The present invention relates to fin type compound parabolic concentrators (FT-CPCs) for use as static concentrators in cost effective solar photovoltaic (PV) systems. More specifically, the invention includes geometries that limit the angle of reflected rays striking the PV cell. This is called a θi θo FT-CPC or a θo FT-CPC where θi is the angle of rotation of the semi-parabolic reflectors of the concentrator and θo is the critical angle of the PV cell.
A system based on FT-CPCs is disclosed in the L'Esperance U.S. Pat. No. 4,024,852 and a number of reflector shapes for such concentrators are disclosed in the Winston U.S. Pat. No. 4,002,499. These reflector shapes including a FT-CPC have since come to be known as ideal concentrators.
There are two main types of compound parabolic concentrators using flat absorbers. The first is a fin type (FT-CPC) as shown in
The FT-CPC shown in
The PT-CPC shown in
A major advantage of the FT-CPC over the PT-CPC is that in the former, the exit aperture is on both sides of the fin. As a result,
Ideal concentrators are a subset of non-imaging concentrators. Non-imaging concentrators are better suited than imaging concentrators for solar collectors because they can be designed to have a flat response over a range of angles. Ideal concentrators have a response that is either 100% or 0%. They are “ideal” in that they achieve a theoretical limit, specifically, the highest concentration ratio for a given field-of-view. This theoretical foundation is important because if expensive solar cells are to be replaced with relatively inexpensive reflectors, then the fin-type compound parabolic concentrator with its bifacial fin is the ultimate static concentrator.
All conventional reflector-absorber configurations fall short of this theoretical limit, sometimes significantly. This limit can also be derived from the second law of thermodynamics. For a two dimensional trough, the theoretical limit is:
CR1mx=1/sin θi (1)
where 2 θi is the field-of-view.
An intuitive method of constructing a fin-type compound parabolic concentrator is presented in
The fin-type compound parabolic concentrator can be constructed by independently rotating the right side and the left side of the parabola about the focal point. In
As described in the Winston U.S. Pat. No. 4,002,499, the gap between the semi-parabolic apexes can be completed with an involute of the absorber. The involute of a flat or fin absorber is a circular arc centered on the focal point. Completing the gap with a circular arc results in exit angles, the angle at which light rays are incident on the absorber, to be unconstrained. Reflected light strikes the absorber at all angles between ±90°. This is characteristic of the classic ideal fin-type compound parabolic concentrator.
From
The ray trace illustrated in
With the classic ideal concentrators shown in
A θo concentrator limits the angle of light striking the absorber to ±θo about the perpendicular to the absorber surface. Limiting the angle is important because light striking real absorbers at high incidence angles or grazing angles cannot be absorbed.
Silicon solar cells have an index of refraction in the neighborhood of n2=3.5. As a result, some of the light striking the solar cell after passing through air n1=1.0, or glass n1=1.5 will be reflected back off the photovoltaic surface. This reflection is particularly acute for light rays striking the solar cell at high angles of incidence. There is a critical angle θo beyond which most of the light will be reflected off the surface.
The incidence angle reflection is illustrated in
Referring to
The concept of limiting both the input and output angles as a θi θo ideal concentrator is apparent from the prior art. That is, both the input aperture ±θi and the output aperture ±θo are defined and limited. The θi θo concept was first disclosed in the Rabl U.S. Pat. No. 4,130,107. A notable generalization from this prior art is that the theoretical maximum concentration ratio is now:
CR2mx=sin θo/sin θi (2)
If all exit angles were allowable, θo=90° and equation 2 becomes equation 1.
Prior art relating to θi θo focused on innovations for limiting exit angles for a plate type compound parabolic concentrator and a tube type ideal concentrator. The specific innovations are quite different for different types of ideal concentrators. Such prior art does not refer to or provide guidance for a fin-type compound parabolic concentrator. In addition to not providing guidance about how to construct a θi θo fin-type compound parabolic concentrator, the prior art does not provide any assurance that an ideal θo fin-type compound parabolic concentrator exists.
Accordingly, it is an object of the present invention to create the geometry that limits the angle of light striking a bi-facial photovoltaic cell when a flat bi-facial photovoltaic cell is employed as the fin in a fin-type compound parabolic concentrator.
