Patent Application
20180067292
This invention relates to radiation concentrators that employ parabolic reflectors which may be used as solar collectors, to collect solar energy in the form of thermal energy which in turn may be converted to electric energy as its receiver attains high temperature. The radiation concentrators may be a dish type (3D) concentrator or a trough type (2D) concentrator.
The efficiency of a solar collector depends upon the concentration achieved by it. The efficiency increases, as the concentration achieved increases. The ideal concentration (theoretically achievable maximum) for a 3-D concentrator is 1/sin2θ and for a 2-D concentrator it is 1/sin θ; where θ is half the angle subtended by the radiation source (2θ).
The solar collectors according to the prior art include imaging concentrators like parabolic reflectors and non-imaging concentrators like compound parabolic concentrators (CPC), compound elliptical concentrators (CEC) etc. The non-imaging concentrator, CPC gives ideal concentration when employed as a trough type (2-D) concentrator. But for concentrating radiation from very far away sources like the sun, the CPC will have to be impractically tall; and hence cannot be use as an ideal solar collector. Imaging concentrators, such as parabolic reflectors are much more compact but they cannot achieve high concentrations that non-imaging concentrators can deliver.
For an imaging concentrator employing parabolic reflector, the receiver may be a flat receiver or an Omni-directional receiver (a conventional Omni-directional receiver has a circular cross section). For a trough type solar collector with a flat receiver, the maximum concentration achieved is 50% of the ideal concentration; when the rim-angle of the parabolic reflector is 45°. For a trough type solar collector with an Omni-directional receiver, the maximum concentration achieved is about 32% of the ideal concentration; when the rim-angle of the parabolic reflector is 90°.
One way to increase the concentration of a parabolic reflector with a flat receiver, according to the prior art, is to combine the parabolic reflector with a non-imaging concentrator, like CPC or CEC, as primary and secondary reflectors/concentrators respectively. CPC is a special case of CEC and CEC is more suitable to be used as secondary concentrator. The combination of parabolic primary with CEC secondary reflector gives a maximum concentration, for 2θ=0.52°, of approximately 96% of the ideal concentration, for rim angle approximately 9°. A parabolic reflector of rim angle approximately 9° has its focus at a very large distance which makes it an impractical design. A more practical design of the combination of parabolic primary with CEC secondary reflector has a parabolic reflector of rim angle approximately 45° and 40° which gives an approximate combined concentration of 70% and 75%. A parabolic reflector of rim angle approximately 45° and 40° will have its focus, and thereby the receiver, away from the aperture; which makes the design difficult to track the sun.
In the combination of parabolic primary with non-imaging concentrator secondary, the surface of the secondary reflectors has to touch the receiver, it causes the loss of heat from the receiver; also much heat is radiated from the other side of the flat receiver tube or plate. Reflectors which are made of glass will not be able to withstand the thermal shock and breaks. There are some designs with insulators between receiver and the secondary reflectors and around the flat tubular receivers. This makes the shape of the secondary reflector imperfect and prevents a part of the incoming radiation from reaching the receiver surface. And as no insulator is a perfect insulator some heat will still be lost by conduction. Also heating on one side can cause the receiver to bend.
For a parabolic reflector with an Omni-directional receiver, one way to increase the concentration according to the prior art, is to reshape the receiver to better fit all the edge rays reflected by the parabolic reflector. This receiver captures all the rays reflected off the parabolic reflector and is smaller than what a conventional circular receiver would need to be to do the same. Yet the concentration achieved is much lower than the ideal concentration.
The combination of parabolic primary with secondary concentrators having multiple entry apertures is another way to increase the concentration of a parabolic reflector with an Omni-directional receiver. In this combination, the primary and secondary concentrators are divided into sections. Each section of the secondary concentrator collects light from corresponding section of the primary concentrator. This kind of device has been proposed with a large number of divisions for primary and secondary concentrators. Though this kind of device can have the primary reflectors of rim angle up to 90°, the secondary concentrator becomes complex and difficult to manufacture. The secondary reflectors have to touch the receiver surface which makes it impossible to use a secondary reflector having the perfect shape for this device. Also the light rays passing through the secondary undergo multiple reflections and thereby reducing the energy of the light beam.
