge from a dam, which is ideal for hydroelectricity. 
       Means for Solving the Problems 
       [0028]    In order to solve the problem shown above, NISHIOKA Toshihisa, the inventor of this invention, as the professor of the marine engineering field at the Georgia Institute of Technology and the Faculty of Maritime Sciences, KOBE University Graduate School of Maritime Sciences, has promoted the power generation for utilizing the marine power, in which tidal current energy is a huge amount but does not have enough speed for power generation. NISHIOKA Toshihisa has been studied the possibility of high efficient and stable power generation system utilizing tidal current energy, he invented the following marine power generation system. 
         [0029]    In order to achieve the above-mentioned object, the present invention of a marine power generation system comprises; a water intake structure for taking in seawater, wherein the water intake structure comprises an aperture part for taking in seawater, and an upper seawater guide part for sending the taken-in seawater from the aperture port to an inner portion; a seawater drop pipe comprising an upper terminal and a lower terminal for supplying a dropping space undersea for dropping the taken-in seawater from the upper terminal which is connected with the upper seawater guide part to the lower terminal; a power generator installed in the water drop pipe for generating electricity power by motion of the dropping seawater and/or by a water pressure of the taken-in seawater; a submersible body connecting with the lower terminal of the seawater drop pipe for supplying a seawater tank space for temporarily storing the discharged seawater from the lower terminal of the seawater drop pipe; and a forced discharge part for discharging the stored seawater from the seawater tank space of the submersible body to the outer undersea forcibly. 
         [0030]    According to the above-mentioned configuration of the first invention of the marine power generation system, the seawater drop pipe is set vertically in the ocean, so it can supply the space in vertical with a difference between ocean surface and the submersible body similar to a water discharge from a dam, which is ideal for hydroelectricity. The marine power generation system utilizes this dam like space, and the general power generator used in the general hydroelectric dam for power generation. The water intake structure may be a structure floating on the ocean surface apart from the shore by its buoyance or may be a structure constructed on the shore if the aperture part is located in the sea. In either type, seawater taken-in from the aperture part is led to the seawater drop pipe through the upper seawater guide part. The seawater drop pipe should be set vertically for securing the vertical difference, so if the aperture part is constructed on the shore, a cliff is preferable place. 
         [0031]    Next, there are two configuration mechanisms for installing the submersible body as the “forced discharge part”. 
         [0032]    The first configuration of the forced discharge part comprises a cylinder, a piston for reciprocating in the cylinder, a first check valve connecting the cylinder to the seawater tank space of the submersible body while preventing a countercurrent, a second check valve connecting the cylinder to the outer undersea of the submersible body while preventing a countercurrent, and a power transmission mechanism for transmitting the power to the piston for piston motion by utilizing the power of the tidal current flowing around the submersible body. 
         [0033]    The forced discharge part is operated as a piston-cylinder style seawater forced discharge mechanism including an intake phase for taking in the seawater from the seawater tank space to the cylinder, and a discharge phase for discharging the seawater from the cylinder to the outer undersea of the submersible body by pumping in synchronization with the piston reciprocating movement. 
         [0034]    The piston-cylinder style seawater forced discharge mechanism may be called a “submersible piston-cylinder mechanism” working in the seawater discharging piston-cylinder cycle in which seawater is taken in the cylinder and discharged from the cylinder to the outer undersea of the submersible body by piston motion. It may be categorized as a “natural power discharge” utilizing the tidal current. 
         [0035]    As shown above, the piston-cylinder style seawater forced discharge mechanism discharges the stored seawater from the seawater tank space of the submersible body at low water pressure to the outer undersea of the submersible body at high water pressure forcibly. The piston motion can be achieved when the drive mechanism receives the strong pressure by the tidal current. The speed of the tidal current is not important, but rather its pressure. 
         [0036]    The above-mentioned piston-cylinder style seawater forced discharge mechanism can be installed in parallel. Plural piston-cylinder style seawater forced discharge mechanisms are installed around the submersible body in parallel, and each piston-cylinder style seawater forced discharge mechanisms is operated by a piston-cylinder cycle for discharging seawater from the seawater tank space of the submersible body to the outer undersea of the submersible body. The total seawater discharge amount can be enhanced by using the parallel piston-cylinder style seawater forced discharge mechanisms. The volume of the seawater tank space of the submersible body is limited, so the drop seawater amount taken-in from the upper seawater guide part is limited to the amount equivalent to the discharged seawater from the seawater tank space of the submersible body to the outer undersea forcibly by the piston-cylinder style seawater forced discharge mechanism. The seawater amount that can be used for the power generation at the seawater drop pipe is limited to the discharge amount. 
         [0037]    Therefore, this system includes a seawater flow amount control part for controlling the taken-in seawater amount dropping from the upper seawater guide part through the seawater drop pipe as the amount corresponding to the discharge ability of the piston-cylinder style seawater forced discharge mechanisms. The seawater flow amount control part controls the dropping seawater amount from the upper seawater guide part to the seawater drop pipe for allowing the piston-cylinder style seawater forced discharge mechanism to catch-up the discharge of the seawater to the outer undersea in order to prevent the whole system from being filled with the seawater. 
         [0038]    Next, the second configuration of the forced discharge part comprises an electric pump style seawater discharging mechanism comprising a pump mechanism for discharging the seawater by pumping from the seawater tank space of the submersible body to the outer undersea of the submersible body, and a power generator installed to the outside of the submersible body driven by the tidal current around the submersible body. 
         [0039]    In short, the electric pump style seawater discharging mechanism discharges the seawater forcibly to the outer undersea of the submersible body by the electric power obtained by the power generator installed to the outside of the submersible body driven by the tidal current around the submersible body. 
