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11НОВЫЕ КОНСТРУКЦИИ ВЕТРОЭНЕРГЕТИЧЕСКИХ УСТАНОВОК С ВЕРТИКАЛЬНОЙ ОСЬЮ ВРАЩЕНИЯ
NEW DESIGNS OF VERTICAL AXIS WIND TURBINES
Статья поступила в редакцию 05.01.10. Ред. рег. № 688 The article has entered in publishing office 05.01.10. Ed. reg. No. 688
УДК 621.311.24
ТЕХНИЧЕСКИЕ ОСОБЕННОСТИ И ПРЕИМУЩЕСТВА ВЕТРОЭНЕРГЕТИЧЕСКИХ УСТАНОВОК ПРОИЗВОДСТВА ООО «ГРЦ-ВЕРТИКАЛЬ»
1 2 Р. Холстед , Е. Соломин
1Эмпайр Магнетикс, Инк. США 94928, Рохнерт Парк, б-р Коммерции, д. 5830 Тел. 17075842801, факс: 17075843418, e-mail: [email protected] 2OOO «ГРЦ-Вертикаль» 456300 Челябинская обл., г. Миасс, Тургоякское шоссе, д. 1 Тел.: +79123171805, факс: (351) 2647694; e-mail: [email protected]
Заключение совета рецензентов: 15.01.10 Заключение совета экспертов: 20.01.10 Принято к публикации: 25.01.10
Статья содержит описание совместных достижений российских и американских ученых в разработке ветроэнергетических установок с вертикальной осью вращения.
Ключевые слова: ветроэнергетика, установки с вертикальной осью вращения.
TECHNICAL FEATURES AND ADVANTAGES OF SRC-VERTICAL WIND TURBINES
R. Halstead1, E. Solomin2
'Empire Magnetics, Inc. 5830 Commerce Blvd, Rohnert Park 94928, USA Tel. 17075842801, fax: 17075843418, e-mail: [email protected] 2"SRC-Vertical", Ltd. 1 Turgoyaksky road, Miass, Chelyabinsk reg., 456300, Russia Tel.: (912) 317-1805, fax: (351) 264-7694; e-mail: [email protected]
Referred: 15.01.10 Expertise: 20.01.10 Accepted: 25.01.10
The article describes the mutual achievements of Russian and American scientists of Empire Magnetics, Inc. and SRC-Vertical, Ltd. in the development of new design of vertical axis wind turbines and alternators.
The basic goal of the 3 kW six blade turbine, was a product that would be suitable for Urban or Suburban use, in class 3 wind conditions or above. (Three Kilowatts is the name plate rating at 11 meters per second wind speed).
In general the efforts on the US side were directed on the alternator design and improvement, the Russian side had covered the aerodynamic and mechanical issues.
Some of the requirements:
- The unit must be nearly silent. (Tests to date confirm that we met this goal);
- The unit should not kill birds. (However we can't avoid those committing suicide in the mating season);
- The unit should be cost effective. (Studies show that in mass production we can meet cost goals);
- The unit should make sense in distributed power application. (This is an elusive requirement that changes from site to site, and with the prospective buyer).
Implications:
To capture power from lower wind speeds the blades need to be highly efficient, they need to be self starting, they need to be able to extract power as they cut across the wind in both directions over a wide range of angles. The turbine needs to deal with variable wind directions.
The aerodynamics specialists spent a great deal of effort to calculate various blade shapes, computer models
International Scientific Journal for Alternative Energy and Ecology № 1 (81) 2010
© Scientific Technical Centre «TATA», 2010
were made, then blade segments were fabricated for testing in hydrodynamic facility. The result is a specific blade shape that has been defined for patent application. The following is a photo of a blade segment inside the water tube and an overview of the hydrodynamic test facility (Fig. 1, 2).
