COLLECTION OF SOLAR RADIATION AND ITS CONVERSION INTO ELECTRICAL POWER

Abstract: Apparatus and methods for collecting and concentrating solar radiation for the generation of electrical power, the apparatus (1) comprising a rotatable reflector dish (3) which is pivoted to one side so as to be positioned between a vertical position or lowered to a horizontal position. When horizontal, the dish is protected by a surrounding weathershield (13) for protection against high winds. The reflector dish (13) is preferably parabolic and mounted so as to concentrate and reflect the solar radiation into a receiver (5).

Description: COLLECTION OF SOLAR RADIATION AND ITS CONVERSION INTO ELECTRICAL POWER This invention relates to apparatus for collecting and concentrating solar radiation for the generation of electrical power. In particular, the invention particularly concerns the mounting of a solar reflector dish. In addition, features of the construction of a solar radiation receiver and of a parabolic reflector dish are also described. Systems are known for the generation of electrical power through the conversion of thermal energy produced by the concentration of solar energy by a suitable reflector, such as parabolic trough systems, planar mirror array systems and parabolic reflector dish systems. In a parabolic reflector dish system, one or more parabolic dishes, each having a reflective surface, are driven in azimuth and elevation so as to track the diurnal and seasonal movement of the sun in order to collect and concentrate solar radiation in or on a suitable receiver. At the receiver, the thermal energy produced by the concentration of solar radiation is usually conducted away from the receiver to a heat engine, generator or the like for the production of electrical power. In order for a parabolic reflector dish system to be of greatest effect, the area of the parabolic reflector dishes must be as large as possible. This is achieved either by having a large number of small reflector dishes or by having one, or only a small number of, very large reflector dish. In systems comprising many reflector dishes, the cost of providing an equivalent number of drive systems so that each dish accurately tracks the movement of the sun becomes prohibitively expensive. Accordingly, large parabolic reflector dishes are frequently employed. Unfocussed solar radiation has a peak solar flux of 1000W/m 2 at ground level. At a latitude of between 30° and 35°, the average power of unfocussed solar radiation during each day is approximately 700W/m 2 . At a typical system efficiency of about 30% therefore, in order to produce 1 Mw of electrical power, a parabolic reflector dish of about 4- 5000m 2 , or of about 75m diameter would be required. In order to mount and to move such a dish so as to track the sun a large dish support structure is required. However, a large dish presents a considerable cross-section to any wind, and so as to remain accurately aligned to the sun, the dish support structure must be made sufficiently strong to resist wind forces. In very high winds, both the dish and the support structure are liable to be damaged, thus both need to be strengthened to resist the effect of occasional high winds. Reinforcing the dish and the support structure increases the mass of the dish which leads to an increase in their cost and also in the costs of the systems which move the dish so as to track the sun. It is also usual for parabolic dish reflectors to be gimbally mounted, i.e. so as to pivot about an axis lying in a plane at the centre of the dish. Such a mounting enables the dish to be moved in both azimuth and elevation using simple drive means, and the mass of the dish can be counterbalanced. However, the gimbal mounting of a dish means that the dish must be supported so as to pivot about an axis which is at a height above the ground equal to the radius of the dish. In the event of high winds, such a dish cannot be moved out of the path of the wind, and therefore the dish support structure has to be reinforced adding to the mass and cost thereof . Due to such factors, current parabolic solar radiation reflector dishes are limited in size to between 10m and 15m in diameter. Accordingly, several such dishes are required in order to produce a significant amount of power (25 dishes of 15m diameter being required to produce the same power as a single 75m diameter dish) , which becomes prohibitively expensive, as explained above. In accordance with the present invention, apparatus for collecting and concentraring solar radiation comprises a reflector dish, means for rotating the dish about a first, substantially vertical axis, means for pivoting the dish about a second axis, the second axis lying in a plane parallel to but not coincident with the vertical axis, the pivoting means being effective to pivot the dish to any position between a first position in which the circumference of the dish lies in a substantially horizontal plane and a second position in which the circumference of the dish lies in a substantially vertical plane, and a weathershield extending around the vertical axis so as at least partially to surround the circumference of the dish in the first position, the height of the weathershield being such that the upper edge thereof is level with or extends above the circumferential edge of the dish in the first position. With such an arrangement, in which the second axis is preferably substantially horizontal, the reflector dish and the support structure therefor need only be strong enough to resist the effect of moderate winds, which keeps the cost of the apparatus, including that of the means for moving the dish so as to track the movement of the sun, to a minimum. In the event of high winds, the dish can be lowered to the first, horizontal position so that it is below the upper level of the weathershield, which protects the dish from wind damage. The weathersheild extends sufficiently downwardly to prevent winds from getting under the dish and the whole apparatus, when the dish is in the horizontal position, presents a low profile to wind from any direction, enabling dishes of large diameter to be used even in locations where high winds occur relatively frequently. Those skilled in the art will appreciate that there are many possible kinds of dish support structures which can be employed to provide the "non-gimbal" dish mounting in accordance with the invention. For the mechanically least complex dish support structure however, the second axis is preferably fixed in relation to the first, vertical axis . Suitably the dish is mounted so as to pivot with respect to a cradle, which cradle is rotatable about the first, vertical axis. The weathershield may be mounted so as to rotate with the cradle. This is advantageous where the second axis lies within the circumference of the weathershield; it will be readily appreciated that in order for the weathershield not to shield or obstruct the lower part of the reflector dish when in the second, raised position, a part of the weathershield corresponding to that portion subtended by the second axis when the dish is in the second position may be cut away. Such an arrangement lessens the mass of the weathershield and, because it is mounted to rotate with the cradle, when the dish is lowered because of a high wind the whole apparatus can be rotated so as to locate the cut away portion of the weathershield on the leeward side of the apparatus, to keep wind out from under the dish. For the apparatus to present the lowest possible profile to a high wind, the lower edge of the weathershield is preferably close to the ground and the height of the second axis above the lower edge of the weathershield is preferably equal to or greater than the maximum horizontal distance between the second axis and the circumference of the dish when the dish is in the first horizontal position. This means that, in the