the Lux Wind Turbine

 

The Lux Wind Turbine is a six bladed turbine that revolves around a vertical axis.  Several cables are connected between each blade in a cross pattern, which supports each blade along its entire length. This rotor is a rigid structure never seen before in the wind industry.  Due to this cross cabling, the central structure/tower can be removed, which is the main reason why the turbine can be built using less materials.

+ Comparing Vertical & Horizontal Axis Wind Turbines

Several decades ago, scientists were in disagreement as to which type of turbine would be beneficial to the industry. After extensive research on both types of turbines, the HAWT won the battle, not because it was a great turbine but because scientists at that time were unable to solve the major problems associated with the VAWT and they were also fixated on efficiency.

Comparing vertical & horizontal axis wind turbines
The Coefficient of Power (Cp) measures the efficiency of the turbine and is the amount of energy extracted divided by the total amount of energy in the wind. Even though the Cp of a HAWT is higher than the Cp of a VAWT (about 50% versus 45%) this should not be a factor when evaluating the two types of wind turbines. The only evaluation tool should be the Levelized Cost of Electricity (LCOE), which is the sum of all the costs over a lifetime/sum of all electricity produced over a lifetime. The Lux Wind Turbine is expected to have ½ the LCOE as a HAWT. There are many reasons for this expectation but the main ones are discussed in the following paragraphs.

Consider a few basic engineering principles to compare the two types of wind turbines. HAWT use cantilevered blades and a cantilevered tower, which means, each of the blades and tower are supported only at one end. This is similar to sweeping a floor with one hand at the end of the broom stick. Engineers avoid using cantilevers wherever possible because cantilevers require more material at the support end to hold the item in place. Structures that require the least amount of material are supported in many places. This basic principle can be seen in truss bridges, buildings and most all other structures built by mankind.

In contrast, the multiple (six or more) Lux Wind Turbine blades are supported at each end and cables are attached at various positions along the blades entire length. This fully supported system is in direct contrast to the cantilevered blades and tower of the HAWT. The cables cross each other and when looking from the top down, each cable forms a hexagon, which is close to a circular shape. This shape ensures that the aerodynamic drag from the rotating cables is kept to a minimum. The cross cables and blades create a rigid structure similar to the shape of a football. This type of structure is also similar to the geodesic dome, which uses the slogan “doing more with less”. The rotor can be built to almost any size, using fewer materials than the HAWT blade and tower system.

There is one other major point that I want to make. The rotor and nacelle, which contains the mechanical and electrical components, of a 5 MW NREL reference turbine, has a weight of 350,000 kg (770,000 pounds). The position of the nacelle is more than 100 meters (328 feet) above the ground mounted on top of a tower. Building a tower and foundation to keep the rotor and nacelle in this position is an unnecessary cost, especially when comparing it to the Lux Wind Turbine, which has all the mechanical and electric components at ground level. In addition to this, the entire 350,000 kg must be turned into the wind every time the wind changes direction. The Lux Wind Turbine does not need to be turned into the wind because it will accept wind from all directions, even the up and down drafts. Companies today are putting 6-8 MW HAWT wind turbines offshore in water up to 60 meters (200 feet) deep and on floating platforms! Again, our turbine, with a low center of gravity, is better suited for these locations.

+ Advantages

• The blade and cross cable system eliminates or reduces all problems associated with previous Vertical Axis Wind turbines including reduced vibrations, torque ripple and premature blade failure. The power output is improved, especially in low winds, by using an advanced blade profile and by building a rotor with a larger swept area. This is practical because the blade and cable system is light in weight and therefore relatively inexpensive. The ½ cost analysis includes this larger swept area.

• The tower at the bottom of the rotor is short but the equator of the rotor, on megawatt machines, is as high or higher than the hubs of conventional turbines, therefore, taking advantage of higher wind speeds that occur at higher elevations.

• All of the mechanical and electrical components are at ground level. This makes it easier to erect and also reduces maintenance costs.

• A yaw system is not required because this turbine accepts wind from all directions.

• The blades do not need to be pitched, which eliminates the need for the large diameter slewing bearings, retainers and hydraulic components. The blade speed and power output is controlled by aerodynamic stall.

