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Articles for Keyword "photovoltaic"

A Guide To Photovoltaic Panels

Posted on May 23, 2011

Information about PV panels, how to interpret manufactures’ data and how to select the correct mounting angle. This article is based around autonomous and semi-autonomous systems that use PV panels to charge a bank of lead-acid batteries.

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Part 1: Photovoltaic (PV) Cells

Posted on May 23, 2011

1.1 The I-V Curve Semiconductor solar cells convert sunlight into electricity using the photovoltaic effect. Incident light falls on the cells and creates mobile charged particles in the semiconductor which are then separated by the device structure to produce electrical current. The vast majority of solar cells are made from crystalline silicon. Single crystal cells are the most efficient however, cheaper multicrystaline cells are also popular. Even cheaper amorphous silicon cells are also available and used widely for small consumer products but rarely used for power systems. A single PV cell will produce between 1 and 1.5W at a voltage of 0.5 to 0.6V under standard test conditions. Standard test conditions are: an irradiance of 1kW/m2, standard reference AM1.5 spectrum[1], and a cell temperature of 25°C. A characteristic I-V curve is shown in figure 1. The important points are: Short Circuit Current (ISC) – This is the maximum current that the cell can provide and it occurs when the cells is short circuited. Unlike other small scale electricity generating systems PV cells are not harmed by being shorted out. Open circuit Current (VOC) – This is the maximum voltage that exist between the cells terminals and is obtained when there is no load connected across them. Maximum Power Point (PMax) – The point on the I-V curve at which maximum power is being produced by the cell. Note that since the graph is not a straight line, the power produced will vary depending on the operating voltage (figure 2); although the voltage at any point on the graph can still be calculated using P=IV. PMax occurs on the ‘knee’ of the I-V curve. In Practice PV cells do not operate under standard conditions. The two parameters that have the most bearing on their performance are temperature and irradiance. 1.2 The Affects Of Temperature Figure 3 shows the effects of temperature on the I-V curve of a PV cell. ISC increases slightly with temperature by about 6µA per °C for 1cm2 of cell, this is so small that it is normally ignored. However, a more significant effect is the temperature dependence of voltage which decreases with increasing temperature. Typically the voltage will decrease by 2.3mV per °C per cell. 1.3 The Affects Of Irradiance Solar irradiance is a measure of the sun’s energy, under standard conditions the amount of energy reaching the Earth’s surface on a clear day is taken to be 1kW/m2. The amount of irradiance reduces with the slightest amount of haze and becomes quite small on over cast days. ISC is directly proportional to the irradiance: so that if irradiance halves so does ISC. The voltage variation is very small and usually ignored The power produced under different conditions, as a function of voltage, is shown in figure 5. Figures 4 and 5 clearly show that the voltage at which PMax occurs does not vary much with irradiance. Irradiance values are normally given as an average per day, so that the average global irradiance may be 4.5kW/m2...

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Part 2: PV Panels and Manufactures’ Data

Posted on May 23, 2011

2.1 Quoted Values PV cells are connected in series to produce PC panels. These usually contain 36 or 72 cells to match 12 and 24V systems respectively. 36 cells in series will produce a panel rated at about 75W and 72 cells will produce a panel that is rated at about 160W. Panels must be able to produce a voltage higher than that of the battery bank (the nominal system voltage) otherwise the batteries will not charge, panels for a 12V system normally have VOC in the region of about 17V. Values of ISC for panels will vary from make to make but will be approximately the same for a single cell, 36 cells or 72 cells. The I-V curve for a panel therefore looks the same as that for a single cell, only the voltages are larger (figure 6). PMax is the preferred point of operation however, if the temperature is too high this may not be possible. If a voltage below PMax, in the linear section of the I-V curve (figure 6), is acceptable the effect of temperature can be eliminated and the output current is dependent only on irradiance. Some modern charge controllers have maximum power point tracking that will alter the voltage across the panel to find the maximum power output for any given conditions. Other charge controllers rely on a charging voltage being set manually (e.g. 15V for a 12V battery bank) and you will have to take whatever current is available at that voltage. Manufactures provide data for ISC, VOC and PMax, also the characteristic I-V curve can usually be obtained. These figures are quoted for standard conditions: and irradiance of 1kW/m2, spectral distribution of AM1.5 and a cell temperature of 25°C. Panels are never used under perfect standard conditions and the manufactures’ data must be altered to find the true power output under relevant conditions. Figure 6 illustrates how a PV panel’s output changes with temperature and irradiance, this curve if for a typical panel from a 12V system. 2.2 Fine Tuning Manufactures’ Data 2.2.1 Voltage VOC must be calculated for the operating temperature (TC), for each cell it drops by about 2.3mV for each °C over 25°C. For a panel with n cells connected in series: Specification sheets may quote a value for the Temperature Coefficient of Voltage for particular makes, for example for a BP 585 panel it is -80±10mV per °C. Note that this is almost exactly the same as -2.3mV when multiplied by 36 cells. The voltage at the maximum power point (VM) does not vary much with irradiance and can be estimated as 80% of VOC under standard conditions. 2.2.2 Current ISC is directly proportional to irradiance (G). Therefore the short circuit current at the given irradiance (ISC(G)) is given by: ISC does not vary much with temperature and this effect is normally ignored. However, manufactures’ specification sheets often provide a Temperature Coefficient of ISC, for example this is 0.064±0.015% per °C for a BP 585 panel;...

