DESCRIPTION [Title of Invention] SOLAR HEAT COLLECTOR, SOLAR THERMAL POWER GENERATION SYSTEM, AND SEAWATER DESALINATION SYSTEM [Technical Field] [0001] The present invention relates to a solar heat collector that collects thermal energy of sunlight. [Background Art] [0002 Utilization of so-called renewable energy, such as sunlight, solar heat, wind power, geothermal heat, and tidal power, has attracted attention in relation to recent problems of depletion of fossil fuel, a rise in crude oil prices, review of energy security, and global warming. In the world, European countries have taken the lead in adopting a national policy of spreading renewable energy as an alternative to nuclear power generation and thermal power generation. [0003] Among renewable energy, sunlight and solar heat are highly effectively utilized in areas where an amount of sunlight more than or equal to a certain amount is obtained throughout the year. Particularly in areas where sunlight can be stably obtained throughout the year, such as Southern Europe, Middle East, and North Africa, utilization of solar - 1heat as energy has been shifting to experimental and practical stages. In Spain, solar thermal power generation plants have already started running. [0004] In solar thermal power generation, energy from the sun is concentrated 30 to 800 times by a reflecting mirror or the like, and the collected energy is converted into thermal energy by using a heat exchange medium or the like. Steam is generated by the thermal energy to generate power in a steam turbine. Solar thermal power generation is roughly divided into a trough type, a Fresnel type, a tower type, and a beam-down type according to the differences among light collecting methods, specifically, among light collecting devices. While the trough type is commercially used at present, the Fresnel type and the tower type are considered promising in terms of power generation efficiency and initial investment. In Japan, beam-down sunlight collecting systems are being developed as a further improvement on tower type systems. The beam-down systems have advantages of high heat collection efficiency and simple system configuration. [0005] In these systems, a device called a solar heat collector is necessary to convert sunlight collected by the light collecting device into thermal energy. PTL 1 and PTL - 2- 2 disclose examples of solar heat collectors for tower type light collecting systems. PTL 1 describes the structure of a heat collection panel that constitutes a solar heat collector, and PTL 2 describes the material and structure of a heat collection panel in which a plurality of heat collection tubes are combined. [0006] PTL 3 and PTL 4 disclose examples of solar heat collectors for beam-down sunlight collecting systems. A low-melting-point metal is held as a heat exchange medium in a heat-resistant container, a black carbon material (PTL 3) or a light absorption copper plate (PTL 4) is floated on a surface of the metal, and sunlight absorbed by the material or the plate is transferred into the heat exchange medium. A heat exchange tube is laid in the heat exchange medium, water is conducted through the tube, and steam is generated by heat from the heat exchange medium. [0007] PTL 5 and PTL 6 disclose other examples of solar heat collectors. PTL 5 discloses a solar heat collector including a cavity-shaped housing and a heat collecting tube laid all over an inner surface of the solar heat collector, and a solar heat collector in which a heat exchange medium flows down as a liquid film along an inner surface of a substantially barrel-shaped or substantially trumpet-shaped - 3 housing. However, the cavity-shaped solar heat collector is difficult to shape, and the solar heat collector in which the liquid film flows down has a problem in that entry of dust from the outside is inevitable. Further, PTL 6 discloses a solar heat collector shaped like a basket or a frustum in which modules including substantially trapezoidal heat collecting tubes are combined. However, an object of the invention of PTL 6 is to quickly recover a heat exchange medium circulating in the solar heat collector in case of an emergency, and. this object is different from the object of the present invention. [Citation List] [Patent Literature] [0008] [PTL 1] W02007/088031 [PTL 2] U.S. Patent No. 5,862,800 [PTL 3] Japanese Unexamined Patent Application Publication No. 2010-85079 [PTL 4] Japanese Unexamined Patent Application Publication No. 2010-85080 [PTL 5] W02006/025449 [PTL 6] Japanese Unexamined Patent Applicaiton Publicaiton No. 2011-43128 [Summary of Invention] [Technical Problem] -4- [0009] The present invention has been made under these circumstances, and an object of the invention is to provide a technique of obtaining high heat collection efficiency with a simple structure in a solar heat collector for use in a solar energy utilization system. [Solution to Problem] [0010] A solar heat collector according to the present invention collects sunlight