U.S. patent application number 13/081829 was filed with the patent office on 2012-10-11 for sun tracking solar concentrator.
Invention is credited to Richard A. Hutchin.
Application Number | 20120255540 13/081829 |
Document ID | / |
Family ID | 46965125 |
Filed Date | 2012-10-11 |
United States Patent
Application |
20120255540 |
Kind Code |
A1 |
Hutchin; Richard A. |
October 11, 2012 |
SUN TRACKING SOLAR CONCENTRATOR
Abstract
A sun tracking solar concentrator includes a one or two-sided
linear Fresnel lens imprinted on a rollable sheet that is curved to
form as a cylindrical arc surface. A one sided lens has a first
zero line or a center point that transmits sunlight through without
any refraction. A two sided lens also has a second zero line that
is perpendicular to the first zero line. The Fresnel lens may be
spooled onto rollers at its two straight ends. The first zero line
or the center point may be positioned along the cylindrical arc by
rotating one or both of the rollers. This mechanism aimed at
providing horizontal tracking of the sun as it moves from East to
West. Vertical tracking is accomplished by a tiltable mount coupled
to the two rollers.
Inventors: |
Hutchin; Richard A.;
(Calabasas, CA) |
Family ID: |
46965125 |
Appl. No.: |
13/081829 |
Filed: |
April 7, 2011 |
Current U.S.
Class: |
126/600 |
Current CPC
Class: |
G02B 5/04 20130101; H01L
31/0543 20141201; F24S 23/31 20180501; Y02E 10/47 20130101; Y02E
10/52 20130101; F24S 50/20 20180501; Y02B 10/20 20130101; F24S
30/452 20180501; G02B 3/08 20130101 |
Class at
Publication: |
126/600 |
International
Class: |
F24J 2/46 20060101
F24J002/46 |
Claims
1. A sun tracking solar concentrator comprising: a Fresnel lens
comprising a linear arrangement of prisms with a common
non-refracting first zero line, the Fresnel lens being curved to
form a substantially cylindrical arc surface such that the
non-refracting first zero line is parallel to a cylinder axis; and
a mechanism for positioning the non-refracting first zero line
along the cylindrical arc surface and parallel to the axis of the
cylinder.
2. The sun tracking solar concentrator as in claim 1 wherein the
mechanism positions the non-refracting first zero line so that a
plane tangent to the cylindrical arc and including the first zero
line is substantially perpendicular to incident solar rays.
3. A sun tracking solar concentrator comprising: a Fresnel lens
comprising a radial arrangement of prisms with a common
non-refracting center point, the Fresnel lens being curved to form
a substantially cylindrical arc surface; and a mechanism for
positioning the non-refracting center point on the cylindrical arc
surface.
4. The sun tracking solar concentrator as in claim 3 wherein the
mechanism positions the non-refracting center point so that a plane
tangent to the cylindrical arc and including the non-refracting
center point line is substantially perpendicular to incident solar
rays.
5. A sun tracking solar concentrator comprising: a single sided
Fresnel lens comprising a linear arrangement of a chain of prisms
with a common non-refracting first zero line, the single sided
Fresnel lens being curved to form a substantially cylindrical arc
surface such that the non-refracting first zero line is parallel to
the cylinder axis; and a mechanism for orienting the non-refracting
first zero line so that a plane tangent to the cylindrical arc and
including the first zero line is substantially perpendicular to
incident solar rays.
6. A sun tracking solar concentrator comprising: a two sided linear
Fresnel lens, comprising a chain of prisms with a common
non-refracting first zero line on a first side for concentrating
rays of the sun along a first axis and a chain of prisms on a
second side for concentrating rays of the sun along a second axis
perpendicular to the first axis, the two sided linear Fresnel lens
being curved to form a substantially cylindrical arc surface such
that the non-refracting first zero line of the first side is
parallel to the axis of the cylinder; and a mechanism for orienting
the non-refracting first zero line of the first side so that a
plane tangent to the cylindrical arc and including the first zero
line is substantially perpendicular to incident solar rays.
