English English.   Español  Español.

The application of Appropriate Technology

Articles for Keyword "I-V curve"

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...

Read More

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...

Read More