• According to Dr. John Dabiri at California Institute of Technology, counter rotating Vertical Axis Wind Turbines can be spaced closer together than conventional Horizontal Axis Wind Turbines https://arxiv.org/pdf/1010.3656.pdf. This is advantageous because most high wind speed sites are already occupied by widely spaced conventional wind turbines. • The blades on the prototypes are made from aluminum, which are extruded at relatively low costs. However, since the blades experience only small deflections, they could be made from a wide range of materials or a combination of materials. Conventional wind turbine blades have large deflections, therefore, their material selection is limited.

• The blade profile is constant from one end to the other. Manufacturing this blade is much easier than manufacturing the conventional wind turbine blade, which has a profile that changes in width and curvature along its entire length.

• The blades can be made in sections and assembled like tent poles. This is possible because the blades are always in compression, unlike all other wind turbine blades that change from tension to compression through each cycle. The blade sections are easy to transport and assemble.

+ Validation

The Institute of Aerospace Research, a branch of the National Research Council (NRC), in Ottawa, Canada developed computer models of the Lux turbine and tested these models for aerodynamic and structural performance. The first model they analyzed was 40 meters in diameter and had a power output of 1MW. The results of the analysis showed the Lux Wind Turbine performed well and the blades had a life expectancy well in excess of 25 years. NRC then scaled the computer models to a diameter of 160 meters with a power output of 16MW. The positive results observed in the 1MW turbine analysis were repeated with the larger turbine.

IOPARA, a Vertical Axis Wind Turbine consulting company, in Montreal, used their CARDAAV software, which is well respected around the world, to predict the power output of several Lux Turbine models with and without cross cables. The power curves created from this software were confirmed with data collected from the prototypes. The power loss from the cross cables was low, as expected.

+ Product Development

Work to date on the Lux Wind Turbine has been personally carried out and funded by Mr. Glen Lux B.E. Development of the Lux Wind Turbine began in 2004 with several turbine designs being tested, culminating in the current design.

The first generation of the 40KW Lux Wind Turbine was operational for 1½ years. It was then lowered and 7 other models were assembled and tested for various periods of time. Variations of blade curvature, blade offset angles, solidity ratios, blade profiles, drivelines and air brakes, were applied to these models, with the goal of optimizing turbine performance.


There are two options to hold the rotor in place.  

+ Guy Cable Supported Rotor

The least expensive option uses 3 guy cables that go from the top of the rotor to an anchor on the ground. This turbine does not need a central structure/tower or a robust foundation to keep the rotor upright. The guy cable and anchor system is only a fraction of the cost of the traditional robust cement foundation and tower system. This turbine can be used in most rural areas and could reduce the cost of the support structures in offshore locations as well.

+ Lattice Supported Rotor

In locations where it isn’t feasible to use guy cables, the rotor is supported by a lattice structure positioned along the vertical axis that rotates with the blades. The cross cable pattern is still utilized so the blades have very little movement even in hurricane wind conditions. This system requires a robust foundation but the blade, lattice tower and cross cable system is expected to have a cost that is significantly lower than the conventional wind turbines.

The Drive System Options

 

Roughly speaking, when we double the diameter of a wind turbine the swept area and power output increases by a factor of 4.  The rotational speed is cut in half and the torque on the output shaft is 8 times greater.  Therefore, as wind turbines increase in size, the speed increaser becomes disproportionately more expensive.  The main factor when selecting a gearbox is the torque required for the application.

The rotational speed of the output shaft on the NREL 5 MW reference wind turbine is 12.1 rpm.  When it is producing 5 MW the torque is about 4,000,000 newton meters or 3,000,000 foot pounds.  The combination of this slow speed and high torque is rarely seen, if at all, in any other machine built today.   

Speed increasing gearboxes convert this slow speed, at high torque, to the desired generator speed but can cost 25% or more of the overall cost of the wind turbine.

Direct drive generators rotate at the same speed as the output shaft but require a large number of poles and a massive structure that is equally as expensive as the gearbox.  Both the direct drive generator and the gearbox need a frequency converter to connect a variable frequency current to a constant frequency electrical grid.  The cost of the converters can also be a high percentage of the overall cost of the wind turbine.  Here are two more options.


Hydraulic Pump

 

Hydraulic systems on some wind turbines use a pump, connected to the slow rotating output shaft, to convert the wind energy to a high pressure fluid flow, which connects to one or more hydraulic motors that drives one or more generators at a constant speed.  This system does not require a frequency converter; however, the pump is also massive and therefore, expensive.

This picture shows the pump as being multiple times bigger than the motors.  For this reason, the pump is the most expensive part of the system.  One way to lower this cost is to use the reciprocating motion of hydraulic cylinders to create a high pressure flow.  The piston rod of each cylinder is connected to a crank driven by the output shaft and the Cap-end is mounted to the frame of the turbine. 

