Title: A Focused Solar Energy Collection System to Increase Efficiency and Decrease Cost
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of the provisional applications:
1) Serial number 61/280644 110609, filed 2009 November 6
FEDERALLY SPONSORED RESEARCH Not applicable SEQUENCE LISTING OR PROGRAM Not applicable
Title: A Focused Solar Energy Collection System to Increase Efficiency and Decrease Cost. Field of Invention
This invention pertains to the collection, concentration, and delivery of disperse solar energy to a central location for use in electrical power generation. It describes an efficient and cost effective means of doing so.
Solar thermal energy is gradually gaining attention as a better way of supplying renewable clean energy. The technology described in this patent is applicable to large scale solar power production in giga-watt quantities. A power tower configuration is one form of large scale solar thermal collection facility. It consists of a field of flat mirrors (heliostats) on the ground, each aimed so as to redirect the suns scattered rays to shine upon a central (receiver which is mounted on a central tower). The Barstow facility (Solar 1 and 2) is an example of this type of facility. The heat collected by the receiver can be used to drive a power generation unit, typically a steam turbine, or can be stored for later use, thereby extending the power output capability past daylight hours.
While this scheme has its advantages, there are still several inefficiencies to this configuration in its current form. The largest inefficiency is the cosine effect. Sunlight falls on the earth with a fixed amount of energy per unit area facing the suns rays. Depending upon where a mirror is positioned on the ground, it may have to be placed at a considerable angle to the suns rays in order to reflect the suns rays directly at the receiver. This angle reduces the amount of sunlight the mirror is able to collect by a factor equal to the cosine of the angle. This factor can be as much as 25%. Consequently each mirror must be oversized to accommodate its worst angle condition. Typical heliostat arrangements try to circumvent this inefficiency by placing the mirrors to the side of the tower opposite the sun which reduces the reflection angle. Yet a substantial cosine effect is also produced by the suns relative change in position in the sky throughout the day. So the brunt of this effect cannot be avoided by simply skewing the heliostat distribution.
The heliostats should be arranged so as to deliver maximum energy to the receiver. However, the attempted circumvention of the cosine effect by skewing the mirrors to one side of the receiver exacerbates other inefficiencies. Scattering of the reflected rays; due to solar beam divergence, mirror optical imperfections, atmospheric considerations, or due to aiming error can be minimized by placing the mirrors closer to the receiver. Each of these causes introduces error in the reflected angle of the sunlight, resulting in a portion of the reflected light missing the receiver. Since the error is angular, it gets worse with distance. Thus scattering effects are minimized with a mirror configuration that is concentric (closest) to the receiver. The shadowing effect - where one heliostat casts its shadow on a neighboring heliostat - also becomes worse with distance. This is because each heliostat must reflect sunlight over the heliostats between it and the receiver. So the further from the receiver, the more land is required per heliostat.
The shadow effect problem may be ameliorated to some extent by making the tower higher. However, tower costs become more expensive than linear with height and this can be
prohibitive. This is driven in part by the weight of the receiver apparatus at the top of the tower, because the receiver must be sized to the dimensions of each flat mirror panel on the heliostats and it must have some thermal mass to reduce the susceptibility to overheat. Attempting to reduce tower costs by using a down mirror at the tower top to direct the incoming sunlight to a receiver at ground level becomes extremely complicated to orchestrate. If flat mirrors are used on the ground, then the down mirror on the tower must consist of many flat facets; one for each mirror panel on the ground and just as large. So the down mirror doubles your mirror area and results in an excessively huge down mirror. As result, this configuration has been largely avoided.
Use of a parabolic down mirror would require parabolic up mirrors, because each up mirror must converge its light to the down mirrors focal point to avoid the light being scattered by the down mirror. But parabolic up mirrors are not conducive to large angle reflections. An angle of incidence that is not parallel to the up mirrors parabolic axis of revolution will result in scatter losses and changes in focal length. These scatter losses are then magnified by the added distance the light travels - tower to the ground. One additional but significant disadvantage to using parabolic heliostats in this fashion is that the shape of the parabola must vary with distance from the receiver in order to match the focal length to that distance. So each heliostat is unique to its position, which drives up the cost.
We have attempted to present a brief, but fairly complete picture of some of the difficulties faced by the designer of a Power Tower solar thermal system. This patent solves many of those difficulties by introducing a system that eliminates the cosine effect on the heliostat and minimizes scatter and shadow effects; thereby increasing system efficiency and decreasing system cost.
