English .   Español  .

The application of Appropriate Technology

# Articles for Keyword "solar power calculations"

## 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;...

## Part 3: Fixed Panel Angles

Posted on May 27, 2011

## 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...