As the piston in the cylinder moves back and forth, one side of the piston pressurizes the fluid while the other side is filled with fluid.  Each end of the cylinder has two ports with one way valves, at each port, guiding the fluid through the cylinder.

The table below shows the cylinder dimensions for random scenarios.

The cylinders are small in comparison to the largest cylinders built today and almost any machine shop anywhere in the world has the capability of building or repairing them.  Since the flow rates are larger than most hydraulic applications none standard motors could be considered for driving the generators. Impact turbines such as the Pelton wheel can produce power at an 82% efficiency when the pressure is 2,500 psi.  Efficiency usually increases as the pressure increases so the Pelton wheel may be beneficial especially if the flow from more than one turbine travels through the wheel.

The cost to build the prototype hydraulic pump for the Lux wind turbine, not including labor for installation, was 1/5 the cost of a positive displacement pump with an equivalent flow rate.  Some of the cost savings was attributed to the outer frame, containing the cylinders, which was already in place and only required minor changes.  Also, commercial grade cylinders with a maximum pressure of 3,000 psi were used in the demonstration project, which would not be adequate for long term operation.  High quality components are necessary to ensure long time periods between maintenance schedules.  Also, the material and machining costs were at retail prices, which did not include any discounts for volume purchases.

Items to consider when designing this pump:

  • Number of cylinders per row
  • Number of rows
  • System maximum pressure
  • Size of bore
  • Length of stroke
  • Materials for seals and other components
  • Hose and Tube connections
  • Ease of replacing a cylinder
  • Environmental control
  • Sensors for leakage detection


TRACTION DRIVE

 

The Lux wind turbine showing the Traction Drive at the bottom

 

A magnified view to the Traction Drive.

The Traction Drive is used to increase the rotational speed of the output shaft.  A large diameter wheel is attached to the output shaft and one or more small diameter wheels are forced against the large wheel, creating friction, forcing the small wheel to rotate at a higher speed.   The Traction Drive is similar to a locomotive transferring 5 MW of power from the steel wheels to the steel rail.  This drive system has been used by the transportation industry for over a century. 

The system is easy to build on a VAWT because there are no space limitations at ground level and the ground or foundation can be utilized to support the drives.  Recyclable bushings mounted on the outer diameter of all the wheels can be replaced after excessive wear rather than replacing the entire wheels.  The large diameter wheel could have segmented bushings simplifying the process even further. 

The contact area of a locomotive wheel and rail is the size of a dime.  This small area is due to the shape of the wheels and rail that is needed to keep the wheels centered and to prevent the wheel flange from contacting the rail.  The contact area on the Traction Drive, however, can be adjusted to maximize adhesion between the surfaces, thus reducing surface wear.  Also, the rail system has to contend with vegetation and other adverse environmental conditions, which reduces the coefficient of friction.  The environment around the Traction Drive can be controlled and the coefficient of friction can be maximized at all times.

The coefficient of rolling resistance of the steel wheel is low, which provides a power loss similar or lower than a multistage gearbox.  A Traction Drive with several small wheels, each driving a generator can be engaged or disengaged as the wind speed changes.  The drive system can also be orientated to help diminish or remove the overturning moment at the bottom of the rotor of a VAWT.

A Traction Drive system was installed on several of our prototypes and has proven to be a viable alternative to the gearbox system.  Future models will combine the mounting brackets for the blades with the large diameter wheel.  The hub and wheel diameter will be approximately 1/5 the rotor diameter.  The small wheel diameters will be 1/10 the large wheel diameter yielding output shaft speeds, 10 times the rotor speed.

Development History

2003-2005

Experimented briefly with a HAWT, then built and tested several H style VAWT.  The experiments clearly demonstrated the vibration problems with 2 and 3 bladed turbines. Started experiments with 5 or more blades of a VAWT.

2006

Built a mobile wind tunnel consisting of a diffuser, test section, contraction zone and fan.  Conducted experiments on blade profiles, bluff bodies, small VAWT with solid aluminum blades and aerodynamic drag on cables at various skew angles.

2007  

Started experimenting with horizontal cables attached to the blades of rotors with 5 or more blades.  

The blades on each rotor started at the bottom of the central structure, went outward and diagonally upward, then vertical and diagonally back to the central structure.  The joints from the diagonal and vertical blades were adjustable so data was collected using various blade offset angles.

