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We have an ever-increasing thirst for energy. In the past fossil fuels - coal, oil, gas, wood and peat have in the main provided this energy. However, fossil fuels have two major problems, escalating costs and pollution. Alternatives to fossil fuels have been sought in two areas nuclear energy and renewable energy. The problems associated with nuclear energy are now well known and both economic and safety aspects may prevent the further expansion of nuclear power in most counties.
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There are many renewable forms of energy including, solar, wind, water and biomass. This document explores all four but focuses on small scale wind power.
Wind energy can be described as second hand solar energy or stored solar energy. This is because wind is made from the energy falling on the earth from the sun. Some of the sun's energy in the form of solar radiation is absorbed by the earth's atmosphere and the air is heated up. Hot air is less dense than cold air and is consequently lighter. It has a lower pressure than cold air, which means that as the air is heated cold air is drawn in. This heating is uneven leading to the air circulating around the earth producing vast movements of energy. The energy can be harnessed by large wind turbines and equally by small units supplying electricity to single houses, farms, workshops, boats, caravans, etc.
Small wind chargers with rotor sizes of less than 3 metres are available from several manufacturers as are plans for DIY units. Typical applications for the electrical energy generated by wind power include:
1. Charging batteries for low energy devices like lighting, radio, hi-fi, tv, etc.
2. Supplying power to remote locations such as caravans, boats and yachts, out houses and workshops.
3. Maintaining electricity for animal fencing, fish farming, irrigation, chicken layers, meteorological recording stations, radio repeater units and many more.
Generating your own power from the wind, however small that amount is, can be very satisfying and in addition help solve the problem of supplying power to remote locations without pollution.
The wind has been used to power machines capable of grinding corn, pumping water and producing electricity for hundreds years. There are records of Persian and Japanese wind machines as long ago as 200 BC. Wind power probably has its origin in the ancient civilisations of China, Tibet, India, Afghanistan, and Persia. The first written evidence of the use of wind turbines is that of Hero of Alexandria, who in the third or second century BC described a simple horizontal-axis wind turbine. From contemporary sources we also know that windmills have been used in the 11th & 12th century in England. Also from a contemporary eyewitness (1190) we know that German crusaders brought the skills of building windmills to Syria. From this, we may assume that this technology was generally known all over Europe since the Middle Ages.
The two most familiar types of wind machines are the traditional windmill used for grinding corn, once common place in Europe and the water pumping windmills that provided water to farms and towns in the U.S.A. and seen on many Western films. It is estimated that in the 1930s there were 6 million of these fan type mills in use in America.
Pitstone Windmill stands in the north east corner of a large field near the parish boundary of Ivinghoe and Pitstone in Buckinghamshire. It is thought to have been first built circa 1627 as this date is carved on part of the framework. This is the earliest date to be found on any windmill in the British Isles. It should be remembered that such a structure would have had to have frequent repairs made to it, so it is quite possible the mill predates 1627.
The design of the mill is what is known as a post-mill. This means the whole superstructure of the mill rests on one main post. This post arises from ground level through brick and a foundation chamber; the post then acts as a pivot for the timber built structure above with the sails. Consequently, the upper section of the mill and sails can be turned towards the direction of the wind. The mill machinery in the upper rotating section was reached by a long flight of external steps.
For many hundreds of years corn grown in the two adjoining villages was ground at the mill into flour. In 1874 the mill was bought by Adelbert Wellington Brownlow Cust, 3rd Earl Brownlow who owned the nearby Ashridge Estate. He subsequently let it to a local farmer, who ran a successful milling business from the mill.
In 1902
the mill was seriously damaged during a furious gale, damaging it beyond the
price of economic repair. Circa 1922 the now derelict
ruined mill was bought from the Ashridge Estate by a farmer whose land was close
to the mill. In 1937
he donated it to the National
Trust. However, it was not until 1963 that a band of
volunteers began to carry out renovations at their own expense. After seven
years of hard work in 1970
after an interlude of 68 years the mill once again ground corn. Today owned by
the National Trust the windmill is open to the public on Summer Sunday
afternoons.
The
development of the water-pumping type windmill in the USA
was the major factor in allowing the farming of vast areas of North America,
which was otherwise devoid of readily accessible water, and also allowed the
extension of rail transport systems,
throughout the world, into areas where water could be pumped up from underground
to supply the needs of the steam
locomotives of those early times. They are still used today for the
same purpose in some areas of the world where grid electricity is not a
realistic option.
