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Research and development issues for small wind turbines
PERGAMON
Renewable Energy 16 (1999) 922-927
RESEARCH AND DEVELOPMENT ISSUES FOR SMALL WIND TURBINES P. D. CLAUSEN and D. H. WOOD
Department of Mechanical Engineering, The University of Newcastle. University Drive, Callaghan, NSW 2308, AUSTRALIA.
ABSTRACT This paper discusses some of the major research and development issues for small wind turbines whose upper power limit we take arbitrarily as 50 kW. The basis for the comparison is the mature technology available for large turbines which has yet to be fully diffused downwards. After defining and categorising the important features of small turbines, the paper considers the major issues of blade aerodynamics, especially starting performance and materials and manufacturing methods. We conclude with a brief discussion of current developments in technology, which should increase the level of small turbine technology towards that of large machines. 0 1998 Published by Elsevier Science Ltd. All rights reserved.
KEYWORDS wind turbines; small wind turbines; research and development; starting performance; blade materials; blade manufacturing.
INTRODUCTION Modem large wind turbines, used mainly mature technology. Maximum efficiency 20% of the Betz limit when the efficiency size of large turbines is steadily increasing
in wind farms to produce electricity for large grids, represent a of power generation is typically around 47%. that is well within of the generator is factored out. The power output and physical with each new generation.
Small capacity wind turbines, that is turbines with a power output up to 50 kW, have to date not received the same engineering attention as their large counterparts. These turbines are used typically in remote area power systems, often in conjunction with a diesel/electric generators and a battery system to supply power to a single user or a small grid. Whereas the basic aerodynamics of the blades is independent of size, small turbines have a number of unique problems. The most important are: the starting behaviour with its complex combination of unsteadiness, low operating Reynolds number, and high angles of attack; yaw behaviour and over-speed protection. Blade rotational speed typically increases with decreasing size which results in the blade loading being dominated by centrifugal forces. Furthermore, the small blades can be difficult to make with high tolerances. 0960-1481/99/S-see front matter @ 1998 Published by Elsevier Science Ltd. All rights reserved. PII: SO960-1481(98)00316-4
WREC 1998
923
This paper will describe the basic feature of small wind turbines and finish by mentioning the worldwide efforts being made to improve small turbine technology. The presentation of material is biased towards our work - see Web-Site (1) in the listing at the end of the paper - a bias which results partly from the limited amount of public information available on small wind turbines. We hope this paper will help to rectify this undesirable situation by stimulating further publications and exchange of information.
DEFINING FEATURES OF SMALL WIND TURBINES Small turbines usually rely on aerodynamic torque for starting, and on a tail fin to point the blades into the wind. These two features are so common that they can be taken as defining a small turbine. As shown in Table I, we have divided small turbines into three categories based on typical use: micro, powering electric fences, remote telecommunications, equipment on yachts and the like; mid-range, principally used to power a single remote house; and mini for small grids for remote communities. Table 1. Operating Parameters of Small Wind Turbines
Category
W (kw)
R (m)
max. rotor speed O-pm)
typical uses
generator type(a)
Examples
micro
I
1.5
700
mid-range
5
2.5
400
electric fences, yachts remote houses
mini
20+
5
200
Permanent magnet (PM) PM or induction PM or induction
Web-site (1) Web-Site (2) Web-site (3)
mini grids, remote communities
Table I summarises typical operating parameters for our three categories. It is emphasised that the entries in the table are typical values and there are wide variations in them all. The example turbines have been chosen for two reasons: firstly, they are, apparently, the outcome of current turbine technology, and secondly, a description is available on the web. A list of relevant web sites is given before the reference list at the end of this paper. Each category in Table I has particular features which can be understood partly in terms of the dependence of the important performance parameters (for geometrically similar blades of constant density operating at the same tip speed ratio) on the blade radius, R, as shown in Table 2. A notable omission from the table is the noise output which we will discuss briefly and separately. It is immediately obvious from Table 2 that a major problem with micro turbines is their starting torque, which is also of considerable imprtance for mid-range turbines, but much less so for mini turbines.
