Patent Issued for Low Cost Focussing System Giving High Concentrations (USPTO 9244262)
By a News Reporter-Staff News Editor at Journal of Engineering -- A patent by the inventors Bailey, Paul B. (
Oxford
, GB); Dadd, Michael W. (Oxford, GB); Stone, Charles R. (
Oxford
, GB); Jelley, Nicholas A. (
Oxford
, GB), filed on June 1, 2011, was published online on January 26, 2016, according to news reporting originating from
Alexandria, Virginia
, by VerticalNews correspondents.
Patent number 9244262 is assigned to ISIS Innovation Limited (
Oxford
, GB).
The following quote was obtained by the news editors from the background information supplied by the inventors: "Diminishing fuel reserves and the effects of climate change have highlighted the need to make the best use of all available energy sources. Solar energy is probably the most significant renewable energy source--it also has the advantage of minimal environmental impact. The development of technology for exploiting solar energy is very important for future energy production.
"Solar energy can be utilized in a number of ways but for the production of 5-500 kW electrical power in dry sunny regions, where water is scarce, there are two leading possibilities: concentrated photo-voltaic (CPV) systems; and solar heated
Stirling
engines driving electrical generators.
"What is needed for both are low cost solar concentrators that can produce high concentrations of at least 500 and preferably up to values in excess of 1000. For photo-voltaics, high efficiency Photo-Voltaic materials are expensive and high concentrations allow them to be used more cost effectively. For
Stirling
engines, high efficiency requires high heater temperatures and this can only be achieved with high levels of concentration.
"Currently solar concentrators used to provide these high levels of concentration use designs that require reflective surfaces that are fully three dimensional. For example, in US2006266408 (Solfocus) a two stage concentrator is described that can provide high concentrations for Photo-Voltaic devices. The two mirrors are made out of glass and are silvered to give the required reflective coatings. The mirror profiles have curvature in two directions.
"For
Stirling
engine applications, designs have centered on single stage parabolic reflectors that produce a point focus. Spherical reflectors have also been used--for high values of f/D (focal length/dish diameter) an approximate point focus is produced with a high enough concentration factor to be used in these applications. Alternatively, arrays of spherical reflectors have been used that approximate a parabolic reflector. Again the various reflector profiles have curvature in two directions.
"Relatively expensive manufacturing techniques and materials are generally required to form the reflector profiles having curvature in two directions to the required accuracy. In addition, handling and transport of the reflective components from the factory to the installation site can be difficult, due to their bulk and potential fragility.
"As well as the design of the basic concentrator system, other aspects of a complete solar generator system that need to be addressed concern the design of the cavity or window type receivers for Solar Stirling generators, aspects such as the angle of incidence for Photo-Voltaic generators, and more generally non-imaging 'secondary' concentrators for both.
"The principles used for the design of cavity receivers and non-imaging concentrators, such as Winston cones, are well established and known to those skilled in the art. See, for example, the following: 1) Pitz-Paal Robert. High Temperature Solar Concentrators. in Solar Energy Conversion and Photoenergy Systems in Encyclopedia of Life Support Systems. Eolss Publishers,
Oxford UK
2007; and 2)
Roland Winston,
Juan C. Minano,
Pablo Benitez, (with contributions by
Narkis Shatz and
John C. Bortz), Nonimaging Optics,
Academic Press, 2004. (ISBN 0-12-759751-4). Also commercial software exists that greatly facilitates the design of these components. See, for example, ZEMAX,
Optima Research Ltd, 8 Riverside Business Park, Stoney Common Road,
Stansted
, CM24 8PL,
United Kingdom
e.g. ZEMAX (ref 4). These tools are particularly useful for more detailed requirements such as attaining uniform temperature of the heater assembly or uniform illumination of the photo-voltaic cell."
In addition to the background information obtained for this patent, VerticalNews journalists also obtained the inventors' summary information for this patent: "It is an object of the present invention to provide a system capable of providing the high concentration factors necessary for high efficiency photo-voltaic/
Stirling
generators while being reasonably compact and/or having the potential for low cost manufacture and convenient handling/transportation.
"According to an aspect of the invention, there is provided a focussing system for concentrating radiation onto a target surface, comprising: a first reflective element forming part of a conical surface axially aligned along a first alignment axis, the first reflective element being positioned such that when planar radiation is incident on the first reflective element in a direction parallel to the first alignment axis, the planar radiation is focussed towards a first focus lying along the first alignment axis, wherein said part of the conical surface is contained within a sector having an included angle of less than 180 degrees; and a second reflective element having a reflective surface that at all points is flat in a direction parallel to a single reference direction, the second reflective element being positioned between the first reflective element and the first focus such that, when planar radiation is incident on the first reflective element in a direction parallel to the first alignment axis, radiation reflected from the first reflective element onto the second reflective element is focussed towards a second focus.