The
many-bladed wind turbine atop a lattice
tower made of wood or steel was, for many years, a fixture of the rural
landscape throughout rural America. These mills, made by a variety of
manufacturers, featured a large number of blades so that they would turn slowly
but with considerable torque. A tower-top gearbox
and crankshaft converted the rotary motion into
reciprocating strokes carried downward through a pole or rod to the wellhead below.
In
areas not prone to freezing weather, a pump jack (or standard) was mounted at
the top of the well below. This was the connection between the windmill and the
pump rod, which generally went through the drop pipe to the cylinder below. The pump jack provided a means
for manual operation of the pump when the wind was not blowing. Some pump jacks
provided a sealed connection, allowing water to be forced out under pressure,
but many had a simple spout allowing water to flow away in a trough by gravity.
The
drop pipe and pump rod continued down deep into the well, terminating at the
pump cylinder below the lowest likely groundwater
level. A suction tube usually continued a short distance more. This arrangement
allowed wells as deep as 400 feet
to be constructed, though most were much more shallow.
The
number of moving parts led to the whole arrangement to be rather trouble prone,
and "well men", as they were called in the early days, had a
profitable business in repair and maintenance work.
The
wind turbines and related equipment are still manufactured and installed today
in remote parts of the western United States where electric power is not readily
available. The arrival of electricity in rural areas, brought by the REA
in the 1930s through 1950s,
made these windmills obsolete in the Midwest and other more built-up areas. The
mills and towers remained for a time. Today, most are gone, victims of storms,
rust, and progress.
As the use of fossil fuels developed and the use of electricity generated from coal and oil became widespread in the early 20th Century, renewables began to contribute less and less to the world’s energy needs. Research continued in many counties with advances being made in both the theoretical and practical use of wind energy. Hence, the revival of the wider interest in wind power after the 1970s did not start from scratch, but could build on a solid foundation of theories and practical experiences. When the new era of wind energy was initiated in the 1970s new materials and technologies were available. Composite materials such as fibreglass showed to be very suitable for the blades, and electronics were developed to control the wind turbine. In the 1980s investment was once again made in Europe and America into the production of electricity from very large windmills (often known as wind turbine or wind turbines). Wind parks consisting of groups of wind turbine were built in California, Sardinia, Orkney, Ilfracombe in Devon, Carmarthan Bay and Richborough in Kent. These experimental sites were funded by public and private money. By the end of 1996 a total of 6200 MW grid connected wind turbine capacity was installed around the world. In 1996 1200 MW were added.
The main application for mechanical farm wind pumps is drinking water supply. The markets for this type of machines include USA, Argentina, South Africa and New Zealand.
Cumulative
global wind energy generating capacity topped 31,000 megawatts
(MW) in
2002. Some 6,868 MW of new capacity were installed worldwide during the year, an
increase of 28%, according to preliminary estimates by the
American
Wind Energy Association and the European Wind Energy Association. Wind plants now power the equivalent of 7.5 million average American
homes or 16 million average European homes, worldwide. Global wind power
generating capacity has quadrupled over the past five years, growing from 7,600
MW at the end of 1997 to an estimated 31,128 MW at the end of 2002 - an increase
of over 23,000 MW. Wind is now the world's fastest-growing energy source on a
percentage basis, with installed generating capacity increasing by an average
32% annually for the last five years (1998-2002). The slightly slower rate of
28% in 2002 was primarily due to a lull in the U.S. market.
Wind has been assessed as one of the most promising renewable energy sources for electricity generation in many countries including the UK. As energy use in colder climates is proportional to both temperature and available daylight, the greatest proportion of energy use is in the winter months. (In hotter climates energy use is often greater in the summer due to cooling and air conditioning requirements). This is favourably met by wind energy with three-quarters of the potential energy wind can supply is available between November and April.
Originally wind generators were built right next to where their power was needed. With the availability of long distance electric power transmission, wind generators are now often on wind farms in windy locations and huge ones are being built offshore. Since they're a renewable means of generating electricity, they are being widely deployed, but their cost is often subsidised by governments, either directly or through renewable energy credits. Much depends on the cost of alternative sources of electricity.
