Amid heightened concerns over global warming, utilities everywhere are looking for cost-effective ways to generate electricity without burning fossil fuels and producing greenhouse gases. At the Nevada Solar One power plant in Boulder City, Nev., powerful and precise hydraulic actuators are a key element in converting the sun’s energy into electricity.

Nevada Solar One power plantThe 64-MW solar thermal power plant spans more than 300 acres, making it the world’s third larges t solar energy field. It generates approximately 130 million kW-hr of electricity annual1y, reportedly enough to power about 14,000 homes.

Solar thermal plants generate electricity in a different manner than the familiar photovoltaic solar panels common on rooftops. Instead, the facility uses 760 parabolic troughs — holding about 182,000 curved mirrors — to concentrate sunlight on glass and steel receiver tubes. Fluid circulating through the tubes reaches temperatures as hot as 750°F and is used to generate steam, which drives a turbine and generator to produce electricity.

Gilbert Cohen, senior vice president of developer Acciona Solar Power (ASP), a unit of Spain’s Acciona Group, says the plant is creating a lot of interest because it provides a renewable energy alternative with no fossil-fuel emissions. And the sizeable amount of electricity Nevada Solar One produces, says Cohen, illustrates the potential for more parabolic trough systems in the southwestern U.S.

New design
Another significant factor is that this is the largest solar plant of its kind to be built in the U.S. in more than 16 years, and it incorporates a precise, powerful, simple, and low-maintenance hydraulic motion system. Many older solar plants rely on electromechanical drives to rotate the collectors and track the sun. Although they met the performance and cost bogeys of the time, maintenance and performance drawbacks encouraged leaders in the solar industry to search for better solutions. For the Solar One plant, ASP teamed with Parker Hannifin, Cleveland, to devise a more-powerful and rugged hydraulic motion-control system that meets its objectives for low-cost power generation.

“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 thermal fluid,” explains Woodie Francis, a Parker product manager for hydraulic rotary actuators. There is only a small tolerance band around the focal point and thermal heating efficiency falls off dramatically outside this band, he says. “Some things that can cause deviation from the focal point are backlash within the actuator, deflection and wind up of the mirrored array, and manufacturing tolerance variations from one array to the next.” Electromechanical drives have worse backlash than hydraulic versions, he adds.

Complicating matters, parabolic mirrors can act as large sails. High winds produce significant torques that try to rotate the panel from its commanded position. Torque-output limitations and, again, backlash inherent in mechanical systems made it difficult to precisely focus sunlight on the collectors in the wind. And to make the new plant more economical, APS increased the number of mirrored arrays to focus more sunlight and generate more heat, which increased wind loads acting on the larger surface area of the panels. This required more torque from each drive — challenging the capability of electromechanical drives, says Francis.inally, electromechanical systems were too frail. Because the devices have rigidly interconnected components, high winds could backload and damage the drive — forcing plant operators to curtail operations when winds hit critical speeds.