Solar energy collection system

A solar collector utilizes multiple reflections of light passing down a tapered, pyramidal-type structure made of highly-reflective mirrored surfaces. A right-angled truncated reflective pyramidal structures have been discovered to have many properties which make them superior to existing concentrator geometries. The use of a tapered, pyramidal-type structure creates multiple reflections which appear at the collector output in the form of a Buckminster-Fullerene display, providing improved collector efficiency and amplification when compared to prior art “concentrators” of the Fresnel lens or parabola type.

TECHNICAL FIELD

The present invention relates to a solar energy collection system and, more particularly, to a solar collector of a preferred tapered geometry that allows for significantly improved concentration efficiency, while also relaxing the requirements on tracking the movements of the sun; the inventive system also provides a means of reducing energy generation costs well below those associated with standard fossil fuels.

BACKGROUND OF THE INVENTION

Most present day energy usage is derived from the sun. This comes largely from the burning of fossil fuels. Such usage has caused vast environmental problems, starting from as early as the Industrial Revolution and continuing today at an almost unabated pace. Indeed, the subsequent neglect of these growing problems has given rise to the present-day situation, calling for rapid remediation on the scale of a few years.

Direct use of solar energy in photovoltaic (PV) and thermal systems is probably the most desirable—yet least used—of the so-called “green technologies” under consideration for overcoming environmental problems for home, industrial and/or large-scale usage. High construction costs and the difficulty of achieving high solar cell efficiencies are the principal factors in preventing the extensive use of most presently-available systems. Most efforts to reduce costs of solar systems are centered on improving the efficiency and cost of the solar cells themselves. For example, extensive work is underway on improving cells made of single crystal silicon and other PV materials (such as, for example, plastic films based on polysilicon, organic PV material, inorganic PV material, and the like). New physical properties, such as large charge multiplication and high voltage charge extraction, are also being studied. The present cost of production of the best solar PV cells is approximately $3-$6 per watt, which is prohibitive when compared with a current cost of about $0.50 per watt for fossil fuels. Presently, government subsidies are attempting to make up the difference in cost in order to advocate for the solar cell alternative.

Most current solar systems for residential or business use are based on large arrays of planar, flat-plate solar PV panels set out on rooftops. Also, thin-film PV systems are being tested on vast stretches of desert floor for large-scale power plant use, for example. The flat-plate design requires that the active area of the collector be essentially equal to the area of the PV material exposed to one sun radiation. That is, there is a one-to-one ratio of active collector area to PV cell area. The cost of a one-sun flat plate module is mostly governed by the cost and efficiency of the PV material that is used to cover the active module area. Therefore, in order to reduce the cost of a flat-plate module, the PV base material must be made less expensive, or more efficient, or both. Many organizations are investigating thin-film photovoltaic technologies to address the issue of lowering the cost of the PV base material. All thin-film approaches thus far have lowered the cost of the PV material, but at the expense of module efficiency.

Other approaches to achieve solar generation involve use of solar concentrating systems. These systems generally use parabolic mirror collectors or Fresnel lenses in various configurations to focus and concentrate the sun's light onto small-area PV cells, or fluid-filled thermal absorbers for driving turbines or other heat-generating systems. The concentration ratio is defined as the input power:output power and in these designs may vary from 1.5:1 to over 1500:1. Traditionally, design approaches for concentrating collectors have been large and bulky, using Fresnel lenses or large area parabolic reflectors. These arrangements have large single-element focusing optics, requiring highly accurate and expensive feedback-controlled solar tracking mechanisms.

One exemplary prior art solar concentrator that addresses some of these concerns is described in U.S. Pat. No. 6,276,359 issued to S. Frazier on Aug. 21, 2001. The Frazier arrangement comprises a “double reflecting” solar concentrator that utilizes a primary parabolic reflective surface in combination with a secondary reflective surface. The incident light reflects off the secondary surface away from the primary parabolic surface's natural focus point toward a second focal point positioned on (or substantially near) the surface of the primary parabolic reflective surface. This optical path results in a narrower field of view at the receiver, which can improve the costs of some PV arrangements. The efficiency of this arrangement, however, remains limited in terms of the angle of acceptance of the incoming radiation and the need to accurately track the movement of the sun to provide a practical arrangement.

