Urban wind turbines: the case study of the city of London
12
Introduction
Energy from wind is a source of renewable energy that has the
potential to be utilized on a wider basis. Energy capture by wind
turbines is highly dependent upon the local wind regime. The
majority of the installed wind energy generating capacity in the
UK is large scale turbines located at costal locations, inland areas
of open terrain and near large bodies of water.
The potential for small-scale production in urban environments is
less well utilized and understood. Leading factors for the use of
urban and building mounted turbines are the requirements to
reduce carbon dioxide emission to meet the building regulations
imposed by European Community. Heating in the UK is mainly
generated by electrical power [21], if offices are considered: this
further increases the electricity demand in the cities.
Wind energy has been identified as one of the most cost effective
means of localized renewable power generation. The main
concerns with the use of turbines in urban environments are due
to the limited information on noise and limitations to the energy
yield due to the wind regime in urban environments. This study
has focused on the wind regime and potential energy yield at the
site of an urban installation.
The factors that affect the wind regime are complex. They include
terrain, local obstructions and the building that the turbine is
mounted upon, each will affect the wind and therefore the
energy capture. The first step to understand the influence of the
urban environment was to determine what the expected wind
regime would be and how this compares with the measured data
from a test site.
Urban wind turbines: the case study of the city of London
13
The realization of the weather station useful for data collecting
has been done in cooperation with GasDynamics staff. This
installation allows achieving real wind data and comparing them
with the estimated value.
Data from a wind turbine recently installed at an urban test site
was collected and analyzed with GasDynamics team supervision.
Wind data, power and energy yield from a building mounted
turbine has been included in the analysis. The installation has
been described in some detail and showed by pictures.
The aims of the study were to compare predictions of mean wind
speed, wind speed distribution and energy yield with measured
data from the test site. To achieve these, the following objectives
were identified:
1. Present critical review information on small scale wind
turbines, current sources of wind data, estimating wind resource
at a site and theoretical methods for predicting turbine energy
yield (Chapter two, paragraph two and chapter three, along
paragraphs two and three).
2. Compare actual recorded wind data with available estimates to
determine which estimates are most appropriate for urban wind
turbines (Chapter three and four, paragraph one).
3. Compare actual recorded energy yield of the test turbine with
the predicted yield (Chapter four, paragraph four).
CHAPTER ONE
WIND AND ENERGY
In the first part of this chapter it is exposed a physical reason of
whence events the wind depends. It will be given an overview of
energy actual situation around the world, hitting issues and their
relative solutions of increasing energy demand.
A brief history about how the wind has been utilized during
ancient time is described in paragraph 1.1.1; nowadays
applications are also presented.
Later on, the theory and the equations which stand behind a
wind turbine are exposed, together with all the fundamental
components, those made up the wind device.
The cost involved in a wind turbine installation is analyzed,
followed by an overview of some aspects of electric energy
generation by wind turbines.
Chapter one: Wind and energy
15
1.1 Wind: formation and utilization
Almost all renewable energy comes from the sun. About 1 to 2
per cent of the energy coming from the sun is converted into
wind energy. How?
The sun warms up the air. The regions around equator are heated
more by the sun than the rest of the globe. Hot air is lighter than
cold air and will rise up into the sky, until it reaches
approximately 10 km altitude and will spread to the North and to
the South. If the globe did not rotate, the air would simply arrive
at the North Pole and the South Pole, sink down and return to the
equator, forming the trade winds.
Since the globe is rotating, any movement on the Northern
hemisphere is diverted to the right, as it looks from a ground
position. A similar diversion happens in the Southern hemisphere,
oriented to the left. This apparent bending force is known as the
Coriolis force (see Fig. 1.1).
Figure 1.1: Action on the wind of the Coriolis’ force (red arrow) [2]
Chapter one: Wind and energy
16
Another effect needs to be considerate. Around 25˚ up to 30˚
north and south, near the tropics, the winds calm again in the so
called horse latitudes, creating belts of high pressure. This effect
is caused by the air cooling during the rise from the equator
towards the troposphere. The air needs to go somewhere, so it
blows towards the equator as the trade winds, and towards the
middle latitudes.
Meanwhile, at the poles, the cold causes air to fall down,
increasing the air pressure to cause the polar highs. The
phenomenon is explained in the next picture (Fig 1.2).
