The G168 VAWT

Background

Vertical Axis Wind Turbines (VAWTs) have been around as long as Horizontal Axis Wind Turbines (HAWTs). Both have their origins in machines that turned millstones or pumps centuries before electricity was being generated. By the early 1980s, investments and progress on HAWT and VAWT technology was close to equal. From this investment base came the foundational computer modeling and international certification (IEC 61400) requirements needed for both technologies to succeed.

The VAWT industry has encountered difficulties reaching commercialization in large part because of the absence of rigorous field testing, data collection and computer model validation. Sandia National Labs in the U.S. produced an important body of engineering and design work on curved-blade Darrieus-type prototypes from the 1970s to the 1990s. That work is now being used in VAWT aerodynamic and aeroelastic modeling software by wind turbine technology companies such as Wind Harvest International and Nenuphar, and by aerodynamic modeling consultancies such as Iopara Inc. in Montreal. These tools are critical to the engineering design of VAWTs.

In its early years, WHI co-founder and turbine inventor Bob Thomas applied his experience as an engineer in the aerospace industry to wind turbine R&D. Over the course of designing, building and operating 13 prototypes, he learned a great deal about straight-bladed VAWTs, and in the process discovered and patented the “coupled vortex effect” (CVE). WHI’s most recent VAWT prototypes have used strain gauges to validate fatigue life, frequency response, aerodynamic and extreme load computer models.

Key to WHI’s current modeling success have been the contributions of Dr. David Malcolm. Soon after retiring in 2013 from Det Norske Veritas, a world-renowned wind turbine engineering and certification company, Dr. Malcolm started working with WHI as its senior engineer. With his help, and with the computer models he developed starting with his work with Sandia National Labs and on the FloWind VAWTs, WHI’s engineering team finished the Design Evaluation for its G168 VAWTs and submitted it to the U.S ICC-SWCC (Small Wind Certification Council) in February 2017.

Certification

Wind Harvest International (WHI) has submitted the Design Evaluation of the G168 v1.1 VAWT to the U.S ICC-SWCC, a key part of the certification process. Once the turbine is built and installed on the foundation awaiting it at the Advanced Wind Turbine Testing Facility, it will be put through the Safety and Function, Power Performance, Acoustic, and six-month Endurance Test. Then it will be certified.

During this testing period, UL (Underwriters Laboratories) will install strain gauges on the G168 and produce third-party validated data that will be used to certify WHI's Design Evaluation and its underlying aeroelastic model.  After this has been completed, the G168 would be one of the few, if not the only, VAWT to meet full IEC 61400 certification requirements.

Description

WHI’s G168 is a straight-bladed VAWT, which has evolved from earlier versions and modern aerodynamic modeling. It has three aluminum blades, each 14 m (46 ft) long, and three sets of horizontal arms with a diameter of 12 m (39 ft). which brings its rotor-swept area to 168 square meters (1808 square ft) before the blade tip fairings have been added.

The G168 VAWT is cantilevered from the ground through two sets of radial thrust bearings and one set of axial load bearings, resulting in a blade tip height of 18.26 m (59.9 ft) with a basic 4 meter tower.  The rotor turns a direct drive synchronous generator in conjunction with a power converter that allows variable speed operation. It has a dual, fail-safe braking system that uses a resistor and pair of spring-loaded caliper brakes.

The G168 prototype (v1.0) was field-tested in Denmark at the national Nordic Folkecenter for Renewable Energy. Using strain gauges and other instrumentation, critical data was secured to validate the aeroeleastic modeling used to perfect the G168 v1.1.

The G168 v1.1 is designed to operate in IEC Wind Turbine Class II winds and withstand 50-year extreme wind speeds of 59.5m/s  (133 mph). With a total weight of 12,000 kg (26,500 lbs), it has a design life of 20+ years, with the blades and the core of the turbine engineered to last significantly longer.

Though it will first be certified with a 70kW capacity generator, it can be used with generators with nameplate capacities of 50 to 75kW, the choice depending on the wind resources at the installation site and  the Power Purchase Agreement. Customers will be able to choose various height towers to optimize cost effectiveness in different wind shears.


Variable Speed

The G168 is a variable speed turbine, which allows the rotor to operate at maximum aerodynamic efficiency over a range of wind speeds. It also allows limiting the maximum power in higher wind speeds by restricting the rotor speed in combination with aerodynamic stall. The variable speed capability is achieved through the use of a power converter together with a synchronous generator.

The relationship between wind speed, rotor speed, and electrical power is shown in Figure 1. Note that while the values in Figure 1 are theoretical, they are based on field data and code validation.

Figure 1. Electric Power and Rotor Speed vs. Wind Speed

Energy Production
The projected annual energy production of the G168 turbine is shown in Table 2. The wind speeds refer to average wind speeds at the center of the rotor. The values in the table come from theoretical calculations and are consistent with the power curve in Figure 2.

