New small turbine technologies - Wind Power Generation and Wind Turbine Design - page 394

where R , V D and D are rotor radius, design wind speed and design rotor speed,

respectively. Therefore, when design wind speed V D is fi xed, the following relation
is obtained:

Ω

l
Ω=

V
R

DD

(4 )

D

The smaller the turbine becomes, the faster the rotor rotates. This means a SWT is
a high-speed turbo-machine and a particular attention shall be paid on control and
safety even though the machine is very small. All the design issues for mechanical
safety are well described in IEC 61400-2.
Safety issue relates also to the operation and environmental conditions, because
SWTs are very easy to be built on domestic roofs, for example. For any kind of
WT, an open area with free wind fl ow without obstructs nearby is the optimal for
the WT and safest for human beings.

3 Purposes of use of SWTs

Among all, the objectives of the use of SWT(s) must be identifi ed fi rst. Some
SWTs are very interesting as rotating toys or can be impressive artistic monuments
in public parks or in front of buildings. There are also many wind engines pumps
or mills. However, the primary important technical aspect of a SWT is its feature
as an energy producing machine.
Drag-driven WTs such as Savonius rotors are also often used. These WTs are of
high-torque/low-speed type and their effi ciencies are not as high as lift-driven
WTs of low-torque/high-speed type. Among vertical-axis wind turbines (VAWTs),
Darrieus rotor is lift-driven type with an elegant shape. Straight Darrieus rotor is
also lift-driven type. If carefully designed, these lift-driven VAWTs may also have
not far less effi ciency than horizontal-axis wind turbines (HAWTs). As technical
grounds are quite same between lift-driven VAWTs and HAWTs, main efforts are
made to describe the technical issues of HWATs.
Those who conduct a technical design of a SWT had better refer to IEC standard
“IEC 61400-2, Wind turbines - Part 2: Design requirements for small wind tur-
bines”, which fully describes safety philosophy and engineering integrity with all
technical requirements for SWTs.
However, due to various reasons, most of SWTs are technically and commer-
cially behind LWTs.
When IEA WIND, Topical Expert Meetings “Challenges of Introducing Reli-
able Small Wind Turbines” was held in Stockholm in fall 2006, 17 experts from 8
countries came to such a conclusion that to create a technically and commercially
reliable common market for SWTs is crucial. As a result, a new task (Task 27)
entitled “Development and Deployment of Small Wind Turbine Quality Label-
ling” was initiated in 2008 under the cooperative activities of IEA R&D WIND
(IEA Implementing Agreement for Co-operation in the Research, Development,
and Deployment of Wind Energy Systems). Considering the present situation that
although internationally accepted IEC standards relevant for SWTs industry
already exist (IEC 61400-2: 2 nd Ed:2006), its application is still not common and