reflected by a reflecting portion and converts the sunlight into heat, and includes a housing formed of an insulating material and having an opening portion through which the sunlight is incident, the opening portion being provided on a side close to the reflecting portion, and a heat collection passage provided on a bottom face portion of the housing such that a heat exchange medium is conducted therethrough to receive the sunlight incident through the opening portion and to convert the sunlight into the heat. The housing is structured such that an optical path of the sunlight incident from the reflecting portion through the opening portion reaches only the bottom face portion. A peripheral side surface in the housing may be inclined from an edge of the opening portion or a portion near the edge to a peripheral edge of the bottom face -5portion. The opening portion and the bottom face portion of the housing may be shaped like a circle or a regular polygon. A ratio r 2 /r 1 of the maximum distance r 2 between two points at the peripheral edge of the bottom face portion to the maximum distance r, between two points at the edge of the opening portion may be 1 to 2. The heat collection passage may be provided all over the bottom face portion. An auxiliary reflecting portion may be provided at a position opposing the opening portion to reflect the sunlight from the reflecting portion into the opening portion. A sub-heat collection tube through which the heat exchange medium is conducted to receive radiant heat from the heat collection passage may be provided on a peripheral side surface in the housing. [0011] A solar thermal power generation system according to the present invention includes the above-described solar heat collector, a steam generation mechanism that generates steam from thermal energy collected by the solar heat collector, and a steam turbine mechanism that is driven by the generated steam to generate power. [0012] -6- A seawater desalination system according to the present invention includes the above-described solar heat collector, a steam generation mechanism that generates steam by thermal energy collected by the solar heat collector, a passage member through which the generated steam is conducted, a seawater distillation unit that evaporates seawater by heat from the passage member, and a desalination cooling unit that cools the evaporated seawater into fresh water. [Advantageous Effects of Invention] [0013] According to the present invention, in the solar heat collector, sunlight reflected by the reflecting portion is caused to be incident on the bottom face portion of the housing through the opening portion at the top of the housing so as to heat the heat exchange medium in the heat collection passage provided on the bottom face portion, and an optical path of the sunlight incident from the reflecting portion through the opening portion reaches only the bottom face portion. For this reason, light beams having high solar energy density do not touch the peripheral side surface. Hence, the light beams reach the bottom face portion without any energy loss, and heat can be collected from the sunlight to the heat exchange medium with high efficiency. [Brief Description of Drawings] -7- [0014] [Fig. 1] Fig. 1 is an external perspective view of a solar heat collector 1 according to a first embodiment of the present invention. [Fig. 21 Fig. 2 is an external plan view of the solar heat collector 1 of the embodiment. [Fig. 31 Fig. 3 is a front view of the solar heat collector 1 of the embodiment. [Fig. 4] Fig. 4 is a transverse sectional plan view of the solar heat collector 1 of the embodiment. [Fig. 5] Fig. 5 is a partly cutaway perspective view of the solar heat collector 1 of the embodiment. [Fig. 6] Fig. 6 is an enlarged schematic view of a section around a bottom face portion of a housing 11 in the solar heat collector 1 of the embodiment. [Fig. 7] Fig. 7 is a cross-sectional side view of the solar heat collector 1 of the embodiment. [Fig. 8] Fig. 8 schematically illustrates an application of the solar heat collector 1 of the embodiment to a beam down sunlight collecting system. [Fig. 9] Fig. 9 is an operation view illustrating an energy distribution of the solar heat collector 1 of the embodiment. [Fig. 101 Fig. 10 schematically illustrates an application of a solar heat collector 1 according to a - 8 second embodiment. [Fig. 11] Fig. 11 is an external perspective view illustrating a modification of the housing 11 in the solar heat collector 1. [Fig. 12] Fig. 12 is an external perspective view illustrating a modification of the housing 11 in the solar heat collector 1. [Fig. 13] Fig. 13 includes an external perspective view and cross-sectional side views illustrating modifications of the housing 11 in the solar heat collector 1. [Fig. 14] Fig. 14 is a transverse sectional plan view illustrating another exemplary arrangement of a heat collection tube 15 in the solar heat collector 1. [Fig. 15] Fig. 15 is schematically illustrates