Рис. 1. Фрагмент лопасти в гидродинамической трубе Fig. 1. Blade segment inside the water tube
Рис. 2. Общий вид гидродинамической трубы Fig. 2. Hydrodynamic test facility
Рис. 3. Распределение скорости ветра Fig. 3. Wind speed distribution
Since most people live in areas of class three wind conditions or lower, we determined that, in areas where people live, peak wind speeds occur less than 1.5% of the time. Building a windmill system to capture these relatively rare peak power moments greatly increases the cost of the system, without great benefit to the users. Below are examples of wind speed distributions in specific locations (Fig. 3).
From a mathematical point of view, due to the increase in power as the cube of the wind velocity, the 1.5% of the time with high wind speeds represents a significant portion of the total wind power available over the course of the year. However building a system to capture and use this peak power is not practical. We find that while power producers want to maximize the amount of power produced and sold, the users of power are more interested in having smaller amounts of power on a more reliable basis. Thus wind farms with large turbines operated for power sellers have a very different set of economics than power consumers, particularly off grid power consumers.
To capture smaller amounts of power with greater frequency, it is necessary that the wind turbine be self starting in very light wind conditions. To accomplish this requires a combination of factors; the air foils must be of the correct shape, they must be placed in the correct position, the bearing friction must be very low and the alternator must not present resistance until after the system is at a working speed.
The directly driven alternator designed by Empire Magnetics Inc. has no switched iron in the magnetic circuit, so unlike other permanent magnet alternators, this design has zero cogging, presenting near zero resistance. In addition there is no magnetic attraction or loads placed on the bearing until power is being consumed from the alternator. These two factors allow the turbine to begin turning in very light wind conditions. By using a large diameter direct drive alternator design we eliminate efficiency losses and costs associated with gears or other mechanical drive systems. Since we have no switched iron in the magnetic system, it is feasible to use the structural iron of the windmill as components in the
magnetic design, this integration allows cost reductions not possible with other alternator designs (Fig. 4).
The mechanical bearing system is placed at the middle of the assembly, where the wind loading on the top of the assembly is approximately equal to that on the bottom. This arrangement in conjunction with the specific bearing design results in very low friction in light wind conditions, exactly what we needed for the intended purpose.
Рис. 4. Магниты генератора, смонтированные на ступицу ротора Fig. 4. Magnets mounted to wind turbine hub
Рис. 5. Лопасть ветроколеса Fig. 5. Profile of blade
The scientists at SRC-Vertical and Empire Magnetics had focused on development of the exact shape of the blade (Fig. 5). In addition to extensive math modeling, blades of different profiles were tested in the hydrody-namic test facility. The result is a high efficiency, self starting, bi-directional air foil. In addition it has some unique operational features as shown in the following graph. These features allowed us to create a control system that is specific to the turbine and blade design. In the middle of the graph is a curve depicting the non linear effects on power output of adjusting the rotational speed
of the turbine, with respect to the wind speed. By taking advantage of this feature we can adjust to maximize power output below 3 kW, or decline to generate power above 3 kW.
The way to obtain more power in light wind conditions is to make the turbine larger, but to do so without a significant cost increase. From the outset we planned to make the air foils by a low cost mass production process, extrusion or pultrusion are the obvious candidates. So we designed the air foils with constant cross sections in order to accommodate a low cost manufacturing process. Once the machines are set up and running the blade can be made in long production runs, with sections cut to length after they exit the machine. This focus on a manufacturing process as well as a technical design gives us the flexibility to make turbine blades of different lengths at very low cost.
Since our blades are low in costs, the turbine is a relatively low cost component as compared to the rest of the wind power system, over sizing the turbine to gather wind power available most of the time, provides more reliable and more useful power, without a great cost penalty. However avoiding the cost associated with capturing the peak power requires strict management discipline. The technical analysis of momentary or even annual power production tempts many designers into trying to capture peak power, it is only when a business minded manager places cost constraints upon the technical designers that we approach the economic goals of a practical wind system. In particular the electronic power components must be undersized as compared to the turbine in order to obtain economic operation in light wind conditions.
