Resources

Installing Small Wind Turbines

This is a guide, for any installation the particular manufacturers instruction must be followed.

1 - Siting

Small Wind Turbine (SWT) positioning is based on particular compromises outlined below. The influence of these factors is learned by experience, although some rules-of-thumb apply. A poorly sited turbine may have increased installation and maintenance costs and / or mediocre energy capture. Therefore taking extra care in the planning stage could pay significant rewards.

Local Topography and Wind Obstructions

This is possibly the most important factor to consider when micro-siting a SWT. Consider the effect of hills, valleys, buildings, trees etc on the prevailing wind. Both weak and turbulent wind flow are to be avoided. It is best to have minimal disturbances to the wind particularly in the prevailing wind direction. The ‘region of highly disturbed flow’ in Figure 1, must be avoided. Ridges or steep slopes should be avoided at all costs. Such features have severe turbulence which may reduce the life of a SWT and the potential energy capture. Gradual, smooth, slopes accelerate the wind which may increase energy capture.

Figure 1: The effect of obstructions on turbulence. (Source: Gipe, P. Wind Energy Basics p72).

Distance to Occupied Buildings

All turbines produce some noise, it is advisable to install SWTs away from any occupied buildings etc. A general rule of thumb is allow 100m and if possible install downwind of the predominant wind direction. It is not advisable to install a SWT on the roof of an occupied building. A SWT is a rotating machine, some sort of vibration will be transmitted to the building at all operating speeds.

Distance to Grid or Battery Room

The SWT should be located as close as possible to the battery room (or to a power line for grid connected) to save on conductor and civil works cost and reduce power loss.

2 - Planning Approval

States and territories and local councils have differing requirements. These local rules should be studied to ascertain the likelihood of approval. If your site is appropriate for your area an Application to Commence Development should be sent to the local Council. A general application contains:

  • A completed application form to be signed by the owner of the land
  • A report containing details of the proposed development with diagrams of tower and foundation
  • A map showing the location of the proposed turbine in relation to it’s surroundings
  • The appropriate planning service fee

The council will then seek further assessment from government bodies, such as the Heritage Council, the Main Roads Department and Bush Forever. Obtaining planning permission for installation of a SWT can be lengthy, though NSW has taken steps to simplify the process.

3 - Tower Types

There are two basic types of tower, self supported and guyed. These can both be either tubular or lattice designs. A guyed tower, shown in Figure 2, is generally lower in cost since the tower and foundation require less material, note there are 5 separate parts to the foundation. This design allows erection via the levering action of a gin-pole with a tow-up vehicle. Self supported towers can also use a gin pole with 2 foundation parts, tower and gin pole anchor foundations. Many self supported towers require a crane to erect them, the entire structure is lifted into place.

Figure 2: The effect of obstructions on turbulence. (Source: Gipe, P. Wind Energy Basics p72).

4 - Civil Works

After selecting the proposed site to install the turbine, the next step is to determine the exact location of the various foundations both for the cantilever type and guyed type guy anchor blocks.

Site Preparation

It is important to follow the manufacturers’ plans regarding the layout of the foundations. The best orientation of the foundations should be determined, so the tower may be easily lowered (i.e. nothing in the lay down path including rotor area) and an adequate tow-up path exists or a winch may be used. A winch may need a separate anchoring foundation. The exact locations of the foundations and the cable path to the grid or battery room should be pegged out.

Foundations

For a guyed tower the guy foundations provide a restraining force to oppose the guy tension. The tower foundation takes the axial force, acting down the tower. For a self supported tower the foundation provides a resisting moment (twist) to keep the tower upright, this foundation is more substantial for the same height and turbine size to the sum of those of a guyed tower. Both foundation types must be able to withstand the loads caused by the maximum design wind speed acting on the structure. Soil type significantly affects foundation design, soils that have high clay content can withstand greater bearing loads than loose sandy soils. Moisture content of the soil also changes the mechanical properties.

Concrete

Most guyed tower foundations and all self supported towers used with wind turbines are made by casting concrete into holes in the ground. Details, grade of concrete, reinforcements and the dimensions of the block are provided by the manufacturer and must be adhered to. The concrete is poured into the hole where it will bear against undisturbed ground, few sites are level so it is common that wooden shutters (a low box open at the top and bottom) is used so that the concrete level may be raised above that of the surrounding ground. See Figure 3. Concrete foundations need to cure before being loaded. Curing time depends on many factors it is good practise to leave them to cure for a week.