Table 2. Dependence of Important Parameters on Blade Radius
Parameter Reynolds Number (Re) Power Output Centrifugal Loads Starting Torque Inertia of Blades
Dependence R R’ R’ RJ RS
On the other hand, the small dependence of the chord Reynolds number, Re, on R means that low operating Reynolds numbers occur on all small turbines, at least during starting. For example, the tip Re for the 5 kW turbine described by Bechly et al. (1996) is only 15,000 when the blades are stationary in a 3 m/set wind but rises to 450,000 at rated conditions (IO m/set). According to Giguere and Selig (1998) the adverse effects of low Re usually occur below Re = 500,000, although they emphasise that this limit is approximate and aerofoil-dependent.
924
WREC 1998 AERODYNAMIC ISSUES FOR SMALL TURBINES
Consider first the simplest possible case: a stationary turbine that has negligible frictional torque in the drive train and negligible “cogging” or other resistive torque arising from the generator. Then, as the wind starts blowing, the angular acceleration (rate of increase of rotor speed) is given by the aerodynamic torque divided by the rotational inertia of the blades. Starting is, therefore, independenr of the number of blades and is maximised by minimising blade inertia by minimising blade weight. Table 2 shows that microturbines have the smallest aerodynamic starting torque because of the R’ dependence, and what there is has often to overcome a significant cogging torque which is characteristic of some permanent magnet (PM) generators. In this case, starting is improved by increasing the number of blades, so it is not surprising that micro-turbines often have many blades, and in practice, many start only at moderate wind speeds, say, greater than 5 mlsec. On the other hand, mini-turbines naturally have high starting torque but a much higher inertia, so they should begin to rotate easily but take a longer time to reach operational speed.
Pitch Angie = 35”
‘Or
I
0 10
15
20
25
30
Time (set) Fig. 1. Turbine starting at 35” pitch.
0
10
20
30
40
50
Time (set) Fig.2. Turbine starting at zero pitch
60
little Unfortunately research has been undertaken on blade starting, with the only reference we are aware of being Ebert and Wood (1997) who analyzed the 5 kW turbine described by Bechly er al. (1996). From the measurements of wind and rotor speed, Ebett and Wood (I 997) found that the magnitudes of the nondimensional parameters controlling the effects of unsteadiness were sufficiently small to justify a quasi-steady analysis. In further work on the same blades, the results shown in Figs. I and 2 were obtained for two different pitch angles, defined as the angle between the tip chord and the plane of rotation. Figure 1 shows that the blades, at large pitch, take about IO seconds to reach their maximum speed, just over 50 r.p.m., at a high wind speed of around 8 m/set. Figure 2, for the pitch giving rated performance, indicates the blades take around 50 seconds to start at about the same high wind speed, for which the operational rotor speed is about 300 rpm. This long time is due partly to the high resistive torque in the I6 kW generator but it is obvious that there is a significant trade-off between starting and operational performance which is likely to remain until a cheap and reliable system of pitch adjustment can be developed. Although not shown in the figures, it is noteworthy that, at least, the initial acceleration (up to about 50 rpm in both figures) can be predicted with reasonable accuracy using a quasi-steady blade element analysis in spite of the high angles of incidence - initially 90” in Fig. I - and low Re.
WREC 1998
925
Part of the difliculty in calculating starting performance is the dearth of information about aerofoil behaviour at low and very low Re - less than, say, 40,000 - and high incidence - up to 90”. As indicated by the typical Re values quoted at the end of the previous Section, low Re effects on operational blades (at lower incidence) can also be important, as documented by Giguere and Selig (1998). To maintain high lift and low drag at low Re requires a thin aerofoil to minimise the acceleration over the upper surface that produces a laminar separation bubble which, in turn, causes most of the performance degradation at low Re. At very low Re, the optimum thickness approaches zero, eg Laitone (1997). a situation that is likely to give rise to structural problems! Noise measurement and prediction for small turbines is another undernourished area. A limited number of measurements are available; for example Wood (I 997) demonstrated that the turbine described by Bechly et al. (I 996) produces about half the sound power (the strength of the noise source) of a human voice at 5 m/set and that simple data correlations for the sound power developed for large turbines, eg Wagner et al. (1997) apply also to small turbines of good aerodynamic design.
