PRIMARY CONCENTRATOR WITH ADJUSTED ETENDUE COMBINED WITH SECONDARIES ASSOCIATED TO MULTIPLE RECEIVERS AND WITH CONVECTION REDUCTION
The present invention relates to primary and secondary concentrators of solar radiation. New primary concentrators of adjusted etendue combined with secondary concentrators are presented, for single of multiple receivers, capable of attaining the maximum possible concentration. The present invention also refers to devices which reduce the thermal losses by convection of the receivers.
The present invention relates to primary and secondary solar radiation concentrators. The invention relates to new primary etendue adjusted concentrators, combined with secondary ones, for simple or multiple receivers, able to reach the highest concentration value possible. The present invention also relates to devices to reduce thermal losses due to convection at the receivers.
BACKGROUND OF THE INVENTIONLarge-scale solar power plants can produce large quantities of electric power. This means that large quantities of sunlight must be collected.
Some of these plants may simply collect sunlight without concentrating it, as in the case of using flat photovoltaic panels exposed to the sunlight. However, when using high-efficiency solar cells or thermodynamic cycles, some degree of concentration is needed to increase efficiency and, therefore, typically also some tracking of the sun.
Sunlight is concentrated using optics. There are cases in which a large number of small optics are placed side by side, each one of them with its own receiver. That is the of, say, a set of parabolic primaries with solar cells at the foci, or adding kaleidoscopes to improve irradiance uniformity on the cell [Daniel Feuermann and Jeffrey M. Gordon, Solar Fiber-optic mini-dishes: a new approach to the efficient collection of sunlight, Solar Energy Vol. 65, No. 3, pp. 159-170, 1999]. In other cases, a smaller number of larger receivers is used, but then large optics are needed. The problem with large optics is that they are hard to assemble and move to track the sun. One possible way around this problem is to replace the large optic by a large number of small optics that mimic its behaviour. One example of this process is found in the tower power plants which have a large number of small mirrors called heliostats that reflect the light to a large receiver. In this case, instead of a large parabolic primary, these plants have a “Fresnel” primary composed of many small mirrors. [J. I. Ortega, J. I. Burgaleta, F. M. Tellez, Central Receiver System (CRS) Solar Power Plant using Molten Salts as Heat Transfer Fluid, Proceedings 13th International Symposium on Solar Power and Chemical Energy Technologies ISBN 84-7834-519-1, Edit. M. Romero, D. Martinez, V. Ruiz, M. Silva, M. Brown, M. Snachez, M. Romero, Methodology for generation of heliostat field layout in central receiver system based on yearly normalized energy surfaces, Solar Energy 80, pp 861-874, 2006].
A further possibility is to have trough receivers onto which light is concentrated using trough optics. Also in this case parabolic primaries may be used [E. Rojas, A. Fernandez, E. Zarza, Theoretical evaluation of parabolic trough designs for industrial applications, Proceedings 13th International Symposium on Solar Power and Chemical Energy Technologies ISBN 84-7834-519-1, Edit. M. Romero, D. Martinez, V. Ruiz, M. Silva, M. Brown], or alternatively “Fresnel” primaries composed of long linear heliostats running in the direction of the receiver [Patente U.S. Pat. No. 4,131,336: Miller et al., Primary reflector for solar energy collection system, 1978, Solar thermal power plants, Renewable Energy World 06/2003 pp. 109-113]. The heliostats may track the sun keeping the receiver illuminated by concentrated sunlight. However, the heliostats shade each other, especially those further away from the receiver, and the light that is shaded is lost. The concentration of these primaries may be increased by secondary optics, such as the TERC secondaries [J. M. Gordon and Harald Ries, Tailored edge-ray concentrators as ideal second stages for Fresnel reflectors, Applied Optics, Vol. 32, No. 13, pp. 2243-2251, 1993]. These concentrators will, however, only approach the theoretical maximum concentration in the limit case of a primary composed of infinitesimal heliostats, a severe practical limitation.
In the prior art, an improvement over a simple “Fresnel” primary is to intersect two heliostat fields in an arrangement know as CLFR (Compact Linear Fresnel Reflector) [David R. Mills and Graham L. Morrison, Compact linear Fresnel reflector solar thermal power plants, Solar Energy Vol. 68, No. 3, pp. 263-283, 2000; U.S. Pat. No. 5,899,199: David Mills, Solar Energy Collector system, 1999, U.S. Pat. No. 6,131,565: David Mills, Solar Energy Collector system, 2000]. In this arrangement, instead of a single receiver there are several receivers. The heliostats are all the same size and those closer to a first receiver redirect the light to it. Those more spaced apart, alternatively redirect the light to the first receiver and to a second receiver. This creates a W shaped heliostat field in the areas more spaced apart from the receivers where the odd heliostats reflect light to one receiver, while the even heliostats reflect light to the other receiver. This approach, however, still does not adjust the etendue of the incoming radiation with that reflected to the receivers and, therefore, there will always be either some shading of light or areas of the heliostat field not fully illuminated when seen from the receivers.