an application of the solar heat collector 1 to a tower-type sunlight collecting system. [Fig. 16] Fig. 16 schematically illustrates an application of the solar heat collector 1 to a steam power generation system. [Fig. 17] Fig. 17 schematically illustrates an application of the solar heat collector 1 to a seawater desalination system. [Description of Embodiments] [0015] [First Embodiment] -9- As illustrated in Fig. 1, a solar heat collector according to an embodiment of the present invention includes a housing 11. As illustrated in a plan view of Fig. 2 and a side view of Fig. 3, an upper surface of the housing 11 is formed as an opening portion 12 having a circular aperture, and a bottom face portion 14 on an inner side of the housing 11 is square. The size of one side of the square that forms the bottom face portion 14 is set to be more than the diameter of the opening portion 12, and a peripheral side surface extending from an edge (aperture edge) of the opening portion 12 to the bottom face portion 14 is formed as an inclined surface 19. Such a shape of the housing can be formed by connecting equal division points for dividing the circle of the opening portion 12 into n-number of equal parts to corresponding equal division points for dividing the square of the bottom face portion 14 into n-number of equal parts by straight lines and interpolating gaps between the straight lines. The number n is a natural number, and preferably, the number of equal division points is as large as possible. The inclined surface 19 formed in this way is a curved surface formed by straight lines, and is used in, for example, the field of architecture. The internal shape (three-dimensional outline) of the housing 11 corresponds to the external appearance. As the - 10 material that forms the housing 11, a heat insulating material, such as ceramic fiber, is used. As the heat insulating material, a material having a thermal conductivity of 0.2 W/(mK) or less is given as an example. [0016] All over the bottom face portion 14 of the housing 11, a heat collection passage, such as a heat collection tube 15, through which a heat exchange medium 16 is conducted to receive sunlight incident from the opening portion 12 and to convert the sunlight into heat is provided. In relation to the piping layout of the heat collection tube 15, for example, when one end side and the other end side of one side 14a of the bottom face portion 14 of the housing 11 in Fig. 4 are designated as a left side and a right side, respectively, two heat collection tubes 15a and 15b juxtaposed in a right-left direction penetrate, from the outside, through a portion of the peripheral side surface corresponding to the center of the one side. Also, one of the heat collection tubes, that is, the heat collection tube 15a is routed in a left half area of the bottom face portion 14, and the other heat collection tube 15b is routed in a right half area of the bottom face portion 14. [0017] The routing layout of the heat collection tube 15a will be described in detail with reference to Figs. 4 and 5. The - 11 heat collection tube extends straight frontward from the one side along the bottom face portion 14, bends at a side (opposing side) 14b opposing the one side, and extends straight toward the one side. By repeating such a bending pattern, the heat collection tube 15a is routed to the left side, and covers the left half area of the bottom face portion 14. After the heat collection tube 15a reaches a left inner portion of the bottom face portion, as viewed from the center of the one side, it is bent upward, is routed along three sides of the left area of the bottom face portion 14 above the layout area of the heat collection tube 15a that covers the left half area, and is then routed outside from the peripheral side surface corresponding to the opposing side. [0018] For example, after being bent upward, the heat collection tube 15a is routed along the sides of the square from the left inner portion of the bottom face portion 14 to the center of the one side, is further bent upward, and is routed along the three sides of the left area. Since the heat collection tube 15 is symmetrically laid out, the other heat collection tube 15b is similarly routed. The routing area where the heat collection tube 15 is routed before being bent upward, that is, the routing area where the heat collection tube 15 is laid along the bottom - 12 face portion 14 functions as a main heat collection tube that is directly irradiated with sunlight to heat a heat exchange medium. The routing area where the heat collection tube 15 is bent upward and is routed along the sides of the bottom face portion (along the peripheral side surface) at a position higher than the main heat collection tube functions as a sub-heat collection tube to be heated by radiant heat from the main heat collection tube. [00191 When the heat collection tube is thus laid on the flat surface, production is facilitated and the production cost can be