This system minimizes the cosine effect of traditional power tower systems. Parabolic heliostats are used, pointing always at the sun. The parabolic shape converges the reflected sunlight upon a close coupled concave quartz lens, placed with its focal point at the heliostat focal point in order to straighten the converging rays into a narrow beam of concentrated sunlight. The beam is reflected again by a mirror (on servos), redirecting it to a central receiver that is close to the ground. Each parabolic heliostat is pointed directly at the sun, eliminating its cosine effect and utilizing its full reflective potential at all times. The outgoing beam is reflected (by a small flat mirror) directly towards a central receiver at a height slightly taller than the heliostat height. This last reflection is subject to the cosine effect, but since the mirror is so small, the expense of over-sizing the small mirror is minimal. Because each heliostat is pointed directly at the sun, the mirror distribution may be freely optimized to minimize the scatter effect and the shadow effect. More specifically, the heliostats can be positioned concentric to the receiver; minimizing the distance to the receiver in order to reduce scatter effect. The heliostats can also be placed closer together since the only consideration with respect to shadow effect is a clear path to the sun. Each heliostat directs its energy to the receiver at a height above its neighboring heliostats. So there is no limiting (clear path to the tower) consideration which would require heliostats that are further from the receiver to be spaced further apart from each other.
Furthermore the heliostat structure is very strong and lightweight due to its wire braced truss construction. This is very important because the heliostat must be rigid for accuracy and lightweight for low cost The end result is an accurate, low cost system that maximizes the amount of sunlight that can be gathered from a given amount of real estate and delivered to a central receiver. And it does so while minimizing the mirror area of its heliostats.
The system consists of multiple heliostats surrounding a central receiver placed close to the ground. Each Heliostat (shown in Figure 1) consists of a parabolic mirror (1) mounted on a motorized stand (2). The stand is equipped with actuators (2a); providing a means of rotating the heliostat about the vertical and elevating the heliostats central aiming axis (3) (the axis of the parabolic surface of revolution) to angles above or below the horizon. The lightweight support structure for the mirror (1) consists of a central shaft (3a) with integral hub (3b) running along the central aiming axis (3) and an array of guy wires (4) attached to the ends of the central shaft (3a) and to lugs embedded in the mirror (1). The mirror (1) attaches to the central hub (3b) also via lugs embedded in the mirror. The tensioned guy wires (4) place both the central shaft (3a) and the mirror (1) in compression, thus forming a rigid truss that holds the mirror (1) accurately parabolic. The parabolic mirror (1) consists of a shaped structural core (such as balsa wood) sandwiched between layers of fiberglass and lined with a mirrored surface. Metal lugs are embedded in the fiberglass for attachment points. Thus the mirror (1) is capable of sustaining the compression forces of the truss. The mirror (1) can be fabricated in gore shaped panels for ease of transportation. Vents at the panel join lines can be used to spoil wind loads.
A concave (diverging) quartz lens (5) is positioned perpendicular to the central aiming axis (3) with its focal point coincident to the focal point of the parabolic mirror (1). The lens is mounted on the end of the central shaft (3a) using curved struts (3c) that minimize shadowing. Surrounding the lens is a ring shaped actuator (6) which rotates a side mirror mount (6a). A planar side mirror (7) is mounted upon the side mirror mount (6a) and is tilted by a side mirror actuator (8). These actuators give two angular degrees of freedom to the side mirror (7) and provide a comprehensive means to aim the planar mirrors (7) normal in any direction (using spherical coordinates).
Referring to Figures 1 and 2, the heliostat stand actuators (2a) are used to aim the heliostat directly at the sun. Aim is quantified in polar coordinates by rotation about the axis of the heliostat stand (2) and elevation above the horizon. Aim is accomplished very precisely via worm gear actuators using stepper motors with built in encoders. For any given date, time, longitude and latitude, the aiming angles are readily calculated using known astronomical relationships. Incoming solar rays, traveling parallel to the heliostats central aiming axis (3) are reflected from the parabolic mirror (1) towards the parabolas focal point to enter the lens (5) where they are collimated to travel approximately parallel to the central aiming axis (3). At this point the suns incoming rays have been collected into a concentrated beam of sunlight traveling along the central aiming axis (3) directly towards the sun.
