Wind power

Mankind has been harnessing the wind for more than two millennia. Windmills have ground grain, drained marshes, sawn timber, and even heated water by churning. The first windmill specifically designed to generate electricity was erected in Ohio in 1888, and in 1931 a 100 kW wind turbine in Yalta, in the former USSR, began feeding power into the local 6.3kV grid. Wind farms are now seen as the quickest and easiest route to reducing dependency on fossil fuels for national electricity generation. However, controversy still surrounds both their actual effectiveness and their potential impact on the local tourist

Wind Turbines Cause Global Warming
Wind farms are becoming larger and more common, but still represent only a small fraction of the world's electricity generating capacity (Reproduced with permission from Michael J. Pasqualetti)

trade and/or local wildlife. Generous government subsidies in most developed countries have spurred technological progress, and the latest generation of very large turbines are beginning to live up to the dreams of their adherents. At the end of 2006 the total world wind farm capacity was calculated to be nearly 75,000 megawatts, still less than 1% of world electricity consumption.

At the other end of the spectrum, there has been a parallel blossoming of interest in small domestic wind turbines, usually rated at 100 kW or below. A bewildering variety of designs have been offered, with a number of frankly misleading claims as to their potential performance made by manufacturers. Recently it has become fashionable to have a roof-mounted wind turbine installed, particularly amongst aspiring politicians. Small turbines are even available from DIY stores. More significantly, a number of building integrated wind turbine designs are beginning to leave the drawing board. Research into various types of medium-scale wind turbine technologies is beginning to bear fruit. For most projects above the domestic scale, wind energy is now worth considering as at least part of a package of renewable energy alternatives.

Recent experience all over the world has confirmed what the old windmill builders knew well; extracting energy from the wind is never straightforward. These days we have detailed maps of average wind speeds available for many countries, but these are only a starting point. The key to successful wind turbine installation lies in the micrositing. Windmills were built on hills, or on flat, featureless, almost treeless plains, where the wind had a good 'fetch' , an unobstructed run through the windmill's sails. Height helps, as wind velocity increases with distance from the energy-sapping ground - although windmills were never built on really tall hills. What is really needed is steady, predictable wind with as little turbulence as possible.

So the most predictable and productive wind farms to date are those built off shore. Wind farms on high altitude moorland also perform well. Least favourable is the urban environment. Not only do closely packed buildings and trees suck massive amounts of energy out of the wind immediately above them, they also generate highly unpredictable turbulence. A typical small wind turbine battered by a turbulent air stream rarely produces enough electricity to make it more than a token gesture to a greener lifestyle. And retrofitted building mounted small wind turbines can also feed unacceptable levels of noise and vibration into the building. On the other hand, some buildings with certain types of roof will see wind speed increase over the roof when the orientation is favourable. This accelerated airflow can be utilised if the installation is correctly designed. Buildings in industrial estates or on the urban fringes could well experience less turbulent flow than those in town centres - but trees can still be a major problem for turbines at low level.

Whatever the location, the same basic laws apply. Available power is related to the cube of the average wind speed. Double the wind speed, and eight times as much power is available. Double the height of a turbine by mounting it on a tower, and average wind speed will increase by around 10% - which implies a 30% plus increase in power generated. Unfortunately, much of the energy generated will come in short bursts at higher wind speeds. Typically, half the total energy will be produced in just 15% of the operating time, making either connection to the national grid or some form of energy store essential for economic operation (see Chapter 1 and Chapter 15). A useful rule of thumb is that wind turbines are worthwhile where the average wind speed is greater than 4.5 m/s - at turbine height. For individual installations it is not enough to depend on average wind speed maps, especially where the local topography is far from flat and featureless. A proper survey measuring wind speeds in various locations on the project site for a whole year would be the ideal solution, as experience has shown that in some locations moving the wind generator less than 50 m can double its output.

By far the most common and most developed way of extracting energy from the wind is the horizontal axis wind turbine (HAWT). This will typically be a three-bladed design -although versions with anything from one to 20 blades have been tried - with the blades mounted on a shaft that turns some form of electricity generator (see below). This assembly is mounted on a tower. Most designs have the blades rotating upwind of the hub to minimise the effects of the inevitable turbulence downwind of the tower. These blades have to be relatively stiff or mounted well away from the tower or tilted upwards, to minimise the risk of blades flexing under load and striking the tower - which, alternatively, could be raked forward to achieve the same goal.

