Precision Actuator Puts New Spin on Solar Power

Twenty-five miles south of Las Vegas, Nevada Solar One, a new 64-MW solar thermal power plant, recently went online. Spanning more than 300 acres, it’s the world’s third largest solar-energy field and will generate approximately 129 million kW-hr of electricity annually, reportedly enough to power about 14,000 homes.

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

Also significant: 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 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 used Parker Hannifin, Cleveland, to devise a more-powerful and rugged hydraulic motioncontrol 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, ASP 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.

Finally, 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.

Parker says its hydraulic drive system addresses these shortcomings. Even when occasional wind gusts generate torque that exceeds design limits, the actuators withstand backlash through “clutching” action inherent in hydraulic systems with pressure-relief valves. The solar troughs slip and rotate in a controlled fashion without damaging motion-control components, and realign and begin tracking again when the wind subsides.

Each of the plant’s 760 solar collectors has its own hydraulic drive and electronic controls. The centerpiece is a hydraulic rotary actuator based on the industrial HTR Series from Parker’s Pneumatic Div., Wadsworth, Ohio. The rack-and-pinion actuator harnesses linear motion from opposing hydraulic cylinders operating at 3,000 psi that move a rack gear back and forth. The mating pinion gear rotates 240° and produces 300,000 lb-in. of torque — enough to move the solar array when winds exceed 40 mph, yet hold position within 0.1°. The actuator housing also acts as a primary structural element between the solar panels and support pylons.

Positive-displacement gear pumps built by Parker’s Oildyne Div. in Rockford, Ill., supply highpressure fluid to the actuators. A low-speed, 1.5-gpm pump, driven by a1/3-hp, single-phase electric motor, powers the actuator as it positions the solar array to track the sun.

ASP control software contains data corresponding to the theoretical sun position for any time, day, and year. The controller uses this data to point the collectors at the sun at startup and subsequently throughout the day. To maximize receiver-tube efficiency, the hydraulic motion system must track the sun in miniscule increments — in this case 0.1° steps. Based on the sun’s speed of travel across the sky, this corresponds to the electronic control commanding the pump/motor to pulse approximately every 24 sec. An inclinometer for each array supplies a feedback signal confirming the correct position. And because the heating tube is a continuous circuit that traverses the solar collector assemblies, all arrays must move at precisely the same time.

At the end of the day, a high-speed 3.75-gpm pump engages and quickly returns the panels to the home position, ready to begin tracking the next morning. A mechanical lock secures the troughs for the night or in extreme weather conditions.

Because downtime in a power plant is expensive, Parker took steps to ensure reliability. For instance, the workhorse HTR actuators often run up to 10 million cycles/yr in industrial applications. By comparison, the solar-power plant has only one operational cycle per day — or less than 10,000 cycles in 20 years. With this extreme servicelife safety factor, plant operators are highly confident that the hydraulic drives will eliminate maintenance and life issues encountered with electromechanical systems.

Each actuator’s self-contained fluid system uses a multifunctional gear oil with special additives to power hydraulics and cool and lubricate the gears. There are no filters to change, which eliminates one maintenance headache. To ensure a clean system, Parker meticulously washes components before assembly, filters the oil when it’s installed, and adds screens over the pump inlets and outlets. Because the duty cycle is quite benign, as are the general loads on gears, wear debris is not considered an issue. The unit also incorporates oversized tapered-roller bearings and dual seals at every critical interface. All these factors suggest that the design will deliver nearly maintenance- free operation for more than 20 years, says Francis.

According to ASP officials, solarthermal power is slightly more expensive than wind power but cheaper than photovoltaic, somewhere between 9 and 13 cents/kW-hr. But solar thermal holds several advantages over wind. One is that except for the troughs, the rest of 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 — unlike wind, where the best wind resources are often far from where the electricity is consumed. As they’re scaled up in the future, costs of 7 cents/kW-hr are a reasonable target, experts say.

Because trough technology relies on sunshine, future designs will include methods to store the hot fluid and use it to keep the turbines running into the night. Technology advances may someday let solar energy be used around the clock.

MAKE CONTACT
Acciona Group, acciona-energia.com
Parker Hannifin, parker.com