BLADE MATERIALS AND MANUFACTURING METHODS Small wind turbine blades are currently manufactured from solid and laminated timber, glass-fibre laminated composites and carbon fibre laminated composite. All turbine blades experience a high number of flexing cycles so it mandatory for the blade material to have an extremely long fatigue life. For the short blades of micro and mid-range turbines, good quality, knot free solid timber, for example hoop pine in Australia and Sitka Spruce or basswood in the USA, Sagrillo (1995), is attractive because it has a long fatigue life and reasonable purchase cost. Timber may not, however, be the most desirable material for complex blade shapes which are dictated by the need for high aerodynamic efficiency. For example, undercamber (concavity in the lower surface), which is common to many high-performing aerofoil sections, is often very difficult to create in a timber blade. Furthermore, each blade must be machined separately and it is necessary to maintain dimensional tolerances even when the tip chord is only a few centimetres. Most large turbine blades consist of a shell structure constructed from laminated composites and/or laminated timber. As far as the authors are aware very few small turbine blades are manufactured from laminated timber as it is likely to be too expensive. Laminated composites require moulds with high dimensional integrity which can be cut using a computer-controlled machine centres. Blade shape toolpaths are becoming easier to create due to the advances in functionality and user-friendliness of Computer aided manufacture (CAM) software. This coupled with the decreasing cost of high performance computers has given smaller organisations the chance to build high quality, high efficiency small wind turbine blades. Computer aided engineering tools such as finite element analysis can be used to optimise the lay and amount of fibre use within the blade to create sufficiently strong yet lightweight blades. As an example the 2.5 metre blades described in Bechly et al. (1996) weigh only 4.6 kg each which are significantly lighter than blades on most commercially available 5 kW machines. With careful design, a blade with low mass has low rotational inertia which leads to easier starting. It seems at present that the major barrier to the use of composites is the high capital cost of moulds and the investment in developing the manufacturing techniques which must be recovered over a limited output. With increasing demand, the cost of blades should decrease significantly. There exist a number of techniques to manufacture composite small wind turbine blades. Composite blade construction techniques include hand lay-up, resin transfer moulding and injection mouldigg the technique used is a function of the size of the blade, number created as well as expertise available. We chose to use Resin Transfer Moulding to construct our 2.5 metre long blades as it was the most cost effective technique
WREC 1998
926
to manufacture a relatively small number of blade, the blades created had high structural integrity and consistent properties and expertise was locally available. For their AIRM range of wind turbines, Southwest Windpower, Web-site (I), use a direct injection moulding process to create the carbon fibre blades.
CURRENT AND FUTURE DIRECTIONS It appears that a worldwide effort is producing, and will continue to produce, small wind turbines that are approaching the technological sophistication of large machines. For example, the Danish company Olsen Wings, Web-Site (4). have recently released a 3.4 m long composite blade with deployable tip-brakes. The U.S. Department of Energy through the National Renewable Energy Laboratory launched its Small Wind Turbine Project in 1995 to support the commercial development of four turbines in the range 8 - 40 kW, Forsyth (1997). The Australian Co-operative Research Centre for Renewable Energy (ACRE) has recently begun a project to develop mid-range to mini-sized turbines for which we will be supplying the blade designs and assisting in developing the manufacturing techniques. We have just completed making the moulds for a I m long blade rated at 600 W as the first step in this project. The blade shape, determined using our design and analysis programs, was processed by a commercial CAD/CAM software package to generate the tool paths for cutting the two Aluminium half moulds (split along the leading and trailing edges) by a numerically controlled milling machine. The moulds include resin runners for manufacturing along with special treatments of the joints to ensure maximum dimensional integrity of the critical leading and trailing edges. We are currently developing a fatigue test procedure for small wind turbine blades and plan to use this procedure to verify the structural designs of our blades. Here we will measure the aeroelastic response of an operating blade on our 5 kW wind turbine. Measurements will be taken periodically over 8 - I2 months in a range of wind conditions for statistical relevance. In the long term this information will assist in the structural design optimisation of our blades. Although space limitations, and lack of expertise, preclude an adequate description, it must be pointed that advances in microprocessor-based control systems have also made fundamental contributions to improving the technology level of small turbines in two main ways. Firstly, small turbines which do no? have to maintain grid-synchronous frequency, can be maintained at optimum tip speed ratio over a wide range of wind speeds, eg Bechly et al. (I 996). This is achieved very simply by sensing the turbine shaft speed and then setting the alternator field current proportional to this speed. (It was assumed that sensing the wind speed directly would be too costly in a commercial version of their turbine.) Secondly, a field excited generator can also be used for over-speed protection at high wind speed by loading the blades to drive them away from optimum performance. This again requires sensing the shaft speed and possibly the power level but has been proven to be very effective.
