Direct drive superconducting wind generators - Wind Power Generation and Wind Turbine Design - page 304

to the new generation of offshore wind farms currently under construction and
planning. Recently there has been a development of direct drive generators by a
number of turbine manufacturers in order to simplify the mechanical drive train
and avoid reliability problems with gearboxes. Direct drive generators are required
to operate at the very low speed of the turbine rotor, and hence very high torque.
Since it is torque rather than power that predominantly determines the size of the
generator, they are signifi cantly larger than high speed generators. To date, most
commercial direct drive generators have been very large conventional synchro-
nous machines, most notably from the turbine manufacturer Enercon, but recently
there has been the introduction of permanent magnet direct drive generators that
are smaller and lighter for the same rating.
During the 1990s electrical machines began to be developed using high tempera-
ture superconducting (HTS) materials, the attraction being a signifi cant reduction
in the size and weight of the machines. Recently a 36.5 MW, 120 rpm HTS propul-
sion motor for the US Navy was tested, with a shaft torque similar to that of existing
direct drive wind turbine generators. However, the cost of HTS wire has been too
high for a cost sensitive market such as wind energy. A second generation of HTS
wire (sometimes known as tape) is beginning to come into commercial production,
offering an order of magnitude reduction in the cost of HTS wire when produced in
volume. This new type of HTS wire opens up the possibility of using HTS genera-
tors in wind turbines to make the next signifi cant step up in turbine power rating
without the additional penalty of higher mass at the top of the tower.
2 Wind turbine technology
2.1 Wind turbine market
The wind energy market has been growing rapidly since the mid 1990s, with new
installed capacity growing at an average rate of 28% in the years 1997-2004, and
an average of 34% in the years 2005-2007 [1]. Total installed capacity stood at
93 GW by the end of 2007, and the wind industry expect this to increase to between
490 and 2400 GW by 2030 [2].
Along with this growth in installed capacity there has been a growth in the size
and rating of wind turbines, with the largest turbines now being installed (2008)
rated at 5 MW. Larger turbines are under development, with Clipper Windpower
Plc developing the 7.5-10 MW Britannia turbine in the UK for the offshore wind
market [ 3 ].
The UK, like many northern European countries is densely populated, so the
number of acceptable sites for onshore wind farms is limited. However, the UK is
surrounded by large areas of shallow sea with some of the best wind resource in
the world. European countries are committed to increasing the share of renewables
in energy consumption to 20% by 2020 [4]. In the UK and many other northern
European countries, a large proportion of this energy is planned to come from
offshore wind, and it is the UK government's intention to install up to 33 GW of
offshore wind power to meet the 2020 target [5].