         [0040]    The same as the piston-cylinder style seawater forced discharge mechanisms, the above-mentioned electric pump style seawater forced discharge mechanism can be installed in parallel. Plural pump style seawater forced discharge mechanisms are installed around the submersible body in parallel, and each pump style seawater forced discharge mechanisms is operated by pumping for discharging seawater from the seawater tank space of the submersible body to the outer undersea of the submersible body forcibly. The total seawater discharge amount can be enhanced by using parallel pump style seawater forced discharge mechanisms. 
         [0041]    It is preferable that the second configuration of the forced discharge part includes the seawater flow amount control part for controlling the taken-in seawater amount dropping from the upper seawater guide part through the seawater drop pipe as the amount corresponding to the discharge ability of the piston-cylinder style seawater forced discharge mechanisms. The seawater flow amount control part controls the dropping seawater amount from the upper seawater guide part to the seawater drop pipe for allowing the piston-cylinder style seawater forced discharge mechanism to catch-up the discharge of the seawater to the outer undersea in order to prevent the whole system from being filled with the seawater. 
         [0042]    Moreover, it is preferable that the marine power generation system includes a ventilation pipe for securing the ventilation for both or either of the space lower than the installation part of the power generator to the seawater drop pipe and the seawater tank space of the submersible body, in order to prevent the whole system from being filled with the seawater. 
         [0043]    It is an effective countermeasure for securing the ventilation to the space lower than the installation part of the power generator to the seawater drop pipe and the seawater tank space of the submersible body in order to prevent the whole system from being filled with the seawater and keep the whole system from an inoperable state. 
         [0044]    Next, general use power generator for hydroelectric power generation can be applicable to the power generator for this marine power generation system. 
         [0045]    There are various types of the power generator for hydroelectric power generation. Roughly saying, there are two types. One is an impulse water turbine. The other is a reaction water turbine. 
         [0046]    The impulse water turbine works by converting the pressure head to velocity head. In short, the impulse water turbine receives the kinetic energy of the water stream and converts it to the kinetic energy of the turbine. As the impulse water turbine, the Pelton turbine and Opening round flow water turbine are known. If the generator employs the impulse water turbine such as the Pelton turbine, it is possible to obtain the power by impacting the generator turbine by the dropping seawater from the seawater drop pipe. 
         [0047]    The reaction water turbine works by the pressure head. In short, the pressure head is applied to the reaction water turbine and the applied water pressure is converted to the kinetic energy of the turbine. As the reaction water turbine, the Francis turbine, the propeller turbine and the Kaplan turbine are known. If the generator employs the reaction water turbine such as the Francis turbine, it is possible to obtain the power by subjecting the generator turbine to the seawater pressure introduced from the seawater drop pipe. 
         [0048]    Next, a marine power generating method comprises; setting vertically a seawater drop pipe for supplying the space for dropping a seawater to undersea through it installing a power generator generating power by a seawater falling kinetic energy or a seawater pressure energy introduced to the space of the seawater drop pipe via an aperture part; installing a submersible body for supplying a seawater tank space for temporarily storing the discharged seawater from the lower terminal of the seawater drop pipe; installing a forced discharge part for discharging the stored seawater from the seawater tank space of the submersible body to the outer undersea forcibly; taking in the seawater from the aperture part, dropping the seawater through the seawater drop pipe, generating power with the generator installed in the seawater drop pipe, discharging the seawater taken-in the submersible body from the seawater drop pipe from the submersible body with the forced discharge part. 
       Effect of the Invention 
       [0049]    According to the present invention of the marine power generation system, the seawater drop pipe is set vertically in the ocean, so it can supply the space with in vertical difference between ocean surface and the submersible body similar to a water discharge from a dam, which is suitable for hydroelectricity power generation. 
         [0050]    The forced discharge of the seawater dropped in the submersible body to the outer undersea is conducted by the piston-cylinder style discharge mechanism with the first check valve and the second check valve for preventing a countercurrent. It may call a “natural power discharge” utilizing the power of the tidal current flowing around the submersible body. If the forced discharge of the seawater is conducted by the pump style discharge mechanism, it may call as a “forced power discharge” using electricity obtained from the power of the tidal current flowing around the submersible body. 
         [0051]    The piston-cylinder style discharge mechanism discharges the seawater from the seawater tank space of the submersible body with low water pressure to the outer undersea of the submersible body with high water pressure. However, the piston motion can be achieved if there is strong water pressure in the tidal current. The velocity of the tidal current is not important. The seawater amount to be dropped through the seawater drop pipe depends on the volume of the vacant space in the seawater tank space of the submersible body obtained by the piston-cylinder style discharge mechanism. The seawater dropping through the seawater drop pipe can be dropped at a suitable velocity and suitable water pressure for the hydroelectric power generation 
         [0052]    The same as the piston-cylinder style discharge mechanism, the pump style discharge mechanism also discharges the seawater from the seawater tank space of the submersible body with low water pressure to the outer undersea of the submersible body with high water pressure. The electricity for driving the pump style discharge mechanism is obtained by the generator installed around the submersible body, and the seawater in the seawater tank space of the submersible body is discharged forcibly. The seawater amount to be dropped through the seawater drop pipe depends on the volume of the vacant space in the seawater tank space of the submersible body obtained by the pump style discharge mechanism. The seawater dropping through the seawater drop pipe can be dropped at a suitable velocity and the suitable water pressure for the hydroelectric power generation. 
         [0053]    As shown above, according to the present invention of the marine power generation system, the tidal current which is relatively low velocity can be converted to the suitable velocity and the suitable water pressure for hydroelectric power generation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0054]      FIG. 1  is a schematic view of the configuration of the marine power generation system  100  of the present invention. 
           [0055]      FIG. 2  is a schematic view showing the seawater flow. 
