With aluminum, it's all about give and take. Aluminum production consumes a great deal of electricity, but the material itself gives back as a major component of power transmission systems.
The metal has been the material of choice for electrical transmission applications for more than 100 years for investor-owned utilities, rural electric authorities and electrical cooperatives, said Ian Hewett, president of the Alcan Cable unit of Alcan Inc., Montreal.
Aluminum cable accounts for the vast majority of the 212,000 miles of existing transmission lines that exceed 230 kilovolts in North America, Hewett said.
"The key advantages of aluminum are its light weight, high strength and reliable performance over many diverse operating environments around the world," Hewett said. "Aluminum cables are the most economical solution for transporting power over long distances compared to copper or other conductive materials."
He pointed to the price and performance advantage of aluminum cable for transmission applications compared to other materials. "It takes 2 pounds of copper to equal 1 pound of aluminum for the same current-carrying capacity, and the tension to support copper is much greater than aluminum. The higher material cost of copper and the increased tension requires more steel in the towers and more concrete in the foundations." And at less than half the price of copper cable, aluminum cable offers substantial cost savings when used in long-distance electrical transmission, Hewett said, although he could not provide actual cost comparisons. (From a pure metal standpoint, copper is nearly three times more expensive than aluminum $3.30 per pound for copper vs. $1.15 for aluminum based on recent London Metal Exchange three-month settlement prices.)
The market for aluminum transmission cable will grow as the country expands power transmission capabilities. The Edison Electrical Institute said member companies plan to increase transmission investment by nearly 60 percent to $31.5 billion between 2006 and 2009 vs. 2002 to 2005. The institute's figure represents the total spending forecast for transmission projects, including towers, engineering, construction and more. No dollar figure for the cable or aluminum components was available.
Despite the fact that aluminum cable has been in service for more than a century, Alcan continues to develop new products, Hewett said. "We have introduced our HiTemp line of ACSS (aluminum conductor, steel support) conductors that can be operated up to 250 degrees Celsius with low sag. This gives customers a cost-effective option to relieve line sections that are overloaded or would be in emergency circumstances."
Another new product making its way into the market is the ACCC (aluminum conductor, composite core) transmission cable from Composite Technology Corp., Irvine, Calif. In place of the steel core used in traditional cable, Composite Technology's new cable uses a carbon-fiber core for added strength and functionality.
Although the cable, introduced in 2003 on a demonstration basis, is three times more expensive than ACSR (aluminum cable, steel reinforced) products, it offers specific advantages, according to Benton Wilcoxon, the company's chief executive officer. The composite core of the new cable is less than one-quarter the weight of existing steel cores but is twice as strong, and it utilizes trapezoidal-shaped bands of aluminum for conductors vs. the round strands found in conventional cable products, which allows a greater amount of aluminum to be used.
"For any given diameter, we can put in more aluminum conductor material, resulting in greater conductivity with the same size cable," Wilcoxon said. The resulting combination of the composite core and the shaped conductor is a cable that can not only carry more voltage, but can run cooler at similar loads. This minimizes thermal expansion and "line sag," which was at the root of the 2003 blackout that swept across much of the northeast United States and parts of Canada.
The new ACCC cable suffers less "line loss" than ACSR cable, Wilcoxon said. With lower line loss, ACCC cable can deliver as much power as ACSR cable with lower generator loads. The new cable, according to Wilcoxon, can allow power providers to dial down generators in proportion to the lower line loss by as much as 10 percent.
The market seems to have taken notice of Composite Technology's product. Wilcoxon expects 2007 cable sales of around $30 million, 10 times the $3-million level seen in 2005. The bulk of the company's sales are in China, which is rapidly building grid capacity as it undergoes its own industrial revolution.
Composite Technology's cable is manufactured under contract by traditional aluminum wire companies, including the Quebec facility of General Cable Corp., Belgium's Lamifil NV and Midal Cables Ltd. of Bahrain. Composite Technology makes the core from aircraft-grade composite, and then the various contractors wind the specially shaped aluminum conductor around it.
Any bare overhead transmission line (as they are known) needs a grounding wire, and supplying this falls into the purview of Conex Cable LLC, DeKalb, Ill. "You need something to protect transmission cables from lightning strikes," said Jerry Bliss, Conex's sales and marketing consultant. About 90 percent of overhead bare transmission cables use ground wires, with aluminum cables serving in this role as well as in the actual transmission of electricity, he said.
Superior corrosion resistance, especially in marine or coastal applications, and greater conductivity makes aluminum cable ideal for serving as ground wires, Bliss said. The size or ratio of ground wire to transmission cable is a function of the voltage being carried on the line, he added.
With power transmission lines carrying ever-higher loads, ground wires have grown in importance. As the level of power being transmitted grows, aluminum's superior electrical properties in this application will become more important, Bliss said.