second, vertical position the lowermost edge of the dish comes close to but does not impact the ground. The apparatus is provided with means for rotating and pivoting the dish so as to track the sun's movement as is known in the art. In addition, means may be provided to sense the wind speed and direction and to lower the dish to the first position when the sensed wind speed exceeds a predetermined speed, i.e. a wind speed which might damage the dish. The means for rotating the dish would, as described above, be adapted to rotate the dish so that any cut away section of windsheild lay in the lee of the apparatus, with the second axis perpendicular to the sensed wind direction. It is well known that wind speeds may increase over a short period of time; consequently the means to lower the dish to the first position may be adapted to lower the dish quickly. The means to rotate the dish may also be adapted to turn the apparatus quickly so as to put the cut away portion of the weathershield to leeward. Alternatively, as the rotation of the dish so as to track the sun is relatively slow, requiring only a lower power rotational drive, a removable portion of weathershield may be provided, either to cover the cut away portion of weathershield or the arc that portion passes through, which may be less expensive than providing a high speed rotational drive. A further problem with conventional apparatus is the efficient conversion of the solar radiation into electrical power. This problem is of particular significance where solar radition is collected over a large area and reflected onto and concentrated in a small area. The concentration of a large amount of solar energy in a small area produces a high concentration of thermal energy and there is a physical limit to the maximum solar power density which a receiver apparatus can tolerate without damage. Preferably the reflector dish is a point focus parabolic dish and a receiver is provided, located adjacent the focal point of the dish, for absorbing the solar radiation collected and concentrated thereon by the reflector dish and for generating electrical power. Suitably, the receiver has an aperture through which the solar radiation passes into a cavity, which may be a so-called "black body absorber" and which extends behind the aperture. The cavity is defined by walls which are preferably formed of a material adapted to absorb a majority of the solar radiation incident thereon (and to reflect a minimum of the solar radiation) and to conduct heat to a working fluid contained in a pressure vessel surrounding the cavity walls, means being provided to circulate heated working fluid to a heat engine, or generator, for converting thermal energy carried by the hot working fluid into electrical power, the circulating means returning cooled working fluid back to the pressure vessel . Preferably the interior of the cavity wall is shaped so as evenly to distribute thereover the density of solar power incident thereon resulting from the concentration of solar radiation thereon by the parabolic reflector dish. The receiver may be mounted to one or more legs extending outwardly from the receiver dish outwith the reflected ray path, the circulation means extending between the receiver and the generator via one or more of the said leg(s) . Preferably shield means are provided so as to protect the exterior of the receiver and the or each leg in the region adjacent the aperture from concentrated solar radiation incident thereon, to reduce the possibility of damage. The thermal energy contained in concentrated solar radiation can be so great as to melt most metals. Suitably, the cavity walls are formed of molybdenum or of an alloy thereof. Such materials have excellent strength and thermal conductivity up to temperatures of 1100°C, and they are not highly reflective. Accordingly, the cavity walls can be relatively thin, which facilitates the conduction of thermal energy to the working fluid. The cavity walls are preferably coated with a high-temperature matt coating, to minimise reflections. The working fluid may be water, or it may be sodium. Sodium is a preferred working fluid because it has a low vapour pressure (typically about 1.2 atmospheres at 900°C) , however, it solidifies at about 91°C. Accordingly, means are provided to heat and to liquefy sodium in the circulation means and in the generator to enable the working fluid to flow throughout the system when the dish is first aligned towards the sun. Sodium in the receiver is, of course, liquefied by the solar radiation incident on the receiver. Suitably, the generator is a steam generator, of the type used in nuclear power reactors, in which thermal energy is transferred from the sodium working fluid to water, the resulting high pressure steam being used to drive a turbine to create electric power. A generator may be associated with a single reflector dish and its respective receiver or, as is known in the art, the heated working fluid from several dishes may be directed to a single large generator. When molybdenum is exposed to air at high temperatures it has a tendency to oxidise. Accordingly, the cavity contains an inert gas or a vacuum. In order to achieve this, a window is provided over the aperture of the receiver. Such an arrangement also minimises the loss of thermal energy out of the cavity by convection and conduction. The material of which the window is formed must be gas or air tight and must transmit radiation of wavelengths between 290nm and 4000nm. Glass, for example, is not a suitable material, because it is relatively opaque to infra red radiation, which is where a significant proportion of the energy in solar radiation lies. Sodium Chloride is a suitable window material, as is common in all infra red transmitters. Alternatively, quartz may be used, although this is expensive and difficult to fabricate. Preferably, the window is paraboloid in shape, so that the solar radiation concentrated thereon by the reflector dish impacts the window as near to the normal as possible. Such an arrangement minimises the reflection of solar radiation away from the receiver by the window. Preferably, the interior of the cavity is symmetrical about the axis of symmetry of the reflector dish and iε shaped so as to distribute evenly thereover the density of solar radiation incident thereon. This may be achieved by shaping the cavity wall to take account of two factors, namely that increasing the angle of incidence of solar radiation on the cavity walls spreads that radiation over a larger area of cavity wall, thereby reducing the density, and increasing the internal radius of those parts of the cavity walls where the density of solar radiation is high will also reduce the density. The cavity walls may therefore be configured such that, proceeding along the axis of symmetry of the reflector dish from the aperture to the distal end of the cavity, the cavity wall has at least the following shaped portions in the stated order: a first portion substantially parallel to the axis of symmetry; a second portion diverging from the axis of symmetry; a third portion surrounding the point of maximum internal radius of the cavity where the wall turns so as to converge on the axis of symmetry, and a fourth portion gradually converging to meet the axis of symmetry and so as gradually to increase the angle of incidence of solar radiation incident thereon to about 90°. The first portion of the cavity wall compensates for any misalignment of the reflector dish, or portions thereof, causing the concentrated solar radiation to be reflected away from the receiver rather than accurately into the aperture. The second portion is effective to spread the incident solar radiation over a larger area by increasing the angle of incidence thereof. The internal radius of the receiver also increases in this portion, which also spreads the solar radiation over a larger area. The