reduced, compared with a case in which theheat collection tube forms a three-dimensional shape like the cavity-shaped light collector disclosed in PT x. As the material of the heat collection tube 15, a material that is not highly reactive to the heat exchange medium 16 conducted therein and has a high thermal conductivity is selected. Specifically, a material having a thermal conductivity of 16 W/(m-K) or more, such as stainless steel, is selected as an example. For example, molten salt is used as the heat exchange medium 16. For example, the temperature of the heat exchange medium flowing out from the housing 11 during operation of the solar heat collector 1 ranges from 150 to 800 0 C. As the molten salt, nitrate, such as potassium nitrate or sodium nitrate, is - 13 given as an example. In this case, for example, the temperature of the heat exchange medium, that is, nitrate flowing out from the housing ranges from 150 to 6000. Preferably, a heat insulator is provided around a lower half of the heat collection tube 15. For example, a heat insulator 17 may be laid on the bottom face portion 14 of the housing 11 to cover the lower half of the heat collection tube 15, as illustrated in Fig. 6, or the lower half of the heat collection tube 15 may be directly buried in the bottom face portion 14 formed of a heat insulating material. In contrast, when the upper half of the heat collection tube 15 and a front surface of the heat insulator 17 to be irradiated with light are colored in black, for example, by applying a heat-resistant black coating agent thereon, high heat collection efficiency can be obtained. [0020] Further, the structure of the housing 11 and dimensions of the parts will be described with reference to Fig. 7. When a diameter of the opening portion is taken as r, as illustrated in Fig. 7, a diameter r 2 (diagonal length) of the bottom face portion 14 of the housing 11 is set to be r, to 2ri. The peripheral side surface 19 in the housing 11 is set such that light does not reach the peripheral side surface 19, regardless of the incident point and incident - 14 angle of the light incident on a below-described reflecting mirror. In other words, the inclination angle of the peripheral side surface 19 is set to provide an angle between the peripheral side surface and a horizontal surface in the housing smaller than the minimum incident angle of the light into the housing 11 so that the peripheral side surface 19 of the housing 11 is not visible, regardless of the portion of the reflecting mirror viewed by the human eyes and the viewing angle of the human eyes. [0021] For example, the condition of the diameter of the bottom face portion and the range of a depth h are set as follows. As an example, to heat a heat exchange medium (specific heat C) flowing at f (kg/hr) from T, to T 2 (OC) , a thermal energy Q (MW) should be absorbed by the heat exchange medium. In contrast, when a light collecting mirror having a focal point is used, for example, a solar energy of Qo (MW) is supplied to a circular area having a radius R from the focal center and an area of So (M 2 ) in a focal portion (the center portion of the opening portion of the housing in the embodiment). The density of the solar energy in the focal portion is the highest at the focal center and decreases toward a peripheral edge portion. As illustrated in a graph of Fig. 9 described below, the relationship between the separate - 15 distance from the center of the opening portion 12 and the density of the solar energy is represented by a curve shaped like a wide-based mountain. As the position is shifted from the focal portion in a light traveling direction, the mountain-shaped portion is leveled, the energy density in the center portion decreases, and the area where the solar energy is supplied increases. [0022] As a result, at a position shifted from the focal portion by h, in the light traveling direction, the area where the solar energy of Qo (MW) is supplied becomes S(> SO) (M 2 ). Therefore, the layout area of the heat collection tube 15 at the depth h is set to include an irradiation area (an area of the bottom face portion irradiated with light incident from the opening portion) having the area S, in which the solar energy Q (MW) incident from the opening portion is greater than the thermal energy Q (MW) capable of raising the temperature of the heat exchange medium to a desired temperature (Qo > Q). By considering the heat resistance of the heat collection tube 15, the energy absorption efficiency and thermal conductivity of the heat collection tube 15, the heat transfer coefficient between the heat collection tube 15 and the heat exchange medium 16, and the heat transfer - 16 loss and radiant heat loss from the heat collection tube 15 to the atmosphere in the housing 11 and comprehensively considering the ease of production, the shape, area (S'), and depth h from the opening portion of the layout area of the heat collection tube 15 are determined. When the depth h and the shape of