This is a fundamental limitation of these optics and is independent of the size or shape of the heliostats. The concentrations attained by these optics are much lower than the theoretical limit.
To solve the etendue mismatch problem between the etendue of the light received by the primary and the etendue the primary should ideally redirect towards the receivers, new primaries are needed.
The present invention discloses two different ways of improving the primary: changing its overall shape and changing the size and shape of its heliostats. To increase concentration and approach the theoretical limit, the new primaries must be combined with new secondary optics.
When changing the overall shape of the primary, the heliostats are placed on a wave shaped trough surface and the size and shape of the heliostats is a function of the position in the heliostat field. The heliostats may also be flat, in which case the smaller the heliostats, the higher the concentration the primary can provide.
The size and shape of the heliostats may be adjusted in order to increase concentration, which can further be augmented with the use of a secondary. The heliostats constitute a discontinuous primary and, in order to design a continuous secondary, a continuous primary is developed. The heliostats are inter connected by flow lines resulting in a continuous primary (broken line) for which a continuous secondary can be designed. The portions of primary along flow lines can then be removed, leaving the initial heliostats present at the start of the procedure. In this concept the primary is conceived (for the purpose of secondary design) has a continuous but broken mirror, i.e. in steps, portions of which follow flow lines and other portions are transverse to those flow lines [Pablo Benitez, Juan Carlos Minano, Maikel Hernandez, On the analysis of microstructured surfaces, SPIE Proceedings, Vol. 5529, Nonimaging Optics and Efficient Illumination Systems, pp. 186-197, 2004]. This type of design is common in Fresnel Lens design, which can also be combined with secondaries to increase their concentration [M. Collares Pereira, A. Rabl and R. Winston, Lens-mirror combination with maximal concentration, Applied Optics, Vol. 16, No. 10, pp. 2677-2683, 1977, M. Collares Pereira, High temperature solar collector with optimal concentration: non-focusing Fresnel lens with a secondary concentrator, Solar Energy, Vol. 23, pp. 40-9420, 1979, Ralf Leutz, Akio Suzuki, Atsushi Akisawa and Takao Kashiwagi, Design of a nonimaging lens for solar concentrators, Solar Energy, Vol. 65, No. 6, pp. 379-387, 1999]. Other more elaborated types of stepped optics are also possible [Julio Chaves, Manuel Collares-Pereira, Ultra flat ideal concentrators of high concentration, Solar Energy Vol. 69, No. 4, pp. 269-281, 2000, Julio Chaves and Manuel Collares-Pereira, Ideal concentrators with gaps, Applied Optics, Vol. 41, No. 7, pp, 1267-1276, 2002 Julio Chaves, Introduction to Nonimaging Optics, CRC Press, Taylor and Francis Group, 2008]. Continuous secondaries may also be directly designed from discontinuous primaries (a set of heliostats) in which case joining the primary heliostats by flow lines is not required. In this version the heliostats may be on the wave shaped surface or on a flat one. The secondary concentrators conceived for these new primaries are continuous, from portion to portion, in accordance with the piece wise nature of the primary.
In the case of a single receiver, the concentrators thus obtained compare favourably to traditional combinations of Fresnel reflectors and secondaries, increasing the concentration on the receiver close to the theoretical maximum, even for primaries composed of large size heliostats. In the case of multiple receivers, these new concentrators also compare favourably to CLFRs, once again achieving concentrations on the receivers close to the theoretical maximum, while having lower losses.
The secondary mirrors typically touch the receiver and a method for preventing/solve such problem is also disclosed.
The invention further includes devices for reducing convection losses in the receiver. These include specially shaped mirrors and transparent covers.
SUMMARY OF THE INVENTIONThe present invention relates to an optical system with primary concentrator of the Fresnel type and secondary concentrator adjacent to the receiver, the secondary-receiver set being above the primary and characterized by the primary concentrator containing a stepped flow-line optics which shape verifies the fact that it reflects a set of edge rays tangent to the receiver and the other set of edge rays into the direction of the secondary concentrator which has a shape that (in turn) reflects those edge rays tangent to the receiver producing theoretically there maximum concentration and in which the secondary concentrator is truncated.