"The part of a conical surface may alternatively be described as part of the surface of a cone and is a first order aspheric surface defined by z=ar, where z is the displacement of the point on the surface from the vertex of the cone along the cone's axis, r is the radial distance from the point on the surface to the cone's axis, and a is a constant.
"Preferably, the first focus is a line focus (i.e. a focus having an elongate form approximating a portion of a line). As such a line focus is most constrained in directions within a horizontal plane (when the first alignment axis is vertical), this focus may be referred to as a 'horizontal focus'.
"Preferably, the second focus is a point focus (i.e. a focus that is spatially constrained in all directions, thereby approximating a point). As such a focus is spatially constrained in directions within a vertical plane relative to the first focus, this focus may be referred to as a 'vertical focus'.
"The second reflective element may be formed from a parabolic mirror with an axis of curvature perpendicular to the first alignment axis--such a mirror has the shape of part of a parabolic trough.
"This arrangement thus uses reflective elements that can be readily formed from flat sheet by simple bending--such surfaces are called developable. For a developable surface, three mutually orthogonal planes can be chosen in which the curvature of the surface is only non-zero in one of the planes: for the surface of a cone, the xy plane, where z is the axis of the cone; for the surface of a parabolic trough, the xy plane, where z is along the base of the trough.
"Developable geometries of this kind can be referred to as 'two-dimensional' and are characterized generally by surface geometries built up from locally flat linear elements (i.e. elements that have no curvature in a direction parallel to their length). In the case of the first reflective elements, which comprise a reflective surface corresponding to a part of a cone (a first order aspheric surface), the linear elements would run continuously from a lower extremity of the element to an upper extremity of the element, and be aligned towards the tip of the cone. In the case of the second reflective elements, where all parts of the surface are flat along the same reference direction, the linear elements are (infinitesimally thin) parallel strips. Such 'two-dimensional' geometries provide several advantages relative to the alternative 'three-dimensional' geometries (i.e. geometries having local curvature that cannot be achieved by simple bending) that are used in the prior art in the most similar contexts.
"Firstly, the manufacturing processes and materials necessary to form the 'three-dimensional' geometries tend to be more expensive. A 'two-dimensional' design, in contrast, can be manufactured from reflective sheet by the simple process of bending. A 'three-dimensional' design has to start with a material that can be machined or deformed, and a more complex manufacturing process is required to accurately achieve the required geometry. Glass is a material that is frequently used as the basis of a 'three-dimensional' concentrator. The shape can be achieved by deforming a plate into a mold by a process called slump forming The reflective surface is achieved by the coating of the glass with a thin reflective coating--this process is familiar in the silvering of conventional glass mirrors; however, the material is not robust.
"Secondly, a drawback of focussing systems based on reflectors having 'three-dimensional' curvatures is associated with the handling and transport of concentrator components. With a 'two-dimensional' design it is possible to pursue a 'flatpack' concept where reflective elements and other components can be transported in a compact, easily handled flat form. The focussing system can be assembled at the chosen site and reflective elements formed into the appropriate geometry by relatively simple bending procedures. With 'three-dimensional' designs it is unlikely that the geometry can be formed on site so it will be necessary to handle and transport finished 'three-dimensional' optical components. This is clearly a more difficult and expensive task, particularly if the components are manufactured from an easily damaged material such as glass. Existing solar concentrators, that do have a 'two-dimensional' design, tend to have insufficient concentration for high efficiency photo-voltaic or
Stirling
engine applications.
"More generally, the provision of separate reflective elements provides greater flexibility in comparison to systems that rely on a single reflective element, because of the relative freedom to orient and/or position the reflective elements to achieve different focus positions.
"For example, the first and second reflectors may be configured so that the second focus is above the second reflector, such that radiation approaches the focus from below. This arrangement may be useful where the target for the focussed radiation is within a housing provided for restricting upward movement of gas in the region of the target (to reduce convective losses). For example, this approach enables radiation windows to open out downwards and/or be positioned in a lower part of the housing, which naturally restricts escape of hot gases relative to arrangements where this is not the case. As a further example, the reflective elements may be arranged so that the second focus is in close proximity to one or both of the first or second reflective elements, which makes it easier to provide a light, low cost support structure.
"Depending on the context, the second focus can be arranged relative to the first alignment axis so as to be radially outside of one or both of the first and second reflective elements, or radially inward of one or both of the first and second reflective elements. At the same time, the second focus can be arranged to be longitudinally nearer to the source of the incident radiation than one or both of the first and second reflective elements, or can be longitudinally further from the source than one or both of the first and second reflective elements.
"As discussed above, the reflective elements can be formed by simple bending or rolling of flat strips of material. Preferably, the flat strips are rectangular to facilitate manufacture. The strips may also be formed from aluminium, which can be shaped efficiently, and is light and strong.
