Patent Application
20100212719
The invention relates to conversion of solar radiation into useful forms of energy (electricity or heat). Specifically it relates to sunlight concentrators that can be used as passive light collectors (that do not need active tracking of the sun), that take the form of flat or gently curved concentrator panels or fields, and allow a high concentration factor in the range of 5-1300. The invention further relates to heat storage systems that allow the sunlight to be used overnight for solar concentrator plants, and systems that allow a concentrator field to be located at sea or on a lake, or suspended above ground using a light support structure. Finally the invention relates to methods for increasing the photovoltaic conversion factor possible with any given photovoltaic conversion medium, such as CIGS thin films and silicon cells.
Relative to a panel that covers the whole incidence area with photovoltaic cells or a photovoltaic thin film, the purpose of using a concentrator is to reduce the component cost of the photovoltaic material without adding the expense of a tracking device. The invention reduces the photovoltaic area with a factor that can be scaled to between 1/5 and 1/7000 of the panel area, with preferred embodiments in the range of 1/10 to 1/1300. Because the concentrator itself relies only on simple production methods and inexpensive materials, it can be produced at a reduced cost per area unit relative to covering the same area with a photovoltaic material of the same conversion efficiency.
Current flat-panel concentrators have limited usefulness because they require the panel surface to stay orthogonal to the sun in order to work. They must therefore be mounted on active tracking systems (heliostats). The invention overcomes this limitation, and efficiently utilizes sunlight at incidence angles of at least 45 degrees. This property further allows surface reflectance losses to be efficiently constrained using either a single axis tracking device or no tracking device.
There are currently no solar thermal power plants that do not require active tracking. Current heat storage systems are subject to substantial heat loss due to the simplicity of their construction, which typically takes the form of a hot core of a latent heat medium surrounded by an insulating layer, or a core of sufficiently low thermal conductivity that it does not require insulation (e.g. graphite, concrete).
U.S. Pat. No. 4,440,153 and U.S. Pat. App. 20060274439 both describe concentrators based on filled, parabolic mirrors that require active tracking. U.S. Pat. App. 20060274439 teaches a flat panel modular concentrator based on a plurality of parabolic filled mirrors and the use of a Cassegrain two-mirror arrangement for tracking-based solar concentration. These patents are outside the scope of the present invention, which does not extend to neither imaging optical systems nor parabolic mirrors that require tracking.
U.S. Pat. No. 6,700,054, Int. Pat. No. WO 00/07055, and U.S. Pat. App. 20050081909 all describe a solar concentrator in the form of a tapered lightguide. U.S. Pat. App. 20050081909 teaches a flat panel modular static concentrator based on a plurality of conical or parabolic mirrors. This patent application is outside the scope of the present invention because when construed as short tapering light tubes, said conical or parabolic mirrors are not curved or curvilinear as taught herein, but strictly linear, and the concentration factor is ×3, and thus outside the range that defines the present invention. When construed as deep parabolic mirrors, the patent application remains outside the scope of the present invention since the system described herein does not extend to parabolic or conical mirrors.
U.S. Pat. No. 6,994,082 and U.S. Pat. App. 20080047546 teach an inflatable balloon formed from one clear and one reflecting film, such that either an oblate, spherical form or a parabolic form results, capable of concentrating light onto an internal PV cell. The method requires active tracking. While being a container embodiment of a mirror concentrator, the patent application is outside the scope of the present invention because the oblate ellipsoid is substantially and functionally different from the ellipsoidal shape described herein, and because said patent application does not specify or include any method of giving the balloon the shape described by the present invention. While said patent application claims a solar concentrator in the form of a balloon in general, the present invention describes a solar concentrator in the form of a closed container, of which a balloon is an embodiment. Thus the use of a balloon as a preferred embodiment of the present invention does not constitute an infringement of said patent application. Said patent teaches a combination of inflation and tensile support fibers that give the balloon a parabolic shape, but the tensegrity method described in the present invention does not rely on inflation, and the present invention does not extend to parabolic mirrors.
U.S. Pat. No. 6,274,860, U.S. Pat. No. 6,958,868, and U.S. Pat. App. 20070107770 describe flat panel static concentrators based on holographic principles. In the case of U.S. Pat. No. 6,274,860 the method is capable of reaching a concentration factor of ×6. Said concentrator panels have substantially similar properties to the panel concentrator described herein, but the use of holographic methods is outside the scope of the present invention.
JP Pat. No. 2005123036 describes a planar, modular static mirror concentrator. While the panel has many of the same properties as the concentrator panel described herein, said patent employs a very different mirror shape that functions substantially differently from the present invention.
The invention is a system for utilizing solar energy, consisting of a solar energy concentrator, which concentrates and transports sunlight, combined with either a heat storage system or a photovoltaic electricity conversion system. The heat storage system may for instance be used to drive a steam-based turbine continuously overnight in a solar thermal power plant.
The basic element of the system is a flat concentrator panel or field that allows a low light incidence angle and therefore does not require a tracking device. The panel may be planar, or have a gentle curvature. The system may for example be applied in the form of roof tiles, vehicle surfaces, solar panels floating in the sea or on lakes, or a field of solar thermal concentrators.
The concentrator has two embodiments, a light tube system and a mirror system. The mirror concentrator panel consists of round or trough-shaped mirrors with a co-adapted photovoltaic or thermal converter located at their center. The light tube concentrator panel is modular, and based on a hierarchical arrangement of curved tubes that transport light either to a photovoltaic converter, or to a heat storage unit.
