Vertical or Horizontal Axis Wind Turbines (VAWT or HAWT)?

What’s better – VAWT or HAWT?

(based on materials of scientists of “Yuzhnoye” Design Bureau (Ukraine): M.Galas, PhD, U.Dimkovec, N.Akaev, I.Kostyukov)

Currently Horizontal Axis Wind Turbines (HAWT or propellers) cover more than 90% of wind turbine World park. About 100 companies produce these machines. Vertical Axis Wind Turbines (VAWT) were developed much slower due to several reasons:

VAWT was designed later than HAWT (Savonius rotor in 1929, Darrieus rotor in 1931, Musgrove rotor in 1975).

Till recently it was erroneously considered that for WAWTs it is impossible to get the ratio of maximum linear speed of operating parts (blades) to the wind speed more than 1:1 (for HAWTs this ratio is 5:1). This data is only true for slow speed rotors of Savonius type which use different resistance of blades when moving along the wind and against it.

It leaded to wrong theoretical conclusion that the maximum coefficient of wind use of VAWT is less than of HAWT, and as the result this type was not developed for more that 40 years. In 1960 Canadian and later American scientists experimentally proved that this concept is not true for Darrieus rotor which uses lift power of blade. Maximum ratio of speed of operating parts (blades) to the wind speed for these machines is more than 6:1. The coefficient of wind use is not less than of HAWT.

Also there is still some “traditional” vision of wind turbines in the form of propellers.

Finally the Vertical Axis Wind Turbines didn’t get enough volume of theoretical and experimental knowledge in aerodynamics.

Vertical Axis Turbines intensive development has started from 1980. Their power range extends constantly. Today almost all countries have Darrieus type Vertical Axis Wind Turbines.
Classic design with curvilinear blades prevails in USA and Canada. Great Britain and Romania prefer rotors with rectilinear blades parallel to axis of rotation. VAWT company (Great Britain) has achieved the most success. In 1986 it tested 40 kW wind rotor (outside diameter, OD is 14 meters) in Sardinia island. VAWT-130 (450kW, 25m OD) was produced serially in the same year. Currently this company works on prototyping VAWT-850 (500kW) and VAWT -2400 (1.7MW, 67m).

In Russia the developers of Vertical Axis Turbines are “SRC-Vertical”, CAGI, “Hydroproject” Holding, Institute of Electric Energy. Several turbines such as VL-2M, VDD-16 and others show very good results.

Why designers and scientists choose Vertical Axis Wind Turbines for the development more and more often?

In literature they mention the independence of wind direction as the main feature or advantage. It means that the design can be seriously simplified. For developing countries it is even more attractive due to the limitation of high technologies use.
As the argument they say that the design of VAWT does not require some turning systems and aggregates.

However the experience of design, development and operation of VAWTs has shown that it’s not just one advantage of Vertical Axis Turbines. These types of Turbines have different technical solutions, lots of parameters cannot be applied from one to another.

Some of distinctive features of VAWT and HAWT are resented below from different points of view. The discussion is basically about traditional propeller HAWT and Darrieus VAWT with straight blades.

DEPENDENCE OF EFFICIENCY ON WIND DIRECTION

The maximum efficiency of HAWT can be reached when the axis of wind wheel (rotor) and wind direction are collinear. Necessity of orientation on the wind requires some extra mechanisms and system of orientation control of constant tracing of wind status, searching the direction with maximum wind potential, turn of wind wheel to this direction and keeping it in the best position.

Orientation system makes the aggregate more complex and reduces its reliability (up to 13% of failures pertain to orientation control system). 

In addition it is almost impossible to orient the wind wheel efficiently when the wind is changing its direction, due to the delay of mechanism of orientation action. For medium and big Wind Turbines of 30-40 m diameter the efficiency of orientation on the wind reduces because of non-complanarity of the difference of wind flow speed along the length of blades, which makes impossible the setting of the wheel (rotor) to the optimal orientation direction. As the result the production of energy and efficiency of the whole Wind Turbine is less due to the reduction of wind flow energy use.