second portion is important, since the outer edges of the parabolic reflector dish receive the majority of the solar radiation incident on the dish and concentrate that radiation on the second portion of the cavity wall, and in order to be able to construct the cavity walls of homogeneous material and to avoid "hot spots" of concentrated energy in the cavity walls it is necessary to ensure that the energy density of the solar radiation is uniform by reducing the density in this portion. In the third portion, the internal radius of the cavity walls is greatest. However, this portion receives the minority of the concentrated solar radiation from the annulus on the reflector dish about half way along its radius. This area of the reflector dish receives and concentrates less solar radiation than does the outermost portion of the dish. In order to provide uniform density of solar radiation along this portion of cavity wall, it is therefore necessary to increase the density, which is achieved by increasing the angle of incidence of the solar radiation incident thereon. The majority of the solar radiation concentrated on the fourth portion of the cavity wall is provided by the innermost area of the reflector dish. This part of the dish collects and concentrates proportionally less solar radiation than the outer parts of the dish, and therefore to account for this the angle of incidence of the solar radiation on the fourth portion is increased, and the internal radius of the cavity reduced, so as to increase the density of incident solar radiation in order to render the density more uniform. This shape of cavity not only evens out the solar radiation density over the cavity wall but also cuts down on convection and black body radiation losses from the receiver. The internal cavity is so shaped as to retain any reflected solar radiation within the cavity rather than to reflect radiation out of the cavity through the window. The reflector dish is preferably parabolic, the exact shape of the parabola being determined so as to represent a compromise between a shape which gives a large surface area, in order to collect and concentrate the maximum solar radiation for a given dish diameter, and a shape giving a dish of minimum "depth", or height when the dish is in the first, horizontal position, so that the apparatus presents the least surface area to a high wind when the dish is in the first position. In order to provide a dish which is inexpensive, light yet sufficiently strong and easy to construct accurately, the dish preferably comprises a plurality of reflective planar segments, or sheets, mounted to and supported by a frame. The frame may comprise a plurality of support members extending radially inwardly from the outer circumferential edge of the dish toward the centre thereof and having a planar segment supporting surface formed as a sector of a parabola or in a series of linear steps approximating thereto, adjacent support members being connected. The maximum size of the planar segments is limited by the size of the receiver, by the accuracy with which each planar segment is aligned so as to reflect solar radiation incident thereon into the aperture and by the overall shape of the parabolic dish. Suitably, each support member has a connecting member mounted thereto so that when the connecting members of adjacent support members are connected together to form the frame, the connecting members form a circle, or cylinder, centred on the centre of the dish and with a radius of about 3/4 the radius of the dish. Each planar segment may be mounted so as to span the radial distance between two support members, and adjacent support members may be connected in such a way as to subtend a substantially equal angle at the centre of the dish. So as to reduce the weight of the dish whilst maintaining its structural integrity, it is not necessary for all of the support members to extend the full distance from the outer circumferential edge of the dish to the centre thereof. Instead, some of the support members may only extend to a point half way along a radius of the dish. This takes advantage of the fact that, as the radius of a circle increases, the circumference also increases, and vice versa. Accordingly, a planar segment of a size sufficient to span the distance between two adjacent support members at the outer circumference of the dish would span the distance between three adjacent support members at half the radius (and between four adjacent support members at one quarter the radius, and so on) . Each planar segment having, when mounted to the frame, an outer edge furthest from the centre of the dish which is substantially perpendicular to a line extending from the centre of the said outer edge to the centre of the dish, the length of the outer edge of each planar segment is preferably equal to or less than a predetermined maximum length substantially equal to the distance between adjacent support members at the outer circumferential edge of the dish. The predetermined maximum length is, as stated above, dependent upon the size of the receiver. Suitably, a first support member is interposed between a second support member which extends from the circumferential edge of the dish to the centre thereof and a third support member, the first support member extending radially inwardly only to a point where the distance between the second and third support member is equal to the predetermined maximum length. Such an arrangement not only reduces the overall number of planar segments, but reduces the lengths and hence the weights of at least some of the support members, without compromising the structural strength of the frame supporting the dish. Only some of the support members need extend the complete distance from the outer edge of the dish to the centre thereof; the exact number of such support members is dependent upon factors such as the overall size of the dish and the maximum size of the receiver (which determines the maximum size of the planar segments) , as are the sizes of those support members which are not as long as the radius of the dish. Advantageously the frame comprises a plurality of sets of support members, each set comprising a series of support members having lengths l x , 1 2 , 1 3 l x , wherein x is an even number and wherein the length of each support member is defined by the equations (2) 1_ = L where n is even ( 3 ) 1 (x + I , = - 1 2 X (4) where n is less than x and not even, 2 l n = M L where n + 1 is exactly divisible by 4 = 7 /β L where n +3 is exactly divisible by 8 = 15 16 L where n + 7 is exactly divisible by 16 = 3132 L where n + 15 is exactly divisible by 32 (5) 1„ = 1 ( x + 2 - m where n > x + 2 L being the distance from the circumferential edge of the dish to the centre thereof. It will be appreciated that the features of "non- gimbal" mounting of a reflector dish, of the construction of a solar radiation receiver and of the construction of a parabolic reflector dish as described above represent significant improvements over known apparatus for the conversion of solar radiation into electrical power. It will also be understood by those skilled in the art that each of these features is capable of being employed independently of the others. Embodiments of apparatus in accordance with the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a parabolic reflector dish apparatus in accordance with the invention; Figure 2 is a plan view of one embodiment of part of the supporting frame for the apparatus of Figure 1; Figure 3 is a plan view of another part of the supporting frame for the apparatus of Figure 1; Figure 4 is a schematic side view of the frame shown in Figures 2 and 3 with the dish in a first, horizontal position; Figure 5 is an enlarged view of the circled area of Figure 2; Figures 6 and 7 are side views similar to that of Figure 4 but showing the dish in an intermediate and a second, vertical