           [0056]      FIG. 3  is a schematic view showing the configuration if a Pelton turbine is employed in the generator installed in the seawater drop pipe  20 . 
           [0057]      FIG. 4  is a schematic view showing the configuration if a Francis turbine is employed in the generator installed in the seawater drop pipe  20 . 
           [0058]      FIG. 5  is a schematic view showing the configuration if the ventilation pipe  42  with employed to the seawater drop pipe  20  and the submersible body  40 . 
           [0059]      FIG. 6  is a schematic view showing the forced discharge part  50  employing the piston-cylinder style discharge mechanism of the Embodiment 1. 
           [0060]      FIG. 7  is an operation of the seawater intake phase  1 . 
           [0061]      FIG. 8  is an operation of the seawater intake phase  2 . 
           [0062]      FIG. 9  is an operation of the seawater discharge phase. 
           [0063]      FIG. 10  is a schematic view showing the forced discharge part  50   a  employing the pump style discharge mechanism of the Embodiment 2. 
           [0064]      FIG. 11  shows the problem of the tidal current generation in prior art. 
           [0065]      FIG. 12  shows a perspective view showing the tidal current generation system disclosed in the prior art 2. 
           [0066]      FIG. 13  is a perspective view showing the submarine style tidal current generation system disclosed in the prior art 3. 
           [0067]      FIG. 14  is a perspective view showing the submarine style tidal current generation system disclosed in the prior art 4. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0068]    Some embodiments of a marine power generation system according to the present invention are described below with reference to the relevant drawing. Needless to add, the claims of the present invention include but are not limited to the application, configuration, or quantity shown in the following embodiments. 
       Embodiment 1 
       [0069]    The marine power generation system  100  in embodiment 1 according to the present invention is described. 
         [0070]      FIG. 1  is a schematic view of the configuration of the marine power generation system  100  according to the present invention. 
         [0071]      FIG. 2  is a schematic view showing the seawater flow. 
         [0072]    As shown in  FIG. 1 , the marine power generation system  100  comprises a seawater intake structure  10 , a seawater drop pipe  20 , a generator  30 , a submersible body  40  and a forced discharge part  50 . As shown in  FIG. 2 , there are seawater flows inside and outside of the marine power generation system  100 . The marine power generation system  100  generates power by the taken-in seawater flow drop in the system and the tidal current flow around the system. 
         [0073]    Hereinafter, each component is explained, then the operation of the marine power generation system  100  is explained. 
         [0074]    Each component is explained as follows. 
         [0075]    The seawater intake structure  10  may be a structure constructed on the ground along the coast or a sea surface floatation object floating by its buoyance on the sea surface away from the coast. 
         [0076]    The seawater intake structure  10  can take in the seawater via the aperture part  12 , and the seawater intake structure  10  is either the construction on the ground or the sea surface floatation object. At least one portion of the aperture part  12  is located below the seawater line, and the seawater can be taken into the inside. Moreover, there is the upper seawater guide part  13  for sending the taken-in seawater from the aperture port to inner portion. The size and the length of the upper seawater guide part  13  are not limited. It can be applicable if it can send the required amount of seawater to the seawater guide pipe  20   
         [0077]    In this embodiment, as shown in  FIG. 1 , the seawater intake structure  10  is a vessel shape type, and it can be floated on the sea by the buoyance of the sea surface floatation object  11 . In the configuration shown in  FIG. 1 , the shape of the seawater intake structure  10  is disclosed as the Xuanwu ship shape. A normal vessel does not have any structure for taking-in the seawater and sending the taken-in the seawater to drop to undersea at all. However, the marine power generation system  100  of the present invention includes the aperture part  12  for taking-in the seawater and the upper seawater guide part  13  for sending the taken-in seawater to the further inner portion for use in the power generation. 
         [0078]    If the water intake structure  10  is the structure constructed on the ground along the coast, the seawater drop pipe should be set vertically for securing the vertical difference, and using the coast topography such as a cliff is desirable. 
         [0079]    In this configuration, the water intake structure  10  includes the seawater flow amount control part  14  for controlling the taken-in seawater amount. As described later, the marine power generation system  100  generates power by introducing the seawater to the sea water drop pipe  20  and driving the generator  30 , so if the sea water drop pipe  20  is filled with seawater, the generator  30  operation is stopped. Therefore, the amount of taken-in seawater should be limited to the discharge ability of the forced discharge part  50 . Therefore, the seawater flow amount control part  14  controls the amount of seawater taken-in to the sea water drop pipe  20 . 
         [0080]    The seawater drop pipe  20  is a pipe, as shown in  FIG. 1 , the upper terminal of the seawater drop pipe  20  is connected to the water intake structure  10 , and the lower terminal of the seawater drop pipe  20  is connected to the submersible body  40 . In short, the seawater drop pipe  20  is the structure between the water intake structure  10  and the submersible body  40 . 
         [0081]    The seawater drop pipe  20  supplies the space for dropping the taken-in seawater from the upper seawater guide part  13  to the submersible body  40 . In short, the seawater drop pipe  20  supplies the space with vertical difference between the ocean surface and the submersible body. 
         [0082]    The structure strength is designed to withstand the water pressure in the ocean. For example, stainless wireframe spectra pipe object can be employed as the material of the seawater drop pipe  20 . The stainless wireframe spectra pipe object is lightweight and flexible, and has the mechanical strength 10 times as large as iron. In addition, the stainless wireframe spectra pipe object is resistant to corrosion, and the metal fatigue does not happen on the stainless wireframe spectra pipe object. These merits are important for marine use. 
         [0083]    As shown above, the seawater drop pipe is set vertically in the ocean, so it can supply the space with vertical difference between the ocean surface and the submersible body  40  similar to a water discharge from a dam. In this model, the sea surface can be regarded as the water surface of the reservoir. The inner wall of the seawater drop pipe  20  can be regarded as the wall of the dam. The seawater taken-in the seawater drop pipe  20  can be regarded as the water mass for utilizing the hydroelectric power generation. 