U.S. Pat. No. 6,666,207 issued on Dec. 23, 2003 to E. Arkas et al. discloses a solar concentrator formed into the shape of a spiral horn, where the horn is adapted to concentrate, by multiple reflections from the internal light-reflecting surface of the horn, solar energy incident within a predetermined range of angles. In particular, a preferred embodiment of the Arkas et al. design utilizes a spiral horn having the geometry of the well-known “Golden Spiral”. While potentially interesting from a design point of view, the formation of such a spiral horn had extensive manufacturing difficulties which may result in a cost-prohibitive option.

Long parabolic troughs are used in many conventional solar collector systems, where only the elevation is feedback-controlled (that is, azimuthal control is not a concern). The design of such a trough system is based on a technique called “non-imaging optics”. This type of analysis considers principally the power concentration features of solar collectors and totally neglects the imaging features which can often be complex and highly aberrated.

The state of the art approaches have not adequately addressed the issues of optical efficiency, optical cost, heat dissipation, solar tracking tolerance and size and weight concerns. Although interest in solar energy usage is high, experts predict it will take years (varying from a few years to a few decades) and large investments of capital and possibly government subsidies to significantly reduce our dependence on fossil fuels.

SUMMARY OF THE INVENTION

The present invention addresses the needs remaining in the prior art and discloses a new type of solar collector system that is capable of converting solar energy into electrical energy and heat at a very low cost and high efficiency.

A solar collector has been formed in accordance with the present invention which utilizes multiple reflections of light passing down a tapered, pyramidal-type structure made of highly-reflective and planar mirrored surfaces. In particular, right-angled truncated reflective pyramidal structures have been discovered to have many properties which make them much superior to existing concentrator geometries.

It has been discovered that the use of a tapered, pyramidal-type structure creates multiple reflections which appear at the collector output in the form of a Buckminster-Fullerene display, providing improved collector efficiency and amplification, while being much less expensive than prior art concentrators of the Fresnel lens or parabolic type.

It is an advantage of the solar collector system of the present invention that the truncated pyramidal collector is less sensitive to the movement of the sun than the prior art conventional arrangements. In particular, the truncated pyramidal collector of the present invention is more than an order of magnitude less sensitive to the sun's acceptance angle than parabolic and other lens-like collectors. This insensitivity to acceptance angle eliminates the need for the above-mentioned expensive two-axis tracking apparatus as required for use with conventional parabolic collectors (which are known to be extremely sensitive to small collector misalignments).

Additionally, the reduced sensitivity to acceptance angle allows for the system of the present invention to operate at quite high efficiency even under cloudy or hazy skies. These are conditions where parabolic collectors are inoperable. In particular, it has been observed that the truncated pyramidal collector of the present invention exhibits increased collection levels under hazy conditions, as a result of increased scattering of light within the haze coupled with the structure's ability to capture the scattered rays and reflect them towards the solar receiver. This results in a significant increase in collected power over conventional solar parabolic systems. Moreover, the wide acceptance angle of the inventive arrangement allows for the collectors themselves to be relatively “low precision” devices—tolerant of flaws in collector geometry, such as distortions from rippled mirror surfaces. Indeed, these types of flaws render conventional systems inoperable.

It is another advantage of the present invention that the tapered pyramidal geometry of planar, mirrored sidewalls is significantly less expensive to manufacture and implement than the prior art “spiral horn” arrangement described above. Indeed, the square-based truncated pyramidal solar collector embodiment of the present invention can be formed as a ziggurat-like structure resembling a series of terraced steps that is lightweight and inexpensive. The circular conic embodiment of the present invention is readily fabricated for relatively small dimensions (such as may be utilized in residential applications), but may be more difficult in larger sizes—where any of the multi-sided truncated pyramidal arrangements (triangular, square, rectangular, pentagonal, or the like) may be more appropriate.