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Part 3: Fixed Panel Angles

Posted on May 27, 2011

Most small PV systems have the panels fixed so that they do not track the sun across the sky throughout the day. For fixed panels the maximum power output can be achieved when their surfaces are perpendicular to the sun at solar noon. Note that due to the vagaries of national time keeping solar noon is unlikely to be 12pm, rather it it the point in time at which the sun is at its daily zenith. Panels should be fixed on the north-south axis since at all times of year, at solar noon, the sun will be directly on this line. If your system is in the northern hemisphere and above the Tropic of Cancer (i.e. has a latitude greater than +23.45°) your panels will always be inclined to face south because the sun’s daily zenith will always be in the southern skies. Similarly, if your system is in the southern hemisphere and below the Tropic of Capricorn (i.e. has a latitude less than -23.45) your panels will always be inclined to face north. Note that outside the tropics the sun is never directly overhead. If your system is in the tropics (i.e. has a latitude between -23.45° and +23.45°) matters are not so simple. At the equator the sun is directly overhead at solar noon on the equinoxes (21st-23rd March and 22nd-23rd September), but reaches its daily zenith in the northern skies from March to September and the southern skies from September to March. In the northern tropics (i.e. latitudes between 0° and +23.45°) as the latitude increases the sun follows a similar pattern, although it will be directly overhead on days that approach the summer solstice (21st-22nd June). If your site is on the Tropic of Cancer the sun’s daily zenith will always be in the southern skies and will be directly overhead on the summer solstice. The sun in the southern tropics (i.e. latitudes between 0° and -23.45°) is in the southern skies for some of the year between the autumnal equinox and the vernal equinox and will be directly overhead on dates approaching the winter solstice as the latitude decreases, until at the Tropic of Capricorn the sun is always in the northern skies and directly overhead on the winter solstice. The Arctic Circle is at 66.5° and the Antarctic Circle is at -66.5°, beyond these latitudes the sun will be completely absent for some of the year and ever present at other times. When the sun never sets it circles in the sky, never being directly over head. The above discussion illustrates that PV panels sited in the tropics will need to be inclined to face south for some of the year and north at other times. Figure 8 shows a diagram that makes choosing panel angles relatively simple. Find find latitude of your site on the y-axis on the right hand side, then follow the line that corresponds most closely to your site and using the numbered days on the x-axis read the...

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Part 4: PV Panel Arrays and Wiring

Posted on May 27, 2011

When the panel angles are connected together they are known as an array. The voltage of the array must be matched to the voltage of the battery bank (if one is being used). Typically the bulk charging of a 12V battery bank will be done at about 15V. It is clear from figures 10 and 11 that both the 85W and 160W panels of 15V will deliver about 5A at 25°C therefore paying for 160W panels would be a waist of money. When panels are connected together in parallel, shown in figure 12a, they will operate at the same voltage: if a parallel array of 85W panels are charging a twelve volt battery bank all of the panel will be operating at the charging voltage (i.e. about 15V). The current from each panel in a parallel array are added together so that two 85W panels in series will produce (2 x 5) 10A at 15V giving (10×15) 150W. Note that the panels have not been down rated from the manufacturer’s specifications. Panels connected in series, as depicted in figure 12b, work the other way round so that the voltage will be the sum of the voltages across both panels but the same current will flow through both panels. Therefore, two 85W panels in series can comfortably operate at 30V charging a 24V battery bank with 5A, once again the total power is (5×30) 150W. However, if these two panels in series were connected to a 12V battery bank they would operate at 15V and still produce about 5A, thus the power produced would only be (15×5) 75W. Panels can be combined in series and parallel to get the desired current at the battery bank voltage (figure 12c). If batteries are not being used and the panels are connected to a electricity supply grid through an inverter the panels are usually connected in a long series string. This has the advantage of keeping the current small and thus losses can be kept to a minimum and thinner, cheaper wires can be used. This is not possible with a battery system because the voltage across each panel is summed so that the operating of voltage of a 20 panel series array may be about 300V. A moderately sized 12V system will require about ten 85W panels in parallel, producing about 50A. This is quite a large current therefore quite thick wires are needed to connect the panels together and to convert the panels to a battery charger. Wires with a large diameter cause a smaller voltage drop and will not burnout when substantial currents are fed through them. Another consideration when wiring PV panels is that at night or when in deep shade the cells tend to draw current from the batteries rather than sending current to them, this effect obviously causes the batteries to lose charge. Most charge modern controllers contain diodes to prevent the flowing of a reverse current however, if the charge controller does not take account of...

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Photovoltaic Water Pumps

Posted on Oct 25, 2011

Short technical brief by the GTZ on photovoltaic pumping systems. In many developing countries, the inadequate supply of drinking and irrigation water is a severe problem. In rural areas with no access to grid power, national water authorities and private farmers have to rely on hand pumps and diesel-driven pumps, many of which are out of service due to technical defects or a lack of fuel. As a rule, hand-operated pumps are the least-cost option for low consumption rates and low pumping heads. If hand pumps cannot satisfy the demand, diesel driven pumps are commonly used for drinking and irrigation water supply. These pumps stand in competition with photovoltaic water pumps (PVP), which present themselves as a reliable and environmentally-sound alternative means of water delivery. Read the whole of this document as a PDF file (144...

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