         [0084]    The generator  30  is a generator installed in the seawater drop pipe  20  and generates electricity by the seawater kinetic energy of pressure energy. The general use power generator for hydroelectric power generation can be applicable to the generator for this marine power generation system  100 . 
         [0085]    There are various types of the power generator  30 . Roughly saying, there are two types. One is an impulse water turbine. The other is a reaction water turbine. 
         [0086]    The impulse water turbine works by converting the pressure head to velocity head. In short, the impulse water turbine receives the kinetic energy of the water stream and converts the energy to the kinetic energy of the turbine. As the impulse water turbine, the Pelton turbine and Opening round flow water turbine are known. 
         [0087]    The Pelton turbine is one of the impulse turbines. It is driven by impacting the bucket installed around the runner with the water flow. The general Pelton turbine is driven by impacting the bucket with the jet water flow from the nozzle. However, the Pelton turbine in the seawater drop pipe  20  is driven by impacting the bucket with the dropping seawater flow through the seawater drop pipe  20 . 
         [0088]    The Opening round flow water turbine is a very familiar turbine that has been widely used as a general water wheel since ancient times. It is driven by impacting the plate installed around the runner. 
         [0089]      FIG. 3  is a schematic view showing the configuration if the Pelton turbine is employed in the generator  30  installed in the seawater drop pipe  20 . The bucket of the Pelton turbine is installed in the seawater drop pipe  20  where the dropping seawater passes through. The introduced seawater to the seawater drop pipe  20  drops rapidly and violently collides with the bucket of the Pelton turbine. The Pelton turbine rotates rapidly. The runner which the Pelton turbine installed to be rotated by impulse turn, the generator  30  (not illustrated) is rotated along with the runner and the electric power is obtained. 
         [0090]    Next, the reaction water turbine works by the pressure head. In short, the pressure head is applied to the reaction water turbine and the applied water pressure is converted to the kinetic energy of the turbine. As the reaction water turbine, the Francis turbine, the propeller turbine and the Kaplan turbine are known. 
         [0091]    The Francis turbine has the spiral runner turbine that receives the running-in seawater from the radial direction flow and discharges the running-out seawater in the axial direction flow. This Francis turbine is widely used as the hydroelectric power generation because the structure is relatively simple and the maintenance is easy. 
         [0092]    The propeller turbine has the plural runner blades installed on the rotary axis with skew, which receives the running-in seawater from the axial direction flow and discharges the running-out seawater to the axial direction flow. 
         [0093]    The Kaplan turbine is categorized as the propeller turbine. It can change the blade skew depending on the difference in vertical direction and the dropping water amount. 
         [0094]    The cross-flow turbine is the turbine which the running water flows crosswise relative to the blade. 
         [0095]      FIG. 4  is a schematic view showing the configuration if the Francis turbine is employed in the generator  30  installed in the seawater drop pipe  20 . The eccentric positions of the Francis turbine face the seawater drop pipe  20 , and the dropped seawater is accumulated at the part of the seawater drop pipe  20  near the upper portion of the generator  30  that employs the Francis turbine. Large seawater pressure is applied to the portion of the Francis turbine facing the seawater drop pipe  20 . The Francis turbine rotates by reaction turn. Every blade is installed to the runner with skew. The seawater will be turn from the eccentric positions of the Francis turbine to the deep inside of the center portion along the axis. Then the applied seawater is discharged to the lower portion of the seawater drop pipe  20  as shown in  FIG. 4 . The runner, to which the Francis turbine is installed, is rotated by reaction turn, and the generator (not illustrated) is rotated along with the runner and the electric power is obtained. 
         [0096]    Next, the submersible body  40  supplies a seawater tank space for temporarily storing the discharged seawater from the lower terminal of the seawater drop pipe  20 . The submersible body  40  is connected with the lower terminal of the seawater drop pipe  20 . The submersible body  40  is located undersea, so the structure strength of the submersible body  40  is designed to withstand the water pressure in the ocean. The seawater tank space  41  is a space for storing the discharged seawater from the lower terminal of the seawater drop pipe  20 . However, the discharged seawater is further discharged to the outer undersea by the forced discharge part  50  in turn, so the seawater tank space  41  is a temporary storing space. 
         [0097]    If the submersible body  40  is filled with seawater, the marine power generation system  100  approaches its limit. If the seawater keeps on flowing through the seawater drop pipe  20 , the marine power generation system  100  will stop because whole system becomes filled with the seawater. Therefore, the seawater flow amount control part  14  for controlling the taken-in seawater amount becomes important. Moreover, the ventilation pipe  42  is useful for the countermeasure. 
         [0098]      FIG. 5  ( a ) is a schematic view showing the configuration if the ventilation pipe  42  is employed with the seawater drop pipe  20  and the submersible body  40 .  FIG. 5  ( b ) is a schematic view showing the configuration if the ventilation pipe  42  is employed directly with the submersible body  40 , next to the seawater drop pipe  20 . 
         [0099]    As shown above, if there is no ventilation pipe  42  to the submersible body  40  directly, the ventilation should be conducted via the seawater drop pipe  20  in which the seawater drops through. However, the taken-in amount of seawater is larger than that of the discharged water, the seawater is gradually filled with seawater. Therefore, the taken-in amount of seawater is controlled by the seawater flow amount control part  14  and the ventilation is secured by the ventilation pipe  42 . 
         [0100]    It is preferable that the ventilation pipe  42  is connected to the space lower than the installation part of the power generator  30  to the seawater drop pipe  20  and the seawater tank space of the submersible body  40  for securing the ventilation. 