The wide spread use of large wind turbines is limited by two factors.
1 - The relative generating cost compared with fossil fuels.
2 - The environmental impact of large structures.
Improving technology and escalating fossil fuel costs and the second by the exploitation of sites remote from human habitation including off shore sites are addressing the first factor.
Wind energy is clean and safe it does not produce greenhouse gases in the same way as fossil-fuelled generating plants do. Wind energy has little or liabilities related to decommissioning of obsolete plants, unlike nuclear power. The environmental impact of wind energy has been investigated thoroughly in both Europe and the USA. Particular areas of concern that have been researched include noise emissions; the sun's reflection from the blades; and the threat to birds. Opinion surveys indicate that the majority of citizens in most European countries favour renewable energy sources such as wind power. Opinion surveys in areas of Denmark and UK with wind farms indicate that 70 to 80 % of the population is " supportive" or "unconcerned" with respect to the turbines.
Wind power has a number of advantages, these include:
· The economics of wind energy are good and improving - relative to other methods of generating electricity. For example, the energy invested in the production of a typical wind turbine has a "pay back" time (energy balance) of less than half a year of operation.
· Wind energy is a domestic source of energy and as with other renewable energy sources can improve a nation's degree of self-sufficiency.
· Wind turbines can be installed fast, plants of, 50 MW, can be in operation in less than a year from signing the contract. Wind turbines are modular and therefore more power can be added quickly as demand increases.
· Wind power has proved to be a reliable technology.
· Wind power is not only applicable in the industrialised areas and countries, but is an ideal technology for the electrification of rapidly industrialising countries.
· Wind power application can include all types of systems: grid connected wind farms, hybrid energy systems, and stand-alone applications such as battery chargers.
· The technological complexity of operating and maintaining wind turbines does not differ from that of other electrical machines in rural, developing communities: desalination plants, water pumps, etc. Consequently, today wind power is being included in the energy planning of the rapidly industrialising nations.
· Land-based wind energy has the potential of covering six times the world's electricity consumption, or one time the world's total energy consumption.
· The energy consumption for production, installation, operation and decommission of a wind turbine is usually earned back within 3 months of operation.
· Conventional and nuclear electricity production receive massive amounts of direct and indirect subsidies. If a comparison is made on real production costs, wind energy is competitive in many cases. If the so-called external costs are taken into account, wind energy is competitive in most cases. Furthermore, wind energy costs are continuously decreasing due to technology development and scale enlargement. On the other hand, the hidden costs of decommissioning nuclear power stations, and waste disposal are now coming to the fore.
· Studies show that the number of birds and bats killed by wind turbines is negligible compared to what's due to other human activities such as traffic, hunting, power lines and high-rise buildings. For example, in the UK with a few hundred turbines, about one bird is killed per turbine per year; 10 million per year are killed by cars alone.
http://www.bwea.org/media/news/birds.html
· After decommissioning wind turbines, even the foundations are removed.
· Conventional and nuclear plants also have sudden unpredictable outages. Statistical analysis shows that 1000 MW of wind power can replace 300 MW of conventional power.
· The creation of a "burst electricity" industry, where excess electricity can be used extremely cheaply on windy days for opportunistic production, such as electrolysis for hydrogen fuel, and other processes that are efficient with intermittent electricity usage. This can prevent windmills from being forced to idle during days of excess power availability.
· Existing European hydropowerplants have the capacity to store enough energy to supply one month's worth of European energy consumption. Improvement of the international grid would allow using this at relatively short term at low cost. Furthermore, geographically spread wind turbine parks used together produce power much more constantly. On the longer term, the electricity might be used to produce hydrogen. This could be used with fuel cells to produce electricity at times of low wind supply and as fuel for transport.
· Improvement in energy efficiency should go hand in hand with the use of renewable energy.
· Wind turbines are beautiful, graceful machines that symbolise humans in harmony with the natural world.
· More recent wind farms have their turbines spaced further apart, due to the higher capacity of the individual wind turbines. They no longer have the cluttered look of the early wind farms.
· It is possible to hold a conversation directly underneath a modern wind turbine without any difficulty whatever and without raising one's voice. The modern turbine is quieter than its predecessors owing to improvements in the blade design. It makes a gentle "swish swish swish" sound that is quite pleasant and soothing. In addition, when it is windy the background noise of rustling trees, etc., exceeds the turbine noise.
