SEE THE LIGHT: THE EMERGING ROLE OF ALUMINUM IN SOLAR POWER

The uncertainty of global energy supplies, the rising cost of fossil fuel, and climate change are all hot topics spurring calls for increased use of solar and other sustainable energy generation sources. Coincidentally, advancement in solar technologies, government incentives and consumer demand for clean, renewable energy are helping to make solar applications competitive and ubiquitous.

But as with all new technologies, displacing the incumbent is a challenge. Traditional technologies typically
are the low-cost leaders, benefiting from discounts created by economies of scale and production infrastructure that has been paid for many times over.

Such is the case in the energy market. For solar energy generation to effectively compete with, or displace, fossil fuels without significant increase in government tax incentives and rebates, it will require dramatic improvement in solar collection, storage and distribution technologies, as well as cost reductions in manufacturing and key factor inputs. Eventually, economies of scale will follow, lowering the cost of production and leading to proliferation of solar around the globe.

In the solar industry, where concentrating solar power (CSP) technologies lead in large-scale applications, solar field and collection devices are generally recognized as one of the areas for continuous design improvement and cost reduction.

A CSP system generally comprises a reflector, a receiver for collecting/absorbing the reflected and refocused solar radiation, along with support structures for the collector and suitable mechanism for tracking the sun. In utility-scale CSP generation, the solar field accounts for at least 50 percent of the power plant?s total cost.

Solar collector assemblies (SCA) consisting of a rigid truss assembly or space frame structure to support the reflectors, receivers and system for tracking the sun constitutes approximately 25 percent of those costs. The distinguishing characteristics of an SCA are the structure topology and architectural expressions that support
a particular reflector type (e.g., trough, dish, etc.).

While concentrating the sun?s rays relies on parabola-shaped reflectors, it is the supporting metal architecture ? trusses, frames, arches, etc. ? that guarantees reflector alignments and provides the strength needed to withstand common forces, stresses, movements and wind loads.

Steel has traditionally been the favored metal for construction of SCA support structures. However, new designs and fabrication techniques for aluminum have provided viable alternatives. State-of-the- art aluminum space frames in commercial operations have achieved the multiple goals of much higher optical efficiency, lower life-cycle cost and minimal ecosystem disruption.

Although steel generally has a lower initial cost of production, innovative aluminum designs have mitigated this. Aluminum surpasses steel in cost total efficiency by being less expensive to transport and assemble. It also has an advantage in operations and salvaging periods.

Aluminum?s emerging role in CSP solar equipment showcases extruded aluminum?s versatility and its advantages in solar structures based on its ability to:

? Deliver precise machining to provide a high strength-to-weight ratio;

? Provide suitable support using fewer parts produced with high recycled content;

? Minimize transportation and assembly costs;

? Ensure quick assembly and accurate alignment and movements in the field;

? Provide full recyclability at end of life.

According to the U.S. Dept. of Energy, in 2007 the United States produced 72.87 quadrillion BTU. While solar capacity in America more than doubled between 2000 and 2007, it still represents only a very small part of overall U.S. electricity production. 

In 2007, just six percent of America?s total energy output was generated from renewable sources (excluding hydropower). Of that, less than two percent was derived from solar. The U.S. had 419 MW of CSP capacity and 874 MW of photovoltaic. Yet, America was the world?s third largest producer of solar power worldwide, behind only Germany and Japan. It?s obvious from these figures that countries around the globe still have a long way to go to get anywhere near fully realizing solar?s potential. A 2005 study by Greenpeace International, the European Solar Thermal Industry Association and IEA SolarPACES suggested that CSP capacity could reach nearly 37,000 MW worldwide by the year 2025. 

In the U.S. funding has risen sharply from $5 million in 2001 to $1.569 billion in 2007. The global recession and credit crisis may slow solar deployment, but there is still significant capacity due to be commissioned by 2012. According to the Washington, DC-based Solar Energy Industries Association trade group, the U.S. stimulus package and other incentives have helped spur 6,000 MW of solar thermal projects this year. Included in those incentives are tax credits and grants that will allow plant owners to be reimbursed for 30 percent of the plant?s cost. They can also apply for federal loan guarantees. 

Based on solar project applications, an estimated 4.4 GW of CSP is scheduled for development in the U.S. The largest proposed project in the U.S. is the 290 MW Starwood Solar I plant near Phoenix, AZ. As CSP solar generation inches toward critical mass and its technologies become more efficient, the cost has dropped from 20-25 cents per kilowatt- hour (kWh) in 1984 to 10-18 cents per kWh in 2009. This pricing compares favorably with today?s photovoltaic solar (21-38 cents per kWh), but lacks in competitiveness with fossil fuels and wind in some markets. In 2007, the cost of wind power ranged between 5 and 8.5 cents per kWh. The cost of electricity from coal was just $0.006 per kWh for the same period, while oil and natural gas were about $0.05 per kWh.

STRUCTURAL NEEDS

Designers must balance the need for the highest possible optical efficiency with design integrity, minimal raw material use, lean manufacturing processes, easy assembly and total life-cycle costs. They must also consider how the solar field and materials will impact local water sources and ecosystems. 

Economically viable CSP support mounts must include the following characteristics:Stiffness ? must have a robust, rigid frame capable of maintaining geometry of reflector at all times to deliver high optical efficiency.Motion ? must have high angular tolerance to enable one or two-axis tracking requirements.Weight ? must be heavy and capable of withstanding acts of nature, including severe winds, hail, snow, sandstorms, etc., to ensure extended service life.Strength ? must be capable to withstanding dead weight and loads without deformation while supporting large collector assemblies tracking the sun.Modularity ? must have an adaptable design with fewer components and low material cost to enable system to be optimized to suit the size needs, transportation challenges and site conditions.Ease of fabrication ? must have lightweight members that can be prefabricated and transported easily for final assembly at the site.O & M ? high mean time between repairs or replacement of components. All forces acting on a collector must be transmitted through the support framework or auxiliary structures and be dissipated into the ground. In early support designs, it was deemed imperative to use heavy steel structural members to maintain the stiffness and geometry of the collector at all times. Recent advancements in aluminum space frame design have, however, debunked that notion.