Environmental Engineering Reference
In-Depth Information
constant low temperature, except for periods of prolonged shutdown, and therefore
does not see continuous thermal cycling [29]. Moreover, the operating temperature
is such that the chemical processes that are responsible for the ageing of the elec-
trical insulation have all but ceased.
The HTS windings need to be cooled by a closed loop cryogenic cooling sys-
tem, with a cryocooler providing the cooling power. Cryocoolers with a suitable
power rating are available as off-the-shelf commercial products. The present gen-
eration of cryocoolers do require periodic maintenance with intervals similar to
those of many other turbine components; however in Europe there are projects
working on the development of low maintenance or maintenance free cryocoolers.
Converteam Ltd. is involved in one of these projects.
4.2 Commercial exploitation of HTS wind generators
In order for an HTS generator to be commercially competitive in the wind turbine
market, a number of prerequisites must be met.
4.2.1 HTS wire
It must be possible to manufacture HTS wire in large volume at low cost.
The volume of HTS wire required for a viable HTS wind market is many times
greater than the present HTS production capacity. The production process of the
previously commercially available BSCCO-2223 wire would not be scaleable to
the required volume at the required cost. HTS wind generators will rely on the
development of the 2nd generation (2G) of HTS wire described below.
The cost of HTS wire is normally stated as the cost of the wire needed to carry
an amount of current over a certain distance, typically in $/kAm - the cost to carry
1000 A over 1 m. There is a further complication in that the current carrying capac-
ity of the wire depends on its operating temperature and the operating magnetic
fl ux density, therefore it is conventional to use the current carrying capacity at
77 K (boiling point of liquid nitrogen) with no applied magnetic fi eld. In 2008 the
cost of HTS wire was around 130 $/kAm. In order to be cost effective in HTS
generators for wind turbines the cost needs be in the range 10-20 $/kAm.
4.2.2 Generator design
The generator design must be optimised for low cost volume production .
The majority of cryogenic design experience has been with low volume special-
ist applications where cost is much less important. The exception to this has been
the MRI scanner market using LTS magnets, which manufacture moderately high
volumes. Design for manufacture techniques, and careful selection of materials
and components, must be used to obtain a volume manufacture generator design.
4.2.3 Cooling system
The cryogenic cooling system must be reliable with extended maintenance
intervals.
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