Orientation system tears the rigid connection between gondola (wind aggregate housing) and the mast (tower). It’s obvious design disadvantage which brings self-oscillations and the difference of frequency parameters of moving and motionless parts of propeller Wind Turbine. It all reduces the reliability of turbine and increases depreciation expenses.

Efficiency of Vertical Axis Wind Turbine does not depend on wind direction. I.e. there is no need in mechanism and system of orientation. Inequality of the wind flow along the height leads to some unimportant leveling of turning torque on the blade.

COEFFICIENT OF WIND POWER USE

Theoretically proved that the coefficient of wind power use of ideal wind rotor (wheel) of any wind turbine (both HAWT and VAWT) is 0.593. It can be explained by the fact that rotors of both turbine types use the same effect of lifting force appearing when the wind is flowing around the profiled blade. The maximum coefficient of wind energy use on HAWT has reached 0.4 recently.
Wind Turbines designed by SRC-Vertical, have 0.38.

Experiments of Russian Wind Turbines have shown that the number 0.4 – 0.45 can be reached. It’s a solvable task. 

It means that the coefficients of wind power use of these types of Wind Turbines are almost the same.

START OF OPERATING (SELF START)

It is considered that starting torque of propeller doesn’t equal to 0 (zero), i.e. no outside power source is needed. However practice shows that the HAWT rotor self-starts when it is enough directed on the wind. Side wind cannot start the propeller and outside power source is required for turning the gondola on the wind.

For a long time it was considered that starting torque of VAWT is equal to 0. I.e. it was considered that they are not self-starting. However the scientists of SRC-Vertical developed Darrieus rotor which self-starts on 3.5-4 m/sec wind speed. The self-starting torque of these turbines is much higher than 0. Moreover these turbines start from even small wind gust.

Nevertheless sometimes big wind turbines are equipped with Savonius rotors for guaranteed self-start.

The more complex the turbine, the less the reliability. Extra aerodynamic devices can reduce the power of Wind Turbine. It is even worth than the need of extra power for self-start. Recently the designers take this fact into account. For instance the developers of MOD turbines (4.4 kW) rejected one of the main advantage of HAWT (self start) and mounted blades on rotor with settled step. This solution was intended on more efficient operation of turbine in nominal range of the wind speed. For starting they used the approach used before for VAWTs (short time switching of alternator to motor mode for rotor acceleration).

RATIONALITY OF POWER STRUCTURE OF WIND TURBINE

Inertial loading on blade of the HAWT is applied along the blade, i.e. by the most disadvantageous way. Hub and support-bearing module are compact and small.

Inertial loading on the blade of VAWT is directed across the blade, along traverse. Hub and bearing module have relatively big size.

It means that VAWT is not as rational as HAWT. It is heavier than HAWT.
However for megawatt class Wind Turbines it’s necessary to take into account the alternating loading.
First, aerodynamic loads on the blade of HAWT in top and bottom position are different because of the difference of wind speed along the length of the blades. The blade operates in different forces and transfers the pulsing rotating torque to the hub.
Second, the gravitation influence increases a lot. The pulsing aerodynamic and gravitational loads reduce the vibro-stability of blade, hub and support-transmission system.
Coriolis forces increase when the turbine is orienting on the wind.

DESIGN OF BLADE

All cross-sections of HAWT propeller are operating in different energy status because they have different rotating speeds and angles of attack. This difference is much less if the cross-sections are rolled relatively each other. Inertial loading leads to the necessity of narrowing the profile from butt to the end.

So the propeller blade is more complex than straight (orthogonal, symmetrical relatively the chord plane of VAWT).
On another hand the assembling of parts of the fiber plastic blade of VAWT is very complex task due to the flange joints arrangement..