According to a primary object of the invention, fin-type compound parabolic concentrator has geometries that limit the angle of reflected rays incident on the fin. The angle is limited to less than or equal to a critical angle θo. To achieve this it is necessary to truncate the reflector and alter the geometry in the vicinity of the apex. These geometries are consistent with theoretical limits and are a close approximation to an ideal θi θo fin-type compound parabolic concentrator. The invention includes three main components. First, the top of the reflector is truncated. It is common practice to arbitrarily truncate fin-type compound parabolic concentrators for mechanical convenience in order to avoid designs that are awkward to build because they are very deep. A novel aspect of this invention is to provide limits, so that truncating to the limit results in little optical loss because rays reflected off the outer limb of the reflector strike the absorber at angles>θo. Truncating beyond the limit involves optical loss because reflected rays strike the absorber at angles<θo.
Second, the apex geometries are modified to limit the angle of reflected rays. A fin-type compound parabolic concentrator apex geometry is determined by the reflection behavior of limiting rays. Limiting rays are incoming rays at different angles θ<θi that pass just above the focal point. A limiting apex geometry limits the reflected limiting ray to strike the absorber at an angle less than or equal to some critical angle θo. An optimum geometry, the θo apex geometry, is a specific equation that maximizes concentration ratio consistent with the constraint of limiting rays striking the absorber to angles≦θo. Another geometry, referred to as βt apex geometry uses a simple straight line tangent. This is a crude but useful approximation that limits reflected rays to less than θo but reduces the concentration ratio to less than the optimum. The invention also relates to a range of ad hoc apex geometries between the optimum and the crude approximation.
Third, connection geometries are provided between the limiting apex geometry and the semi-parabolic reflectors. The combination of the limiting apex geometry and the connection geometry are incorporated into a bottom reflector. The invention includes three different connection structures depending on the relationship between the design parameters θi and θo. For example, if
θi=90°−θo
the limiting apex geometries are tangent to and connect with the semi-parabolic wall at the semi-parabolic apex. If
θi<90°−θo
the connection geometry is a straight line that is tangent to both the semi parabolic wall as some F distance from the apex and to the limiting apex geometries. If
θi>90°−θo
the connection geometry is a circular arc that is tangent to both the limiting apex geometries and the semi parabolic apex.
It is not possible to build an “ideal” θi θo fin-type compound parabolic concentrator. A small number rays will directly strike the absorber without reflection at angles greater than θo. For this reason a θi θo fin-type compound parabolic concentrator is not theoretically an “ideal” concentrator, but it is a close approximation.
The invention relates to a fin-type compound parabolic concentrator which is designed to limit the light striking the photovoltaic cell absorber to an angle less than or equal to a critical angle θo. There are three components to the invention: truncating the semi-parabolic reflectors of the concentrator; configuring the limiting apex geometry of the bottom reflector of the concentrator; and connecting the apex geometry with the semi-parabolic reflectors.
Referring to
The bottom reflector has a configuration adjacent to the apex which limits reflected light to angles≦θo. As will be developed in greater detail below in connection with
In
In
The outer portion 138 of the semi-parabolic reflector 108 is not very useful because light reflected off this portion strikes the absorber at high incidence angles. This light is not absorbed by the absorber but rather is reflected back into space. Thus, the outer portion 138 can be removed with little reduction in optical efficiency of the concentrator. The limiting point is at 152, the point where the extreme ray 144 is reflected to strike the absorber at the critical angle θo. The distance A is the height of the truncated reflector above the focal point 104. From basic geometry, the distance A can be shown to be:
A=2 sin θo/[1−sin(θo−θi)] (3)
where
The most efficient fin-type compound parabolic concentrator would have a height above the focal point given by equation (3). Practical fin-type compound parabolic concentrators have tolerances, rounded corners, and dead space to avoid exposed reflector metal edges. With tolerances, the working portion of the reflector should be as high or higher than equation (3) but not shorter. Shorter geometries introduce significant optical losses. Taller geometries add cost but collect little or no additional light.
Truncation has value for fin-type compound parabolic concentrator configurations with high concentrations and wide fields of view in accordance with the following relation between parabolic rotational angles and absorber critical angles:
Truncating the top of the reflectors results in little optical loss because rays reflected off the outer portions of the reflectors would strike the absorber at angles>θo and hence the ray would be reflected off the photovoltaic cell and not absorbed.
