Data processing: generic control systems or specific application – Generic control system – apparatus or process – Digital positioning
Reexamination Certificate
2001-05-21
2004-03-09
Khatri, Anil (Department: 2121)
Data processing: generic control systems or specific application
Generic control system, apparatus or process
Digital positioning
C700S059000, C700S061000, C700S302000, C250S203400, C250S548000, C126S574000, C126S601000, C356S139010
Reexamination Certificate
active
06704607
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to methods and apparatus for controllably positioning a solar concentrator and, more particularly, to methods and apparatus for controllably positioning a solar concentrator that take into account at least one of a gravitational residue error, azimuth transfer function, and elevation transfer function error and an error due to atmospheric refraction.
BACKGROUND OF THE INVENTION
There is currently a large domestic and international market for clean non-polluting generated grid and remote electrical power, such as the electrical power generated by solar energy generating systems. This demand is anticipated only to grow. For example, over the next 12 years, the average growth of power consumption in California is estimated to be 700 MW per year, while Arizona consumers are anticipated to demand an additional 200 MW per year between the years 2000 and 2010. In addition, the state of Nevada has one of the fastest growing energy needs per capita in the United States with its electrical needs estimated to grow in excess of 200 MW per year. In the solar belt states alone, the estimated growth is anticipated to be more than 1000 MW per year. Furthermore, an estimate by the World Bank of the international electrical market in the solar belt countries is for growth of more than 2000 MW year.
Of this growth in power consumption, at least a portion will be solar energy. For example, the state of Arizona has decreed that 1% of all generated electrical power must be solar generated. This requirement creates the need for 350 MW of grid electricity in this state from solar energy alone. Other states in the solar belt, such as California, Nevada, New Mexico, etc., have or are expecting similar legislation.
A variety of solar-to-electrical energy conversion systems have been developed with the most cost-effective systems being concentrating solar energy systems that focus the energy of the sun to a relatively small area. One exemplary concentrating solar energy system is a Stirling dish developed by McDonnell Douglas Corporation. A Stirling dish includes a plurality of reflective facets disposed side-by-side upon a support frame to define a reflective surface. The reflective surface typically has a parabolic or spherical shape. The parabolic reflective surface of the Stirling dish concentrates the incident solar energy upon a power conversion unit that is located at the focal point of the reflective surface. In this regard, the power conversion unit is generally mounted upon the distal end of a support arm that extends forwardly of the Stirling dish. The support frame that carries the plurality of reflective facets and the support arm that carries the power conversion unit are mounted upon a pedestal which, in turn, is secured to a foundation within the earth. The pedestal permits the Stirling dish to move in both an azimuth rotational plane and an elevation rotational plane such that the Stirling dish can track the sun throughout the day. Thus, the Stirling dish also generally includes an azimuthal drive and an elevational drive, including an elevation activator, for providing the desired movement in response to azimuth and elevation commands issued by a controller or the like. Typically, these commands attempt to drive one Stirling dish to a position at which a centerline defined by the reflective surface is aligned with the sun. Other types of concentrating solar energy systems exist, however, including heliostats and other sun tracking solar concentrators.
There are two general types of tracking control systems for use with concentrating solar energy systems, namely, open loop and closed loop control systems. In a closed loop tracking control system, a sun sensor is aligned to the centerline defined by the reflective surface. As such, the sun sensor generates error signals between the centerline of the reflective surface and the line-of-sight to the sun, i.e., the sun reference vector. While closed loop tracking control systems can be effective, closed loop tracking control systems are generally quite expensive due to the addition of a sun sensor and the attendant cabling, additional interface electronics and increased operational and maintenance costs attributable to the additional hardware. Further, closed loop tracking control systems have difficulty maintaining track during periods of cloud cover. In this regard, if the reflective surface is not pointing at the sun when the sun comes out from behind the clouds, the concentrating solar energy system may be damaged. As such, open loop tracking commands must be calculated during the period of time in which the sun is behind the clouds. Additionally, further problems arise in instances in which the face or lens of the sun sensor becomes dirty, such as from dust, sand or other airborne particles. In this regard, a sun sensor relies upon the shading of the solar cells to obtain an error voltage. As such, a sun sensor having a dirty lens will unevenly illuminate the solar cells which, in turn, creates tracking errors and also loss of track during low sun irradiance levels. Closed loop tracking control systems also suffer from an additional cost of aligning the sun sensor to the centerline defined by the reflective surface and maintaining this alignment over time. Furthermore, the sun sensor oftentimes serves as a roosting place for birds which can cause additional problems, by altering the alignment of the sun sensor or soiling the lens of the sun sensor. As such, most concentrating solar energy systems utilize an open loop tracking control system.
In an open loop tracking control system, the position of the sun is calculated by a set of ephemeris equations. The reflective surface is then commanded to point toward the position of the sun. As a result of the open loop nature of this system, there is no feedback from the solar concentrator that the reflective surface is actually pointing at the sun. Unfortunately, the command coordinate system, i.e., the coordinate system in which the commands that direct the position of the solar concentrator are formulated, and the concentrator coordinate system, i.e., the actual coordinate system defined by the physical construction of the solar concentrator, are generally somewhat misaligned. As such, the centerline defined by the reflective surface may not be pointing directly at the sun even though commands have been issued that would have caused the reflective surface to point at the sun if the concentrator coordinate system and the command coordinate system were identical. Typical sources of error that will cause the misalignment of the command coordinate system and the concentrator coordinate system are pedestal/foundation tilt errors, elevation pivot point manufacturing tolerances, azimuth and elevation reference errors, gravity bending of the structure, atmospheric bending of the sun rays, reflectivity surface misalignment errors, elevation actuator offset errors and errors inherent in the mathematical model utilized by the control system.
The deleterious effect of these errors can be reduced by increasing the manufacturing tolerances and the installation tolerances. However, the increase in these tolerances will greatly increase the cost of a concentrating solar energy system such that the resulting concentrating solar energy system will no longer be economically competitive with either non-concentrating solar systems or conventional power systems. The deleterious effect of these errors can also be reduced by modifying the pointing commands that position the reflective surface in an attempt to compensate for the misalignment errors.
In this regard, some concentrating solar energy systems automatically provide a fixed bias of correction in the azimuth direction and a fixed bias of correction in the elevation direction. While the application of a fixed amount of error correction is somewhat helpful, the azimuth and elevation errors vary throughout the day and year as the position of the sun and the solar concentrator changes. In this regard
Kiefer James A.
Stone Kenneth Wayne
Alston & Bird LLP
Barnes Crystal J.
Khatri Anil
The Boeing Company
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