Numerical and Experimental ......(9)

A PV module consists of solar (PV) cells permanently wired in series and/or parallel to form a single power unit, and is often packed for protection from the environment while permitting
$z7H#@,yT u VH0sunlight to reach the active surfaces of PV cells. When solar radiation is incident on a semiconductor, the photons may transmit their energy to the valence electrons. Each time a photon breaks a bond an electron becomes free to move through the lattice. The absent electron leaves behind a vacancy (hole) that can also ‘move’ through the lattice as the electrons move around it. These holes can be considered like particle (positive). The movement of electrons and holes, if passing through an external circuit, can be just like any other current source.绿色建筑博客lv*i@g

A solar cell is made from a semiconductor material such as silicon or gallium arsenide which sometimes acts as conductor, other times as insulator. When solar radiation is absorbed it 8 causes electrons to break loose from atoms, leaving atoms with a positive charge (hole). Under certain conditions the electrons and holes move in opposite directions, creating a current. In a solar cell the free electrons and hole move randomly and they eventually may recombine, or reach regions of semiconductor containing different type of impurity, called p-n junctions (p indicates conduction by holes and n by electrons). When they reach the junction, they will move through an external circuit connected at the p-n junction, creating a current (Athienitis and Santamouris 2002).绿色建筑博客Z
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When a load is connected to an illuminated solar cell the current that flows is equal to the photogenerated current IL due to generation of carriers by the sunlight minus the diode or dark绿色建筑博客 W$TE.qz3_e
current ID due to recombination of carriers driven by the external voltage:绿色建筑博客;C;K||Q,l9qO

The current I is also often defined using short-circuit current ISC and open circuit voltage VOC. This definition allows us to write the characteristic I-V curve of the cell in the following form:绿色建筑博客$s l,Ol
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where,绿色建筑博客Xu)m#C1[%KFc#V z#w
ISC is the short circuit current绿色建筑博客E*}rB*h*y
VOC is the open circuit voltage
e'ul0B} A_R0e is the charge of an electron (1.602*10-19 C)
n8~M.R k&p^0m is the ideality factor (1<m<2)
Sjp-] e0K is the Boltzman constant (1.381*10-23 J/K)绿色建筑博客7kvm&?!G.oNhY!M8O0S
T is the temperature of the cell绿色建筑博客QB v*l2P\Z^%|

From this formulation 2.2 or the I-V characteristic curve we can find the maximum-power point as indicated at point A in Figure 2.1. For usual practical solar cells, the maximum-power point is close to the elbow of the characteristic curve.绿色建筑博客!CVMj*V\;~w

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Figure 2.1 I-V characteristic curve of PV cell (Lorenzo 1994)绿色建筑博客c'PJ4|!GT0^{