Multi Tailed Metal Wind Vane
Old and new - wind turbines and an agricultural windmill (Reproduced with permission from Michael J. Pasqualetti)

Small upwind HAWTs will have a tail vane to turn the blades into wind: larger models will have a wind sensor and a servomotor to ensure alignment, although these can become overwhelmed by a gusty, turbulent airflow, typical of urban locations. Downwind designs do exist, and align automatically, simplifying manufacture and maintenance. The blades on downwind HAWTs can be made more flexible than upwind alternatives, allowing them to flex and spill wind when a gust strikes. Against that, as blades pass through the mast's turbulence the forces on them will vary suddenly, causing extra stresses on the hub.

Wind Shear Wind Turbine
An established downwind HAWT - the 15 kW Proven 15 (Reproduced with permission from Proven Energy)

All HAWTs suffer from the same problem of asymmetry of blade loading. Wind shear, the variation of wind speed with height above ground, usually means that, at any given moment, there will be a measurable difference in air velocity between the lowest and highest points of a blade's rotation, even on a relatively small installation. On the large wind farm HAWTs this difference is considerable. Thus the aerodynamic forces on individual blades will fluctuate significantly as they rotate, and these forces have to be resisted by the hub and tower. Moreover, as the turbine swings backwards and forwards to follow the wind there will be gyroscopic forces acting on the blades, which again will put stresses into the hub and tower, and its foundations. These cyclic and random stress variations have caused fatigue failures at the hub in a number of early HAWT installations.

Better understanding of the forces involved has led to the development of much more rugged designs, and fatigue failures are now rare. The three-bladed option is popular, largely because it minimises cyclic variations. It should be noted that a three-bladed HAWT would only extract around 3% more energy out of the wind than a two-bladed design, at the price of greater initial cost and more complex erection. Medium-sized two-bladed designs with developed versions of the so-called 'teetering' hub, which reduces hub stresses, are now commercially available, although it is claimed they are noisier than equivalent three-bladed designs.

Alternative forms of HAWTs have also been the subject of much experimentation. Small domestic-scale units are now available in the US with two widely separated twin-bladed rotors mounted on the same shaft, one upwind and one downwind of the mast. These co-axial designs are claimed to be significantly more efficient than single rotor designs at low wind speeds, and there are proposals for much larger wind farm sized versions. In Japan a turbine that uses a so-called 'loopwing' is on the market. This looks remarkably like a giant food mixer lain on its side, and is claimed to be quieter, safer, and more efficient than conventional small HAWTs.

Counter-rotating co-axial designs have also been tried, although none are yet available commercially. Prototypes with both sets of blades on the same side of the tower, and on opposite sides, have shown that they can tap more of the wind's energy over a wider range of wind speeds than a single-rotor design. One experimental installation in California is said to be up to 40% more efficient than a comparable single-rotor system, but at the price of greater complexity and cost.

Ducted rotor or diffuser augmented wind turbines (DAWT) also offer greater efficiency, at a similar price. Surrounding a rotor with a duct that is shaped to accelerate the wind flow through the rotor significantly, is said to enable it to operate at higher speeds in a wider range of wind speeds. In practice, it would seem to make little sense to mount a DAWT on a tower as an alternative to a conventional HAWT. To achieve significant augmentation the duct has to be as long as possible, preferably at least seven times the rotor diameter, which poses a formidable alignment problem in rapidly shifting winds. And for the same cost and material usage a bigger HAWT, which would probably produce more power overall, could be constructed. However, both DAWTs and counter-rotating co-axial HAWTs might have a role to play where turbine diameter has to be restricted for some reason, as they can achieve more power from the same diameter than a conventional HAWT. This can make them some of the more interesting options for building integrated wind turbines (see below).

All HAWTs can trace their ancestry back to the earliest European windmills, which were first recorded in the thirteenth century. The most primitive types used fabric sails spread by timber frames, which could easily be furled in high winds. By the time the classic windmill fell out of favour more than 500 years later, the sails were sophisticated aerodynami-cally efficient fabrications made up of timber shutters that could be adjusted while the sails were in motion. In high winds all the shutters opened to minimise the loads on the system. Modern HAWTs also have to have some form of speed-limiting device to avoid overloading. Earlier medium-sized HAWTs depended on self-regulation; as wind speed and the loads on the blades increased, they began to warp, until turbulence grew rapidly and the blades effectively 'stalled'. This is a noisy process, and large modern HAWTs generally reduce the angle at which the blades meet the wind - the angle of attack - as wind speed increases, to keep the torque at the hub constant. Either individual electric servomotors or hydraulic power is used to accomplish this feathering or furling, and there will be a back-up system that will furl the blades if main power fails.