Figure 3: Guy wire anchors cast into the concrete block. Source: B. Brix, Westwind Turbines.

Some manufacturers provide foundation components to be cast into the concrete. Others prefer first casting the concrete blocks and then bolting hardware to the blocks. Components cast into the concrete must have their locations double checked before the concrete starts curing and becomes un-workable.

Ground Anchors

Some small turbines use ground anchors to restrain the guys, this reduces the cost. Ground anchors are available in various designs, but the operating principle is similar in that they rely on the passive soil resistance and mass of ‘entrapped’ earth to resist uplift. One type of anchor resembles an auger, which is screwed into the ground. Soil type and the moisture content affect the restraining load offered. Anchors are usually driven into the ground parallel to the guy wire so all load is taken as pure tension.

Electrical Cabling

Electrical standards stipulate the depth to bury electrical cabling (normally 600mm) and other requirements such as safety signage that will be required. Figure 4 shows the laying of a 100 m turbine cable for the Exmouth wind farm.

Figure 4: Trenching for the underground cable. Source: B.Brix, Westwind Turbines.

Occasionally, particularly with grid connected systems, a concrete pad may be required to secure electrical equipment. These usually require electrical conduits cast into the concrete so that the underground cable may be neatly connected.

5 - Small Wind Turbine Assembley

There are two basic parts to assemble, the tower and the nacelle – turbine, these are usually assembled in this order.

Tower

As previously mentioned there are two tower types, guyed and free standing. There are also various assembly methods, these will be addressed below. For all towers the sections are laid out on the ground in the correct order. Before joining these sections (see below) a draw wire should be threaded through, this is to draw the power cable thru once the tower is assembled. There are some lattice towers but they are so few they will be omitted here.

Self Supported

Most free standing towers have sections joined with a slip joint. A slip joint is where two tapered sections join by the lower section slipping into the upper section. These joints need to be axially compressed, a winch or jack is used to do this. Before compressing the joints it should be checked the sections are in the correct orientation to each other. For tilt towers, with a gin pole, the bottom section should be fitted to the hinge in the foundation first and each section fitted in sequence. Then the gin pole fitted to either the tower or the foundation depending on the design, larger turbines may need a HIAB or a small crane to do this (See 1.5.1.2 for more detail). Once the tower is assembled it should be checked over and have a trial standing prior to fitting the turbine. For towers without the tilt option, before compressing the joints all sections should be checked for correct orientation to each other. Depending on the manufacturers’ instruction the tower can either have the turbine fitted or be first lifted via a crane onto the foundation. Some free standing towers have bolted joints, for these follow the bolting procedure below (in 1.5.1.2) in place of winch compressing above.

Guyed

On tilt-up guyed towers the sections are first laid out on the ground in the correct sequence. Firstly the tower base is fitted onto the central hinge assembly and then each section is bolted on in succession. Most tubular style tower have flanged joints using bolted connections. The gin pole is also hinged at or near the tower hinge. A sheave (pulley) system connecting the guy wire end of the gin pole to the foundation block is used to increase the mechanical advantage of the towing vehicle. Once the sheave system and the guy wires have been attached the gin pole may be lifted to the vertical position. A HIAB or small crane may be needed to lift the gin pole on larger towers Rigging screws or turnbuckles are usually fitted on the ground level end of all guy wires to allow for fine adjustment in length. Again once the tower is assembled it should be checked over and have a trial standing prior to fitting the turbine. During this trial lift the tension of the guys should be monitored

Nacelle - Turbine

Some manufacturers supply a small trestle structure the tower should be supported, off the ground, with this trestle so there is significant clearance for fitting the turbine. The first stage of installing the turbine onto the tower usually involves bolting the nacelle onto the tower top. The tower electric cable can be connected to the turbines output leads either before or after bolting the nacelle onto the tower top. Attaching the tail boom and fin (if used) and then the blades is next. The method of doing so differs for each manufacturer, the installation manual should be followed, particularly the torques needed for the blade attaching bolts. Some manufacturers provide a nose cone to cover the hub of the turbine; this is primarily for aesthetic purposes but also may shield the blade pitch mechanism from water and dirt.

