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Power Electronics in Solar Energy Systems Part II I - How does a solar / photovoltaic cell work? - Photovoltaic cells are made of various semiconductor materials which become electrically conductive when supplied by heat or light. The majority of solar cells produced are made out of Silicon (Si) which exist in sufficient quantities and do not add any burden on the environment. - Doping technique is used to obtain a surplus of positive charge carriers (p-type) or a surplus of negative carriers (n-type). When two layers of different doping are in contact, then a p-n junction js formed on the boundary. Figure 1 – Doping Process - An electric field is built up which then causes the separation of charge carriers released by light. Light is composed of small packets called photons. When these photons bombard our cell, many electrons are freed within the electric field proximity, which then pull the electrons from the p-side to n-side. Through metal contacts, an electric charge can be tapped. If the outer circuit is closed, then direct current flows Figure 2 – P-N juntion Figure 3 – Solar cell Cross section Figure 4 – Solar cell with connected load - A solar cell is approximately 10 x 10 cm, protected by transparent antireflection film. Each of these cells produces around 0.5 V (for Silicon). The voltage across a solar cell is primarily dependent on the design and materials of the cell, while the electrical current depends primarily on the incident solar irradiance and the cell area. (Solar Irradiation is the process by which an object is exposed to the sun’s radiation.) - The efficiency of operation of a solar cell is determined by the electrical power output divided by the power provided by the light source. That is η = [Po(electrical) / Po(light)] x 100 - Over 95% of solar cells have efficiencies of about 17%. However solar cells, having power conversion efficiencies as high as 31% have been developed over the last decade in laboratory environment. - In order to obtain the appropriate voltages and outputs for different applications, single solar cells are interconnected in series (for larger voltage) and in parallel (for larger current) to form the photovoltaic module. Then several of these modules are connected to each other to form the photovoltaic array. Figure 5 – Solar Cell, module, Panel and Array II – PV cell characteristics - The I-V (current-voltage) characteristics curves of solar cells are nonlinear and follow the general shape and equation shown below. Where: I – PV cell current Isc – PV cell short circuit current (V = 0) Voc – open circuit voltage A and B – constants e – Euler’s constant (2.7128…) Im - maximum PV cell current Vm - maximum PV cell voltage IMPP – current at maximum power point VMPP – voltage at maximum power point Solar Insolation – is the solar irradiance measured at a given location on earth with a surface element perpendicular to the sun’s rays, excluding diffuse insolation (the solar radiation that is scattered or reflected by atmospheric components in the sky). Insolation has the unit Watts per square meter (W/m2) or Kilowatt per square meter (kW/m2). Figure 6 – PV cell I-V Characteristics Figure 7 – PV cell P-V Characteristics - Photovoltaic arrays are usually mounted in a fixed position and tilted toward the south to optimize the noontime and daily energy production. The orientation of fixed panels should be carefully chosen to capture the maximum energy for the season or year. Photovoltaic arrays have an optimum operating point called the MAXIMUM POWER POINT (MPP) as shown on the graphs of figures 6 and 7. - Figure 7 shows that power increases as the voltage is increased, reaching a peak value before decreasing as the resistance increases to the point where current drops off. According to the maximum power transfer theory, this point is where the load is matched to the solar panel’s resistance at a certain level of temperature and insolation. As shown in figure 8, the I-V curve changes as the temperature and insolation levels change, thus the MPP will vary accordingly. - It has been shown that the open circuit voltage increases logarithmically while the short circuit current increases linearly as the insolation level increases. Moreover, increasing the cell’s temperature decreases the open circuit voltage and increases slightly the short circuit current. This then makes the cell less efficient. - Since solar power is relatively expensive, it is important to operate panels at their maximum power conditions. Thus, PV system will operate more efficiently with systems that adjust automatically their loads to match the PV resistance. In addition, panels would change orientation to track the sun. We need then to control either the operating voltage or current to get maximum power from the PV panel at the prevailing temperature and insolation conditions. Figure 8 – PV panel insolation and temperature characteristics