Such complexity is really only cost-effective on large installations. A more realistic alternative on medium-sized projects is electrical braking, in which electrical energy is drained off into a resistor, which converts the kinetic energy of the blades into heat - which can be utilised for many purposes. This system, also known as dynamic braking, allows the turbine to run at constant speed even when wind speeds are high. Passive pitch control is used on the Iskra AT5-I, a 5.4m diameter three-bladed HAWT rated at 5kW; springs in the hub balance centrifugal force from the rotating blades against torque loads from the generator to maintain blade pitch at the optimum setting.

If HAWTs are the descendants of mediaeval European windmills, vertical axis wind turbines (VAWT) have an even more ancient lineage. One of the earliest applications of wind power was to grind grain, and millstones have a vertical axis. To the Persians, and subsequently the Chinese, it made perfect sense to arrange fabric sails around a central shaft connected directly to the upper millstone. This was some 3,000 years ago. Even then, the windmill builders were aware of the fundamental problem with VAWTs - for part of their rotation

Passive pitch control features on the 5kW Iskra AT5-I (Reproduced with permission from Iskra UK)

Drag Vawt Panemone
Panemone-based designs have been around for centuries

the blades are travelling against the wind. In the Persian panemone, a wall shields half the assembly, allowing it to rotate against the wind direction with as little drag as possible.

More sophisticated versions of the panemone are still intriguing wind energy enthusiasts to this day. All VAWTs share the same basic advantages: the heavy generator is at ground level and much more accessible for maintenance, the assembly needs no yaw mechanism to keep it pointing into the wind, and vulnerability to high wind speeds is much reduced. The expensive tower used by HAWTs is eliminated: transport and erection costs are usually much lower.

Against these benefits must be set the inherent drawback of lower overall efficiency due to the drag from the blades travelling forward against the wind for part of the rotation. Efficiency is also compromised by the fact that most VAWTs operate closer to the ground than HAWTs, in a more turbulent and less energetic airflow. The latest designs are all aimed at reducing these drawbacks and maximising the benefits, and most are variations on a number of basic themes.

Simplest of all is the Savonius turbine, little more than a sophisticated version of the twin scoop rotors used for anemometers and roof vents. With three scoops or more a Savonius turbine will always self start, and there has been much work done to improve its basic efficiency. Perhaps the most interesting variant is the TMA design from the US, originally a , multi-Savonius, concept with fixed vanes to concentrate the wind on the rotor. This has now evolved into a much simpler and more efficient version, with a slender 'twin scoop' rotor surrounded by three offset 'stators', which both accelerate the wind speed hitting the rotor and act as the main support structure. Claimed advantages include the ability to operate efficiently over a much wider wind speed range than HAWTs and thus to have a significantly higher capacity factor, and to offer no threat to birds and bats.

Direction of rotation

Direction of rotation

Savonius Turbine Rate Curve

Principle of the Savonius turbine (Reproduced with permission from Schnargel)

Principle of the Savonius turbine (Reproduced with permission from Schnargel)

'Eggbeater' designs have also been much developed. Collectively known as Darrieus turbines, the earliest versions used simple fixed pitch blades arcing out from a central shaft, and were relatively efficient but were subject to large variations in torque and consequent stresses on the central tower. Other drawbacks were the need for assisted starting -sometimes by a small coupled Savonius turbine - and the loads on the central bearing produced by the downforce from the guy wires used to stabilise the tower.

Later developments took two main routes to improve the basic design. More fixed pitch blades were added, often in a helical arrangement, to minimise torque fluctuations and aid self-starting. A good example is the British quietrevolution QR5 design. The basic model has three tapered S-shaped blades, a direct drive permanent magnet generator and measures 5 m high by 3.1 m diameter. Unable to self-start, the QR5 senses wind speed via an anemometer and the control system passes a current through the generator to start the turbine moving when wind speed reaches a predetermined value. In a typical urban situation, mounted either on a mast at least 9 m high or 3 m above a building roof the QR5 is claimed to generate at least 10,000 kWh a year in suitable locations.

Vawt 100kw
An original approach to VAWT design - the 2007 version of the TMA unit (Reproduced with permission from TMA Wind)

An alternative approach sees the blades made straight and vertical, mounted at the end of radial arms, and often endowed with variable pitch. This latter feature enables self-starting, and produces a relatively flat torque curve as well as helping speed limitation in very high winds. Some, like the Italian Ropatec range, have their generators between their fixed pitch vertical blades and can be 'stacked' one above the other as needed.

