The major cost item in photovoltaic solar panels is the amount of photovoltaic material used. A concentrator reduces the amount of material used by concentrating the photons to fall on a much smaller area. If the geometric concentration level is 500, then the amount of material required is reduced by a factor of 500. As the concentration level increases the accuracy of tracking to keep the light focused on the solar cell is also increased. Optical efficiency also decreases as the concentration level is increased.
| Concentration with refraction | Concentration with reflection |
Fresnel lens are commonly used to focus the photons. The cell is placed between the fresnel lens and the focus. If the area of the fresnel lens is A and the area of the cell is R, then the concentration level C is A/R. At concentration level C the cell is placed
F/sqrt(C) distance from the focus, where F is the focal length. There are losses every time light is refracted or reflected. To simplify discussion these losses are not considered here.
| Ray-trace of 400x concentration at front focus with the cell radius 85% bigger. The cell has to be 242% bigger to capture 99% of the photons. The hot spot at the center is more than 2000x. The acrylic fresnel lens is 395mm x 395mm, with F/D of 0.835, 395 grooves, refraction index of 1.49 and Abbe number of 55.3 | Ray-trace of 400x concentration at back focus with the cell radius 54% bigger. The cell has to be 136% bigger to capture 99% of the photons. The high spot at the center is greater than 400,000x and approaches infinity as the spot gets smaller. |
However as photons converges to the focus point, it tends to cross itself, creating hot spots. Shorter wavelength (UV) photons are more refracted than longer wavelength (infra-red) photons causing the shorter wavelength photons to be more focused to the center and the longer wavelength photons are diffused away from the center.
At the back focus, the longer wavelength photons are concentrated at the center and the shorter wavelength photons are diffused over a large area.
Various technologies (secondary optical elements, secondary flux modifiers, homogenizer, TIR-R Total Internal Refraction and Refraction) are used to make the concentrated light more uniform in intensity and color.
Heat is produced when the energy of the photon exceeds the band gap of the photovoltaic material. Solar cells lose efficiency for each oC rise in temperature. More electricity and less heat are produced if the band gap of the photovoltaic material and the energy of the photon are matched closely. The energy of the photon is a function its wavelength.
Multi junction devices are more efficient because they capture a larger portion of the solar spectrum by layering single junction cells of decreasing band gap. The top layer has the highest band gap. The theoretical efficiency (before optical and other losses) increases as the number of junctions is increased, reaching 86% for infinite number of junctions. A severe limitation of MJ cells is that the photocurrent produced by each layer must be matched because each layer is connected in series and the maximum current is limited by that produced by the weakest layer. Another limitation of MJ cells is that each layer must be optically transparent to allow the photons that it cannot use to pass to the layer below.
The methods described here aim to achieve high concentration level with minimal optical loss and more uniform intensity. These methods capture the concentrated shorter wavelength photons at the front and the concentrated longer wavelength photons at the back. This method uses fresnel lens to split the solar spectrum and capture more of the photons with three separate solar cells. The first cell at the front focus will capture more of the shorter wavelength photons and less of the longer wavelength photons. In the middle cell after the focus, the longer wavelength photons are now more focused to the center and the shorter wavelength photons to the edge. The middle cell has a hole in the center to pass the red and infra-red photons to the third cell behind it.
Silicon can be used as the photovoltaic material for all three cells. The light at the first solar cell will be quite uniform in colour, therefore 3J cells can be used at the front focus. For the second solar and third cells higher band gap materials are placed at the edge, using lower band gap materials as you move towards the center.
The heat load is now separated into three areas, with the third cell having the heaviest heat load because of the infra-red. Cooling is achieved by attaching liquid cooled copper heat sink to the back of the cells. The cells and the heat sink will need to be insulated from the environment to protect them and the environment. The heat sink for the front cell must be tapered to be smaller towards the focus. The heat sink for the middle cell is hollow, the hole is bigger towards the third cell. The front coolant tubes must present a flat surface to the front to reduce reflection losses. The concentrated infra-red at the third cell can be feed instead to a photovoltaic cavity converter
or to drive high temperature chemical reactions.
Mirrors can be placed on the heat sink behind the first and second solar cell to reflect back light from the second and three solar cells to increase efficiency further.
