Abstract:
A solar thermal electric energy system utilizes a solar receptor having a double paned glass window with cooling liquid circulating therethrough. This serves as a window for a solar heat receptor cavity having a number of water or steam carrying tubes therein. A portion of the tubes operate at a temperature range suitable for heating boiler water and another portion of the tubes is at a higher temperature which serves as steam superheat. The liquid flowing through a pair of window panes is triethylene glycol and in addition to cooling the window has an index of refraction which matches the window to provide the most efficient conduction of solar energy to the heat recepting tubes behind the window. The power plant itself includes heat saving jackets around the steam headers to provide for boiler feed water preheat and has both high pressure and low pressure turbines to utilize most efficiently the water being heated by the solar receptors.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to a solar thermal electric power plant. Such a power plant normally includes a solar collector, such as heliostatic mirrors, which concentrates the reflected sunlight on a number of receptors mounted on towers. Some type of fluid is heated in the receptor and connected to a steam turbine type power plant. 
     Prior designs of solar electric plants have been of a relatively small scale and have not been competitive with either large scale fossil fuel or nuclear fuel power plants. Solar power has, of course, obvious advantages over fossil fuel plants in that there is no depletion of nonrenewable resources and over nuclear power plants from both contamination and safety aspects. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is, therefore, a general object of the present invention to provide an improved solar thermal electric power plant. 
     It is another object of the invention to provide a power plant which can be built at reasonable costs with available materials and conventional engineering design techniques. 
     In accordance with the above objects there is provided a solar energy system comprising solar collector means. Solar receptor means include a double paned window means with a flow space between the panes and positioned to receive solar energy from the solar collector means. Means provide for the flow of a cooling liquid in contact with the panes in the flow space. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified elevational view of solar collectors and a solar receptor embodying the present invention; 
     FIG. 2 is a detailed cross-sectional view of one embodiment of the solar receptor of FIG. 1; 
     FIG. 3 is a perspective view of an auxiliary structure of FIG. 2; 
     FIG. 4 is a detailed cross-sectional view of another embodiment of the solar receptor of FIG. 1; 
     FIG. 5 is a cross-sectional view taken along the line 5--5 of FIG. 1; 
     FIG. 6 is a schematic diagram of a power house used in the present invention; and 
     FIG. 7 is a schematic cross-sectional view of the fusion storage system used in the power house of FIG. 6; 
     FIG. 8 is a plan view of a strip type solar reflecting mirror; and 
     FIG. 9 is a plan view, partially cut away, of a strip type solar receptor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a simplified elevational view of a solar collector 10 which consists of a field of heliostatic (i.e., following the sun) tiltable mirrors 11 which focus and concentrate sunlight onto to a solar heat receptor 12 mounted on a tower 13. The configuration and design of the mirrors 11 in conjunction with the receptor 12 and tower 13 are described and claimed in a copending application in the name of the present inventor entitled Solar Collector System, filed Nov. 1, 1976, Ser. No. 737,489, now U.S. Pat. No. 4,117,682. In general, the mirrors face south and are automatically controlled to follow the movement of the sun to concentrate the maximum amount of solar energy during the day onto the solar heat receptor 12. 
     One form of receptor 12 is shown in cross-section in FIG. 2 and includes a double paned window 16 having separate glass panes 17 and 18 with a flow space 19 between the panes. The windows are typically of low iron glass and are coated on the outside, that is at the air glass interfaces 17a, 18a, with a magnesium fluoride optical coating to minimize reflection. The interpane space is filled with triethylene glycol (TEG) which enters the space at 21 and exits at 22. These inputs and exits are connected to the power plant to provide preheat for the power plant&#39;s boiler feedwater. 
     The preferred liquid to be used in the interpane space 19 is colorless so that it does not absorb energy from the sun and should have an index of refraction close to that of the panes 17 and 18. This allows the maximum amount of concentrated sunlight to enter the cavity area 23 behind the window 16. Thus, with a low iron glass window which has an index of refraction of approximately 1.50 one of the group of the following may be used (in addition to the TEG which has a refractive index of 1.456): butyrophenone, o-toluidine, chlorobenzene, diethylene glycol, triethylene glycol, tetraethylene glycol and trimethylene dibromide. In addition, a suitable mixture of two liquids can provide an equivalent refractive index. 
     Thus, in summary the double paned window structure performs a triple function: the interpane fluid provides valuable heat to the steam preheaters in the power house; the fluid cools the windows for extended life; and because of the relatively low temperature of the glass, the infrared and convection losses to the environment are greatly reduced. Moreover, because of the nearly equal refractive indices of the glass and fluid the reflection from the inside surfaces of the glass is negligible. Thus, the window transmits more than 95% of the solar energy to the heat receptors in the cavity 23. 
     Because of the importance of keeping the glass of the window cool the fluid pumping rate is controlled to hold the outlet temperature at 200° C. 
     The interpane fluid may also serve yet another purpose of cooling tower supports 13. The pipes 26 and 27 are in close thermal connection with the support tower (for example, welded to it) and are in series with the input union 21. 