In one embodiment, the optical system is characterized by the receiver having a convex face and also a planar one in which the secondary receiver touches only in one point and in which the planar face may be substituted with a concave one resulting in a secondary that does not touch the receiver.
In another embodiment, the referred optical system is characterized by the absence of some or all of the stepped optics that go with the flow lines, leaving only those portions that cross the flow lines, this is, the heliostats and in which the eventual continuous (not step like) portions of the primary concentrator may also be divided into heliostats.
In another embodiment, the referred optical system is characterized by the heliostats being able to rotate about themselves to track the apparent diurnal motion of the sun.
In another embodiment, the referred optical system is characterized by combining at least two optical systems as those referred above thus forming a concentrator with multiple receivers.
In an embodiment the referred optical system is characterized by the fact that the heliostats being placed on a wave like surface, with cylindrical geometry, substantially corresponding to a curve that optically conserves etendue. In another preferred embodiment, the referred optical system is characterized by the mirrors of the primary concentrator being curved or flat. Still in another preferred embodiment the referred optical system is characterized y the global form of the primary being flat.
Preferably, the referred optical system is characterized by all receivers being substantially at the same height and the angles measured from the vertical that passes through one of the receivers substantially being about 33±10° with the line that, leaving the receiver, passes through the point on the primary that marks the transition between one and two receivers, and about 61±5° with the line that goes through the intermediate point of the primary optics and about 71±5° with the line that passes in another transition point from one to two receivers. In a most preferred way the previous optical system is characterized by the angles as measured from the vertical that goes through one of the receivers being: 32.6° with the line that leaving the receiver, passes through the point on the primary that marks the transition between one and two receivers, of 61.2° with the line that goes through the intermediate point of the primary optics and 71.2° with the line that passes in another transition point from one to two receivers. In a more preferred embodiment the previous optical system is characterized by not comprising secondary concentrators by the receivers.
In another embodiment said optical system is characterized by the receivers having a larger size than they would if sized to correspond to the ideal or maximum concentration.
The present invention also relates to an optical system characterized by comprising transparent covers, substantially perpendicular two flow lines and mirrors (mirrored on both sides) that substantially follow the flow lines, to reduce the convection by the receiver.
In a preferred embodiment of the present invention the referred optical system is characterized by comprising mirrors and/or transparent covers with the shape of broken lines that substantially follow flow lines or are transverse to them and in which the covers may be simple or double.
The present invention refers to the use of any optical system as defined above, characterized by the optical system being intended to concentrating the solar radiation.
The above and other aspects, features and advantages of the present invention will be apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:
A better understanding of the characteristics and advantages of the present invention will be obtained with reference to the detailed description of the invention and corresponding figures, which are illustrative of the way in which the principles behind the invention are used.
DETAILED DESCRIPTION OF THE INVENTIONThe present invention relates to new etendue-adjusted primary concentrators combined with secondary concentrators, for simple or multiple receptors, able to reach the maximal possible concentration. The present invention also relates to devices able to reduce convective thermal losses from the receivers.
In prior art these concentrators may have only one receiver (as in the case Linear Reflectors of the Fresnel type-LFR) or they may have multiple reflectors (as in the case of Compact Linear Fresnel Reflectors-CLFR). The present invention shows improvements in the general primary shape in the case of multiple receivers. It further shows new concentrators of the primary-secondary type, coming very close to the theoretical concentration limit, even when the primary is formed by large heliostats (something impossible in prior art). These new concentrators are also applicable in the case of multiple receivers. The present invention further shows devices to reduce thermal convective losses from the receiver(s).
The present invention describes a new type of Fresnel primaries so-called “etendue adjusted”. These primary shapes have a wave type configuration, conserving “etendue” and are characterized by the fact that the etendue of the incident radiation is perfectly adjusted to the etendue of the reflected radiation towards the receivers (in particular in the case that only one receiver exists the shape is no longer a wave but a parabola (prior art)).
The corresponding reflector mirror corresponds to a large number of small structures that follow the etendue conserving curve and reflects the light/radiation to the receivers. A limiting case of these new primaries occurs (as in prior art) when they are formed by an infinite number of heliostats. New secondaries can then be designed for these primaries coming close to the theoretical limit for concentration. Although these new sets represent a step ahead when compared with prior art, because of their reduced etendue mismatch, they have a reduced practical interest. As discussed below the present invention also shows geometries for primary structures with large (finite) and practical sizes.
