Solar photovoltaic energy
Practical Action
China, India and other developing countries are emerging as major solar PV manufacturers.
The dominant application for PV in developing countries is the solar home system (SHS). This
involves the installation of PV systems of 30 to 50 peak watts (Wp), costing about $300 to $500
(U.S.) each, in individual homes, mainly in rural areas.
Apart from SHS, other applications of PV in developing countries include 1) PV-powered remote
telecommunications equipment; 2) rural health clinic refrigerators; 3) rural water pumping; solar
lanterns and 4) PV battery-charging programmes, which allow rural residents to purchase or rent
batteries to provide electricity to their homes, and then recharge them at PV-powered charging
stations. A few attempts have been made to establish PV-powered village power grids in developing
countries, such as in Sagar ‘Solar Island’ off the cost of India (see later).
Cost of solar PV
The process of producing efficient solar cells is costly due to the use of expensive pure silicon and
the energy consumed, and cost has been the major barrier to the widespread uptake of PV
technology. As materials technology improves, costs are slowly dropping, making PV technology more
attractive. Since 1976, costs have dropped about 20% for each doubling of installed PV capacity, or
about 5%/year. Module prices have fallen from $30/Wp in 1975 to less than $1/Wp in 2012. Costs
rose slightly in 2004 due to high demand (which outpaced supply) and the rising cost of silicon. The
expectation is that the cost of PV will continue to come down as mass production increases and
technologies evolve.
Note: Cost of PV modules is usually given in terms of Peak Watt (Wp), which is the power rating of
the panel at peak conditions - that is at 1kWm-2 irradiance at 25ºC.
Technical issues
The nature and availability of solar radiation is described in the technical brief Solar Thermal Energy.
Once the solar energy has arrived reaches the surface of a photovoltaic cell, the electrons become
energised in proportion to the intensity and spectral distribution (wavelength distribution) of the
light. When their energy level exceeds a certain point a potential difference is established across the
cell. This is then capable of driving a current through an external load, such as a light or radio.
PV modules and arrays
When light falls on the active surface, the electrons in a solar cell become energised, in proportion to
the intensity and spectral distribution (wavelength distribution) of the light. When their energy level
exceeds a certain point a potential difference is established across the cell. This is then capable of
driving a current through an external load.
All modern, commercial PV devices use silicon as the base material, mainly as mono- crystalline or
multi-crystalline cells, but more recently also in amorphous form. Other materials such as copper
indium diselenide and cadmium telluride are being developed with the aim of reducing costs and
improving efficiencies. A mono-crystalline silicon cell is made from a thin wafer of a high purity
silicon crystal, doped with a minute quantity of boron. Phosphorus is diffused into the active surface
of the wafer. At the front electrical contact is made by a metallic grid; at the back contact usually
covers the whole surface. An anti-reflective coating is applied to the front surface. Typical cell size
is about 15cm diameter.
The modules in a PV array are usually first connected in series to obtain the desired voltage; the
individual strings are then connected in parallel to allow the system to produce more current. The
modules are then protected by encapsulation between glass and a tough metal, plastic or fibreglass
back. This is held together by a stainless steel or aluminium frame to form a module. These
modules, usually comprised of about 30 PV cells, form the basic building block of a solar array.
Modules may be connected in series or parallel to increase the voltage and current, and thus achieve
the required solar array characteristics that will match the load. Typical module size is 50Wp and
produces direct current electricity at 12 V (for battery charging, for example).
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