ACKNOWLEDGMENTS Our work described in this paper was funded by the ACRE, the Australian Research Council, and the N.S.W. Office for Energy. Major contributions to the work were made by Matthew Bechly, Dr Paul Ebert and Christian Mayer. We thank Philippe Giguere for his comments on this paper.
WREC 1998
927
LIST OF RELEVANT WEB-SITES I.
2. 3. 4.
http://www.windenergy.com/Air.html http://www.eng.newcastle.edu.au/me/wmd! http://www.dewi.de/wtsuppliers/swframe.html http:Nwww.cybemet.dkusers/olsen/wings.html REFERENCES
Bechly, M.E., Clausen, P.D., Ebert, P. R., Pemberton, A. and Wood, D.H. (1996). Field testing of a prototype S kW wind turbine. Wind Energy Conv. 1996 (Proc. 18th BWEA ConjI). (M. B. Anderson, ed.), pp. 103 - I IO. M.E.P., London. Ebert, P.R. and Wood, D.H. (1996). The starting behaviour of a small horizontal-axis wind turbine. Renew. Energy, 12. pp. 245 - 257. Forsyth, T.L. (1997). An introduction to the small wind turbine project. In: Windpower 1997 Proceedings, pp. 23 I - 239, Am. Wind Energy Assoc. Giguere, P. and Selig. M. S. (1998). Low Reynolds number airfoils for horizontal axis wind turbines. Wind Eng ‘g, (to appear). Sagrillo (1995) Apples and Oranges: An update. Home Power, 47, pp. 36 - 47. Wagner, S., Bareiss, R. and Guidati, G. (1997) Wind Turbine Noise. Springer-Verlag. Berlin. Wood, D. H. (1997). Noise measurement and prediction for small wind turbines. In: Solar ‘97 (Proc. 35’h Ann. Conf Aus’n N.Z. Solar Energ. Sot.), (T. Lee, ed.), Paper 153.
Renewable Energy 16 (1999) 922-927
RESEARCH AND DEVELOPMENT ISSUES FOR SMALL WIND TURBINES P. D. CLAUSEN and D. H. WOOD
Department of Mechanical Engineering, The University of Newcastle. University Drive, Callaghan, NSW 2308, AUSTRALIA.
ABSTRACT This paper discusses some of the major research and development issues for small wind turbines whose upper power limit we take arbitrarily as 50 kW. The basis for the comparison is the mature technology available for large turbines which has yet to be fully diffused downwards. After defining and categorising the important features of small turbines, the paper considers the major issues of blade aerodynamics, especially starting performance and materials and manufacturing methods. We conclude with a brief discussion of current developments in technology, which should increase the level of small turbine technology towards that of large machines. 0 1998 Published by Elsevier Science Ltd. All rights reserved.
KEYWORDS wind turbines; small wind turbines; research and development; starting performance; blade materials; blade manufacturing.
INTRODUCTION Modem large wind turbines, used mainly mature technology. Maximum efficiency 20% of the Betz limit when the efficiency size of large turbines is steadily increasing
in wind farms to produce electricity for large grids, represent a of power generation is typically around 47%. that is well within of the generator is factored out. The power output and physical with each new generation.
Small capacity wind turbines, that is turbines with a power output up to 50 kW, have to date not received the same engineering attention as their large counterparts. These turbines are used typically in remote area power systems, often in conjunction with a diesel/electric generators and a battery system to supply power to a single user or a small grid. Whereas the basic aerodynamics of the blades is independent of size, small turbines have a number of unique problems. The most important are: the starting behaviour with its complex combination of unsteadiness, low operating Reynolds number, and high angles of attack; yaw behaviour and over-speed protection. Blade rotational speed typically increases with decreasing size which results in the blade loading being dominated by centrifugal forces. Furthermore, the small blades can be difficult to make with high tolerances. 0960-1481/99/S-see front matter @ 1998 Published by Elsevier Science Ltd. All rights reserved. PII: SO960-1481(98)00316-4
WREC 1998
923
This paper will describe the basic feature of small wind turbines and finish by mentioning the worldwide efforts being made to improve small turbine technology. The presentation of material is biased towards our work - see Web-Site (1) in the listing at the end of the paper - a bias which results partly from the limited amount of public information available on small wind turbines. We hope this paper will help to rectify this undesirable situation by stimulating further publications and exchange of information.
