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Solar Panel - The different technologies

Solar Panel - The different technologies

There are three common types of solar cells: monocrystalline, polycrystalline and thin film.

Both mono and polycrystalline cells are made from wafers cut from blocks of silicon, which are then modified by a process known as ‘doping'. This involves heating the cells in the presence of boron and phosphorus, which changes the structure of the silicon in such a way as to make it a semiconductor. This is the same method which is used to make computer chips.

Once the wafers have been doped, they then have a fine array of electrically conductive current-collecting wires applied to each side of them.

Thin film technology uses a differenttechnique, and involves the deposition of layers of different materials directly onto metal or glass. The most common thin-film panels are the amorphous silicon type, which are found everywhere from watches and calculators right through to large mains-grid connected PV arrays.

Flexible panels are a spinoff of amorphous technology. These are manufactured on a plastic or thin metal substrate and can be rolled up or attached to curved surfaces. They are commonly used for camping and boating, but are generally quite expensive on a dollarper watt basis, although larger ones designed for mounting on buildings are competitive with conventional rigid panels.

As far as material use is concerned, crystalline panels use a great deal more semiconductor material than an equivalent output thin film panel. This is because a lot of material is lost in the process of cutting the silicon boule or billet into slices (wafers). The cutting is done with a diamond saw, which may well have a blade thicker than the resulting wafers, so more than half of the silicon may be lost in this process.

Amorphous panels don't have this problem and so may use less than 1% of the semiconductor material as a crystalline panel. An example is the Kaneka thin film modules. These have an active material thickness of just 0.3 micrometres.

Compared to a typical crystalline cell thickness of 100 to 200mM, this is as little as 1/600th of the silicon, and that doesn't take into account the silicon wasted by the cutting process for crystalline cells.

Why is silicon use such an issue?

There are two reasons. The first is the embodied energy of the silicon—it takes a lot of energy to make the highlypurified silicon used in solar panels.

The second is the fact that high-grade silicon suitable for this sort of use is often in short supply due to the demand for it in both solar cells and integrated circuits, which keeps the price higher than it should be.

The miniscule amount of silicon used in thin film panels should allow them to be more cost effective, and you have to wonder why this isn't the case at the moment, although the high demand for solar panels in general most likely has a lot to do with it!

Panel ratings

There are a number of different ratings on solar panels, so let's have a look at what they are and what they mean. Rated (peak) power:  This is the maximum sustained power output of the panel, assuming a level of insolation (strength of light falling on the panel) of one kilowatt per square metre. In general, the solar panel's rating is the rated peak power.

Nominal voltage (Vn): The system voltage that the panel is designed to be used in. A 12 volt panel is designed for a 12 volt system, but will produce voltages well above 12 volts. Some panels can be rewired to suit six or 24 volt systems.

Other panels are designed for grid-interactive systems, and have nominal outputs of 48 volts or even higher.

Voltage at peak power (Vp): This is the voltage measured across the panel when the panel is producing peak power.

Current at maximum power (Im): The maximum current available from the panel at peak power.

Open circuit voltage (Voc): The maximum voltage available from the panel with no load attached. This is usually around 21 volts for a 36 cell, 12 volt unit. Short circuit current (Isc): The current obtained when the output of the panel is short circuited with an insolation level of 1000 watts per square metre at a panel temperature of 25°C.

Temperature at rated power:  This is the temperature that the solar panel manufacturer rates their panels at. Most panels are rated to put out their maximum power at 25°C, which is a rather unrealistic figure given that the panel temperature under typical Australian conditions can be up to 70°C. Figure 1 shows how cell temperature affects power output for crystalline panels.

Current-voltage (IV) curves: These are graphs of output voltage versus current for different levels of insolation and temperature. They can tell you a lot about a panel's ability to cope with temperature increases, as well as performance on overcast days. Examples of IV curves can be seen in Figure 1.

Obviously, the most important ratings when doing calculations for a power system are the voltage and current at maximum power. A system is rarely calculated using panel wattage ratings, as this is a function of both the voltage and current. Some panels are rated at slightly higher or lower voltages than others, and this affects the amount of current available. The open circuit voltage and short circuit current ratings are important from a safety point of view, especially the voltage rating. An array of six panels in series, while having a nominal 72 volt rating, can output over 120 volts DC— more than enough to be dangerous.

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