the bottom face ("diameter of the bottom face portion") are designed under this condition, the slope of the peripheral side surface is determined so that light incident from the opening portion does not impinge on the peripheral side surface. As is clear from the above-described distribution of the solar energy, the solar energy in the area near the peripheral edge portion is less than in the area near the center portion, when viewed in the transverse section of the housing 11. Hence, even if the heat collection tube 15 is provided on the peripheral side surface of the housing 11, the heat collection efficiency is low. Accordingly, in this embodiment, roughly speaking, a group of light beams having a high solar energy density are guided to the bottom face portion 14 of the housing 11 where the heat collection tube 15 is laid, while avoiding energy loss due to application of the light beams on the peripheral side surface of the housing 11. This achieves a high heat collection efficiency. As an example, in a solar heat collector that can collect a thermal energy of 50 MW, when the diameter ri of - 17 the opening portion 12 is taken as 10 m, h is 6 m and the diameter r 2 of the bottom face portion 14 is 10 to 20 m. [0023] Further, the blackness of the peripheral side surface in the housing 11 is preferably increased, for example, by applying a heat-resistant black coating agent onto the peripheral side surface. [0024] While the structure of the solar heat collector 1 has been described above, an example in which the solar heat collector 1 is actually used as a part of a sunlight collecting system as illustrated in Fig. 8, and the operation of the solar heat collector 1 will be described. [0025] Fig. 8 illustrates a case in which the above-described solar heat collector 1 is used as one mechanism in a beam down sunlight collecting system. As to the structure, a plurality of heliostats 21 (primary sunlight reflecting mirrors) are installed on the ground, and a light collecting mirror 22 (secondary sunlight reflecting mirror) is provided immediately above the solar heat collector 1. The heliostats 21 are set to reflect sunlight from the sun S to the light collecting mirror 22, and the light collecting mirror 22 is set to further reflect the sunlight into the opening portion 12 of the solar heat collector 1. - 18 - More specifically, the light collecting mirror 22 is shaped like an ellipsoid or a hyperboloid (an ellipsoid in the example of Fig. 9), and is adjusted and set so that a second focal point F of the ellipsoid or the hyperboloid is located in a plane of the opening portion 12 of the solar heat collector 1, for example, at the center of the plane. In contrast, the heliostats 21 change their positions by changing the angle according to the time or latitude so that reflected light passes through a first focal point 3 of the above-described light collecting mirror 22. As illustrated in Fig. 8, it is geometrically clear that the sunlight from the sun S passing through the first focal point 3 and reflected by the light collecting mirror 22 passes through the second focal point F. [0026] In this way, the sunlight from the sun S is collected at the second focal point of the light collecting mirror 22, that is, near the opening portion 12 of the solar heat collector. By thus concentrating the sunlight, the area of the opening portion 12 can be reduced. Fig. 9 illustrates a measurement result of the energy density in the solar heat collector 1 in the actual system. This figure shows that the energy density is particularly high around the second focal point F. [0027] - 19 - The sunlight collected by the light collecting mirror 22 and entering a housing interior 13 does not directly reach the peripheral side surface 19 in the housing 11, reaches the bottom face portion 14, and is absorbed by the heat collection tube 15. In this example, the solar energy is absorbed with high efficiency because the surface of the heat collection tube 15 is covered with a black coating. Also, since the lower side of the heat collection tube 15 is insulated from heat, the loss of radiant heat to the bottom face portion 14 of the housing 11 can be reduced. The solar energy absorbed by the heat collection tube 15 on the bottom face portion 14 of the housing 11 is converted into heat, and increases the temperature of the heat collection tube 15. Then, the heat collection tube 15 transfer the heat to the heat exchange medium 16 conducted therein. The heat collection tube 15 (main heat collection tube) on the bottom face portion 14 of the housing 11 emits a part of the absorbed energy as radiant heat into the housing interior 13, and a part of the heat collection tube 15 (sub heat collection tube) provided on the peripheral side surface near the bottom face portion recovers the radiant heat. In this way, the heat exchange medium 16 flowing in the housing 11 converts the solar energy into heat with high efficiency, and flows out from the housing 11. For example, the sub-heat