The Z-40-A, developed by Scaled Composites (above), was
the first aileron-controlled wind turbine in the world.

Type: large commercial pylon-mounted wind turbine



Powerplant: 550 kW

Significant date: began operation in September 1995

The largest wind turbine built in the U.S. when it came out, the Z-40 Bladerunner was developed by Zond Energy Systems, Inc. (now a subsidiary of Enron Wind Energy Corporation) at Tehachapi, California, under the Value Engineered Turbine (VET) program (part of the NTTR program), under the auspices of DOE's Wind Turbine Development Program, and with funding assistance from the U.S. Department of Energy's Near-Term Turbine Research (NTTR) program. The VET work involved fabrication and testing of the first prototype of the 550-kW machine, a phase that was completed in about 18 months.

Development of the 500 kW Z-40 was initiated in March 1993. The Z-40 was an original design and not a clone of the Vestas V39 as some have claimed. Zond could have used the V39 as a model thanks to one unit Zond operated at its Sky River wind plant. Unlike the Vestas design, though, the Z-40 used an integrated drive train more like that of WindWorld, Tacke, and Kenetech's KVS 33 than that of a Vestas V39. In fact, the Zond gear box was manufactured by the same company that built the Kenetech transmission, Milwaukee Gear.

Zond built two versions of its prototype turbine. The first version was the Z-40A, which used a three-bladed, upwind, rigid hub machine with active yaw drive and aileron control (the first commercial turbine to do so), to provide aerodynamic braking and peak power modulation, and sat atop a low-cost, free-standing open truss tower. After an accelerated 21 month development effort, an aileron controlled prototype was fabricated and subjected to an extensive series of tests. This prototype uses a unique partial span aileron system developed by Zond for regulation of power and control of rotor rpm. During development testing, the prototype was thoroughly evaluated. The testing of the Z-40 prototype has included power curve verification, evaluation of aileron performance, assessment of power quality, measurement of structural loads, and assessment of turbine noise emission. Turbine component testing includes blade fatigue, aileron and aileron linkage fatigue, measurement of blade modal properties, static blade testing, and gearbox fatigue testing. After the prototype began operating, blade testing and certification of the machine ensued.

The second version was the Z-40-FS (later called Z-40B) which used conventional full-span pitch control system and sat atop a tube tower. Zond planned to compare the two turbine control strategies during prototype testing, but eventually both configurations were produced.

The Z-40 is a state-of-the-art 550-kW machine with a three-bladed, 40-meter (132 ft.) diameter rotor, which stands over 140 feet tall, and in windy locations can produce as much energy each year as is consumed by over 200 homes. It was designed to withstand the rigors of the harsh environment, and is designed to withstand peak wind speeds of over 150 mph. The Z-40 is unusual in that the power generation is controlled by varying the rotational speed of the fibreglass blades. It uses airfoils designed by NREL and ailerons for power regulation, a fully integrated drive train and yaw system, and a custom-designed control system

The Z-40 also uses a three-bladed rotor mounted upwind of the tower. The 63-foot blades are “pitched” in winds over 30 mph to enable them to “spill” excess power and continue operating at a constant rotation for maximum output. The use of three-bladed rotors, mounted upwind of the tower has been shown to be structurally and economically sound, while eliminating the tower’s interference with the wind and excess noise experienced by downwind designs.

Heated wind sensors mounted atop each turbine constantly monitor wind and weather conditions. In response to shifting wind directions, large motors turn the horizontal-axis turbine to face always into the wind. The turbines operate in wind speeds of 10-65 mph and are built to withstand gusts as high as 150 mph (the highest gust GMP ever recorded at the Searsburg site since 1983 was approximately 125 mph, in July of 1995). The turbines are designed to spin at a constant speed of approximately 29 revolutions per minute (rpm). They produce their rated output, 550 kilowatts, in wind speeds of 30 to 65 miles per hour.

The turbines “pitch” their blades in winds over 30 mph to enable them to “spill” excess power and continue to operate at a constant speed of rotation for maximum power output. This ability to adjust the angle of the blades to the wind is also useful in making adjustments that permit maximum power extraction in the winter when the blades are coated with rime ice, a relatively lightweight kind of ice. In most parts of the country the wind blows the strongest in the winter months and the weakest in the summer months. This pattern fits well with northern-based energy suppliers like GMP, where need for electricity and its value is the highest in the winter months.

The winds at mountain tops are known to be the most strong and persistent. A question frequently asked is what portion of the time turbines are generating electricity. This value tends to be less variable than you would think looking at the variability of energy production across the months of the year. In the low wind months of summer the facility is producing energy about 65-70 percent of the time. In the windy winter months energy production can be as high as 80% of the time. Because the turbine rotors turn in winds that are too light for power production, the percentage of time they are actually turning is even higher.

Prototype turbines ran for 5,072 hours and produced 687,000 kWh, until the first production units were built for utilities Central and South West Corporation, in west Texas, and Green Mountain Power Co., in southern Vermont. Zond was the first to sell wind-generated electricity to Southern California Edison Company, now one of the world's largest purchasers of wind energy. Other units were soon installed abroad, the first European installation being in Ireland, on a private natural site. China, Mexico, the United Kingdom, and Greece followed. The Z-40 received type certification in 1997. Because there was no U.S. certification body for wind energy technology at the time, Zond worked with Germanischer Lloyd (GL), a European certification agent based in Germany, for the Z-40's power performance after testing using procedures prescribed by the IEC (International Electrotechnical Commission), and also a certificate for fatigue testing. NREL performed power performance and loads tests on the Z-40 leading to certification.

