Transcript Document
Power Generation from Renewable Energy Sources Fall 2012 Instructor: Xiaodong Chu Email：[email protected] Office Tel.: 81696127 Flashbacks of Last Lecture • There are two conditions for the actual PV and for its equivalent circuit: – The current that flows when the terminals are shorted together (the short-circuit current, ISC) – The voltage across the terminals when the terminals are left open (the open-circuit voltage, VOC) Flashbacks of Last Lecture • The PV equivalent circuit includes both series and parallel resistances as q(V I RS ) V I RS I I SC I 0 exp 1 kT RP Flashbacks of Last Lecture • The basic building block for PV applications is a module consisting of a number of pre-wired cells in series, all encased in tough, weather-resistant packages • Multiple modules can be wired in series to increase voltage and in parallel to increase current, the product of which is power referred to as an array Flashbacks of Last Lecture • Example 8.3 of the textbook: you should master it! Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves • The output of a PV module can be reduced dramatically when even a small portion of it is shaded • External diodes can help preserve the performance of PV modules – The main purpose for such diodes is to mitigate the impacts of shading on PV I –V curves – Such diodes are usually added in parallel with modules or blocks of cells within a module Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves • Consider an n-cell module with current I and output voltage V shows one cell separated from the others • In the sun, the same current I flows through each of the cells • In the shade, the current source of the shaded cell ISC is reduced to zero; the voltage drop across RP as current flows through it causes the diode to be reverse biased, so the diode current is also zero Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves • Consider the case when the bottom n − 1 cells still have full sun and still carry their original current I so they will still produce their original voltage Vn−1 • The output voltage of the entire module VSH with one cell shaded will drop to VSH Vn1 I ( RP RS ) • The voltage of the bottom n − 1 cells will be n 1 Vn 1 V n • Then n 1 VSH V I ( RP RS ) n Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves • The drop in voltage ΔV at any given current I , caused by the shaded cell, is given by 1 V V VSH V 1 V I ( RP RS ) n V V I ( RP RS ) n • Since the parallel resistance RP is much greater than the series resistance RS V V IRP n Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves • At any given current, the I –V curve for the module with one shaded cell drops by ΔV Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves • Example 8.6 of the textbook: you should master it! Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves • The voltage drop problem in shaded cells could be to corrected by adding a bypass diode across each cell • When a solar cell is in the sun, there is a voltage rise across the cell so the bypass diode is cut off and no current flows through it—it is as if the diode is not even there • When the solar cell is shaded, the drop that would occur if the cell conducted any current would turn on the bypass diode, diverting the current flow through that diode • Since the bypass diode, when it conducts, drops about 0.6 V, the bypass diode controls the voltage drop across the shaded cell, limiting it to a relatively modest 0.6 V instead of the rather large drop that may occur without it Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves • In real modules, it would be impractical to add bypass diodes across every solar cell, but manufacturers often provide at least one bypass diode around a module to help protect arrays, and sometimes several such diodes around groups of cells within a module • These diodes do not have much impact on shading problems of a single module, but they can be very important when a number of modules are connected in series • Just as a single cell can drag down the current within a module, a few shaded cells in a single module can drag down the current delivered by the entire string in an array Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves • When any of the cells are shaded, they cease to produce voltage and instead begin to act like that cause voltage to drop as the other modules continue to try to push current through the string • Without a bypass diode to divert the current, the shaded module loses voltage and the other modules try to compensate by increasing voltage, but the net effect is that current in the whole string drops • If bypass diodes are provided, current will go around the shaded module and the charging current bounces back to nearly the same level that it was before shading occurred Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves • Bypass diodes help current go around a shaded or malfunctioning module within a string • This not only improves the string performance, but also prevents hot spots from developing in individual shaded cells • When strings of modules are wired in parallel, a similar problem may arise when one of the strings is not performing well • Instead of supplying current to the array, a malfunctioning