The heat storage method is a method for extenuating heat loss from a hot core. An insulating container is multilayered such that an inner zone acts as a secondary heat storage zone that supplies heat to the hot core during extraction of energy.
The invention includes a PV concentrator field capable of being positioned above ground and therefore allowing dual use of the land area, and a PV concentrator field capable of floating in the sea or on a lake.
Systems and methods in accordance with various embodiments of the present invention can overcome the aforementioned and other deficiencies in existing photovoltaic and solar thermal systems and devices, by changing the way in which light is collected and directed towards the photovoltaic and heat storage elements, as well as changing and controlling the interaction between the light and photovoltaic elements, and changing the way in which heat is lost from heat storage units.
Said systems do this by having the following properties imparted by the methods of the invention (claim 1):
The concentrator has high tolerance to light incidence angle down to 45 degrees, and thus there is no critical need for an expensive tracking device or other moving parts. This property also allows the system to function under diffuse light. Preferred embodiments of the system include a static system, and the use of a simple one-axis tracking system for the purpose of reducing reflective surface loss.
The concentrator is an efficient light collector, in that it takes the form of a flat, or very gently curved panel, which means that all elements on it receives the same amount of light and hence all are fully active at any time.
The concentrator works optimally in the concentration range of 6-30 times. It can also be made to deliver a very high concentration factor. For example, the mirror embodiment will still function efficiently at 300 times concentration. The light tube embodiment will still function efficiently at 50 times concentration.
The concentrator can be made from inexpensive materials, using inexpensive mass production processes such as injection molding. On the other hand, by using more expensive high quality reflective coatings or transparent materials of high transmissivity, significant improvements of internal light loss can be achieved. Thus there is a direct trade-off between cost and the main efficiency bottleneck of the system. If there is only moderate limitation on the area available, the concentrator can be very low cost per square meter, while if it is strictly limited to, say, the area of a car roof and has to be made very thin, the embodiment can be made more efficient, but also more expensive. The system can therefore be adapted to a range of conditions, and optimized for efficient cost control under any given constrains.
The electricity converter will consistently deliver a high conversion ratio due to a high degree of control over the exit light incidence angle onto the photovoltaic cell surface. The ratio is higher than obtainable by covering the whole panel surface with the same photovoltaic material. The largest relative gain will be achieved if CIGS or other thin film materials are used, and for a concentration factor in the range of 6-30, which minimizes internal light loss, avoids heat problems, while remaining highly cost-effective in terms of use of photovoltaic materials.
The heat converter is capable of storing sufficient heat energy for continuous overnight use, provided it is scaled to the capacity required. It works by having a secondary heat storage zone with low thermal conductivity in contact with a hot core that supplies heat for external use.
The present invention is described more fully hereinafter with reference to accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.
In one of the preferred embodiments of the present invention, the concentrator takes the form of an assemblage of light tubes. It is known to those in the art that a light tube embodies a method for directional transport of light. Light enters at one end and exits at the other, and hence light tubes are transparent at both ends and internally. Light tubes are either hollow or filled. If hollow, they are covered or coated on the inside with a high-reflectance material, and light transport occurs by wall reflection. If filled, they do not have walls, but are made from a highly transparent material such as glass (e.g. water-white glass or acrylic) and transport light by the well-known principle of total internal reflection below a critical angle
The use of filled tubes is a preferred embodiment where the panel needs to be thin, such as in a concentrator panel covering an automobile roof. In this case each tube has to be small, and wall thickness can therefore be a substantial source of inefficiency. Where there is no constraint on the thickness of the panel and the objective is to cover a very large area and minimize light loss over large transport distances, hollow tubes are the less expensive option and therefore a preferred embodiment.
In all
The basic arrangement of light transport tubes described in claim 2 is termed a singular concentrator unit. The unit is defined by having a single light exit point, which may terminate either within the unit in an inbuilt energy converter, such as a photovoltaic cell, or connect to an external heat exchange unit via a single exit tube.
The tube arrangements of
Alternatively, the exit tube may be a hollow diffuser, and the entry tubes manufactured as a set of hollow tubes or acrylic rods that are heated and drawn for tapering and curvature such as for example shown in
The invention includes several methods for reducing internal light loss within the concentrator while maintaining a high concentration factor. The attainable efficiency and concentration factor is limited by total travel distance of the light because of refractive light loss at the tube walls of hollow tubes or loss due to imperfect transmissivity of any filled tube material.
Claim 2 discloses a method of reducing light loss within a hollow tube concentrator by increasing tube diameter between entry and exit point for the light, and thus decrease the number of reflection points. For a filled tube concentrator increasing tube diameter likewise allows maintaining a constant optimal light flux density throughout the concentrator.
Claim 3 discloses a method of reducing light loss within the concentrator. A modular concentrator unit is defined as comprising a plurality of singular concentrators with their converters connected in series or parallel. They terminate in one or more external exit points for the resulting electric current generated within the modular concentrator panel. This arrangement gives a lower concentration factor than a singular unit of the same area, but also reduces light loss since the mean light travel distance is reduced. The use of a modular arrangement thus gives control over several parameters of the concentrator panel, and allows precise optimization for specific uses.