To prevent damage to the system, we incorporated controls that actually prevent generating extra power, as opposed to trying to dispose of the power peaks when they occur. While this appears counter intuitive, the result is a capped power output from the windmill. On rare occasions when the power available in the wind exceeds the system capacity, we simply decline to generate the extra power. This result is depicted in the following graph (Fig. 6). X axis = wind speed in meters per second. Y axis is watts x 100. Power curve of 3 kW six blade windmill, with control systems.
0 4 8 12 16 20
Wind speed, m/s
Рис. 6. Зависимость мощности от скорости ветра Fig. 6. Power curve
International Scientific Journal for Alternative Energy and Ecology № 1 (81) 2010
© Scientific Technical Centre «TATA», 2010
To achieve this we have developed a control system that takes advantage of features incorporated into the shape of the airfoils. The following graph shows the power output of the turbine, as a result of wind speed and turbine rotational speeds. The nonlinear feature in the middle region of the graph is the key to a control algorithm which allows us to limit the power output of the turbine over a wide range of wind speeds.
To achieve this control, we have a set of electronics which determines the amount of power available from the turbine, and then seeks to produce the maximum amount of power possible from the winds that are available, up to the maximum. The result is a small amount of power available on a more consistent basis than the competition. When we look at applications, most users of small wind systems would rather have a modest amount of power every day than to have a large amount of power 5 days per month. While we were unable to do anything about wind variablility, we were able to greatly increase the number of days, and the number of hours per day, when we produce some useful power. This feature is especially critical for off grid situations.
At the other end of the scale, we designed the turbine to survive at 60 meters per second wind speed (135 m/h). Generally speaking this is better than the homes that the people using the windmill are likely to be living in. Since wind power increases as the cube of the wind speed, addition a few MPH to the top end of this scale has a very big increase in the overall cost of the product.
Because our turbine catches wind from any horizontal direction the turbine captures more power in light and variable winds than HAWT (Horizontal Axis Wind Turbines) in the same conditions. When wind directions change for a HAWT the unit must overcome the gyroscopic forces to point into the wind. If the propeller is not facing directly into the wind, the efficiency drops of as a function of the cosine of the angle to the wind. At 15 degrees off the wind direction, the propeller efficiency drops by 20%, at 30 degrees the drop is 40%. The gyroscopic force of the rotating propeller makes it slow to respond to changes in wind direction, as such the HAWT in light and variable wind conditions is very seldom operating at peak efficiency. In the case of our turbine, the gyroscopic force works to our advantage, the turbine acts as a flywheel picking up energy from any wind gust while smoothing the power output over time. We do not suffer an efficiency loss with a change in wind direction.
In distributed power applications, one of the primary ways to reduce cost is to mount the turbines on existing structures. Commercial buildings in particular are likely locations for such turbines. In addition to being silent, our turbine design has some significant advantages in this situation, in comparison to HAWT designs.
If we look at the volume of space consumed by a propeller type turbine, it is basically a sphere, where the diameter of the sphere is the diameter of the turbine blade, plus the off set from the center line of the mast. While our VAWT defines a rectangle. If we were to define our turbine as a square that had the same height and
width as the diameter of the sphere consumed by the HAWT, the swept area of our turbine would be 27% greater just due to the shape. However by using a rectangle the swept area can be 2 or 3 times greater using the same amount of roof area. When faced with the problem of a fixed amount of roof area, the HAWT cannot take advantage of height without also consuming width. See the comparison graphic below on Fig. 7.
CD
Рис. 7. Сравнение объемов ветроэлектростанций равных мощностей Fig. 7. Volume of consumed space
Since our turbine has he blades at the OD the whole blade is doing work at the maximum radius.
This increased torque arm allows slower rotational speed, resulting in quiet operation, longer bearing life, less stress on the blades, fewer harmonic vibrations in the blades and lower abrasive wear on the blade surfaces.
If one looks at the standard propeller design, you will find that the air speed near the hub is quite low, as the radius increased outward from the hub, the air speed is increasing. What this means is that from the hub to the tip of the propeller, a wide spectrum of different vibra-tional frequencies are excited in the blade structure. This is one of the reasons why propellers are so very complex and expensive to manufacture.