"After we built the first production units," Zond spokesman Craig Christensen said, "we realized that reducing the weight of the blades would be a good way to reduce the overall turbine cost. We found that we could take 500 pounds of laminate out of each blade, out of a total weight of about 2,000 pounds. We went ahead and built a prototype and sent it to the National Wind Technology Center for fatigue testing. We quickly realized that we had gone a little too far, and put some of the laminate back in." Following the modifications, Christensen said, the blade was successfully fatigue-tested for 1 million cycles each of flap bending and edge bending, equivalent to the fatigue expected during a 30-year design lifetime.

For operation in Vermont, each blade and is specially coated black to absorb the sun's energy to assist in ice removal during the winter months. The blades are also coated with a clear Teflon-like coating to resist ice adhesion. These techniques were pioneered at GMP's first wind turbine test facility at Little Equinox mountain in Manchester, Vt., in 1990-1994. In addition to the blades, the Z-40 turbines used in Vermont incorporate other features to allow them to perform reliably in Vermont's winter climate. These features include cold-temperature steel for the towers, gearbox oil and hydraulic fluid heaters, and heaters for the control systems. With this equipment the turbines will be able to operate in temperatures down to -40° Fahrenheit. The turbines have a nacelle to cover and protect the generator and drive train and maintenance workers from the weather. A tubular tower provides a weather-protected environment for field personnel to perform service and maintenance work.

Zond offered a 40-meter (128 ft.), 24-sided tubular hightower as standard with its Z-40 turbine and provided a 50-meter tower as an option. Not including the foundation, the turbine and tower components weigh almost 60 tons, with each blade each blade weighing approximately 4,250 pounds. Towers of other heights could be considered in response to specific site conditions, as trees or other obstructions and terrain could dictate taller towers.

Zond’s pitch-controlled turbines (such as the Z-40B) do not have internal blade conductors, whereas the aileron-controlled turbines (Z-40A) have metallic rods inside the blades, which increases the risk from lightning. Fiberglass has sometimes splintered along blade cracks, and the load-carrying skin laminate has been torn by lightning strikes. All of the turbines were off line during July and August of 1998 for repairs to their ailerons, and in part upon request of the TVP host utilities and sponsors, Zond tested a newly-designed internal blade conductor retrofit on several turbines at its 194 MW facility in Storm Lake, IA. Although little is known of Burt Rutan's and Scaled Composites' involvement with the Z-40 design, it is likely, given their expertise on aircraft design, that they worked especially on the aileron-controlled version. The Z-40 is currently off production but production could be re-initiated if the market exists.

More powerful wind turbine generator models were later marketed as the Z-750 kW Series ('750' indicating their kilowatt power): these were the Z-46, Z-48 and Z-50. Zond’s variable-speed Z-750 wind turbines are three-bladed, upwind, active yaw, pitch-regulated machines which come in three rotor diameters: 151, 157, and 164 feet (or 46, 48, and 50 meters, hence their model numbers). Larger rotors are used in lower wind speed areas. Airfoils (cross-sectional blade shapes) developed by NREL are used on the Z-750 kW Series turbines.The Z-750 Series was of course based on the previously successful Z-40 and on experience gained through the installation and operation of over 2500 wind turbines in the USA since 1981.

The Z-50 has obtained necessary field verification and is currently available on the market. The Z-50 is a variable speed, variable pitch wind turbine and can be equipped with a cold weather package that allows operation down to -40º C (-40º F). One difference between the Z-40 and the Z-50 is in the weight, 132 and 217 kips, respectively, which can be a cost advantage during shipping and installation. Annual operating and maintenance cost for the Z-40 and Z-50 is $15,000-20,000 and $17,000-23,000, respectively. The Z-750 kW Series turbines are designed in accordance with the International Electrotechnical Commission 61400-1 standard for large wind turbines.

In the Near-Term Research and Testing program, Zond has a subcontract with the National Renewable Energy Laboratory (NREL) to lower the cost of energy (COE) and improve performance and reliability of the Z-750 kW Series. Concepts under consideration include resident transfer molding (RTM), an advanced blade fabrication technique that could cut the cost of blades by 20%, and integrating the turbine's controller and converter, which could result in a similar reduction in the cost of those components. Zond Systems was acquired by Enron Corporation in 1997 and was subsequently renamed Enron Wind Corporation. The installed GMP wind turbines retain the Zond name. Today, there are more than 5,000 wind turbines located in the Tehachapi/Mojave wind resource area. Operations and maintenance of the Z-40 are provided by B9 Energy (O&M) Ltd.


NOTE: the following specs were issued in 1997

Annual Capacity: 20%
Cost - $900 to $1000/kW currently
- $600-700/kW high volume commercial production (year 20)

- Intermittent energy production
- Large, highly visible structures
- Lowest cost renewable technology available today
- Avian interaction must be considered

1995: 594,640 kWh
1996: 8,614,249 kWh
2000: 11,500,000 kWh (estimated)


Tower heights and rotor diameters of the Zond Z-750 Series
Wind Turbine Generators (Z-46, Z-48 and Z-50)

The Z-40-FS can be adapted to all sorts of weather conditions.