         [0101]    The forced discharge part  50  discharges the stored seawater from the seawater tank space  41  of the submersible body  40  to the outer undersea forcibly. The structure of the forced discharge part  50  is not limited if it can discharge the taken-in seawater from the submersible body  40  to the outer undersea. 
         [0102]    For example, as the configuration of the forced discharge part  50 , there is a piston-cylinder type discharge mechanism and a pump style discharge mechanism. In this embodiment 1, the marine power generation system  100  of the present invention employs the piston-cylinder style discharge mechanism as the forced discharge part  50 . The other configuration, which employs the pump style discharge mechanism as the forced discharge part  50 , is shown in Embodiment 2. 
         [0103]      FIG. 6  is a schematic view showing the forced discharge part  50  employing the piston-cylinder style discharge mechanism of the Embodiment 1.  FIG. 6  shows the basic operation, so fine details of the mechanical structure and configuration are omitted. 
         [0104]    As shown in  FIG. 6 , the forced discharge part  50  employing the piston-cylinder type discharge mechanism comprises a cylinder  51 , piston  52 , the first check valve  53 , the second check valve  54 , the third check valve  55 , the power transmission mechanism  56 , and the discharge part  57 . 
         [0105]    The cylinder  51  is a cavity for taking-in the seawater to be discharged to the outer undersea temporarily. The cylinder  51  has mechanical strength for withstanding the high water pressure as described later. The inner diameter of the cylinder  51  is the same as the outer diameter of the piston  52 . The inner diameter of the cylinder  51  is “D” in this description. 
         [0106]    The installation portion of the cylinder  51  may be set anywhere relative to the submersible body  40 . In this example, it is installed at the end portion of the submersible body  40 . 
         [0107]    The cylinder  51  is connected to the seawater tank space  41  of the submersible body  40  via the first check valve  53 . The cylinder  51  is also connected to the outer undersea via the second check valve  54 , and the taken-in seawater is discharged from the seawater tank space  41  of the submersible body  40  to the outer undersea 
         [0108]    In the configuration shown in  FIG. 6 , there are the cylinder  51  divided into two cylinder cavities by the piston  52 . The first cylinder cavity near the seawater tank space  41  of the submersible body  40  is defined as the first cylinder  511 . The second cylinder cavity near the outer undersea is defined as the second cylinder  512 . 
         [0109]    The piston  52  has a cylindrical shape. The piston  52  can be operated in piston motion in the cylinder  51 . The outer diameter corresponds to the cylinder  51 . The outer diameter of the piston  52  is “D” in this description. 
         [0110]    In the configuration shown in  FIG. 6 , there is the water pass  521  connecting the front side and rear sides through the piston  52 . As described later, the water pass  521  of the piston  52  is utilized to pass the taken-in seawater of the first cylinder  511  to the second cylinder  512  in the piston-cylinder operation cycle. 
         [0111]    The various technologies used in the piston engine mechanism can be applied as the cylinder  51  and piston  52 . For example, if a piston ring attachment is applied to the piston  52 , the piston ring attachment can better maintain the seawater pressure during the seawater discharge operation. 
         [0112]    The first check valve  53  is a check valve for connecting the cylinder  51  and the seawater tank space  41  of the submersible body  40  that limits flow to one direction. The example shown in  FIG. 6  is described as the hinge type check valve for understanding easily, but the structure of the check valve is not limited to the hinge type check valve. 
         [0113]    As described later, the first check valve  53  is opened when introducing the seawater from the seawater tank space  41  of the submersible body  40  to the first cylinder  511  in the reciprocating cycle of the piston. The first check valve  53  is closed when transmitting the seawater from the first cylinder  511  to the second cylinder  512  for preventing the backflow of seawater from the first cylinder  511  to seawater tank space  41  of the submersible body  40 . 
         [0114]    The second check valve  54  is a check valve for connecting the cylinder  51  and the outer undersea around the submersible body  40  that limits flow to one direction. The example shown in  FIG. 6  is described as the hinge type check valve for understanding easily, but as with the first check valve, the structure of the check valve is not limited to the hinge type check valve. In the configuration shown in  FIG. 6 , the second check valve  54  is installed to the discharge part  57  of the submersible body  40 . 
         [0115]    As described later, the second check valve  54  is opened when discharging the seawater from the second cylinder  512  to the outer undersea via the discharge part  57  in the reciprocating cycle of the piston. The second check valve  54  is closed when transmitting the seawater from the first cylinder  511  to the second cylinder  512  for preventing the backflow of seawater from the outer undersea to the second cylinder  512 . 
         [0116]    The third check valve  55  is a check valve for controlling the flow of the water pass  521  connecting the front side and the rear side of the piston  52 . 
         [0117]    As described later, the third check valve  55  is opened when transmitting the seawater from the first cylinder  511  to the second cylinder  512  in the reciprocating cycle of the piston. The third check valve  55  is closed when discharging the seawater from the second cylinder  512  to the outer undersea via the discharge part  57  in the reciprocating cycle of the piston. 
         [0118]    The power transmission mechanism  56  is a mechanism for transmitting the power to the piston  52  for operating the reciprocating cycle of the piston by converting the energy of the tidal current around the submersible body  40 . The structure of the power transmission mechanism  56  is not limited as long as it can convert the energy of the tidal current to the kinetic energy of the piston cylinder cycle. In this configuration, the power transmission mechanism  56  includes a propeller  561  that is rotated by the energy of the tidal current flowing around the submersible body  40 , a rotary axis  562  of the propeller, a conversion mechanism  563  for converting the rotating energy obtained by the rotary axis  562  to the kinetic energy of the piston cylinder cycle. 
         [0119]    The discharge part  57  is an opening to the outer under sea. The discharge part  57  connects the outer undersea and the second cylinder  512 . 
