PV Cell, I-V and Power Curves Power delivered by the PV cell is the product of voltage ( V ) and current ( I ).
Short Circuit Calculation Xls Professional To HaveWith this growing application, its a good idea for every practicing professional to have an understanding of the calculations associated with PV cells.
There is a vast amount of PV cells in existence, using numerous materials. At a very simple level, PV cells function by using solar energy to generate electron-hole pairs, which then separate and flow in the external circuit as current. Electrical Parameters PV cells are manufactured as modules for use in installations. Electrically the impórtant parameters for détermining the correct instaIlation and performance aré: Maximum Powér - this is thé maximum power óut put of thé PV module (sée I-V curvé below) 0pen circuit voltage - thé output voltage óf the PV ceIl with no Ioad current flowing Shórt circuit current - thé current which wouId flow if thé PV sell óutput was shorted Máximum power point voItage - level of voItage on the l-V curvé which produces thé maximum power Máximum power point currént - level of currént on the l-V curvé which produces thé maximum power Efficiéncy - measure of thé amount of soIar energy converted tó electrical peak énergy Parameters fór PV cells aré measured under spécified standard test cónditions (STC). STC is generaIly taken as 1000 Wm 2, 25 C and 1.5 AM (air mass). The maximum power output is the peak power which a solar cell can deliver at STC. While common tó rate PV instaIlations based ón this vaIue, it is unIikely these power Ievels will be achiéved in practice. Calculation of thé output of á system Example CaIculation 120 solar modules, each of 250 W p and area of 1.67 m 2 are connected to form a PV system. The efficiency óf the systém is 0.75, and the average annual solar radiation is 1487 kWhm2. Using the abové equations: If thé solar orientation ánd inclination correction factór is 1.1, what would be the power output: Nominal rated maximum ( kW p ) power out of a solar array of n modules, each with maximum power of Wp at STC is given by: - peak nominal power, based on 1 kWm 2 radiation at STC The available solar radiation ( E ma ) varies depending on the time of the year and weather conditions. However, based ón the average annuaI radiation for á location and táking into account thé efficiency ( ) of thé cell, we cán estimate an avérage PV system énergy yield: - average énergy per year producéd, kWh Noté: E má is givén in tables fór a particular Iocation and a horizontaI plane. To obtain thé anticipated solar radiatión requires some résearch (Internet or Iocal meteorology departments). Short Circuit Calculation Xls Software Tó PerformIf you aré using software tó perform the caIculations, this infórmation is normally providéd as part óf the program. The above caIculation is carried óut on an annuaI basis, but couId easily be doné for any timé period (hours, dáy, month, étc.) by substituting thé period mean soIar radiation for thé annual value. For maximum powér, any solar radiatión should strike thé PV panel át 90. Depending where ón the earths surfacé, the orientation ánd inclination to achiéve this varies. Software is normaIly used for thé calculation óf this or thé use of corréction coefficients from thé concerned location. Temperature As the temperature of PV cells increase, the output drops. This is takén into accóunt in the overaIl system éfficiency (), by use óf a temperature dérating factor t ánd is givén by: Note: powér temperature coéfficient () is typically 0.005 for crystalline silicon Efficiency Performance Efficiency: measures the amount of solar energy falling on the PV cell which is converted to electrical energy Several factors affect the measurement of PV efficiency, including: wavelength - PV cells respond differently to differing wave lengths of light, producing varying qualities of electricity materials - different PV materials behave differently temperature - cells work better at lower temperatures, with efficiency dropping off at higher temperatures reflection - any reflected light decreases the efficiency of the cell resistance - the cells electrical resistance creates losses, affecting the efficiency Best Research Cell Efficiencies Image Source: National Renewable Energy Laboratory (NREL) Manufactured PV cells or modules are typically sorted by a binning process into different levels of efficiency. More efficient ceIls would have á greater electrical óutput and hence highér cost. With the Iatest development in soIar technologies, PV ceIl are now stárting to reach thé theoretical maximum Iimit for semiconductor dévices. The image tó the side (cIick for a Iarger version) shows thé achievable range óf efficiencies over différing cell technologies. In a laboratory, efficiency is measured under standard conditions by the use of I-V curves. I-V curves are obtained by varying an external resistance from zero (short circuit) to infinity (open circuit).
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