· Wind turbines have epistemological value, as well as artistic value. As a form of sculpture, wind turbines are a dynamic (moving) art form. As an epistemological sculpture, they also make visible the process of producing electricity. There is an obvious beauty to seeing the process and understanding how it works. Urban wind turbines like the 750 kW Lagerwey in Toronto are popular gathering places where people come to sit and contemplate, and enjoy the restful beauty of a sculpture in motion, or to bring their children to what essentially amounts to a good outdoor exhibit that's open 24 hours a day.
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Wind power alone cannot satisfy the world's increasing demand for electrical power. But wind energy represents a feasible supplement in a diversified energy supply portfolio. The best wind turbines start working at less than 4mph with most cutting in between 8 and 12 mph. Generally wind speed decreases inland and is consequently higher at the coast. The speed is also reduced by trees and buildings and is therefore less in a city than in open countryside. An average wind speed through the year of 10mph is usually required for a wind machine to produce electricity economically. Wind measurement devices can be hired or bought and are known as anemometers. They usually consist of small propellers or cups that produce a small measurable electric current.
The following image is a wind resource map for the USA.
The wind resource estimates on this map refer to exposed
areas such as hilltops and plains. More
detailed wind resource information, including the Wind Energy Resource Atlas of
United States, published by the U.S. Department of Energy (DOE), can be found at
the National Wind Technology Center web site
The DOE
Windpowering America website at
http://www.eren.doe.gov/windpoweringamerica/
Local
airports can also often provide data however average wind speeds increase with
height and may be 15%–25% greater at a typical wind turbine hub-height of 80
ft (24 m) than those measured at airport anemometer heights.
Observation of
trees can also provide a good indication of wind speeds. The Beaufort Scale
helps provide information on actual wind speeds whereas the Griggs-Putman index
of deformity provides information on average wind speeds.
One of the first scales to estimate wind speeds and the effects was created by Britain's Admiral Sir Francis Beaufort (1774-1857). He developed the scale in 1805 to help sailors estimate the winds via visual observations. The scale starts with 0 and goes to a force of 12. The Beaufort scale is still used today to estimate wind strengths.
|
The
Beaufort Scale for use on land |
|||||
|
Beaufort
Force
|
Description
|
Effects
on land
|
Speed
|
||
|
knots |
km/h
|
mph |
|||
|
0
|
Calm
|
Smoke rises vertically. |
Less
than 1 |
Less
than 1 |
Less
than 1 |
|
1
|
Light
Air
|
Direction of wind shown by smoke drift, but
not by wind vanes. |
1
- 3
|
1
- 5
|
1
- 3
|
|
2
|
Light
breeze
|
Wind felt on face; leaves rustle; ordinary
vanes moved by wind. |
4
-6
|
6
- 11
|
4
-7
|
|
3
|
Gentle
breeze
|
Leaves and small twigs in constant motion;
wind extends light flag. |
7
- 10
|
12
- 19
|
8
- 12
|
|
4
|
Moderate
breeze
|
Raises dust and loose paper; small branches
are moved. |
11
- 16
|
20
- 29
|
13
- 18
|
|
5
|
Fresh
breeze
|
Small trees in leaf begin to sway; crested
wavelets form on inland waters. |
17
- 21
|
30
- 39
|
19
- 24
|
|
6
|
Strong
breeze
|
Large branches in motion; whistling heard in
telegraph wires; umbrellas used with difficulty. |
22
- 27
|
40
- 50
|
25
- 31
|
|
7
|
Near
gale |
Whole trees in motion; inconvenience felt
when walking against the wind. |
28
- 33
|
51
- 61 |
32
- 38
|
|
8
|
Gale |
Breaks twigs off trees; generally impedes
progress. |
34
- 40
|
62
- 74 |
39
- 46
|
|
9
|
Strong
gale |
Slight structural damage occurs (chimney-pots
and slates removed). |
41
- 47
|
75
- 87 |
47
- 54
|
|
10
|
Storm |
Seldom experienced inland; trees uprooted;
considerable structural damage occurs. |
48
- 55
|
88
- 101
|
55
- 63
|
|
11
|
Violent
storm
|
Very rarely experienced; accompanied by
wide-spread damage. |
56
- 63
|
102
- 117
|
64
- 73
|
|
12
|
Hurricane
|
Whole hangars disappear.