Claim 4 discloses a method of simultaneously reducing light loss and chromatic aberration within the concentrator. It refers to the observation, in accordance with Snell's law, that there is less light loss within the curved section relative to the light loss of a straight tube between the end points of the same diameter (from either wall refraction or transmissivity loss) if the dimensionless curvature (normalized by tube diameter) has a value of 4 or larger (
Claim 4 further refers to the observation that said principle can be used as s method for keeping light incidence angles consistently below the critical angle of total internal reflection, which for glass and acrylic is approximately 48 degrees measured from the tube wall (
The use of an initial lens introduces chromatic aberration, a refraction phenomenon known to reduce the efficiency of photovoltaic cells. Claim 4 further refers to the observation, in accordance with Snell's law, that the use of a curved entry tube section or any curved or curvilinear tube section according to the principle disclosed in claim 4, acts to eliminate or minimize chromatic aberration by random chromatic re-mixing and re-focusing (
In a preferred embodiment, a modest degree of tube tapering, e.g. with linear, parabolic, elliptic or catenary tapering curvature, is used as a final concentrating step of the exit tube (
If the tapering section further is curved according the principle of claim 4, it is possible to increase the concentration factor to at least 5.5 without increasing light losses using a planar surface layer (
It is further possible to orient the entry tubes at another angle to the surface layer than orthogonal. In a preferred embodiment of the invention the entry tubes are oriented 45 degrees to the surface layer. This arrangement is used if the concentrator is positioned vertically, as a wall panel or tile.
A tube connection method is disclosed in claim 5 and shown in
The light control tube controls the light flux by causing ray trajectories to become more parallel and their reflection points to converge on regularly spaced light spots within the tube (
In general, the larger the diameter of the tubes, the less reflective and refractive contact the light will have with the tube wall (i.e. the more of the distance traveled by ray trajectories is spent within the tube medium) for any given tube length. Light loss reduction is therefore achieved when the light flux cross-sectional area in the system is increased, by increasing either the total or the average tube cross-sectional area of the hollow tube embodiment (claim 2, 5).
If panel thickness is not a major constraint, using tubes with a large diameter therefore allow the furthest light transport for the least amount of relative light loss. In a typical embodiment of this form, hollow tubes with a reflective internal coating further allow very large panels to be relatively lightweight and made from inexpensive materials. Hence a hollow tube arrangement is the preferred embodiment of the invention for delivering energy to a solar thermal power plant.
In one such preferred embodiment, light is first transported from entry tubes into a light control tube and from there into an exit tube of the kind described in claim 5(i). Said exit tube then terminates in a heat distributor. A plurality of light control tubes connect to a single exit tube, and this arrangement forms a singular concentrator unit (
In a preferred embodiment of said power plant, the turbine is located between a plurality of concentrators, with an arrangement of pipes transporting a hot fluid, such as steam, from a plurality of heat storage units (60) to the turbine (70).
The direction of light flux within each concentrator may be either radial or central, as shown in
In one preferred hollow tube embodiment each element that consists of entry tubes leading into one light control tube is made from extruded blocks of an embedding material. The blocks act as molds that slot into each other, such that the hollow interior structure of the concentrator results when the blocks are fitted together. The embedding material may for example be a rigid foam such as expanded polystyrene or PUR.
In a preferred hollow tube embodiment the reflective surface is made up of a plurality of flexible aluminized plastic sheets. Said sheets may be small relative to the wall curvature, and made from an inexpensive reflective material such as for example an aluminized plastic film (e.g. Mylar), or a laminate that is flexible but not crease-prone. Such a laminate can for example have an aluminized plastic film surface layer, or a dielectric mirror coated surface, and a further aluminized plastic sheet (e.g. Heat Shield) underneath. It is possible that that said laminate requires the use of plastic materials with an unusually high melting point. These flexible tiles are placed in a tiling arrangement, and affixed for example with Velcro, or fitted at the corners into slits in screws with a reflective head, protruding from the surface of the embedding material. To further improve reflectivity, for example in the infrared range, a plurality of flexible tiles with different reflectance properties can be stacked.
Light may escape from a system based on total internal reflection because the reflection is only total when the light reflects below the critical angle. Refracted light can re-enter the concentrator if it is reflected from the bottom and sides of the panel casing (claim 5(iv)). The system will achieve this most efficiently if the bottom consists of ridges orthogonal to the direction of light movement with side angles of 45 degrees (41) (
It is known in the art that refracted light can also be used for passive heating or cooling of a surface, in this case the surface underneath the panel. Said internal casing reflection increases the amount of solar energy that is prevented from reaching the surface below. This is particularly effective for passive cooling if the reflective material is capable of reflecting infrared light, such as an aluminized plastic sheet or film (e.g. Mylar and Heat Shield), or a combination of said materials. Conversely, if instead a heating function were to be desirable, the casing can easily be manufactured as a heat sink by being made from, or incorporating an element of a heat-absorbing material, and having a dark, non-reflective inner surface. Secondary far-infrared emittance from this material will be unable to escape through the concentrator, and hence heat is efficiently trapped in the casing, from where it will transmit to the surface beneath the panel. This allows the panel to have a dual use, e.g. generation of electricity as primary function and heating of water or passive cooling as a secondary function.
