Patent Application
20070246095
1. Field of the Invention
This invention relates to apparatus for generating electrical power from solar radiation, and, more particularly, to such apparatus including a reflector concentrating solar radiation on a transducer producing electrical energy, in which the reflector and the transducer are moved to follow the relative movement of the sun.
2. Summary of the Background Art
The most common method for directly converting solar energy to electricity is the photovoltaic cell, in the form of a thin, flat disk-shaped or square element that is arranged within an array of elements formed in a flat solar panel. Generally, the solar panel is not moved to maintain alignment with the sun, but is arranged in a fixed location at an angular orientation providing maximum electrical output on a basis averaged over daily and yearly operation. The electrical output of such a solar panel is naturally limited by the efficiency of the photovoltaic cells and by the total area of the photovoltaic cells exposed to the ambient level of solar radiation.
It is particularly desirable to substantially increase the electrical output of a number of solar cells above the level that can be provided with the cells merely exposed to ambient sunlight, particularly since the cost of the photovoltaic cells represents a significant portion of the total cost of providing electrical power from solar energy, and additionally because the worldwide availability of photovoltaic cells is limited. It is known that photovoltaic cells are capable of efficient operation at light levels higher than the level of ambient sunlight, so a number of patents describe means for providing individual light concentrators that focus light on the surfaces of individual photovoltaic cells within a solar panel, In such an arrangement, the individual photovoltaic cells are spaced apart from one another so that light can be collected from an area larger than the area of the individual cell on which the light is focused. Examples of such an arrangement are found in U.S. Pat. No. 4,002,031, which shows individual lenses concentrating solar energy directed to individual cells, and in U.S. Pat. No. 4,106,952, which shows a transparent sheet extending over spaced apart cells, with the sheet including a portion shaped as a concentrating lens over each cell. Another such arrangement is found in U.S. Pat. No. 4,148,298, which shows an array of photovoltaic cells, with each cell facing but toward a curved reflector serving as a collector and concentrator.
Whet is needed is an apparatus achieving a relatively high ratio of concentration of radiant energy from the sun without requiring that the individual photovoltaic cells must be spaced apart from one another. Mounting such cells close to one another provides for a more compact structure of heat receiving elements, making simple arrangements for cooling and electrical connections possible.
It is well known that a parabolic reflector reflects parallel rays to its focal point when the optical axis of its parabolic surface is aligned with the parallel rays. The patent literature includes a number of descriptions of apparatus using this principle to concentrate energy from the sun's radiation to produce heat, with the parabolic reflector being pivoted at two axes to track the movement of the sun relative to the earth. For example, such devices are described in Published International Pat. App. No. WO 2004/099682 A2 and in U.S. Pat. No. 5,275,149. As described in U.S. Pat. No. 4,198,826, in a Fresnel reflector design, the reflector surface may be divided into a number of concentric annular surfaces, each of which is shaped as a portion of a cone so that parallel light rays are reflected to a common focal point. The reflective surfaces may be further broken up into an array of mirrors individually angled so that parallel rays are directed to a common point, as described in U.S. Pat. No. 3,466,119, or reflected off one or more additional surfaces to a common point, as described in U.S. Pat. No. 4,784,700.
Since such arrangements of reflectors reflect sunlight from a relatively large area to a small area around a focal point, the levels of light and heat are higher than those required for efficient operation of photovoltaic devices over long periods of time. Thus, another arrangement, such as a circulating fluid carrying heat away from the heated area and providing for its use through a remote heat exchanger, or heating a surface of a boiler producing steam to drive an engine, is required. Thus, what is needed is an apparatus using a reflector to direct sunlight to an array of photovoltaic cells arranged with the reflector, with the area of the cells being substantially larger than the area in the vicinity of the focal point that is heated by such reflector systems of the prior art. Additionally, it is desirable to incorporate the array within the reflector, so that it is not necessary to provide a structure extending outward from the reflector to a focal point.
In accordance with a first aspect of the invention, an electrical power generator is provided, including a concave reflector, an energy receiving structure, a stationary mounting structure, a first pivotal mounting structure, and a motor. The concave reflector has a reflective surface extending along a surface of revolution formed about a central axis, with the reflective surface reflecting light from a light source aligned with the central axis inward, toward the central axis, within the concave reflector. The energy receiving structure, which is attached to the concave reflector, extends within the concave reflector, having a photovoltaic surface facing outward from the central axis of the central axis in directions perpendicular to the central axis. The first pivotal mounting structure attaches the concave reflector to the stationary mounting structure while allowing pivotal movement with the sun during movement of the sun relative to the stationary structure.
Preferably, the photovoltaic surface of the energy receiving structure includes a plurality of photovoltaic cells attached to a cell support structure to face outward from the cell support structure, perpendicular to the central axis, with the cell support structure including a plurality of channels through which a coolant is circulated. For example, the cell support structure may include a plurality of walls with each wall including a plurality of channels extending between opposite ends, an outer manifold, an inner manifold, at least one inlet tube for a flow of cooling fluid into one of the channels, at least one outlet tube for a flow of cooling fluid outward from another one of the channels, and a number of passages for a flow of cooling fluid between the channels.