TURNING OF BLADES

The turning of blades of HAWT propeller is polished and used not only for braking of the wind wheel (together with general frictional brake) but mainly for the mean of searching of optimal angle of blade setting, to keep the wind rotor on the level of maximum possible number of revolutions without damaging. The system of blade turn makes the design of the Wind Turbine much more complex as it requires the system of continuous tracing of the rotation speed, turning devices with drives for each blade and automatic control of blade turn angle. Such systems for propeller HAWTs are required to avoid the hazardous operation duty of wind rotor.

The turn of the blade of HAWT was very efficient not for only braking but supporting the optimal angle of attack in all positions of the blade on the circle of rotation. The Wind Turbines with such approach are not widely distributed because the heavy blade while going round along the circle, should make several swinging, being oriented on the wind... In addition the devices and mechanisms are so complex that the production is slow and expensive.

Finally the Wind Turbine becomes very complex and expensive.

Instead the efficiency of VAWT is similar to the efficiency of HAWT in the most efficient duty, without any blade turn. It’s a main advantage of VAWT!

SWEPT AREA ON THE UNIT OF BLADE LENGTH

The swept area of HAWT is the area of circle made by rotating blade ends. For VAWT this area equals the area of rectangle with the sides equal to length of blade and diameter of Wind Turbine (wind rotor or wheel). I.e. the swept area of VAWT is more flexible than of HAWT as it can be adjusted (or changed) by the dimensions not only of blade length, but also by diameter of Turbine, which makes the tactic possibilities of Turbine design wider.

Energy taken from the unit of blade length of HAWT, is different along the length from butt to the end, mainly because of different angle velocities (from 0 in the butt region to maximum value on the end).

The value of energy taken from the VAWT blade is almost not changing along the length of blade. The small difference appears due to the difference of wind flow, inconstancy of wind speed along the height.
There are several more reasons of losses of energy taken from blade – not optimal angle of attack in different positions of blade on the rotation circle, reduction of wind wheel rotating torques in positions when the blade is moving along the flow, and reduction of torques coming from blade which is going in the aerodynamic shadow of the mast (tower).

So it can be concluded that the efficiency of taking energy of the wind by the blades of both types of Wind Turbines, will be about the same.

It’s necessary to note that for small (up to 5 kW) Wind Turbines the angle of attack is setting optimal depending on the wind conditions of the local region.

In big Wind Turbines it can be controlled depending on the duty of operation. When starting it should be bigger, when the angle velocity increases, the angle of attack is decreasing.

Such system makes the efficiency as well as the cost higher.

HIGH-SPEED DEGREE

The most widely distributed turbines among the propeller HAWTs, are the high-speed turbines (up to 5-7 modules), with less than 4 blades. They provide the highest coefficient of energy use, i.e. they are the most efficient. The high degree of high-speed requires extra special devices and aggregates which limit the angle velocity of wind rotor in accurate limits and avoid the damage of rotor and transmission, what makes the Turbine more complex.

The constancy of high speed of rotation makes the transmission connections between wind wheel and alternator more simple, and the quality of electric energy is higher without extra transforming or converting circuits.

At the same time the constancy of operating speed of rotation limited by the inertial strength of blades, means the limitation of operating wind speed (usually within 12-15 m/sec), and operation of turbine in optimal duty on the fixed wind speed, which reduces the efficiency.

For HAWT with big diameter the influence of non- complanarity of wind speed along the height and gravitation forces causing pulsing loading in blade material and transmission support mechanisms, become higher. The bigger the high-speed degree, this influence is bigger. The high speed determines the higher requirements for dynamic stability of all rotating elements, durability of construction, accuracy of fabricating, quality of assembling and lubrication, and balancing of rotating parts and modules.