A light concentrator may the form of a singular or a plurality of concave elliptical, catenary or cycloidal mirrors, each forming a light attractor basin by reflection. These basins take the form of concentrator units that are either round or trough-shaped in outline (claim 8). A round concentrator unit is preferably circular in plan view, but may also consist of a plurality of sector-shaped segments at an angle to each other. A single concentrator unit, or a plurality of said units interlinked or placed within an external support structure such that they form a flat or gently curved panel with the properties stated in claim 1, are referred to as the mirror embodiment of the concentrator.
Hereafter the case of a plurality of mirrors formed from a single sheet (claim 9i and 9ii) is referred to as a “plural mirror”, and the case of a single self-contained mirror (claim 9iii) is referred to as a “singular mirror”. Said mirror shapes may be produced as concave hollows (hereafter referred to as “hollow mirrors”) by a process such as vacuum or injection molding of a plastic sheet, followed by coating with a high-reflective material. Alternatively, the mirror cavities may be produced as convex protusions of one surface of a sheet (hereafter referred to as “filled mirrors”). In the case of filled mirrors, the method of reflection is total internal reflection and the sheet must therefore be transparent. It can for example be made from acrylic resin using injection molding. A filled mirror can also be made from glass (e.g. white water glass), using a glass molding method. With small scale protusions the plural concentrator may further be produced by thermoforming of a thin plastic sheet or film, whereupon the converter units for example are deposited using an inkjet method. Alternatively, the converter units and the grid that connects them may be positioned on the surface of a separate layer or back panel. Said panel or layer may further be transparent and the gap between the concentrator sheet and back panel sheet may be evacuated for insulation purposes.
Examples of plural mirror embodiments of the present invention are described in
A preferred plural hollow mirror embodiment is made using twin sheet thermoforming. One sheet includes at least one reflective layer, such as an aluminized plastic film. The other sheet is transparent, such as an acrylic sheet or ETFE film. During forming, the reflective sheet is pressed against a forming tool that contains a plurality of tightly packed hollows shaped according to claim 8. The shape of the tool should facilitate the forming of a small hole in the center of each hollow in the sheet. The acrylic sheet may be of uniform thickness, or have thicker regions and thinner regions that correspond to the pattern of hollows, positioned such that the thicker regions act as lensoidal elements in the final plural concentrator. The two sheets are welded together at contact points where the forming tool protrudes maximally, which may be at points or ridges. To complete the functioning concentrator, converters are fitted into the holes at the bottom of each mirror.
Another preferred plural hollow mirror embodiment uses twin sheet vacuum forming of films rather than sheets. Instead of a reflective sheet, a laminated film is used, including at least one reflective layer, such as an aluminized plastic film, and one electricity-conducting layer in the form of a flexible grid connecting regularly spaced PV converters. For example, the reflective plastic film may have regularly spaced small holes through which rod-shaped PV converters are inserted from the electric grid layer. The lamination tool has holes to accommodate the converters. The twin sheet structure is subsequently formed from this laminate and a transparent film of uniform or varied thickness, such as an acrylic or ETFE film, pressing the laminate against a forming tool that contains a plurality of hollows shaped according to claim 8 and sized such that each converter is positioned at the bottom of each hollow. The protusions of the tool form a lattice of ridges, which generate a continuously sealed contact with the transparent layer around each hollow. The resulting structure is an inflated bubble wrap that encloses a plural concentrator.
Where the concentrator consists of separate and self-contained concentrator units (claim 10), a preferred embodiment consists of at least two parts; a container where the inside forms the reflector basin shape, and a converter (
The lid may be made by injection molding of acrylic resin or another transparent plastic, and the membrane may be a transparent ETFE film. The container may be made by vacuum forming of any suitable plastic, such as HDPE. It may for example be co-formed with an aluminized plastic film (e.g. Mylar), or by vacuum forming of a single aluminized thermo-plastic sheet. The container may also have a separate aluminized plastic film affixed to the inside, for example by chemical bonding. Said film will then first have been cut to fit the reflector form. Alternatively the mirror concentrator, whether in the form of a singular or plural mirror, can be made by twin sheet vacuum forming of a transparent sheet and a reflective sheet (for example an aluminized sheet), in which case the connection between them is permanently sealed by edge welding.
Separate and self-contained round units may for example be used as large, but lightweight concentrators suitable for small and medium-sized power plants, wherein each unit typically has more than one square meter incidence area, and wherein a plurality of such units are affixed to an external support structure, or each unit is affixed to neighboring units.
In another preferred embodiment of the round or trough-shaped concentrator unit, the separate and self-contained concentrator unit is an inflatable container made from two plastic films welded together at the edges, one transparent, and one with a reflective surface on the inside. Said arrangement when inflated forms a balloon, which is a separate container filled with air that has a transparent surface layer and a round reflector basin. When said container is fabricated in the simplest possible way, using two round films of the same size and shape, the reflector profile will be elliptical with the vertical axis orthogonal to the axis containing the geometric focal points. This geometry yields a highly inefficient concentrator.