The present invention also describes new primary-secondary concentrators for single receivers. The primary is composed by a set of heliostats (movable mirrors able to track the sun). To increase primary concentration it may be combined with a secondary one. To facilitate the design of a continuous secondary, the heliostats are interconnected by flow lines, resulting in a continuous primary with a stepped shape, a continuous and broken line. A continuous secondary may then be designed for this now continuous primary. A continuous primary is not a necessary condition and a continuous secondary may also be designed even for a discontinuous primary (this type of optics is also described in the patent. However a continuous primary makes the design more intuitive. Once the secondary designed, the primary portions along flow lines can be removed, leaving the primary just with the initial heliostats.
The primary reflector is thus conceived to be a flow line stepped reflector, consisting of continuous of portions, as a broken line, which parts either follow or cross flow lines. These individual portions are curved in general, but some may be flat. The underlying shape of the primary, as a whole, may be curved or flat.
In particular configurations of the present invention, the resulting optics, primary-secondary, comes close to the theoretical concentration limit on the receiver. In contrast, prior art could only (theoretically) reach maximum concentration on the receiver only with a primary of infinitesimal structures. In the primary-secondary set, the primary is conceived to reflect one set of edge rays into directions tangent to the receiver. The other set of edge rays is reflected by the primary into the direction of the secondary and this one, in turn, redirects them in a way that they also end up tangent to the receiver.
In case, for instance, of large solar systems, the sections of stepped reflectors along flow lines, may be eliminated, thus resulting a discontinuous reflector, containing only the portions that cross the flow lines. These portions may now be considered as heliostats that “track” the apparent motion of the Sun. This set by itself is not ideal and the heliostats may be extended a little to recuperate some of the lost radiation. The secondary initially designed for the continuous reflector can now be used with the resulting curved heliostats, constituting high efficiency optics, coming close to the theoretical concentration limit on the receiver.
Some of these concentrators for a single receiver may now be combined, originating concentrators for multiple receivers. The process is similar to that of combining LFR to get CLFR, in which several LFR juxtapose on each other, intersecting and forming a CLFR. In a simpler solution consists on having all heliostats on a straight line. A more elaborated solution the underlying primary shape may be different, just as a curve that conserves etendue. In these optics the heliostats can be placed on a wave shaped line (surface), reflecting solar radiation towards different receivers. The size and shape of the heliostats are a function of their position on the wave line (surface). This method permits matching the incident radiation etendue with the one reflected towards the different receivers. The result is a Fresnel primary with very little blocking, and thus with very small optical losses. In contrast, prior art does not contemplate the matching of the incident etendues to the reflected ones at and by the primary and so the resulting optics are less efficient due to radiation blocking between heliostats.
The present invention also explores the relative dimensions of a reflector optimized for two receivers, in which all components (portions) are placed on a straight line (flat surface). For a final optimal (in the theoretical limit) concentration of the present invention, obtained with the help of the secondary concentrators described in the present invention, the receiver can no longer be flat and have only one absorbing side, as it can no longer accommodate all the reflected radiation etendue from the primary towards it.
The present invention further describes a methodology to create a separation (gap) between the mirror and the receiver, to prevent a thermal bridge and the consequent thermal losses.
The present invention further describes ways to reduce the convective losses around the receiver, through the use of covers/transparent surfaces, substantially perpendicular to flow lines and mirrors that substantially follow those flow lines.
The present invention relates to optical design with two-dimensional geometry, that may be implemented in a practical way applying to these designs a translation symmetry (or a rotational one when only a design with a flat receiver exits).
The term “étendue” of the radiation that crosses a curve (two-dimensional geometry) relates to the integral of the projected length with the angular aperture of the radiation: U=∫∫dxcosθdθ in which U is the étendue, dx is an infinitesimal length along the curve, θ is the angle the propagation direction makes with the normal to dx and dθ the angular aperture occupied by the radiation that crosses dx. In a more general way the curve may be immersed in a medium with refractive index n, and in that case etendue is defined by U=∫∫ndxcosθdθ.
In the present invention the term “etendue matching curve” relates to a curve to which small structures may be added and which redirect the light towards one or more receivers and for which the incident light etendue coincides with the “etendue” of the redirected light towards the receivers.
Additionally, the term “stepped optics” or “optics in steps”, relates to optics that consists of a mirror that follows a flow line, followed by one that crosses the flow lines which captures and re-directs a portion of light and then another mirror along a flow line, followed by another mirror crossing the flow lines capturing and redirecting the light and so on. A particular case occurs when the optics crossing flow lines is a simple mirror (curve or flat). In that case the stepped optics is a continuous mirror, continuous but broken, either following or crossing the flow lines.
The term “simple mirror” relates to any surface with only one reflecting face, and “double mirror” relates to any surface mirrored on both sides thereof.
