With over 200,000 wind turbines now operating worldwide, and global
installed capacity expected to reach
663MW by 2020, wind power is on a roll.
Accordingly, I thought I’d take a look at some of the key
engineering issues affecting wind power technology, and
which new developments might grow important over the
Regardless of the design, the maximum power that
can be extracted from the wind can be calculated by
Betts’s law. According to the law, no turbine can capture
more than 59.3% of the kinetic energy in the wind, for a
theoretical maximum power efficiency of 0.59.
Current generation wind turbines have power
efficiencies between .35 and 0.45. After taking
other losses into account (generator, bearings, power
transmission, etc.), only about 10 to 30 percent of wind
power is actually converted into usable electricity.
Since the wind does not blow continuously, the
annual capacity factor—the ratio of actual output to
output at 100 percent capacity—of a wind site is much
lower than unity.
In a 2009 study, it was estimated that the average
capacity factor of Germany’s wind farms was 18. 3
percent. In Britain, a windier country, it was 26.1
percent, so each 10 GW of installed capacity delivered
about 2. 6 GW on average.
Unsurprisingly, the wind farm power output varies
widely over any given year: in Germany, for example,
the maximum instantaneous wind power output in
2013 was 26GW at 6pm on December 5th, in contrast
to the minimum power output of 0.128 GW at 2pm
on September 4th, only . 5 percent of maximum. This
is true over shorter time periods, too: in Britain, total
output during the course of a single day (June 16th)
went from around 2. 5 GW down to almost zero.
These figures naturally represent extremes, and
improved forecasting can predict short-term fluctuations
to some extent, but unpredictable
variation in power output means that wind
farms cannot completely replace more
conventional systems such as fossil fuels or
nuclear power plants. Instead, they should
be viewed as a means to save fuel and
reduce CO2 emissions during times when
they are generating power.
Recognizing this, Germany, the country
with the highest installed wind power
capacity in Europe (over 39 GW), is
planning on opening 14 coal-fired power
stations and 27 gas-fired stations by 2020,
with a total capacity of 56 percent of the
country’s power needs. Incidentally, 39
GW, while impressive, only gets Germany
to the number three position on the
worldwide windpower list behind China
(115 GW) and the US (66 GW).
This rise in windpower installations has
not been without problems, though. On
the technical side, issues include lowered
efficiency due to the build-up of dead
insects, forced shutdown due to high
winds, ice buildup, lightning damage, oil
leaks, and fire due to overheated bearings.
In addition, the average load factor
steadily declines with age. A 2012 analysis
of wind farm performance in UK and
By Paul Pickering, Technical Contributor
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