         [0120]    As described later, the discharge part  57  is opened when discharging the seawater from the second cylinder  512  to the outer undersea in the operation of the reciprocating cycle of the piston. The discharge part  57  is closed when transmitting the seawater from the first cylinder  511  to the second cylinder  512  via the third check valve  55 . 
         [0121]    The reciprocating cycle of the piston of the forced discharge part  50  including the piston-cylinder type seawater discharge mechanism is described phase by phase. 
       [Seawater Intake Phase  1 ] 
       [0122]    The seawater intake phase  1  is a phase for taking-in the seawater from the seawater tank space  41  of the submersible body  40  to the cylinder cavity  511  of the cylinder  51 , synchronizing with the reciprocating motion of the piston  52 . 
         [0123]      FIG. 7  ( a ) to  FIG. 7  ( c ) show the operation of the seawater intake phase  1 . 
         [0124]      FIG. 7  ( a ) shows the start state of the seawater intake phase  1  and the start state of the seawater discharge phase. The piston  52  is located at the place (most right-hand place of the cylinder  51 ) minimizing the first cylinder cavity  511  and maximizing the second cylinder cavity  512 . 
         [0125]    As shown in  FIG. 7  ( a ) to  FIG. 7  ( b ), the piston  52  starts to move for enlarging the first cylinder cavity  511  and reducing the second cylinder cavity  512 . The first check valve  53  is opened, and the seawater flows from the seawater tank space  41  of the submersible body  40  to the cylinder cavity  511  of the cylinder  51 . The third check valve  55  is closed. Therefore, the volume of seawater taken-in from the seawater tank space  41  of the submersible body  40  corresponds to the enlarged volume of the first cylinder  511 . 
         [0126]      FIG. 7  ( c ) shows the state where the piston  52  is located at the place (most left-hand place of the cylinder  51 ) maximizing the first cylinder cavity  511  and minimizing the second cylinder cavity  512 . 
       [Seawater Intake Phase  2 ] 
       [0127]    The seawater intake phase  2  is a phase for taking-in the seawater from the first cylinder cavity  511  of the cylinder  51  to the second cylinder cavity  512  via the seawater pass  521 , synchronizing with the reciprocating motion of the piston  52 . 
         [0128]    The seawater taken-in the tank space  41  of the submersible body  40  is taken in the second cylinder cavity  512  of the cylinder  51  by these two phases of the seawater intake phase  1  and the seawater intake phase  2 . 
         [0129]      FIG. 8  ( a ) to  FIG. 8  ( c ) show the operation of the seawater intake phase  2 . 
         [0130]      FIG. 8  ( a ) shows the start state of the seawater intake phase  2 . The piston  52  is located at the place (most left-hand place of the cylinder  51 ) maximizing the first cylinder cavity  511  and minimizing the second cylinder cavity  512 . 
         [0131]    As shown in  FIG. 8  ( a ) to  FIG. 8  ( b ), the piston  52  starts to move for reducing the first cylinder cavity  511  and enlarging the second cylinder cavity  512 . The first check valve  53  is closed, so the seawater in the first cylinder cavity  511  is blocked from reverse flow to the seawater taken-in the tank space  41  of the submersible body  40 . The third check valve  55  is opened, so the seawater flows from the first cylinder cavity  511  of the cylinder  51  to the second cylinder cavity  512  via the seawater pass  521 . The second check valve  54  is closed, so the seawater is blocked from flow from undersea to the second cylinder cavity  512 . 
         [0132]      FIG. 8  ( c ) shows the state that the piston  52  is located at the place (most right-hand place of the cylinder  51 ) where minimizing the first cylinder cavity  511  and maximizing the second cylinder cavity  512 . 
       [Seawater Discharge Phase] 
       [0133]    The seawater discharge phase is a phase for discharging the seawater from the second cylinder cavity  512  of the cylinder  51  to the outer undersea via the discharge part  57 , synchronizing with the reciprocating motion of the piston  52 . 
         [0134]      FIG. 9  ( a ) to  FIG. 9  ( c ) show the operation of the seawater discharge phase. 
         [0135]      FIG. 9  ( a ) shows the start state of the seawater discharge phase and the start state of the seawater intake phase shown in  FIG. 7 . The piston  52  is located at the place (most right-hand place of the cylinder  51 ) maximizing the second cylinder cavity  512  and minimizing the first cylinder cavity  511 . 
         [0136]    As shown in  FIG. 9  ( a ) to  FIG. 9  ( b ), the piston  52  starts to move for reducing the second cylinder cavity  512  and enlarging the first cylinder cavity  511 . The first check valve  53  is opened and the third check valve  55  is closed, so the seawater is blocked from flow from the second cylinder cavity  512  to the first cylinder cavity  511  via the seawater pass  521 . 
         [0137]    The opening and closing of the second check valve  54  depends on the magnitude correlation between the seawater pressure of the outer undersea and the seawater pressure of the second cylinder cavity  512 . The seawater pressure of the second cylinder cavity  512  can be controlled by impressing the pressure power by the power transmission mechanism  56 . Therefore, the opening and closing of the second check valve  54  depends on the pressure of the terminal of the piston applied by the outer undersea and the pressure of the propeller  561  applied by the outer undersea plus the tidal current power. 
         [0138]    If the effective area of the propeller  561  is larger than the terminal area of the piston  52 , the following relation is satisfied. 
         [0000]      (the outer undersea around the submersible body 40)&lt;(the seawater pressure of the second cylinder cavity 512) 
         [0139]    When the above relationship is satisfied, the second check valve  54  is opened, the seawater of the second cylinder cavity  512  in higher pressure is discharged to the outer undersea in lower pressure. 
         [0140]    As described above, the seawater is taken-in from the tank space  41  of the submersible body  40  to the first cylinder cavity  511  simultaneously. 
