flexible substrate. The firm also sells small wallet-type
cells that can be used to charge mobile phones in areas
where electricity is not available.
Meanwhile, the Australian firm Dyesol is collaborating with the steel giant Corus to commercially manufacture dye-sensitized solar cells on steel so that they
can be incorporated into roofs. At a time when virtually all of the construction and solar-cell industry is
going through a significant slowdown in activity and
short-term demand, the partnership promises to be the
first solar-cell technology that can produce electricity
as cheaply per kilowatt-hour as conventional gas- or
oil-fired stations in the normal light conditions experienced in most cities around the world.
rotor diameter (m)
We should not forget though that nanotechnology
can help other renewable sources of energy too, such
as wind power. As the power output of a wind turbine
increases with the length of the turbine’s rotor blade,
the search is on for light but strong materials that can
be used to make blades as long as possible (figure 2).
For example, researchers at the German firm Bayer
MaterialScience have developed a new composite material consisting of aluminium powder reinforced with
carbon nanotubes that has a tensile strength similar to
that of steel but only half the density. When incorporated into a rotor blade, it is some 10–30% lighter and
20–30% stronger than traditional non-reinforced aluminium and epoxy blades.
These and other nanocomposite materials could
even replace conventional metallic moving parts in
tidal- and wave-power plants. Researchers at the UK
firm TidalStream, for example, have designed turbines
that are mounted on semi-submersible buoys tethered
to the seabed – rather than floating on the surface or
mounted on towers attached to the seabed – using technology and components from the wind industry. Elsewhere in the renewable sector, nanotechnology could
also play a role in hydro-energy power plants – for example, via novel water-repelling nano-scale materials
that can be coated onto turbine blades to prevent them
from corroding.
2 Wind: where size matters
Airbus A380
wingspan 80 m
160 m
126 m
112 m
15 m
1985 1987 1989 1991 1993 1995 1997 1999 2001 2003 2005
0.05 0.3 0.5 1. 3 1. 6 2 4. 5 5
?
8/10
year of first operation and power (MW)
The power output (red) from wind turbines increases with the diameter of the rotor blade – a
15 m diameter blade produces about 0.05 M W, while the most recent 126 m blades generate
about 5 MW. Rotor size (and hence power output) has risen steadily over the last 30 years,
such that 160 m blades, producing 8–10 MW, should one day be available.
No trouble in store
Although some of the developments described so far
are still some way from being widely deployed, nanotechnology has already had a significant impact on
existing power sources, particularly in relation to
“supercapacitors” – devices that not only have a high
power density, like a conventional capacitor, but also
a high energy density, like a battery (figure 3). Supercapacitors are common in laptops, where they can prevent the memory from being erased if the batteries run
out. They typically consist of two electrodes made of
highly porous “activated” carbon, which has a high surface area and can store lots of charge, suspended in a
solution of long-chain polymer molecules.
When a voltage is applied across the electrodes, elec-
trons in the polymer solution move towards one elec-
trode, whereas the positively charged ions move towards
the other. This creates two separate layers of opposite
charge in the solution, which are kept nanometres apart
using a thin plastic film. The stored energy can be re-
leased in a matter of seconds – compared with hours
for rechargeable batteries – by simply turning off the
applied voltage. This makes the charged layers move
back together and recombine, thereby releasing elec-
trons into the external circuit. Commercial supercapaci-
tors can be charged and discharged over half a million
times without their performance being degraded.
Chunsheng Du and colleagues at the University of
California, Davis, have recently developed a way of
storing even more energy in a supercapacitor. Their
device consists of a closely aligned suspension of carbon nanotubes deposited on nickel foil. The nanotubes
have such a large storage area that these devices could
have a power density of 30 k W per kg, compared with
4 k W per kg for the most advanced devices currently
commercially available. The researchers are currently
seeking to patent their invention.
Many companies are also trying to develop new
higher-powered rechargeable batteries, which is not
surprising given that the global market for portable
rechargeables – dominated by lithium-ion batteries –
is set to grow from $1bn in 2008 to an estimated $9.1bn
by 2015. For example, Nanotecture, a spin-off company
from Southampton University in the UK, has developed extremely thin, high power, rechargeable batteries based on novel liquid-crystal technology. The
batteries, which are just 150 µm thick, can be designed
to fit into small spaces and are a cross between batteries and supercapacitors. Nanotecture’s patented supercapacitor technology uses a hybrid design in which the
negative electrode consists of activated carbon and the
positive electrode is a nanoporous nickel-hydroxide-based material.
With pore diameters of about 3 nm and wall thick-nesses of about 5 nm, the anode has a surface area
available for electrochemical reactions that is some
200 times that of materials without such a nanoporous
surface. The sizes can be controlled and optimized to
enhance battery properties like power and energy densities for specific applications. The device can be
charged and discharged very quickly, which is ideal for
high-current pulse applications such as flash photography with mobile phones.
To take third-generation
solar cells from
the lab bench
into mass
production,
we will need to
find ways of
manufacturing
them cheaply
enough