DEFINING FEATURES OF SMALL WIND TURBINES Small turbines usually rely on aerodynamic torque for starting, and on a tail fin to point the blades into the wind. These two features are so common that they can be taken as defining a small turbine. As shown in Table I, we have divided small turbines into three categories based on typical use: micro, powering electric fences, remote telecommunications, equipment on yachts and the like; mid-range, principally used to power a single remote house; and mini for small grids for remote communities. Table 1. Operating Parameters of Small Wind Turbines
Category
W (kw)
R (m)
max. rotor speed O-pm)
typical uses
generator type(a)
Examples
micro
I
1.5
700
mid-range
5
2.5
400
electric fences, yachts remote houses
mini
20+
5
200
Permanent magnet (PM) PM or induction PM or induction
Web-site (1) Web-Site (2) Web-site (3)
mini grids, remote communities
Table I summarises typical operating parameters for our three categories. It is emphasised that the entries in the table are typical values and there are wide variations in them all. The example turbines have been chosen for two reasons: firstly, they are, apparently, the outcome of current turbine technology, and secondly, a description is available on the web. A list of relevant web sites is given before the reference list at the end of this paper. Each category in Table I has particular features which can be understood partly in terms of the dependence of the important performance parameters (for geometrically similar blades of constant density operating at the same tip speed ratio) on the blade radius, R, as shown in Table 2. A notable omission from the table is the noise output which we will discuss briefly and separately. It is immediately obvious from Table 2 that a major problem with micro turbines is their starting torque, which is also of considerable imprtance for mid-range turbines, but much less so for mini turbines.
Table 2. Dependence of Important Parameters on Blade Radius
Parameter Reynolds Number (Re) Power Output Centrifugal Loads Starting Torque Inertia of Blades
Dependence R R’ R’ RJ RS
On the other hand, the small dependence of the chord Reynolds number, Re, on R means that low operating Reynolds numbers occur on all small turbines, at least during starting. For example, the tip Re for the 5 kW turbine described by Bechly et al. (1996) is only 15,000 when the blades are stationary in a 3 m/set wind but rises to 450,000 at rated conditions (IO m/set). According to Giguere and Selig (1998) the adverse effects of low Re usually occur below Re = 500,000, although they emphasise that this limit is approximate and aerofoil-dependent.
924
WREC 1998 AERODYNAMIC ISSUES FOR SMALL TURBINES
Consider first the simplest possible case: a stationary turbine that has negligible frictional torque in the drive train and negligible “cogging” or other resistive torque arising from the generator. Then, as the wind starts blowing, the angular acceleration (rate of increase of rotor speed) is given by the aerodynamic torque divided by the rotational inertia of the blades. Starting is, therefore, independenr of the number of blades and is maximised by minimising blade inertia by minimising blade weight. Table 2 shows that microturbines have the smallest aerodynamic starting torque because of the R’ dependence, and what there is has often to overcome a significant cogging torque which is characteristic of some permanent magnet (PM) generators. In this case, starting is improved by increasing the number of blades, so it is not surprising that micro-turbines often have many blades, and in practice, many start only at moderate wind speeds, say, greater than 5 mlsec. On the other hand, mini-turbines naturally have high starting torque but a much higher inertia, so they should begin to rotate easily but take a longer time to reach operational speed.
Pitch Angie = 35”
‘Or
I
0 10
15
20
25
30
Time (set) Fig. 1. Turbine starting at 35” pitch.