         [0141]    This outlines the operation of the reciprocating cycle of the piston operated in the forced discharge part  50  employing the piston-cylinder type seawater discharge mechanism. 
         [0142]    The forced discharge part  50  can use plural piston-cylinder type seawater discharge mechanism. Each piston-cylinder type seawater discharge mechanism can drive in parallel. Plural sets of the piston-cylinder type seawater discharge mechanism are arrayed around the submersible body  40 . Each piston-cylinder type seawater discharge mechanism is driven by each reciprocating cycle of the piston, and seawater is taken-in from the tank space  41  of the submersible body  40  and seawater is discharged to the outer undersea. The total seawater discharge ability becomes large. 
         [0143]    The power generating ability of the whole marine power generation system is described. 
         [0144]    The tank space  41  of the submersible body  40  can be regarded as a “buffer” for storing the seawater flow used for the power generation in the seawater drop pipe  20 . Therefore, the discharge ability depends on the discharge ability of the forced discharge part  50  for the tank space  41  of the submersible body  40 . 
         [0145]    The marine power generation system  100  of the present invention of Embodiment 1 utilizes the tidal current as the natural resource, whose speed is relatively low but whose energy amount is huge. The low velocity tidal current is caught by the propeller  561  and converted to the reciprocating cycle of the piston via the power transmission mechanism  56 . The tidal current energy that is relatively low velocity can be converted to the reciprocating cycle of the piston at a multiple velocity of the tidal current velocity by adjusting the element such as the transmission rotary ratio and the cam reciprocating motion conversion ratio. 
         [0146]    One example is shown. 
         [0147]    The cross-sectional area D of the cylinder cavity  51  is 3 m 2  (radius of 1 meter). The maximum width L1 of the first cylinder cavity  511  is 9 meters. The maximum width L2 of the second cylinder cavity  512  is 9 meters. The tidal current speed is 3 meters per second (it is calculated from the speed of the Straits of Dover at 12 kilometers per hour). The transmission rotary ratio and the cam reciprocating motion conversion ratio is 1:3. The discharge ability per second is calculated as follows. 
         [0148]    The piston  52  moves 9 meters per second calculated by the tidal current 3 meters per second and the transmission rotary ratio and the cam reciprocating motion conversion ratio of 1:3. The maximum width L1 of the first cylinder cavity  511  is 9 meters and the maximum width L2 of the second cylinder cavity  512  is 9 meters. Therefore, the piston reciprocating cycle can be driven for one turn per second. 
         [0149]    The capacity of the second cylinder cavity  512  is 27 m 3  calculated by the cross-sectional area D of the first cylinder cavity  51  and the maximum width L2 of the second cylinder cavity  512 . In conclusion, the discharge ability of the system is 27 m 3 . The discharge ability of the forced discharge part  50  with one set of the piston-cylinder style seawater discharge mechanism is also 27 tons per second. In the above calculation, the velocity of the tidal current is assumed as 3 meters, which comes from the maximum velocity of the tidal current of the Straits of Dover. However, the maximum velocity does not always match the Straits of Dover. If the Kuroshio Japan Current is utilized instead of the Strait of Dover current, the discharge ability is calculated as follows. The velocity of the Kuroshio Japan Current is about 6 to 7 kilometers per hour, about 1.5 meters per second. The condition obtained in Kuroshio Japan Current is about half that of the Straits of Dover. Therefore, if the Kuroshio Japan Current is utilized as the power source, the discharge ability of the forced discharge part  50  with one set of the piston-cylinder style seawater discharge mechanism is about 14 tons per second. The discharge ability of the forced discharge part  50  is about 14 tons per second, so 14 tons of seawater can introduced to the seawater drop pipe  20  and can be utilized for power generation of the generator  30 . 
         [0150]    It is said that the fourth Kurobe dam in Japan discharges 10 tons of water per second for power generation. So, the power generation by the marine power generation system  100  employing the discharge ability of the forced discharge part  50  with one set of the piston-cylinder style seawater discharge mechanism roughly compares to the power generation of the fourth Kurobe dam in Japan. 
         [0151]    The marine power generation system  100  can employ the plural sets of the piston-cylinder type seawater discharge mechanism in parallel. If four sets of the piston-cylinder type seawater discharge mechanism are employed, the discharge ability will be 500,000 tons per second in total. The marine power generation system  100  can take in the seawater to the seawater drop pipe  20  corresponding to the discharged amount of the forced discharge part  50 . So the seawater of 500,000 tons per second can utilize for power generating. As shown above, the fourth Kurobe dam utilizes 10 tons of water per second for power generation, and it is said that the generation power of the fourth Kurobe dam is 300,000 kW. Therefore, the marine power generation system  100  whose discharge ability is 500,000 tons per second can generate 1,500,000 kW. 
         [0152]    The large scale power generation can be achieved by utilizing the tidal current, which is the huge power source with strong seawater pressure but relatively low velocity, for obtaining the discharge ability by converting the reciprocating cycle of the piston. The capacity obtained by the forced discharge part  50  for discharging the seawater to the outer undersea is utilized as the “seawater drop discharge” through the seawater drop pipe  20  in turn. 
         [0153]    In the above-shown configuration, any one or the combination of the seawater intake structure  10 , the seawater drop pipe  20 , the power generators  30 , the submersible body  40 , and the forced discharge part  50  can be used in parallel. Moreover, the plural sets of marine power generation system  100  can be operated simultaneously. 
       Embodiment 2 
       [0154]    The second marine power generation system  100   a  in embodiment 2 according to the present invention is described. In the second marine power generation system  100   a  shown in this embodiment 2, the pump mechanism is employed in the forced discharge part. 
         [0155]    The marine power generation system  100   a  shown in this embodiment 2 discharges the seawater in the forced discharge part  50   a  by the pump mechanism  58 . 
