Solar Energy

Appendices Solar energy Black silicon dry etching – textured surface on multi cell Black silicon wet etching – textured surface on multi cell Source: GCL-Poly Source: GCL-Poly Black silicon texturing Black silicon texturing improve multi cell efficiency by 0.6ppts to 0.7ppts improve multi cell efficiency by 0.6ppts to 0.7ppts Source: GCL-Poly 17 November 2020 [email protected] 201 Appendices Solar energy Appendix 14: Solar tech: Round-shaped ribbon Round-shaped ribbon A thinner, round-shape A thinner, and round-shape ribbon for the MBB rather than a flat ribbon could be ribbon reduces shading and used - this reduces shading and increases the photons hitting the surface of the increases the photons semiconductor. With a round-shape wire, incident light gets reflected at various hitting the surface of the angles, and more light hits the cells. At the same time, according to CPIA, the usage semiconductor of silver paste is reduced significantly for MBB (9BB reduces silver paste usage by 25% versus 5BB), at the same time increasing power output by 2% and reducing micro-cracks. A flat rectangular ribbon blocks light A round-shape ribbon reflects light Source: LG Source: LG 202 [email protected] 17 November 2020 Appendices Solar energy Appendix 15: Solar tech: Half-cut solar cells Half cut solar cell modules Another small tweak to the Another small tweak to the cell-module fabrication technique is half cut cells, which cell-module fabrication is done by cutting a full-size solar cell into two half-size cell with an infrared laser. technique is half cut cell The logic is simple, with half-cut cells, the electrical current flowing in each busbar is halved, and for that reason the amount of internal losses in a solar module with half-cut cells is only a quarter of a full-sized cell module. With half-cut, module With half-cut, module power output is boosted by another 2-3%, a tweak that is power output is boosted by straightforward and logical. CPIA foresees that half-cell will increase its share from another 2-3% 21% in 2019 to over 70% by 2025, of course, with full-size cells seeing diminishing market share. Half-cell modules will be Half-cell modules will be the mainstream the mainstream (%) Overlapping Half-cut Full-size 100 90 80 70 60 50 40 30 20 10 0 2018 2019 2020F 2021F 2022F 2023F 2025F Source: CPIA With half-cut, module Modules with half-cut cells versus full-size cells power output is boosted by another 2-3% Source: Jinko Solar 17 November 2020 [email protected] 203 Appendices Solar energy Joule’s Law - Power dissipation = I^2 x R Source: Jinko Solar Half-cut cells lower the hot spot temperature Half-cut cell enables Hot spot refers to the heating up of a solar cells when the cell is shaded, either by better reliability tress, leafs, bird droppings, or by other solar modules. With half-cut cells, because the current is half of a full-size cell, the hot spot temperature could be substantially reduced (10-20Ԩ, according to Longi), which enables better reliability. Having a hot spot is in no way good, but half-cut is an effective mitigation. Half-cut cell reduce hot spot temperature Source: Longi The long explanation: Recall that solar cells are connected in a series in a solar module (and the fact that a solar cell is essentially a diode). When a particular cell is shaded while other cells in series are unshaded, the illuminated cells are operated in “forward bias” (i.e. electrons following from the P-region to the N-region, which is triggered by photons), and the energy collected by the unshaded cells would be sent to the shaded cell, which is operating as if a diode was in “reverse bias”. This shaded cell will dissipate the energy in the form of heat, which is referred as “hot spot”. If the duration is long enough, the hot spot would bring irreversible degradation of solar modules. 204 [email protected] 17 November 2020 Appendices Solar energy Half-cut lowers operating temperature The operating temperature For a similar reasons, even if a hot spot does not exist in a module, the operating in the half-cell module is temperature in the half-cell module is going to be lower, simply because the going to be lower working current is 50% of its full-cell counterpart. Lower operating temperature Source: Longi Working temperature is Working temperature is ~2 degrees lower for half-cell modules ~2 degrees lower for half-cell modules Source: Jinko Solar Half-cut cells arrangement offers less shading impact Shading causes hot Shading causes hot spot issues, but another serious consequence is that the power spot issues output of other solar cells connected in the same series will be affected. This is because the solar