Other and further advantages and aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.

DETAILED DESCRIPTION

A novel way of concentrating radiant energy using multiple reflections of light passing down a tapered pyramidal-type structure made from highly-reflective mirrored surfaces is the subject of the present invention. It has been discovered that tapered structures such as these have many properties that make them not only useful, but superior to existing solar collectors.FIG. 1(a) illustrates an exemplary embodiment of a right-angled truncated pyramidal solar collector10formed in accordance with the present invention. In this case, collector10utilizes a square geometry entrance port and exit port, with tapered reflective sidewalls disposed therebetween. That is, collector10is formed of a set of four tapered, planar reflective sidewalls12,14,16and18. Collector10is shown as having an entrance20of dimension L (and area, therefore, of L2). Collector10has a length S, tapering downward to an exit22of dimension D (and area of D2). Although not explicitly shown, it is to be understood that in implementation, a transparent covering is placed over entrance20to prevent rain, snow, debris, etc. from entering collector10and obstructing its reflective properties.

In accordance with the present invention, parallel light rays, such as from the sun, enter collector10within an acceptance angle that transports all of the rays at entrance20toward exit22, propagating along collector axis CA. Depending on the angle of the incoming rays with respect to axis CA, they will make a number of reflections of increasing angle with respect to mirrored surfaces12,14,16and18as they proceed down the length of concentrator10to exit22. At exit22, the rays are concentrated and collected by a PV panel or a thermal absorber (not shown). This optical behavior is similar to the “walk-off” that occurs in misaligned planar mirrors in open laser resonators. This behavior can also be thought of as a version of the “barber-shop effect” when two slightly tilted mirrors on opposite walls of barber shop reflect multiple images. Another very useful way of viewing this reflective behavior is as a simple application ofFermat's Principle of Least Timein which each ray follows a single straight line path from a “virtual sun” source to the output face. Each “virtual sun” is a mirror image of the real sun, located the same perpendicular distance behind the reflecting mirror, as the real sun is in front of the mirror.

FIG. 2is a cross-sectional side view of collector10, illustrating the parameters L, D and S and showing the relationship that is used to configure the concentrator in accordance with the present invention. The angle α is defined as the taper angle of the collector sidewalls with respect to the collector axis CA. As shown, the four parameters L, D, S and α are used to define the geometry of the truncated, right-angled pyramidal collector based on a square-shaped input of area L2and a square-shaped output of area D2. Indeed, the length S is defined by the geometry as:

S=(L2-D2)tan⁢⁢α.(1)
Using this relation, it is possible to calculate any parameter of the collector given the other three. Alternatively, characteristics of the collector's behavior can be determined by, for example, by holding one of the parameters and allowing the others to vary.

FIG. 3is the master diagram, basic to the understanding of the present invention, illustrating the relationship between the parameters as shown in equation (1). In the master diagram ofFIG. 3, the parameter α is shown along the y-axis; the quantities (S/D) and log(S/D) are displayed on the x-axis. By selecting different values of (L/D), the corresponding values of α can be computed from equation (1). These values are plotted as a series of curves showing all possible collector parameters for each of these chosen values of (L/D). This master diagram is the basis used to derive a preferred set of structural dimensions forming the collector of the present invention. It is to be understood, however, that the master diagram ofFIG. 3is associated with the particular geometry of square-based collector10ofFIG. 1(a). When using other collector geometries (i.e., triangular-based truncated pyramid, pentagon-based truncated pyramid, or the like), a different set of relationships will be created and used to determine a preferred set of dimensions L/D, S and α. For example,FIG. 1(b) illustrates an alternative collector10C of the present invention, formed of a conic structure with a circular entrance11of radius R and area πR2and a circular exit13of radius r and area πr2. As shown, entrance12of the arrangement ofFIG. 1(a) can be inscribed within entrance11of collector10C. Indeed, for a conical collector with a circular cross-section, the incoming rays will spiral about the axis and give rise to a ring-type pattern of images emerging from circular exit13.