collection tube is laid in a range - 20 corresponding to a lower 20% part of the peripheral side surface. Since the peripheral side surface 19 in the housing interior 13 is covered with a black coating, reflection of the heat radiated from the heat collection tube 15 is suppressed, and the radiant heat loss from the opening portion 12 is reduced. [0029] For example, the temperature of the heat exchange medium 16 in the heat collection tube 15 rises from 350 0 C to 550 0 C while the heat exchange medium 16 flows around in the housing interior 13. In this way, the solar energy is finally converted into thermal energy and is stored in the heat exchange medium 16, and the thermal energy can be utilized. [0030] According to the above-described embodiment, in the solar heat collector, sunlight reflected by the reflecting portion is caused to be incident on the bottom face portion 14 of the housing 11 through the opening portion 12 at the top of the housing 11 and to reach only the bottom face portion 14 so as to heat the heat exchange medium 16 in the heat collection passage provided on the bottom face portion 14. For this reason, since a group of light beams having a - 21 high solar energy density do not touch the peripheral side surface 19, they reach the bottom face portion 14 without energy loss. Thus, heat from the sunlight can be collected to the heat exchange medium 16 with high efficiency. While the energy convects in the housing 11 and the energy is radiated from the portion near the opening portion 12, since the peripheral side surface 19 is inclined, a configuration factor Gij of the peripheral side surface 19 to the opening portion 12 (the ratio of energy reaching the opening portion 12 to the energy radiated from the peripheral side surface 19 in this case) decreases, and the loss of radiant heat from the opening portion is reduced. [0031] By aligning the height of the opening portion 12 with the position of the second focal point F where the area of the light beams incident from the reflecting portion becomes the smallest, the area of the opening portion 12 can be minimized. As a result, it is possible to reduce convection loss in which gas heated in the housing 11 flows to the outside and radiation loss in which energy radiated from the heat collection tube 15 on the peripheral side surface 19 and the bottom face portion 14 in the housing 11 is radiated to the outside through the opening portion 12. [0032] Since the housing serving as a body of the solar heat - 22 collector has a simple structure and the heat collection tube can be easily laid out, as described above, cost reduction in terms of production, repair and maintenance can be expected. Further, since the opening portion is narrower than the bottom face portion, entry of dust and the like into the housing can be suppressed. [0033] [Second Embodiment] A solar heat collector according to a second embodiment of the present invention will be described below. Fig. 10 illustrates a solar heat collector 1 of the second embodiment, in which a secondary light collector 23 for reflecting light around an opening portion 12 into the opening portion 12 is provided on an outer peripheral edge of the opening portion 12. A focal point F 2 of the secondary light collector 23 is set near the center of the opening portion 12. By collecting light in an area in the secondary light collector 23 where the luminous density is not relatively low and concentrating the light at the center portion of the opening portion 12, the luminous density in the center portion can be further increased, and the area of the opening portion 12 can be reduced. Therefore, the heat transfer area is reduced, and the total size of the housing can be reduced. [0034] - 23 - [Modifications] Both the opening portion 12 and the bottom face portion 14 of the housing 11 may be circular, as illustrated in Fig. 11. In this case, the housing 11 is shaped like a truncated cone in appearance. Alternatively, both the opening portion 12 and the bottom face portion 14 of the housing 11 may be shaped like a regular polygon, for example, a regular hexagon, as illustrated in Fig. 12. In this case, the housing 11 is shaped like a truncated pyramid in appearance. Further alternatively, the housing 11 may be cylindrical in appearance, as illustrated in Fig. 13(a), and the inner space may be shaped like a truncated cone, as illustrated in Fig. 13(b). For example, even when the external shape is formed as illustrated in Fig. 13(a) and the inner space is shaped like a circular column as illustrated in Fig. 13(c), the housing 11 may have that structure as long as the optical path of light incident from the opening portion 12 reaches only the bottom face portion 14. Alternatively, a housing 11 including a polygonal opening portion 12 and a circular bottom face portion 14 may be used. [0035] As to the arrangement layout of the heat collection tube 15, for example, the heat collection tube 15 may be spirally routed from an outer peripheral side on a