Tong W. Wind Power Generation and Wind Turbine Design
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Design and Development of Megawatt Wind Turbines 255
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CHAPTER 7
Design and development of small wind turbines
Lawrence Staudt
Center for Renewable Energy, Dundalk Institute of Technology, Ireland.
For the purposes of this chapter, “small” wind turbines will be defi ned as those
with a power rating of 50 kW or less (approximately 15 m rotor diameter). Small
electricity-generating wind turbines have been in existence since the early 1900s,
having been particularly popular for providing power for dwellings not yet con-
nected to national electricity grids. These turbines largely disappeared as rural
electrifi cation took place, and have primarily been used for remote power until
recently. The oil crisis of the 1970s led to a resurgence in small wind technology,
including the new concept of grid-connected small wind technology. There are
few small wind turbine manufacturers with a track record spanning more than a
decade. This can be attributed to diffi cult market conditions and nascent technol-
ogy. However, the technology is becoming more mature, energy prices are rising
and public awareness of renewable energy is increasing. There are now many
small wind turbine companies around the world who are addressing the growing
market for both grid-connected and remote power applications. The design fea-
tures of small wind turbines, while similar to large wind turbines, often differ in
signifi cant ways.
1 Small wind technology
Technological approaches taken for the various components of a small wind
turbine will be examined: the rotor, the drivetrain, the electrical systems and
the tower. Of course wind turbines must be designed as a system, and so rotor
design affects drivetrain design which affects control system design, etc. and so
no component of a wind turbine can be considered in isolation. In general small
wind turbines should be designed to IEC61400-2, Design Requirements for Small
Wind Turbines [ 2 ].
258 Wind Power Generation and Wind Turbine Design
Figure 1: Annual deployed UK wind systems (credit: British Wind Energy
Association).
Figure 2: On-grid vs. Off-grid UK wind systems (credit: British Wind Energy
Association).
Design and Development of Small Wind Turbines 259
Figure 3: Whisper H40 (credit: AWEA, Southwest Windpower).
Figure 4: AIR Marine (credit: AWEA, Southwest Windpower).
260 Wind Power Generation and Wind Turbine Design
1.1 Small wind system confi gurations
Just as has been the case with large wind technology, a number of attempts have
been made to design vertical axis wind turbines – none of them commercially suc-
cessful as of yet. Proponents of this technology for small wind point out important
advantages: the ability to take cope with turbulent wind (as is found more often in
small wind applications, due to lower towers, building mounting, etc.) and lower
turbine noise. It remains to be seen as to whether a commercially successful verti-
cal axis small wind turbine will emerge, and vertical axis machines will not be
discussed further.
Unlike large wind turbines, which now exclusively use upwind designs (the
blades upwind of the tower), there are successful upwind and successful down-
wind machines in the small wind turbine market. An early downwind design was
the Enertech 1500 1.5 kW machine (which sold about 1200 units in the early
1980s, Fig. 5 ), the forerunner of the AOC 15/50 50 kW turbine (which sold between
500 and 1000 units in the 1980s and 1990s, Fig. 6 ), which was the basis for the
current Entegrity EW50. The Scottish company Proven successfully use the
downwind approach in its line of turbines ( Fig. 7 ).
Virtually all small wind turbines use passive yaw control, i.e. the turbine
requires no yaw motors and associated controls to orient the machine into the
wind. In the case of upwind machines, a tail is used to keep the rotor upwind of
the tower. The tail is often hinged to facilitate overspeed control (see Section
1.2.2). The tail becomes mechanically unwieldy as turbine size increases above
Figure 5: Enertech 1500 (credit: American Wind Energy Association).
Design and Development of Small Wind Turbines 261
Figure 6: AOC 15/50 (credit: AWEA, David Parsons).
Figure 7: Proven 600W (credit: AWEA, Leslie Moran).
262 Wind Power Generation and Wind Turbine Design
about 8 − 10 m rotor diameter. Downwind passive yaw machines can be found up
to about 15 m rotor diameter. Large wind turbines virtually are all of the active
yaw, upwind design.