The collection efficiency, or amplification factor A, is given by (L/D)2for the arrangement ofFIG. 1(a), assuming that all the power entering collector entrance20(defined by area L2) passes through collector exit22(defined by area D2). In the master diagram ofFIG. 3, the (L/D) values range from approximately one to one hundred, with an amplification A of approximately (1-104) for values of α of approximately (1°-20°, with (S/D) values ranging from (0-600).

For a given value of α, the master diagram ofFIG. 3shows that there is only a limited range of possible collector geometries for which there is no reflection of any input power. This fact implies that there are two zones: a first zone, shaded and labeled “I” in the master diagram ofFIG. 3, in which all of the input power is concentrated by the collector and exits through collector exit22, and a second zone, labeled “II”, in which an increasing fraction of the input power is retro-reflected back out through collector entrance20. For some collectors, not all of the input power exits the collector but instead is retro-reflected back out through collector entrance20. This is due to the fact that for these collectors the many reflections that occur as the light proceeds down the collector can increase in angle with respect to axis until it exceeds 90°, at which point it retro-reflects. This behavior is discussed below in paragraph [0043]. Therefore, the parameters within “zone I” are those which are selected to form collectors in accordance with the present invention. Regardless of the geometry of the entrance/exit of the collector structure (i.e., circular, square, triangular, pentagonal, etc.), there will always be a preferred “zone I” in the associated master diagram where there is no reflection of input power.

Still referring to the master diagram ofFIG. 3, a line is drawn across the master diagram at the value α=15°, which illustrates the range of (L/D) values over which zero-reflection, one-reflection, and two-reflection beamlets are generated. In general, additional power output can be obtained with the same dimension D of exit22, but using a smaller collector angle α. The smaller angle provides a larger amplification by virtue of the fact that larger values of (L/D) occur for smaller angles of α. This then leads to higher power output. This higher power output does come at the expense of a larger length S of collector10.

For example, with reference to the value of α=2.5°, it can be seen that an amplification of A=2500 can be achieved from an (L/D) value of 50. Theoretically, there is no limit to the amount of power that may be collected. This master diagram is based on a purely geometric optics model where diffraction effects are neglected. Diffraction effects, however are negligible for essentially all pyramidal-type collectors.

FIG. 4(a) is an exemplary diagram of the actual paths of several axial input rays as they pass through a configuration of collector10with a taper angle α=15°. A first set rays within the interior portion of entrance20that aligns with exit22will pass straight through the collector and exit as a undeflected “beamlet”, having made zero reflections off of the mirrored surfaces, as shown inFIG. 4(a). Another set of rays, immediately outside this first set, will make one reflection along a sidewall (for example, sidewall18) before exiting collector10(for the region between L/D=1 and L/D=2) and are shown inFIG. 4(a) as a “one-reflection beamlet”. Continuing in a similar fashion, another set of rays will reflect off of two, opposing sidewalls (e.g., sidewalk14and18) of collector10before exiting at collector exit22as a “two-reflection beamlet” (as shown inFIG. 4(a)).

In general, rays entering parallel to collector axis CA make successive reflections at increasing angles of 3α, 5α, 7α, etc. with respect to reflective surfaces12and16. These same rays are also shown inFIG. 4(a) as making angles 0, 2α, 4α, 6α, etc. with respect to the collector axis.FIG. 4(b) depicts the normalization of L/D=6/2 to 3/1, giving (L/D)2=(6/2)2=9. The diagrams ofFIG. 4(b) illustrate entrance face20, showing the relationship between L and D and the exemplary process used to normalize the ratio of L/D. Keeping in mind the diagram ofFIG. 3, it is shown that for an exemplary angle α=2.5 degrees, the ratio L/D=50 in zone I can be reached with an amplification A=2500. Thus, there is no theoretical limit to the amount of power which can be collected from arrangements configured from parameters within zone I of the master diagram ofFIG. 3(preferably, removed from the boundary area between zones I and II to minimize absorption and scattering losses). One works at smaller angles of α, or simply scales up all collector dimensions for a given collector. In practice, other factors, discussed hereinbelow, should be taken into consideration, as discussed in paragraph [0047] hereinbelow.