     Cavity 23 is formed by the thermal insulation 28 which provide a semi-circular enclosure which terminates at the window 16. Within the cavity 23 are a number of water or steam carrying tubes which are coated with black heat absorbing material. The panel of tubes at the back of the cavity area 23 marked C and D are the main absorber of solar energy. The tubes in front designated A and B form a lower temperature screen and they intercept only about half of the incoming light. The visible light reflection from the front tubes is a major loss component and is kept low by the geometry of the cavity 23 and by a high absorption coating. Visible light reflected from the back panel is mostly absorbed by other tubes, but is also kept low by a high absorption coating. Specifically, tubes A and B are coated with a low infrared emmissivity intermetallic aluminum-nickel and C and D with a high visible absorption coating of intermetallic aluminum-chromium. The tubes are typically constructed of stainless steel. The infrared radiation from the back of the panel in the C and D tubes&#39; location is mostly absorbed by the walls, by other back tubes and by the front tubes. The infrared radiation by the front tubes A and B is absorbed by the window 17. However, this radiation is kept low by the relatively low tube temperature. For example, the incoming fluid such as water might enter at 320° C. from the power house, circulates through the A tubes and then through the B tubes, indicated and exits at 360° C. The low emissivity coating for infrared on the front tubes minimizes heat transfer from the front tubes to the window. 
     High pressure water flows out from the power house in the A tubes at a minimum temperature and at 200 atmospheres pressure. Such high pressure prevents the water from boiling. In the solar receptor module the water is passed from the A to the B tubes reaching a maximum temperature of 360° C. and returned from the B tubes to the power house. Fifty-five percent of the heat required by the thermodynamic system is provided by this water. The heat exchange fluids flowing outside of the receptors are in certain headers which are heavily insulated. Thus, the conductive heat loss is negligible, and the piping costs in addition to the receptors is low. 
     Dry steam flows out from the power house in the C tubes and returns through the D tubes. These are the tubes in the rear portion of the cavity 23 as opposed to the intermediate location of the A and B tubes. The radiation heat exchanges in the receptor cavity 23 are a net flow from the D tubes to the A and B tubes and the window. Thirty-one percent of the heat required by the thermo-dynamic system is provided by the returning dry steam in the D tubes which exits at 500° C. 
     Because the pressure is above atmospheric in the interpane fluid and the space 19, a static pre-stress frame illustrated in FIG. 3 may be placed in contact with the surface 18a of window 16. This prevents deflection of the window due to the triethylene glycol fluid changing its pressure. The frame of FIG. 3 consists of several supports with a single horizontal support but may include many other similar rigid configurations. 
     FIG. 4 shows a variation of the heat receptor of FIG. 2 with a rectangularly shaped cavity 23&#39;. However, the major change is a window 16&#39; which includes a plurality of sandwiched double paned windows with the cooling liquid flowing successively from the outer panes to the inner panes. Specifically, the first input at 29 is shown as being at 143° C. which passes through the first interpane space 19&#39; and exits at exit 31 at 165° C. It is then coupled to a fluid input 32 the fluid circulating through the interior interpane space 19&#34; exiting at 33 at 200° C. The advantage of this construction is that a larger temperature difference may be allowed between the input and outputs of the cooling fluid. In addition, since the outer pane facing the solar collectors can be relatively cooler less energy is re-emitted. 
     The embodiment of FIG. 4 also includes a cooling jacket formed of the pipes 34 embedded in thermal insulation 28&#39; through which the TEG liquid circulates. This provides for greater efficiency since heat normally lost from the cavity 23&#39; is utilized in the boiler water preheat function. 
     Because of the widely differing temperatures of both the water and super-heated steam circulating in the heat receptor it is most efficient to fully insulate all pipes leading to and from the tower 13 and moreover to utilize any heat loss that exists through the insulation as a preheat for lower temperature liquids. This is illustated in FIG. 5 which is a typical piping bundle to the tower or in the associated access tunnels. It shows the various piping A through F with the D piping, for example, at the relatively high temperature of 500° C. which is steam exiting the tower for the power plant. Several layers of thermal insulation 38 surround the pipes. In general the incoming pipes going to the tower are heated by the oppositely flowing liquid which is at a higher temperature. The various fluids and their temperatures are shown by the associated table in FIG. 5. 
     FIG. 6 illustrates a typical power plant using the heat receptors of either FIGS. 2 or 4. The various solar receptors are indicated as SR W , which refers to the solar receptor 16 or 16&#39;; in other words, the TEG liquid flowing through the double paned windows, and as solar receptors SR 1 , SR 2  and SR 3  which are water or steam tubes of increasingly higher temperatures which are taken from the appropriate sections of the heat receptor cavity 23. 
     Referring now to the specific building blocks of the power plant a warm feedwater reservoir unit 51 which is at approximately 50° C. has its output pressure raised to 200 psi by a first feedwater pump 52 and passes through a first preheater 53. This preheater is a heat exchanger for the low pressure TEG for SR W  which is the heat source. As indicated, the triethylene glycol exits preheater 53 at 140° C., is circulated through the window solar receptor and heated to 200° C. and returned to preheater, it thus acts as a heat source for raising the feed water temperature to 180° C. at the exit 54. A storage unit 56 is in parallel with the incoming and outgoing TEG pipes. 
