Page 25 - Energize Issue 1 2023

TECHNICAL

Wind turbines for urban applications

The growth of distributed energy systems in the urban environment has created the

need for a wind turbine that, like rooftop solar PV, is invisible and silent, and which could
supplement rooftop PV.

By Mike Rycroft, Energize

he challenge to produce a wind turbine with the same The VAWT has several advantages:
capacity as rooftop PV has led to the development of a • The design is omnidirectional, and does not have to be faced
Trange of vertical-axis wind turbines. into the wind
An advantage of wind as a complementary energy source to • Low minimum operational or cut-in speed
solar power is that it blows continuously, and although variable, • Low susceptibility to the turbulence level of wind
there is a distinct advantage in using wind in combination with • Less visual intrusiveness. VAWTs can be invisible when
solar for behind-the-meter applications. Traditional horizontal housed in an enclosure. The vertical axis wind turbine rotates
axis wind turbine designs would be unsuitable for rooftop in a horizontal plane which is less visually disturbing than a
application. This has resulted in various vertical wind turbine HAWT which rotates in the vertical plane. In many designs
designs with blades that spin about a vertical axis to generate the visual profile is the same whether the turbine is rotating
electricity. In the past, the low efficiencies of such devices or stationary.
have prevented them from widescale deployment, but recent • Lower noise
developments and cost factors have made vertical axis wind • The generator is located at the base of the turbine (at ground
turbines (VAWT) an attractive option. level) and not at the mast top, which simplifies mounting and
access for maintenance.
Requirements for an urban wind turbine: • Compact construction
• Visually non-intrusive
• Silent operation Although the VAWT is less efficient than a horizontal axis version,
• Aesthetically and architecturally acceptable in profile and compact size and lower cost per unit of electricity, plus other
appearance positive factors, make them the unit of choice for mounting on or
around buildings.
While horizontal axis wind turbines (HAWT) cannot meet The VAWT has been around for centuries in various forms.
the above requirements, vertical axis wind turbines have the The original design appeared in what was then Persia and was
potential to meet all of them. used to drive domestic mills. Mounted on the roof of a dwelling,
VAWTs are considered to be a better choice for cities and the device used an arrangement of wooden sails mounted in a
isolated semi-urban areas. Several attributes make VAWTs tunnel arrangement to harness the power of the wind. Some are
suitable for integration in urban areas; these include good still in use today. A later development used hinged sails which
performance even in weak and unstable wind, being noiseless made the mill omni-directional.
and safe, as well as being aesthetically acceptable. VAWTs can be divided into drag-type and lift-type depending
A wide range of VAWT designs have appeared, both in on the aerodynamic force that drives the rotation. The impulse
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research fields and on the market, but few seem to have or drag-type turbine relies on the force of the wind acting
been successful. Many types have failed because the basic directly on the blade to drive the turbine, and the reaction
construction is visually unattractive. However, the last few years turbine or lift-type relies on aerofoil dynamics to drive the blade.
has seen significant research in and development of VAWT The two basic types are known as the Savonius model (drag-type
technology, judging by the number of papers published on the SWT) and the Darrieus model, (lift-type DWT) and although there
topic. are a number of variations on each, the basic principle remains
Current development is aimed at improving efficiency by the same.
modifying and improving the basic design and investigations In general, lift-types are known to be more efficient than
cover different design options. Further research is expected drag-types, though normally they generate low torque in low
to make VAWTs a viable, dependable and affordable power tip-speed ratios (TSRs) and are consequently unsuitable for self-
generation technology for many low power decentralised start. SWT are known for high start-up torque, no low-frequency
applications. There are numerous VAWTs available on the market noise pollution, simple construction, low cost, and operation in a
in the range below 10 kW maximum output, a suitable size for wide range of wind conditions. These factors make the Savonius
urban installations. type more popular in small wind turbine applications.

energize | Issue 1 2023 | 25