Structural Design Reduces Solar-Electric Power-Plant Costs and Improves Solar-Generating Efficiency

The challenge that has always faced alternative-energy producers is cost per megawatt (MW) compared to cheap fossil fuels. But rising natural- gas prices, political pressure, and the add-on costs of environmental regulation continue to shrink the gap between these two energy sources. Alternative energy is finally coming of age.

Alternative-energy production is also getting more competitive thanks to improved materials and designs. Such is the case of Nevada Solar One, a 64-MW concentrating solargenerating facility that came online in July 2007. Using a new aluminum frame design, the facility’s output exceeds design specs by up to 15%.

Concentrated solar
Located south of Las Vegas in Boulder City, Nev., Nevada Solar One (NS1) is the world’s third-largest solar-energy field. It spans more than 400 acres and generates approximately 130 million kW-hr of electricity annually.

Instead of the photovoltaic solar panels commonly seen on building rooftops, the facility uses parabolic troughs holding curved mirrors to concentrate sunlight on glass and steel receiver tubes. Fluid circulating through the tubes reaches temperatures as hot as 735°F and is used to generate steam, which drives a turbine and generator to produce electricity.

One advantage solar thermal holds over other renewable resources is that, except for the troughs, the power plant is a standard design widely used by electric utilities. And generating capacity can be built close to where the power is needed. This is in contrast to wind-power-generation facilities. Because they must be near the best wind resources, they are often far from where the electricity is consumed.

NS1 is creating a lot of interest because it provides a renewable-energy alternative with no fossil-fuel emissions. In fact, the plant eliminates CO2 emissions equivalent to taking 20,000 cars off the road, according to the operators. And the sizable amount of electricity NS1 produces illustrates the potential for more parabolic- trough systems in southwestern U.S.

Solar-thermal power-plant efficiency depends heavily on how well the parabolic-mirror array concentrates and maintains the sun’s energy at the focal point of the tubes carrying the heat-transfer fluid. There is only a small tolerance band around the focal point and thermal-heating efficiency falls off dramatically outside this band. Deflection of the mirrored array and manufacturing variations from one array to the next are two reasons for deviation from the focal point.

Complicating matters, parabolic mirrors can act as large sails. High winds produce significant torques that try to rotate the panel from its commanded position. So the structural framing that supports the mirrors had to be strong, built to exacting tolerances, and yet stay within tight budget constraints.

A better mousetrap
Concentrating solar-power designs have been in commercial use since 1985, but no new solar-power plants had used this technology for more than a decade before construction began on NS1. While parabolictrough technology is 50 to 75% cheaper than photovoltaic solar collection for large-scale power plants, it simply wasn’t cost efficient enough to compete with fossil fuels during the last decade. Once power-generation cost structures began shifting, the technology was reevaluated.

NS1 developer Solargenix (now part of Acciona Solar Power, a unit of Acciona S.A. based in Madrid, Spain) realized that if they could improve the technology’s performance and reduce the costs for frame manufacturing, installation, and maintenance, they could be more cost competitive with fossil fuels. The company approached Gossamer Space Frames, Huntington Beach, Calif., for a new way to build the troughs and frames that would meet these needs.

Gossamer’s metal structure using shear pins instead of nuts, bolts, and welds met Solargenix’s basic requirements and took advantage of aluminum’s strength and low weight. Gossamer used MultiFrame, a graphical structural-engineering analysis software from FormSys, Fremantle, Australia, to create and engineer the frame, and Inventor, an AutoDesk 3D solids program, for modeling and detailing.

The challenge then came down to manufacturing. “We needed a partner that had specific extrusion presscircle diameters, production capabilities for a variety of complicated shapes with extreme tolerances, and the ability to meet just-in-time delivery schedules,” said Gossamer President, Glenn Reynolds.

Gossamer turned to Hydro Aluminum’s Extrusion Americas unit to economically produce the complicated shapes with the exacting tolerances necessary for correct alignment and assembly.

Hydro recommended 6061 T6 aluminum for its strength and machinability, and also used metal with 70 to 80% recycled content. Manufacturing recycle-blended aluminum requires only 5% of the energy, yet the material retains all the performance characteristics of virgin aluminum.

Hydro manufactured 36 separate components, including connectors and frame parts, at its Phoenix plant. It then shipped the extruded parts to the company’s facility in Guaymas, Mexico, for punching, multispindle drilling, and CNC fabrication. All parts then returned to Phoenix for final inspection before shipping to the NS1 site for assembly. Hydro produced over 40,000 lb of aluminum components per day — more than 7 million lb in all — in less than nine months.

Exacting performance
Compared to other proposed support structures, the final space frame used 50% fewer parts, eliminated the need for welding, and did not require field alignment for the mirrors. This let workers assemble the frames in one-third the time. The aluminum frames have an approximate 3:1 reduction in weight compared to steel frames, which are widely used in Europe. The weight savings substantially reduced shipping and assembly costs, but did not compromise the finished frames’ torsional strength. For instance, the assemblies can withstand 85-mph winds in the upright position.

Because the solar frames are big — about 8-m long and 4-m high — there is always the concern over less-thanperfect mirror alignment. This can be caused by warping, poor connections, improper assembly, or a number of other factors. But the Gossamer/ Hydro design produced near-maximum reflectance.

The National Renewable Energy Laboratory recommends a combined “slope error” (mirror error plus frame-alignment error) of ~3.0 milliradians or less for solartrough arrays. NS1 operates with a combined slope error near 2.0 milliradians. This translates to a focus improvement of 34 to 38% over NREL recommendations.

This means the trough frames are quite close to theoretically perfect performance. The net effect is a real output increase of 4%, or approximately 2.5 MW. During its first peak summer season, NS1 actually had to dump power to keep its system balanced.

Acciona has contracted Hydro to manufacture identical frames for three new solar plants currently under construction in Spain.