Solar radiation collection with high concentration level, high optical efficiency and spectrum splitting.
The 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. The accuracy of tracking to keep the light focused on the solar cell increases with concentration level. Optical efficiency decreases as the concentration level increased.
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| Concentration with refraction |
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| 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.
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Ray-trace of 400x concentration at front focus with the cell radius 84% bigger. The cell has to be 237% bigger to capture 99% of the photons. The hot spot at the center is 1481x. 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 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. |
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Ray-trace of 400x concentration at back focus with the cell radius 41% bigger. The cell has to be 98% bigger to capture 99% of the photons. The high spot at the center is greater than 174,000x and approaches infinity as the spot gets smaller. 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.
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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.
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| 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. The spokes on the second cell show how much of the light (2.5%) misses that solar cell. |
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| 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 35 slices. The width of both cells is 150mm.
The result of the calculations are as follows:
Total available energy 777 Watts
Acrylic Fresnel Lens width 1000 mm, focal length 1000mm (f/1), 2500 grooves
777W/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
777.37W/m2 refractive index at 589.2nm(1.492) m(20), eff(97.500%)
SunHalfAngle(0.26657o)
Total input energy 777 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 BlueGreen Red NIR IR
390 492 622 760 1110 4000
mm W W % % cm2 % % % % % %
Cell 1 Concentration Level 4.3x, distance from lens 516.6mm, 0.52f,
radius 272.7mm
0 9.0 21.1 21 3 53 106 106 3 3 14 21 19 29 14
1 15.6 21.1 42 5 53 53 71 5 3 14 21 19 29 14
2 22.2 21.1 63 8 53 35 52 8 3 14 21 19 29 14
3 28.8 21.1 84 11 53 26 42 11 3 14 21 19 29 14
4 35.4 21.1 105 14 53 20 34 13 3 14 21 19 29 14
5 42.0 21.1 126 16 53 17 29 16 3 14 21 19 29 14
6 48.6 21.1 147 19 53 14 26 19 3 14 21 19 29 14
7 55.2 21.1 168 22 53 13 23 22 3 14 21 19 29 14
8 61.8 21.1 189 24 53 11 20 24 3 14 21 19 29 14
9 68.4 21.1 211 27 53 10 18 27 3 14 21 19 29 14
10 75.0 21.1 232 30 53 9 17 30 3 14 21 19 29 14
Cell 2 Concentration Level 38.5x, distance from lens 1161.3mm, 1.16f,
radius 91.0mm, Center hole radius 75.0mm
11 18.2 21.1 253 33 50 26 17 10 0 0 0 13 50 37
12 20.6 21.1 274 35 56 91 19 3 0 0 15 24 40 21
13 22.7 21.1 295 38 55 93 20 3 0 4 21 21 36 19
14 24.7 21.1 316 41 54 90 21 3 0 9 20 20 34 18
15 26.7 21.1 337 43 53 87 22 3 0 11 19 20 33 17
16 28.6 21.1 358 46 53 83 23 3 2 10 18 19 33 17
17 30.4 21.1 379 49 53 78 24 3 2 10 18 19 33 18
18 32.3 21.1 400 51 53 75 25 4 2 10 18 19 33 18
19 34.1 21.1 421 54 53 72 25 4 2 10 18 19 33 18
20 35.9 21.1 442 57 53 69 26 4 2 10 18 19 33 19
21 37.6 21.1 463 60 52 67 27 4 2 10 18 19 33 19
22 39.4 21.1 484 62 52 65 28 4 2 9 17 19 33 20
23 41.0 21.1 505 65 52 64 28 4 2 9 17 18 34 20
24 42.7 21.1 526 68 52 63 29 4 2 9 17 18 34 21
25 44.3 21.1 547 70 51 62 30 4 2 9 16 18 34 22
26 45.8 21.1 568 73 50 63 30 4 1 8 15 17 34 24
27 47.3 21.1 590 76 52 60 31 4 1 8 16 18 35 22
28 49.0 21.1 611 79 56 55 31 5 2 9 17 19 39 15
29 50.8 21.1 632 81 66 48 32 6 2 10 18 21 46 3
30 52.8 21.1 653 84 64 40 32 7 2 11 21 25 41 0
31 55.2 21.1 674 87 60 34 32 8 2 13 25 30 31 0
32 58.1 21.1 695 89 56 27 32 10 3 15 30 37 14 0
33 61.7 21.1 716 92 52 20 31 14 3 20 39 38 0 0
34 66.7 21.1 737 95 48 13 30 20 4 27 54 14 0 0
35 75.0 21.1 758 98 44 7 28 37 7 43 49 0 0 0
36 116.1 19.4 777 100 38 1 17 247 25 68 6 0 0 0
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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 Raymond Warner, Jr.
