Solar Energy
The energy from the sun can be exploited directly in the form of heat or first converted
into electrical energy and then utilized. Accordingly the solar energy is classified into
solar thermal and solar photovoltaics (PV).
Solar thermal has numerous applications like water heating, drying vegetables and
agricultural products, cooking etc. In Nepal the solar water heaters are being extensively
used in urban areas. The applications of solar dryers and cookers have found moderate
use simply because of the low level of dissemination of these technologies.
The solar PV, on the other hand, is extensively used not only in the developing countries
but also in highly developed countries. The application of solar PV is virtually unlimited.
Countries like Germany, Japan and United States of America have initiated highly
subsidized rooftop programs for solar PV. The level of subsidy is up to 65% of the total
system cost. In Nepal solar PV is extensively used for communications, home lighting,
drinking water pumping etc. The installed capacity of Solar PV in Nepal now exceeds 3.4
MWp mark and over 93,000 households are electrified using this technology.
Considering the positive impact that solar PV can bring to the rural population of the
developing countries like Nepal, the Government of Kingdom of Denmark has supported
Energy Sector Assistance Program (ESAP) to promote alternative energy sources,
including PV. ESAP target was to subsidize installation of 25,000 Solar Home Systems
within a time span of 5 years. Similarly, a sizeable project with assistance from European
Union (EU) is being implemented to promote institutional Solar PV in Nepal.
The solar PV can be considered the only form of electricity that can be generated any
time and anywhere provided sunshine is available. The earth receives more energy from
the sun in just one hour than the world uses in a whole year. The annual total amount of
solar energy incident on the surface of the earth is estimated to be about 795 x 1012 MWh,
which is 8300 times greater than the global energy demand in 1991. The Environmental
savings from the Photovoltaic modules are highlighted in table 3.1 below:
Table 3.1 Environmental Savings from Photovoltaic Modules
Description Savings of one 50Wp module *
Electricity saved per year 90 kWh
Electricity saved per life of PV module 2700 kWh
Barrels of oil saved over lifetime of PV module 4.8 barrels
Pounds of coal saved over lifetime of PV module 2700 lbs
Carbon Di-oxide kept out of the air over life of PV 4000 lbs
Sulfur Di-oxide kept out of air over life of PV 23.3 lbs
* Based on:
Coal required to produce 1 kWh = 1 lb
Carbon Di-oxide emission = 1.5 lb/kWh
Photovoltaic (PV) Technology
Photovoltaic (PV) Technology is a process of generating electrical energy from the
energy of solar radiation. The principle of conversion of solar energy into electrical
energy is based on the effect called photovoltaic effect. The smallest part of the device
that converts solar energy into electrical energy is called solar cell. Solar cells are in fact
large area semiconductor diodes, which are made by combining silicon material with
different impurities. The sand, a base material for semiconductor, is the most abundantly
available raw material in the world. The ordinary sand (SiO2) is the raw form of silicone.
The solar energy can be considered as a bunch of light particles called photons. At
incidence of photon stream onto solar cell the electrons are released and become free. The
newly freed electrons with higher energy level become source of electrical energy. Once
these electrons pass through the load, they release the additional energy gained during
collision and fall into their original atomic position ready for next cycle of electricity
generation. This process of releasing free electrons (generation) and then falling into
original atomic position (recombination) is a continuous process as long as there is the
stream of photons (solar energy) falling onto the solar cell surface.
History of Development of PV Technology
The birth of PV technology dates back to 1839 AD when Edmund Becquerel, the French
experimental physicist, discovered the photovoltaic effect while experimenting with an
electrolytic cell made up of two metal electrodes placed in an electricity conducting
solution—generation increased when exposed to light.
In 1876 William Adams and R. Day discovered that the junction of selenium and
platinum also exhibit photovoltaic effect. This discovery led the foundation for the first
selenium solar cell construction in 1877.
The photovoltaic effect remained theoretically unexplained until the great scientist Albert
Einstein described this phenomenon in 1904 along with a paper on his theory of
relativity. For his theoretical explanation of photo-electric effect, Albert Einstein was
awarded a Nobel Prize in 1921.
Another breakthrough in development of PV technology was the discovery of the method
for monocrystalline silicon production by Polish scientist Czohralski in 1918. This
discovery enabled monocrystalline silicon solar cells production. The first silicon
monocrystalline solar cell was constructed only in 1941.
In May 1954 The Bell Laboratories of USA (Researchers D. Chapin, C. Fuller and G.
Pearson) published the results of discovery of 4.5% efficient silicon solar cells.
First commercial photovoltaic product with 2% efficiency was introduced in 1955 by
Hoffman Electronics-Semiconductor Division. The cost of a 14 milli Watt peak power
solar cell was US$ 25 (or US$ 1,785 per Watt). The efficiency of commercially available
solar cell increased to 9% in 1958.
The first PV powered artificial satellite of the earth, Vanguard I, with 0.1 W of solar cell
occupying an area of approximately 100 cm2
and powering a 5 mW back-up transmitter
was launched in 17 March 1958. Three more PV powered satellites were launched in the
same year. The first PV powered telephone repeater also was built in Americus, Georgia,
USA in the same year.
Sharp Corporation was the first company to develop the first usable PV module (group of
solar cells put together in a single module) in 1963.
By 1974 the cost of PV power came down to US$ 30 per watt from US$1785 per watt in
1955. With the dramatic reduction in the cost, the PV power once affordable only in
space vehicle became an alternative source of electrical energy for terrestrial applications.
The fig. 3.1 below illustrates the decrease in price (US$ per peak watt) of solar PV with
time.
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