FIG. 5is a photographic output image obtained for the arrangement ofFIG. 1(a), looking through collector10from entrance20to exit22. The open innermost square area shown inFIG. 5(labeled by its dimension “D” in the image) is clearly the illuminated edge of exit22. The radial ray-like lines arise from the illuminated junction of the four converging reflective sidewalls12,14,16and18, which then reflect multiple times in the four sidewalls. The spherical nature of the image itself is an interesting discovery. This behavior had been fully confirmed using a laser pointer to trace along the path of an exemplary light ray and is understood as coming from the multiple reflections of the edges of entrance20behaving as a light source. In actuality, the “sphere” is comprised of a multi-sided polygon, formed of many linear segments of smaller size. The size of the “sphere” has been found to be relative to the area of entrance20and provides an intuitive indication of the amplification factor A associated with collector10.

By using the pyramidal-type collector of the present invention, uniformity of intensity is achieved by making the ratio of the input face to output face (L/D) an integral value. In this case, the input area L2can be divided into an N×N array of N2smaller areas, each of area D2, where with parallel input light each of the N2sub-areas is imaged onto collector entrance20. For ideal geometry, this guarantees uniformity of intensity at the output.FIG. 6is a diagram of collector entrance20for the case of N=7, showing the location of all possible input rays. The rays discussed above in association withFIG. 4are shown as the shaded beamlets in the view ofFIG. 6. Of the 49 beamlet regions in this arrangement, the shaded area accounts for 13 of the beamlets. These include the “straight through” beams from the central square (associated with L/D=1) and the other six beamlets diverging along the x and y axes. The remainder of the beamlets will reflect off of all surfaces12,14,16and18and result in a fan of diverging beamlets as they leave exit22, as shown inFIG. 4(a). In this case, the arrangement yields an image including one original sun and 48 “virtual suns”. Again, this illustrates the collector efficiency improvement of the present invention over the prior art.

In implementation, there are other factors to be considered which make the use of small angles of α, and the associated large amplification, impractical. In particular, it is unfavorable to have many emerging beamlets exiting collector10at large angles with respect to collector axis CA. Such beamlets will strike the absorbing solar cell (or absorbing thermal fluid) at small angles of incidence where surface reflectivity is high. Another undesirable factor is that reflections close to normal incidence will contribute to local heating and scattering loss, while making little or no contribution to amplification (on a per reflection basis).

In contrast, favorable collector parameters may comprise the following values: (L/D)=15, A=225, (S/D)=105 and D=10 cm. These values will yield an L of 1.5 m and a value for S of 10 m, with an input power of 1.5 kW at “one-sun” insulation. To work at yet higher power, additional PV collector units may be added, or all of the collector dimensions scaled upward by the same factor.

The above analysis has related to the use of pyramidal collectors with rays entering parallel to the collector axis. In the case where the sun's rays enter at an angle with respect to the axis, it can be shown that the collected power falls only slowly over an acceptance angle approaching 2α. This value of 2α is associated with the pyramidal collector geometry of the present invention, and is more than an order of magnitude less sensitive to angle than prior art collector geometries (e.g., Fresnel lenses and parabolas). As a result, the insensitivity to angle eliminates the need for expensive, feedback-controlled two-axis tracking apparatus, as used with conventional solar collector systems (which are extremely sensitive to small collector misalignments).

Instead, a relatively inexpensive two-axis tracker (no feedback required) can be used to orient the inventive collector during the full course of the day while operating at close to full power. Another major attribute of the large collection angle of 2α is that the collection system can continue to operate at quite high efficiency—even under cloudy or hazy skies—where parabolic collectors are inoperable. A further benefit of the wide acceptance angle is that the collectors themselves need not be high-precision devices and are tolerant of flaws in collector geometry, such as distortions from rippled mirror surfaces.