A preferred embodiment may therefore use a dome-shaped thermoforming tool in the form of an ellipsoidal, cycloidal, or catenary dome (hereafter referred to as a “dome”) to form the aluminized film, for example an ETFE film. Without ability to stretch, the metalized film will fold locally, and these folds are then thermoformed using an inverted plug, since plastic layers are everywhere in contact inside the folds. To ensure the flattened folds stay in place, an extra plastic film may be welded to the non-reflective side. If the aluminized film is not pliable enough to allow orderly folding, an alternative embodiment is manufactured in the following way. First, a plurality of deep, wedge-shaped incisions are cut into the round sheet that will form the reflector. (
The dome tool is then removed, and the film is thermoformed with a transparent film using the twin sheet method to create a sealed and inflatable container with a reflector shape according to claim 8. In a final step, the converter and air valve may be inserted through a small central hole in the reflective layer, and the connection sealed, or they inserted earlier in the process, as described in the bubble wrap embodiment (
Another preferred embodiment is a round or trough-shaped concentrator in the form of a self-supporting tensile or tensegrity structure (
To manufacture a singular concentrator unit utilizing said tensegrity structures, a reflective bag is pulled over a dome tool, then a ribcage is pulled over the bag and welded to the back of it, such that the central ring is centered on a central opening of the bag. A plastic ring is then pulled over the outside of the ribcage. If there is only one such ring, its diameter is slightly larger than the large opening of the container, and it is affixed to the ribcage, for example by welding, in a position at the tips of the ribs or close to the tips. More than one plastic ring of different diameters may be used in order to ensure the resulting structure has and maintains the correct shape after the dome is removed. The dome is removed from the resulting container, which may stay open or be closed, for example by welding a transparent plastic film, such as an EFTE film, to the container rim. Finally the converter is placed within the container through the small opening. If the container remains open, the converter may be coated with a protective layer of ETFE.
In another embodiment of a tensegrity structure, the ribs may be tangential to the central ring instead of radial. The tensegrity structure can then be formed by folding two tangential ribcages over a dome and welding them together, such that the ribs form a rhombic pattern. In a further embodiment, a concentric ribcage if formed from a plurality of rings of different diameters, connected via radial or tangential spokes to form a tensegrity ring structure in three dimensions. The ring structure may be formed by welding the ribs to a second ribcage and push the structure into a dome shape using a dome plug. When either the concentric or rhombic ribcage is pulled over a separate inner ribcage and the two welded together at the central ring, a tensile open container skeleton is formed. The reflective bag may be affixed to the inside of it, or sandwiched between the ring structure and the ribcage. The resulting container may be closed by a transparent surface layer, such as an ETFE film, which may further be held in tension if the skeleton sets into a shape with a slightly larger diameter after completion of the container.
The one-mirror method (claim 11i) focuses the light onto a vertical line at the center, and thus requires a converter shape capable of utilizing this fact, e.g. a rod-shaped converter (32). The concentration factor is typically in the range of ×15-25 with a rod-shaped converter, and ×5-15 with a plural converter (33). The method gives substantial control over the range of angles with which most of the light reaches the converter.
The two-mirror method (claim 11ii) allows the use of a small PV cell or continuous set of small PV cells (31) and therefore a high concentration factor (claim 12ii), typically in the range of ×20-200 but gives little control over the angle with which the light reaches the PV cell (
In the case of the trough-shaped concentrator unit, the two-mirror method concentrates the light along the horizontal centerline of the trough, and hence requires the electricity converter to take the form of a strip of PV cells in said position. The use of trough-shaped concentrator units requires less reflector and container material than round concentrator units, and allows a better utilization of available land area when a tracking device does not have to be accommodated.
In the case of a trough-shaped concentrator unit, the one-mirror method concentrates light onto a vertical plane at the horizontal centerline of the trough. This embodiment allows the mirror concentrator to be used for generating thermal energy. The converter may then for example take the form of a plurality of parallel pipes (55), located in said vertical plane, that transport steam or another heat transport fluid to a heat storage unit or a turbine (
Said separate and self-contained trough-shaped units may for example be used as large concentrators suitable for large solar thermal power plants, wherein each unit typically has more than a thousand square meter incidence area.
We now describe the transparent surface layer and container or casing common to both light tube and mirror embodiments of the concentrator panel. The basic purpose of the surface layer is to act as an isolating and protective cover for the light concentrator system. In one embodiment the surface layer has only this basic purpose (claim 14i). In this case it could be cut from a sheet of acrylic, glass, or other transparent material, or be a stretched polymer film or inflated bubble. It could also take the form of a surface coating of a transparent polymer. Said polymer film and surface coating could for example be made from the self-cleaning material ETFE.
When the surface layer has the Fresnel or planoconvex lens arrangement described by claim 14(ii) and 14(iii), the layer also functions to make the conditions of light entry into the concentrator more efficient under conditions of low incidence angle. In a preferred embodiment with this function there is a gap between the tube openings and the surface layer (
Where the concentrator collects thermal energy, it is known in the art that improved infrared light transparency may be achieved by doping of the surface layer and any filled concentrator embodiment with Germanium (Ge), and that improved conversion of infrared light to electricity may be achieved using Ga—As/Ge PV cells. Furthermore, it is known in the art that if a solar concentrator uses a material that over time degrades and becomes less transparent due to UV light, such as a plastic with moderate UV resistance, a polaroid or other UV-reflective coating of exposed concentrator surfaces may be used to reduce the problem. For the same purpose, the surface layer may be made from a material with low UV transparency, such as ETFE or glass in order to limit exposure of the concentrator.