position, respectively; Figures 8, 9 and 10 are views of a second embodiment of a frame mechanism for supporting and moving the dish; Figure 11 is a perspective view of the apparatus incorporating the mechanism of Figures 8 to 10; Figure 12 is a side view of another embodiment of a frame mechanism for elevating the dish; Figure 13 is a perspective view of a preferred method of supporting the reciever. Figures 14 and 15 are schematic diagrams showing the concentration of solar radiation produced by a parabolic reflector dish in a parallel-sided receiver cavity; Figures 16 and 17 are side and perspective schematic views of a preferred configuration for a solar radiation receiver in accordance with the invention; Figures 18 to 21 are cross-sectional or schematic views illustrating preferred features of the receiver of Figures 16 and 17; Figures 22a and 22b are, respectively, perspective and end elevation views of a support member forming part of a frame for supporting a prabolic reflector dish; Figures 23a and 23b are views of alternative configurations for part of the support member of Figures 21a and 21b; Figure 24 is a schematic plan view of a portion of a parabolic reflector dish comprising a plurality of support members and of reflective planar segments; Figure 25 is a schematic plan view of a portion of a parabolic reflector dish in accordance with the invention comprising a plurality of reflective planar segments and a plurality of support members. Figures 26a and 26b illustrate alternative forms of planar segments for use in a parabolic reflector dish; Figure 27 shows a method of fixing the reflective planar segments of Figure 26b to the support members of Figures 22a and 22b, and Figure 26 shows a method of aligning reflective planar segments forming part of a parabolic reflector dish. Figure 1 shows an embodiment of an apparatus 1 in accordance with the invention for collecting and concentrating solar radiation for the generation of electrical power. The apparatus comprises a parabolic dish 3 for reflecting solar radiation into a receiver 5 mounted in a position adjacent the focal point of the dish by a number of legs 7. The dish is mounted to a cradle, or chassis, 9, which includes a mechanism for elevating the dish 3 and the receiver 5, and which is described further below. The cradle 9 is mounted so as to rotate around a number of tracks 11 and has mounted to it a weathershield 13 which substantially surrounds the dish 3 when it is lowered to a horizontal position. The apparatus 1 works in the following manner. The cradle 9 is driven to rotate and elevate the dish 3 into a position facing the sun, whereupon solar radiation incident on the dish 3 is collected and reflected so as to concentrate the solar radiation on, or in, the receiver 5. The receiver 5 is adapted to absorb thermal energy which is then converted to electrical power. This is achieved by pumping a working fluid, such as water/steam, which is heated in the receiver 5, to a turbine or heat engine (not shown) , where the thermal energy in the working fluid is used to drive an electrical generator. The cooled working fluid is then circulated back to the receiver 5. The circulation lines for the working fluid advantageously pass along, or within, one or more of the legs 7 and, via a flexible linkage (not shown) at the base of the apparatus 1, to and from the generator. Heated working fluid may circulate from more than one such apparatus 1 to a single generator. The rotational and elevational drive means (not shown) are effective to move the dish in azimuth and elevation so as to catch the diurnal and seasonal movement of the sun, as is known. In the event of a high wind, sufficient to damage the dish 3 or to topple the apparatus, the dish 3 is lowered so that it is horizontal. The height of the weathershield 13 is such that, when the dish 3 is horizontal, the top edge of the weathershield 13 is level with, or preferably slightly above, the circumferential edge of the dish 3. This prevents the wind from getting under the dish 3, and possibly causing damage to it. It can be seen from Figure 1 that the weathershield 13 has a portion cut away. This portion corresponds to the lowermost portion of the dish 3, when in the raised position as shown, so that that portion of the dish is not in the shadow of the weathershield 13. This allows the dish 3 to be elevated by being pivoted about an axis which is as low as possible, which means that the apparatus 1 (with the dish 3 lowered) presents as low a profile as possible to a strong wind. Referring to Figure 2, the dish chassis 9 is supported by and rotates on five concentric circular tracks 11, similar to railway tracks. The chassis 9 comprises two parallel beams 15 which run on tracks 11. Between and perpendicular to beams 15 are two similar beams 17, which also run on tracks 11. In the same plane as and connecting the ends of one end of beams 15 and both ends of beams 15 and 17 around the circumference are beams 19. On the long arcs between beams 15 and between beams 17 beams 19 go to a point directly above and run on the outermost of the tracks 11 at points 21. Mounted on and perpendicular to beams 19 are a plurality of triangular frames 23 (see Figure 5) . As shown by arrow 24 they are mounted at varying distances across beams 19, so that column 25 lies directly above the outermost track 11. Thus columns 25 form a circle above that track. To the columns 25 cladding sheets are attached to form the weathershield 13. Frames 23 only extend to points 27 shown in figure 2. Ballast boxes 29 are mounted on beams 15 and 17 and are filled with earth or other suitable material to stabilise the chassis 9. Mounted on the chassis 9 is a structure 31 (see Figure 3) for supporting and elevating the dish 3 . This structure 31 consists of two pairs of parallel beams 33. The gap between each pair of beams 33 is directly above beams 15. On the ends of and raised above, beams 33 are pivots 35. These pivot with two pivots on beams 15 and lie directly above the outermost but one of the tracks 11. Beams 37 lie between and perpendicular to beams 33. Referring to figure 4, on beams 15 is a rack 39 served by a pinion 41. Pinion 41 is on the end of column 43 and lies directly above the central track 11. Column 43 is telescopically extendable to about 180% of its original length by a further rack and pinion system 45 (see Figure 7) . The other end of column 43 from pinion 41 is pivot 47 between beams 33. Beam 49 connects pivot 47 to pivot 35, and runs just below the dish 3 and through the dish support members (described later) . Beam 49 serves to strengthen the dish support structure. As pinion 41 moves along rack 39 it raises the dish support structure about pivots 35 until column 43 is vertical, as shown in figure 6. At this point the pinion lies directly above the innermost track 11, column 43 and the rack and pinion means 45 then extends telescopically, until the dish 3 is vertical as shown in figure 7. The parabolic dish 3 comprises a reflective covering supported by a number of radially extending support members, to be described in more detail below. Adjacent support members are connected along a circle 51 shown in Figure 3 . The circle 51 is centered on the square formed by a line at the midpoint between each pair of beams 33 and the two outermost beams of beams 37. Pivots 47 are directly below the point where circle 51 intersects the midpoint between each pair of beams 33. At point 53 (figure 3) , where beams 33 meet the two outermost beams 37, are mounted the ends of legs 7 which support the receiver. The motors which rotate the dish chassis and drive the rack and pinion systems need not be very powerful because the movement needed to track the sun is only very slow. However if high winds develop the dish 3 must be lowered and rotated quickly. The lowering could be achieved by disengaging the motors and using a controlled braking