6 - Erection and Comissioning

After all of the bolted and electrical connections have been re-checked the tower is ready for final raising. If possible short circuit the generator (if it is of the permanent magnet type) prior to raising the tower. This is usually possible at the terminal box at the tower base where the tower electric cable connects. The tower should only be raised in relatively low wind strengths. After the tower has been raised, the gin pole is fastened to its foundation. After doing this the tow-up cable can be removed. Guy wires should be tightened to the manufacturers specifications. The tower should be checked for vertical in two planes at right angles. This may be done by using a spirit level or more accurately by holding a plumb bob on a string at a distance from the tower and eying the edge of the tower to the vertical string. Guyed towers are susceptible to being deformed by unevenly tensioned guy wires, thus it is important to also check the tower for straightness while the tower is being adjusted. The riggings screws should be locked to prevent them from unscrewing during service. Some rigging screws have locking nuts provided; in any case it is good practise to pass a loose loop of wire through adjacent rigging screws to prevent them from loosening. Any shackles that have been used should also be wired to prevent them from loosening. The turbine may now be put into service.

The electrical short that was previously installed may be removed. If there is sufficient wind the rotor will begin to turn. There should be no undue noises or vibrations as the rotor turns. If there is a significant vibration or noise the turbine should be shorted and the problem investigated. If the generator is turning a voltmeter may be used to read the voltages between each phase, (for a three phase output machine), with the generator disconnected from any load. These voltages should be all identical for a given rotational speed. This may be a difficult measurement to obtain in gusty conditions when the rotor speed is varying with time since the open circuit voltage of a permanent magnet generator is proportional to its rotational speed. If the turbine is in circuit, the currents in each of the three phases should be measured to see if there is a phase balance. It is good practise to inspect the turbine paying particular attention to guy tension after the turbine has been in service for a couple of days and after the first strong wind.

Resources

Small Wind Policy in Australia

The Federal Government of Australia assists small wind turbine (SWT) technology through a national renewable energy target (RET) using Renewable Energy Credits (RECs) (Office of the Renewable Energy Regulator 2008). Under the RET scheme, a SWT system is able to create, sell and trade RECs in one or five year deeming periods, using a formula specified by the Office of the Renewable Energy Regulator (ORER) in Figure 1.

Figure. 1: Australian SWT system REC calculation formula. (Office of the Renewable Energy Regulator 2006)

REC creation for SWT systems is based on the rated power output of the system (in kW which is determined by the SWT manufacturer), and the wind resource (Office of the Renewable Energy Regulator 2006). If a wind resource is unavailable for the region where the SWT is installed, a base annual wind resource of 2000 hours is used in the calculation (Office of the Renewable Energy Regulator 2006). The five year deeming period REC valuation and up-front sale value of a 1 kW SWT system installed in May 2009 with a 2000 hour/year resource is estimated to pay the REC holder $409AUD (SolarPay.com.au 2008). As of November 2009, 1264 unique RECs have been created by SWTs using the ORER’s small generation unit (SGU) method. Assuming a five year deeming period and the base figure of 2000 hours, this an estimate of the total SWT capacity installed in Australia since 2001 is around 133 kW. The Federal Government also currently supports SWT systems through the National Solar Schools Program which allows for up to $30,000 to be spent on a school renewable energy and efficiency program, which can include a SWT (Department of the Environment 2008).

The Federal Government support also extends to SWT research and development through funding the National Small Wind Turbine Centre (NSWTC).

 

State Policy

At a State level, NSW has been the most active in small wind policy. A discussion paper on ‘Planning for Renewable Energy Generation – Small Wind Turbines’ was released for public comment in April 2010 on the Department of Planning website (NSW Department of Planning, 2010). The discussion paper proposes to make changes to the current NSW environmental planning policy to streamline the approval for small wind turbines. Under the proposed changes, a complying development certificate for installing small wind turbines can be approved in 10 days. The policy will allow pole-mounted wind turbines with a total combined capacity of less than 10 kW in residential zones and less than 60 kW in rural and industrial areas. Height restrictions are imposed; the total height (including height that the blade tip reaches) is limited to 15 m in residential zones and 25 m in rural zones.

