A current method for the large scale collection and concentration of solar energy is the use of heliostat mirrors to reflect the sun's rays to a central receiver. By utilizing multiple heliostat mirrors, each one reflecting to a common point, concentration of solar energy is achieved.
In known systems, heliostat mirrors may be in a fixed position surrounding the tower. The mirror surfaces are typically controlled in two degrees of motion to position the surface of the mirror with respect to the tower. Each heliostat mirror has a control system which tracks the motion of the sun with respect to the centrally located receiver. The mirror is on a fixed base and the reflective surface of the mirror is continuously moved to maintain the solar reflection from the surface of the mirror onto the receiver. The purpose of positioning the heliostat mirrors being to reflect and direct the sun's rays to a designated central collection point, known as a central target receiver or a power tower. In order to accomplish this, the heliostat mirror requires a surface area of reflective mirror, two axes of motion, a servo motor for each axis of motion and a control system for positional calculation and motion control of the two axes.
In some installations, the array of heliostats encompasses a full 360 degree area around a receiver. This is a very expensive installation due to the large number of heliostats required in order to populate the array and surround the receiver. Also, because the position of the sun is changing throughout the day, at any point in time more than half of the array has a very low performance due to their fixed placement within the array.
Other installations have attempted to address the issue of heliostat cost by reducing the number of mirrors. A reduced number of mirrors are controlled as a gang in that the heliostat array is a single movable unit that is positioned on a track. The array then rotates concentrically about the receiver on a horizontal plane of motion. The mirrors in the array are not only controlled as a single unit about the receiver, but are also movable as a single unit about a vertical axis to rotate their surface position with respect to the receiver. These installations are not practical for an installation having a large radius from the central tower. Also, large arc segments of mirrors will not target the receiver with adequate accuracy.
Furthermore, known concentric designs do not always take into account the requirement of a full 360° for rotation about the central receiver. In global implementations, particularly for solar tracking in latitudes between the Tropic of Cancer and the Tropic of Capricorn which comprises a large global market segment for solar thermal applications, known concentric designs may not capable of tracking the sun. For this special case, the solar path is not always southerly or northerly, but both depending on the day of the year. In fact, the solar path may be north or south of the central receiver and at one point, will be directly overhead. Without the capability of a full circle of motion for the array, known concentric designs will not operate within this equatorial band.
Another major factor in a solar thermal concentration system is the environmental impact of the installation. In known systems, a large area of land is necessary and that land must be dedicated to the sole purpose of solar thermal power collection. Additionally, the typical ground preparation for installation involves invasive grading and leveling practice which completely destroys the indigenous habitats and ecology of the terrain. Furthermore, the dedicated area is permanently covered by the mirror array, limiting ground exposure to natural heal and light provided by the sunlight, which negatively impacts the environment.
There is a need for a solar thermal concentration system and method that is cost effective to install and maintain, fully utilizes the collection possibilities as the earth orbits the sun, and has minimal adverse impact on the environment.