system. A high speed drive system would have to be provided to rotate the dish 3 into the wind. The above embodiment has been designed to be as low as possible when the dish 3 is in the lowered position, to keep the weathershield 13 low and in the slower winds near the ground. This is a trade-off between keeping pivots 35 below the dish 3, and as far under the dish 3 as possible to keep beams 33 short and to reduce the bending moment on them, and keeping the lowermost edge 55 of the dish (see figure 7) above tracks 11 when the dish 3 is in the duly raised position. Point 55 could be kept high by putting beams 15 and therefore each pair of beams 33 closer together but this would lengthen beams 33 and also reduce the twisting resistance of the dish support structure. It may be more cost effective to raise beams 33 until their ends are at the same level as pivot 35. Beams 49 would then not be needed, but the dish 3 would be around twice as high in the lowered position, requiring the weathershield 13 to be twice as high. This alteration would have the advantage that the means used to raise the dish 3 need not be so compact in the lowered position, because there would be more room between beams 33 and 15. It could also be more cost effective to allow beam 49 or even pivot 35 to protrude through the dish 3 and lose only a small fraction of the total reflective area of the dish 3, and this alteration would make the dish 3 and weathershield 13 lower than that described above. The curvature of the dish 3 is also a trade off. If the dish 3 is deep the receiver 5 need not be as high, but deep dishes have several disadvantages. The area of the dish is greater for the area of sunlight it intercepts, and this requires more reflective material and is more expensive to manufacture. Deep dishes are also higher in the lowered position so higher weathershields are required. Perhaps most importantly, the angle at which the reflected rays of solar radiation strike the receiver 5 from the outer edges of the dish (which is where the majority of the solar radiation is reflected from) is shallower. This means that rays reflected from the outer edge of the dish 3 strike the receiver 5 where they may only be reflected twice in the receiver 5 before leaving it. After only two reflections the ray will still carry a significant aount of energy, which will be lost. Thus, the dish 3 is preferably kept shallow and one with a radius equivalent to x=0.2 giving a height of y=0.04 is chosen (the dish surface being defined by the equation y=x 2 ) . The gradient at the outer edge of the dish 3 is then 0.4 which is about 22°, thus light rays strike the receiver 5 at about 45°. This places the receiver 5 about 1.25 dish radii above the bottom centre of the dish 3. The apparatus described above would be suitable for operation in wind speeds up to approximately 15 m/s . For a dish of 75m diameter, such a wind would produce a force of about 65-85 tonnes, when the dish 3 is fully raised and square on to the wind. Accordingly, the dish 3, receiver 5 and support structure 31 are preferably designed to weigh a similar amount. The ballast boxes 29 help prevent the structure from falling over in high winds. In the event of high winds, the dish 3 is lowered to the horizontal position so as to be protected from the wind by the weathershield 13. The apparatus 1 is also rotated so that point 57 (see Figure 2) faces the wind, putting the open arc (between points 27 in Figure 2) of the weathershield 13 to leeward. Alternative means of raising the dish may be provided, such as multiple stage bydraulic rams or single stage rams in combination with rack and pinion systems. A removable weathershield (not shown) could be placed around the arc where the weathershield 13 is not present . This may be more cost effective than providing a high speed rotating drive to rotate point 57 into the wind, should high winds be likely to develop without warning. Figures 8 to 11 illustrate an alternative mechanism for supporting and elevating the dish 3. Beams 15', similar to beams 15 in Figure 2, extend through the weathershield 13 which extends as a substantially complete circle, broken only above beams 15' . Members 59 are mounted to the base of the dish 3 and are pivotally mounted, at point 61, to the end of other members 63. The other ends of members 63 are pivotally mounted, at point 65, to the ends of beam 15' . A rack 67 is fixed to the top of each of beams 15' and pinions 69 fixed to the ends of members 59. Rotation of the pinions 69 is effective to raise the dish 3 from the lowered position shown in Figure 8 towards the vertical (Figures 10 and 11) , Further tracks lla, lib are provided, outside the circumference of the weathershield 13, for the ends of beams 15' to run on. Figure 12 illustrates another mechanism for supporting and elevating the dish 3. As in Figue 2, beams 15" extend only within the circumference of the weathershield 13. At one end of the beams 15" is fixed a post with a pivot 15. Beams 49 and 71 extend from this pivot and the dish skeleton sits on beam 33 which interconnects beams 49 and 71. Additional beams 72 and 74 are added between beams 71 and 33 for strength. Beams 33 are interconnected by beams 37 as are beams 74 by beams 76. A ram is connected between beams 71 and post 73 to elevate the dish 3 to the required position. The post 73 is preferably sufficiently high to prevent the lowermost edge 55 of the dish 3 from touching the ground. The receiver is preferably supported as shown in Figure 13 by beams 7 and 8 as shown in this Figure. The underside of beams 8 are coated on firebrick 111 as is a portion of the undersides of beam 7. The arrangement minimizes the blocking of reflected solar radiation by the beams. Beams 8 are preferably 0.08 dish diameters long. The dish 3 is effective to reflect and concentrate solar radiation onto the receiver 5. Conventional receivers work by transferring thermal energy from the concentrated solar radiation to a working fluid, which heated fluid is used to drive an electrical generator. Most often, the working fluid is water and the generator comprises a steam turbine. In order to minimise the loss of energy due to reflection of radiation away from the receiver, these are commonly formed with a cone-shaped cavity. Radiation enters the cavity through an aperture at the base of the cone and any reflected radiation tends to be reflected onto another portion of the cone rather than lost out of the receiver. A problem with cone-shaped cavities is that solar radiation tends to be unevenly distributed over the surface of the cavity. This creates hot spots which reduce efficienty and can damage the material from which the cavity is formed. Various shapes of cavity have been used in order to even out the distribution of solar radiation, such as cylindrical or continuously curved cavities, with varying degrees of success. Two factors which affect the uniformity of solar radiation concentrated within the receiver are the size and shape of the parabolic reflector dish. Because the area of the dish increases in proportion with its radius, the outer part of the dish collects and concentrates a greater amount of solar radiation than does the inner part. For example, if the radius of a circle is divided into 10 equal lengths, about 19% of the area of the circle is in the annulus formed by the outermost 10% of the radius, about 17% of the area is in the next 10%, about 15% in the next, about 13% in the next and so on down to about 1% in the innermost circle. About half the area of the circle is accounted for by the outermost 30% of radius thereof. For the parabolic dish described above, where the gradient of the dish at the outermost circumference is about 0.4, i.e. about 22°, light rays would enter the receiver at angles to the horizontal (assuming the dish is horizontal and the sun directly overhead) , i.e. between about 46.4° and 88° (the body of