Figure 1.2: Geostrophic wind [1]
Synthesizing, Coriolis force and the pressure gradient work
against each other: pressure gradient pushes wind outward,
while the Coriolis’ effect moves winds to the right or left. Often
these two forces reach a balance, causing winds to blow sideways
along the pressure gradient, not getting any further from a high
Chapter one: Wind and energy
17
pressure or closer to a high pressure. This type of wind is known
as a geostrophic wind.
Figure 1.3: Prevailing winds [3]
All the trends explain above causes the so called prevailing winds,
visible on the figure 1.3.
1.1.1 Wind harvest among the years
Wind is one of the primary fonts of energy which human kind has
been used since ancient times. Sail boat has always been moved
by blowing winds: the New World was explored by wind powered
ships.
Indeed, wind was almost the only source of power for ships until
Watt invented the steam engine in the 18th Century. [4]
Chapter one: Wind and energy
18
Figure 1.4: a traditional windmill
Another apparatus capable of wind energy harvesting is the
windmill (figure 1.4 and 1.5). They date back many centuries. It
has been reported that the Babylonian emperor Hammurabi
planned to use the first version of wind turbines for irrigation in
the seventeenth century B.C.
The Persians were using windmills extensively by the middle of
the seventh century A.D.
These early machines were undoubtedly crude and mechanically
inefficient, but they served their purpose well for many centuries.
The earliest application of a wind turbine in Europe happened on
the year 1191 and it was recorded in England. The application
was agriculturally related. The first corn grinding wind turbine
was built in Holland in 1439. There were a number of
technological developments through the centuries. By 1600, the
most common wind turbine was the tower mill.
A distinctive wind turbine, called the American multi bladed wind
turbine, was developed later, in the mid 1800’s, to front a
Chapter one: Wind and energy
19
morphological problem which happens in the West. Over there
they had nice grazing lands, but with no surface water. Ample
ground water stays underneath the surface only a few meters.
A smaller wind turbine for pumping water was developed to solve
such an issue. An estimated 6.5 million units were built in the
United States between 1880 and 1930 by a variety of companies.
Many of these are still operating satisfactorily.
Figure 1.5: A Dutch windmill
Recent wind turbines utilize energy withdraw from the wind to
convert it into electrical energy.
Since several years, energy contained inside the wind contributes
significantly to supply part of the power demand made by
industrial sites, farms, private houses and so on. This happens in
pretty much all the industrialized and wealthy countries all over
the world.
Chapter one: Wind and energy
20
1.1.2 Energy standings
The Energy Information Administration (EIA), a section of the
United States of America energy department (DOE), stated that
the industrialized countries used more than half the world’s total
energy consumption in 2001. They are the big consumers and
have the prosperity and knowledge to switch to renewable
energy sources. Moreover, most people in the industrialized
countries are aware of the vanishing fossil energy reserves and
the environmental impact of the huge consumption of fossil
energy nowadays. Changes in energy source and consumption
behavior will probably be initiated soon. Consequently, the need
for renewable energy in prosperous countries will rise.
It is not certain that the temperature rise in the last decade is
caused by the emission of carbon dioxide from burning fossil
energy reserves. Experts differ in their opinion whether or not
climate change is a result of our pollution. Nevertheless, there
are serious indications that the consequences of the greenhouse
effect may not allow the burning of remaining fossil energy
reserves. The earth’s population increases rapidly and energy
demand follows. It has been predicted an average annual of two
percent increase in energy consumption over the coming twenty
years, largely realized using fossil energy.
1.1.3 Renewable energy potential
There are more reasons to switch to renewable energy instead of
fossil energy. Fossil reserves vanish.
There is much controversy on the timeframe since this largely
depends on the definition of global reserves and global energy
Chapter one: Wind and energy
21
consumption in the future. Nevertheless, in the future production
decreases while consumption increases. This will have a huge
impact on energy prices and community. Not “running out” but
“peak production” of fossil energy determines the turning point.
Furthermore, the unstable situation in the Middle East is
responsible for considerable variations in the fossil energy price
since the major countries largely depend on their fossil fuel
production. A switch to renewable energy can decrease this
dependence.
However, there is another important reason for switching to
renewable energy from fossil energy: the cost of the energy. A
fair comparison of the price of energy should be based on the
total cost of energy. This includes external costs, such as the costs
of people getting ill because of pollution.