An interesting variation on the Darrieus principle is the Windstar from the US. Basically a variable pitch vertical blade design with the tower supported by an external steel

Rotates Image When Wind Hits

Airspeed due to rotation

Principles of the Darrieus turbine (Reproduced with permission from Graham UK)

Airspeed due to rotation

Principles of the Darrieus turbine (Reproduced with permission from Graham UK)

framework, the Windstar is said to work best when installed in linear arrays of three or more, with the blades passing within 600 mm of each other. Dubbed linear array vortex turbine systems, the claimed advantage is a massive boost to the efficiency of the internal turbines at wind speeds below 20 m/sec. Current versions on offer are 15 m high and up to 23 m in diameter, and are seen as ideal partners to large HAWTs in wind farms, operating in the turbulent air beneath the HAWTS.

Solwind Vawt
VAWTs can be mounted on towers, as shown by this 4 blade, 6 kW Solwind unit (Reproduced with permission from Alvesta)

Darrieus type VAWTs may also be mast mounted, as in the case of the Solwind Four Winds unit from New Zealand. Four or six fixed pitch blades drive a low-level generator via a right angle gearbox and vertical shaft. Self-starting is said to occur at wind speeds in excess of 1.5m/sec, outputs on standard versions reach 9kW. Solwind VAWTs became available in Europe in 2007 from Alvesta, and versions yielding up to 100 kW are under development.

VAWTs are becoming an increasingly attractive option for both building mounted wind turbines (BMWT) and building integrated wind turbines (BIWT). In some cases multi-bladed Darrieus turbines are actually mounted horizontally on rooftops, to minimise the forces transferred to the building structure. The Dutch led the way with the WindWall; a more recent development due to come onto the market in 2007 is the 520 H Aeroturbine from Chicago-based Aerotecture. The company's first product was the vertical axis 510 V Aeroturbine, which features a translucent helical Savonius rotor fabricated from Lexan (polycarbonate) coupled with a small Darrieus turbine. Standing 3 m high, the 510 V is claimed to generate around 1kW in 50 km/hour winds. Two 510 Vs mounted horizontally back to back make up the 520 H. In this form the turbine needs to be aligned with the prevailing wind. TMA turbines (see above) can also be mounted horizontally on rooftops.

Aeroturbine
Aerotecture's 520H Aeroturbine is one recent example of a VAWT turned on its side to better integrate with the building (Reproduced with permission from Kurt Holtz, Lucid Dreams Productions)

When located on the corners or sides of buildings, VAWTs can be visually much less obtrusive than HAWTs on rooftops, and more resistant to the inevitable turbulence in urban locations. Vibration can still be a problem, however, especially in retrofit situations.

Retrofitted rooftop-mounted small urban HAWTs have an unhappy history. Problems with vibration and turbulence are widespread, and overall performance has regularly fallen far short of optimistic manufacturers' predictions. Mounting the HAWT on a rooftop tower or mast helps to reduce the turbulence problem, although such an installation could fail to find favour with the planning authorities. It can also feed significant structural loads into the building. There is also concern about the health and safety implications of a blade failure, seen as much more likely in the highly turbulent urban environment. Some small HAWT manufacturers have tackled these problems head on by developing designs like the Swift, where the blade tips are linked by a circular rim, and the Combined Augmented Technology Turbine, where the power producing rotor is surrounded by a short duct and sits behind a free rotor which is claimed to 'process, rapidly veering and turbulent air before it hits the main rotor. Another approach is to use gears at the hub to turn a vertical shaft inside the tower, which then drives a generator at the base of the tower. This is claimed to increase reliability and minimise stresses at the rotor hub.

If space permits, the usual advice is to site the turbine as far away from any obstruction as possible - this obviously includes the building it is supplying. A set-off distance at least ten times the height of the obstruction is recommended. The fact that trees could gain significantly in height during the turbine's lifetime should not be forgotten. Long set-offs lead to energy losses in transmission from the turbine to the building, or heavier gauge cabling might have to be specified, adding to the expense. The only alternative way of maximising turbine efficiency is to mount it on a tower at least 10 m higher than the height of any obstructions closer than the ideal minimum distance. Experience with HAWTs, however, indicates that towers taller than three times the rotor diameter are rarely economic.