or shaded string can withdraw current from the rest of the array • By placing blocking diodes, the reverse current drawn by a shaded string can be prevented Photovoltaic Materials and Electrical Characteristics–Shading impacts on I– V curves Photovoltaic Materials and Electrical Characteristics–Crystalline Silicon Technologies • There are a number of ways to categorize photovoltaics – Thickness of the semiconductor – Extent to which atoms bond with each other – Heterogeneity of p and n material Photovoltaic Materials and Electrical Characteristics–Crystalline Silicon Technologies • On the thickness of the semiconductor – Conventional crystalline silicon solar cells are relatively thick—on the order of 200–500 μm – An alternative approach to PV fabrication is based on thin films of semiconductor—on the order of 1–10 μm Photovoltaic Materials and Electrical Characteristics–Crystalline Silicon Technologies • On the extent to which atoms bond with each other in individual crystals – Single crystal, the currently dominant silicon technology – Multicrystalline, in which the cell is made up of a number of relatively large areas of single crystal grains, each on the order of 1 mm to 10 cm in size – Polycrystalline, with many grains having dimensions on the order of 1 μm to 1 mm – Microcrystalline, with grain sizes less than 1 μm – Amorphous, in which there are no single-crystal regions Photovoltaic Materials and Electrical Characteristics–Crystalline Silicon Technologies • On whether the p and n regions of the semiconductor are made of the same material – With the same material, they are called homojunction PVs – When the p–n junction is formed between two different semiconductors, they are called heterojunction PVs Photovoltaic Materials and Electrical Characteristics–Crystalline Silicon Technologies • Multiple junction solar cells are made up of a stack of p–n junctions with each junction designed to capture a different portion of the solar spectrum – The shortest-wavelength, highest-energy photons are captured in the top layer while most of the rest pass through to the next layer – Subsequent layers have lower and lower band gaps, so they each pick off the most energetic photons that they see, while passing the rest down to the next layer • Very high efficiencies are possible using this approach – Lab examples of multi-junction cells have demonstrated performance over 42% Photovoltaic Materials and Electrical Characteristics–Crystalline Silicon Technologies Photovoltaic Materials and Electrical Characteristics–Crystalline Silicon Technologies • Concentrated photovoltaic (CPV) technology uses optics such as lenses to concentrate a large amount of sunlight onto a small area of solar photovoltaic materials to generate electricity • Unlike traditional, more conventional flat panel systems, CPV systems are often much less expensive to produce, because the concentration allows for the production of a much smaller area of solar cells Photovoltaic Materials and Electrical Characteristics–Crystalline Silicon Technologies • Today, the vast majority of PV modules (85% to 90% of the global annual market) are based on wafer-based crystalline silicon • Crystalline silicon PV modules are expected to remain a dominant PV technology until at least 2020, with a forecasted market share of about 50% by that time Photovoltaic Materials and Electrical Characteristics–Thin-Film Photovoltaics • Conventional crystalline silicon technologies require considerable amounts of expensive material with additional complexity and costs needed to wire individual cells together • Competing technologies are based on depositing extremely thin films of photovoltaic materials onto glass or metal substrates • Thin-film devices use relatively little material, do not require the complexity of cell interconnections, and are particularly well suited to mass-production techniques Photovoltaic Materials and Electrical Characteristics–Thin-Film Photovoltaics • Their thinness allows photons that aren’t absorbed to pass completely through the photovoltaic material, which offers two special opportunities – Their semitransparency means that they can be deposited onto windows, making building glass a provider of both light and electricity – They also lend themselves to multiple-junction, tandem cells in which photons of different wavelengths are absorbed in different layers of the device Photovoltaic Materials and Electrical Characteristics–Thin-Film Photovoltaics • Currently, thin-film cells are not as efficient as crystalline silicon —especially when they are not used in tandem devices • While the likelihood of significant reductions in module costs are modest for conventional crystalline silicon, many opportunities remain to increase efficiency and dramatically reduce costs using thin-film technologies Photovoltaic Materials and Electrical Characteristics–Thin-Film Photovoltaics • Thin films currently account for 10% to 15% of global PV module sales • They are subdivided into three main families – amorphous (a-Si) and micromorph silicon (a-Si/μc-Si) – Cadmium-Telluride (CdTe) – Copper-Indium-Diselenide (CIS) and Copper-Indium-GalliumDiselenide (CIGS) Photovoltaic Materials and Electrical Characteristics–Thin-Film Photovoltaics