7. The sun tracking solar concentrator as in claim 6 wherein the
first side of Fresnel lens faces inward and the second side faces
outward.
8. The sun tracking solar concentrator as in claim 6 wherein the
Fresnel lens is imprinted on a rollable sheet.
9. The sun tracking solar concentrator as in claim 8 wherein the
Fresnel lens is spooled onto rollers.
10. The sun tracking solar concentrator as in claim 9 wherein the
rollers are rotated to position the non-refracting first zero line
of the first side along the cylindrical arc.
11. The sun tracking solar concentrator as in claim 9 wherein the
rollers are tilted to orient the non-refracting first zero line of
the first side with respect to the ground.
12. The sun tracking solar concentrator as in claim 9 wherein the
rollers are coupled to each other and pivot around a common axis to
orient the non-refracting first zero line of the first side with
respect to the ground.
13. A sun tracking solar concentrator comprising: a linear Fresnel
lens imprinted onto a rollable sheet and comprising a chain of
prisms with a common non-refracting first zero line on one side of
the sheet for concentrating rays of the sun along a single
dimension, the linear Fresnel lens being curved to form a
substantially cylindrical arc surface such that the non-refracting
first zero line of the linear Fresnel lens is parallel to the
cylinder axis, and a pair of rollers onto which the Fresnel lens is
spooled, the rollers being configured to rotate to position the
non-refracting first zero line.
14. The sun tracking solar concentrator as in claim 13 further
comprising a tiltable mount coupled to the pair of rollers.
15. A sun tracking solar concentrator comprising: a two sided
linear Fresnel lens imprinted onto a rollable sheet and comprising
a first chain of prisms with a common non-refracting first zero
line on a first side of the sheet for concentrating the rays of the
sun along a first axis and a second chain of prisms on a second
side of the sheet for concentrating rays of the sun along a second
axis perpendicular to the first axis, the two sided linear Fresnel
lens being curved to form a substantially cylindrical arc surface
such that the non-refracting first zero line of the first chain of
prisms is parallel to the cylinder axis; and a pair of rollers onto
which the two sided Fresnel lens is spooled, the rollers being
configured to rotate to position the non-refracting first zero line
of the first side.
16. The sun tracking solar concentrator as in claim 15 further
comprising a tiltable mount coupled to the pair of rollers.
17. The sun tracking solar concentrator as in claim 15 further
comprising at least a pair of ribs configured to curve the two
sided Fresnel lens substantially into the cylindrical arc
surface.
18. The sun tracking solar concentrator as in claim 17 further
comprising sprockets traversing along the ribs and coupled into
perforations formed along a curved side of the lens, the sprockets
maintaining the Fresnel lens in the shape of the cylindrical arc
surface under tension.
19. The sun tracking solar concentrator as in claim 15 wherein the
two sided linear Fresnel lens is cut into at least two strips that
run perpendicular to the non-refracting first zero line of the
first side and each strip is spooled onto a pair of rollers.
20. The sun tracking solar concentrator as in claim 19 further
comprising at least three ribs configured to curve the strips
substantially into the cylindrical arc surface.
21. The sun tracking solar concentrator as in claim 20 further
comprising sprockets traversing along the ribs and coupled into
perforations formed along curved sides of the strips, the sprockets
maintaining the strips in the shape of the cylindrical arc surface
under tension.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The field of the present invention relates to solar
concentrators, in particular to those that use flexible Fresnel
lenses and track the movement of the sun.
[0003] 2. Background
[0004] Most of the US landmass has solar potential varying between
3-8 kWh/m2 per day. If one could only convert 30% of the solar
energy incident upon a typical 6 m.times.6 m two-car garage area in
a state like Colorado, one could power the whole house, based on
the average US daily consumption rate of 32 kWh and send an
additional 16 kWh back into the grid for others every day. This
potential remains to be realized. To date, much progress has been
made, but the amount of electricity generated from solar
technologies remains very low.
[0005] The two main technologies for harvesting solar power are (1)
photovoltaics and (2) solar thermal. Photovoltaic devices convert
solar energy directly to electricity. Solar thermal devices
concentrate and convert the solar energy to heat which is then
converted to electricity.