Table 2. Annual Energy Production

Average Wind Speed
Single VAWT With Coupled Vortex Effect
6.0 m/s (13 mph)114 MWh135 MWh
7.0 m/s (16 mph)167 MWh190 MWh
8.0 m/s (18 mph)219 MWh252 MWh

Table 3. Operational Parameters

Wind speeds
Cut-in wind speed4.5 m/s (10 mph)
Cut-out wind speed25 m/s (56 mph)
Rated wind speed14 m/s (31 mph)
Reference wind speed (max 10-min mean)42.5 m/s (95 mph)
Power
Rated power (at 14 m/s)~70 kW
Rated torque10.3 kN m
Rotor speeds
Rated rotor speed65 rpm
Max rotor speed (over speed)74 rpm

Manufacturing and Installation

Multiple manufacturers will be involved in the production of each G168 VAWT. Components will be shipped to a central facility where the turbines will be assembled and made ready for shipping to project sites. In the U.S., Patriot Modular is fabricating and assembling the first G168s that will be installed at the Advanced Wind Turbine Testing Facility in West Texas.

Only a few components are available “off-the-shelf” (e.g., SKF bearings, caliper brakes, fasteners, generator). The remaining pieces are custom-made. The costliest components include the central mast, composed of three 22-inch diameter pipes with forged flanges attached to each end by full penetration welds, and the nearly 3-meter-long forged and machined drive shaft to which the central mast connects.

The blades are extruded by Nedal Aluminum in Holland. The fiberglass fairings are handmade by Superior Glass Works in Oregon. The steel blade arms and base tower can be produced by any quality fabrication facility.

By assembling most of the turbine off-site, it can be installed and ready for shakedown testing within two days of delivery. The base tower acts as the VAWT’s nacelle, with the driveshaft installed and connected to the generator, and all power electronics, controls and brakes installed before delivery. The final step is to hoist the fully covered base tower onto a “lowboy” semi-trailer for local delivery.

At the project site, the base tower is lifted and set onto its foundation anchors and bolted down. Then the central mast is attached to the drive shaft. The blade arms with their sheet metal fairings are installed by two workers in a bucket lift. The blades are then lifted by the crane and attached to the blade arms. Finally, the fiberglass fairings are attached. This completes the installation, with the VAWT ready for pre-operational testing.

WHI intends to find local companies to do the assembly and simple fabrication in each country or territory in which its VAWTs are installed. The parts requiring more sophisticated manufacturing will be shipped to that assembly facility. The more work that can be done within a local facility, the easier it is to complete the installation in the field.

Prototypes

The design and engineering of the WHI VAWT Model G168 evolved directly from the combination of modern computer modeling paired with field-tested results from WHI’s utility-scale prototypes installed in California, Finland and Denmark over the past 10-plus years. Many key mechanical components featured in the G168 were born from earlier small-scale prototype turbines designed, built and tested by co-founder Bob Thomas in California over the course of roughly 30 years of building VAWTs. Collectively, these prototypes operated for over 40,000 hours, with the majority of early turbines installed in the demanding wind conditions found in California’s San Gorgonio Pass.

Many of the key lessons learned from the designs of Bob Thomas are incorporated in the G168. Computer modeling funded by the California Energy Commission and completed by IOPARA Inc. showed that WHI could use lower-solidity rotors in its VAWTs and still create the coupled vortex effect. Field-testing of the 636G-3 prototype (Finland, 2013–2014) proved that a design using only three aligned blades would not produce a problematic torque ripple, and, in addition, that lower-solidity VAWTs would realize a higher efficiency (Cp max).

The biggest change in the G168 v1.0 was the single-rotor, H-type design that was based on the aerodynamic modeling of Charliaos (Haris) Kotsarinis and the cantilevered mechanical engineering calculations of Ionut Munteanu. With the electrical engineering of Pablo Paz, the G168 incorporated a variable speed drive and modern electronic controls. Until this iteration, first field tested in Denmark in 2015–2016, all WHI prototypes had external support structures to hold the bearing at the top of the rotor mast in place, either with guy-wires or with arms to aerodynamically shaped stators. Changing to a cantilevered design required a significant increase in the size and length of the drive shaft and in the sophistication and strength of the bearings but resulted in an overall less expensive turbine to manufacture and one that could be installed in more types of terrain.

The stories of each of the main prototypes can be found on their individual pages.

WHI Prototypes

Camarillo Model, Camarillo and Bald Mountain, Calif.
Installed 1976

Windstar 256, Sandberg, Calif.
Installed 1979

Windstar 480-4, Concord, Calif.
Installed 1984

Windstar 480-5, Antelope Valley, Calif.
Installed 1986

Windstar Model 530, Whitewater, Calif.
Installed Spring 1988, removed Jan. 1994

Windstar Model 1066, Whitewater, Calif.
Installed Feb. 1991, removed Jan. 1994

Model 530G –The Vortex Turbine Array, North Palm Springs, Calif.
Installed 2001 and 2002

Model 636G-3, Lilla Bakstar, Finland
Installed 2012 and 2013

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