Further, the term “double cover/transparent surface” relates to one that is formed by two single transparent surfaces.
The present invention will be further clarified with reference to the accompanying figures.
In the case in which the incoming radiation 804 has angular aperture 2θ0, the radiation 805 redirected to the left has angular aperture 2θ1 and the radiation 806 redirected to the right has angular aperture 2θ2 the shape of the etendue-conserving curve is governed by the equation for the conservation of etendue that can now be written as: sinθ1cos(φ1−α)+sinθ2cos(φ2+α)=sinθ0cosα.
This is a common procedure when designing nonimaging optics. The right receiver 1831 has the same shape as the left one (bound by points 1801, 1802 and 1803).
For this particular shape of receiver, the Fresnel reflector curve starts with a parabola 1804 with axis parallel to 1810 and focus 1801. The vertical though point 1805 is the same as through point 1802. The Fresnel reflector then continues between points 1806 and 1808 as another parabola 1807, also with axis parallel to 1810, but now with focus 102. The central portion of the Fresnel receiver is an etendue conserving curve 1821 for two receivers, extending from point 1808 to its symmetrical 1822. With reference to a generic point 1823 on this curve, the light reflected to the left receiver 1830 is bound by ray 1826 tangent to the receiver at point 1802 and by the other edge ray 1827. The light reflected to the right receiver 1831 is bound by ray 1824 tangent to the receiver at point 1820 and by the other edge ray 1825. This portion of the curve is calculated using a similar geometry to that shown in
As point 1823 moves towards point 1808 where curve 1821 ends, ray 1824 tends to ray 1828. This ray 1828 is tangent to curve 1821 at point 1808. Choosing a point 1808 further up on curve 1807 to start curve 1821 would result in less compact Fresnel primary. On the other hand, choosing a point 1808 further down on curve 1807 to start curve 1821 would result in light losses due to shading of some light by curve 1821 (in this case, ray 1828 would intersect curve 1821).
It is possible to determine how much etendue is reflected by the Fresnel mirror in the direction of the left receiver 1830 and how much is reflected in the direction of the right receiver 1831. All the light falling on parabolas 1804 and 1807 is redirected in the direction of 1830 (although in the present configuration not all this light will hit the receiver). On the other hand, for the light hitting curve 1821 between points 1808 and 1822, half of it is reflected towards 1830 and the other half towards 1831. This means that the light redirected in the direction of 1830 by the Fresnel reflector on 1821 corresponds to the etendue of the light falling on half the curve 1821. The total etendue of the light redirected towards 1830 is then given by U=2Rsinθ where R is the horizontal distance between point 1805 and midpoint 1829 of curve 1821.
Due to secondary truncation, now some light is lost because part of the secondary mirror is no longer there to redirect it towards the receiver. The final concentration of the optic also decreases accordingly. The receiver and the secondary mirror also shade the primary, further decreasing the final concentration. For small acceptance angles such as those needed for the collection of sunlight, these losses are rather small.
If the system had no losses, the etendue of the light reaching receiver 1830 would equal that of the light falling on the primary between point 1829 and its symmetrical 2006.
This optical system is extended left and right by reflection symmetry with 2007 and 2008 as the axes of symmetry.
To the left of the vertical line 2401, the bottom surface is also a broken line but now designed in a different way. The lines 2412 are parallel to the edge rays 2413 inside the lens (and, thus, have no optical function) while lines 2411 cross the flow lines of the incident radiation.
In this example, the primary is formed of a parabolic mirror 2504, a flat flow-line mirror 2505, and another mirror composed of two sections: a flat section 2506 and a parabolic section 2507. These two sections (2506 and 2507) share a common derivative at point 2513. Both parabolas (2504 and 2507) in the primary have axes parallel to edge rays 2517 and focus 2502.
The secondary is formed of three sections 2508, 2509 and 2510. An edge ray coming from the left is reflected at a point 2511 on parabolic section 2504 in the primary towards a point 2512 on the secondary. This point is calculated in such a way that it reflects that ray towards the edge 2502 of the receiver. For the reflected edge rays at point 2513 of the primary, one goes into direction 2514, while the other is reflected again by the flow-line 2505 into the direction 2515, parallel to 2514. The parabolic arc 2509 with axis parallel to 2514 and 2515 concentrates these edge rays towards edge point 2502 of the receiver. Points 2516 in section 2510 of the secondary are calculated in such a way that they reflect towards the edge 2502 of the receiver the edge ray they get from the primary parabolic arc 2507. Primary and secondary meet at point 2518.