Aeolian Altechnica Building
Mast mounting away from the building is usually a better solution than HAWTs on rooftops (Reproduced with permission from Proven Energy)

There will be few sites where such generosity of space is available, although schools with playing fields and shopping complexes with large surface level car-parks might be exceptions. Even on the urban fringes it might well be the case that some form of building-integrated turbine is the most effective option - and the easiest to get past local conservation groups and planning authorities. BIWTs are relatively recent developments, and many have yet to progress beyond the concept or prototype stage, although such experience as is available suggests that this could be a very useful option for building designers.

One of the simplest and least obtrusive is the ducted wind turbine design developed by the University of Strathclyde. A rotor is mounted on a vertical axis inside a duct curved through 90 The inlet to the duct is mounted on the face of the building while the outlet is on the roof; the vertical shaft passes through the wall of the duct to drive a generator below. Orientation is obviously critical, and it may be that turbines have to be installed on more than one face of the building to yield worthwhile power.

A much bolder approach is to design the whole building around the wind generator. One of the most dramatic concepts is the WEB Concentrator, in which two aerodynami-cally shaped tall buildings are linked by large ducted HAWTs. A smaller-scale experimental version with a single rotor proved to be remarkably insensitive to wind direction and to perform better than expected at low wind speeds. Development continues, but obviously such an extreme building would be relatively expensive to construct.

More practical perhaps are the various concepts developed by Altechnica of the UK -collectively dubbed Aeolian Planar Concentrator devices. These use wing-like aerofoils to accelerate wind speed through both HAWTs and VAWTs, which can be free standing or mounted above the roof. Integrating them with the building proper is an exciting alternative. The simplest version is the Aeolian Roof. In new build the main roof can curve upward towards the centre, where a row of relatively small turbines, either HAWTs or VAWTs, are topped by a horizontal planar concentrator. This has a flat upper surface, which can support solar PV arrays. A similar, if less effective installation can be fitted to an existing pitched roof, and protective grilles or screens can be specified if there is a perceived risk to health and safety. These will reduce the energy yield, but the basic system is said to be remarkably tolerant of oblique winds, making orientation not so critical. Another alternative is to install a similar array arranged vertically at the corners of tall buildings.

Wind turbines can generate both alternating and direct current. Early large HAWTs and VAWTs invariably used gearboxes at the hub to speed up the input from the relatively slow rotation of the blades to one that suited the generators available. HAWTs in the 50 m diameter class are now available with highly efficient low speed direct drive generators, cutting capital and maintenance costs, and most wind turbine manufacturers are looking seriously at this option. Small HAWTs have almost always used direct drive generators to produce DC power, a legacy of their early role as battery chargers. For most purposes what is needed is AC power at grid frequency and voltage. One simple approach, used in most early wind farms, is to govern the speed of the turbine within very close limits so that a simple, cheap induction generator produces AC power at a frequency very close to grid frequency. This severely reduces the turbine's ability to extract power from a wide range of wind speeds. Later generation large turbines use more sophisticated AC or DC generators and control systems to supply precisely regulated current at prescribed frequency and voltages. There is some loss of energy in the regulation process, but this is more than compensated for by the greater range of operating speeds possible.

In the early days of wind generated electricity there was a regrettable tendency to rate turbines by the potential output of the generator itself. This could be completely misleading. The power produced by a turbine is determined almost entirely by the mass of air that can pass through its blades. This is a function of the swept area, which in HAWTs is the area of the circle described by the blades. Thus, for convenience HAWTs are usually classified by blade diameter, which is then linked to potential output.

Table 5.1 Typical relationship between HAWT rotor diameter and output

Diameter (m)

Output (kW)

9

15

15

50

20

100

30

225

40

500

50

600

VAWTs are harder to classify, as each type has its own unique characteristics. The whole situation is complicated by the fact that there is no internationally recognised standard testing or classification system. Manufacturers rate their products at different wind speeds, anything from 10 m/sec to 15 m/sec or more. As the output from any particular wind turbine at any particular location is probably influenced more by its micro-siting than by its basic design, specifying an installation that will be likely to meet a project's needs is fraught with problems.

A number of factors have to be considered, of which the unit's rated capacity is only the starting point. The next most common figure usually quoted is the predicted annual output for any given average wind speed. Basing any site-specific calculations on national wind maps can result in subsequent disappointment, as actual average wind speed. Therefore, actual output achieved may be significantly different to that predicted from the wind maps, especially if local obstructions generate energy-sapping turbulence. For anything but the smallest and most token of wind turbine installations a proper wind speed survey is essential.