[0006] Photovoltaic devices can be in the form of flat panels that
are exposed to sunlight or concentrated photovoltaics (CPV) systems
that employ sunlight concentrated onto photovoltaic surfaces.
Concentrating solar energy leads to increased efficiency in
photovoltaics (from 15% to 38.5%) and reduces costs since much less
photovoltaic device area is required. Concentration ratios can
range from 2 to 800 times, i.e. 2-800 suns. Likewise, in solar
thermal systems, the sun's rays must be highly concentrated
(100-1000 suns) for efficient electric power generation. Both
systems use optical techniques to focus incident sunlight into a
small beam. Higher concentration generally means more efficient
power generation. Moreover, in a high concentration design,
tracking is critical to keep the sunlight focused onto the small
solar cell or a hot spot.
[0007] The three main categories of existing technologies for solar
concentrators are parabolic troughs, dish reflectors, and Fresnel
lenses and reflectors. Parabolic troughs concentrate incoming light
along one dimension leading to a line of concentrated light.
Parabolic dishes, on the other hand, concentrate along two
dimensions. Fresnel lenses and reflectors can be linear resulting
in one dimensional concentration or radial leading to two
dimensional concentration. Concentration along two dimensions is
required for use with CPV solar cells to make them cost effective.
This makes radial Fresnel lenses and dish reflectors the main
candidates. Dishes are made of metals which make them expensive and
heavy and thus unsuitable for distributed applications. Radial
lenses are inexpensive and are the concentrator of choice for
home-owner solar technology. There are radial Fresnel lenses on
weather-tough acrylic currently available on the market. Even
though these lenses are light and inexpensive, external moving
parts are needed to orient them to track the sun which raises the
cost of the end product significantly. The trackers are also large
and bulky and thus not suitable for many applications, especially
distributed applications, such as those set up in remote locations
or camps, mounted on rooftops, or installed in backyards.
[0008] Both photovoltaic systems and solar thermal systems would
benefit greatly from sun tracking solar concentrators built with
common inexpensive and lightweight materials. The present invention
is aimed at addressing this need.
SUMMARY OF THE INVENTION
[0009] The present invention is directed toward a sun tracking
solar concentrator which includes a single or double layer linear
Fresnel lens that can be used with a solar thermal or photovoltaic
energy conversion system. In case of a single layer linear Fresnel
lens, the rays of the sun are concentrated along one dimension onto
a narrow line straddling the focal line of the lens. In case of the
double layer Fresnel lens, the solar rays are concentrated along
two dimensions onto a spot centered at the focal point of the
lens.
[0010] The surface of the Fresnel lens is curved to form a
cylindrical arc surface. During use, the axis of the cylinder is
preferably positioned such that it lies substantially in the plane
perpendicular to the East-West axis. The East-West axis may be
defined as the line that connects the two points on the horizon,
the first point being where the sun rises and the second being the
point where the sun sets on the Spring and Autumn equinoxes.
[0011] The linear Fresnel lens surface has a first zero line
substantially parallel to the cylinder axis where the solar rays
incident upon it pass through with little or no refraction
perpendicular to the cylinder axis. The solar rays incident at
other locations are refracted perpendicular to the cylinder axis by
the chain of prisms of the Fresnel lens towards the focal area.
[0012] The linear Fresnel lens surface may also have a second zero
line substantially perpendicular to the cylinder axis where the
solar rays incident upon it pass through with little or no
refraction parallel to the cylinder axis. The solar rays incident
at other locations are refracted parallel to the cylinder axis by
the chain of prisms of the Fresnel lens towards the focal area.
[0013] Based on the described geometry, tracking the sun may be
accomplished by accommodating two angles: First is the azimuth
angle as the sun travels from East to West during the day. The
second is the elevation angle as the sun rises, traverses its daily
path overhead, and sets. The azimuth and elevation tracking
mechanisms maintain the solar concentrator in its preferred
orientation, which is achieved when the Fresnel lens is positioned
such that a plane tangent to its surface and passing though the
first zero line is substantially perpendicular to the incident
solar rays.