"The first or second reflective element may be formed from a plurality of reflective elements. In both cases, the reflective elements within each group can be spaced apart to allow air to flow between the individual elements, thereby reducing wind forces and enabling a lighter, lower cost construction, while not altering the collecting area. If the individual elements are simply displaced vertically (parallel to the first alignment axis), relative to where they would have been had they been joined integrally, then the `footprint` or collecting area of each reflective element (i.e. of each element formed from a plurality of strips) is not altered.
"The use of multiple second reflective elements makes it possible to achieve greater than unity concentration of rays reflected from the second reflective elements even when the secondary reflective elements are formed from plane mirrors. It is thus possible to realize high concentrations whilst benefiting from the cost savings associated with the relative ease of manufacture, transport and on-site installation of planar mirrors in comparison with mirrors that have to be bent into the required shape.
"The shapes of the reflective surfaces of the reflective elements can be adapted so as to produce a uniform power distribution at the second focus. This can be achieved by controlling the overall shape of the reflective elements or by applying a suitably shaped non-reflective mask to the reflective elements.
"The reflective elements may be formed into the required shape by either using curved guides or by clamping nominally flat strips/facets of material in such a way as to impose the boundary conditions that force them to bend as desired. The reflective elements may thus be formed at the location where the solar focussing system is to be deployed, thus avoiding careful (and therefore expensive) transportation of pre-shaped components. The reflective elements may also be tuned on site, by adjusting the clamping conditions, for an optimal focus, or tuned to correct for damage of the flat starting materials during transportation or for general wear and tear and/or fatigue.
"According to a further aspect of the invention, a multiple target focussing system may be provided, which consists of two or more focussing systems, each configured to produce different second foci. The different second foci may be spatially separated from each other, for example along a direction parallel to the first alignment axis of one or more of the focussing systems in the multiple target focussing system and/or along a direction perpendicular to the first alignment axis of one or more of the focussing systems in the multiple target focussing system. In this way, it is possible simultaneously to concentrate radiation onto more than one target in a flexible manner.
"According to a further aspect of the invention, there is provided a solar powered system, comprising: a heat driven engine; a heat exchanger surface for receiving solar energy to drive the heat driven engine; and a focussing system according to an embodiment of the invention that is configured to concentrate solar radiation onto the heat exchanger surface.
"The heat exchanger surface may be in the form of heater tubes within a cavity defined by a housing, for example.
"Preferably, the reflective elements are arranged so that light will be incident on the heat exchanger from below, which makes it easier to control convective heat losses. Where this is the case, the solar focussing system is preferably configured to direct radiation in an upwards direction towards the secondary focus. A concentrator and/or homogenizer may be provided to further control the degree of focus and/or distribution of radiation incident on the heat exchanger.
"The solar powered system may further comprise a second heat exchanger surface, spaced apart from the first heat exchanger surface, for receiving solar energy to drive a different part of the heat driven engine to the first heat exchanger surface. In this case, a multiple target focussing system according to an embodiment of the invention may be provided for independently focussing solar radiation onto each of the heat exchanger surfaces.
"This approach may be useful where the heat driven engine is a linear multi-cylinder
Stirling
engine comprising a plurality of longitudinally spaced apart heat exchangers (e.g. one for each cylinder), for example. Using sets of separate reflective elements rather than a single reflector provides the necessary flexibility for efficiently dealing with this situation, even where the axis of the linear engine is parallel to the incident solar light.
"Such an arrangement can also be useful where it is desired to focus light onto different heat exchangers that are separated from each other in a direction perpendicular to the first alignment axes of the reflective elements (and to the incident solar light). For example, this approach might be useful where a linear multi-cylinder
Stirling
engine is to be oriented horizontally. This approach might also be useful where different sets of reflective elements are to be used to focus light onto different elements of a single heat exchanger (e.g. heater tubes) where these elements are displaced significantly from the axis of the heat exchanger.
"According to a further aspect of the invention, there is provided a solar powered system comprising: a photo-voltaic generator; and a focussing system according to an embodiment of the invention which is configured to concentrate solar radiation onto the photo-voltaic generator. The system may have a housing to contain the photo-voltaic generator, with means for dissipating rejected heat and for protecting the photo-voltaic generator from damage. In preferred embodiments the receiving face of the photo-voltaic generator is directed downwards so that unwanted debris is discouraged from accumulating on the receiving surface. As described above, the focussing system can be adapted to focus light upwards onto such a surface by suitable positioning and orientation of the reflectors. A secondary element may be provided to help ensure that the solar flux is optimally focussed onto the surface of the photocell. This element could be a concentrator or a homogenizer."
URL and more information on this patent, see: Bailey, Paul B.; Dadd, Michael W.; Stone, Charles R.; Jelley, Nicholas A.. Low Cost Focussing System Giving High Concentrations.
U.S.
Patent Number 9244262, filed June 1, 2011, and published online on January 26, 2016. Patent URL:
http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=9244262.PN.&OS=PN/9244262RS=PN/9244262
Keywords for this news article include: ISIS Innovation Limited.
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