Focusing solar collector

A focusing solar collector which utilizes an elongated parabolic shaped mirror made from a sheet of drapable material so that when draped in a catenary like curved configuration, the sun's rays may be focused on a linear target which is axially aligned with the axis of the mirror. Means are provided for adjusting the angle of the drape of the catenary like curve in order to maintain the focus of the sun's rays on the linear target as the relative diurnal position of the sun to the collector changes. The optimum catenary like curve for the range of the drape angles involved is achieved by using a non-linear distribution of weight along the cross-section of the draped mirror material.

FIELD OF THE INVENTION 
This invention relates to an improved solar collector. More particularly, 
this invention relates to a focusing solar collector of the type utilizing 
an elongated parabolic shaped mirror. 
DESCRIPTION OF THE PRIOR ART 
An example of a focusing solar collector which utilizes an elongated 
parabolic mirror is disclosed in U.S. Pat. 2,247,830 issued on July 1, 
1941 to C. G. Abbott. The Abbott solar collector discloses a mirror having 
a rigid shape. 
The book "Direct Use of the Sun's Energy" by Farrington Daniels, 1964 
edition, describes on pages 42-57 a number of focusing solar collectors. 
The author describes the use of aluminized plastic film for the reflective 
surface. However, the aluminized plastic film is applied to the surface of 
a rigid parabolic form. He also cites an example of using air pressure to 
stretch a plastic film into a spherical form as a means of adjusting the 
focus during the process of building a rigid mirror. He then points out 
that when an aluminized plastic film is stretched, the stretching reduces 
it reflectivity appreciably. 
Other prior art of general interest are the following U.S. Pat. Nos.: 
3,974,824 issued Aug. 17, 1976 to Smith; 
4,000,734 issued Jan. 4, 1977 to Matlock; 
4,015,585 issued Apr. 5, 1977 to Fattor; 
4,022,188 issued May 10, 1977 to Cohen et al. 
The prior art cited above is representative of the state of the art with 
respect to focusing solar collectors in so far as it applies to the 
present invention. All prior art focusing solar collectors use mirrors 
having rigid shapes. 
DESCRIPTION OF THE INVENTION 
Before describing the present invention, several of the design problems 
solved by the invention will be discussed. 
The energy absorption capacity of a solar collector is proportional to the 
absorption area upon which the solar energy is received. In order to be 
practical, a solar collector must have a relatively large energy 
absorption area. A solar collector having an elongated parabolic mirror of 
a rigid shape is consequently relatively large and heavy, requiring a 
relatively expensive and powerful focusing mechanism. Therefore, there is 
an advantage to providing a design which has a light weight mirror so that 
the focusing mechanism required is relatively less expensive and less 
powerful. 
The cost of producing an elongated parabolic mirror having a rigid shape 
includes the cost of the rigid form and the cost of preparing the 
reflecting surface. When an aluminized plastic film is used for the 
reflecting surface, the cost also includes the film and the mounting of 
the film. Therefore, there is an obvious cost advantage in being able to 
use an aluminized plastic film without the need for a rigid form or the 
cost of mounting the film on the rigid form. 
In most focusing collectors, the target is rigidly mounted so as to avoid 
the need for movable joints through which the heated medium must pass as 
it conveys the heat collected to other apparatus. Therefore, the focusing 
mirror structure must be such that it can be focused onto a rigidly 
mounted target. In addition, the target should be mounted in a position 
which does not block the path of the sun's rays received by the mirror. 
In the discussion of the prior art, it was pointed out that a spherical 
parabolic mirror made of a plastic film could be brought into focus on a 
target by applying air pressure and then made into a rigid mirror. No 
suggestion was made that air pressure could be utilized as a means for 
automatically controlling the focus of the mirror. If air pressure were 
used for automatically focusing an aluminized plastic film, it would 
require a supply of air under pressure and the controls for applying a 
pressure to the mirror. It is not obvious how the same principle could be 
applied to shape an elongated parabolic mirror. For example, a 
cylindrically shaped mirror might readily be adjusted in diameter by air 
pressure. However, the stretching of the plastic film results in a 
substantial reduction in the reflectivity of the aluminized surface. It is 
therefore an advantage to be able to adjust the focus of a mirror made of 
aluminized plastic film without stretching the plastic film or requiring a 
source of air under pressure and the controls essential to controlling the 
inflation pressure. 
It is an object of this invention to provide a novel focusing solar 
collector which has a light weight parabolic shaped mirror adapted to be 
focused on a target. 
It is a further object of this invention to provide a novel focusing solar 
collector which has a light weight parabolic shaped mirror having an 
adjustable shape for maintaining the focus on the target for changing 
positions of the sun with respect to the collector. 
