Renewable surface heliostat type solar mirror

A support surface having the shape of a desired mirror surface for use in reflecting solar light and a sheet of flexible mirror material with a portion covering the support surface. A reel device for selectively changing portions of the flexible mirror material on the support device, and a device for adhering the sheet of flexible material to the curvature and surface of the support surface. The mirror also includes devices for preventing wind lifting of the flexible material from the support surface and for correctly positioning the solar mirror relative to the sunlight.

BACKGROUND OF THE INVENTION 
Arrays of mirrors of the heliostat type are being used to direct solar 
light to a receiver. To make utilization of solar energy practical, it is 
necessary that a large number of heliostats are used. Thus, a large number 
of mirrors surfaces must be constructed, arrayed, cleaned, protected from 
the elements and eventually replaced. 
Mirror surfaces are expensive to construct to the desired configuration and 
the surfaces are oftentimes subject to destruction by the elements in a 
relatively short period of time. Mirror surfaces that are not subject to 
destruction in short periods of time are often prohibitively expensive and 
are required to be made from exotic and expensive materials. Yet even 
using these materials, the mirror surfaces have to be cleaned often 
because of the elements. Where the mirror surfaces may cover an area in 
the order of several acres, the task of maintaining the mirror surfaces in 
good condition, cleaning their surfaces and replacing the surfaces from 
time to time becomes prohibitively expensive. 
Because of these limitations and because of the low power of solar energy, 
oftentimes the operational expenses involved in converting solar energy 
into usable power are so large that solar energy becomes a hopelessly 
non-competitive power source. It is therefore an advantage to have a 
renewable surface heliostat type solar mirror in which the surfaces can be 
kept reasonably clean and selectively replaced, in a manner that is less 
expensive than constructing and replacing expensive mirror surfaces and 
keeping the mirror surfaces clean. 
SUMMARY OF THE INVENTION 
In a preferred embodiment, a renewable surface heliostat solar mirror has a 
support means with a surface for supporting a sheet of flexible mirror 
material. The surface is a substantially planar surface that is curved for 
reflecting solar light to a receiver in a manner determined by the system 
and the focal length of the distance between the mirror and the receiver. 
The mirror material is a strong material that may, for example, be made of 
Mylar having a metalized or other suitable flexible mirror surface secured 
thereto. The mirror material is rolled onto and off of positioning reels 
that are positioned at opposite ends of the support surface. 
A means is employed for adhering the flexible mirror material to the 
support surface. In one embodiment, this comprises a vacuum means that 
draws a vacuum through holes in the support surface thus creating a 
suction on the back side of the flexible mirror material, causing the 
mirror material to be drawn against the conform to the support surface. In 
another embodiment, an eletro static field is set up across the mirror 
material and the support surface, whereby the electro static field causes 
the sheet material to adhere to the support surface. In the latter 
embodiment, the polarity of the field is such that the mirror surface 
carries a charge that tends to repel the accumulation of dirt and other 
particles on the surface of the mirror, tending to keep the mirror surface 
clean or allowing the wind or other means to quickly and easily blow such 
dirt particles from the surface of the metalized mirror surface. 
The support surface has a width that is only slightly larger than the width 
of the sheet of flexible mirror material. So means are provided along the 
edge of the support surface to prevent the wind from getting under the 
edge of the sheet material and lifting it from the support surface. Such 
means are provided in one embodiment by longitudinal lip or a resilient 
member secured to the lip that presses against the flexible mirror 
material preventing accumulation of dirt between the lip and the mirror 
material. 
The support surface is supported by a triangular support that is pivoted at 
the apex of the triangle. A positioning device pivots the triangular 
support and the mirror at an angle on the pivot point, with a gradual 
movement that allows the mirror to track the movement of the sun. Also a 
pair of rails are provided for supporting the mirror in vertical movement, 
which gradually allows the mirror surface angle to be changed to provide 
optimum reflection of the solar energy to the receiver and yet maintain 
the correct focal length between the center of the mirror surface and the 
receiver. 
It is therefore an object of this invention to provide a new and improved 
method and apparatus for providing a renewable surface heliostat type 
solar mirror. 
It is another object of this invention to provide a heliostat solar mirror 
in which the mirror surface can be selectively changed or renewed 
periodically. 
It is another object of the invention to provide a heliostat type solar 
mirror in which a flexible mirror material may be selectively positioned 
on the mirror surface. 
It is another object of this invention to provide a heliostat type solar 
mirror that is less expensive to operate, has a selectively replaceable 
mirror surface, and has a mirror surface that does not have to be cleaned 
except on rare circumstances.