This could well reveal that the projected turbine will only develop something between 20 and 40% of its theoretical annual output - a figure derived by multiplying its rated capacity in kilowatts by the number of hours in the year. Smaller turbines operating in largely turbulent airflows will struggle to meet even these figures. These annual capacity factors are site specific - those quoted by manufacturers are at best averages and at worst highly optimistic. The table below details a list of typical test results on an HAWT.

Even if a 12-month survey of a particular site reveals excellent potential for wind power, the next step may turn out to be the most frustrating. Currently there is a massive gap in

Energy Production and Carbon Dioxide Savings from Iskra Wind's AT5-1 Turbine, based on Test Site Results after Inverter

Annual

Annual

Expected

CO2 saving

Expected

CO2 saving

average

average

Yield per

per year;

Yield per

per year;

wind

wind

turbine;

Tonnes/year

turbine;

Tonnes over

speed; m/s

speed;

MWH/year

MWHr over

20 years

(at hub

mph

20 years

height)

3

6.72

1.84

1.05

36.8

20.90

3.5

7.84

3.13

1.78

62.6

35.56

4

8.96

4.75

2.70

95

53.96

4.5

10.08

6.64

3.77

132.8

75.43

5

11.20

8.74

4.96

174.8

99.29

5.5

12.32

10.94

6.21

218.8

124.28

6

13.44

13.15

7.47

263

149.38

6.5

14.56

15.31

8.70

306.2

173.92

7

15.68

17.35

9.85

347

197.10

7.5

16.80

19.24

10.93

384.8

218.57

8

17.92

20.93

11.89

418.6

237.76

8.5

19.04

22.41

12.73

448.2

254.58

'Conversion factor: 0.568 kg CO2/kWh for grid electricity - see UK Building Regulations L2A, para 22. Reproduced courtesy of Iskra UK.

Field test results post-inverter for a HAWT (Reproduced with permission from Iskra UK)

Field test results post-inverter for a HAWT (Reproduced with permission from Iskra UK)

wind turbine availability. Units up to 20kW are common, and a number of manufacturers offer the larger wind farm turbines in capacities from 225kW to 5,000 kW.Demand in the intermediate size range has historically been low, and the number of manufacturers supplying suitable turbines is also low, while costs could be correspondingly high. The problem comes when the analysis for a particular project throws up an answer that falls into the size gap. For example, a typical modern energy efficient office development may consume up to 200 kilowatt-hours of electricity a year per square metre of floor area. To supply say 20% of that from wind energy implies an installation with an annual output equivalent to 40kW hours per square metre. Assuming a generous capacity factor of 30% means that a relatively modest office development of 1,000 m2 needs a turbine with a theoretical annual output of more than 130,000 kW hours. Meeting this demand would require thirteen 5 m high VAWTs such as the QR5, three or four HAWTs with 9 m diameter blades, for example, or possibly two 15 m diameter HAWTs on 30 m high towers. A 20 m HAWT would seem to be the simplest answer. The units described would probably be rated as 15 kW, 50 kW and 100 kW, respectively. Larger developments would need larger capacity, and the choice is limited.

Where the site is suitable and the planning authorities complacent, the best answer for the medium-sized project for the moment at least might well be a secondhand 225 kW

HAWT from one of the early wind farms. Wind farm operators are constantly upgrading their installations to take advantage of the improving performance and economics of ever larger HAWTs, and the earlier units are available at reasonable prices with at least ten years of potential trouble-free service left in them. By the time they need replacing more appropriate turbines might be on the market. A 225kW machine is likely to have a 30 m diameter rotor and be mounted on a 35-50 m high tower.

Objections to such an installation will typically be based on visual intrusion, noise, flicker and danger to bird life. The latter now seems to be exaggerated, although controversy continues, and the danger to bats seems to be greater than expected. VAWTs are said to be safer in this respect, as the birds and bats usually perceive them as a more solid obstacle. Older HAWTs are noisier than the current generation of large wind farm turbines, but most fears about noise pollution have so far turned out to be unfounded. The flickering shadows from the revolving blades of an HAWT are also annoying to some people.

Altechnica
Development of building integrated wind turbines continues (Reproduced with permission from Kurt Holtz, Lucid Dreams Productions)

Small- and medium-scale wind power has so far failed to realise its theoretical potential. Many pioneering entrepreneurs have tried to develop the technology to the point at which it is a realistic option for every building developer. Some are beginning to succeed. Wind power on its own will never be the ideal solution; coupled with other technologies, especially an efficient energy store, it is definitely worth considering for more than a symbolic contribution to a greener future.

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Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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