The protective container or casing of the concentrator panel (claim 15) may for example be made from plastic, glass, ceramics, or a metal, using well-known methods such as vacuum forming. The surface layer may be affixed to the casing by mechanical pressure, for example using snap-on features or regularly spaced screws along the edge, and sealed with a silicon or a rubber ring. If the layer is affixed to the container with chemical bonding, this may for example take the form of a silicon wedge around the periphery. If an airtight sealing method is used, the enclosed space may be evacuated, and the resulting difference in air pressure contribute to the strength of the seal. Evacuation gives the panel an additional insulating quality. If the casing is made from a transparent material and a filled concentrator is used, the panel may further be used as a translucent glazing.
The casing may also function as an energy co-generator and means of active cooling of the converter. In the latter case it contains a separate chamber wherein a heat transport fluid can circulate, either within the chamber as a whole, or inside a pipe arrangement fitted into the chamber.
Finally, depending on their use, concentrator panels in accordance with the present invention can be used for conversion of sunlight to electricity at a multiplicity of scales. The concentrator may take the form of a thin sheet-like panel for a car roof, using for example the filled light tube or mirror embodiment, or a roof tile shape, or a relatively deep box, for example when a hollow tube embodiment is used for a solar power plant. The mirror embodiment may further take the form of a large structure with the dimension of a thick sheet consisting of separate and self-contained concentrator units. These may be placed close together either within an external or internal frame or without any enclosing or interlinking frame other than a common substrate, such that they form a planar or gently curving concentrator field.
Claims 7 and 8 describe how a modular concentrator unit terminates in a singular converter, which may be a single photovoltaic cell that for example is square or round in outline. The PV cell may be positioned orthogonally to a straight or curved exit tube length axis. However, in a preferred embodiment the cell is placed at an angle to the tube length axis that differs substantially from 90 degrees, and is combined with a curved or curvilinear exit tube (
Said arrangements functions as a method of increasing PV conversion efficiency relative to a cell placed orthogonally to the exit tube centerline. Typical absorption coefficients of inorganic semiconductors imply light penetration depths of order 100 nm. However, only photon trajectories that terminate in a narrow band of 10 nm around the n-p interface contribute to the electric current (
An incidence angle exists for which the combination of increased zone thickness and depth position is optimal, and the present invention allows configuration such that most light reaches the PV converter at this angle. Hence a spatial converter shape and arrangement relative to a concentrator is disclosed herein as a method of optimizing PV energy conversion for any concentrator that uses light tubes or mirrors.
The converter may be either singular or plural (claim 7iii). A singular converter consists of a single PV cell or a plurality of PV cells arranged to form a continuous surface. A plural converter consists of a plurality of PV cells, for example in the form of PV thin films, that are not in continuity, but connected in parallel or series such that they connect to a single electricity outlet. A plural converter has a spatial arrangement of said cells that functions as a method of reducing or minimizing the mean light incidence angle.
A converter as described herein consists of one or a plurality of PV cells mounted on one or a plurality of rigid substrates that supports and holds each PV cell in their prescribed position relative to each other and the concentrator (
Claims 8 and 13 describe a singular converter in the shape of a rod (
Claims 8 and 14 describe a plural converter in the form of a radial and concentric arrangement of converter elements (331) (
The heat storage system of claim 17 consists of a hot core embedded in an insulator, and the application of two methods for reducing heat loss from the core (claim 17ii and 17iv). The system thus maintains the core temperature above a critical threshold for a specific time interval under continuous extraction of energy in the absence of external heat supply.
The methods follows from Fourier's law of heat conductance. The heat equation describes the heat profile between a hot and a cold region according to Fourier's law and the law of conservation of energy. The control parameter is the ratio of thermal conductivity to the product of specific heat capacity and density. The lower this ratio, the lower the temperature gradient.
Method 1: A qualitative three-component structure is provided, composed of a hot core, an outer insulating zone, and a transitional zone between them, such that the transition zone is insulating relative to the hot core by having lower thermal conductance, and heat storing relative to the outer insulating zone by having a larger product of heat capacity and density. This zone thereby functions both as a secondary heat storage element and an insulating layer. The heat storing capacity of the transition zone is further increased by using a material that undergoes a phase change above the critical temperature.
The purpose of this arrangement is to reduce heat loss over a range of core temperatures and thus maintain a sufficiently high core temperature for as long as possible. Since heat is extracted from the core as thermal work, the temperature of the core decreases not just by heating up the surroundings, but also due to work. Gradually the temperature becomes higher in the transition zone than in the core itself, and heat begins to flow from the transition zone back into the core. If a phase-change material is used in the transition zone, the secondary heat inflow is prolonged.
The inner hot core contains a material with both high thermal conductance and a large product of high heat capacity and density (e.g. concrete) that may also be a phase change heat material (e.g. saltpeter salt).
Preferred embodiments of the invention do not relate to specific materials, only their specific heat flow and heat capacity properties, so that many different materials may be used in any given zone or layer to achieve the physical properties prescribed by the method for said zone or layer.
Method 2: Provide a quantitative zoned arrangement of the material surrounding the hot core, wherein the zonation is guided by the temperature profile of a uniform material extending from the hot core to the outside surface of the heat storage unit.
Method 2 consists of five rules:
1. Minimize the sum of the product of the thickness of the two zones and their heat equation proportionality factor.
2. Where there are more than two zones, they must be arranged according to a decreasing product of [thermal conductivity×heat capacity×density] from the inside to the outside of the whole structure.
