Precast Solutions

Expanding U.S. Wind Power
Precast concrete supports the weight of renewable energy.
By Sue McCraven

Within the past few years, wind power in the United States has become a viable alternative energy source to fossil fuels. In 2007 alone, wind-generated electrical capacity increased by 45 percent, or an additional 5,000 megawatts, representing 30 percent of all new U.S. energy capacity. Twenty-six states have passed standards that identify renewable energy targets for electric companies, with targets up to 25 percent of total power generation.

Legislative mandates aside, the reality of wind power’s remarkable growth in the United States is undeniable. A surprising example of wind power’s expanding capacity comes not from our windy coastlines or mountain tops, but from America’s heartland.

MidAmerican Energy Co. began building wind turbines just four years ago and currently operates more than 460 wind turbines in Iowa, making it No. 1 in ownership of wind energy generation capacity among the country’s regulated utilities. Florida Power & Light’s non-regulated subsidiary derives thousands of megawatts of power from wind farms and is the nation’s largest developer of wind towers. Yet Iowa and Florida rank behind Texas and California in total wind power generation. There is unprecedented growth in the U.S. and worldwide wind power market, and untapped potential for precast concrete wind tower and foundation systems for taller and more efficient wind turbines.

Why wind power is exploding
Wind-generated power is not only here and growing, it is increasing at phenomenal rates. Currently, wind turbine supply shortages have affected numerous projects, but this situation is expected to improve as manufacturers ramp up production to meet demand. According to the American Wind Energy Association (AWEA), the U.S. wind energy industry in 2008 is expected to match or exceed the nation’s unprecedented 2007 growth in wind power. To keep perspective, this projected 2008 growth represents just over 1 percent of the country’s total electrical supply; however, we need only to look to wind power developments in Europe – particularly in Germany, Spain and the Netherlands – to read the writing on the wall.

Soaring prices for oil, the ongoing energy demands of Mideast military operations, devaluation of the dollar and the vagaries of international energy politics all support an irrefutable rationale for unfurling our sails to the wind.

Not only is wind and advanced wind power technology available today, but wind-harvested power from just a handful of states, according to the U.S. Department of Energy, could provide all the power needs of the entire country. Think of all the green advantages of a non-carbon fuel like wind power: no air pollution, no increase in global warming and no depletion of limited fossil fuels. Yet with all the environmental advantages of wind power and possessing both the vast landmass and offshore regions on which to generate it, America is not at the forefront of this abundant green energy technology.

Europe leads in wind power capacity and engineering
Today, Europe contributes more than 60 percent of the world’s wind energy capacity and more than 54 gigawatts, or 4 percent, of the European Union’s (EU) electric supply, according to the Global Wind Energy Council. With the current price of oil at more than $110 per barrel and growing, it is not surprising that more than 20 percent of Europe’s total wind-generated power was installed in one year, in 2007.

The European Commission calls for a target of 20 percent of EU energy from renewable sources and a 20 percent reduction in greenhouse gas emissions by 2020. In 2007, Spain installed more than 3,000 megawatts of wind turbine power, almost half of all the additional power in Europe. More than 20 percent of Denmark’s electric power is generated by onshore and offshore wind towers, but Germany continues to retain its EU leadership with more than 22,000 megawatts in wind-generated power. Offshore wind power promises to be a major player in Europe’s wind power capacity in the near future. Europe leads the world in not only producing wind energy but in designing and engineering exciting new precast concrete wind tower systems.

Netherlands develops advanced precast tower system
Advanced Tower Systems (ATS), Netherlands, a joint venture of Mecal BV and Hurks Groep BV, has developed a precast concrete segmental tower with tubular steel sections on top.

Mecal is an independent engineering company with more than 10 years of experience in wind turbine technology, and Hurks Bouwgroep is a large general contractor and builder of high-tech precast concrete construction. “Larger and higher wind turbines are the global trend in the rapidly expanding wind energy market,” explains Frans Brughuis, managing director for ATS. “We have developed an economically viable alternative to steel towers for wind turbines of 1.5 megawatts or more and at hub heights (tower elevations) of 80 to 150 meters (263 to 492 feet).”

Larger hub heights (more than 1.2 times the rotor diameter) of up to 160 meters (525 feet) are more likely to deliver up to 60 percent efficiency, or energy yield, than shorter wind towers (see more in-depth here). “At heights of more than 100 meters, or about 330 feet, the atmosphere is characterized not only by higher wind speed,” asserts Brughuis, “but by more constant wind speeds and less turbulence.” Less turbulence translates into lower fatigue loads on the wind rotor and therefore more energy production efficiencies.