The results of a simulation of the light intensity and distribution for 36 on three cells.
fresnelx w6000 g6000 l5012 r13 R25 c12:75 c24:100 c34:100 e99
777.37W/m2 refractive index at 589.2nm(1.492) m(20),
eff(99.000%) SunHalfAngle(0.26657o)
Total input energy 27985 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 BlueGreen Red NIR IR
390 492 622 760 1110 4000
mm W W % % cm2 % % % % % %
Cell 1 Concentration Level 343.8x, distance from lens 4741.7mm, 0.95f,
radius 182.6mm
0 26.1 769.6 770 3 49 464 464 21 16 19 20 15 21 9
1 30.4 769.6 1539 6 51 1295 683 8 10 20 21 17 22 10
2 34.7 769.6 2309 8 50 1128 787 9 11 20 21 16 22 10
3 38.9 769.6 3078 11 50 1009 833 10 12 20 21 16 21 9
4 43.1 769.7 3848 14 50 914 848 11 14 20 20 16 21 9
5 47.3 769.6 4618 17 50 823 843 12 13 20 20 16 21 9
6 51.6 769.6 5387 19 50 741 827 13 11 22 20 16 21 9
7 56.1 769.6 6157 22 50 659 802 15 8 24 21 16 22 10
8 60.6 769.6 6927 25 51 599 773 17 3 28 21 16 22 10
9 65.2 769.6 7696 28 51 539 740 18 0 29 22 17 22 10
10 70.0 769.6 8466 30 51 486 707 20 0 28 22 17 23 10
11 75.0 769.7 9235 33 52 438 672 23 0 26 23 18 24 10
Cell 3 Concentration Level 162.3x, distance from lens 5405.4mm, 1.08f,
radius 265.7mm
12 23.9 769.6 10005 36 37 553 661 18 0 0 0 0 43 57
13 31.7 769.6 10775 39 52 720 665 14 0 0 0 9 56 36
14 37.9 769.6 11544 41 50 729 669 14 0 0 0 19 47 35
15 42.5 769.6 12314 44 42 858 678 12 0 0 1 18 36 45
16 45.9 769.6 13084 47 33 1065 693 9 0 0 3 13 29 56
17 49.6 769.6 13853 50 45 886 702 11 0 0 5 15 40 40
18 52.7 769.6 14623 52 44 979 712 10 0 0 4 13 40 44
19 56.8 769.6 15392 55 61 711 712 14 0 0 0 15 61 24
20 61.1 769.6 16162 58 84 614 707 16 0 0 0 8 90 1
21 67.0 769.6 16932 61 84 421 686 24 0 0 0 2 98 0
22 75.7 769.6 17701 63 80 255 639 39 0 0 0 0 100 0
23 100.0 769.6 18471 66 74 74 484 134 0 0 0 7 93 0
Cell 2 Concentration Level 365.3x, distance from lens 5274.2mm, 1.05f,
radius 177.1mm, Center hole radius 25.5mm
24 28.9 769.6 19241 69 62 1700 475 6 0 0 10 35 46 9
25 32.8 769.6 20010 72 59 1305 487 8 0 0 14 52 25 10
26 37.0 769.6 20780 74 56 1061 497 9 0 0 18 71 1 10
27 41.9 769.6 21549 77 54 821 504 12 0 0 24 66 0 10
28 47.4 769.6 22319 80 53 642 508 15 0 0 30 61 0 9
29 53.8 769.6 23089 83 53 483 507 20 0 2 38 55 0 6
30 61.6 769.6 23858 85 52 353 500 28 0 5 49 42 0 4
31 71.3 769.6 24628 88 52 244 484 41 0 10 76 13 0 1
32 83.6 769.6 25398 91 50 165 457 60 0 17 83 0 0 0
33 100.0 769.6 26167 94 47 105 416 95 0 26 74 0 0 0
34 124.4 769.6 26937 96 45 58 353 172 0 42 58 0 0 0
35 173.1 769.6 27706 99 42 22 248 455 0 80 20 0 0 0
36 253.3 278.7 27985 100 40 3 143 1074 0 100 0 0 0 0
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| 4 x 4 array of fresnel lens constructed from 4 different fresnel lens. | |
| Front Cell radius 75mm | |
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| Back Cell radius 75mm | |
| Single HMJ Cell radius 152mm | |
| HMJ (Horizontal Multijunction) cell with total of 100 slices producing 60 volts. No front surface grid, front or back contact, bus bar or bypass diode is required. High voltage/low current. Wafer size solar cell (75mm to 450mm diameter). Run 'fresnelx w1000 s105 r50 c50:75 c99:75' to show simulation for front and back cells. Run 'fresnelx w1000 s105 c99:150' for single cell. | |
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.
Philip Wong BSc BE(UNSW)
Phone/fax: 61 2 9805 0356
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
Lens
Fresnel Lens Manufacturer. fresneloptic.com
Fresnel Lens Manufacturer. ntkj.co.jp 2.9m x 5m lens
Fresnel Lens Distributor. 3dlens.com
Cells
Spectrolab Terrestrial Concentrator cells
Indium Gallium Nitride Solar Cells
Tracking
Precise Tracking. Precision Solar Technologies
References
Optical Design software. optenso.de
Persistence of Vision Raytracer
Reference Solar Spectral Irradiance: ASTM G-173
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Last updated: Mon, 25 Jan 2021 08:28:43 GMT |