With customer choice now available in the deregulating US power market, renewable energy is finally finding a firm foothold. PEi examines one of the latest wind energy projects to be completed, one whose very presence illustrates the growing enthusiasm of electricity consumers towards renewables.

In 1993, TXU Electric and Gas investigated the level of demand for renewable energy in Texas. Encouraged by the enthusiasm of its customers towards ‘green’ energy, TXU examined opportunities for wind power generation, and in December 1998 unveiled the $40 million Big Spring wind power project near Midland, Texas.

Developed by York Research, Big Spring consists of 46 turbines with a total capacity of 34 MW. The final phase of the project, completed in April 1999, saw the commissioning of the largest commercial wind turbines in the world – four Vestas V66 turbines standing nearly 80 m tall above the elevated plateaux of west Texan ranch land.

TXU, an investor-owned utility with assets of over $40 billion, believes that the project is testament to the fact that as power technologies advance, electricity generated by renewable resources will become more common and economic. The company has already launched TU Renew, a renewable energy programme offering customers the choice of buying wind-generated electricity, which has already been taken up by residents in the Waco area where the scheme was initially offered. TXU operates wind turbines at Energy Park, located near Dallas/Fort Worth International airport, which produce about 800 000 kWh per year, and also uses landfill methane gas to generate electricity.

Change in the air

The electric industry in the USA is currently experiencing an unprecedented transformation from a strictly regulated, monopoly structure to a deregulated, market-based system. Several variables have contributed to these changes, one of which is an increasing concern for environmental well-being. Twenty-one individual states have enacted electricity restructuring legislation; six of these have a renewable portfolio standard written into their restructuring bills.

Texas is a benchmark state in the deregulation process and in promoting renewable energies. It has one of the most aggressive renewable portfolio standards. Senate Bill 7 calls for 2000 MW of new renewable energy generated capacity by 2009. That addition alone would almost double the amount of wind generated electricity in the USA today. According to a 1991 survey compiled by the Pacific Northwest Laboratory, Texas has the second strongest statewide wind resource in the USA with the potential to produce over 1190 billion kWh of electricity annually.

In 1993 TXU Electric and Gas, the Big Spring utility customer, began investigating the benefits of wind power when it received an overwhelming approval for the use of renewables from their customers. In 1998 a new customer advisory group found that TXU customers preferred clean energy (renewable and efficient energy) over fossil fueled power by a factor of 10 to 1, and that over 80 per cent of their customers said they would pay at least $1 more per month on their utility bills for renewables.

The positive results from this poll reiterated that the demand for renewables from Texas citizens was significant and this was one of the motivators for building Big Spring.

In October of 1997, developer York Research acquired 100 per cent of the partnership interests in the Big Spring project from a previous developer who was not able to meet the project requirements. The acquired power purchase agreement requires TXU to purchase power from Big Spring for 15 years and provides for two 5-year option extensions available, exercisable by either party.

York Research Corporation develops, constructs, owns and operates cogeneration and renewable energy projects worldwide, and through North American Energy Conservation, a subsidiary, markets natural gas in the northeastern USA. Evolving over the last 15 years from an environmental consulting company to a developer of energy projects, it is currently exploring both wind and thermal project opportunities in North America, Europe, and the Caribbean. It sees vast possibilities in both the renewable and cogeneration markets.

Site design

The Big Spring project is built upon approximately 23 km2 of northwest Texas ranch land located roughly 80 km east of Midland. Elevated plateaux, or mesas, rise 60-90 m above the surrounding areas. The winds accelerate rapidly as they move up over these terrain features. Annual average hub-height wind speeds range anywhere from 29.6 to 35.7 km/h over the site. The site was configured to align the machines according to the prevailing wind direction, which is south by southwest. Around 70 per cent of the energy will come from this direction. York chose to space the turbines with significant separation in order to avoid energy losses and turbulence induced by machine to machine interaction.

The Big Spring project was financed through a $150 million private placement portfolio bond offering underwritten by Credit Suisse First Boston in August of 1998. In total, $30 million of the bond proceeds were allocated to the construction of the $40 million Big Spring project, while $10 million came from internal funding through York Research. The remainder of the proceeds were used to build York’s other wholly-owned $100 million InnCogen natural gas project in Trinidad and Tobago, and $20 million was used for general corporate purposes.

Site mobilization at Big Spring began in June 1998 with construction commissioning in July 1998. York acted as the general contractor for the entire project. The substation, electrical collection system, civil works, and SCADA system were subcontracted to different vendors. The turbines were manufactured by Vestas Wind Systems AS, of Lem, Denmark, a world-leading manufacturer of wind turbine technology.