To make use of convection the coolant will enter from the bottom of the heat sink and exit from the top. The coolant will pass through the front blue cell first, followed by the middle green cell, then the third infra-red/red cell. The electrical connections must be behind or inside the coolant tubes.
Light concentrated by reflectors also have the same problem with non-uniform intensity that is also solved by using the light at both the front and back focus. Reflective concentrators produces uniform color flux, which makes them more suitable for use with MJ (Multijunction) cells.
This method also applies to line focus fresnel lens instead of the point focus fresnel lens as described. The light pattern falling on the solar cells depends on the material of the fresnel lens, focal length, refraction index, Abbe number, number of grooves and the concentration level.
Less grooves means less tip losses. This method applies to both imaging and
non-imaging optics.
The light intensity and colour at each of the focus planes can be calculated using an appropriate algorithm. At each focal plane, the program can calculate the light intensity and colour as a function of the distance from the centre of the solar cell. This may be used to determine the width/radius for each slice forming the solar cell, so the energy falling on each slice is equal. The program may implement the algorithms described in Proceedings of SPIE - Volume 3781, ‘Fresnel lens solar concentrator design based on geometric optics and blackbody radiation equation’ by Michael D. Watson and Robert R. Jayroe, Jr., October 1999, pp. 85-93. This program can also calculate the concentration level for each focus plane so that the light intensity does not exceed a nominated intensity level at any of the slice. The program can also calculate the concentration level for a given solar cell radius at each focal plane.
For a particular lens, the optimum concentration level for the middle cell is when mostly infra-red and red light falls on the third cell. The optimum concentration level can be increased by using higher focal length, higher refraction index material for the fresnel lens or less color dispersive (higher Abbe number) material for the fresnel lens.
The ratio between electricity produced and the quality / temperature of the thermal energy produced can be actively controlled by regulating the flow rate of the coolant.
| Ray-trace using an acrylic Fresnel of 1000 x 1000 x 2 mm, a focal length of 1000 mm, a groove pitch of 0.2 mm, a refractive index of 1.49 and an Abbe number of 55.3 (BHPA1000-2 from http://www.bhlens.com/fresnel_lens.aspx). The spokes on the second cell show how much of the light (2.5%) misses that solar cell. | Example of a three cells solar collector. Looking down, fresnel lens is at front and the solar cells at the back |
The program calculates the parameters so that about a third of the energy falls on the first cell. The two cells are divided into 36 slices and the intensity of each slice is limited to about 100x. The width of both cells is 150mm. The intensity of the light at the
center 5% of the second cell is very low and that region cannot be used.
The result of the calculations are as follows:
Total available energy 888 Watts
Acrylic Fresnel Lens width 1000 mm, focal length 1000mm (f/1), 2500 grooves
887.65W/m2 ASTM G-173-03 Direct Normal solar spectrum
Refraction index 1.492 at 589.2nm
Cauchy Equation n(nm) = 1.4779 + 5049.6 / nm ** 2 – 6.9486e7 / nm ** 4
Sun half angle 0.26657 degree
‘Slice’ is the slice number. 1 is the inner most slice.
‘Radius’ is the radius of the slice.
‘Power’ is the maximum energy that is available at that slice.
‘Tot W’ is the cumulative energy in Watts.
‘Tot %’ is the cumulative energy in percentage.
'QEff %' is the quantum efficiency.
‘Ix’ is the Light intensity for that slice. 1 means 1 sun concentration level.
‘Ax’ is the average concentration level for all cells. Area of Input/Area of all cells.
‘Area’ is the area of the slice.
‘UV’, ‘Blue’, ‘Green’, ‘Red’, ‘NIR’ (Near Infrared) and ‘IR’ (Infrared) is the percentage energy contribution of these wavelengths.