There are combinations of parabolic reflector primary with various Tailored Edge Ray Concentrator secondary reflectors with an Omni-directional receiver, according to the prior art. This method enables us to design simple secondary optics that attains high concentrations at large primary rim angles. For example, for a rim angle of 90°, an acceptance angle (2θ) of 0.007 rad (0.4°) and a concentration of 70% of the ideal maximum. The shading of the primary by the secondary is about 2%. All the light reflected by the primary reaches the secondary. However, in this method also, the secondary reflectors have to touch the receiver surface and the light rays undergo multiple reflections by the secondary reflector.
The invention incorporates a primary reflector consisting of two uneven confocal parabolic reflectors on either side of the axial plane of the system; where axes of both the uneven parabolic reflectors are directed to the centers of diagonally opposite halves of the radiation source. As the uneven parabolic reflectors focus the radiation from their diagonally opposite half of the source to their common focus, a receiver placed at the common focus absorbs a half of these rays directly. The receiver in this invention is shaped to fit in the envelope, formed by the upper edge rays and the middle rays (the middle rays are those rays which pass through the angular bisectors of the angles formed between the upper and lower edge rays reflected from the primary reflector) from both halves of the radiation source. Such a receiver absorbs the upper half of radiation reflected from all points of the primary reflector. The secondary reflector employed in this invention has an upper and a lower surface. Upper surface is shaped along the normals to the reflected middle rays from both the uneven parabolic reflectors and the lower surface is shaped along circular arcs with their centers at the lower edge of the receiver. Such a secondary reflector reflects the lower half of radiation reflected from all points of the primary reflector to the receiver. But the secondary reflector blocks some radiation from reaching the primary reflector; as a result the effective concentration achieved by the concentrator is reduced
An object of the invention is to device a main radiation concentrator, which can be paired with an auxiliary concentrator, incorporating parabolic primary reflectors that give good concentration ratio, for small acceptance angle, at rim-angles near 90°; so that the receiver of the system is very close to the aperture. Another object of the invention is to design a radiation concentrator, which achieves high concentration of radiation; with minimum energy loss due to multiple reflections. Yet another object of the invention is to design a radiation concentrator, which incorporates a secondary reflector that is not in contact with the receiver and is simple and easy to manufacture. Yet another object of the invention is to design an auxiliary radiation concentrator that can be paired with the main radiation concentrator in order to concentrate the light that would have been blocked by the secondary reflector of the main concentrator to the common receiver. Yet another object of the invention is to design a system of radiation concentrator, which incorporates a receiver that has a minimum surface area and absorbs all the radiation that enters the aperture of the system; that would be concentrated by the main and the auxiliary concentrators of the system.
The invention incorporates a main radiation concentrator that concentrates most of the radiation to a common receiver and an auxiliary radiation concentrator that concentrates the radiation that would have been blocked by the secondary reflector of the first concentrator to the common receiver. The primary reflector of the first concentrator consisting of two confocal parabolic reflectors on either side of the axial plane of the system; where axes of both the parabolic reflectors are directed to the centers of diagonally opposite halves of the radiation source. As the parabolic reflectors focus the radiation from their diagonally opposite half of the source to their common focus, a receiver placed at the common focus absorbs a half of these rays directly. The receiver in this invention is shaped to fit in the caustic curves formed by the upper edge rays and the middle rays (the middle rays are those rays which pass through the angular bisectors of the angles formed between the upper and lower edge rays reflected from the primary reflector) from both halves of the radiation source. Such a receiver has a surface area much smaller than the surface area of the re-shaped receivers in the systems according the prior art and absorbs the upper half of radiation reflected from all points of the primary reflector. The secondary reflector employed in the first concentrator has an upper and a lower surface. Upper surface is shaped along the normals to the reflected middle rays from both the parabolic reflectors and the lower surface is shaped along circular arcs with their centers at the lower edge of the receiver. Such a secondary reflector reflects the lower half of radiation reflected from all points of the primary reflector to the receiver. Also, this secondary reflector does not touch the surface of the receiver. Also the secondary reflector has a gap directly above the receiver which allows an auxiliary concentrator to be paired with it, in such a way that the auxiliary concentrator concentrates and directs the radiation, which would have been blocked by the secondary reflector of the main concentrator, through the gap to the receiver. The second concentrator may be a combination of reflectors facing each other or a lens or any combination of reflective or/and refractive elements that concentrates radiation to a point after the concentrator along the line through which the radiation travels.