In a residential application, a square pyramidal collector, such as collector10ofFIG. 1(a), may be directly mounted onto a house, or located in an area immediately adjacent to the house.FIG. 7illustrates an exemplary collector10mounted onto a boom100. A first cable102is attached to boom100at point P, near entrance20of collector10. First cable102then passes through a pulley104at the top of an associated pole106. A small motor108is connected to first cable102at ground level and used to continuously (i.e., no feedback) vary the elevation angle of collector10at a constant rate. If pole106can itself be rotated about a vertical axis, the aximuthal angle of collector10can also be varied using the same motor through a second cable (not shown) to rotate pole106about its axis and thus also rotate collector10. This relatively simple two-axis tracking system is considerably less expensive and complicated than the tracking systems required for conventional solar concentrators based on parabolic reflectors or Fresnel lenses. Moreover, the pyramidal collectors of the present invention are lightweight and inexpensive, requiring only a simple mechanical support structure (indeed, a ziggurat-type structure has been used in construction of an exemplary collector).

In situations where space is limited and access is difficult, the collector of the present invention may be disposed in a horizontal configuration with a reflective mirror (having an unobstructed view) disposed at the entrance and used to direct the incoming solar radiation into the collector.FIG. 8illustrates this particular embodiment. Collector10is disposed horizontally, and is slightly raised off the ground by a support arrangement150(such as a pair of blocks). A PV/thermal receiver160is shown in this embodiment as disposed behind exit face22of collector10, in a position to receive the collected and concentrated solar rays. The separate rotatable mirror200is disposed in front of entrance face20of collector10and is positioned to receive the incoming solar rays and direct them into collector10, as shown. Similar to the tracking arrangement described above, a simple two-cable and motor system may be used to direct the movement of rotatable mirror200to follow the movement of the sun relative to entrance20of collector10.

A system useful for implementing a larger number of the inventive collectors is shown in a top view inFIG. 9. As shown, the multiple arrangement includes eight collectors, denoted10-1,10-2, . . . ,10-8, each with its own feeding mirror200-1,200-2, . . . ,200-8. The set of collectors is arranged, in this embodiment, as a “ring”, feeding a single PV (or thermal) receiver disposed in the center of the ring. In this particular embodiment, a single length of quartz tubing155is efficiently used as a single thermal receiver for the multiple collector arrangement, allowing for continual heating to take place.

FIG. 10depicts the ideal thermodynamic power conversion for a solar collector of the present invention, useful in understanding the efficiencies of the collector of the present invention. In the case of the embodiment ofFIG. 9, where the multiple collectors are disposed in a ring arrangement, an amplification factor A in the range of 100-1000, and with a temperature of 700° K, efficiencies between 50% and 70% can be obtained.

It has been determined that the use of a pyramidal collector in accordance with the present invention provides easy access to heated water, and can make good use of this heat energy to provide building and hot water heat for residences and businesses (since the hot water need only be pumped into a standard water heater for later use).FIG. 11shows the design of an exemplary hybrid PV and hot water system absorption chamber300for use with the truncated pyramidal collector10of the present invention. In order to protect the arrangement from the elements, a quartz window302is placed across exit face22of collector10, with an O-ring304used to seal the connection between collector10and absorption chamber300. In this particular embodiment, an outer insulating housing304defines the boundary of chamber300. Any type of conventional insulative material may be used for this purpose.

In accordance with the operation of chamber300, an incoming liquid (e.g., water, molten salt, oil, or any other fluid used in thermal systems) enters chamber300through inlet tubing306. In a preferred embodiment, a metal with a high melting temperature (for example, tantalum) is used to form tubing306. The incoming fluid is then exposed to the solar radiation concentrated by collector10, is heated, and then flows out through an outlet port308in tubing306. In the formation of this hybrid system, direct electrical energy is created by using a PV cell310which is disposed adjacent to quartz window302.