Turning now to the configuration of the bottom reflector, the geometry thereof preferably limits the angle of reflected rays near the apex. With a conventional fin-type compound parabolic concentrator as shown in
Three limiting apex geometries of the bottom reflector for limiting the angle that rays strike the absorber to less than a critical angle θo are disclosed. The preferred embodiment is referred to as the θo apex geometry. A second embodiment is referred to as βt apex geometry, and a third embodiment includes a range of geometries.
As noted earlier, the conventional fin-type compound parabolic concentrator has no constraints on the angle of incidence of light striking the fin absorber. That is, the rays striking the fin have incidence angles ranging from ±90° and equation (1) applies. The difficulty with unconstrained incidence angles is illustrated in
For circular arcs and practical absorbers, limiting rays are not absorbed whenever (90°−θ)>θo. This occurs next to the absorber where θ˜0. The challenge is to find a new shape 254 for the bottom reflector 256 with a local slope β such that limiting rays are reflected to strike the absorber at the critical angle θo as illustrated by reflected ray 246.
β(θ)=45°+(θ−θo)/2 (4)
At the intersection of the θo apex geometry of the bottom reflector 256 with the absorber 202 (
βf=45°−θo/2 (5)
The geometry extends away from the absorber out to a tangent point where the reflected ray 246 is returned to the focal point. There are two solutions for βt, the slope of the θo apex geometry of the bottom reflector at the tangent, depending on the relationship between θi and θo:
βt=45°+(θi−θo)/2 when θi<90°−θo (6)
βt=90°−θo when θi≧90°−θo (7)
Integrating the slope as in equation (4) results in the following equation for the θo apex geometry of the bottom reflector:
r(φ)/ro=cos−2 α (8)
where
The circular arc of the bottom reflector 356 intercepts the absorber 302 perpendicular to the absorber at point 318 which is along the centerline of the bottom reflector. The θo apex geometry intercepts the absorber at angle βf according to equation (5) at point 370. The βt apex geometry intercepts the absorber at angle βt according to equation (7) at point 366. The βt apex geometry is a simple straight line with slope βt intercepting the semi-parabolic reflector 308 at a tangent point 360 coincident (for the condition θi=90°−θo) with the semi-parabolic apex 316 of the reflector. The axis 324 of the reflector passes through the apex.
The ad hoc geometries are shown by the hatched area of
As the angle of arrival θ of the limiting ray increases to approach the design maximum angle θi, the angle at which all reflected rays 342, 346, 358 strike the absorber approaches θo.
The profile of the different geometries of the bottom reflector is determined by limiting rays that pass just above the focal point. As shown in
Limiting the angle at which reflected rays strike the absorber decreases in the concentration ratio. In
In another embodiment of the invention, the bottom reflector is configured in such a way as to include an additional portion to connect the limiting apex geometry with the semi-parabolic reflector walls. The bottom reflector includes both the limiting apex geometry and the connection geometry. How this is accomplished depends on the relationship between the design parameters θi and θo.
Referring to
For the unique case where θi=90°−θo, the apex geometry of the bottom reflector is connected with the semi-parabolic reflectors at the semi-parabolic apexes, one of which is shown at 316 for the semi-parabolic reflector 308 in
Referring now to
The profile of the connector reflector portion 474 between the axis 424 of the semi-parabolic wall 408 at the tangent point 460 and the tangent 476 to the semi-parabolic wall 408 is determined by extreme angle rays 478. Arriving at the extreme angle θi, these rays are all reflected (see rays 446) to the angle θo by a straight line at the tangent angle
βt=45°+(θi−θo)/2 equation (6)
With a conventional fin-type compound parabolic concentrator, the semi-parabolic wall would have its apex at 416 and transition into a circular arc to reflect limiting rays back to the focal point. With the θo concentrator of the invention, the semi-parabolic wall extends only to the tangent point 476 where it transitions into the straight connector reflector portion 474.
It should be noted that for the particular values of θi and θo used in
A θo fin-type compound parabolic concentrator for the condition θi≧90°−θo is shown in
It should be noted that for the particular values of θi and θo shown in
While the preferred forms and embodiments of the invention have been illustrated and described, it will be apparent to those of ordinary skill in the art that various changes and modifications may be made without deviating from the inventive concepts set forth above.
This application claims the benefit of U.S. provisional application No. 61/024,277 filed Jan. 29, 2008.
Number | Date | Country | |
---|---|---|---|
61024277 | Jan 2008 | US |