0
10
20
30
40
50
Time (set) Fig.2. Turbine starting at zero pitch
60
little Unfortunately research has been undertaken on blade starting, with the only reference we are aware of being Ebert and Wood (1997) who analyzed the 5 kW turbine described by Bechly er al. (1996). From the measurements of wind and rotor speed, Ebett and Wood (I 997) found that the magnitudes of the nondimensional parameters controlling the effects of unsteadiness were sufficiently small to justify a quasi-steady analysis. In further work on the same blades, the results shown in Figs. I and 2 were obtained for two different pitch angles, defined as the angle between the tip chord and the plane of rotation. Figure 1 shows that the blades, at large pitch, take about IO seconds to reach their maximum speed, just over 50 r.p.m., at a high wind speed of around 8 m/set. Figure 2, for the pitch giving rated performance, indicates the blades take around 50 seconds to start at about the same high wind speed, for which the operational rotor speed is about 300 rpm. This long time is due partly to the high resistive torque in the I6 kW generator but it is obvious that there is a significant trade-off between starting and operational performance which is likely to remain until a cheap and reliable system of pitch adjustment can be developed. Although not shown in the figures, it is noteworthy that, at least, the initial acceleration (up to about 50 rpm in both figures) can be predicted with reasonable accuracy using a quasi-steady blade element analysis in spite of the high angles of incidence - initially 90” in Fig. I - and low Re.
WREC 1998
925
Part of the difliculty in calculating starting performance is the dearth of information about aerofoil behaviour at low and very low Re - less than, say, 40,000 - and high incidence - up to 90”. As indicated by the typical Re values quoted at the end of the previous Section, low Re effects on operational blades (at lower incidence) can also be important, as documented by Giguere and Selig (1998). To maintain high lift and low drag at low Re requires a thin aerofoil to minimise the acceleration over the upper surface that produces a laminar separation bubble which, in turn, causes most of the performance degradation at low Re. At very low Re, the optimum thickness approaches zero, eg Laitone (1997). a situation that is likely to give rise to structural problems! Noise measurement and prediction for small turbines is another undernourished area. A limited number of measurements are available; for example Wood (I 997) demonstrated that the turbine described by Bechly et al. (I 996) produces about half the sound power (the strength of the noise source) of a human voice at 5 m/set and that simple data correlations for the sound power developed for large turbines, eg Wagner et al. (1997) apply also to small turbines of good aerodynamic design.
BLADE MATERIALS AND MANUFACTURING METHODS Small wind turbine blades are currently manufactured from solid and laminated timber, glass-fibre laminated composites and carbon fibre laminated composite. All turbine blades experience a high number of flexing cycles so it mandatory for the blade material to have an extremely long fatigue life. For the short blades of micro and mid-range turbines, good quality, knot free solid timber, for example hoop pine in Australia and Sitka Spruce or basswood in the USA, Sagrillo (1995), is attractive because it has a long fatigue life and reasonable purchase cost. Timber may not, however, be the most desirable material for complex blade shapes which are dictated by the need for high aerodynamic efficiency. For example, undercamber (concavity in the lower surface), which is common to many high-performing aerofoil sections, is often very difficult to create in a timber blade. Furthermore, each blade must be machined separately and it is necessary to maintain dimensional tolerances even when the tip chord is only a few centimetres. Most large turbine blades consist of a shell structure constructed from laminated composites and/or laminated timber. As far as the authors are aware very few small turbine blades are manufactured from laminated timber as it is likely to be too expensive. Laminated composites require moulds with high dimensional integrity which can be cut using a computer-controlled machine centres. Blade shape toolpaths are becoming easier to create due to the advances in functionality and user-friendliness of Computer aided manufacture (CAM) software. This coupled with the decreasing cost of high performance computers has given smaller organisations the chance to build high quality, high efficiency small wind turbine blades. Computer aided engineering tools such as finite element analysis can be used to optimise the lay and amount of fibre use within the blade to create sufficiently strong yet lightweight blades. As an example the 2.5 metre blades described in Bechly et al. (1996) weigh only 4.6 kg each which are significantly lighter than blades on most commercially available 5 kW machines. With careful design, a blade with low mass has low rotational inertia which leads to easier starting. It seems at present that the major barrier to the use of composites is the high capital cost of moulds and the investment in developing the manufacturing techniques which must be recovered over a limited output. With increasing demand, the cost of blades should decrease significantly. There exist a number of techniques to manufacture composite small wind turbine blades. Composite blade construction techniques include hand lay-up, resin transfer moulding and injection mouldigg the technique used is a function of the size of the blade, number created as well as expertise available. We chose to use Resin Transfer Moulding to construct our 2.5 metre long blades as it was the most cost effective technique
WREC 1998
926
to manufacture a relatively small number of blade, the blades created had high structural integrity and consistent properties and expertise was locally available. For their AIRM range of wind turbines, Southwest Windpower, Web-site (I), use a direct injection moulding process to create the carbon fibre blades.