It is also possible to construct a primary-secondary combination of first concentrator where the rim angles of the primary parabolic reflectors are near 90°. In such a construction, the receiver will be very close to the aperture. As this design of radiation concentration system gives good concentration for small acceptance angle and as being an uncomplicated design, it can be easily implemented in a trough or dish type solar collector system.
Some exemplary embodiments of the present invention is illustrated by way of example in the accompanying drawings in which like reference numbers indicate the same or similar elements and in which:
To clearly illustrate the features, geometry and principles of the invention the cross sectional view of various exemplary embodiments of the invention and the trajectory of light through them are described in the following part. The various embodiments described are trough type radiation concentrators, suitable for different distant radiation sources which subtends different angles at the aperture of the concentrator.
Please refer to
As shown in
The parabolas, the left parabola (5a) and the right parabola (5b), are rotated in opposite directions, keeping the common focus (4) as the center. The rotational angle (157) of the parabolas are defined by the angle formed between the axis (101a) of the left parabola (5a) or the axis (101b) of the right parabola (5b) and the axis of the system (103); which is ¼th the angle subtended by the radiation source. The axis (101a) of the left parabola (5a) and the axis (101b) of the right parabola (5b) pass through or points towards the midpoints of the diagonally opposite halves of the radiation source; when the axis of the system (103) is aligned towards the center of the source. In such an arrangement the left parabola (5a) focuses on the right half of the radiation source and similarly the right parabola (5b) focuses on the left half of the radiation source.
In this exemplary embodiment the left parabola (5a) extends from its apex (6a) towards the axis of the system (103) and the right parabola (5b) extends from its apex (6b) towards the axis of the system (103); and are joined together to form the vertex of the primary reflector (7). The angle formed between the axis of the system (103) and the axis (101a) of the left parabola (5a) and similarly, the angle formed between the axis of the system (103) and the axis (101b) of the right parabola (5b) are termed as the inner rim angles. The inner rim angles are denoted by −θ/2; also the angle subtended by the radiation source is denoted as 2θ. The angle formed between the axis (101a) and the parabolic radius (104), from the left edge (8a), of the left parabola (5a) and similarly, the angle formed between the axis (101b) and the parabolic radius (105), from the right edge (8b), of the right parabola (5b), are termed as the outer rim angles. The outer rim angles can have any value less than 90-20 degrees, which is denoted by ψR.
The left parabola (5a) and the right parabola (5b) of the primary reflector need not extend towards the vertex when the auxiliary concentrator is used, as the part of the radiation that would have reached the points on the primary reflector near the apexes (6a & 6b) of the parabolas would be directed to the receiver by the auxiliary concentrator (161). The primary reflector of the exemplary embodiment shown in
Any point on the left parabola (5a) and the right parabola (5b) corresponds to some value for the parameter ψ; which is the angle between the parabolic radius from that point and the axis of the same parabola. And the parameter ψ varies from ψR, the outer rim angle, to −θ/2, the inner rim angle, of both the left parabola (5a) and the right parabola (5b).