|
>64
|
>119
|
>74
|
You
can have varied wind resources within the same area. In addition to measuring or
finding out about the annual wind speeds, you need to know about the prevailing
directions of the wind at your site. If you live in complex terrain, take care
in selecting the installation site. If
you site your wind turbine on the top of or on the windy side of a hill, for
example, you will have more access to prevailing winds than in a gully or on the
leeward (sheltered) side of a hill in the same area. You also need to consider
existing obstacles such as trees, houses, sheds, out buildings and electrical
pylons. Don’t forget to plan for future obstructions such as new buildings or
trees that have not reached their full height.
Room
will be needed to raise and lower the tower for maintenance, and if your tower
is guyed, you must allow space for the guy wires. Whether the system is
stand-alone or grid-connected, you will also need to take the length of the wire
run between the turbine and the load (house, batteries, water pumps, etc.) into
consideration. A substantial amount of electricity can be lost as a result of
the wire resistance—the longer the wire run, the more electricity is lost.
Using more or larger wire will also increase your installation cost. Your wire
run losses are greater when you have direct current (DC) instead of alternating
current (AC). So, if you have a long wire run, it is advisable to invert DC to
AC.
The
economics leads us to the second consideration, which is the cost of the system,
and whether the power produced is less per unit than electricity supplied to you
from the mains. This point may not be decisive in deciding whether wind power is
for you as the satisfaction of building and running a wind power unit may be
enough on its own without any financial benefits, in addition you will be
generating pollution free power. Each site evaluation will take into account
different components but generally they are as follows.
·
The total cost of
installing the system - the wind generator itself, wiring, a tower or mountings,
control system, batteries and an inverter should a 240-volt ac supply be
required.
·
The cost of installation
at the site, such as scaffolding, should the unit be placed on top of an
existing building and the cost of a survey of the best location using wind
measurement instruments.
The costs should
be set over a year and the cost of maintenance added. It is then simply a matter
of dividing these costs by the kWh produced by the system. However when
comparing the unit cost against the cost of electricity supplied via the mains
it is worth considering that the power produced is tax free i.e. you do not have
to earn the money to pay for it and therefore save income tax and any other
taxes like VAT in the UK. Furthermore the annual cost will not rise
significantly, where as electricity supplied to you may rise in the future.
To decide what unit would be practical it is wise to consider the average wind speed, the size of the site, the power required and the cost of the unit. In order to evaluate these it is perhaps best to look at some of the fundamentals behind windmill design. The actual position of the windmill on the site should take into account the position of existing and future buildings. Areas of turbulence caused by buildings subject the windmill and tower to undue stress.
Small-scale wind power is particularly suitable for remote off-grid locations where conventional methods of supply are expensive or impractical.
Most small wind turbines generate direct current (DC) electricity. Off-grid systems require battery storage and an inverter to convert DC electricity to AC (alternating current - mains electricity). A controller is also required to ensure the batteries are not over or under-charged and can divert power to another useful source (e.g. space and/or water heaters) when the battery is fully charged. It is common to combine this system with a diesel generator for use during periods of low wind speeds. A combined wind and diesel system gives greater efficiency and flexibility than a diesel only system. It allows the generator to be used at optimum load for short periods of time to charge batteries when there is little wind, rather than by constant use at varying loads.
Wind systems can also be installed where there is a grid connection. A special inverter and controller converts DC electricity to AC at a quality and standard that is acceptable to the grid. No battery storage is required. Any unused or excess electricity can be exported to the grid and sold to the local electricity supply company.
Systems up to 1kW will cost around £3000 whereas larger systems in the region of 1.5kW to 6kW would cost between £4,000 - £18,000 installed. These costs would be inclusive of the turbine, mast, inverters, battery storage (if required) and installation, however it's important to remember that costs always vary depending on location and the size and type of system.
Turbines can have a life of up to 20 years but will require service checks every few years to ensure they continue to work efficiently. For battery storage systems, typical battery life is around 6-10 years, depending on the type, so batteries may have to be replaced at some point in the system's life.