[0014] The sun tracking solar concentrator may exhibit local
invariance of the angle of incidence as long as the preferred
orientation of the solar tracker is maintained. This local
invariance renders the angle of incidence of solar rays at any
given point on the solar concentrator lens substantially constant
despite the movement of the sun across the sky as long as the
preferred orientation is maintained. The substantially constant
angle of incidence allows for the optimization of the cylindrical
Fresnel lens design prism by prism resulting in enhanced optical
efficiency.
[0015] Accordingly, an improved sun tracking Fresnel lens solar
concentrator is disclosed. Advantages of the improvements will
appear from the drawings and the description of the
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] In the drawings, wherein like reference numerals refer to
similar components:
[0017] FIG. 1A schematically illustrates the Fresnel lens concept
as known in the prior art;
[0018] FIG. 1B schematically illustrates a linear Fresnel lens as
known in the prior art;
[0019] FIG. 1C schematically illustrates a radial Fresnel lens as
known in the prior art;
[0020] FIG. 1D schematically illustrates a section of the Fresnel
lens prism chain with nomenclature conventions as known in the
prior art;
[0021] FIG. 1F schematically illustrates a linear Fresnel lens with
support structure to hold or suspend the lens as a cylindrical arc
surface as known in the prior art;
[0022] FIG. 1G schematically illustrates the underside of the
linear Fresnel lens shown in FIG. 1F;
[0023] FIG. 1H schematically illustrates the rays refracted as they
pass through the linear Fresnel lens shown in FIG. 1F;
[0024] FIG. 2 schematically illustrates the movement of the sun
across the sky on different days of the year as known in the prior
art;
[0025] FIG. 3 schematically illustrates a cylindrically shaped
linear Fresnel lens that concentrates sunlight along one dimension
and utilizes an external pan and tilt mechanism to track the sun as
known in the prior art;
[0026] FIGS. 4A and 4B schematically illustrate a first sun
tracking solar concentrator that concentrates sunlight along one
dimension;
[0027] FIGS. 5A and 5B schematically illustrate a second sun
tracking solar concentrator that concentrates sunlight along two
dimensions;
[0028] FIG. 6 schematically illustrates a first modified sun
tracking solar concentrator;
[0029] FIG. 7 schematically illustrates a second modified sun
tracking solar concentrator;
[0030] FIG. 8A schematically illustrates a third embodiment of a
sun tracking solar concentrator;
[0031] FIG. 8B schematically illustrates the folded view of the
solar concentrator shown in FIG. 8A;
[0032] FIGS. 9A and 9B schematically illustrate a rolling mechanism
for a sun tracking solar concentrator;
[0033] FIGS. 10A, 10B, and 10C schematically illustrate the local
invariance of the angle of incidence for a sun tracking solar
concentrator; and
[0034] FIGS. 11A and 11B schematically illustrates a bundle of
solar rays that pass through a single prism of a Fresnel lens.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] Turning in detail to the drawings, FIG. 1A illustrates in a
series of steps (a)-(d) how a Fresnel lens 10 can be constructed by
collapsing a continuous surface plano convex lens 20 into an
equivalent power Fresnel lens 10. This concept is well known to
those skilled in the art. As shown, the Fresnel lens consists of a
chain of prisms. Commonly, the prisms can be arranged linearly in
rows or radially in concentric circles. A linear arrangement shown
in FIG. 1B is commonly called a linear Fresnel lens 12 and provides
one dimensional concentration of incident sunlight 90 onto an often
narrow rectangular area 14 around the focal line of the lens. A
radial arrangement shown in FIG. 1C is commonly called a radial
Fresnel lens 16 and provides two dimensional concentration of
incident sunlight 90 onto a small rectangular area 18 around the
focal point of the lens.