fresnelx w1000 g2500 l1000 r12 c11:75 c36:75 e97.5 887.65W/m2 refractive index at 589.2nm(1.492) m(20), eff(97.500%) SunHalfAngle(0.26657o) Total input energy 888 Watts Acrylic Fresnel Lens Width 1000mm x 0.150mm, Area 1.00m2, Focal Length 1000mm (f/1.000), 2500 grooves Slice Radius Power Tot Tot QEff Ix Ax Area UV Blue Green Red NIR IR 390 492 622 760 1110 4000 mm W W % % cm2 % % % % % % 0 887.65 888 100.0 50.8 1.0 1 10000.00 5.6 13.0 19.6 18.2 27.5 16.0 Cell 1 Concentration Level 4.267x, distance from lens 515.921mm, 0.5159f, nominal radius 273.113mm 1 9.029 24.04 24 2.7 50.8 105.8 106 2.56 5.9 13.4 19.7 18.1 27.2 15.6 2 15.611 24.04 48 5.4 50.7 53.2 71 5.10 5.9 13.4 19.7 18.2 27.0 15.7 3 22.193 24.04 72 8.1 50.8 34.6 53 7.82 5.9 13.4 19.7 18.2 27.2 15.7 4 28.784 24.04 96 10.8 50.8 25.7 42 10.55 5.9 13.4 19.7 18.2 27.1 15.7 5 35.376 24.04 120 13.5 50.8 20.4 34 13.29 5.9 13.4 19.7 18.1 27.2 15.7 6 41.973 24.04 144 16.3 50.8 16.9 29 16.03 6.0 13.4 19.7 18.1 27.1 15.7 7 48.570 24.04 168 19.0 50.8 14.4 26 18.77 6.0 13.4 19.7 18.1 27.1 15.7 8 55.172 24.04 192 21.7 50.8 12.6 23 21.52 6.0 13.4 19.7 18.1 27.1 15.7 9 61.777 24.04 216 24.4 50.8 11.2 20 24.27 6.0 13.4 19.7 18.1 27.1 15.7 10 68.386 24.04 240 27.1 50.8 10.0 18 27.03 6.0 13.4 19.7 18.1 27.1 15.7 11 75.000 24.04 264 29.8 50.8 9.1 17 29.79 6.0 13.4 19.7 18.1 27.1 15.6 Cell 2 Concentration Level 42.618x, distance from lens 1153.181mm, 1.1532f, nominal radius 86.423mm 12 16.771 24.04 288 32.5 45.3 30.7 18 8.84 0.0 0.0 0.1 10.9 46.0 43.0 13 19.151 24.04 313 35.2 53.3 100.8 19 2.69 0.0 0.0 12.6 23.4 39.0 24.9 14 21.232 24.04 337 37.9 52.4 102.6 20 2.64 0.0 2.8 19.8 20.6 34.6 22.3 15 23.179 24.04 361 40.6 51.8 99.7 21 2.72 0.0 7.8 18.7 19.4 32.8 21.3 16 25.048 24.04 385 43.3 51.3 95.6 22 2.83 0.3 10.0 18.0 18.8 31.9 20.9 17 26.861 24.04 409 46.0 50.8 91.6 23 2.96 1.8 10.1 17.5 18.4 31.4 20.8 18 28.629 24.04 433 48.8 50.5 87.8 24 3.08 2.8 9.8 17.2 18.1 31.2 20.9 19 30.364 24.04 457 51.5 50.3 84.3 25 3.21 3.3 9.6 16.9 17.9 31.1 21.2 20 32.067 24.04 481 54.2 50.1 81.1 26 3.34 3.4 9.5 16.6 17.7 31.2 21.6 21 33.740 24.04 505 56.9 49.9 78.3 27 3.46 3.4 9.3 16.4 17.6 31.3 22.1 22 35.379 24.04 529 59.6 49.6 76.1 28 3.56 3.3 9.1 16.1 17.4 31.4 22.8 23 36.977 24.04 553 62.3 49.2 74.5 28 3.63 3.2 8.8 15.8 17.2 31.4 23.5 24 38.531 24.04 577 65.0 48.7 73.5 29 3.69 3.1 8.6 15.4 16.9 31.4 24.7 25 40.028 24.04 601 67.7 47.9 73.3 30 3.70 3.0 8.3 14.9 16.5 31.2 26.2 26 41.472 24.04 625 70.4 47.3 73.3 31 3.70 2.9 8.0 14.4 16.1 31.2 27.5 27 42.888 24.04 649 73.1 47.6 72.2 31 3.75 2.8 7.8 14.2 16.0 31.9 27.3 28 44.427 24.04 673 75.8 53.4 64.1 32 4.22 3.0 8.4 15.5 17.6 36.5 18.9 29 46.095 24.04 697 78.5 60.6 57.1 32 4.74 3.3 9.1 16.9 19.4 42.4 8.9 30 47.955 24.04 721 81.3 66.0 49.3 33 5.50 3.6 10.2 18.9 22.1 45.0 0.1 31 50.119 24.04 745 84.0 61.4 40.6 33 6.67 4.2 11.8 22.1 26.5 35.4 0.0 32 52.628 24.04 769 86.7 57.8 33.4 33 8.10 4.8 13.7 25.9 31.9 23.7 0.0 33 55.719 24.04 793 89.4 53.7 25.7 33 10.52 5.9 16.8 32.3 40.0 5.1 0.0 34 59.699 24.04 817 92.1 49.9 18.8 32 14.43 7.5 21.4 42.3 28.8 0.0 0.0 35 65.348 24.04 841 94.8 45.9 12.2 30 22.19 10.5 30.2 55.1 4.2 0.0 0.0 36 75.000 24.04 865 97.5 41.4 6.4 28 42.55 17.3 50.6 32.1 0.0 0.0 0.0
