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. This lets the plant continue operating in more-adverse weather conditions than was possible in the past.

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 3000 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 high-pressure fluid to the actuators. A low-speed, 1.5-gpm pump, driven by a 0.33- 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 start-up 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 seconds. 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.Hydraulic rotary actuators

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. Both pumps are reversible and include internal valving and hydraulic circuits that ensure pressure-relief thermal pressure-relief compensation against trapped volume heating affects within the fluid.

Built-in reliability
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 per year 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 service-life safety factor, plant operators are highly confident that the hydraulic drives will eliminate maintenance and life issues encountered with the electromechanical systems.Hydraulic rotary actuators

In addition, the entire flow and pressure-control hydraulic circuits are made of 0.5-in. diameter stainless- steel tubing. Prebent tubing and fittings with judiciously positioned bends helped ensure assembly flexibility.

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/kWh. 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 seven cents/kWh 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.

For more information, visit: Acciona Group at www.acciona-energia.com/default.asp; and Parker Hannifin at www.parker.com. For a video tour of the plant, visit www.nevadasolarone.net.