It is another object of this invention to provide a novel focusing solar 
collector which has a light weight parabolic shaped mirror adapted to be 
adjustably focused on a rigidly mounted target. 
It is a still further object of this invention to provide a novel focusing 
solar collector having a mirror which is inexpensive to construct. 
It is still another object of this invention to provide a novel focusing 
solar collector which requires a minimum amount of energy to operate. 
The objects of this invention are achieved by forming a novel elongated 
parabolic mirror by draping an aluminized plastic film to form a modified 
catenary curve. Inasmuch as no rigid form is used, the parabolic mirror is 
inexpensive to make and light weight. 
The term "catenary" is defined as the curve assumed by a cord when the cord 
is uniformly loaded along its length. In the case of a very flat arc 
produced by a taut cord, the equation for such a catenary curve closely 
approximates that equation for a parabola. The invention departs from the 
above definition in two ways. First, the elongated parabolic mirror is 
formed by draping a plastic film, thus adding a width dimension not found 
in a cord. Secondly, for the axis of the parabola to be aligned with to 
the sun's rays, the angle of drape is greater than that at which a 
suspended cable will approximate a parabolic curve. Therefore, the 
cross-section of the pliable film is made non-uniform so as to compensate 
for the range of angles through which the plastic film may be moved in 
order to maintain the focus of the rays on a rigidly mounted target which 
is located so that it does not block the path of the rays received on the 
reflective surface of the mirror. 
The adjustment for focusing the rays on the target may be performed either 
manually or automatically. The automatic control system is preferably one 
which is actuated by sensing elements located in the proximity of the 
target so as to maintain a focusing position which directs the maximum 
available energy to the target area.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIGS. 1 and 2, the focusing solar collector 10 of this 
invention is enclosed in casing 11, which has a transparent panel 12 along 
one side thereof through which the rays of the sun pass to impinge on the 
reflective surface 13a of a sheet of drapable material 13. Drapable 
material 13 may be a plastic film having one surface aluminized to provide 
a reflective surface 13a. Drapable material 13 is attached at its lower 
edge by cleat 14 to the upper surface of rib 15, which is mounted on the 
inner surface of casing 11. Thus the mounted edge of drapable material 13 
is raised by rib 15 above the surface of casing 11. The upper edge of 
drapable material 13 is attached to bar 16. Thus drapable material 13 
extends parallel to the longitudinal axis of casing 11 and drapes at an 
angle to the horizontal axis of casing 11. On each end of casing 11 is 
mounted arced track 17 arranged so that rollers 18 rotatively mounted on 
each end of bar 16 will travel along the upper surface of arced track 17. 
The center of an arced track 17 is on the axis parallel to the lower edge 
of drapable material 13. 
Mounted on the inner surface of casing 11 in a central position near the 
top thereof are control circuit 19 and positioning motor 20. One end of 
cord 21 is attached to pulley 22 of positioning motor 21 and the other end 
thereof to bar 16. Sensing elements 23, 24 and 25 are connected to control 
circuit 19 and sensing elements 26, 27, and 28 (not show in FIG. 1, see 
FIGS. 2, 4 and 5) are also connected thereto. 
Mounted longitudinally within casing 11 in the space adjacent the upper 
edge of the transparent panel 12 parallel to the longitudinal axis of 
casing 11 is target chamber 29 which is covered by transparent panel 30 to 
provide a "green house" effect. Within chamber 29 is mounted black body 31 
which absorbs the energy of the sun's rays and is adapted to transfer that 
energy to other apparatus (not shown). By way of example, black body 31 
may be a conduit through which either air or water flows and absorbs 
energy for transfer to the other apparatus for utilization. 
The operation of focusing solar collector 10 will now be explained with 
reference to FIGS. 1, 2 and 3. The mirror surface 13a of drapable material 
13 is oriented so that its longitudinal axis is at right angles to solar 
south. The angle .theta..sub.1 of the sun's rays 32 from horizontal 
depends upon the latitude at which focusing solar collector 10 is located, 
the season of the year and the time of the day. At a latitude of 
40.degree., for example, the useful range of angle .theta..sub.1 may vary 
from 10 degrees in the winter to 73.5 degrees in the summer. Therefore, in 
order that the sun' rays 32 which impinge on mirror surface 13a of 
drapable material 13 will be reflected along path 33 to impinge on black 
body 31, the drapable material 13 must assume a shape which is 
substantially parabolic regardless of the drape angle .theta..sub.3 
represented by a line through the edges A and B of drapable material 13. 