Vertical Axis Wind Turbines do not have such problems as their design provides low speed operation. In all known experiments including the search of maximum efficiency of wind use, the high-speed didn’t increase 2.5 – 2.8 modules. The significance of this fact will be understandable if the designers will take into account that all characteristics (including the coefficient of energy use) of VAWT are almost the same as of HAWT. The 2-3 times reduction of high-speed improves operating conditions of mechanisms due to reduction of dynamics, simplification of requirements for transmission supporting elements, excluding mechanisms and systems providing the constancy of rotating speed. The high-speed reduction allows operating with coefficient of use of wind speed in all wind speed range, i.e. increase the efficiency with rather simple design of blade. Operating range of wind speed for low speed Wind Turbines extends up to 20-25 m/sec. However it’s necessary to have in mind that rotating torques become higher when the speed is low, which leads to increase of material mass for Wind Turbine blades due to longer traverses, bigger hub and transmission. It’s necessary to understand that the alternating frequency of Turbine rotation causes the necessity of including converters into the electric circuit for improvement of quality of energy and for coordination of it with grid parameters.

Vertical Axis Wind Turbine with straight blades can be high-speed. The only limitation is the strength of blades on transversal inertial loading and vibro-loading. The tendency of development of more light and high durable materials makes the development of high-speed VAWTs of Darrieus type more and more attractive.

ALTERNATOR AND GEARBOX (MULTIPLIER) LOCATION

The big advantage of VAWT is the possibility of placing the alternator and multiplier on the base of turbine, excluding the angle transmission of the rotating torque. Powerful, may be even multi-module transmission of rotating torque, can make the requirements for repairs more simple (no limitation on mass and dimensions), and requirements of operation more light (no vibration, no jerk). When the equipment is located on the ground, the montage and transmission of energy conditions improve greatly. The energy production becomes simple.

It is considered that the angle transmission will reduce the reliability of HAWT turbine. Thus the equipment is placed in rotating gondola. The complexity with repairs, operation and montage increase a lot. Alternator rotates together with gondola and makes the system of energy transmission very complex. To avoid the twist of power bus, it’s required to limit the turn of gondola, to bring in the collector transmission or untwist the bus. In all these cases there are several extra devices and mechanisms which make the Wind Turbine more complex.

It’s necessary to note that the transmission of rotating torque on the foundation (base) level can be transmitted by long transmission shaft. However the advantage of the bottom based equipment location compensates the complex design of shaft, even if the shaft will be post-gearbox, i.e. high-speed. When the shaft is off-gearbox (low speed), it does not make the design more complex.

In Wind Turbines produced by SRC-Vertical there is no multiplier (gearbox). It increases the efficiency (because each gearbox reduces it). Alternator and part of power electronics are located in the hub housing. It makes the servicing more complex, but increases the efficiency.

RELIABILITY

HAWT propellers use the achievements of aviation techniques, in particular the design of blades, system controlling the angle of blade setting, transmissions. So it can be considered that these Wind Turbines are studied and their reliability is known as high. However it is obvious that VAWTs (especially big) promise even higher reliability. The reason of such opinion is the significant simplification of design, reduction of level of requirements for transmission fabrication, simplification of montage and operation, as well as the features of VAWTs (no mechanisms and system of orienting of gondola on the wind, no system controlling the angle of blade setting, no problem with energy transmission from alternator, and the alternator and multiplier are placed in some cases on foundation).

The high level of reliability of complex structure is based on the high level of technology development. This fact is very important for evaluation of optimality of variants of co-operation of different companies for fabrication of modules and aggregates. It’s hard to predict that the simple and reliable design of Wind Turbine will be more expensive than the more complex one, in spite of material capacity.

POWER

Recently the power of Wind Power Units in the World Wind Power industry is increasing. It can be explained by the following reasons: Since the size increases, the cost of energy taken from 1 square meter of swept area, is getting down, expenses on operation and technical servicing are getting less, the property area is getting less. The efficiency of the whole construction is growing.

However the growth of HAWT is limited. It has the upper limit of 3-4 MWatts as the blades are exposed to centrifugal and bending forces alternating in value and direction. This fact limits the dimensions of blades, reduces the reliability of HAWTs and makes the operating (life) time less.

So the growth of power means the change of design of Wind Turbines. The most preferable solution is the Vertical Axis structure which limits are much more than of HAWTs, in accordance with theoretical development.