CURRENT AND FUTURE DIRECTIONS It appears that a worldwide effort is producing, and will continue to produce, small wind turbines that are approaching the technological sophistication of large machines. For example, the Danish company Olsen Wings, Web-Site (4). have recently released a 3.4 m long composite blade with deployable tip-brakes. The U.S. Department of Energy through the National Renewable Energy Laboratory launched its Small Wind Turbine Project in 1995 to support the commercial development of four turbines in the range 8 - 40 kW, Forsyth (1997). The Australian Co-operative Research Centre for Renewable Energy (ACRE) has recently begun a project to develop mid-range to mini-sized turbines for which we will be supplying the blade designs and assisting in developing the manufacturing techniques. We have just completed making the moulds for a I m long blade rated at 600 W as the first step in this project. The blade shape, determined using our design and analysis programs, was processed by a commercial CAD/CAM software package to generate the tool paths for cutting the two Aluminium half moulds (split along the leading and trailing edges) by a numerically controlled milling machine. The moulds include resin runners for manufacturing along with special treatments of the joints to ensure maximum dimensional integrity of the critical leading and trailing edges. We are currently developing a fatigue test procedure for small wind turbine blades and plan to use this procedure to verify the structural designs of our blades. Here we will measure the aeroelastic response of an operating blade on our 5 kW wind turbine. Measurements will be taken periodically over 8 - I2 months in a range of wind conditions for statistical relevance. In the long term this information will assist in the structural design optimisation of our blades. Although space limitations, and lack of expertise, preclude an adequate description, it must be pointed that advances in microprocessor-based control systems have also made fundamental contributions to improving the technology level of small turbines in two main ways. Firstly, small turbines which do no? have to maintain grid-synchronous frequency, can be maintained at optimum tip speed ratio over a wide range of wind speeds, eg Bechly et al. (I 996). This is achieved very simply by sensing the turbine shaft speed and then setting the alternator field current proportional to this speed. (It was assumed that sensing the wind speed directly would be too costly in a commercial version of their turbine.) Secondly, a field excited generator can also be used for over-speed protection at high wind speed by loading the blades to drive them away from optimum performance. This again requires sensing the shaft speed and possibly the power level but has been proven to be very effective.
ACKNOWLEDGMENTS Our work described in this paper was funded by the ACRE, the Australian Research Council, and the N.S.W. Office for Energy. Major contributions to the work were made by Matthew Bechly, Dr Paul Ebert and Christian Mayer. We thank Philippe Giguere for his comments on this paper.
WREC 1998
927
LIST OF RELEVANT WEB-SITES I.
2. 3. 4.
http://www.windenergy.com/Air.html http://www.eng.newcastle.edu.au/me/wmd! http://www.dewi.de/wtsuppliers/swframe.html http:Nwww.cybemet.dkusers/olsen/wings.html REFERENCES
Bechly, M.E., Clausen, P.D., Ebert, P. R., Pemberton, A. and Wood, D.H. (1996). Field testing of a prototype S kW wind turbine. Wind Energy Conv. 1996 (Proc. 18th BWEA ConjI). (M. B. Anderson, ed.), pp. 103 - I IO. M.E.P., London. Ebert, P.R. and Wood, D.H. (1996). The starting behaviour of a small horizontal-axis wind turbine. Renew. Energy, 12. pp. 245 - 257. Forsyth, T.L. (1997). An introduction to the small wind turbine project. In: Windpower 1997 Proceedings, pp. 23 I - 239, Am. Wind Energy Assoc. Giguere, P. and Selig. M. S. (1998). Low Reynolds number airfoils for horizontal axis wind turbines. Wind Eng ‘g, (to appear). Sagrillo (1995) Apples and Oranges: An update. Home Power, 47, pp. 36 - 47. Wagner, S., Bareiss, R. and Guidati, G. (1997) Wind Turbine Noise. Springer-Verlag. Berlin. Wood, D. H. (1997). Noise measurement and prediction for small wind turbines. In: Solar ‘97 (Proc. 35’h Ann. Conf Aus’n N.Z. Solar Energ. Sot.), (T. Lee, ed.), Paper 153.