The turbines were installed and commissioned by Vestas American Wind Technologies of North Palm Springs, CA. The Big Spring project is broken down into Phases I, II, and III. Phase I consists of 16 Vestas V47 660 kW turbines, Phase II consists of 26 Vestas V47s, and Phase III consists of four Vestas V66 1650 kW turbines, the largest commercial wind turbines in the world.

Vestas received the DKK210 million ($30 million) order for the 46 wind turbines in May 1998 after six months of negotiations. It began to manufacture the units at its Ringk bing facility in the third quarter of 1998.

The assembly of the first turbine in December, 1998 marked the beginning of Phase I construction which was commissioned by TXU in February, 1999. Phase II was commissioned in March, 1999 and Phase III was commissioned in April 1999. The Big Spring facility received its certification of readiness for full commercial operation from TXU on May 27, 1999.

Standing tall

The projected annual electricity generation for Big Spring is 117 million kWh, enough to power 7300 homes for one year. The two turbine models used at the facility are the 660 kW V47 and the

1650 kW V66, the largest commercial wind turbine model in the world. The tubular, free standing, rolled steel plate towers of the V66 are just under 80 m tall and the rotor diameter is 65 m wide. The V66 has a 3417 m2 rotor swept area, a 4.0 m/s startup wind speed, a 17.7 m/s optimal operating wind speed, and a 24.99 m/s cut out wind speed.

The smaller V47 has a tower height of 64.9 m, a rotor diameter of 46.9 m, and a rotor speed of 28.5 r/min. Its rotor wind swept area is 1518 m2, and it has a startup wind speed of 4.0 m/s, an optimal operating wind speed of 17 m/s, and a cut out wind speed of 19.8 m/s.

Both turbine models use three rotor blades which are made of epoxy and fibreglass composite.

The crosswind separation of the machines at Big Spring is nominally 3.5 rotor diameters. The row to row spacing of the machines is well in excess of ten rotor diameters in order to minimize the impact of turbulence caused by adjacent rotors. Each turbine has a total footprint area of approximately 18.3 m x 18.3 m, including the foundation footprint of 6 m x 6 m, a crane pad, and padmount transformer.

The turbine control system used in the Big Spring project was supplied by Vestas and is described as ‘state of the art’. The control system monitors turbine starts and stops under normal operating conditions and also protects the turbine under extreme emergency conditions such as faults caused by a loss of grid load while under power or a component failure. In addition, the control system manages the power output of each turbine by pitching the blades and changing the generator slip to maximize energy production while minimizing loads at wind speeds greater than 50 km/h.

The control system is operated by a digital computer that uses well-proven programmes developed by Vestas. Portions of the control system are located both in the base of the tower and in the nacelle of the wind turbine. These are linked by fibre optic lines to minimize interference and damage from lightning.

A key feature of the Vestas control system is ‘OptiSlip’, which controls loads and spikes from the turbines under high wind speeds. OptiSlip allows the wind turbine to operate in a similar way to a variable speed machine. This prevents the drive line of the machine from experiencing severe torque spikes, which can be as high as 100 per cent of rated load, during peak wind power conditions.

Wind gusts are not stored by the turbines, which would produce load spikes if their speed were not controlled. But by allowing the rotor to increase rotational speed by ten per cent, the torque is kept at a near constant. This allows the machine to adjust blade pitch to reduce aerodynamic lift and the resultant thrust. As the gust dies, rotor speed slows, and the blade pitch is again adjusted to maintain constant peak power. Peak power wind conditions will only occur about five per cent of the time, or 430 h per year.

York also favoured the modular component design of the drive train used by Vestas whereby the generator, transmission, mainshaft and housings are mounted on top of a chassis. Even though there are separate components, the cost differential when compared to an integrated design is negligible. York believes that the real benefits of the modular design will be realised in terms of the cost of long term operations and maintenance.

For example, the replacement of a $500 gearbox bearing in a modular system only requires the removal of the gearbox. However, performing the same replacement on an integrated machine would require higher labour and equipment costs, as the entire machine would need to be removed. The net result is lower long term repair costs for a modular system over the 25-30 year life of the machines.

The Big Spring project will use an Advanced Data Management System (ADMS) supplied by Second Wind, Inc. of Somerville, MA. This will monitor and record virtually every aspect of the project performance and allow the operators full remote control of the facility. The Second Wind ADMS was developed in conjunction with the National Renewable Energy Laboratory.

Portions of the ADMS are being supplied to this project under an agreement between York Research and the Turbine Verification Programme (TVP). The TVP is a joint effort involving the Electric Power Research Institute, the National Renewable Energy Laboratory, and the US Department of Energy. Performance data from this project will be provided to TVP in order to disseminate wind plant operational information to other utilities throughout the country.

Lightning strikes to wind blades are a significant risk to wind power plant operators. Vestas uses a lightning protection system which integrates lightning conduction in the blades with directed pulse pathways to the grounding system.