It is known that silicon-based PV cells need to be maintained below a temperature of 60° C. to maintain efficient PV generation. When PV cell310is operating at a peak efficiency of about 25%, for example, the remaining 75% of the energy will appear as heating and be transferred to the circulating liquid. In the arrangement as shown inFIG. 11, perhaps 80% of this remaining 75% of the energy will be converted to heated fluid. As a result, the exemplary system ofFIG. 11provides a total energy conversion of 25%+(0.8)(75%), or 25%+60%, yielding an overall efficiency of 85%, which is useful for residential and business applications. Indeed, this performance greatly exceeds current solar systems.

In large-scale energy production systems, where high temperature liquids (molten salt, for example) are used, it may be preferable to forego the inclusion of a PV cell (such as cell310), and rely solely on the generation of thermal energy through fluid circulation.

As mentioned above, the specific embodiment of the present invention illustrated inFIG. 1(a) may be fabricated in a ziggurat-like construction technique in the form of a series of terraced steps from the input aperture to the output aperture that creates a rigid, yet lightweight, structure of the kind needed for low-cost pyramidal collectors.FIG. 12(a) shows an exemplary ziggurat-like pyramidal collector, showing the terraced steps configuration. There are known problems associated with the fabrication of such pyramidal structures. However, using thin, flexible, silvered foils (approximately 0.010″ thick) for the flat mirrors, and a 0.07″ aluminum roof flashing (or thin plywood) as backing material, it is possible to make quite high quality mirrors with low distortion. As shown inFIG. 12(a), cross-bars30are attached to the outside of sidewalls12,14,16and18at intervals as shown to prevent the mirrors from collapsing inward toward the collector axis CA. Square plywood plates with square holes, shown as elements32, are also connected to the outside of the collector at intervals to prevent the mirrors from collapsing outward. The plastic mirrors are glued to the aluminum flashing (or plywood) with a material such as a spray glue. There is very little rippling of the plastic film material upon drying. Liquid glues are mostly water and not as appropriate for this application. Rubber cement has also been found acceptable. In combination, ziggurat elements30and32make a very rigid structure, even with quite flexible materials (such as the plastic mirrors), if they are supported on a small scale. For larger collectors, as one scales up the thickness of the collector sidewalls, the cost/watt stays constant inasmuch as the collected power increases at the same rate as the increase in collector cost. Other equipment construction methods can be devised within the spirit and scope of this invention; for example, replacing the outer plates that prevent outward collapse by strong tape (such as Gorilla-brand tape) bound around the outside of the collector.FIG. 12(b) is a photograph of an exemplary collector including a ziggurat-like support structure, showing the entrance face of the collector in this view. In formation, relatively thin plywood (quarter-inch, luan material) was used in the construction with the mirrored inner surfaces formed of extremely thin (5-10 mil) reflective film. The ziggurat structure provides an impressive degree of rigidity to a structure formed of relatively “thin” materials.FIG. 12(c) is a view of the same structure from the exit face. The thickness of the materials used for this exemplary arrangement are sufficient for a collector sized for residential purposes. Obviously, when designing larger collectors for industrial or larger-area applications, the thickness of the materials will scale accordingly (while still achieving collector efficiencies orders of magnitude better than conventional arrangements).

While the above discussion has been associated with the use of the collector geometry ofFIG. 1(a), it is to be understood that the same principles apply to various other pyramidal collector geometries.FIG. 13illustrate various other suitable arrangements, withFIG. 13(a) showing a simple triangular collector10-T embodiment of the present invention. A truncated pentagonal pyramid solar collector10-P is shown inFIG. 13(b). In each instance, a Buckminster-Fullerene type of solar image is created at the exit face of the collector, whereFIGS. 14(a) and14(b) illustrate the images for the arrangements ofFIGS. 13(a) and (b), respectively.

Although various preferred embodiments of the inventive solar collector have been shown and described, it will be appreciated by those skilled in the art that changes may be made to these embodiments without departing from the principles and the spirit of the invention, the scope of which is defined by the claims appended hereto.