Referring now to the drawings and to FIGS. 1, 2 and 3, the heliostat type 
solar mirror has a mirror support member 14 on which is mounted a flexible 
mirror material 68. This mirror material 68, see FIGS. 5 through 10, may 
comprise any suitable flexible mirror material and specifically may be a 
Mylar film having a metallized mirror surface thereon. The mirror surface 
has a reasonable life under the elements involved including dew and 
sunlight. The support surface 14 has motor operated reels at each end of 
the curved planar surface, which are located in housings 36 and 40. The 
motors 38 and 42 rotate the reels for positioning the given portion of the 
flexible mirror material onto the surface of the support means 14. For 
example, the reel motor 38 may be the rewind motor and motor 42 the wind 
motor. The motors 38 and 42 cooperate to either position new material onto 
the surface of the support means 14 or to replace the new mirror material 
with older and previously used flexible mirror material, such as at night. 
The support member 14 includes a sheet holding means for holding the 
flexible sheet material in adherence to the surface. In one embodiment, 
see FIGS. 5 through 9, the support means comprises a honeycomb panel 
having enclosed cells 78 that communicate with an isogrid of holes 80 
through the upper surface of the support means 14. Apertures 82 in the 
walls of the cells 78 provide air communication between the cells. A 
blower 70 is positioned in a housing 44 with an opening 76 to the cells 
78. The blower draws air through the apertures 80 and through openings 82 
and discharges the air through the hole 72 around flapper valve 74. When 
the flexible mirror material 68 is in position, this material closes holes 
80 allowing a negative pressure condition to be created by blower 70 in 
the cells 78 and holes 80. This creates a suction force on the flexible 
mirror material 68 across the entire upper surface of the support member 
14 that secures or adheres the flexible mirror material against the 
surface with the mirror configuration of the surface. When blower 70 has 
created the desired vacuum in cells 78, a known pressure sensing device 75 
detects the negative pressure limit and shuts down the blower 70 before 
the blower creates sufficient suction force through apertures 80 that will 
damage the flexible mirror material 68. Flapper valve 74 closes off hole 
72 to maintain the negative pressure condition. When a given lower limit 
of negative pressure is reached, then detector 75 reenergizes blower 70 to 
return the negative pressure in cell 78 to the desired higher limit. Thus, 
a given negative pressure range is maintained across the upper surface of 
the support means 14. 
In another embodiment, see FIG. 10, the support member 14 is a conductor 
104. The flexible mirror material comprises an insulative base 106, with 
an electrically conductive, metallized, mirror film surface 108. A 
suitable potential through an electrical circuit 110 and switch 112 is 
applied to the respective conductor surfaces creating an electro-static 
field and creating the adherence force between the sheet material 68 and 
the support plate 104. This creates sufficient force to hold the flexible 
mirror material into adherence with the upper surface of the support 
member 104. The electrical circuit is selectively energized and 
de-energized, thus selectively releasing the adherence of the flexible 
mirror material 68 to the surface of the support means 14 and allowing the 
reel motors 38 and 42 to move the film material as desired. 
The mirror material 68 is wound at each end on a reel 87 having a shaft 86, 
and end flanges 91, as in FIG. 6. The flange 91 has circumferential 
notches 114 to receive a detent 116 actuated by a solenoid 118, to lock 
the reel once the mirror material is positioned as required. It is also 
desirable to maintain a light tension on the flexible material, so that 
the surface does not become wrinkled or uneven and reduce the reflective 
efficiency. This is accomplished by a tension roller 120 mounted in 
housing 40 on a spring biased arm 122 to bear against the material. A 
similar arrangement, but for reverse rotation, is installed in housing 36 
for two way movement of the mirror material. 
The operation of the reel motors 38 and 42 are automatically controlled by 
timing means, illustrated in FIG. 11. In general, it is desirable to move 
a new portion of the flexible mirror material 68 onto the surface of 
support means 14 whenever the mirror material has become sufficiently 
deteriorated. However, it is also desirable that the new mirror surface 
only remain on the support surface 14 during times when actual reflection 
of solar energy is desired. At other times, the older and prior used 
surface of the flexible material can be inserted onto the support surface 
14. This would, for example, be at night, during storms or at other times 
when the film surface may be damaged by storms, elements, dew or the like. 
It is not necessary that the newer and better surface be subjected to such 
elements, and this movement will lengthen the life of the flexible mirror 
surface. In doing this, the rewind reel is rewound a sufficient number of 
revolutions to wind back the exact desired length of the previously used 
flexible mirror material. To aid in this, see FIG. 9, a capstan roller 90 
holds the material 68 against a measuring roller 92 connected to a known 
measuring device 94. This is used in connection with the control circuit 
of FIG. 11 to maintain exact positioning of the flexible material, 
notwithstanding the difference in size of the reflective feed diameters 
because of reeling on and off of lengths of the flexible mirror material. 