the receiver itself prevents solar rays reaching the central portion of the dish) . As the outermost 30% of the dish reflects about 50% of the solar radiation incident on the dish, half the solar radiation enters the receiver cavity between the angles of 46.4° and 58.7°. For a parallel sided receiver, which is shown in Figures 14 and 15 for clarity, radiation reflected between the angles of 46° and 58° is incident on the receiver wall at a greater angle, which means that that radiation is spread over a smaller distance than is the remaining solar radiation reflected by the innermost part of the dish, from the centre to about 70% of the total dish radius. In Figures 14 and 15, light reflected from the dish 3 enters the parallel-sided cavity 81 of the receiver 5a through an aperture 79. The outermost portion 83 of the cavity wall receives little or no reflected solar radiation and is provided so as to allow for any misalignment of the dish 3 relative to the receiver. About 50% of the total reflected solar radiation is concentrated on the next portion 85 of the cavity wall, the remainder of the solar radiation is spread over the next portion 87 of the cavity wall, which is considerably longer than the portion 85. Other factors which affect the design of the receiver cavity are the need to ensure that solar radiation reflected within the receiver does not exit therefrom (preferably by making the cavity of such a shape that each ray is reflected a minimum of three times within the cavity) and to minimise convection and radiation losses . Figures 16 and 17 illustrate a preferred shape for the receiver cavity, which takes advantage of two methods of varying the density of solar radiation. These are that increasing the angle of incidence of solar radiation reduces the radiation density, by spreading it over a larger area, and that increasing the radius of those parts of the receiver cavity where the radiation density is high will also reduce that density. Figure 16 shows, in cross-section, the shape of the internal cavity 81 of a receiver 5 which, taking account of the above-described factors, is effective to even out the density of solar radiation incident thereon. Light is reflected into the cavity 81, which is symmetrical about an axis perpendicular to the aperture 79, via the aperture 79. The initial portion 89 of the cavity wall is cylindrical; if the reflector dish is correctly aligned, no solar radiation is incident on this area, however, it is essential to allow for any minor misalignment of the dish relative to the receiver 5. The next portion 91 of the cavity wall has a slope which spreads the high power density received by this region over a larger area by increasing the angle of incidence of the incoming rays. This slope also increases the radius of the receiver 5, which spreads the rays over a larger area. The slope of the receiver wall then turns at the portion 93, where the power density starts to reduce. This has the effect of slightly concentrating the rays in this area by decreasing the angle of incidence, which is needed because the reduction of power density due to increasing the radius is greatest here. In the portion 95, the slope of the receiver wall gradually reduces to keep it more towards the normal of the incoming rays as the power density reduces further up this region. The decreasing radius (towards the distal end 97 of the cavity 81) also has a concentrating effect in this region. This shape of cavity 81 not only evens out the power density, but also cuts down on convection and black body radiation losses from the receiver. This is achieved because the ratio of the internal volume of the receiver to the area of the receiver mouth is much greater in this design, that in that of conical designs. It can also be seen that this shape is also effective to retain the radiation it receives by reflecting it around within the receiver cavity 81. As shown in Figure 17, each point on the wall of the cavity receives radiation not only from the side of the dish directly opposite but also at oblique angles from any point on the dish visible therefrom. Taking a typical "planar square" segment of the reflective surface of the dish, each point on the wall of the cavity receives radition from the circle traced out by the corners of each such segment around the dish. A cross section of the wall of the cavity is shown in Figure 17. A molybdenum alloy wall 99 is heated directly by the solar radiation. This thermal energy flows through the wall and is transferred to a working fluid such as molten sodium. The molten sodium flows through the receiver 5, from top to bottom, or from bottom to top, discussed later. The sodium is kept in by an austenitic stainless steel shell 101 surrounding the molybdenum wall 99. This in turn is surrounded by insulation, which could take the form of several layers of vacuum insulation 106 within consecutive stainless steel shells 103. If stainless steel is used there is no need for a weathershield to protect the receiver 5. On the outside of the innermost shell of the receiver 5, which is heated by the concentrated solar radiation, are vertical fins, 104 and 105 (see Figures 18a and 18b) . Fins 104 extend down the entire length of the receiver and fins 105 are present where the diameter of the receiver is greatest, to reduce the gap between fins 104. These fins serve several functions. They increase the resistance of the receiver to collapsing in on itself under external pressure. This means that the receiver wall can be thinner, resulting in better heat transfer to the sodium working fluid. They also increase the surface area for heat transfer, and, by providing channels for the sodium to flow along, help prevent the sodium from pooling in one side of the receiver when the receiver approaches the horizontal at either end of the day. The fins extend to touch the stainless steel shell within the limits of thermal expansion and contraction. A molybdenum alloy is chosen because these alloys have excellent strength and thermal conductivity up to 1100 degrees C. Molten sodium is chosen as the coolant, because it has a low vapor pressure, about 1.2 atmospheres at 900 degrees C. This means that pressure on the wall of the receiver is low. Therefore the molybdenum need only be thin to withstand it. The thinner the wall the better, because the thermal energy has to travel a shorter distance before reaching the sodium, so heat transfer is good. Austenitic stainless steel containing 3% molybdenum, which forms the other wall 101 containing the sodium, can withstand temperatures up to 750 degrees C. The vacuum insulation 106 is the preferred means of retaining heat within the sodium. Molybdenum does have disadvantages. If it is exposed to the air at high temperatures, it will oxidise. Therefore it must be in an inert atmosphere or vacuum. To maintain either of these conditions, a gas tight material must be used to enclose the cavity 81 which will transmit radiation of wavelengths 290-4000nm. Glass would be no good for this because it is opaque to infra red, which is where a significant amount of solar energy lies. Sodium chloride is transparent throughout these wavelengths, but is liable to become fogged by moisture in the air. This is common to all infra red transmitters, apart from perhaps quartz, but quartz is difficult to fabricate and expensive. Sodium chloride is the preferred material and it can be polished or replaced if it gets fogged. The shape of the window is important . To minimise reflections of the incoming radiation from the window, the radiation must strike it as near to the normal as possible. Thus it is in the shape of a parabolic dome 107 as shown in Figure 20. A parabolic dome 107 with a diameter at its top edge larger than that of the aperture 79 mouth of the cavity has two benefits. It provides the optimum shape for the incoming radiation falling as near to the normal as possible. The