[0036] FIG. 1D shows a section of the Fresnel lens prism chain with
nomenclature conventions. The prism has two facets, called the
slope facet 30 and the draft facet 32. The distance between the
peaks of the prisms is facet spacing 34. The angle between the line
parallel 38 to the plano surface 40 of the Fresnel lens and the
slope facet 30 is the slope angle 36. The angle between the line 42
perpendicular to the plano surface 40 of the Fresnel lens and the
draft facet 32 is the draft angle 44.
[0037] FIG. 1F illustrates a linear Fresnel lens 100 with support
structure 106 to maintain a cylindrical arc shape. The support
structure 106 may consist of ribs, wires or struts that maintain
the cylindrical shape. The cylindrical outer surface 110 of the
lens 100 pointed towards the sun is smooth. The chain of prisms of
the Fresnel lens 100 are situated on the inside surface 120.
Alternately, the chain of prisms of the Fresnel lens 100 may be
situated on the outside surface 110 with the inside surface 120
being smooth. The material of the lens can be a flexible
transparent polymer. If the material is thin and has a tendency to
bow or sag, it can be held under slight tension. FIG. 1G
illustrates a small cutout of the Fresnel lens with the outer
surface 110' from another perspective, showing the linear
arrangement of prisms 130 and the internal surface cutout area
120'. The lens concentrates incident sunlight 90 in one direction
around a focal line 140. As shown in FIG. 1H, the rays of the sun
incident on the first zero line 150 go through the cylindrical
Fresnel lens with little or no refraction. The remaining rays
incident on the outer surface 110 are refracted towards the focal
line 140. Typically, the ratio of the area of the linear Fresnel
lens 100 surface to the area of the focal line 140 area is between
10 and 100. There are numerous examples of this type of solar
concentrator in the art. Many of these solar concentrators are made
of polymethyl methacrylate (PMMA) which is a transparent
thermoplastic sometimes called acrylic glass.
[0038] FIG. 2 illustrates the movement of the sun across the
celestial sphere 210 for four days during the course of a year for
a location in the Northern hemisphere near latitude 35. The three
solar paths shown are the solar path on Summer solstice 220, the
solar path on Winter solstice 222, and the solar path on the Spring
and Autumn equinoxes 224. Two angles define the position of the
sun. These angles are the elevation angle 230 and the azimuth angle
240. Tracking the sun is important for solar concentrators. The
solar concentrators often have a preferred orientation with respect
to the sun. This preferred orientation is the one which
substantially maximizes the amount of solar energy that can be
concentrated and later converted to other forms of energy. Thus,
many solar concentrators are mounted onto solar trackers that cause
the solar concentrators to assume their preferred orientations with
respect to the incident solar rays 90.
[0039] FIG. 3 illustrates the cylindrical linear Fresnel lens 100
with support structure 106 (shown earlier in FIG. 1F) mounted onto
a solar tracking pan tilt mechanism 310. The pan tilt mechanism 310
is used to maintain the preferred orientation of the Fresnel lens
100 with respect to the incident solar rays 90. The preferred
orientation is when the cylindrical Fresnel lens 100 is positioned
such that a plane 320 tangent to the surface 110 of the cylindrical
Fresnel lens and passing though the first zero line 150 of the
cylindrical Fresnel lens 100 is substantially perpendicular to the
rays 90 of the sun. This serves two purposes: First, the solar rays
90 are focused substantially at the same focal line 140 regardless
of what the azimuth angle 240 of the sun or the elevation angle 230
of the sun is. Second, the solar energy concentrated along the
focal line 140 is substantially maximized. The pan adjustment 340
is substantially coupled to the sun's azimuth angle 240; whereas,
the tilt adjustment 372 is substantially coupled to the sun's
elevation angle 230. There are numerous examples of this type of
solar tracking in the art.
[0040] FIG. 4A illustrates a sun tracking solar concentrator 400
which includes a thin cylindrical linear Fresnel lens 410 spooled
onto rollers 430. The rollers 430 rotate to position the first zero
line 450 of the lens along the cylindrical surface of the lens. The
support structure 416 which maintains the cylindrical surface of
the lens. The support structure 416 may also couple the rollers 430
so that the rollers 430 can pivot around a common axis. This
pivoting allows for a tilt adjustment of the midline 450.