energy captured by the unshaded cells will be dissipated by the shaded cells. A typical standard solar module nowadays consists of 72 cells (6x12 cells), which are usually grouped in 3 x 24 cells, with each group protected by one bypass diode (i.e. 3 bypass diodes in total, at the junction box). If any one of the cells in a group is shaded, the power output from the other 23 cells in the same series will drop to zero, or a 33% power loss. In the most extreme cases, if at each string of cells there is one shaded cell, the power output of the whole module would drop to zero. 17 November 2020 [email protected] 205 Appendices Solar energy One solution to this is to add one bypass diode to every solar cell (i.e. 72 bypass diodes), which would enable the current from the unshaded cells to flow through the bypass diode instead of the shaded cells. Sunpower and AE Solar (and some others) do offer hot-spot-free modules that have bypass diodes for all the cells. The downside is the additional cost of the bypass diodes. By doubling the strings of Here is where the half-cut cells come in. The half-cell module would put together cells, the power loss from 144 half-cells (6 x 24 cells), and the cells are connected in 6 groups, instead of 3 shading would be reduced groups. The cells are arranged in the upper half and lower half, with 2 string of cells connected to one bypass diode in parallel, so still using a total of 3 bypass diodes. By doubling the strings of cells, the power loss from shading would be reduced. Modules built with 210mm solar cells with triple cut Triple-cut a 210mm cell, Another twist to the half-cut cell is “triple-cut” cells, by cutting a piece of 210mm half-cut a 166mm cell solar cells into 3 pieces. The logic is similar to the half-cut modules, lowering the power dissipation in the busbar. Trina Solar has already launched new products with that arrangement to provide 500 watt+ solar modules. The reason that a triple cut is selected for 210mm cells is because half-cutting the 210mm cells would result in higher series resistance otherwise (a half-cut 210mm cell would be bigger than a half-cut 166mm cell, as simple as that). Module power of different cutting forms, versus the number of busbars Source: Trina Solar Source: Trina Solar, PV Tech At the same time, 1/4 cut or even 1/5 cut cells were considered, but the increase in manufacturing complexity outweighs (and higher process risk) the increase in power output. The 1/4 cut would increase open circuit voltage, impacting number of modules per string and increase BOS. Trina also said 1/4 cut or 1/5 cut cells arrangement would require more bypass diodes, but triple cut cells arrangement only requires 3 diodes. Managing the process risk, at the same time strike for higher output Full cell Half cut 1/3 cell 1/4 cell Power (max) 473 watt 495 Watt 500 Watt 502 Watt Mono-facial current 18.2A 18.2A 12.1A 9.1A Voltage (open circuit) 34.3V 34.3V 51.5V 68.7V Process risk Low Low Normal High Source: Trina Solar 206 [email protected] 17 November 2020 Appendices Solar energy Appendix 16: Solar tech: Multi-busbar Multi-busbar (MBB) Busbars and fingers are Busbars and fingers are metalised thin strips printed on the front and back of the metalised thin strips printed solar cell. They are responsible for collecting the electrons flowing from the emitter, on the front and back of the and the busbars are ultimately connected to the junction box at the panels. solar cell In 2019, the mainstream cells were wired with 5 busbar (“5BB”), but CPIA expects a rapid migration to 9BB in the next 5 years. Historically, the number of busbars on a single cell has been increasing, initially at 2BB. The latest cell design aims at more busbars, or MBB technology, with the aim to reduce the series resistance in the cells. For a similar argument, more busbars shorten the travel distance of current on the fingers (or the small grid on the cells). That is why, 12BB (e.g. LG Solar) or even more are busbars are being proposed. More busbars, enabling More busbars, enabling short power collection path, which reduces power loss short power collection path, which reduces power loss Source: Trina Solar 9 busbars quickly becoming 9 busbars quickly becoming the most popular among cell makers the most popular among cell makers (%) Others >9 BBs 9 BBs 5 BBs 4 BBs 100 90 80 70 60 50 40 30 20 10 0 2018 2019 2020F 2021F 2022F 2023F 2025F Source: CPIA 17 November 2020 [email protected] 207 Appendices Solar energy Appendix 17: Solar tech: Encapsulation Encapsulation The cell spacing of a conventional solar panel is 2mm, due to the restriction of ribbon strength.

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