         [0156]    Other components such as the seawater intake structure  10 , the seawater drop pipe  20 , the power generators  30 , the submersible body  40 , and the other components, which are not explained specifically in Embodiment 2, may be the same as that of Embodiment 1. Regarding these other components, descriptions and drawings are omitted here. 
         [0157]      FIG. 10  is a schematic view showing the forced discharge part  50   a  employing the pump style discharge mechanism of the Embodiment 2.  FIG. 10  shows the discharge operation simply, so fine details of the mechanical components and configuration are omitted here. 
         [0158]    As shown in  FIG. 10 , the forced discharge part  50   a  employing the pump type discharge mechanism comprises a pump  58 , a power supply apparatus  59 , and the discharge part  57 . 
         [0159]    It is necessary for the pump  58  to discharge the seawater to the outer undersea against seawater pressure generated at the depth where the submersible body  40  locates. 
         [0160]    The power supply apparatus  59  supplies the electric power for driving the pump  58 . The method of the power supply apparatus  59  for obtaining the electricity to be supplied to the pump is not limited. 
         [0161]    The electricity obtained by the power generation at the submersible body  40  utilizing the tidal current may be supplied to the power supply apparatus  59 , and the electricity obtained by the power generation by the generator  30  installed in the seawater drop pipe  20  may be supplied to the power supply apparatus  59 . Of course, the combination of the above mentioned electricity supplying can be possible. 
         [0162]    For example, as the configuration of the power generation utilizing the tidal current around the submersible body  40 , the propeller components are installed around the submersible body  40  for obtaining the rotation torque for rotating the generator. 
         [0163]    If the electricity is obtained by the power supply apparatus  59  installed around the submersible body  40 , the pump  58  can be operated for discharging the seawater to the outer undersea. 
         [0164]    The discharge ability of the pump  58  is obtained corresponding to the available electricity supplied by the power supply apparatus  59  installed around the submersible body  40 . The amount of the seawater introducing from the sea surface to the seawater drop pipe  20  is limited to the corresponding amount that the pump can discharge by the seawater flow amount control part  14 . 
         [0165]    The amount of electricity obtained by the power supply apparatus  59  installed around the submersible body  40  can be improved along with the development of the tidal current power generation ability. 
         [0166]    Moreover, in addition to the power supply apparatus  59  installed around the submersible body  40 , a part of the electricity obtained by the generator  30  installed in the seawater drop pipe  20  can be utilized. 
         [0167]    In the above configuration, the power supply apparatus  59  can be used in parallel for enhancing the amount of electricity to be supplied to the pump mechanism. 
         [0168]    In the above configuration, a set of the pump  58  and the power supply apparatus  59  can be used in parallel for enhancing the ability of the forced discharge part  50   a.    
         [0169]    Moreover, in the above configuration, any one or the combination of the seawater intake structure  10 , the seawater drop pipe  20 , the power generators  30 , the submersible body  40 , and the forced discharge part  50  can be used in parallel. Moreover, the plural sets of marine power generation system  100   a  can be operated simultaneously. 
         [0170]    Moreover, the discharge by the piston-cylinder type seawater discharge mechanism shown in Embodiment 1 and the pump type seawater discharge mechanism can be combined. 
         [0171]    While some preferable embodiments of the marine power generation system according to the present invention are described above, it should be understood that various changes are possible, without deviating from the technical scope according to the present invention. Therefore, the technical scope according to the present invention is limited only by the attached claims. 
       INDUSTRIAL APPLICABILITY 
       [0172]    A marine power generation system according to the present invention can be used extensively for power generation system. For example, it can be applicable to the place where the tidal current is stable and relatively high velocity. The marine power generation system can be operated by floating the seawater intake structure  10  on the sea surface, sinking the submersible body  40  in the tidal current, and installing the seawater drop pipe  20  vertically. 
       EXPLANATION OF REFERENCE 
       [0000]    
       
           100 : marine power generating system 
           10 : water intake structure 
           11 : sea surface floatation object 
           12 : aperture part 
           13 : upper seawater guide part 
           14 : seawater flow amount control part 
           20 : seawater drop pipe 
           30 : power generator 
           40 : submersible body 
           41 : seawater tank space 
           42 : ventilation pipe 
           50 : forced discharge part 
           51 : cylinder 
           511 : the first cylinder 
           512 : the second cylinder 
           52 : piston 
           521 : seawater pass 
           53 : the first check valve 
           54 : the second check valve 
           55 : the third check valve 
           56 : power transmission mechanism 
           561 : propeller 
           562 : axis of the propeller 
           563 : conversion mechanism 
           57 : discharge part 
           58 : pump 
           59 : power supply apparatus
[Problem] To provide a marine power generation system which is engineered to be able carry out power generation stably and highly efficiently, using the large source of energy even in ocean currents having insufficient speeds to create a water current, similar to a water discharge from a dam, which is ideal for hydroelectricity. [Solution] As an example of a water intake structure ( 10), a sea surface floatation object, floats on the sea by buoyancy, and has an aperture part ( 12) through which seawater is taken therein. A water guide pipe ( 20) is underwater, which secures a space underwater. One end of the water guide pipe ( 20) is connected to the sea surface floatation object ( 10), and the seawater which is taken in from the sea surface floatation object which is the water intake structure ( 10), is introduced downward under the sea surface. Power generators ( 30) that generate power from either the falling motion of the seawater or water pressure are in the water guide pipe ( 20), and carry out power generation. A submersible body ( 40) is in the water, and an accumulation space ( 41) is provided that takes in the seawater which is discharged from another end of the water guide pipe ( 20). The seawater that is taken into the accumulation space ( 41) in the submersible body ( 40) is discharged outward from the submersible body ( 40) by a discharge part ( 50).