Turbines near neighbouring dwellings would have to meet stringent operational noise limits. Depending on the acoustic sound power level of the type of turbine being installed, turbines will have to be at least 25 and possibly up to 200m from neighbouring properties. The customer must ensure that sound power levels of wind turbines have been recorded by an independent testing agency (such as the National Small Wind Turbine Centre) rather than the turbine manufacturer. Similar limits for noise and capacity are applicable for building-mounted wind turbines with an additional height limit of 3m above the roof line. It must be shown that the wind turbines will not affect the structural integrity of the building. A person wishing to install a single pole-mounted turbine of capacity less than 10 kW and height (including height that the blade tip reaches) less than 25m on rural land at a distance of at least 200m from neighbouring properties would be exempt from having to obtain planning approval. Further details are available in the discussion paper on the NSW Department of Planning website (NSW Department of Planning, 2010)

 

References

Office of the Renewable Energy Regulator. Fact Sheet: CALCULATING RENEWABLE ENERGY CERTIFICATES (RECS) FOR SMALL WIND TURBINES. Australian Government 2006 [cited 16 May 2009]. Available from http://www.orer.gov.au/sgu/index.html.

SolarPay.com.au. $43 for Renewable Energy Certificates (REC’s) 2008 [cited 16 May 2009]. Available from http://www.solarpay.com.au/43-for-renewable-energy-certificates-recs.html.

Office of the Renewable Energy Regulator. Fact Sheet: CALCULATING RENEWABLE ENERGY CERTIFICATES (RECS) FOR SMALL WIND TURBINES. Australian Government 2006 [cited 16 May 2009]. Available from http://www.orer.gov.au/sgu/index.html.

Resources

Small Wind Standards and Certification

Review of Existing Standards

NSWTC testing involves standards that are relevant to the safety, reliability, power performance, acoustic testing and certification of small wind turbines (SWTs). The International Electrotechnical Commission (IEC) develops the IEC61400 series of wind energy standards. The primary IEC standards relevant to the NSWTC are IEC61400-2 (2006), IEC61400-11(2006), IEC61400-12-1(2005) (with particular reference to Annex H) as well as the forthcoming second edition of IEC WT 01 (2001) (with particular reference to Annex E). In Australia, the main national standard that is relevant to the work of the NSWTC is AS61400.2 (Int.) (2006), which is identical to IEC 61400-2 (2006) apart from minor editorial changes.The relevant technical committees within Standards Australia that develop standards related to wind energy are EL-048 Wind Turbine Systems and EV-016-Acoustics-Wind Turbine Noise. The American Wind Energy Association (AWEA 9.1, 2009) and from the British Wind Energy Association (BWEA, 2008) are national standards that deal specifically with SWTs. The AWEA and BWEA standards heavily refer to the group of IEC standards mentioned above, but aim to present a more consumer-friendly standard. As a result, the AWEA and BWEA standards have slight variations between the IEC standards, and between themselves.

 

Review of Existing Certification and Labelling Schemes

There are two main certification schemes internationally. One is in the UK which has adopted the BWEA standard, and the other in the USA, which has adopted the AWEA standard. Each scheme has labelling that must be included on any product literature or advertising where product specifications are provided (AWEA 2009, BWEA 2008). However, SWT manufacturers can also obtain full international certification of their products by undergoing the IEC standards. Despite the existence of standards, only a handful of manufacturers of SWTs have certified their products. The main reason for this is the high costs (See Table 1). Anecdotally, SWT manufacturers may perceive that existing national and international standards for SWTs are confusing due to their differing requirements, and that standards were primarily developed to cater for large wind turbines. In 2009, in an effort to increase the number of certified small wind turbines, the Australian Government funded the NSWTC to test SWTs. The results of the tests conducted on each of the SWTs will be publicly available and will aid manufacturers in certifying their turbines. Initial emphasis of testing is on Australian designed SWTs.

Table. 1: Overview of key certification schemes for SWTs.

References

AS 61400.2 – Int. (2006): Wind Turbines - Design Requirements for Small Wind Turbines, Standards Australia.

AWEA (2009) Small Wind Turbine Performance and Safety Standard, version 9.1, American Wind Energy Association.

BWEA (2008) Small Wind Turbine Performance and Safety Standard (29 Feb 2008), British Wind Energy Association.

DR 07153 CP (2007) Acoustics - Measurement, prediction and assessment of noise from wind turbine generators (Draft Australian Standard), Standards Australia IEC61400-2 Edition 2.0 (2006) TC/SC 88, Wind turbines - Part 2: Design requirements for small wind turbines, International Electrotechnical Committee.

IEC61400-11 Consolidated Edition 2.1 (incl. am1) (2006) TC/SC 88 Wind turbine generator systems – Part 11: Acoustic noise measurement techniques, International Electrotechnical Committee.