The edge rays from the right side of the source, upon being reflected from points on the left parabola and the edge rays from the left side of the source, upon being reflected from points on the right parabola, passes through points on the axis of the system (103), above the focus (4); and are termed as the upper rays. As shown in
The rays from the center of the radiation source, upon being reflected from all points on the primary reflector, passes through points on the axis of the system (103), below the focus (4); and are termed as the middle rays. As shown in
The edge rays from the left side of the source, upon being reflected from points on the left parabola and the edge rays from the right side of the source, upon being reflected from points on the right parabola, forms an angle θ with the parabolic radius from the points of their reflection and passes through points, on the axis of the system (103), below the focus (4), and are termed as the lower rays. As shown in
The edge rays from the right side of the source, upon being reflected from points between the apex of the left parabola and the axis of the system (103) and similarly, the edge rays from the left side of the source, upon being reflected from points, between the apex of the right parabola and the axis of the system (103) do not pass through points on the axis of the system (103); but are still termed as the upper rays in conformity with the nomenclature of the other upper rays. In the same way, the edge rays from the left side and from the center of the source, upon being reflected from the point −θ/2 (7) on the left parabola and the edge rays from the right side and the center of the source, upon being reflected from the point −θ/2 (7) on the right parabola, also do not pass through points on the axis of the system (103); but are still termed as the lower rays and the middle rays respectively; in conformity with the nomenclature of the other lower rays and the middle rays. As shown in
As shown in
A method of constructing the right caustic curve (124a) and the left caustic curve (124b) is its approximation by line segments, tangent to the right caustic curve (124a) and the left caustic curve (124b). These tangential line segments are along the entire middle and upper rays from the primary reflector. Every adjacent tangential line segment on the right caustic curve (124a) is along the middle rays from the adjacent points on the left parabola and along the upper rays from the adjacent points on the right parabola. Every adjacent tangential line segment on the left caustic curve (124b) is along the middle rays from the adjacent points on the right parabola and along the upper rays from the adjacent points on the left parabola.
The position of the points of reflection on the right parabola and on the left parabola, relative to the focus and to the axis of the system, can be determined from the parametric equation of parabola rotated through an angle θ/2. The parabolic radius from any point on the right or left parabola, corresponding to a particular value for the parameter ψ, forms an angle ψ+θ/2 with the axis of the system. The trajectory of the upper rays, the middle rays and the lower rays relative to the focus and the axis of the system, can be determined from the position of the points of reflection and the angle formed by them with the corresponding parabolic radius.
Please refer to
The first end point (149) and the second end point (150), of the second tangential line segment (141) are the points where the middle ray (cm) intersects with the middle ray (bm) from ψR and the middle ray (dm) ψ2 on the right parabola respectively. Similarly, the end points of any tangential line segment that is a part of the left caustic curve (124b) are the points of intersection of the ray containing the tangential line segment either with the middle ray or the upper ray, whichever is applicable, from the adjacent points on the primary reflector. The right caustic curve can also be approximated in a similar way. By choosing a closer adjacent point of reflection, more accurate caustic curves can be constructed, as a larger number of smaller line segments tangent to the curves are obtained.
When the auxiliary concentrator (161) is used, the secondary reflector and the auxiliary concentrator blocks a part of the incident radiation, near to its vertex, from reaching the primary reflector both the right and left caustic curves (124a & 124b) will be split into two curves as shown in the
Please refer to
When the auxiliary concentrator is used; as a result of the shading of the primary reflector there won't be any middle rays or upper rays reflected from the points on the primary reflector below the secondary reflector. The last point of reflection on the left parabola that gives the middle rays is the point (20a) vertically below the lower left edge of the secondary reflector (24b). This point will be further referred to as ψ8 of the left parabola. Similarly the last point of reflection on the right parabola that gives the middle rays is the point (20b) vertically below the lower right edge of the secondary reflector (24a). This point will be further referred to as ψ8 of the right parabola. The last point of reflection on the left parabola that gives the upper rays is the point (21a) that intersects with the edge ray from the right side of the source that passes through the lower left edge of the secondary reflector (24b). This point will be further referred to as ψ9 of the left parabola. Similarly the last point of reflection on the right parabola that gives the upper rays is the point (21b) that intersects with the edge ray from the left side of the source that passes through the lower right edge of the secondary reflector (24a). This point will be further referred to as ψ9 of the right parabola.