2008

Build the first Darrieus shaped wind turbine with cross cables and struts.

2009

Hired the NRC to build computer models of this turbine with a diameter of 40 meters.  They also did an aerodynamic and structural analysis of the 40 and 160-meter diameter models.  Removed the central structure or pipe in the middle of the rotor.  The rotor was raised using hydraulics and the support from the pipe.  After the rotor was in place the pipe was removed by using cables and pulleys connected to the top hub.  The pipe weight (2000 pounds) was more than doubled by using this procedure but the blades and cables remained motionless.

2010  

Attached strain gauges on the blades and used equipment from NRC to collect data while the turbine was operating under various wind conditions.  The NRC concluded the stress on the blades was low as expected.

2011 

Constructed the Traction Drive with the small wheel on the outside of the big wheel.  The generator extracted power from this drive system.

2012  

Won first place in a worldwide competition organized by NASA’s media group ‘Tech Briefs’ and attended the awards ceremony in New York. 

Built a Savonius rotor inside the Darrieus rotor to enable the turbine to start operating in low winds.

2013

Built and tested a new traction drive with the small wheel on the inside of the big wheel.

Started the construction of the 60-foot diameter 50 KW wind turbine.

2014  

Completed the 50 KW turbine in January and collected data until March of 2016.

Travelled to Las Vegas and did a presentation at the AWEA annual convention.

2015  

Designed, built and tested a novel aero brake system consisting only of 2 ropes that extended to the equator of the rotor which prevented the turbine from over speeding in potential runaway situations. 

2016  

Designed, built and successfully tested the Lux Hydraulic Pump.  

 

Questions & Answers

 

+ Why will the Lux wind turbine produce more power at a lower cost?

The rotor of the Lux turbine is bigger so it extracts more power, especially at wind speeds between 4 and 10 m/s (9 and 22 mph). This is significant because the majority of power production happens between these wind speeds at most wind turbine locations. The annual kilowatt hours produced by a 2MW Lux turbine is expected to be approximately 10-20% more than most other 2MW HAWT. The Lux turbine has a lower capital cost than HAWT because it does not need a tower, a pitch system or a yaw system. The blades are easier to manufacture and a robust foundation is not required. This is why we say this turbine can produce more power at a lower cost.

+ Problems such as vibrations and torque ripple were common with past VAWT. How does the Lux wind turbine negate these issues?

The Lux turbine uses 5 or more blades instead of 2 or 3. As the blade of a VAWT rotates through one complete revolution, the relative wind direction produces torque, no torque, torque, and no torque. By using multiple blades that are supported with cross cables, this torque pattern is diminished and these problems are eliminated.

+ The Lux turbine only uses a short tower or bottom structure. Does this impair the power output because the equator is lower than the hub height of HAWT?

Although it is true that higher wind speeds are found at higher elevations, the Lux turbine still extracts more power because the rotor is larger. An expensive tower is not required to produce more power. The equator on a megawatt sized Lux turbine can be as high or higher than the hub of most HAWT of equal power rating, therefore, the Lux turbine does utilize the winds at higher elevations.

+ The Lux turbine rotates at a slower speed than other turbines of equivalent rating and will require a more expensive gearbox or direct drive generator. How does this turbine deal with this issue?

First of all, a slower rotating rotor with vertical blades may be easier for birds to see and therefore easier to avoid. We use a Traction Drive system that is similar to the well-proven technology of the locomotive and rail system where steel wheels transfer 6000HP to the steel rails. Our steel on steel Traction Drive is low cost and increases the output shaft speed without using an expensive gearbox. The hydraulic pump using cylinders is also an excellent low cost method for extracting power from a slow rotating output shaft.

+ HAWT must be spaced at large distances from each other to avoid interference. Do the Lux turbines need the same spacing?

Researchers such as Dr. Dabiri at Stanford University have developed computer models and experimented with as many as 18 VAWT in various configurations. They believe that counter rotating VAWT placed in close proximity can extract 10 times more power from a given area of land than can HAWT. More research is required in this area but most people believe turbines like the Lux turbine can be spaced much closer together.

+ Can the Lux turbine operate offshore?

It is an excellent candidate for offshore use because of its very low center of gravity and lightweight. Instead of having the heavy gearbox, nacelle, generator and blades positioned high above the water level it is much easier to float or stabilize a lightweight rotor and have the heavier components such as the Traction Drive, generator and support structure near water level. Also, the components that require maintenance are at water level.