Flow-line 2505 and section 2506 may be given different shapes according to the anidolic optics principles (nonimaging optics). In this case the secondary section corresponding to the primary will no longer be parabolic, but will be calculated with the edge ray principle of anidolic optics.
This optic produces the maximal concentration on the receiver. Radiation incident directly on the mirror that follows the flow line 2505 is reflected into directions other that those of the receiver, but all other light is ideally concentrated on the receiver.
As in the above-mentioned cases, the secondary must be truncated so that radiation may reach the primary.
This combination of primary and secondary is a stepped flow-line optic with walls following flow lines, as the primary is obtained with mirrors generated and placed along flow lines, in a successive way and alternating with other that cross the flow lines. In the limiting case these structures, portions, of primary become infinitesimal and the secondary becomes what is known as a TERC for that primary.
This method for producing the primary shares principles similar to those used in designing the right half of the Fresnel lens, shown in
Since mirrors 2505 along the flow-lines have been removed, section 2506 of the mirror of the primary (as shown in
An edge ray coming from the left and hitting a low point on another primary mirror 2703 is reflected in direction 2709. On the other hand, the edge ray coming from the right and reflected at the same point on the primary is once again reflected by mirror 2702 along the flow line in a direction 2710, parallel to 2709. Both these rays 2709 and 2710 are redirected towards directions tangent to the receiver at point 2708 by parabolic arc 2711 on the secondary mirror.
This optical system is also extended left and right by reflection symmetry with 2804 and 2805 as symmetry axes.
Vertical line 2902 corresponds to vertical line 2707 in
This optic still produces maximum concentration on the receiver 3002. All light falling on the space between end points 3003 and 3004 of the primary mirrors is lost. Also, all the light hitting a point 3005 on mirror 3001 and reflected in directions contained between 3006 and 3007 is shaded by mirror 3008 and lost. The size of the receiver is such that the etendue it can receive matches that reflected by the whole primary (left and right halves) towards it. When that happens, the secondary touches the primary at its end point 3009.
The geometry in this figure may also be considered an alternative to that in
Like in the case of the secondary in
This construction of the primary shares similar principles to those used in the design the left half of the Fresnel lens shown in
Reflection of light by the Fresnel mirror is such that the bisector to the edge rays points towards points towards 3101 (or 3102).
For a point 3104 between points 3103 and 3105 on the reflector, the etendue mismatch between that of the incoming radiation and that of the reflected light towards receiver 3101 is given by ΔU1=2sinθ(1−cosφ1)dx. For another point 3106 between points 3105 and 3108, the etendue mismatch between that of the incoming light and that ideally reflected towards receivers 3101 and 3102 is |ΔU2|=2sinθ|1−cosφ1+cosφ2|dx where |α| is the absolute value of α. The absolute value in the expression for ΔU2 ensures that the etendue mismatch is always accounted for as a positive quantity when integrating it across the Fresnel reflector. From these expressions it can be seen that the etendue of the incoming radiation does not match what ideally the Fresnel reflector should emit towards the receivers. The parameters of the optical system must then be adjusted in such a way as to minimize this etendue mismatch. The height of the receivers (distance from point 3103 to 3101) may be considered as a scale factor of the whole system and, therefore, it can be made equal to one. Being the receivers at a different height, the whole system would be scaled accordingly. The parameters that must now be adjusted are the distance between receivers (distance from 3101 to 3102) and the horizontal coordinate xT of the transition point from one to two receivers.
The total etendue mismatch is proportional to the integral of ΔU1+|ΔU2| from x0 (point 3103) to xM (point 3107). This integral must be minimized relative to the parameters of the optic: horizontal coordinate xTand distance between receivers.
For points 3104 on the primary between point 3103 and 3105, the etendue that the Fresnel reflector should emit towards receiver 3101 is given by 2sinθ cosφ1dx and the etendue of the incoming radiation is 2sinθdx. Therefore, per unit incoming etendue, the primary should emit an etendue of cosφ1. This is represented by curve 3304.
For points 3106 on the primary between point 3105 and 3108, the etendue that the Fresnel reflector should emit towards receivers 3101 and 3102 is given by 2sinθ(cosφ1+cosφ2)dx and the etendue of the incoming radiation is 2sinθdx. Therefore, per unit incoming etendue, the primary should emit a etendue of cosφ1+cosφ2. This is represented by curve 3306.
Curve 3305 is symmetrical to 3304 relative to the midpoint of the Fresnel reflector.
