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SPECIAL REPORT : ALTERNATIVE ENERGY
POWER SYSTEMS DESIGN
2014
NOVEMBER
Efficiency in alternative
power systems
By: Martin Schulz, Infineon Technologies
What efficiency really means and why “good” isn’t good enough
I
n October 1983, a record-
breaking wind generator
was taken into operation
and the world’s largest wind
energy converter called Growian
(artificial German abbreviation
meaning large scale wind power
plant) went live. The 3MW
machine can be considered
an example how the world has
changed since this happened.
Though it was an ingenious
design by the time, the power
harvested by an asynchronous
generator was transferred to
the grid by means of several
gearboxes and conversion
from variable frequency to
fixed frequency involved a
mechanical converter utilizing
rotating machines. Stacking five
mechanical systems resulted
in a conversion efficiency of
less than 80% and more than
600kW of losses were generated.
Today, harvesting, transferring,
storing and using electric energy
is one of the major challenges
industrialized nations face.
Though the scale changes from
Watt to MW, the task itself
remains the same
An issue in watts
Saving energy in a scale of
1W seems to be peanuts but
the number of devices within
this range is enormous. A
mobile phone is one of these
applications. Using an USB-
port, a cell phone charges at 5V
consuming 2.5W. Prior to the
era of high-voltage MOSFETs,
the task would have been fulfilled
using a transformer, a rectifier
and a linear regulator, leading to
a system efficiency of about 50%.
Today, compact switch-mode
power supplies can do the same
task achieving 85% conversion
efficiency. With about 100 million
mobile phones in Germany
alone, charging one hour every
day, the improvement due to
semiconductors sums up to
146,000MWh per year.
A task in under one kilowatt
Personal computers have made
their way into almost every
house in Europe, starting with
the Commodore C64 in 1982.
It took until 2004 to start the
80Plus initiative to foster power
supplies that feature at least
80% efficiency. While most of
these computers operate at a
100W-level, high-power graphic
cards and further accessories can
boost the power consumption up
to 1000W.
Compared to the C64’s power
supply based on transformer and
linear regulators, modern switch
mode power supplies feature a
more complex structure but also
higher efficiency, lower weight
and volume and thus fewer
resources per Watt of output
power. With 66 million privately
owned computers, power
semiconductors contribute to
saving 10,000,000MWh per year
in Germany alone. This quote
would double if the average
efficiency changed from 80% to
90%
A challenge in handling
megawatts
The German “Energiewende”
is a project to eliminate the
need of nuclear power by 2020,
substituting the centralized
power plants using renewable
energies. As any renewable
power source is of fluctuating
nature, energy storage will be
needed. Balancing between
times of production and times
of consumption will become a
key element to achieve stable
supplies with the availability
desired. The challenge for power
semiconductors now becomes
obvious, taking a look to the
flow of energy as depicted in the
scheme in Figure 1:
Energy, harvested from solar
arrays or wind energy converters
is processed by power electronics
to be grid compliant. Comparing
today’s wind converters to the
1983 Growian, efficiency grew by
roughly 20%. An average modern
2MW wind power plant, operated
1000 full power hours per year,
has an additional energy harvest
in a regime of 400,000kWh
due to efficiency improvement,
replacing the mechanical
converter by power electronics.
Germany’s renewable power
generation in 2013 was about
135 billion kWh. Without power
electronics, 27 billion kWh would
have been lost.
Long-distance energy
transmission is most efficient
using High-Voltage DC lines
(HVDC) making AC/DC and
DC/AC conversion part of the
transfer. Storing energy in
batteries (4) again demands AC/
DC conversion while recovering
energy is a DC/AC path. Even
before the energy reaches the
end customer it passed power
electronics five times at least
and was converted seven times
if chemical conversion in the
batteries is taken into account.
Considering 95% conversion
efficiency for each state, 30%
of the initial energy would be
lost. Enhancing the situation in
regards of the power electronic
conversion systems can be done
on different but interacting
levels.
Technical improvements
To a certain extent, adapting
processes or introducing slight
changes to materials can
enhance existing technologies.
Power semiconductor switches,
IGBTs, benefit from thinner wafer
technology as this reduces the
switching losses. Changing the
cell design but remaining with
the same raw materials allows
optimization regarding forward
voltage. Increasing the junction
temperature without sacrificing
Figure 1: Sketch of a supply grid integrating renewable power generation and
battery based energy storage.
Figure 2: Three decades of power semiconductor development