IEC61400-12-1 Edition 1.0 (2005) TC/SC 88 Wind turbines - Part 12-1: Power performance measurements of electricity producing wind turbines, International Electrotechnical Committee.

IEC WT 01 Ed. 1.0 (2001) IEC System for Conformity Testing and Certification of Wind Turbines - Rules and procedures, International Electrotechnical Committee.

ISO/IEC 17025, Edition 2.0 (2005) TC/SC ISO/CASCO General requirements for the competence of testing and calibration laboratories, International Standards Organisation.

MCS 006 (2008) Product Certification Scheme Requirements: Micro and Small Wind Turbines, Issue 1.4, Microgeneration Certification Scheme, UK Department of Energy and Climate Change.

MCS 011 (2008) Product Certification Scheme Requirements: Acceptance Criteria fro Testing Required for Product Certification, Issue 1.4, Microgeneration Certification Scheme, UK Department of Energy and Climate Change.

 

Resources

Small Wind Turbine Design and Manufacture

Design and Manufacture of SWTs

As the small wind turbine market has expanded in recent years many seemingly new turbine designs are appearing. However, most of these fall into one of two broad configurations - horizontal axis and vertical axis. Like most engineered products there is no one single perfect design and the end configuration is a product of many engineering compromises often based on ease/cost of manufacture, efficiency, design life and aesthetics. The information given herein is intended to give the reader a broad understanding of the most common configuration of commercially available small turbines and their advantages and disadvantages.

 

Horizontal Axis Wind Turbines (HAWT)

Upwind and Downwind designs

Horizontal axis turbines may be further sub-classed as upwind (rotor up wind of the tower/yaw axis) or downwind (rotor downwind of the tower/yaw axis). The majority of small turbines sold on the world market today are of the conventional, upwind three bladed, horizontal axis configuration as shown in figure 1. This design nearly always requires a tail to orient the rotor into the prevailing wind. The downwind configuration (see figure 2) uses the drag on the blades to ensure the rotor plane is perpendicular to the wind behind the tower and thereby simplifying the design, by not requiring a tail boom or fin. Both of these systems are termed passive yaw as there is no control system to sense the wind direction and move the rotor accordingly.

Figure 1: Upwind, three bladed, horizontal axis turbine. (http://www.rise.org.au/info/Applic/Smallturbines/WindTurbine.jpg)

Placing the rotor upwind of the tower ensures it is in relatively undisturbed wind. However the blades must be constructed with sufficient stiffness to prevent them from bending backwards in strong winds and potentially hitting the tower. Orienting the rotor downwind will cause the blades to pass through the tower wake or “wind shadow” as they each pass the downward position. This wind shadow, immediately behind the tower, will experience lower wind velocity and higher turbulence which will result in a cyclic loading of the blades and rotor assembly. Some downwind machines make an audible beat as each blade moves through the tower wake. Designers of downwind turbines trade off these disadvantages by being able to construct potentially lower cost, more flexible blades, since they will flex further away from the tower as wind strength increases.

 

Vertical Axis Turbines VAWT

As the name implies, the rotor of the vertical axis turbine revolves around a vertical axis, usually common with the tower. Vertical axis turbines do not need to yaw to face into the wind, hence the design is inherently simpler than HAWTs as no yaw mechanism is required. This feature makes VAWTs more suitable than HAWTs in conditions where the wind often changes direction. However it should be noted that increased turbulence will “rob” any turbine design of potential energy generation. Due to the aerodynamic design whereby the blades angle of attack change as the blades moves through its circular path, VAWTs are inherently less efficient than a HAWT of the same rotor area. Manufacturers of VAWTS cite their competitive advantages, over HAWTs as quieter operation, aesthetics, less moving parts and the ability to capture wind from any radial direction.

Figure 3: VAWT with helical shaped blades (http://upload.wikimedia.org/wikipedia/commons/b/b6/Quietrevolution_Bristol_3513051949.jpg)
Figure 4: Straight blade “H-type” VAWT (http://upload.wikimedia.org/wikipedia/commons/8/81/H-Darrieus-Rotor.png.jpg)

 

 

Controllers and Inverters

All wind turbines whether being battery charging, electric resistance heating or grid connected require an electrical control system. The electrical control system has two main purposes;

  1. to apply a matched load to the generator which ensure that the rotor will operate at the desired operational speed – Maximum Power Point Tracking (MPPT).
  2. to prevent over voltage conditions damaging power electronics in the event that electrical load is lost (inverter being down, batteries at capacity etc)

If the electrical load is disconnected from a generator, then consequently so too is the mechanical load. Under these conditions the turbine will be prone to over-speeding unless a blade pitch system or rotor furling is employed (see below). To minimize blade noise from high rotor speeds controllers often have a load dump diversion circuit that is set up to divert current into a resistive element or a short circuit, once a certain DC voltage is reached. This type of system ensures the generator is loaded even in the event that the normal electrical load connection (inverter etc) is lost.