Another example using a 4 x 4 array of 1500mm x 1500mm Fresnel lenses of four different types to construct a 36m2 Fresnel lens system with a focal length of 5012mm. The solar cell 1 consist of 48 circular slices of silicon. The 48 slices are connected in series from the centre to the edge to produce 28.8 volts. Increased voltage (or lower current) may be generated by increasing the number of slices. The width of each slice is made proportional to the light energy expected at its distance from the centre, so that the current produced by each strip is the same. Slice 35 and 36 are not used because the light intensity is too low and too much material will be required to collect it.
All the solar cells are placed on an aluminium oxide ceramic base that is bonded to a copper heat sink. The second and third cell will be constructed similarly. Such a cell can be constructed on a single 150/200 mm wafer with current microelectronic batch fabrication processes, such techniques are described in US patent 3,994,012 to Warner, Jr.
At the effective concentration level of 415, the optical efficiency is 93.5%.
The results of a simulation of the light intensity and distribution for
36 slices instead of 144.
fresnelx w6000 g6000 l5012 r13 R25 c12:75 c24:100 c34:100 e99
887.65W/m2 refractive index at 589.2nm(1.492) m(20), eff(99.000%) SunHalfAngle(0.26657o)
Total input energy 31955 Watts
Acrylic Fresnel Lens Width 6000mm x 0.405mm, Area 36.00m2, Focal Length 5012mm (f/0.835), 6000 grooves
Slice Radius Power Tot Tot QEff Ix Ax Area UV Blue Green Red NIR IR
390 492 622 760 1110 4000
mm W W % % cm2 % % % % % %
0 31955.32 31955 100.0 50.8 1.0 1 360000 5.6 13.0 19.6 18.2 27.5 16.0
Cell 1 Concentration Level 302.572x, distance from lens 4723.864mm, 0.9425f, nominal radius 194.609mm
1 24.771 878.80 879 2.8 41.8 513.6 514 19.28 47.0 11.7 12.2 9.5 12.8 6.8
2 28.764 878.83 1758 5.5 46.5 1474.2 762 6.72 24.3 16.6 17.4 13.7 18.3 9.8
3 32.901 878.77 2636 8.3 47.0 1235.3 873 8.01 22.2 17.0 17.9 14.1 18.9 10.1
4 37.154 878.82 3515 11.0 47.4 1057.7 913 9.36 20.5 17.3 18.3 14.4 19.3 10.3
5 41.500 878.79 4394 13.8 47.7 921.9 915 10.74 19.3 17.6 18.5 14.6 19.6 10.5
6 45.948 878.78 5273 16.5 48.0 810.1 896 12.22 17.9 18.0 18.8 14.8 19.9 10.6
7 50.461 878.78 6152 19.3 48.2 724.3 866 13.67 16.8 18.6 18.9 14.9 20.1 10.7
8 55.118 878.80 7030 22.0 48.6 641.0 830 15.45 14.2 19.7 19.4 15.3 20.5 11.0
9 59.889 878.82 7909 24.8 48.9 574.3 791 17.24 11.7 21.1 19.7 15.5 20.9 11.2
10 64.812 878.81 8788 27.5 49.3 513.4 750 19.28 7.6 23.6 20.1 15.8 21.4 11.5
11 69.844 878.82 9667 30.3 49.6 465.0 711 21.29 2.0 28.3 20.4 16.1 21.7 11.6