The concentrated sunbeam is then reflected by the planar side mirror (7) to travel horizontally to its target; the receiver. The two actuators (6) and (8) are used to move the planar side mirror (7) to aim the beam at the receiver. Actuator (6) rotates the side mirror mount (6a) about the central aiming axis (3). Actuator (8) rotates the planar side mirror (7) about the side mirror axis (7a), an axis normal to the central aiming axis (3) and in the plane of the planar side mirrors (7) reflective surface. Aiming of the side mirror is thus accomplished via two angles; about the central aiming axis and the side mirror axis. These angles are calculated values based on the dish aiming angles (actuators 2a), heliostat geometry, and receiver location (and using coordinate transformations). These actuators (6) and (8) also consist of worm gears driven be stepper motors with built in encoders, though smaller in size than actuators (2a). As the suns position in the sky varies throughout the day, the heliostat stand actuators (2a) must realign the parabolic mirror (1) to continuously target the sun and actuators (6) & (8) must realign the planar side mirror (7) to redirect the outgoing beam at the receiver. For any given location on earth the angles required to accomplish the aiming are readily calculated based on date, time of day, longitude, latitude, and receiver location.
A topic that should be addressed in gaging the worth of this invention is solar beam divergence. The tremendous distance at which the sun resides relative to the earth lends sunlight the property of being nearly parallel light. However, the sun is of finite size and as such, the light that it casts upon any given point on earth is a converging cone of light that disperses upon reflection. The angle of this cone is very small (.55 degrees), but it does affect the accuracy of reflection and refraction. Incoming sunlight that is not parallel to the central aiming axis (3) of the dish will reflect off any point on the parabolic mirror (1) in an expanding cone of light until it hits the collimating lens (5) - see Figure 4. The sunlight will then be refracted through the lens (5) over a region of the lens that is centered at the point through which a parallel beam would be refracted. Hence the problem of beam divergence is magnified. Not only does the divergence angle propagate from parabolic mirror (1), through collimating lens (5), and bounced off of side mirror (7); non parallel light will refract through the wrong location of the lens and consequently magnify the divergence. It is important to note that this problem becomes worse as the area of the divergent light beam at the lens (5) approaches the diameter of the lens itself. The problem can be reduced by increasing the lens (5) size or decreasing the distance from parabolic mirror (1) to the lens (5).
As the size of the lens (5) increases, its cost can become prohibitive, mainly due to the issue of weight. For this reason, several alternative configurations are included in the claims. The lens (5) may be either a standard double convex lens or a fresnel lens to reduce weight. The lens may also be replaced (see Figures 5 and 6) with a lightweight hyperbolic mirror (5a) which reflectively collimates light from the parabolic mirror (1) back towards the parabolic mirror (1). In this latter configuration, the side mirror (7) is placed either between the parabolic mirror (1) and the hyperbolic reflector (see Figure 5) or behind the parabolic mirror (1) with an aperture placed in the center of the parabolic mirror (1) to clear a pathway for the beam (see Figure 6). The lens (5) or hyperbolic secondary reflector (5a) may be modified to add convergence to the outgoing beam in order to further ameliorate the divergence issue.
I. Since each parabolic mirror (1) is pointed directly at the sun at all times (see Figure 3), the cosine effect is eliminated and the heliostat will always operate at peak efficiency. Hence
a. The mirror (1) area need not be oversized to compensate a cosine effect and this reduces system cost.
b. The heliostat placement need not be skewed to compensate a cosine effect and this allows the most compact heliostat placement pattern - concentric - and thus i. reduces scatter loss by minimizing the average distance of the heliostat to the receiver.
ii. Reduces real estate cost due to denser placement distribution.
II. Because the parabolic mirror (1) is not focused on the receiver (but on a lens (5) in close proximity), its shape is independent of distance from the receiver. So all of the heliostat mirrors (1) can be made identical which reduces system cost.
III. The focal point of the parabolic mirror (1) is designed to be far enough from the mirror (1) to be above the tops of adjacent heliostats at all times of daylight operation. a. This enables the beam to travel directly to a receiver placed close to the ground instead on top of an expensive tower. This reduces or eliminates tower cost.
b. The narrow width of the beam and its height above all parabolic mirrors (1) makes it easier to avoid shadowing effects on light traveling from the heliostat to the receiver.
i. The beam only has to avoid being shadowed by other flat mirrors (7) which are very small and easy to avoid.
ii. And by varying the distance of the side mirror (7) from the lens (5), one can guide the beams of adjacent heliostats to travel to the receiver at slightly different heights in order to avoid shadowing,
The elimination of shadow effects from the heliostat to the receiver reduces the placement constraints on the heliostat enabling closer placement to the central tower, reducing scatter loss and real estate cost.