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[SANET-MG] solar power for the masses
ISIS Press Release 17/01/06
Solar Power Comes of Age
Solar Power for the Masses
Solar power is poised to enter the mainstream energy market with novel
materials that boost energy conversion efficiency and bring down
manufacturing costs. Dr. Mae-Wan Ho
A fully referenced illustrated version of this article is posted on ISIS
members’ website. Details here
Electricity from sunlight
The ability of sunlight to generate electricity was first discovered by
French physicist Andre-Edmond Becquerel in 1839, when he observed that
shining light on certain materials produced an electric current. But it
took just over a hundred years to 1941 before Russell Ohl in the United
States invented a silicon solar cell.
The silicon solar cell (Box 1) is still the predominant model in use
today, representing some 94 percent of the global market. But even with
energy conversion efficiencies as high as 33 percent, silicon-based
solar cells are still too expensive for general use.
The conventional solar cell
The conventional solar cell is made from inorganic crystalline
semi-conducting material such as silicon, which is ‘doped’ (slightly
contaminated with appropriate elements) to form a p-n junction. The p
side of the junction contains an excess of positive charges (holes), the
n side, an excess of negative charges (electrons). This creates an
electric field across the junction.
When sunlight is absorbed in the bulk of the silicon, free electrons and
holes are created, which are accelerated by the electric field to go to
the appropriate electrodes on the top and bottom of the cell (see Fig.
1). On reaching the electrode, the electrons leave the device to drive
the external electric load, returning to recombine with the holes at the
other, counter electrode.
Figure 1. Diagram of a conventional solar cell
The conversion efficiency of the solar cell is defined as the ratio of
the electric power provided to the external circuit to the solar power
incident on the active area of the cell. It is typically measured under
standard simulated conditions.
In recent years, fuelled by the growing global energy demands and to
some extent, by the need to reduce carbon emissions to mitigate global
warming, solar power is gaining in popularity as improvements in design
boost energy conversion efficiency and lower manufacturing costs (see
Box 2). There is a trade-off between the cost of manufacture and the
efficiency, which is expressed in the unit price of electricity
generated. The current cost of about $4/W is still considered too high
for the market. The US Department of Energy has set a target to lower
unit price to $0.33/W; but as the prices of oil and gas are both rising,
solar power will begin to look much more competitive as research and
development continue to improve on efficiency and cost, and especially
when carbon credits from reducing carbon dioxide emissions are factored in.
A major advantage of solar power is that it has minimum impacts on the
environment, which are mostly associated with the manufacturing
processes, and do not require major changes in land use . Solar
panels can be conveniently integrated into existing building structures
and rooftops; and large arrays can be sited in deserts.
Global solar energy status
The world produces 4.6 x 1020 J per year , and the earth’s surface
average solar energy is ~ 4 x 1024 J/year . Thus, even with solar
cells at a low, 10 percent conversion efficiency, the world’s energy
needs can be satisfied with solar panels covering just over 0.1 percent
of the earth surface.
Worldwide, photovoltaic installations increased by 927MW in 2004 , up
from 574MW installed during the previous year. In 1985, annual solar
installation demand was only 21 MW. On the supply side, 742 MW of solar
panels were produced in 2003. But current cumulative solar energy
production accounts for less than 0.01 percent of total global energy
demand, even though it has been growing at about 25 percent per year
over the past 15 years.
Japan manufactured 50 percent of the world’s solar cells in 2003; and
has overtaken the US as the largest net exporter of solar cells and
modules. Four companies account for over 50 percent of solar cell
production: Sharp, Kyocera, BP Solar and Shell Solar. Sharp remains the
largest company, and has shown the fastest growth over the past five
years; Sanyo, fifth largest has shown the second highest rate of growth
over the same period. Solar energy prices have declined on average 4
percent per year over the past 15 years, due to progressive increase in
conversion efficiencies and manufacturing economies of scale.
But to really capture the mainstream energy market, major increases in
energy conversion efficiency and/or reductions in manufacturing cost are
needed; and the prospects look bright for both.
Second generation thin-film technologies
Over the past decade, “second generation” thin-film technologies have
been developed that do not require costly crystalline silicon wafers and
can be manufactured much more cheaply. These include devices based on a
range of new inorganic semi-conducting materials, as well as
multi-junction amorphous (non-crystalline) silicon. Thin-film cells are
fabricated using techniques such as sputtering, physical vapour
deposition and plasma-enhanced chemical vapour deposition.
Multi-junction cells based on amorphous silicon have been the most
successful second-generation technology todate. Amorphous silicon can be
made from waste silicon from the computer chips industry, and devices
can be manufactured at relatively low cost and at high speed with
roll-to-roll processing on flexible stainless steel and other
substrates, which can be easily integrated into roofing materials. These
advantages have helped them capture the 5.6-6 percent of the market not
dominated by crystalline or polycrystalline silicon. One such product on
the market is a triple-junction flexible solar panel  made of three
separate amorphous silicon layers, each with a different bandgap, so as
to harvest light from the entire solar spectrum, and works even in
cloudy conditions. It has a conversion efficiency of 13 percent; and a
test panel averaged nearly 70 percent of its rated maximum output during
the daylight hours of a typically grey British winter (1998/9).
In May 2005, Sharp Corporation, the world’s top manufacturer of solar
panels, announced the introduction of a new polycrystalline solar module
in Japan with the industry’s highest conversion efficiency of 15.8
percent . This sets a benchmark for all second and third generation
Third generation technologies
Third generation technologies are based on new materials, new mechanisms
and concepts in light energy harvesting and conversion. They come in two
kinds: those aimed at achieving very high efficiencies and the rest
aimed at the lowest cost with moderate efficiencies of 15-20 percent. In
the first category are approaches based on quantum dots and new
mechanisms, such as ‘hot carriers’, thermovoltaics and multiple
electron-hole pair creation (“Quantum dots and ultra efficient solar
cells”, this series). These are at the early research stage and yielding
exciting results, but are not yet ready for the market. The second
category includes a wide range of applications based on organic
material, some of which are near to market, or already in market (see
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