The heliostat array is often subjected to high winds. To protect the 
surface edges against lifting by such winds, longitudinal lip members 62, 
see FIG. 5, are positioned along the sides of the support means 14, and 
prevent wind from getting under the edge of the sheet material 68. It is 
possible in sandy areas such as on deserts and the like that there could 
be an accumulation of sand and dirt within the cavity of the lips 62, 
which accumulations could make it difficult to move the sheet material 68. 
In the embodiment of FIG. 6, a resilient edge member 84 is secured to lip 
62 that presses down against film 68 and prevents the entry of dirt, sand 
or the like into the restricted space defined by the lip. In another 
approach, see FIG. 7, the lower end of support member 14, see FIG. 1, has 
a slotted lip arrangement with lip portions 98 having slots 101 
therebetween and turbulence generating projections 103 thereon. This 
creates a wind fence that reduces the ability of wind to lift the edge 
surface of the flexible material 68 while at the same time providing slots 
through which dirt and sand particles can move out from under the lip 
portions 98. It is understood that the reel housings 36 and 40 are 
enclosed to prevent wind contact of the flexible mirror material 68 in 
passing from the surface of support member 14 to reels 87. 
In positioning the heliostat, see FIGS. 1, 2, 3 and 11, the support means 
14 is supported on a pair of V-shaped support legs 32 and 34, with a 
pivotal connection 58 at the apex thereof that secures the respective legs 
32 and 34 to a U-shaped member 20. U-shaped member 20 is supported at each 
end by means of rollers 54 and 56 for rolling movement along the arcuate 
T-rail 16. 
It is necessary to continously move the angle of the support member 14 with 
movement of the sun. This is accomplished by a motor operated screw 
mechanism 46 that is supported on a support member 60 and has a rotatable 
screw 50 that coacts with a pivotal member 52. Operation of the screw 
member 50 continuously pushes the support surface 14 in one direction 
around the pivotal connection 58. Accordingly, during the daytime, screw 
50 gradually moves outwardly rotating the support member 14 in a 
counterclockwise direction. At night, screw 50 moves in the opposite 
direction, rotating the support member 14 in the clockwise direction 
within limits established by the control mechanism. The pivotal member 60 
is supported on T-rail 17 by a U-shaped channel member 21 that has a 
structure and rolling support means similar to that of channel member 20. 
The two channels members 20 and 21 are connected by a pair of brace 
members 48 to form an integral frame. 
It is also necessary because of the movement of the sun in the north and 
south direction, that the support means 14 be rotated vertically. This is 
accomplished by screw member 28 that is rotated inwardly and outwardly in 
conjunction with a pivot holding means 30 that exerts the required force 
on the channel members 20 and 21 to move the heliostat along the arcuate 
rails 16 and 17. The pivotal end 24 of the motor operated screw mechanism 
22 is secured to a bracket 26 on one end of the T-shaped rail member 17. 
The arc of the rails 16 and 17 and the length of the channel members 20 
are such that movement of the channel member 20 and 21 along the rails 
always hold the focal length 15 between the mirror surface of the support 
means 14 and the receiver 12 at a constant focal length. 
In operation, see FIG. 11, the timer and reflector advance and rewind 
control 100 is a known programmed circuit in which the motors 38 and 42 
are operated at given time controlled sequences to move the respective 
portions of the flexible mirror material 68 onto and off of the support 
surface of the support means 14. The control 100 actuates solenoids 118 to 
release the reels 87 each time the motors are to be operated. At the same 
time, the control 100 also provides a release signal to the mirror surface 
holding control 102 wherein a solenoid mechanism 71, see FIG. 5, opens the 
flapper valve 74 and also de-energizes the motor 70, thus releasing the 
holding negative pressure on the flexible sheet material 68. In the 
embodiment of FIG. 10, the mirror surface and hold control operates a 
relay switch to open the switch 112 of the electrical circuit 110. Thus, 
the reel motors 38 and 42 move the film in the manner desired. When the 
film has moved the desired amount, then limit switches detect this 
positioning and through the timer and reflector advance and rewind control 
100 sends a signal through the mirror surface hold control 102 that 
de-energizes relay 71 and opens the switch to the motor 70, see FIG. 5, 
and in the device in FIG. 10 closes the switch 112. 
The sun synch control 105 is separately maintained under a controlled clock 
arrangement to provide the motor 47 with the controlled power to operate 
the screw device 46. The motor control for the screw device 22, not shown, 
is operated only periodically as the change in angle in the vertical 
direction is only slight for the entire year.