The real beauty of this wind power scenario of physics and meteorology is what it all portends for the precast concrete industry. Higher wind towers require stronger and stiffer tower construction. Larger tower circumferences made of tapered steel cylinders pose transportation issues, as a diameter of 4 to 4.5 meters (13 to 15 feet) are limited by overpass clearances on roadways both in Europe and in the United States. Steel towers for land-based sites are generally feasible only for hub heights under 100 meters (328 feet), as higher towers would require steel wall cross sections of more than 30 millimeters (about 11/4 inches) thick or very expensive components. Like the escalating cost of oil, steel prices also have been increasing. ATS has developed a precast concrete tower system that eliminates the transport clearance problem and also enables rapid on-site installation.

What this all means is that for taller and more power-efficient wind towers, precast concrete systems can offer economically feasible alternatives to steel construction.

Other European companies have also developed hybrid towers made of precast concrete and steel, and Spain has installed hybrid towers with heights greater than 100 meters (328 feet). WEC Turmbau GmbH (a subsidiary of Enercon) has developed a new tower construction technique using precast concrete tower ring sections. A minimal number of people are required for tower erection, including the crane operator and four people on the mast-climbing work platform to guide the precast sections into position.

Danish precast concrete foundation piers for wind towers
Since the mid 1990s, Danish firm Per Aarsleff, and more recently its U.K. subsidiary Aarsleff Piling, have installed driven precast concrete piles for hundreds of wind towers in poor soils where ground conditions are not suitable for a cast-in-place concrete foundation. Square precast concrete piles are symmetrically driven into the ground at both inward and outward angles around the tower location in a round or octagonal configuration. The single-piece and segmental precast piles are made by Per Arsleff’s company in Denmark, Centrum Paele, and by Arsleff Piling’s company in the United Kingdom, Centrum Pile Ltd., the largest U.K. manufacturer of continuously reinforced concrete piles fabricated and tested according to the new European Standard (BS EN12794:2005, “Precast concrete products-foundation piles”). Centrum Pile invested extensively in the latest concrete batching, transfer and form-filling equipment and uses a fully robotic pile cage fabricator to produce a standard range of 200 to 400 millimeters (8 to 16 inches) square sectioned piles.

Why precast can out-compete wind tower competitors
1. Transportation via trucks is a problem for larger-diameter steel tower sections. Precast concrete tower sections can be shipped without special transport clearances and are produced locally.
2. Competitive cost favors precast concrete over steel for larger and taller wind towers where the wall thickness and cost of steel fabrication becomes prohibitive.
3. Greater load-bearing capacity is provided by precast concrete piles manufactured under factory-controlled conditions than that of cast-in-place concrete piles.
4. Local materials and labor are used in the fabrication and installation of most precast concrete components as compared with imported steel.
5. No soil disposal and landfill costs are generated by driven precast concrete piles, compared with deep cast-in-place concrete foundations for wind towers.
6. Lower transport costs for precast concrete than for imported steel due to weight and proximity.
7. Shorter on-site construction is possible with prefabricated precast concrete tower segments than with cast-in-place or steel alternatives.
8. Greater vibration-damping capacity favors precast concrete wind tower systems.

Wind power and the precast concrete industry
The National Renewable Energy Laboratory (NREL) and the American Wind Energy Association (AWEA) as well as other alternative energy Web sites provide conclusive data that support strong future expansion in U.S. wind power. Wind power unquestionably is going to play a major role in the way the United States and the rest of the world generates power. Here are some of the reasons why:

• Wind is an inexhaustible and affordable energy source.
• Wind power improves air quality and does not contribute to global warming or climate change.
• Wind-generated power costs are declining (since 1999, wind-generated power cost has dropped almost 50 percent).
• Very high fuel costs and global oil prices remain uncertain.
• Federal and state governments mandate as much as 20 percent wind-generated power.
• Local economic wind farm development and employment can be revitalized.
• Renewable energy is getting strong public support and sentiment.
• National energy security is strengthened.
• It eliminates greenhouse gas emissions and provides other environmental benefits (no pollution typical of fossil-fuel energy generation: SOx, NOx or CO2; no airborne particulates; no mercury emissions; no use of fresh water).
• Wind power represents about 50 percent of all investments in renewable energy today.
• The United States increased its wind capacity by 45 percent in 2007 with more than 5,000 megawatts in new installations, representing 30 percent of all new U.S. energy capacity last year.
• Wind power capacity has exceeded all projections in recent years.
• Europe leads the way in wind power technology and engineering, demonstrating the great economic and environmental advantages of building wind-generated energy sources as rapidly as possible.

Precast concrete systems are poised to play a greater role in the wind energy industry. Unlike projects using imported steel, precast concrete production takes advantage of local resources and labor in fabrication and construction. Precast concrete wind tower foundation piles are less disruptive to the environment than deep cast-in-place concrete mats. Precast concrete production systems take advantage of numerous green initiatives from the use of supplementary cementitious materials to recycling gray water and cement and concrete materials.

For more information on concrete applications for wind power, visit www.concretecentre.com and search for “wind power.”

Sue McCraven, NPCA senior technical consultant, is a civil engineer, technical writer and editor, and environmental scientist who has contributed numerous articles and studies to prominent scientific journals.