Comparing the curves 3304 and 3306 representing the etendue of the light the Fresnel reflector should emit towards the respective receivers with straight line 3303 representing the available etendue at each point, it can be seen that there are points with excessive etendue available and other points with etendue deficit. For the points to the left of 3105, the etendue that the reflector should emit towards receiver 3101 is less than the etendue available (line U=1). This means that some etendue must be lost at the reflector and there will be shading between the microstructures of the primary (infinitesimal heliostats). On the other hand, for points on the Fresnel reflector between 3105 and 3108, curve 3306 sometimes is above line 3303. This means that the etendue the Fresnel reflector should be emitting towards receivers 3101 and 3102 is more than the etendue available. This means that, as seen from the receivers, the Fresnel reflector will have “dark holes” that do not emit light. For the region in which curve 3306 is below 3302, the needed etendue is less than what is available and there is shading of light at the Fresnel reflector.
Point 3105 for which there is a transition from one to two receivers, the vertical distance between curve 3306 and 3303 and between curve 3304 and 3303 are equal to each other and given by 3307, with a value of 0.158 (or 15.8%).
Flow lines 3805 can now be removed and mirrors 3806 given mirror symmetry about vertical line 3804 thought midpoint 3107.
The primary reflects one set of edge rays tangent to the receiver. In the case of a V receiver, this means that to the left of point 3807 one set of edge rays is concentrated to point 3808, while to the right of 3807, this same set of edge rays is reflected towards point 3809. The other set of edge rays is concentrated to point 3809 after reflection on the secondary 3803. Point 3807 is in this example on the straight line through points 3808 and 3809.
The primary 3901 now reflects light to both V receivers and the secondary 3902 was truncated to allow light to reach the primary.
The other set of edge rays reflected by the primary between points 4201 and 4202 is concentrated to the lower tip 4205 of the receiver by a portion of the secondary to the right of hyperbolic arc 4209. This portion of the secondary mirror is designed in the same way as the portion of the secondary mirror touching receiver 3501 in
The portion of the primary between points 4202 and 4203 concentrates a set of edge rays to lower tip 4205 of the receiver, while the other set of edge rays is reflected by the secondary mirror towards the lower tip 4205 of the receiver. Again this is similar to the design of the right portion of the secondary mirror in
Between the transparent cover and the receiver, there are mirrors 4504, mirrored on both sides, shaped as G-lines (or flow lines). These mirrors do not affect the flow of light and, therefore, do not affect the optical behaviour of the optic. G-lines may sometimes be well approximated by simple shapes, such as straight lines.
Both the transparent cover 4503 and the internal mirrors 4504 help reduce the convection around the receiver, reducing thermal losses.
The primary extends from point 3108 on the right to point 4501 on the left. These edge points of the primary are symmetrical relative to midpoint 3103.
Also internal mirrors (mirrored on both sides) 4504 placed along the G-lines help reduce internal convection.
Thermal losses are further reduced by using thermal insulation 4604 on the back of the V receiver. Overheating of the secondary mirrors may be prevented by using heat dissipaters 4605. At point 4606 there should also be some thermal insulation between the receiver and the secondary mirror to prevent a thermal bridge between the two, resulting in a thermal loss.
The preceding description of the presently contemplated best mode of practicing the invention is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The full scope of the invention should be determined with reference to the claims.
Claims
1. An optical system with primary concentrator of the Fresnel type and adjacent secondary concentrator, being the secondary receiver set above the primary, and characterized by the primary concentrator containing a stepped flow-line which shape verifies the condition that it reflects a set of edge rays so as to make them tangent to the receiver and the other set of edge rays in the direction of the secondary concentrator, which shape verifies the condition that it, on its turn, reflects the edge rays it receives from the primary concentrator in directions tangent to the receiver, producing there, theoretically, a maximum concentration, and in which the secondary is truncated.
2. The optical system of claim 1, characterized by the receiver having a face of convex shape and also a flat face, and in which the secondary concentrator touches this flat face only on one point and in which this flat face can be replaced with a concave one, resulting in a secondary which does not touch the receiver.
3. The optical system of claim 1, characterized by some or all of the portions of the stepped flow-line optic which follow the flow lines being absent, leaving only those portions which cross the flow lines, that is, the heliostats, and in which the continuous portions that may exist (not stepped) of the primary concentrator are also divided into heliostats.
4. The optical system of claim 1, characterized by the heliostats being able to rotate around themselves to follow the apparent daily motion of the sun.
5. (canceled)
6. The optical system of claim 1, characterized by the heliostats being placed on a wave shaped surface, with cylindrical geometry, corresponding, basically, to a curve that optically conserves etendue.