If the turbine is grid connected both a controller and an inverter are used. The controller, in its most simple form may be a rectifier mounted on a suitable heat sink, that rectifies the three phase “wild AC” (variable voltage and frequency) from the generator to DC for the inverter. Some SWTs utilize MPPT which optimizes the aerodynamics of the turbine for a given wind speed. By controlling the amount of current drawn from the generator the rotor speed may be effectively controlled. If the wind speed is known or sufficient generator characteristics the controller will be able to operate the rotor at the optimal tip speed ratio, thus optimizing the output of the turbine.

 

Rotor

HAWT and VAWT.

Rotor blades for small wind turbines are most commonly constructed using fibre reinforced composites (glass fibre or carbon fibre reinforced plastic, wood). Blades are usually moulded for increased accuracy and repeatability. Blades less than approximately one metre are often solid in construction however longer blades are usually constructed around a foam or hollow core for material and weight reduction. Engineers may design turbines utilising single or multiple blades. Designs using a single blade with counter weights have been manufactured, however a more complex hub is required to alleviate uneven loading and their asymmetric appearance while rotating often has subjective appeal. Rotors with two and three blades are most commonly used they both have pros and cons see Table 1.

2 Blades

3blades

Pros

Cons

Pros

Cons

Higher RPM = smaller generator (lower cost)

Higher RPM = more blade noise

Better rotor balance

 Extra blade = higher cost

Less blades = lower cost rotor

Higher wind start-up

Better low wind start-up

Lower RPM = larger generator (higher cost)

 

Yaw vibration (HAWT)

 

 

 

 

 

 

Table 1: Pros and cons of 2 and 3 bladed designs.

Usually a significant proportion of the total turbine cost is the cost of rotor blades, hence designs with more than three blades are seldom used other than on very small turbines. It has been shown that the greatest efficiency from a three bladed HAWT is when the tip speed ratio is around six-to-one, (the ratio of the speed of the blades tip to the speed of the wind). From this the airfloil’s angle of attack is calculated, some blades have a twist since the required angle of attack changes along the length of the blade, others are straight since it is cheaper to manufacture. (See Figure 5).

Figure 5: Turbine blades, left with twist to achieve ideal angle of attack for full length, right no twist for cheapest manufacture. (Image courtesy of D Jones).

 

An important design feature of a turbine is the prevention of over speed in high winds. In HAWTs one way to achive this is to pitch the blade. Large wind turbines usually have an active control system to manage this.However,the cost sensitivity of small wind turbines largely makes this complexity unfeasable. Instead passive pitch control is sometimes employed. This works by having a weight and a spring, the weight pulls harder against the spring at higher rotational speeds, this force is from centripital acceleration. The weight and spring are sized so that when the maximum speed is approached the blade starts to twist around its own/pitch axis. This reduces the angle of attack and so lift thus preventing the turbine from over speeding. (See Figure 6.) The lower is in the run position and the upper is pitched.

Figure 6: Turbine pitching mechanism, upper image shows blades fully pitched. (Image courtesy of D Jones) .

The design of VAWTs is somewhat different due to the angle of attack changing as the blade moves around its path. Rotor design is simpler for VAWTs, though the dynamics are more complicated, with the angle of attack continually changing. The blades can be straight, curved and helical or a combination of these. (See Figures 3 and 7.)