radiation travelling through the dome is also diffuse because it travels through the dome further from the focal point of the reflected radiation than if the two diameters were equal. This is important because with the high power densities travelling through the dome any absorbed radiation causes significant heating. The dome shape is also favourable in terms of being a strong design if the receiver is evacuated. An evacuated receiver would be desirable in terms of preventing thermal losses due to convection and conduction. The inner surface of the cavity formed by the wall 99 is provided with a high temperature matt coating (not shown) to reduce the reflection of radiation. This coating may serve to prevent oxidation of the molybdenum, in which case a window may not be required, depending on the cost thereof relative to the savings in conduction and convection losses achieved thereby. Molten sodium used as the working fluid also has disadvantages. When the sun is not shining the sodium cools down and eventually turns solid at 91 degrees C. This does not matter in the receiver 5, as it is soon melted when the sun comes out again, but in the pipes and pumps which carry it to a steam generator (not shown) , it remains solid. Thus a means of heating the pipes and pumps to above 91 C, such as surrounding them with electrical heating coils, powered by a non solar means, has to be employed. Sodium also reacts violently with water, and sodium to water steam generators must be carefully designed. This type of steam generator already operates successfully in fast breeder nuclear reactors which also use sodium as a coolant. To achieve standard power station steam conditions of 540C and 16.5 MPa, with steam superheat and resuperheat, the sodium has to be significantly hotter than this. In fast breeder nuclear reactors which also use molten sodium as a coolant, the sodium has a boiler inlet temperature of 550-600°C. An alloy of sodium and potassium may appear attractive, because it is liquid at room temperature, but it has an adverse effect on the steel used in the pipes. Mercury is another option but this has a higher vapour pressure, and can cause heavy metal poisoning. Also, in the event excess power is generated, heated working fluid (sodium) could be directed to an insulated holding tank to be drawn off when the sun is not shining, to provide continuous power. About 3 times the number of dishes would then be required for continuouspeak power output, rather than just when the sun is shining. Carbon dioxide and helium gases are also used as coolants in nuclear reactors. In an advanced gas cooled reactor, carbon dioxide gas has a peak temperature and pressure of 648 C and 4.4 MPa. The receiver could be redesigned to accomodate these conditions (see Figure 21) . The molybdenum wall 99 could remain about the same thickness, and plural layers of a coolant gas tube 109 would be provided, coiled as close as possible to each other, and around the wall of the receiver. These tubes 109 would have to be thick enough to withstand 4.4 MPa (44 bar) , which would cut down on the ease of heat transfer. The other wall 101 of austenitic stainless steel could remain. A good thermal conductor such as graphite or aluminium powder or granules could fill the gaps between the coils of tube 109. With a uniform power density on the wall of the receiver, the gas could flow in the either up or down the receiver, gradually picking up thermal energy as it did so. Thus there would be no need to return the tube 109, which exited from the hotter lower portion of the receiver back to the top away from the receiver. Instead it could just coil back up the receiver closer to the wall. In any event, it is advantageous for the hot working fluid to exit from the uppermost region of the receiver, to reduce pumping effort . Preferably means are provided to sense the temperature of the working fluid in the receiver and connected via control means to a pump and valve means (not shown) , so as only to circulate the working fluid when it has reached its optimum working temperature. If hot gases were used as the working fluid they could be used directly to drive a Brayton cycle gas turbine generator. Water could be used directly as a working fluid, but at the standard power station steam conditions preferred, about 540C and 16.5 MPa (about 2500psi) , the tube walls would have to be much thicker. If during operation solar tracking is lost, focused solar radiation may track across the receiver, and is of sufficient solar flux to melt steel. Thus any areas susceptible to being tracked by the focused solar image have to be protected with firebrick 111 or the like, see Figure 20. It would be cheaper to provide a disc as shown to protect the outside of the receiver closest to the dish than to coat the whole receiver with firebrick. Figures 22a, 22b, 23a, 23b and 24 illustrate the construction of a parabolic dish for reflecting solar radiation. A frame (a portion of which is shown in Figure 24) for supporting the parabolic reflective surface comprises a plurality of radially-extending support members 113 connected together at the centre of the dish and where rectangular plates 117 abut. Each support member 113 comprises a curved member 115 having a parabolic or near parabolic surface for supporting the reflective surface of the dish and has longitudinal members 119, 121 joining the ends of the curved member 115 to the rectangular plate. Cross-bracing (not shown) may be provided to stiffen the support member 113. The curved member 115 may comprise a flat plate 123 having a curved edge to which another length of strip material 125 is mounted to form a curved T-section member, as shown in Figure 23a. Alternatively, the curved member 115' may be formed in a series of linear steps as shown in Figure 23b, to facilitate the mounting thereto of a reflective surface formed of a plurality of planar sheets, or segments, (this being considerably easier to produce and less expensive than a curved, integral reflective surface) . A portion of a reflector dish comprising such a frame is shown in Figure 24, adjacent support members 113 being joined together as described above, with a plurality of reflective planar sheets 127 mounted thereto. Each planar sheet 127 spans the distance between, and is mounted to, adjacent support members 113. As can be seen in Figure 24, the size of each planar sheet 127 decreases as its distance from the centre of the dish decreases. Thus, a reflector dish constructed as shown in Figure 24 will have a large number of planar sheets 127 of varying size and shape and the dish so constructed will be unnecessarily heavy. The support and elevating mechanism for such a dish would therefore be similarly heavy, and the apparatus unnecessarily expensive. An improved constructional arrangement for a reflector dish is shown in Figure 25. The maximum tangential length of the outermost edge of the outermost planar sheet 127 is determined by the size of the aperture into which solar radiation is to be reflected (the same consideration applies to the maximum radial length of each planar sheet) . Moving radially inwardly from the outermost edge of the dish, at a point equal to half the radius of the dish, a planar sheet 127a having an outer edge of length equal to the maximum tengential length will span the tangential distance between three adjacent support members 115, 115a, 115b. Accordingly, the central support member 115a need only be half as long as the radius of the dish. At a distance of one quarter of the radius of the dish from the centre thereof a planar sheet of maximum length will span the distance between five adjacent support members 115, 115a, 115b, 115a, 115, and therefore the central support member 115b need only be three quarters as long as the radius of the dish, and so on. Thus depending on factors such as the maximum size of the planar segments and the diameter of the dish, the lengths of adjacent support members may be expressed as numerically recurring series representing their lengths as proportions of the dish radius, namely: 1, K, IM, 1, M ; or 1, M, 7 4 , M, 1, M, 7 4 , ; or ι, κ, 7 4 , a, Vβ, %. 