7. The optical system of claim 6, characterized by the heliostats of the primary concentrator being flat or curved.
8. The optical system of claim 6, characterized by the overall shape of the primary concentrator being flat.
9. The optical system of claim 8, characterized by all the receivers being substantially at the same height and the angles measured from the vertical through one of the receivers being about 33±10° with the line which, starting at the receiver, crosses the point of the primary which marks the transition between one and two receivers (angle αT), of 61±5° with the line that crosses the midpoint of the primary optic (angle αM) and of 71±5° with the line that crosses the other point of transition from one to two receivers (angle αR).
10. The optical system of claim 9, characterized by the angles measured from the vertical which crosses one of the receivers being: 32.6° with the line that, starting at the receiver, crosses the point of the primary that marks the transition between one and two receivers, of 61.2° with the line that crosses the midpoint of the primary optic and of 71.2° with the line that crosses the other point of transition from one to two receivers.
11. The optical system of claim 6, characterized by not having secondary concentrators next to the receivers.
12. The optical system of claim 1, characterized by the receivers having a larger size than that they would have if dimensioned for maximum or ideal concentration.
13. The optical system of claim 1, characterized by comprising transparent covers substantially perpendicular to the flow lines and mirrors (mirrored on both sides) which substantially follow the flow lines for convection reduction next to the receiver.
14. The optical system of claim 13, characterized by comprising mirrors and/or transparent covers, shaped as broken lines which substantially follow the flow lines or are substantially perpendicular to them, respectively, and in which the covers may be simple or double.
15. (canceled)
16. An optical system characterized by combining by means of intersecting at least two optical systems chosen from:
- an optical system with primary concentrator of the Fresnel type and adjacent secondary concentrator, the secondary receiver set being above the primary, and characterized by the primary concentrator containing a stepped flow-line optic which shape verifies the condition that it reflects a set of edge rays so as to make them tangent to the receiver and the other set of edge rays in the direction of the secondary concentrator, which shape verifies the condition that it, on its turn, reflects the edge rays it receives from the primary concentrator in directions tangent to the receiver, producing there, theoretically, a maximum concentration, and in which the secondary is truncated, and further characterized by some or all of the portions of the stepped flow-line optic which follow the flow lines being absent, leaving only those portions which cross the flow lines, that is, the heliostats, and in which the continuous portions that may exist (not stepped) of the primary concentrator are also divided into heliostats; or
- an optical system with primary concentrator of the Fresnel type and adjacent secondary concentrator, the secondary receiver set being above the primary, and characterized by the primary concentrator containing a stepped flow-line optic which shape verifies the condition that it reflects a set of edge rays so as to make them tangent to the receiver and the other set of edge rays in the direction of the secondary concentrator, which shape verifies the condition that it, on its turn, reflects the edge rays it receives from the primary concentrator in directions tangent to the receiver, producing there, theoretically, a maximum concentration, and in which the secondary is truncated, and further characterized by the heliostats being able to rotate around themselves to follow the apparent daily motion of the sun;
- thus forming a concentrator with multiple receivers.
17. The optical system of claim 16, characterized by the heliostats being placed on a wave shaped surface, with cylindrical geometry, corresponding, basically, to a curve that optically conserves etendue.
18. The optical system of claim 16, characterized by the heliostats of the primary concentrator being flat or curved.
19. The optical system of claim 6, characterized by the overall shape of the primary concentrator being flat.
20. The optical system of claim 16, characterized by the receivers having a larger size than that they would have if dimensioned for maximum or ideal concentration.
21. The optical system of claim 16, characterized by comprising transparent covers substantially perpendicular to the flow lines and mirrors (mirrored on both sides) which substantially follow the flow lines for convection reduction next to the receiver.
22. A method of concentrating solar energy, said method comprising: concentrating solar energy using an optical system with primary concentrator of the Fresnel type and adjacent secondary concentrator, the secondary receiver set being above the primary, and characterized by the primary concentrator containing a stepped flow-line optic which shape verifies the condition that it reflects a set of edge rays so as to make them tangent to the receiver and the other set of edge rays in the direction of the secondary concentrator, which shape verifies the condition that it, on its turn, reflects the edge rays it receives from the primary concentrator in directions tangent to the receiver, producing there, theoretically, a maximum concentration, and in which the secondary is truncated.
Type: Application
Filed: May 19, 2009
Publication Date: Mar 24, 2011
Inventors: Júlio César Pinto Chaves (Madrid), Manuel Pedro Ivens Collares Pereira (Lisboa)
Application Number: 12/993,478
International Classification: F24J 2/38 (20060101); F24J 2/08 (20060101); F24J 2/00 (20060101);