Introduction to Wind Turbine Technology - Small Wind Turbines: Analysis, Design, and Application - page 22

similar to towers for large turbines, although they are usually assembled, rather
than fabricated, on site. The alternatives of guy-wired tubular towers and lattice
towers are still used but the former are mainly for off-grid applications. Guyed
towers are often the cheapest, and lattice towers are often the easiest to transport.
Tubular lattice towers made from small diameter water pipe are often used where
it is not possible to galvanise the tower components. They have the added benefit
of being easily and accurately pre-fabricated in short sections using simple
equipment. Tower design is covered in Chap. 10 and the important issues in
installation, raising and lowering are discussed in Chap. 12 .

1.9 The Size Dependence of Turbine Parameters

IEC 61400-2 defines a small turbine as having a swept area of less than 200 m 2 ,
which corresponds to a power output of about 50 kW. This definition is as arbi-
trary as it is necessary. There is a further subdivision in that turbines of less than
2m 2 (about 500 W) do not need to have their tower included in the certification
process. Clausen and Wood [ 15 ] have made a further subdivision as shown in
Table 1.6 .
The significance of the division is more apparent from Table 1.6 which shows
the size-dependence of the main turbine parameters. The scaling of the centrifugal
loads follows from the need to keep k independent of turbine size. The dependence
of the noise follows from Eq. 1.4 , and that of the power should be obvious by now.
From Exercise 1.4, all torques, including the starting torque scale as the cube of the
radius. The last two scalings, of blade mass and inertia, follow from assuming that
the blade density and shape stay constant as turbine size changes. Figure 1.13
shows the actual power law dependence is not as strong as indicated in Table 1.7
for large blades made by LM Glassfiber. Brøndsted et al. [ 16 ] found an exponent
closer to 2.66 for, presumably, a different set of blades. It is likely that
improvements in manufacturing techniques have reduced blade mass and lead to
an effective reduction in the exponent; the longer the blade the more recent it is
likely to be.

Table 1.5 Source for data in Fig. 1.12

Label Source Comments

Ginlong (a) www.ginlong.com (accessed 6 July 2010) Web site gives maximum values

Ginlong (b) Measured by author Value the same on two units

Skystream Measured by author on one turbine only www.skystreamenergy.com

AWP a Measured by Wright and Wood [ 17 ] This 500 W turbine features

significantly in Chaps. 8 and 9

Seoltech www.skywindearth.com (accessed 15 Sep

2010)

Web site gives maximum values

a

AWP stands for Australian Windpower which is no longer in production