3. Where there are more than two zones, they must also be arranged according to a decreasing thermal conductivity, and decreasing density from the inside to the outside of the whole structure.
4. The fully developed temperature profile has two regions, an inner plateau and an outer zone of rapid temperature drop. For the outer zone, materials should be chosen with very low conductivity and proportionality factor, and for the inner zone materials should be chosen based on a large product of heat capacity and density.
5. Choose relative thicknesses of zones by a two-step procedure:
First, find thicknesses such that their difference is weighted according to the difference in area under the equivalent sections of the fully developed temperature profile of a uniform material, starting from the boundary of the hot core, given the specified initial temperature difference from the core boundary to outside (when heat supply to the core ceases).
Second, shift the starting point to the edge of the plateau region (at the point where an inner zone has been differentiated), and use the same approach again. This corresponds to the development of the heat profile during the storage period, which eventually peaks in the transition zone as heat is extracted from the core and converted to work. Hence the new start point has a lower temperature than the initial one. The new profile plus the shift distance yields the minimum thickness of the whole zone. Further differentiation into subzones now becomes possible, based on differentiating again between the plateau zone and the temperature drop zone, taking into account that for each new zone rule 1-3 applies. The process can be repeated further, but with diminishing returns for each new zone. If the process is repeated, a stepwise gradation between the initial transition zone and insulation zone results.
In a preferred embodiment, the zones are graded into different subzones with different thermal conductance and heat capacity properties in order to match heat flow and distribution profiles more closely. Where there are steps in properties between layers, and thus accelerated heat flux relative to the flux within the layers, these layer boundaries may have one, or a plurality of membranes or coatings of a heat-reflective material, such as aluminium (e.g. Heat Shield) that reduces radiation heat loss. Also sealed vacuum layers may be used that reduce convective heat loss. Barriers of this kind are most usefully positioned between the transition zone and the insulating zone.
In further preferred embodiments, the transport tubes may terminate within the transition zone or inside the core, but since these tubes become conduits of heat loss at night, in a preferred embodiment the transition zone has three or more layers, where the tubes go through the outer layer, and continue within the middle layer as a heat-absorbing black body cavity, while the inner layer facing the core is everywhere continuous. In another preferred embodiment the tubes pass all the way through the transition zone, and terminate in black-body cavity continuations extending some way into the core.
In other preferred embodiments the heat diffuser tubes may enter the heat storage unit radially or tangentially, so that the heat flow is directed towards the center and has a large contact area within the core and transition zone. Furthermore, the tubes can be closed in the outer zone with blocks of a lightweight insulating material, using a motor arrangement that slide said blocks sideways into the tubes, thus strongly reducing heat loss. A further method of reducing heat loss is to make the heat diffusers taper strongly as they enter the storage unit.
The heat storage unit can be built from relatively inexpensive materials as long as they are stable under operating temperatures. Each zone may be compartmentalized with breeze blocks or a ceramic material. An inner core may be a phase change material that can store latent heat, for instance as molten salt. An outer core may be made from cast iron, or a composite material, for example a mixture of concrete and graphite or corundum, or rubber embedded in asphalt, both combining medium conductivity and heat storage properties, or the whole core may contain a single phase change material. Likewise basalt, gypsum or wax may for example be used for the transition zone, and polystyrene, tufa or pumice may form the insulating zone.
The heat is used to produce steam to drive a turbine via a pressure boiler that in one preferred embodiment is a separate pressure chamber located directly above the roof of the heat storage unit, and covering the area of the core or both the core and some or all of the transition zone. The interface between the heat storage unit and the boiler may be an insulating zone, for example to transition zone level, and contain heat exchange elements in the form of wells or pipes with thermally conductive walls that descend into the core from the boiler. This simple arrangement reduces cost and heat loss. In a preferred embodiment, the boiler is further heated during daytime directly via exit tubes while the heat storage unit is rebuilding temperature. The boiler connects to a turbine, which further connects to a condenser tank.
The Concentrator Support Structure
Unlike current solar thermal plants and PV plants the system disclosed herein allows dual use of the land area physically occupied by the concentrator. There are two reasons for this: First, it is static and thus does not require a fixed, heavy ground support as inertial counterbalance, and second, the modes of concentration provided allows the concentrator to be made from lightweight materials, such as plastic. The methods that provide these properties of the system are further claimed herein as methods of providing the system with a dual land-use capability (claim 19).
A large variety of light weight support structures with limited load-bearing capability are known in the art, employing for example linear elements such as beams, arches, pylons and tensegrity structures. The use of such structures for supporting and suspending a static solar concentrator field above ground is claimed as a part of the invention. Preferred embodiments include the use of a scaffolding of bamboo or impregnated paper rolls. Another preferred embodiment is an open tensile weave of ropes or cables, such as nylon, polyester or manila ropes. For example, interlaced parallel ropes in three directions provide a tensile network of equal-sized triangles, each of which may support one or three concentrator units. The latter may be either interlocked or enclosed within a light external frame, for example made of bamboo. Similarly a rhombic network pattern may support sets of four concentrator units. Alternatively a thin mesh-like weave may be used. In either case, the suspended net is supported at regular intervals by poles or arches affixed to the ground, forming for example a cellular framework. Another preferred support structure embodiment is a thin-shell tensegrity structure, for example in the form of a lattice shell structure, wherein may be inserted for example curved or planar concentrator panels according to the invention.