Last, one of the most important aspects to consider is safety. A windmill in a gale presents a great hazard if there is not a breaking system or proper control of the unit. It is therefore important to ensure that the unit has an adequate mounting if it is to be fixed to an existing building or if onto a tower, that the tower is of suitable strength and is secured by guy-rope type supports. The windmill should be placed in a position so that the revolving blades can not come into contact with any objects especially people, this is particularly important where space is limited such as on a boat. The siting of the unit should also take into account noise and vibration; the rotors make noise as they shed power when the blades stall. The vibration caused could be a problem for instance if the windmill is mounted on an inhabited building.
A windmill takes energy from the wind (fluid) and produces power. The maximum theoretical power is 16/27 of the wind's power this is known as the "Betz limit". The wind’s energy, because it is moving, is in the form of kinetic energy and if all the energy was captured by the propeller or rotor there would not be an airflow consequently it can be said that the "lost" energy is used to keep the air flowing.
The maximum power available P is equal to –
K x D x A x V3
Where
K is a constant (Betz Limit)
D is the density of air (kg/m3)
A is the swept area of the blade (m2)
V is the velocity of the wind (m/s)
It can be seen that by doubling the velocity of the wind the available power is increased eight times (2x2x2) and by doubling the blade diameter the available power is increased four times. When the power required is known and estimating the efficiency of the machine it is possible to size the rotor for the average wind speed available. Equally the efficiency that manufacturers claim can be calculated. It should be noted that when calculating the power available the efficiency of the total system should be used, that is, not only the generator and gearbox but also the transmission and storage.
Wind turbines have "rated outputs" which give the best output at a particular wind speed. But this output must not be taken to mean power available i.e. rated output x hours, as the wind speed is not constant and will vary from around 1/10 to 1/3 of the figure.
Efficiencies achieved in practice are much less than the theoretical 59.3%(16/27), that of a traditional windmill with a small number of sail-like blades is a little over 5%. However since wind is often available in practically unlimited quantities, this "efficiency" can become of little significance.
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Wind units can be divided into two major types, horizontal axis and vertical axis machines.
Horizontal machines some times known as HAWT (Horizontal Axis Wind Turbines) are the traditional conventional design, they consist of a rotor with one to twenty blades driving a generator or a pump either directly or through a gearbox, chain or belt system. A tail vane or fantail is required to direct the machine into the wind.
They are usually more efficient than vertical axis units known as VAWT (Vertical Axis Wind Turbines). Savonius and Darius are two designs of vertical axis machines. This type of unit is often not situated on a tower and does not have to be directed into the wind. Materials and construction are usually cheaper than horizontal axis machines. The Savonius windmill was the brainchild of Sigrid Savonius of Finland. The racing driver of the 1930s said the secret of a good machine was to "add lightness and simplicate". A simple unit can be made by attaching two halves of vertically split oil barrel to a vertical axis this produces a low speed high torque unit that can be used for pumping water and through a gearing mechanism, generating electricity. This design also has the advantage of an aerodynamic effect called the "magnus principal”, suction is formed by the air moving over the convex face of the rotor. This means that there is force acting on the face of the rotor pulling it into the wind.
The Darius windmill was named after its French inventor. It is also known as catenary because of its profile when operating. The mill consists of slim aerofoil section blades taking up an oval or rugby ball shape when spinning due to the centrifugal forces exerted. The design gives a high output for the mass of the structure at relatively low cost. Darius units will often not start to turn by themselves and need either an electric start or use a small Savonius unit attached to the top. As the blades revolve they lose some energy as the head into the wind reducing the output.
Other types of windmill design include an H shape vertical axis machine designed at Reading University by a team lead by Dr Peter Musgrove, the H rotor is centrally mounted and consists of very thin aerofoils of un-tapered section. It has the advantage of being simple and also has a feathering device. Robert Walker in Wales designed an unorthodox windmill. It consists of a cone facing into the wind and the rotor (horizontal) set behind it. The wind speed increases as it passes over and around the cone.
Most of the units available commercially, are of the horizontal type. However DIY plans are available for vertical axis units.
In order to estimate what size system you will require, you
must first work out how much energy you use. This is simply a matter of working
out the energy consumption of all the electrical appliances you use. The power
rating in Watts (found on the makers plate on the back of the appliance),
multiplied by the time in hours the appliance will be used for each day gives
the amount of energy in Watt-hours you will need to generate each day in order
to run the appliances. The more energy you need the bigger and more expensive
the system becomes, so it is worth checking to make sure you are using energy
efficiently at this stage. Fitting low energy light bulbs and using sources
other than electricity for heat (ie. kettles, cookers, etc on gas or wood fuel)
will ultimately make a big difference in the overall cost of the system.