Figure 7: VAWTs, left: "eggbeater" blade shape (http://upload.wikimedia.org/wikipedia/commons/3/3c/Darrieus-windmill.jpg), right: "straight blade" shape. (http://www.nationmaster.com/wikimir/images/upload.wikimedia.org/wikipedia/commons/9/9e/Darrieus.jpg)

 

Generator

Generators need to be designed for a particular rotor speed and torque relationship. The generator may either be directly driven by the rotor or via a gearbox, with the desired speed-up ratio. Nearly all SWTs are direct drive as the inclusion of a gearbox often increases maintenance requirements, complexity and lowers efficiency. As in all electric machines, a rotating magnetic field induces current flow in copper windings (coils). Most SWTs use permanent magnets attached to the rotor to provide the rotating field. Rare earth magnets (Neodymium boron iron) are preferred over ferrite magnets since they have a stronger magnetic field and generators consequently may be designed smaller and lighter. Multi-pole generators are used for direct drive turbines to optimise the generator since the turbine rotor turns relatively slowly. There are two main design topologies for permanent magnet generators - radial and axial flux. The difference is outlined below. Radial flux machines are so named since the working flux is in a radial orientation, perpendicular to the axis of the generators rotation. And so the coils are wound around a core which follows a radial line. To visualise this, from the axis of rotation, with the core removed you could look radially out through the coil at the magnets as they move around. The vast majority of these machines must have a core to carry the flux through the coils. Axial flux machines have flux parallel to the axis of rotation. The coils are wound either around a line parallel to the generator axis. Often these machines have magnets on both sides of the coils. This means there is not a requirement for a core to carry the flux. To visualise this machine imagine 3 disks with a common axis, the centre disk contains the coils around the outer part. The other two disks have magnets at the same radius as the coils. The coils are fixed and the two magnet disks rotate together.

Resources

Small Wind Turbines in the Built Environment

There is an increasing trend of SWTs to be used in the urban environment, particularly in European countries such as the Netherlands and the UK. An Energy Saving Trust research report, ‘Generating the Future ’, completed in 2007, predicted that with a range of policy interventions, up to five million small-scale domestic wind turbines could be installed in the UK by 2020 if turbines are installed widely in urban areas. Subsequently, the Carbon Trust released a report in 2008 which predicted that small-scale wind, when installed at 10 per cent of UK sites (both domestic and commercial) with adequate wind speeds, could produce 1500 GWh annually, which equates to 0.36 per cent of the UK electricity supply.

Urban wind turbines (UWTs) include use on commercial and residential buildings as well as in school grounds. The turbines can be ground-based mounted on poles or they can be mounted on buildings. SWTs on buildings tend to be at the low end of the range of household-size , typically less than 6kW.

Figure 1: Top photo is Aerotecture aeroturbines in downtown Chicago.
Figures 2 and 3: left: one of turbines from Energy Savings Trials in the UK, right: the installation of a small wind turbines at the Port Fairy Consolidated School in Victoria in 2007.

 

Urban wind turbines (UWTs) that are used with buildings can be classified in terms of:

  • Building-mounted wind turbines (BMWTs)
  • Building-integrated wind turbines (BiWTs)
Figures 4-9 : Clockwise from top middle: Scottish Proven (HAWT); Dutch WindWall (VAWT on its side ); Italian Ropatec (VAWT); Finnish WindSide (VAWT); Dutch Turby (VAWT); West Australian WindPod (VAWT on its side). Source: WINEUR Resource Assessment Report 2007.

Of the 3500 SWTs installed in the UK in 2007, 25% were BMWTsIn the UK it is predicted that of the estimated 12,125 small wind systems to be installed in 2010, over half will be building mounted (BWEA 2009).There are a wide variety of designs of building-mounted wind turbines with many manufactured in Europe and a prominence of vertical-axis wind turbines (VAWTs). VAWTs have the advantage of not having to yaw to track the wind, thus reducing visual impact, and lower rotation speeds leading to reduced noise and vibration. The disadvantage of the VAWT design is the lower efficiency compared to horizontal-axis wind turbines (HAWTs).

Figure 10-11: Left: Conceptual design from the WEB (Wind Energy in the Built Environment) Project conducted with the EU Right: In Bahrain, two towers funnel the air flow toward the rotors. Source:.

Building-integrated wind turbines use the shape of the building to speed up the air flow into the wind turbines, and are very much at the concept/prototype stage.

The Case for UTWs:

  • They are cheap – roughly 1/3 cost of PV for residential systems
  • Green electricity generation - CO2 savings , saving of fossil fuel resources;
  • Savings on energy costs - less concerns regarding rising energy prices;
  • Electricity used at site of installation
  •  Less dependence on energy utility
  •  Complement PV systems
  •  Suitable for locations where there is no space for large turbines
  •  Role model function for governments to encourage businesses
  •  Used to meet targets of green building designers
  •  Visible green image – stimulate change of attitude in others
  •  Allows for diverse and innovative designs – municipal art
  • Development of an export product.
  • More experience in this area as UWT market is developing – actual performance data from installations
  • New wind models are a significant improvement over previous models which over-predicted wind speeds.