For example, the concave reflector includes an inner reflective portion an outer reflective portion extending outward from the inner reflective surface. The inner reflective surface portion extends along a first conical surface extending at a solid angle greater than ninety degrees, while the outer reflective surface portion extends along a second conical surface extending at a solid angle of ninety degrees. Preferably, a central support plate holds the energy receiving structure within the concave reflector, while a number of support ribs extending outward from the central support plate are attached to the concave reflector.
Preferably, the electrical generator additionally includes a second pivotal mounting structure, attaching the concave reflector to the stationary mounting structure while allowing pivotal movement of the concave reflector about a second axis of rotation, perpendicular to the first axis of rotation. A first version of the electrical generator also includes another motor, pivoting the concave reflector about the second axis, while a second version has the second axis extending parallel to the direction of travel of the sun relative to the earth, so that manual adjustments in rotation about the second axis can be made more infrequently to compensate for seasonal changes.
The concave reflector 24 has a reflective surface 40 extending along a surface of revolution formed about a central axis 41. It is understood that a surface of revolution is a surface formed by revolving a plane curve about the central axis 41, with the central axis 41 therefore being a feature of the reflective surface 40. The reflective surface 40 reflects light from a light source, such as the sun, aligned with the central axis 41, with such light traveling in the direction of arrow 42 parallel to the central axis 41, inward toward the central axis 41, within the concave reflector 12. The energy receiving structure 14, which is attached to the concave reflector 12, extends within the concave reflector 12, having a photovoltaic surface 44 facing outward from the central axis 41 in directions perpendicular to the central axis 41. A photovoltaic surface is understood to be a surface composed of a material causing a voltage to be generated when the surface is struck by incident light energy.
In the example of
An exemplary version of the cell support structure 74 will now be discussed, with reference being made to
The cell support structure 74 additionally includes three alignment rods 96 extending between the manifolds 84, 86 and between wall sides 98 adjacent to the inlet and outlet tubes 88, 90 of the inner manifold 86. Three alignment rods 100 extend between wall sides 102 that are not adjacent the inlet and outlet tubes 88, 90, with each of the alignment rods 100 also extending between the inner manifold 86 and a bridge plate 104, which serves as a side of a passage 106 providing for a flow of cooling fluid between channels 78 within adjacent walls 80. The bridge plate 104 is held within the slot 94 of the outer manifold 84.
The inner manifold 85 is attached to the inner plate 68 by a number of screws 108 (shown in
Data and instruction storage 128 stores a BIOS (Basic Input/Output System) subroutine 144 and an operating system 146, providing for the operation of the controller 124, including the generation of a time of day clock function generating information on a real time basis indicating the time and date. This information is used within a solar tracking program 148 to develop data indicating the position of the sun relative to the stationary support structure 36 at all times during daylight hours. Mathematical functions for describing the position of the sun at all times are both reliable and well known, with solar movement being probably the first technical problem studied by mankind. The data and instruction storage 128 additionally stores a reflector positioning program 150, which determines error signals indicating differences between actual position of the concave reflector 12, as indicated by data signals from the encoders 136, 138 and the solar position calculated by the solar tracking program 148, and which provides signals for driving the motors 26, 32 to reduce the magnitude of these error signals. In this way, the concave reflector 12 is moved so that its central axis 41 remains in alignment with the sun. During nighttime, the concave reflector 12 is returned by the positioning program 150 to a morning position to begin the process of following the movement of the sun the next day.
Preferably, the controller 124 additionally includes a number of input switches 152, producing signals processed within a switch adapter 152, which are used for initializing the controller 124, with the date and time being set. A small LCD display 156 is provided for indicating the operational state of the system and the present date and time, with signals being processed within a display adapter 158.
During daylight hours, the first and second error signals are calculated as differences between the positions indicated by the encoders 136, 138 and the position of the sun, as calculated by the solar tracking program 148, so that the concave reflector 12 is moved to align its central axis 41 with the sun. During nighttime, the first and second error signals are calculated as differences between the positions indicated by the encoders 136, 138 and a starting position into which the sun will move the next morning, so that the concave reflector 12 is moved to align its central axis 41 with this starting position.
In the electrical generator 10, a vertical second axis 24 was provided to allow the convenience of having a pivot shaft 190 extending within the vertical post 36. However, with this arrangement, the concave reflector 12 must be rotated about both axes 22, 24 to track the movement of the sun during a single day.
Alternately, the system of axes of rotation discussed above in reference to
The present invention is seen to provide a number of advantages over prior art systems including parabolic reflectors focusing solar radiation on a small area. Since the present invention directs solar radiation to an energy receiving structure 14 occupying a much larger area, conventional photovoltaic cells can be used to convert solar energy directly to electrical energy without overheating. Furthermore, a structure holding a heat receiving element outward from the reflector, at the focal point of a parabolic reflector, is not required. Furthermore, the fact that solar energy is directed at a much larger area means that substantial latitude is allowed for variations in the construction of the reflector and in the required accuracy of solar tracking.
While the apparatus has been described and shown in its preferred embodiments with some degree of particularity, it is understood that this description has been given only by way of example, and that many variations can be achieved without departing from the spirit and scope of the invention, as defined in the appended claims.
