Solar photovoltaic energy
Practical Action
Commercially available modules fall into three types based on the solar cells used.
Mono-crystalline cell modules: the highest cell efficiencies of around 15% -18% are
obtained with these modules. The cells are cut from a mono-crystalline silicon crystal.
Multi-crystalline cell modules. The cell manufacturing process is lower in cost but cell
efficiencies of only around 15% are achieved. A multi-crystalline cell is cut from a cast
ingot of multi-crystalline silicon and is generally square in shape.
Amorphous silicon modules. These are made from thin films of amorphous silicon where
efficiency is much lower (10% - 12%) but the process uses less material. The potential for
cost reduction is greatest for this type and much work has been carried out in recent years to
develop amorphous silicon technology. Unlike mono-crystalline and multi-crystalline cells,
with amorphous silicon there is some degradation of power over time.
An array can vary from one or two modules with an output of 10W or less, to a vast bank of several
kilowatts or even megawatts.
Flat plate arrays fixed at a tilted angle and facing towards the equator, are most common.
The angle of tilt should be approximately equal to the angle of latitude for the site. A
steeper angle increases the output in winter; a shallower angle - more output in summer. It
should be at least 10 degrees to allow for rain runoff.
Tracking arrays follow the path of the sun during the day and thus theoretically capture more
sun. However, the increased complexity and cost of the equipment rarely makes it
worthwhile.
Mobile (portable) arrays can be of use if the equipment is required in different locations
such as with some lighting systems or small irrigation pumping systems.
Solar PV systems
While in industrialised countries there has been a rapid increase in grid connected PV systems, in
developing countries the majority of PV systems are stand-alone off-grid systems. The off-grid
systems can be used to drive a load directly; water pumping is a good example. Water is pumped
during the hours of sunlight and stored for later use; or a battery can be used to store power for use
for lighting during the evening. If a battery charging system is used. Electronic control apparatus will
be needed to monitor the system. All the components other than the PV module are referred to as
the balance-of-system (BOS) components. The figure below shows a typical configurations for an off-
grid PV system. Such systems can often be bought as kits and installed by semi-skilled labour.
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