The Case Against UWTs:

  • They might not have an adequate wind resource to work properly - spend long periods inoperative
  • Unlike PV, they have moving parts and require “know-how” in order to maintain the turbine
  • The urban environment has increased air turbulence and may lead to noise and turbine fatigue
  • Safety – turbine fatigue may be a safety issue in an urban environment
  • Value – the energy produced may be small compared to the cost of the turbine
  • They may transmit vibrations to the building
  • Inoperative or unsafe turbines are bad publicity for the wind energy industry
  • Not many SWTs tested and certified.

Conclusion:

  • Initial optimism (“microwind turbines for every home!”) on UWTs has been tempered by results of demonstrations e.g. Warwick Wind Trials (“may make a tangible contribution on suitable sites”)
  • A successful UWT installation requires:
    • Good site selection – proper wind resource assessment
    • Good turbine selection – use of certified products
    • Good building selection – structural assessment
    • Good installation – good placement to avoid turbulence and use of vibration dampeners
  • SWTs are most suited to exposed terrain and/or large distances to neighbouring properties e.g. rural blocks and industrial estates
  • However UWTs are immature technology at this stage - plenty of scope for technological innovation to adapt SWTs to the built environment
Use of more mature technology e.g. large commercial scale turbines within city limits may provide a valid alternative

 

Resources

The Small Wind Market

The Global Small Wind Market

The global market for small wind turbines (SWTs) - those with capacities less than 100 kW - has shown tremendous growth over the last three years. The USA market accounts for 50% of the global market and grew by 78% in 2008. The growth has been attributed to a number of factors including increased private equity investment in manufacturing, raised electricity prices and heightened public awareness of small wind technology. The new long-term USA federal Tax Credits, implemented in February 2009, are expected to produce a 30-fold growth in the industry within 5 years (AWEA, 2009). In the UK, streamlining of the permitting system and the introduction of feed-in-tariffs in 2010 are expected to produce a 60-fold growth by 2020 (BWEA, 2009). In addition to the growth in the industry, the growth in the number of global manufacturers of SWTs has been phenomenal, rising from an estimated 69 manufacturers in 2006, 133 in 2007 and at least 219 in 2008 (AWEA 2008, AWEA 2009). It has to be noted that these figures include companies that have not yet commenced manufacturing.

To place these figures into perspective, there are 36 companies that have actually begun product sales (14 of these are US-based and capture approximately 1/3 of the global small wind market). Continued growth in US and UK SWT markets is expected to be driven by a strong wind resource, rising electricity prices, increasing investment, supportive national and state policies, increasing public awareness, economies of scale, and competitive local markets (AWEA 2009; BWEA 2009). While cost and planning regulations still represent the central barriers to global SWT market growth (AWEA 2008), industry growth and SWT penetration in the US and UK indicate that barriers to SWT systems are lowering in the UK and USA on all fronts.

The Australian Small Wind Industry

In Australia the small wind turbine industry is in its infancy with only one fully commercial SWT manufacturer and two other manufacturers in the final stages of commercialising their product. As per the US and UK, there are also signs of dramatic growth of small wind in Australia with approximately 12 early stage manufacturers, often with less than one year of experience. In a survey of the WA SWT industry, funded by the Western Australian Local Government Association (WALGA) and carried out over the period December 2008 and March 2009, 3 local designers/manufacturers of SWTs were identified. The survey results showed that both manufacturers and designers were interested in the grid-connected residential market, but saw local government and local businesses as their immediate market. This was primarily due to the lack of uniform SWT system planning regulations across the 141 different local councils in WA, and the perceived difficulty of navigating the politics that come with SWTs in residential areas. Growing interest of manufacturers in the urban market was reflected by the fact that the focus of all three interviewed local designer/manufacturers was on developing vertical axis SWTs for specific urban situations. Of the three designer/manufacturers, only one had already installed a turbine in a real world test location and was actively pursuing installations in WA and other countries.

 

References

AWEA (2008) AWEA Small Wind Turbine Global Market Study, American Wind Energy Association.

AWEA (2009) AWEA Small Wind Turbine Global Market Study (Year Ending 2008), American Wind Energy Association.

BWEA (2009) Small Wind Systems UK Market Report 2009, British Wind Energy Association.