The edges A and B of drapable material 13 are located to present an 
effective mirror surface within a rim angle .theta..sub.2 of 90 degrees, 
which is equal to one half of a parabolic curve. 
When the drape angle .theta..sub.3 is zero degrees, drapable material 13 
will assume a parabolic curve provided it is taut and uniformly loaded 
between edges A and B; i.e. is uniform in thickness. However, the further 
the drape angle .theta..sub.3 departs from zero degrees, the greater the 
departure of the assumed curve will be from a true parabola. In order to 
cause the drapable material 13 to assume a curve which is substantially 
parabolic when the drape angle .theta..sub.3 departs from zero degrees, 
the drapable material 13 is provided with some slack and a series of 
weights W.sub.1-W.sub.4 are attached longitudinally to drapable material 
13 at empirically selected distances from edge B and with empirically 
selected weights to produce a nonuniform weight distribution of drapable 
material 13 between edges A and B. The weight of weights W.sub.1-W.sub.4 
and their location necessary to produce the parabolic shape varies as the 
drape angle .theta..sub.3 varies. Therefore, the selection of the best 
combination of weights W.sub.1-W.sub.4 and their location on drapable 
material 13 may be for a range of drape angles .theta..sub.3 best for the 
principle purpose of the focusing solar collector. For example, at 40 
degrees latitude, the range of the drape angle .theta..sub.3 for providing 
heat during the winter months might be from 10 to 50 degrees. 
Referring to FIGS. 1 and 2, drape angle .theta..sub.3 is automatically 
adjusted by positioning motor 20. Sensing elements 23, 24, 25, 26, 27 and 
28 are protected from the direct rays of the sun and are conncted to 
control circuit 19 so that when the path 33 of the reflected rays 
energizes any one of sensing elements 23, 24 and 25 more than sensing 
elements 26, 27 and 28, then positioning motor 20 unwinds cord 21 to lower 
bar 16 along arced track 17 until the energization of each group of 
sensing elements has equalized. In like manner, when any one of sensing 
elements 26, 27 and 28 is energized more than sensing elements 23, 24 and 
25, positioning motor 20 winds up cord 21 to raise bar 16 along arced 
track 17 until the energization of each group of sensing elements has 
equalized. Thus, the path 33 of the reflected rays of the sun is 
maintained focused on black body 31. 
FIG. 4 shows focusing solar collector 10a in which lower edge A of drapable 
material 13 is moved while upper edge B in maintained in a fixed position. 
Lower edge A of drapable material 13 is attached to bar 16a. At each end 
of casing 11 is mounted arced track 17a arranged so that rollers 18a 
rotatively mounted at each end of bar 16a will travel along the upper 
surface of arced track 17a. The center C of the arc of arced track 17a is 
empirically selected for the range of drape angles .theta..sub.3 for a 
particular focusing solar collector 10a. The upper edge B of drapable 
material 13 is attached to bar 35 which is mounted longitudinally in 
casing 11. 
Target chamber 29, transparent cover 30, black body 31 and heat sensing 
elements 23, 24, 25, 26, 27 and 28 are located in the same position within 
casing 11 as in the embodiment shown in FIGS. 1 and 2. 
Positioning motor 20a is mounted below arced track 17a. One end of cord 21a 
is attached to pulley 22a of positioning motor 20a and passes over pulley 
36 attached to the outer end of arced track 17a. The other end of cord 21a 
is attached to bar 16a. 
The operation of focusing solar collector 10a is the same as that of 
focusing solar collector 10, except that the lower edge A of drapable 
material 13 is moved as the means for adjusting drape angle .theta..sub.3. 
FIG. 5 shows focusing solar collector 10b on which both ends A and B of 
drapable material 13 are moved simultaneously. Upper rod 16b is attached 
to upper edge B of drapable material 13 and lower rod 16c is attached to 
the lower edge A of drapable material 13. Each of the rods 16b and 16c is 
mounted on arms 37 and 38 respectively of frame 39. Positioning motor 20b 
is mounted on the upper surface of casing 11. One end of cord 21b is 
attached to pulley 22b of positioning motor 20b. The other end of cord 21b 
is attached to bar 16b. 
Target chamber 29, transparent cover 30, black body 31 and sensing elements 
23, 24, 25, 26, 27 and 28 are mounted in the same positions within casing 
11 as in the embodiments shown in FIGS. 1 and 2. In this FIG. 5, black 
body 31 is indicated as hidden by pivot 40 of frame 39. 
The operation of focusing solar collector 10b is the same as that of 
focusing solar collector 10, except that both the lower edge A and upper 
edge B of drapable material 13 are moved simultaneously as the means for 
adjusting drape angle .theta..sub.3.