CALCULATED WIND SPEED

The calculated wind speed for HAWTs is in the limits of 12-15 m/sec because of strength requirements for inertial loading. “Hydroproject” Institute calculated the wind speeds on the base of data of 200 meteorological stations in Russia. There are several regions (East coast of Arctic ocean, Ohot sea, Kamchatka, Kuril Islands, Kazakhstan, Crimea, etc.) where the calculated nominal speed is 18-20 m/sec. The operating range is up to 30 m/sec. The research of Kazakhstan scientists in Jungar Gates region (annual speed is 8 m/sec, prevalent is 15 m/sec), have shown that the starting speed almost does not influence on the level of energy produced.

If the starting speed will be changed from 4.5 to 7.5, then the energy production will be reduced on 2%. The influence of calculated nominal speed on the energy production is big. For instance the increase of calculated wind speed from 10.4 to 20 m/sec makes the energy production higher by 4 times. It means that for regions with high wind potential the calculated wind speed, taken for normal conditions, is smaller than it should be. Big wind power resources will not be used.

The operating range of wind speed for low speed VAWTs is increased up to 20-25 m/sec, comparing with HAWTs (12-15 m/sec). It means that VAWTs are preferable for regions with high wind potential. Wind Turbines of SRC-Vertical operate in the wide range of wind speed (4-60 m/sec), at nominal speed 10.4 m/sec. When the turbine reaches the nominal speed, the angle velocity is being stabilized by aerodynamic brakes.

ECOLOGICAL ISSUES

Low speed VAWTs have big ecological advantages. All levels of aerodynamic and infra-noise are less, tele- and radio static is insufficient while their operation.
In case of damaging the radius of dispersion of blades is less.
The probability of birds killing is low.

In particular, the level of noise of SRC-Vertical Wnd Power Units is in the limit of 40-50 dB on 10 meters distance from Turbine. To compare the noise from computer fan is 50 dB. The Turbine is almost free of electromagntic oscillations which allows the installation oif such Turbines near the communication centers (including airports where the requirements to the air arte very high due to the navigation devices operation.

MASS CHARACTRISTICS

The characteristics of three Wind Turbines are presented in the table:
NEWECS-45 (Holland, HAWT), VTO- 1250B (Russia, VAWT), D.de Renzo (USA, HAWT)

It’s necessary to note that mass characteristics of D. de Renzo Wind Turbine were calculated by theoretical approaches, so in reality the parameters will be better.

The presented data show that the operating modules (blades, traverses) of HAWT propeller Wind Turbine are lighter than of VAWT, but the hub of VAWT is heavier. The system of transmission of torque of VAWT is lighter than of HAWT, in spite of the transmission shaft in some cases.

Mass of electric circuit and supporting tower (mast) is almost the same for both types.

The total masses of the discussed Wind Turbines are located on the same level. VAWT has the advantages – no gondola, no orientation system and mechanism, no blade turn.

SRC-Vertical Wind Turbines has no transmission shaft. This fact creates extra advantage of these turbines among the rest VAWTs.

It is hard to evaluate and compare the different types of Wind Turbines on the base of simple values (heavier-lighter, simple-complex, efficient-non-efficient). The analysis of the whole complex of characteristics and parameters of Wind Turbines on the base of theoretical and modeling-experimental research of efficiency of both types of turbines, is required in economical and meteorological conditions of the specific region.

The requirements for megawatt class of Wind Turbines are higher, comparing to medium power turbines (less than 1 MW). The main of them are: reliability, ecological cleanness, handly servicing and repairs, life time, etc.). Economical efficiency, the cost of construction, operation costs should also be taken into consideration. VAWT comply with these requirements more than HAWT, because with the same energy parameters it has features which can provide the reduction of energy cost in regions with high wind potential.

According to evaluation of experts, the small Wind Turbines produced by SRC-Vertical (up to 5 kW), also have the same advantages comparing to HAWTs.

From general point of view it’s obvious that it’s impossible to cover the needs of one region (or even country) in wind power, by one size or one type of Wind Turbines. Wind Power Industry will become competitive when several different directions will be approached. This will allow creating the big Wind Turbine market.