circular - 24 bottom face portion, be extended from the center portion to the peripheral side surface, and be led out of the housing, as illustrated in Fig. 14. The passage in which the heat exchange medium 16 is conducted is not limited to a tubular body, and a flat panel body formed in conformity of the shape of the bottom face portion 14 may serve as the passage. The passage of the heat collection tube 15 is preferably provided all over the bottom face portion 14 of the housing 11. Here, the expression "all over the bottom face portion 141" includes, for example, a case in which a space is necessarily formed in a peripheral passage when the heat collection tube 15 is routed, and also includes a structure in which the heat collection tube 15 is regarded as being provided almost all over the bottom face portion 14, because this structure can obtain the advantages of the present invention. While an upper end of the inclined peripheral side surface 19 (side surface) of the housing 11 is not limited to the peripheral edge of the opening portion 12, a flat upper face portion may be provided around the opening portion 12 (for example, the housing 11 has an external appearance illustrated in Fig. 13(a)). In this case, while the upper end of the peripheral side surface 19 is preferably provided near the peripheral edge of the opening - 25 portion 12, the present invention includes a case in which the upper end is not provided near the peripheral edge. The bottom face portion 14 of the housing 11 does not always need to be flat. For example, it is conceivable to curve the center side of the tube laid on the bottom face portion downward and connecting a drain line to a lower end of the curve so that the heat exchange medium is easily drained, for example, during maintenance. [0038] In the above-described embodiments, light is absorbed by the main heat collection tube provided on the bottom face portion 14 of the housing 11 and the side surface near the bottom face portion, and radiant heat from the main heat collection tube is absorbed by the sub-heat collection tube provided in the portion near the side surface and apart from the bottom face portion. According to the mode, it is conceivable that thermal energy radiated from the main heat collection tube 15 is greater than radiant heat that can be absorbed by the sub-heat collection tube. In such a case, the sub-heat collection tube does not always need to be provided. Even when the main heat collection tube is laid on the bottom face portion, a part of the tube near the side surface provided outside the above-described irradiation area may radiate more heat toward the side surface than - 26 absorbed solar energy. In such a case, the main heat collection tube does not need to be provided near the side surface. [0039] Apart from these, according to future technological advances in the material field, it is conceivable to avoid loss of convective heat from the opening portion 12 of the solar heat collector 1 by placing a film or thin plate formed of a highly heat-resistant transparent material at the opening portion. Since the internal temperature of the housing interior 13 becomes 500 0 C or more when the opening portion 12 is closed, the above structure is not realized by existing materials. However, it is considered that the structure can be realized by future advances in the material field. There has already been developed a material called a selection absorption film that selectively absorbs visible light and light with wavelengths around the visible light but does not easily emit radiant heat. At present, the heatproof temperature of this material during use in the atmosphere is about 3000C. However, if an improved material having a higher heatproof temperature is developed in the future, sunlight can be more efficiently absorbed by evaporating a film of the material on the surface of the heat collection tube 15, and heat radiation due to radiant - 27 heat can be reduced. This is presumed to enhance the efficiency as a whole. [ 004 01 [Applications of Solar Heat Collector of the Invention] Applications of the above-described solar heat collector are illustrated in Figs. 15, 16, and 17. Fig. 15 illustrates an application of the solar heat collector to a tower-type sunlight collecting system. The solar heat collector 1 is held in the air by using support devices 24 in a manner such that the opening portion 12 faces down. Sunlight is reflected by heliostats 21 installed on the ground, and the reflected light directly enters the opening portion 12 of the solar heat collector 1. Similarly to the above-described embodiments, the height of the solar heat collector 1 from the ground and the setting of the heliostats 21 are determined so that only the heat collection tube 15 laid on the bottom face portion 14 of the housing 11 receives sunlight reflected by the heliostats 21, but the light is not applied to the peripheral side surface in the housing interior 13. Unlike the application to the beam-down