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Articles for Keyword "solar panels"

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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Solar Panel Angles for Various Latitudes

Posted on May 28, 2011

Choose the line that refers to your latitude from the right-hand y-axis. Read off the angle from the left-hand y- axis that should be used for each month of the year (listed on the horizontal axis). Negative angles indicate that the panel is inclined to face north. You could reset your panel angle every week however setting it four times a year will give good results. Firstly in November set a winter angle, then in February set an equinox angle until April when a summer angle is set and finally reset the equinox angle in August. The correct angles for the beginning and end of each period should be bisected to find the average angle for that period: for a latitude of 10° the winter angle is 28°, the summer angle is -7° and the equinox angle is about 10°. A rule of thumb is that the equinox angle will be about equal to your latitude, the summer angle will be about 15° less and the winter angle will be about 15° greater. You may wish to change the panel angle every month around the equinoxes since the recommended angle is changing rapidly from week to week at these times of year. Note: In the northen hemisphere your panel will normally be facing south (indicated by positive angles). If your site is in the northern hemisphere and within the tropics, negative angles indicate that the panel is facing north. The tropics is the area of the globe on either side of the equator, between latitudes of 23.45 degrees and -23.45...

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Solar Pump Calculation Sheet

Posted on Oct 25, 2011

PDF calculation sheet for designing solar pumping systems. Download this sheet as a PDF file (92 kb)

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