Alternately, the support structure 416 may be mounted onto a tilt
mechanism 470. FIG. 4B shows the rollers 430 and the Fresnel lens
410 in greater detail.
[0041] As with the Fresnel lens 100 of FIG. 3, the preferred
orientation is when the cylindrical Fresnel lens 410 is positioned
such that a plane 420 tangent to the surface of the cylindrical
Fresnel lens 410 and passing though the first zero line 450 of the
cylindrical Fresnel lens 410 is substantially perpendicular to the
incident solar rays 90.
[0042] The rollers 430 rotate to position the first zero line 450
so that the cylindrical Fresnel lens assumes the preferred
orientation. The tilt adjustment 472 may also be necessary to
assume the preferred orientation. The tilt adjustment 472 is
substantially coupled to the sun's elevation angle 230 whereas the
positioning of the first zero line 450 by way of turning the
rollers 430 is substantially coupled to the sun's azimuth angle
240.
[0043] For the thin Fresnel lens to conform to a spool 432, its
material may need to be sufficiently rollable and thin. One
suitable material is a plastic sheet made from the resin
polyethylene terephthalate (PET). Another generic term for this
material is polyester film or plastic sheet. Also, some people
refer to it as Mylar.RTM., which is a registered trademark of
Dupont Tejjin Films. A Fresnel lens may be imprinted onto a plastic
sheet using one of many well-known methods in the art, such as
hot-press embossing.
[0044] FIG. 5A illustrates another sun tracking solar concentrator
500 which includes a thin two sided cylindrical linear Fresnel lens
510 spooled onto rollers 530. FIG. 5B shows the cylindrical linear
Fresnel lens 510 flattened and in greater detail. The rollers 530
position the first zero line 550 of the lens along the cylindrical
surface of the lens. The chain of prisms on the inside of the
cylindrical surface bends the incident rays towards the first zero
line 550 whereas the chain of prisms on the outside of the
cylindrical surface focuses incident rays towards the second zero
line 552. The result is two dimensional concentration of incident
light onto concentration spot 540. Typically, the ratio of the area
of the surface of linear Fresnel lens 510 to the area of the
concentration spot 540 is between 100 and 1000.
[0045] The support structure 516 of this sun tracking solar
concentrator 500 maintains the cylindrical surface of the lens is
mounted onto a tilt mechanism 570. The tilt adjustment 572 is
substantially coupled to the sun's elevation angle 230. The
positioning of the first zero line 550 by way of turning the
rollers 530 is substantially coupled to the sun's azimuth angle
240. As with the Fresnel lens 100 of FIG. 3, the preferred
orientation is when the cylindrical Fresnel lens 510 is positioned
such that a plane tangent to the surface of the cylindrical Fresnel
lens 510 and passing though the first zero line 550 of the
cylindrical Fresnel lens 510 is substantially perpendicular to the
incident solar rays 90.
[0046] Sun tracking solar concentrators may also incorporate
Fresnel lens materials which do not permit spooling around rollers.
FIG. 6 and FIG. 7 illustrate two such modifications. In the sun
tracking solar concentrator 600 shown in FIG. 6, the Fresnel lens
is not spooled onto the rollers 630. Instead, the two rollers 630
are part of a mechanism used to position the first zero line 650 of
the Fresnel lens 610 along the cylindrical arc between lines 660
and 670. The bottom surface 680 of the support structure 116 can be
mounted onto a tilting platform to provide tilt adjustment. In the
sun tracking solar concentrator 700 shown in FIG. 7, the rollers
are eliminated altogether. The Fresnel lens 710 is shaped into a
cylinder. The cylinder is rotated around its axis 720, to position
the first zero line 750 of the Fresnel lens along the cylindrical
arc between lines 760 and 770. The bottom surface 780 of the
support structure 716 can be mounted onto a tilting platform to
provide tilt adjustment. The two sun tracking solar concentrators
600, 700 may provide one or two dimensional concentration. One
dimensional concentration can be achieved with one sided Fresnel
lens and two dimensional concentration can be achieved with a two
sided Fresnel lens as shown in the sun tracking solar concentrator
400 and the sun tracking solar concentrator 500, respectively. The
location of concentrated sunlight is not shown to keep the
illustrations uncluttered.