That is certainly difficult to estimate, but to not incorporate
externalities in prices is demonstrably wrong. Looking at
Denmark, the total costs, including external costs, of a kWh fossil
energy from coal (10 €cent/kWh) are higher than a kWh of wind
energy (6 €cent/kWh) (Figure 1.6) [42].
Chapter one: Wind and energy
22
Figure 1.6: Social cost of energy
Several strong forces indicate the increased use of renewable
energy in the future. People are used to fossil energy availability
and energy density and consume it accordingly, but renewable
energy sources have a much lower energy density.
It is also wise to diversify the sources (solar, wind, hydro, biomass
or geothermal) in order to provide a constant energy source. For
instance, wind energy combined with solar energy provides a
more constant renewable energy supply since there are stronger
winds during the winter when there is less sun.
The energy supply system will change to a decentralized one in
the future: another very drastic change, which needs a huge
effort from the technical community. It does not make sense to
deliver a huge effort in producing renewable energy and then to
consume it carelessly. The best obtainable solution should be to
combine renewable energy production with decreased energy
consumption.
Chapter one: Wind and energy
23
Pulling out advantages and drawbacks of wind energy, positive
aspects are:
• It is renewable, clean, free and available in many parts of
the world
• There is no carbon oxide emissions connected with it:
pollution is definitely reduced to null.
• Costs are going down.
While negative are:
• Size: a turbine is generally huge, in order to utilize most of
the energy contained into the wind.
• Visual impact of such a massive structure is not always
appreciated.
• Noise generation could become fastidious, especially close
to residential area.
• Wind is a stochastic phenomenon, with considerable
intermittence: a user cannot depend completely on it.
• Wind farms are located outside cities: cost of energy
transportation could bear drastically.
From the operating point of view, not all the wind source can be
used: the range of wind speed which make spin the blades of
wind turbines varies from a minimum of 4 m/s up to about 25
m/s. These two speeds are called cut-in and cut-off speed. After
the edge of 25 m/s, functioning could become dangerous for the
turbine integrity and thus for people who may stands close to the
turbine.
Chapter one: Wind and energy
24
All these characteristics give to wind power just an integrative
role into the world of energy fonts: wind energy cannot
substitute completely the supply afforded by thermo electrical
plants. Thus, wind farms cover only the peak request of energy,
which happens usually two times along the day. Wind decrease
during the night, but if the blow is still enough to make the
turbine generate energy, this is normally used in pumping plants,
for instance to pump water up towards a dam.
1.2 Wind turbine: general theory
The principle is basilar: wind blow on the blades, making them
spinning around the rotational axis. The axis is usually connected
to a gearbox and then to a generator, which produces electrical
energy. The output energy is proportional to the third power of
the wind speed.
Moving a little deeper in particular, it is a bit more complicated
than just the air molecules hitting the front of the rotor blades.
Modern wind turbines borrow technologies known from aero
planes and helicopters. The reason why an airplane can fly is that
the air sliding along the upper surface of the wing moves faster
than on the lower surface. This means that the pressure is lower
on the upper surface. This pressure gap creates the lift, or else
the force pulling upwards that enables the plane to fly. The lift is
perpendicular to the direction of the wind (figure 1.7).
Figure 1.7 Lift effect on a wing (black arrow)
1.2.1 Wind turbine components
The complete index of a traditional turbine parts are pointed out
in figure 1.8:
Figure 1.8:
Nacelle
It houses all the generating components, such as gearbox,
generator, shafts, etc.
Rotor
It is made of blades and hub.
Blades
They allow rotational movement.
Chapter one: Wind and energy
25
, caused by faster upper air flow respect
to the lower
List of wind turbine components
Chapter one: Wind and energy
26
Low speed shaft
Straight connected to the rotor, it is either way locked up to the
gearbox, transferring the spinning to it.
High speed shaft
It is hooked up on one side to the gearbox, on the other side to
the generator.
Gearbox
Adapt the lower speed of the rotor to the higher needed by the
generator.
Brake
Take part if some anomalies are encountered by the control
system.
Tower
It sustains the whole upper apparatus, composed by nacelle and
rotor. It is hooked to the ground or to the roof, in case of urban
application.
Anemometer and wind vane
They respectively take care of wind speed and heading
measuring.
Controller
Govern the correct functioning of the system
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