The last point of reflection on the left parabola that gives the lower rays is the point (19a) that intersects with the edge ray from the left side of the source that passes through the lower left edge of the secondary reflector (24b). This point will be further referred to as ψ7 of the left parabola. Similarly the last point of reflection on the right parabola that gives the lower rays is the point (19b) that intersects with the edge ray from the right side of the source that passes through the lower right edge of the secondary reflector (24a). This point will be further referred to as ψ7 of the right parabola. As no light will be incident on the primary reflector (1) between the points ψ7 (19a and 19b) of the left and right parabola (5a and 5b), the left and right parabolas may not extend beyond this points towards the vertex (7).
Please refer to
In the case were the auxiliary concentrator is not used some light will pass through the gap in the secondary reflector vertically above the vertex of the primary, a part of which would further be blocked by the receiver, causing multiple breaks in the caustic curves.
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In case the auxiliary concentrator (161) is not used, the caustic curves (124a and 124b) are broken by the secondary reflector (2) and the receiver (3), there will be multiple break points in both caustic curves. The surface area of the receiver formed by connecting these multiple break points will not be significantly low compared to the surface area of the receiver formed by the unbroken caustic curves. So in this discussion we use a receiver (3) with the cross section along the unbroken caustic curves (124a and 124b) in case where no auxiliary concentrator is used; as shown in
When the auxiliary concentrator (161) is not used, the left most point (41) of the receiver's (3) cross section is along the upper ray (1u) from the apex (6a) of the left uneven parabola (5a) and the right most point (42) of the receiver's (3) cross section is along the upper ray (mu) from the apex (6b) of the right uneven parabola (5b). The width of the receiver's (3) cross section is the distance between the apex (6a) of the left uneven parabola (5a) and the apex (6b) of the right uneven parabola (5b).
Again please refer to
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The distance between the lower left edge (24b) and the lower right edge (24a) of the secondary reflector was considered as the width of the secondary reflector (2) while calculating the caustic break points. From the
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The substantial part of the secondary reflector's (2) cross section is made along two trajectories orthogonal to the middle rays from the left and the right parabola (5a and 5b). Any trajectory orthogonal to the middle rays, with or without the lower section, can be used for the construction of secondary reflectors (2) cross section; however an orthogonal trajectory starting at the said transition points (25a and 25b) combined with the lower section (24a and 24b) gives a simple and efficient secondary reflector. Please refer to
An approximation of the family orthogonal trajectories, including the orthogonal trajectory on the right (122a) and the orthogonal trajectory on the left (122b), may be constructed by the continuum of line segments joining the free ends of an entire set extrapolated middle rays. The term free end of the extrapolated line segment is used in analogy with the free end of the imaginary taut string that traces the involutes.
Please refer to
As the starting point (150), the direction and the length of the second extrapolated line segment (131) can be determined, the free end (135) of the second extrapolated line segment (131) can be located relative to the focus and the axis of the system. Similarly, the free end (136) of the third extrapolated line segment (132), the free end (137) of the fourth extrapolated line segment (133) and the free end (138) of the fifth extrapolated line segment (134) can be located.
An approximation of the left orthogonal trajectory is formed by the continuum of line segments, which are referred to as the orthogonal line segments. The first orthogonal line segment (126) joins the free end (25b) of the first extrapolated line segment (130) and the free end (135) of the second extrapolated line segment (131), the second orthogonal line segment (127) joins the free end (135) of the second extrapolated line segment (131) and the free end (136) of the third extrapolated line segment (132), the third orthogonal line segment (128) joins the free end (136) of the third extrapolated line segment (132) and the free end (137) of the fourth extrapolated line segment (133) and the fourth orthogonal line segment (129) joins the free end (137) of the fourth extrapolated line segment (133) and the free end (138) of the fifth extrapolated line segment (134). The right orthogonal trajectory may also be constructed in a similar way.
By choosing a closer adjacent point of reflection more accurate orthogonal trajectories can be constructed, as a larger number of smaller orthogonal line segments joining a larger number of free ends of the extrapolated middle rays are obtained.