One of the early vertical axis wind turbines was developed by Flow Wind. This full Darrieus design while quite famous, incorporated the same spectrum of vibra-tional frequencies into its design (Fig. 8). To a large degree this is one of the reasons VAWT got a bad name. Another reason related to the blade shape of this particular design is that it was NOT self starting. Yet another is the positive feedback, self destruct cycle in high wind conditions.
Рис. 8. Ротор Дарье Fig. 8. Full Darrieus Turbine
Sandia National Labs put a good deal of work into solving this problem. Reports are available at: http://www.sandia.gov/wind/topical.htm.
As this full Darrieus turbine turns at a faster rate, the centrifugal force on the blades makes them bend increasing the diameter of the turbine. The increase in diameter, generates more torque, which makes the turbine turn faster. If not quickly controlled this positive feedback cycle will quickly lead to self destruction.
The SRC-Vertical turbine is an H-Darrieus, with straight blades was described in Darrieus' 1927 patent is also well known as a Giromill. The early versions of such designs did not have self starting blades, as such they were not very practical. The basic design does greatly reduce the harmonic vibration in the blades.
Supporting the blades to prevent bending/self de-struct cycle is a two part issue. While it is feasible to have multiple support points as shown (Fig. 9).
Each support wire or cross bar increases the drag on the turbine. The SRC-Vertical design greatly reduces the drag, yet provides the necessary support by using rings at the top and bottom.
While we use a direct drive alternator and electronics with a complex algorithm to control the turbine as a way to obtain maximum power output from low wind conditions, due to the cube laws of wind power additional controls are needed at high wind speeds. To accomplish this we have added a centrifugal force activated aerodynamic braking system. The basic principle of operation is the same as the James Watt fly ball governor implemented on the early steam engines. As the turbine speed increases, weights move ailerons on the horizontal support wings, the change in position acts as a speed brake (Fig. 10).
Рис. 10. Аэродинамический тормоз Fig. 10. Aerodynamic brakes
Рис. 9. Ротор H-Дарье Fig. 9. H-Darrieus turbine
If you look carefully at the SRC-Vertical turbine, you will see three vertical air foils on both the top and bottom of the turbine. However you will also see the pattern is offset by 60 degrees around the circle. This particular arrangement has some advantages.
First one needs to know that more blades does not equal more power or greater efficiency. The Russian team did a series of calculations and determined that three air foils offers the optimum trade between power output, drag, wind loads and cost. Adding more air foils increases the cost, increases the drag, and greatly increases the wind loads on the structure, all without a significant increases in power output.
The structure as we have it designed, puts the bearing at the middle of the assembly. The wind loads on the top of the turbine are roughly equal to those on the bottom. The construction reduces the overhung load on the bearings, increasing bearing life.
By offsetting the three air foils on the top circle from the bottom, we get six power pulses per revolution of the wind turbine. This significantly reduces harmonic vibration in the structure making the unit quiet, but also re-
International Scientific Journal for Alternative Energy and Ecology № 1 (81) 2010
© Scientific Technical Centre «TATA», 2010
duces stresses in the structure. The torque ripple is reduced providing power that is easier for the electronics to control.
As a visual comparison: here is a graph of a four blade version versus (Fig. 11) and a six blade version (Fig. 12):
12 000
8000
4000
-4000
.Fx
Pi /
- г у - L
\ ! \ г \
■
90
180
270 V 360
Рис. 11. Ротор с четырьмя лопастями Fig. 11. Four blade version
8000
6000
4000
2000
-2000.
Fx
/ V N V s
N % Г fr i'
0
90
180
270
360
Рис. 12. Ротор с шестью лопастями Fig. 12. Six blade version
So SRC-Vertical in associated with Empire Magnetics has made a quite large research concerning different windmill concepts, designed several types of windmills and tested it different conditions. As results of this work all the scientists has demonstrated large experience in the field of renewable energy and presented some significant consequences for further research and design works.
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- TATA — LXJ
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