[0047] FIG. 8A illustrates another sun tracking solar concentrator
800. Four cylindrically mounted strips 810, 812, 814, 816 of a thin
two sided linear Fresnel lens are spooled onto rollers 830 and 832
which rotate to position the common first zero line 850 of the lens
strips 810, 812, 814, and 816 along the cylindrical surface of the
composite lens. The chain of prisms on the inward facing side of
the cylindrical surface bends the incident rays towards the common
first zero line 850 whereas the chain of prisms on the outward
facing side of the cylindrical surface focuses incident rays
towards the common second zero line 852. The result is two
dimensional concentration of incident solar rays 90 onto
concentration spot 840. The support structure 860 that maintains
the cylindrical surface of the lens is mounted onto two legs 866
and 868 that are further coupled to two hydraulic cylinders 876 and
878 which collectively serve as the tilt mechanism. A heat exchange
engine 842 is thermally coupled to the concentration spot 840. The
heat engine may be a Stirling engine producing electrical
output.
[0048] The support structure 860 is shown with five ribs that
maintain the cylindrical shape of the Fresnel lens strips 810, 812,
814, and 816. The support structure 860 may contain further
supporting beams or braces to reinforce its strength against
external forces, e.g., wind. The support structure 860 may also be
equipped by mechanisms that allow it to be stowed when wind speeds
exceed safe levels. The support structure 860 may also be built so
it can be folded for easy transport or storage as shown in FIG. 8B.
Having a folding support structure 860' is also convenient for
reducing assembly complexity, labor, and time. The dimensions of
the Fresnel lens strips 810, 812, 814, 816 of the sun tracking
solar concentrator 800 determine the solar collection area and
hence the energy output from the Stirling engine. It is expected
that an area of approximately 10 square meters can be used to
generate 1.6 kW of peak electrical power. This size can be achieved
with approximately 50 cm wide strips that roll across ribs with
arclengths of approximately 5.5 meters.
[0049] FIG. 9A illustrates one mechanism for rolling the Fresnel
lens strip 812 in FIG. 8A to position the common first zero line
850 at the desired location. As shown in FIG. 9B, the Fresnel film
strip 812 contains two rows of perforations 920 and 922 along its
length positioned at the top and bottom of the strip 812. The two
rows of perforations 920 and 922 are used for transporting and
steadying the strip 910. They are locked onto two rows of sprockets
930 and 932 positioned on two chains 940 and 942. As the two chains
940 and 942 roll across the two gears 950 and 952, the film 812 is
spooled from one roller 830 to the other roller 832 or vice versa.
The rows of sprockets 930 and 932 hold the film strip 812 in slight
tension to be suspended over the concentration spot 840. The rows
of sprockets 930 and 932 also prevent the film strip 812 from
rubbing against the ribs. A cover may be added to secure the rows
of sprockets 930 and 932 in the rows of perforations 920 and 922
and thus prevent the strip 812 from coming loose.
[0050] FIGS. 10A, 10B, and 10C collectively illustrate a common
property of the sun tracking solar concentrators 400, 500, 600,
700, 800 described above. This common property is referred to
herein as "local invariance of the angle of incidence." This
property can be summarized as follows: The angle of incidence 1060
of solar rays 90 at any single point 1011 on the Fresnel lens 1010
remains substantially constant despite the movement of the sun
across the sky provided that the preferred orientation of the
Fresnel lens 1010 is maintained. As mentioned earlier, the
preferred orientation is when the Fresnel lens 1010 is positioned
such that a plane tangent to its surface and passing though the
first zero line 1050 is substantially perpendicular to the rays 90
of the sun. The substantially constant angle of incidence 1060 at
any single point 1011 allows for the optimization of the Fresnel
lens 1010 design prism by prism as explained further below.
