Solar collector with altitude tracking

#In a publication entitled "Stationary Mirror Systems for Solar Collectors" by H. Tabor which appears in Solar Energy, Vol. II, Nos. 3-4, July-October 1958, at pages 27-33, there is defined an angle in solar geometry called the EWV altitude. In the course of a day, the sun moves over a large angle as it sweeps the sky from sunrise to sunset. It also has a smaller angular movement related to the rise and fall of the sun. In the publication by Tabor this smaller angular variation due to the altitude variation of the sun, which is both a daily and annular phenomenon is termed EWV altitude. Correction of solar collector position to account for the variation of EWV altitude is usually limited to variation in the seasonal tilt of the collector in accordance with the changes in the declination angle of the sun for a particular location. Correction for daily changes in the sun's altitude which would permit a reduced angle of acceptance for a cylindrical, solar radiant energy collector has not been practicable due to the involved equipment necessary.

#In cylindrical, radiant energy collectors, such as described in a publication by R. Winston appearing in Solar Energy, Vol. 16, No. 2, at pages 89-95, (1974) and in a prior U.S. application Ser. No. 492,074, entitled "Radiant Energy Collector," filed on July 25, 1974, now U.S. Pat. No. 4,002,499 and in the above-referred Tabor article, the collector configuration is determined by the angle of acceptance. The concentration factor is roughly inversely proportional to this angle so that the greater the angle, the less the concentration factor but the longer is the period of collection. If a collector could be provided which would allow reduced angle of acceptance without decrease in the hours of collection, higher concentration and simplified design of the collector would be achieved. In particular, in nonimaging cylindrical collection devices, the high concentration factors which are attainable are off-set by the large reflective surface area required for high concentration. By providing a means for varying the daily tilt of the collector according to the variation in the sun's EWV altitude, a truncated, cylindrical collector, i.e. one with shortened side walls and therefore less reflector area, can achieve as good a concentration as the untruncated collector. Thus the reflector utilization factor is improved. Improvement in the reflector utilization factor is also obtained in other concentrators, with increased concentration.

#It is therefore an object in this invention to provide an improved solar collector.

#FIG. 1 is a projection of the sun's trajectory on a vertical, north-south plane with respect to an observer;

#FIG. 2 is an overhead view of a collector utilizing a single heat responsive element;

#FIG. 3 is a section along line 3--3 of FIG. 2;

#Referring to FIG. 1 there is illustrated the position of the sun in the northern hemisphere projected onto a north-south plane, which includes the polar axis, with respect to an observer positioned at point 0. The illustration of the sun's position in the southern hemisphere is the same except north and south are reversed. Line 10 represents the projection of the sun's path at summer solstice with point M being sunrise and sunset, point N being solar noon and point P being midmorning and mid-afternoon positions such as 8:00 A.M. and 4:00 P.M. Line 12 represents the path of the sun at winter solstice with point M' being sunrise and sunset, N' being solar noon and point P' being the same time as point P, say 8:00 A. M. and 4:00 P. M. The EWV altitude angle refers to the altitude of the particular points of the sun's position with respect to the horizontal. In cylindrical, solar energy collectors, the designed acceptance angle determines the hours of collection, with minimum daily hours of collection being at either solstice. Suppose the hours of collection at either solstice are to be from 8 to 4, then energy radiated by the sun at EWV altitudes between points P and N or points P' and N' determines the angle of acceptance. Therefore, the angle of acceptance would be angle A or angle A' which are of the same value. For a cylindrical nonimaging collector, this is more particularly defined in a publication referred to above by Winston. For imaging parabolic, cylindrical collectors, the design is more particularly described in the article by Tabor. Generally, an external mechanism is provided for tilting the collector to correct for changes in the sun's declination so that its optical axis is directed generally to the center of the angle of acceptance. At summer solstice, this would correspond to line 14 which passes through point R. The concentration factor of such cylindrical collectors is roughly inversely proportional to the acceptance angle. Thus, increasing this angle to increase the hours of collection decreases concentration, while decreasing the angle of acceptance to increase concentration, decreases the hours of collection. According to the invention herein disclosed, a mechanism is provided for rotating the collector to allow for a reduction in the angle of acceptance without decrease in the hours of collection. For example, in FIG. 1 with a collector having an angle of acceptance of A/2, rotation between two positions would give the same hours of collection as a collector having an acceptance angle A without rotation between two positions. The two positions would be, first, where the collector was receiving energy from the sun while the sun was at EWV altitudes between points P and R which would occur in the morning and the afternoon, and, second, where the collector was receiving energy from the sun while the sun was at EWV altitudes between points R and N which would occur at midday.

#Referring to FIG. 2 and FIG. 3, there is shown a mechanism for rotating a collector 20 between two positions each day to account for the variation in EWV altitude of the sun. The collector 20 is mounted on axle 22 which is supported by frame 24. The solar collector, which is a device which absorbs and often concentrates energy received at its frontal face 26, is capable of rotating about axle 22. Axle 22 may be positioned away from the center of gravity of collector 20 so that collector 20, due to its own weight, will tend to turn about axle 22. Alternatively a spring or counterweight may be used to give collector 20 a biased position. Stop post 28 limits this tendency so that under no other mechanical influences collector 20 will tend to be in position C shown by the solid lines in FIG. 3.

#Positioning of axle 22 with respect to the earth's polar axis should allow generally for rotation of collector 20 from north to south and from south to north about an east-west horizontal axis. Thus, axle 22 should be generally oriented east to west. The mechanical means for varying the collector tilt includes two heat expansive elements 30 and 31 which are positioned on one side of shadow plate 32, which is mounted on frame 24. Each element 30 and 31 is anchored at one end to anchor plate 34 extending from plate 32. The other end of each element 30 and 31 is coupled to cables 35 and 36 respectively. Cables 35 and 36 extend around rollers 37 and 38 respectively, which are also mounted on frame 24, and each cable is coupled to one end of collector 20 at points 40 and 42.