     A second feedwater preheater 57 pumped by a second feedwater pump 55 is a condensing heat exchanger using turbine exhaust as indicated at 388° C. and 226 psi. A bleed from an intermediate stage of the low pressure turbine 58 is used at night and a bleed from an incoming line 59 through a heat storage unit 61 is used during the day. Such routing is done by valves with the indicated nomenclature for valve open in day only and valve open at night only. Heat storage unit 61 may use molten metal and provide for operation of the solar plant during peak energy hours. 
     A third heater 62 is actually the plant boiler which receives 360° C. water at 3,900 psi from the solar receptor field SR 1  and boils the water in the plant boiler to 335° C. at 2,000 psi as indicated at the exit 63. Thus, dry steam is produced. Solar receptor SR 1  referring to FIG. 2 includes the A tubes at its low temperature end and the B tubes at its high temperature end. Water is used as the heat exchange fluid in this stage because of the abundance of experience in the use of this conventional technology. A water storage unit 64 is provided in parallel between the input and output lines of the plant boiler 62. 
     The next stage is a super heater and includes the solar receptor SR 2  which has as its input steam from exit line 63 of the plant boiler 62 which flows into the C tubes, referring to FIG. 2, of the solar heat receptor, is heated to a higher temperature by the D tubes, then exits on the line 66 at 1,700 psi and 500° C. and supplies high pressure turbine 67. The exhaust 68 from high pressure turbine 67 which is at 394° C. and 700 psi is reheated in a solar receptor SR 3  to 500 psi, 500° C. on line 59 and used as the input to the low pressure turbine 58. This reheat is provided by a suitable bank of solar receptors especially for that purpose and is described in detail and claimed in the above copending application. 
     In order to provide for high system efficiency all of the stream headers have jackets 71, 72 and 73 surrounding them for intercepting any heat from the steam headers which would otherwise be lost. Fluid flow in the jackets is in the opposite direction to the steam flow in order to provide an uphill temperature gradient. The fluid from the jackets 71, 72 and 73 is the heat source of a heat exchanger 80 which is inserted immediately after feed water pump 52. Thus, the feed water is raised perhaps by 10° C. 
     The low pressure turbine 58 exhausts into a spray condenser 74 at 1.84 psi absolute and 50° C. Some of the warm condensate is stored in a warm feed reservoir 51. However, most of the warm condensate is pumped through a heat rejection exchanger 76, a cool feed water reservoir 77 and then by means of a pump 78 back to the condenser 74 serving as the liquid spray for the condenser at the spray input 79. The heat rejection exchanger 76 rejects its heat into a cool pond or reservoir 81 which is kept cool by a dry cooling tower 82. The pond can be large enough to have a time constant of approximately one week and is to be protected or shaded from solar radiation so that it does not heat up during the day. The dry cooling tower has an effective time of operation at night and during high wind velocity conditions. Gravity separation can be used in the reservoirs so that the coolest bottom layer is used in the heat rejection exchanger and the high temperature water is fed into the top layer. The tower pumps from the top layer and feeds into the bottom layer, as illustrated. The dry cooling tower does not need a high power fan because it is not required to reject heat during the day time when air temperatures are high. It operates primarily when the air temperatures are low, or when the wind-augmentation is sufficient. 
     In general, all of the flows and parameters of the power plant of FIG. 6 in conjunction with the solar receptors of FIGS. 2 and 4 can be efficiently controlled by an extremum control system. That is, for example, all of the flows of either steam or water through the solar heat receptors can be shifted slightly, the total output or efficiency of the system measured and if this is a positive change, the change can be made permanent. This can be done until the peak of the extremum curve is reached. The same can be done with other inter-related operating portions of the system such as the actual electrical output, and various steam inputs to the turbine. For example, first and second feedwater pumps 52 and 54 can be controlled to maximize the heat energy sensed by sensor means of the superheated steam at the throttle of the turbine. 
     Heat storage unit 61 is illustrated in greater detail in FIG. 7. A ceramic lined vat 90 contains a mixture of 30% Al and 70% Zinc at a nominal temperature of 510° C. A zone melting control unit has three sequence valves 91 which provide heating fluid to three associated coils 92 each in its own zone of the vat 90; i.e., the zones are of increasing depth. The valve opening sequence in the daytime starts with valve 1 with the most shallow coil etc. A zone freezing control unit 93 has three similar valves for recovering heat during the night time. 
     FIGS. 8 and 9 illustrate a strip type of solar reflecting mirror 100 (FIG. 8) which is associated with a strip type heat receptor (FIG. 9). Such receptor is similar in construction to the receptors of FIGS. 2 and 4 except that it may be 100 meters long. This type of construction is easy to build and is believed to be more efficient for smaller power plants. 
     Thus, an improved solar electric system has been provided which is highly efficient and practical.