How does PV work?
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Theory - figure 1. Radiation from the sun ('sunlight') hits a photovoltaic cell. These cells are made up of two layers (see Figure 2). Depending on the intensity of the sunlight a voltage difference builds up between the upper and lower layer of the cell. This difference is in the order of magnitude of 0.5 V. If you short-circuit the upper and lower layer a current runs of about 3 Amps. If you arrange sufficient cells in series, the result is a PV module or PV panel. Let's say 36 cells in series produce 36 x 0.5V = 18V at 3 Amps = 54Watts Sunlight, especailly in the UK and in the Low Countries, arrives in packets in between between clouds. No problem: the voltage and the amps go up and down a bit, just like a lamp with someone playing with the dimmer.
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Theory
- figure 2.
A funny thing is that sunlight can be considered as light waves (wave theory) or as packets of energy (photon theory). The explanation of the photovotaic effect is easiest when you think of light as energy packets: photons. A solar cell is made up of two layers of semiconducting material: P and N. The boundary between P and N acts as a diode: electrons can move from N to P but not the other way. Photons with sufficient energy hitting the cell cause electrons (-) to move from the P layer into the N layer. An excess of electrons build up in the N-layer while the P layer builds up a shortage. This is the voltage difference that can be used as a power source. As long as the panel is hit by sunshine, the voltage difference is maintained. Isn't it incredible? The photovoltaic principle was discovered in 1839 by Edmund Becquerel in France. Solar panels were used for the first time in 1958 to generate electrical power (0.1 W) for the US Vanguard spacecraft. In 2005, a total of over 1.000.000.000 Wp (1 GWp) is reported to be installed world wide. |
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The amount of power that a photovoltaic produces depends principally on two factors:
Efficiencies of 15% are common amongst commercial photovoltaic cells. This means that of the energy in the light falling onto the cell only 15% is converted into useful electricity. For this reason, photovoltaics are most effective in areas with a large amount of daily sunlight. However, as you can see on the page solar maps, even in the UK there is sufficient daily sunlight to exploit this ‘free’ resource with success.The UK receives on average more hours of sun than Holland! Yet, in Holland like in Germany and Austria there are many more homes with PV panels on their roofs than in the UK. Why? Are the people of the Continent perhaps fools, are they irresponsible green idealists, or are they rationally thinking people? |
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Theory
- figure 3 .
How to squeeze the most out of a solar cell? The diagram on the left is named the MPP-diagram (maximum power performance) of a photovoltaic cell. The red curve is the voltage-performance graph of the cell, the green curve is the current-voltage graph. The best performance (in Watts) is obtained at that voltage at which the current definitely starts to decline: the maximum power point (MPP). Note that the MPP is temperature-dependent. Notice that on a cold day or in a cold environment (outer space!) a photovoltaic cell performs better than on a hot day (0.3% increase per 1 degree C drop in temperature). |
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In practice - Enough with theory! Commercially available photovoltaic panels consist of a number of solar cells. The image on the left shows the front and back of a photovoltaic panel, in this case a Kyocera KC-50. Mounted on the back is the junction box (encircled). If you open this box you will notice the connection screws with in between bypass diodes that protect the solar cells against damage when the panel is only partially illuminated by sunshine. I have sketched four you the DC cables (red, blue) that connect the panel with the DC side of the inverter. This rather expensive electronic device is mounted inside the home to protect it from cold, heat and moist. At the end of the 230V cord from the inverter is a plug that plugs into a wall outlet. Plug in and production can begin. That's all there is. Bloody simple and effective.
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| There exist so many manufacturers of inverters and types of inverter that I have made a separate page dealing with this useful electronic stuff: the page 'the inverter' | |