[0051] FIG. 11A illustrates a bundle of solar rays 1190 that are
incident on a region 1114 of the cylindrical Fresnel lens 1110. As
shown in further detail in FIG. 11B, the region 1114 is located
between surface normal lines 1116 and 1118 of the Fresnel lens
1110. The bundle of solar rays 1190 incident upon the region 1114
passes through substantially only a single prism 1112. The facet
spacing 1134 of the prism 1112 is small enough that the angles of
incidence 1160 of all the rays in the solar ray bundle 1190 are
substantially equal to another. The solar ray bundle 1190 is
refracted first as it enters the surface of the Fresnel lens 1110
and second as it exits the prism 1112. The desired angles of
refraction 1170 for all rays in the solar ray bundle 1190 are also
substantially equal to another. Knowing the angle of incidence 1160
as well as the desired angle of refraction 1170, both measured with
respect to the surface normal 1116 (or 1118) of the Fresnel lens
1110 makes it possible to optimize the design parameters of the
prism 1112. These design parameters are facet spacing 1134, slope
angle 1136, and draft angle 1144.
[0052] One design process that takes advantage of the property of
constant angle of incidence may be described as follows: [0053]
STEP 1. Select material of the Fresnel lens. This will determine
the refractive index. [0054] STEP 2. Select the thickness of the
Fresnel lens. [0055] STEP 3. Select the Fresnel lens focal length,
f number, cylindrical geometry, and dimensions. [0056] STEP 4.
Determine the maximum operational curvature of the cylindrical
Fresnel lens. This is the angle of the arc which is endowed with
Fresnel prisms. Its value is generally between 90 degrees and 180
degrees. [0057] STEP 5. Formulate the initial design for the
Fresnel lens. This design will be optimized. [0058] STEP 6. Divide
the aperture of the Fresnel lens into segments each of which
correspond to a single prism path for the incident solar rays.
[0059] STEP 7. Determine the prism inclination and the angle of
incidence of solar rays per each segment. [0060] STEP 8. Determine
the design parameters of the prism per each segment.
[0061] These steps can be iterated as needed. As already mentioned
in the text description associated with FIGS. 11A and 11B, the
angles of incidence 1160 and the angles of desired refraction 1170
for all rays in the solar ray bundle 1190 are substantially equal.
Any small differences between these angles can be taken into
account for further optimizing the design parameters of the prism
1112.
[0062] Finally, a two layer Fresnel lens for concentrating solar
rays 90 along two dimensions may be replaced with a one layer
Fresnel lens, by arranging the chain of prisms of the Fresnel lens
radially. It has already been mentioned in the text referencing
FIG. 1C that a radial arrangement--commonly called a radial Fresnel
lens 16--provides two dimensional concentration of incident
sunlight 90.
[0063] FIG. 12A illustrates the Fresnel lens 1200 with the radial
arrangement of the chains of prisms around a center 1250. The lens
1200 is laid flat. FIG. 12B illustrates the cylindrical arc shape
in which the Fresnel lens 1200 is to be deployed when used as part
of a solar concentrator of this invention. Referring again to the
cylindrical arc shape of the Fresnel lens 1200 of FIG. 12B, solar
rays incident upon the center 1250 pass through with little or no
refraction. The solar rays incident at other locations are
refracted by the chain of prisms of the Fresnel lens 1200 towards
the concentration spot 1240.
[0064] The preferred orientation of the Fresnel lens 1200 is
achieved when the Fresnel lens 1200 is positioned such that a plane
tangent to its cylindrical arc surface and passing though the
center 1250 is substantially perpendicular to solar rays 90. The
center of the Fresnel lens 1200 can be positioned along the
cylindrical arc as previously described above for the sun tracking
solar concentrators 400, 500, 600, 700, 800.
[0065] Thus, a sun tracking solar concentrator is disclosed. While
embodiments of these inventions have been shown and described, it
will be apparent to those skilled in the art that many more
modifications are possible without departing from the inventive
concepts herein. The inventions, therefore, are not to be
restricted except in the spirit of the following claims.
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