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An embodiment of the invention uses a lens (51) of the same size as the secondary reflector (2) of the as the auxiliary concentrator (161), positioned above the secondary reflector (2) in such a way that its focus is at the common focus (4) or in close proximity to the common focus (4) of the parabolas (5a and 5b) of the primary reflector; as shown in
Another embodiment of the invention uses a combination of concave (parabolic, cylindrical, modified parabolic etc.) first reflector (52) and matching second reflector (53) as the auxiliary concentrator (161), in which the first reflector (52) has an aperture of the same size as the secondary reflector (2), positioned above the secondary reflector (2), in such a way that its focus is at the common focus (4) or in close proximity to the common focus (4) of the parabolas (5a and 5b) of the primary reflector; as shown in
The auxiliary concentrator (161) can be any combination of refractive elements like multiple lenses, where all the lenses arranged with their axes collinear to the axes of the system (103), or where different lenses are configured to focus different parts of the radiation that would have been blocked by the secondary reflector (2) to same or different points in proximity to the receiver (3). Combinations of lenses and mirrors can also function as a good auxiliary concentrator (161).
Different embodiments of the invention, for a particular radiation source subtending an angle 2θ, having primary reflector of different outer rim angle values, have the secondary reflectors and the receivers of different relative sizes. Once the cross section of the concentrator according to this invention is constructed, for a particular outer rim angle and for a particular source, the concentrator can be easily made as a trough type concentrator or a dish type concentrator. These different embodiments also have different concentration ratios and there exists at least one embodiment, for both trough type and dish type, which has the maximum concentration for each radiation source that subtends a different angle, 2θ.
The embodiment of the invention, as a trough type solar collector, has a very small cross section for the secondary reflector (2) and the receiver tube (3) relative to the primary reflector, owing to the fact that the angle subtended by the Sun (2θ) is approximately 0.52°. To find the embodiment that has the maximum concentration ratio, various embodiments of the invention were studied numerically. The various embodiments included the trough type solar collectors, with and without the auxiliary concentrator, with primary reflectors (1) of different outer rim angles and matching secondary reflector (2) and receiver (3). The auxiliary concentrator (161) used in the study was a simple convex lens having the same area as the secondary reflector (2) with different focal lengths placed at different distances from the focus (4) of the primary reflector (1) in such a way that the focus of the lens varied from the lower most point of the receivers cross section (30) to the upper most point of the receivers cross section (29).
The ratio of reduced surface area of the aperture, due to the shading of the primary reflector (1) by the secondary reflector (2), to the surface area of the receiver (3) is termed as the effective concentration ratio. The optimum effective concentration ratio for the trough type solar collector, without the auxiliary concentrator, is achieved when the outer rim angle is approximately 84° and the achieved effective concentration ratio is approximately 178.47; which is approximately 80.99% of the ideal concentration.
The ratio of reduced surface area of the aperture, due to the shading of the primary reflector (1) by the secondary reflector (2), plus the percentage of light from the auxiliary concentrator (161) absorbed by the receiver (3) multiplied by the area of the secondary reflector (2), to the surface area of the receiver (3) is termed as the final concentration ratio. The optimum final concentration ratio for the trough type solar collector, with a convex lens as the auxiliary concentrator, is achieved when the outer rim angle is approximately 85° and when the focal length of the lens is 71.31% of the focal length of the left and right parabolas (5a & 5b) and it concentrates to a point in the axis within the cross section of the receiver. The achieved final concentration ratio is approximately 184.36; which is approximately 83.67% of the ideal concentration.
The trajectory of light reflected from every other point of the primary reflector is referenced by a two letter symbol. The first letter of the two letter symbol represents the following:
The second letter of the two letter symbol represents the following:
| Number | Date | Country | Kind |
|---|---|---|---|
| 2472/CHE/2015 | May 2015 | IN | national |
| 201643020587 | Jun 2016 | IN | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/IB2015/058617 | Nov 2015 | US |
| Child | 15810031 | US | |
| Parent | PCT/IB2017/053540 | Jun 2017 | US |
| Child | PCT/IB2015/058617 | US |