7 4 , *< !< H ' • or 1, H, 7 4 , %, Ve, M, 7 4 , X, 15/16, M, 7 4 , M, 7 / 8 , X , Each series may be represented as a recurring "set", each set comprising a number of support members which is a power of 2, i.e. 2, 4, 8, 16, 32, or 2 X support members. Thus, a dish may be constructed, as shown in Figure 25, which is both lighter and less expensive than conventional reflector dishes comprising a plurality of reflective planar sheets. The principle of construction of a supporting frame for a parabolic dish described above may be applied to dishes of any size. Figure 26a illustrates planar sheets formed of a self- supporting material such as polished aluminium or steel 127a or of glass 127b (Figure 25b) , like a conventional mirror. A glass planar sheet 127b would be supported by a frame 129 of strong but lightweight material, such as aluminium alloy. The planar sheets 127 are mounted to the support members 115, as shown in Figure 27, by an adjustable bolt system 133 passing through holes 131 in each planar sheet 127a or each frame 129 whereby each planar sheet 127 may be aligned so as accurately to reflect light into the receiver by means of a laser 135 (as shown in Figure 28) . The laser 135 is supported by and moved along a beam 137 perpendicular to the axis of symmetry of the parabola, so as to direct a laser beam parallel to that axis onto each planar sheet 127 about the mid point along each side thereof. The sheet 127 is then adjusted, by means of the bolt system 133, so as to reflect light directly into the aperture of the receiver 5. In a conventional mirror the reflective surface is the interface between the backing usually silver or aluminium. However light of wavelengths around 500nm is absorbed by normal or 'green' glass. This gives a maximum reflection efficiency of 85% with a silver back. This can be increased to 94% by reducing the iron contect of the glass. This kind of glass is already used in some solar thermal applications. SORB front face mirrors are coated with a thin layer of silica to protect them. Silica transmits from well into the UV to about 2500-3000nra. The IR between 3000 and 4000 is not absorbed by the atmosphere but would be by the silica. If silica were to be used it could be used for the parabolic dome window, but it could also be difficult to fabricate in large complex sizes. The present invention provides a more efficient and less expensive apparatus for generating electrical power from solar radiation than known systems, and one which is protected from wind damage. It will be apparent that the features of receiver construction and of dish construction may be applied independently of the non-gimbal mounting and weathershield structure herein described.

Claims: CLAIMS 1. Apparatus for collecting and concentrating solar radiation comprising a reflector dish, means for rotating the dish about a first, substantially vertical axis, means for pivoting the dish about a second axis, the second axis lying in a plane parallel to but not coincident with the vertical axis, the pivoting means being effective to pivot the dish to any position between a first position in which the circumference of the dish lies in a substantially horizontal plane and a second position in which the circumference of the dish lies in a substantially vertical plane, and a weathershield extending around the vertical axis so as at least partially to surround the circumference of the dish in the first position, the height of the weathershield being such that the upper edge thereof is level with or extends above the circumferential edge of the dish in the first position. 2. Apparatus as claimed in Claim 1 wherein the second axis is substantially horizontal. 3. Apparatus as claimed in Claim 1 or 2 wherein the second axis is fixed in relation to the first axis. 4. Apparatus as claimed in Claim 1, 2 or 3 comprising a cradle and means to rotate the cradle about the first axis, the dish being mounted so as to rotate with the cradle and to pivot with respect thereto. 5. Apparatus as claimed in Claim 4 wherein the weathershield is mounted to the cradle so as to rotate therewith or mounted so as to move independently of the dish. y6. Apparatus as claimed in any preceding claim wherein the second axis lies within the circumference of the weathershield. 7. Apparatus as claimed in Claim 2, or in any one of Claim 3 to 6 when dependent on Claim 2, wherein the height of the second axis above the lower edge of the weathershield is equal to or greater than the maximum horizontal distance between the second axis and the circumference of the dish when the dish is in the first position. 8. Apparatus as claimed in Claim 2, or in any one of Claims 3 to 7 when dependent on Claim 2, wherein the weathershield does not extend about that portion of the circumference of the dish subtended by the second axis when the dish is in the first position. 9. Apparatus as claimed in any preceding claim comprising means to sense wind speed and direction, the pivoting means being adapted to lower the dish to the first position when the sensed wind speed exceeds a predetermined speed. 10. Apparatus as claimed in Claim 9 wherein the rotating means are adapted to rotate the dish to a position where the second axis is perpendicular to the sensed wind direction. 11. Apparatus as claimed in any preceding claim comprising control means adapted to actuate the rotating means and the pivoting means to move the dish so as to follow the sun through its daily and seasonal motions. 12. Apparatus as claimed in any preceding Claim wherein the dish is a point focus reflector dish, the apparatus comprising receiver means located adjacent the focal point of the dish for absorbing solar radiation collected and concentrated by the dish for the generation of electrical power. 13. Apparatus as claimed in Claim 12 wherein the receiver is supported on beams which project from the dish to the receiver outside the reflected ray path. 14. Apparatus as claimed in any preceding claim wherein the reflector dish is parabolic and comprises a plurality of planar reflective segments mounted to and supported by a frame. 15. Apparatus as claimed in Claim 14 wherein the frame comprises a plurality of support members extending radially inwardly from the outer circumferential edge of the dish toward the centre thereof and having a planar segment supporting surface formed as a sector of a parabola or in a series of linear steps approximating thereto, adjacent support members being connected together. 16. Apparatus as claimed in Claim 15 wherein some but not all of the support members extend from the outer circumferential edge of the dish to the centre thereof . 17. A point focus parabolic reflector dish for collecting and concentrating solar radiation comprising a plurality of planar reflective segments, the frame comprising a plurality of support members extending radially inwardly from the outer circumferential edge of the dish toward the centre thereof and having a planar segment supporting surface formed as a sector of a parabola or in a series of linear steps approximating thereto, adjacent support members being connected together. 18. A method of use of an apparatus as claimed in any preceding claim the method comprising moving the dish in azimuth and elevation so as to track the diurnal and seasonal movement of the sun and in the event of wind speed exceeding a predetermined speed moving the dish to the said first position where the circumference of the dish is at least partially surrounded by the weathershield.

Inventors: HARRISON JOHN, HARRISON, JOHN
Applicants: HARRISON JOHN, HARRISON, JOHN
Application year: 1996
Main classification: F24J2
IPC codes: F24J2