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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.
Box 1
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.
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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 [1]. Solar panels can be conveniently integrated into
existing building structures and rooftops; and large arrays can be sited in
deserts.
Box 2
Global solar energy status
The world produces 4.6 x 1020 J per year [2], and the earth’s
surface average solar energy is ~ 4 x 1024 J/year [3]. 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 [4],
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.
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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 [5] 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 [6]. This sets
a benchmark for all second and third generation solar cells.
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 next article).
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