The concentrator acts to shade the surface below, but many degrees of shading are possible with the system. For example, the concentrator field may have openings between concentrator units, or the concentrator units may be translucent (if panels based on filled embodiments), or the concentrator field may form a closed and watertight roof if the incidence area is complete utilized for sun capture. Hence a number of dual uses are possible. A suspended concentrator field will shield plants from extreme desiccating sunlight, and in general cool the surface and reduce evaporation. Hence the system can be used for the dual purpose of either reforestation or cultivation of specific plants that thrive in semi-shade or deep shade. Furthermore, the area underneath the concentrator can further be fully or partially enclosed, using for example EFTE film, and thus function as a green house, for example in conjunction with hydroponic cultivation. A closed roof embodiment allows the area underneath to be turned into an enclosed space, suitable for example as a storage depot or industrial facility.
The invention includes a simple tracking device. In a preferred embodiment a trough-concentrator field is linked via a drive shaft to a computer-controlled motor that moves the shaft backwards and forwards (
Adaptations that Allow the Concentrator to Float on Water
Unlike current PV plants, the system disclosed herein allows the concentrator field to float on water. The surface of a lake or bay, while relatively sheltered, is a dynamic and corrosive environment that will subject the concentrator field to mechanical stresses, and potentially also water damage and rapid clouding of the surface layer due to salt spray and colonization by birds. In order to overcome these problems the invention comprises a set of adaptive methods that allow the system to function efficiently with low maintenance when located on a lake or in the sea. The system further provides dual use as a trawl-free shelter for fish and includes a method of sustainable fishing.
One preferred embodiment of the floating system takes the form of a floating concentrator field of mono-hull buoyant mirror-based containers able to self-correct their vertical positioning if overturned. Each is given a low centre of gravity sufficient for self-stabilization by affixing to the underside of the container a ballast element, compartment, or object. Each unit is mechanically connected to its nearest neighbors. The connections may be positioned at triple points. They may be rigid, flexible, or jointed, and include a fender.
In another preferred embodiment, the concentrator field consists of a plurality of panels or containers, covering a plurality of fendered buoyant pontoon rafts made for example from foam-filled plastic cylinders or empty or foam-filled steel barrels or that are rigidly connected to each other and support a platform in the manner of a catamaran. The structure may be rectangular like a pontoon boat, or form an angular structure, e.g. a hexagonal or triangular. In the latter case the buoyant pontoon structure may be a single rigid closed unit, or consist of six separate units. The center of the platform may be supported by arched or linear struts or beams that connect a central element to the buoyant structure. The central element may take the form of a ring or angular closed shape (e.g. a hexagonal). It may also be a vertical pole relating to the pontoons in the manner of a tripod.
Said embodiments are suitable for low to medium wave-energy environments, such as lakes or sheltered bays. Another preferred embodiment adapts the catamaran pontoon structure to function in an open marine environment by using a SWATH design for reducing wave impact energy (by positioning the outrigger hulls below the waterline). Furthermore, if a plurality of pontoons is used, each pontoon may be given capacity for absorbing some local wave motion by using joints that allow restricted rotational movement at the central element instead of rigid connections. The structure may be further strengthened by adding spokes, struts, or beams which connect opposite pontoons below the waterline, or connect the pontoons to second central element below the waterline. The two elements may be further connected to a central vertical pole, which may further be attached to a central ballast element.
The buoyant structure is fendered, for example by attaching beams to the pontoons that run parallel to the pontoons. These beams are threaded with small reused vehicle tyres. The beams are further co-threaded with the beams of neighboring units using larger tyres that alternate with the smaller ones. Said use of tyres allow for a fendered mechanism of interlocking neighboring units that flexibly absorbs both compressive and tensile stresses. The cross-bars connecting the beams to the pontoons may have joint connections in order to allow further relative movement. Another embodiment that further allows the structure to absorb rotational movement uses semicircular beams instead of linear ones.
In a preferred embodiment, the surface layer of each floating unit is an ETFE film, stretched over a frame that gives a spire-shape too steep for birds to land. On lakes where birds and salt spray does not present problems, the surface layer may be a curved acrylic lid or a curved ETFE cushion.
Preferred embodiments of the global barrier (claim 20iv) are a static wave breaker extending from the sea bed, and a floating wave breaker in the form of a single or double array of rafts with a low center of gravity, attached to each other and anchored to the sea bed or to land. In the latter case, each raft consists of a material such as plastic or concrete with a prefractal hollow structure or surface indentations, such that wave energy is efficiently dissipated rather than merely reflected. In another preferred embodiment the wavebreaker consist of a chain of wave energy converters anchored to the sea bed.
A preferred embodiment of the fish trapping device (claim 20v) takes the form of a wide enclosure that is open at each end and extends into the water, and wherein the underwater end is blocked by an attached fishing net with openings that correspond to sustainable fish size. The enclosure has wall openings that allow entry of fish larger than the sustainable size, but exit only of fish that are smaller. This one-way effect is caused by the presence of semi-rigid, but flexible spikes lining each opening and oriented at a high angle into the enclosure. Said units are located around the periphery of the concentrator field where they can be accessed by boat.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB08/02239 | 6/25/2008 | WO | 00 | 5/5/2010 |
| Number | Date | Country | |
|---|---|---|---|
| 60946138 | Jun 2007 | US |