To produce the maximum power the generator must turn at the correct speed, it is therefore necessary to match the rotor design to the generator either through direct drive or through a gearbox. There are two types of rotor design.
High solidity, low tip speed ratio - typified by the Cretan sail rotor - the low RPM it most suited to pumping or electrical generation through a gearbox. High starting torque at rest enables good low wind speed performance. They are relatively easy to manufacture from sheet metal, timber and canvas. Wires and stiffening struts can be used with little reduction in performance. Blade design is not critical and accurate balancing is not required. The large amount of material needed to construct the rotor gives a high weight for the diameter and power produced. The large cross section also presents a problem and it is necessary to turn the machine out of high winds.
Low solidity,high tip speed ratio - the design usually consists of a two or three propeller type rotor, they are most often used for electrical generation as no gearbox or only a low cost gearbox is required. Higher efficiencies are usually achieved with this design compared with high solidity designs. The blades are usually costly to make and can be unreliable due to the high speeds and vibration. Impact damage can also be a problem. Straight grained woods, fibreglass and aluminium are all used as construction materials but fracture can still occur. Because very little torque is produced when the blade is stationary starting can be difficult. A small fan can be used to overcome this problem.
As both extremes have advantages and disadvantages, rotors are usually made to incorporate aspects from both designs, an example being a small aerofoil section with several blades.
This is
the wind speed at which the wind generator begins producing. For all practical
purposes, wind speeds below about 6 to 7 mph (3 m/s) provide little or no usable
energy, even though the blades may be spinning. At best, this minimal output
only overcomes the power losses caused by a long wire run
or the
voltage drop due to diodes. We are beginning to see high-tech controllers that
are able to “store” the small amount of energy available at low wind speeds
in the alternator windings. This energy is then pulsed to the batteries in a
manner similar to a pulse width modulated charge controller.
This is
the wind speed at which the wind generator reaches its rated output. Note that
not all wind generators are created equal, even if they have comparable rated
outputs. There is no industry standard for rated wind speed. The listed wind
generator companies rate their turbine output at anywhere from 18 to 31 mph
(8–14 m/s). This may not sound like a big issue until you understand that
there is potentially 511 percent more power in a 31 mph wind than in an 18 mph
wind.
This
measurement is taken at an arbitrary wind speed that the manufacturer designs
for. It tends to be at or just below the governing wind speed of the wind
generator. Any wind generator may peak at a higher output than the rated output.
The faster you spin a wind generator, the more it will produce, until it
overproduces to the point that it burns out. Manufacturers rate their generators
at a safe level, well below the point of self-destruction.
You are
not necessarily interested in the rated output of a wind generator. A turbine
with a high rated wind speed will invariably cost less than one with a lower
rated wind speed, for the same rated output. A higher wind speed gives certain
wattage to the manufacturer at a smaller rotor diameter, smaller physical size
of the generator, and subsequently less weight. All of this means less cost for
the manufacturer, and less cost to you. But remember, it takes a higher wind
speed to achieve that rating. In a 12 mph (5 m/s) average wind speed site, you
will see 18 mph (8 m/s) winds a mere 3 percent of the time. But you will see 31
mph (14 m/s) winds for less than 0.2 percent of the time. Rated output comes to
us from the photovoltaic industry, where panels are tested for output at a fixed
light intensity and a fixed temperature. The wind industry has no such fixed
standards. So, while comparing PVs based on rated wattage makes for great cost
comparisons, comparing rated outputs is a poor way to compare wind generators.
You are far better off comparing swept areas, or the KWH per month of
electricity the different systems will produce at different average wind speeds.
This
figure may be the same as rated output, or it may be higher. Wind generators
reach their peak output while governing, which occurs over a range of wind
speeds above their rated wind speed. Although widely touted by some marketers,
it has limited relevance to the buyer. To quote Hugh Piggott, “Peak or rated
output specifications for small wind turbines can be red herrings unless you
take the rated wind speed into account, and yet these specs are all the
customers seem to want to know about.” Wind turbines are not PVs, don’t
operate in the same manner, and should not be rated in the same way. What you
should be asking is what wind energy engineer Eric Eggleston asked, “What will
this wind generator do at my site with my average wind speed?”