12 75.000 878.78 10546 33.0 49.8 422.0 672 23.46 0.4 28.8 20.7 16.3 22.0 11.8
Cell 3 Concentration Level 162.602x, distance from lens 5405.050mm, 1.0784f, nominal radius 265.468mm 13 21.683 878.78 11424 35.8 32.2 670.3 672 14.77 0.0 0.0 0.0 0.0 37.4 62.6 14 29.534 878.80 12303 38.5 47.9 783.7 679 12.63 0.0 0.0 0.0 8.4 51.4 40.1 15 35.759 878.81 13182 41.3 47.1 775.4 685 12.77 0.0 0.0 0.0 18.3 43.1 38.6 16 40.593 878.78 14061 44.0 41.1 853.7 693 11.60 0.0 0.0 2.1 17.4 35.3 45.3 17 44.136 878.82 14940 46.8 32.6 1049.9 707 9.43 0.0 0.0 3.7 12.9 27.2 56.2 18 47.976 878.80 15818 49.5 38.4 890.8 716 11.11 0.0 0.0 5.7 14.2 32.0 48.1 19 51.433 878.82 16697 52.3 41.7 917.2 724 10.79 0.0 0.0 4.2 13.0 37.1 45.7 20 55.315 878.77 17576 55.0 52.1 760.5 726 13.02 0.0 0.0 0.3 13.4 51.2 35.1 21 60.014 878.79 18455 57.8 80.7 581.4 717 17.03 0.0 0.0 0.0 8.8 86.5 4.7 22 65.845 878.80 19334 60.5 85.4 429.5 696 23.05 0.0 0.0 0.0 1.8 98.2 0.0 23 74.967 878.79 20212 63.3 80.3 245.3 645 40.35 0.0 0.0 0.0 0.0 100.0 0.0 24 100.038 878.78 21091 66.0 74.2 71.8 484 137.84 0.0 0.0 0.0 7.1 92.9 0.0
Cell 2 Concentration Level 373.862x, distance from lens 5271.212mm, 1.0517f, nominal radius 175.074mm, Center hole radius 24.223mm 25 27.641 878.80 21970 68.8 60.2 1778.1 475 5.57 0.0 0.0 10.7 33.4 44.9 11.0 26 31.549 878.85 22849 71.5 57.5 1362.2 487 7.27 0.0 0.0 14.7 49.3 24.5 11.6 27 35.798 878.78 23728 74.3 55.3 1101.3 497 8.99 0.0 0.0 18.5 67.3 2.6 11.6 28 40.666 878.83 24606 77.0 52.4 846.6 505 11.69 0.0 0.0 23.9 63.1 0.8 12.2 29 46.163 878.79 25485 79.8 50.7 660.3 509 14.99 0.0 0.8 29.4 57.9 0.0 12.0 30 52.573 878.79 26364 82.5 50.3 497.9 509 19.88 0.0 3.4 35.7 51.1 0.0 9.7 31 60.332 878.78 27243 85.3 49.7 359.7 502 27.52 0.0 7.0 46.4 39.4 0.0 7.3 32 70.077 878.78 28122 88.0 49.6 248.0 486 39.92 0.0 11.9 73.2 11.1 0.0 3.8 33 82.788 878.78 29000 90.8 49.0 162.2 459 61.05 0.0 18.9 80.5 0.0 0.0 0.6 34 100.000 878.78 29879 93.5 47.1 100.2 415 98.84 0.0 28.7 71.3 0.0 0.0 0.0 35 125.882 878.77 30758 96.3 44.6 53.9 348 183.67 0.0 46.3 53.7 0.0 0.0 0.0 36 178.832 878.77 31637 99.0 41.7 19.5 237 506.89 0.0 84.8 15.2 0.0 0.0 0.0 37 178.832 317.97 31955 100.0 38.3 1.$ 240 0.00 4.8 95.2 0.0 0.0 0.0 0.0
|
4 x 4 array of fresnel lens constructed from 4 different fresnel lens. |
HMJ (Horizontal Multijunction) cell with 12 slices. No front surface grid, front or back contact, bus bar or bypass diode is required. Expected efficiency of 20% at 100x to 2500x concentration. High voltage/low current. Wafer size solar cell (100mm to 450mm diameter) |
Initial application will be in solar farms in regions with high direct insolation to efficiently supply loads up to 7000km away with underground high voltage DC (HVDC) transmission lines. With this method, the major cost item is shifted from the solar cells to the fresnel lens. The cost of the fresnel lens can be reduce by making them as thin as possible.
