太阳能光伏义Lecture note 8.doc

PHOTOVOLTAIC TECHNOLOGYShunt resistanceDefine:rSH = RSH/ RCH normalized shunt resistance Then FFSH FFO1-1/rSH Or more accurately by FFSH FFO 1-(Voc +0.7)* FFo/ (Voc*rSH ) valid for rSH 0.4WhereFFSH is the FF with RSH present (no RS) andFF0 is the FF with RS = 0 and RSH infinite In the presence of both Rs and RSH the FF can still be estimated using the above equation for FFSH, but replacing FF0 with FFS. Effect of Parasitic Resistances: IV equation Considering both Rs and RSH the IV equation becomes:Cell Properties and Design Efficiency and Cost The energy conversion efficiency, , of a solar cell is defined as: maximum power output of a cell / input from the sun, typical 1 kW/m2 = PM/Pin Single c-Si world record is 25% ( Aug 08 ) Industry cell efficiencies are typically lower and in the range of 12- 17%. Many laboratory techniques are not suited to industrial production (too expensive, too slow). Typically lower efficiency indicates lower production costs. simplified method to calculate the cost of electricity : COE = Cm + Cb (1+i) FCR / (S) + 0.016COE = cost of electricity in US $/kW-hCm = PV module cost per unit areaCb = balance of system cost (batteries, inverters, control system, etc.) per unit areai = indirect capital cost factorFCR = fixed charge rateS = annual incident energy = system efficiency modules, including losses due to heating, wiring and mismatch0.016 = power conditioning, operating and maintenance cost (US $/kW-h)Factor affecting efficiency optical losses sources of optical loss in a solar cell:1. shading by top contact;2. surface reflection;3. Reflection form rear contact.Ways to reduce these losses:1. Minimised the top contact coverage .2. Reduce surface reflection by using “antireflection coatings” . “Quarter wavelength” ARC is ideal ( thickness d1 and refractive index n1 such that: d1 = 0/4n1.) ARC is designed for a wavelength 0 = 0.6 m (i.e. close to the peak of the solar spectrum). the optical path difference between these beams due to the ARC introduces a 180 phase difference for = 0. This effect will reduce the reflection particularly for wavelengths close to the desired wavelength 0. If the refractive index of the ARC satisfies the following relation:1 = ( o 2) reflection at 0 will be zero! 3. Surface texturing of the silicon cell to minimize reflection. If the roughened surface has features which are at a “ steep” enough angle, light can be reflected 2 or more times, reduce the overall reflection. For the beam shown if each reflection at the Si surface has a reflectivity of R then the total reflectivity for that beam is R2.The surface of crystalline silicon (of particular crystallographic orientation (100), can be textured chemically. The resulting structure is a surface made up of randomly distributed, upright, square based pyramids.Pimple effect texturing, for polycrystalline cellAn additional benefit of surface texturing is better chance of absorption for weakly absorbed light. This occurs in accordance with Snells law.Where 1 and 2 are the angles as defines in the diagram and n1 and n2 are the refractive indices of the materials on either side of the interface (e.g. air/silicon or glass/silicon).4. Highly reflective rear cell surface reduces absorption of light in the rear cell metal contact. This reflected light then has the chance of being absorbed in the cell.The best option for a rear reflector is one that randomizes the direction of the scattered light. This not only increases the pathlength of some beams, but will cause some of the light to be totally internally reflected at the top surface light trapping.Light trapping is particularity important as silicon wafer thickness are decreasing (from 300m to 150 m) and for thin film solar cells (thicknesses 1- 10 m).Recombination losses The efficiency of a solar cell is also reduced by the recombination of minority carriers before they can be usefully collected by the p-n junction. Recombination can occur via several mechanisms.i) Radiative recombination: the inverse of absorption. An electron in the CB recombines with a hole in the VB and the excess energy is released as a photon .ii) Auger recombination: an electron in the CB recombines with a hole in the VB but the excess energy is given to another electron in the CB (the two electrons collide).Auger recombination is particularly effective in relatively highly doped material (dominant mechanism for Si material doped 1017cm-3).iii) Recombination through traps: defects in the semiconductor material by unwanted impurities (e.g. metals) or crystallographic defects (e.g. grain boundaries, surface