The upwind design appears more popular. The Jacobs design has been around
for many decades ( Fig. 8 ), and Bergey produce a well-known upwind turbine
( Fig. 9 ).
Upwind turbines require a tail vane to orient the machine into the wind, whereas
downwind turbines naturally track the wind without the need for a tail vane. The
rotors on downwind machines are subject to “tower shadow” each time a blade
passes behind the tower. The blade briefl y sees reduced and more turbulent winds
behind the tower, resulting in cyclical moments on the low-speed shaft and turbine
mainframe which do not exist on an upwind machine. This increases fatigue cycles
on the turbine. This must be traded off against the simplicity of the downwind
design. In large wind turbines driven yaw is needed, and there is no reason not to
have the rotor upwind of the tower.
1.2 Small wind turbine rotor design
In general the same issues in blade design exist for small turbines as for large wind
turbines. These are discussed elsewhere in this book, and so only the unique aspects
of small wind turbine blade design will be discussed in this section. These issues
Figure 8: Jacobs 10kW (credit: AWEA, Ed Kennel).
Design and Development of Small Wind Turbines 263
can be put into three categories: rotor aerodynamics, rotor overspeed control, and
rotor manufacturing considerations.
1.2.1 Rotor aerodynamics
In the early days of grid-connected wind turbines, rotors were usually “stall-con-
trolled”, i.e. maximum power was limited via aerodynamic stall. As wind turbines
grew in size, pitch control has become the universal method to limit power output
during high winds. Stall control is still commonly used on small wind turbines.
Figure 10 shows two power curves illustrating the two types of power limitation.
The reason for the use of stall control is simplicity, and therefore low cost. The
blades can be fi xed to the hub without the need for pitch bearings and a pitch
mechanism. Few small wind turbines feature pitch control.
Using blade pitch to effectively “dump” wind when turbine rated power is
reached is a natural and obvious application of blade pitch. Stall control is simple
and elegant, although somewhat less effi cient. It is typically used on a constant
speed turbine, e.g. one with an asynchronous generator. For example, suppose the
blade tip speed is a constant 100 mph. In light winds the blade angle of attack
would be very shallow, i.e. the wind is coming directly at the leading edge (the
blade pitch angle is only a few degrees). In high winds the blade tip would see
wind coming at it from a much steeper angle, and stall would occur, limiting power
output with no moving parts. There are certain airfoils that exhibit a particularly
useful stalling characteristic (e.g. the NACA44 series) which are commonly used
on stall-controlled small wind turbines.
Figure 9: Bergey 10kW with tilt-up tower and furling tail (credit: AWEA, Don
Marble).
264 Wind Power Generation and Wind Turbine Design
With the advent of grid-tie inverters, stall control is becoming less prevalent on
the smaller wind turbines, power curves are beginning to resemble that of the pitch
controlled turbine in Fig. 10 , and power limitation accomplished in conjunction
with the rotor overspeed control (see discussion below). In this case a stalling rotor
design is no longer necessary.
It should be noted that it is diffi cult to get the same high aerodynamic effi ciency
on small turbines as on large turbines. This has to do with the formation of turbulent
boundary layer on the surface of the blade. Towards the leading edge of the blade
the boundary layer is laminar, but at some critical distance l across the blade surface
the fl ow becomes turbulent. This distance is a function of fl ow velocity ( V ), Reynolds
number ( Re ), viscosity ( m ) and density ( m ) as per the following equation:
()/()lRe Vmr=∗ ∗
( 1)
This turbulence reduces pressure drag (the primary effect) and increases friction
drag (a secondary effect), with the net effect being a reduction in drag and an
improvement in rotor effi ciency. Small wind turbine blades have a small chord
length and therefore there is a relatively smaller region across the blade surface
where the fl ow is turbulent compared to large turbine blades, hence the higher
effi ciency from large turbine blades. C
p
values on large turbines can be on the
order of 0.5, whereas on smaller turbines it is on the order of 0.4.
1.2.2 Rotor overspeed control
Turbines (e.g. gas turbines, steam turbines, wind turbines, etc.), when unloaded,
typically only have inertia to limit uncontrolled acceleration. Typically energy
Figure 10: Typical power curves with stall and pitch control.