Often described in wind turbine literature, this term has little bearing on the expected life of a wind generator. Engineers, on paper, design wind generators to survive wind speeds of 120 mph (54 m/s) or more. Unfortunately, wind turbines are not tested for these survival speeds because it is a very difficult thing to test for, or to test repeatedly. Much of the survival speed documentation is not from actually testing turbines at those speeds, but from anecdotal situations. If you watch a wind generator sited on a short tower near trees and building you will see it hunt around continuously, all the while buffeted by the turbulence caused by the short installation height, along with the nearby ground based wind flow interference. It is suspected that more wind turbines are destroyed by turbulence than have seen destroyed in survival-rated high winds. Furthermore, when wind speeds reach 100 mph or more the wind turbine and flying debris rather than the wind itself can often damage tower.
To make electricity an electrical generator is required. The first wind generators used a dynamo. These have now been replaced with alternators designed to give high efficiencies at low speeds. Car alternators can be used however they are designed to run at high speed and are therefore not normally suitable without gearing or modification.
Standard alternators use electric fields that use power typically around ten percent of the power generated and are less efficient than permanent magnet alternators. This is particularly true at low wind speeds. Although permanent magnet alternators are far more suited to the generation of electricity using wind power they are very expensive. As energy is not needed to excite the electromagnetic winding efficiencies of 60 - 80 % can be achieved.
A wind turbine must have a clear shot at the wind to perform efficiently. Turbulence, which both reduces performance and "works" the turbine harder than smooth air, is highest close to the ground and diminishes with height. Also, wind speed increases with height above the ground. As a general rule of thumb, you should install a wind turbine on a tower such that it is at least 30 ft above any obstacles within 300 ft. Smaller turbines typically go on shorter towers than larger turbines. A 250 watt turbine is often, for example, installed on a 30-50 ft tower, while a 10 kW turbine will usually need a tower of 80-120 ft. We do not recommend mounting wind turbines to small buildings that people live in because of the inherent problems of turbulence, noise, and vibration.
The least expensive tower type is the guyed-lattice tower, such as those commonly used for ham radio antennas. Smaller guyed towers are sometimes constructed with tubular sections or pipe. Self-supporting towers, either lattice or tubular in construction, take up less room and are more attractive but they are also more expensive. Telephone poles can be used for smaller wind turbines. Towers, particularly guyed towers, can be hinged at their base and suitably equipped to allow them to be tilted up or down using a winch or vehicle. This allows all work to be done at ground level. The purchaser can easily erect some towers and turbines, while others are best left to trained professionals. Anti-fall devices, consisting of a wire with a latching runner, are available and are highly recommended for any tower that will be climbed. Aluminium towers should be avoided because they are prone to developing cracks. Wind turbine manufacturers usually offer towers and purchasing one from them is the best way to ensure proper compatibility.
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As discussed earlier the two main uses for wind power are water pumping and the generation of electricity. Presumably anyone who purchases or constructs a unit capable of pumping water already has a use in mind. With electricity it can be put to use in different forms.
Most small windmills whether bought off the shelf or DIY will give a 12 volt or 24 volt dc output. dc stands for "direct current", it is the same form as that supplied by a car battery or a torch battery. This can be used to power direct current appliances like radios and electric motors. Many manufacturers supply a range of special appliances adapted to run on dc current from lights to refrigerators. Other dc equipment includes car, boat and caravan accessories. Many small portable televisions also run from a 12v-dc supply.
If this voltage and current type is not suitable fo the appliances you wish to power there is an alternative. An instrument known as an "inverter" can be used to change the 12v or 24v supply into 240volts ac. Ac standing for "alternating current". This is the type of electricity supplied by the main supply in most countries. In the USA, amongst others, the main supply is 115v-ac.
Inverters have two main components, a transformer to change the voltage and electronic circuits to produce the alternating current. It should be noted however that inverters use quite a lot of power in carrying out this conversion. Inverters are sized or rated according to their potential power output, which is measured in Watts. The higher the output the more costly each unit is.
Household appliances use different amounts of power. Generally those which produce heat directly or indirectly such as electric fires or fridges use much more than those wh