sunlight collecting system illustrated in Fig. 8, the reflecting mirror 22 can be omitted in this application. Hence, this application is effective in a case in which it is difficult to install the light collecting mirror 22, for example, in - 28 an area where a gust of wind occurs. [0041] Fig. 16 schematically illustrates an application of the solar heat collector 1 to a steam power generation system. Operation of the system will be described with reference to this figure. A heat exchange medium that stores thermal energy in the solar heat collector 1 is first conducted from the solar heat collector 1 to a steam generator 32 through a heat exchange medium pipe 31. A pipe including water passes through the steam generator 32, and heat exchange between the heat exchange medium and the water is performed in the steam generator 32. Then, the water is converted into steam, and the steam reaches a steam turbine 36 through a steam pipe 35 to drive the steam turbine 36, so that power is generated by a power generator 4 connected to the steam turbine 36. The steam discharged from the steam turbine 36 is cooled into a steam condensate by a cooler 37. The steam condensate flows through a steam condensate pipe 38, and enters the steam generator 32 via a valve 39. Then, the steam condensate is converted into steam again, drives the turbine, and is cooled into a steam condensate. These processes are repeated. In contrast, the temperature of the heat exchange medium subjected to heat exchange in the steam generator 32 is decreased by heat exchange. The heat exchange medium - 29 enters the solar heat collector 1 via a valve 34. The heat exchange medium is converted into a high-temperature heat exchange medium again, is cooled by heat exchange in the steam generator 32, and is returned to the solar heat collector 1. These processes are repeated. This steam power generation system can be an effective alternative to thermal power generation and nuclear power generation. [0042] Fig. 17 schematically illustrates an application of the solar heat collector 1 to a seawater desalination system. Operation of the system will be described with reference to this figure. First, a heat exchange medium that stores thermal energy in the solar heat collector 1 is conducted from the solar heat collector 1 to a steam generator 32 through a heat exchange medium pipe 31. A pipe through which water is conducted passes through the steam generator 32, and heat exchange between the heat exchange medium and the water is performed in the steam generator 32. The water is converted into steam, and enters a seawater storage tank 52 storing seawater through a steam pipe 51. The steam pipe 51 is laid around in the seawater storage tank 52, and the temperature of the seawater is increased by heat of the steam. The steam pipe 51 extended out of the seawater storage tank 52 enters a heat exchanger 53 through - 30 a valve 58. A seawater pipe 55 through which seawater is conducted passes through the heat exchanger 53, and heat exchange is performed between the steam in the steam pipe 51 and the seawater in the seawater pipe 55. By this heat exchange, the steam in the steam pipe 51 is cooled into a steam condensate, and is returned to the steam generator 32 through a steam condensate pipe 54. In contrast, the seawater in the seawater pipe 55 is guided into the seawater storage tank 52 at an increased temperature. In this way, the seawater in the seawater storage tank 52 is heated by two methods. As a result, the seawater is evaporated, and the generated steam is conducted to a steam pipe 59. The steam pipe 59 is guided into a cooler 56, and the steam therein is cooled by the cooler 56, so that fresh water can be obtained. When the amount of seawater in the seawater storage tank 52 exceeds a predetermined amount, the seawater is discharged from a pipe 57, and is cooled into seawater of room temperature by the cooling unit 56. This seawater desalination system is expected to be useful particularly in a coastal area where the rainfall is light and the sunlight amount is large. Further, the seawater desalination system can be combined with the above described solar thermal power generation system. [0043] It is conceivable to apply the above-described solar - 31 heat collector to a refrigerating apparatus. Specifically, in a so-called evaporator in the refrigerating apparatus, the above-described solar thermal energy is thermally transferred to coolant in the refrigerating apparatus to vaporize the coolant. [Reference Signs List] [0044] S: sun 1: solar heat collector 11: housing 12: opening portion 15, 15a, 15b: heat collection tube 16: heat exchange medium 21: heliostat 22: reflecting mirror 23: secondary light collector 32: steam generator 36: steam turbine 37: cooler 4: power generator 52: seawater storage tank 53: heat exchanger 56: cooler F: second focal point -32 - [0044] It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. [0045] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. - 32A -