The land below this type of solar concentrator can still be used, because the diffused background light cannot be focused. Example of such land are parks, car parks, over roads (when we stop using pollution causing cars), cycle ways, schools, shops, residential and degraded land. This method will very efficiently in extracting the direct UV and Infra-red radiation. Concentrated solar power can be very dangerous, take care. Wind load will become a problem as the size of the fresnel lens gets bigger and higher.
An Australian patent application 2008207559 have being filed at the Australian Patent Office on the 27th August 2008 on my behalf by Freehills. The above information was disclosed to Freehills to draft the patent application. I hereby grant the right to any person to personally assemble such a solar collector of up to 100m2 input aperture for their personal use.
27th August 2008
Philip Wong BSc BE(UNSW)
Phone/fax: 61 2 9805 0356
Mobile: 04 2243 8324 (Australia)
Sydney, Australia
ioserver@ioserver.com
Fresnelx Calculates intensity and colour
distribution of solar radiation falling on up to three surfaces. Run "fresnelx
x" to see available commands.
Concentrated Solar Thermal Power with Point Focus Fresnel
lens and reflector
Bifacial concentration on VMJ (Vertical Multijunction)
solar cells
Spectrum splitting concentration on to three solar
cells
Fresnel Lens
Manufacturer. bhlens.com
Fresnel
Lens Manufacturer. fresneloptic.com
Fresnel Lens Manufacturer.
ntkj.co.jp 2.9m x 5m lens
Fresnel Lens
Distributor. 3dlens.com
Photovolt VMJ cell
(100x to 2500x, 20% efficiency silicon solar cells) $5/cm2
AZUR concentrator cells $21/cm2
EMCORE 3J
Concentrator cells $8/cm2
QuantaSol
Spectrolab
Terrestrial Concentrator cells $10/cm2
Sunpower. Their 26% efficiency
250x silicon concentrator cells are no longer available $6/cm2
1 Sun Silicon Solar
Cells WB-28 100mm round $0.06/cm2
Indium Gallium Nitride Solar Cells
Multiple
band gaps Semiconductors
Amonix
Arima Eco Energy
Concentrix Solar, GmbH
Energy Innovations
Entech
GreenVolts
Isofoton
Opel International Inc
RoseStreet Labs Energy,
Inc. Full spectrum concentrator.
Solar Systems (Australia)
sol3g
Soliant Energy
SolFocus
Whitfield Solar
Ausra (CLFR - Compact Linear Fresnel Reflector)
Inspira
Precise Tracking. Precision Solar
Technologies
Solar Tracker
tracker.cat
Celestron
80mm Refracting Telescope, available for AUD$449 from my local electronic
store that can track stars that are light years away with open loop
tracking.
Energy and aesthetics of building integrated
RES (renewable energy sources)
Nonimaging Fresnel lens for
solar concentration
Novel Materials and Devices for Sunlight Concentrating
Systems IBM Research
Photovoltaic semi-conductor devices. Raymond M. Warner, Jr.
The
fresnel lens concept for solar control of buildings
The Solar Race. A multidisciplinary team works fervently to make solar building technology more powerful than ever.
A simple analytical treatment of edge-illuminated VMJ silicon solar cells.
N.H. Rafat, Solar Energy, Volume 80, Issue 12, December 2006, Pages 1588-1599
Optical Design software. optenso.de
Persistence of Vision Raytracer
Reference Solar Spectral Irradiance: ASTM G-173
Swiftech MCP655
12VDC Water Pump, 50,000 hour MTBF, 20L/Min, 60oC
EBP Electric
Booster Pump, 12VDC, 15L/Min, +120oC
Last updated: 08 June 2010