Intelligent actuation allows better system control at solar power plan

Paul Boughton

Novel actuator technologies are giving the level of fine control required to ensure that industrial activities reach new levels of efficiency and cost effectiveness. Sean Ottewell reports.

Rotary actuators play an integral role in the world's third largest concentrated solar power plant, known as Nevada Solar One. The rotary actuators are used to rotate and tilt a series of parabolic mirrored troughs, so that they are able to track the sun's motion for maximum efficiency, and protect the arrays from high winds and dust storms

The 64MW Nevada Solar One plant, located about 25 miles south of Las Vegas, spans 400acres and generates approximately 129million kWh of solar electricity annually, enough to provide power to more than 14000 homes, while emitting near zero carbon dioxide.

"The plant is creating a lot of interest in the energy industry because it provides a renewable energy alternative," said Gilbert Cohen, senior vice president of developer Acciona Solar Power (ASP), a unit of Spain's Acciona Group.

This is because parabolic trough systems are used, which employ a different technology to the photovoltaic solar panels common on rooftops. They use concentrated sunlight, in place of fossil fuels, to provide the necessary thermal energy to drive a conventional power plant. Curved mirrored surfaces concentrate the sun's heat on a receiver tube containing a heat transfer fluid capable of achieving temperatures between 120°C and 260°C. This fluid is used to produce steam that drives a generator system to produce electricity.

Essential to the success of the project is the ability to capture maximum sun exposure with the 760 solar collector assemblies. To enable them to track the sun, Acciona needed a dependable precision actuator system. The electromechanical systems used in other solar fields were unable to deliver the combination of control and robust construction that they were looking for. In contrast, Parker Hannifin offered a hydraulic system that would be powerful enough to track the sun accurately during the day and provide safe, secure stowing of the panels at night.

The self-contained hydraulic system that Parker designed uses the same fluid to power the system and to lubricate it. This design will deliver virtually maintenance-free operation for over 20 years, with no filters to change thanks to an intrinsically clean, sealed design. Each of the 760 solar collectors comprises a customised Parker HTR300 Series rack and pinion hydraulic actuator, control valves, solenoid valves, pumps, cylinders, seals and wear bands, and pre-bent tubing/fitting assemblies.

The hydraulic actuators each drive 12 panels that weigh over 900kg and have a total of 240 curved mirrors. The actuators enable these panels to track the sun in extremely small increments, with small, low-speed displacement pumps providing two short pulses from the reservoir to the unit's motor three times per minute. Furthermore, the system is designed to withstand backlash from high winds, by allowing the solar trough to slip and rotate in a controlled fashion, without damage to the motion control mechanics. This designed 'clutching action' is inherent in hydraulic systems, with the use of pressure relief valves, and allows the solar collector to realign and begin tracking the sun again when the wind subsides.

At the end of each day, the actuators move the arrays in large intervals by engaging high-speed pumps to increase motor speed and return the solar panels to their home position for the following day's tracking of the sun. At this point the hydraulic system's duty cycle, the mechanical locking mechanism, featuring Parker 2H cylinders, stows the parabolic troughs for the night. The Parker actuators provide lockdown capabilities with wind load control up to 84mph to protect the solar arrays against hurricane winds while in stow.

Using Parker technology, the Nevada Solar One plant is able to provide Nevada Power Company and Sierra Pacific Power Company, and ultimately the people of Nevada, with a viable alternative to fossil fuel powered electricity, helping to ensure a more sustainable future

Actuators are also at the heart of Rotork's latest contract - for the Vopak Skarvik 1 project at Skarvikshamnen in the City of Gothenburg.

This is the largest petroleum tank storage project in Scandinavia for 25 years and will be the first fully automated installation of its kind in Sweden. Ten new tanks are being constructed and a new PLC-controlled SCADA system installed to enable Class 1 hazardous products including petroleum to be pumped to and from any location on the site.

Rotork has supplied over 80 ATEX approved IQPro intelligent electric actuators with factory-fitted Profibus cards for flow control duties throughout the site, operating gate valves on the tanks and on a new main manifold and pipe rack that will distribute the products around the plant.

Vopak's project manager Per Follin explains: "We selected Rotork as our actuator supplier following an evaluation of our installed base across other Vopak sites worldwide. Rotork has a vast experience in this field and the reliability, functionality, diagnostic abilities and ease of maintenance of its products were all factors contributing to our decision."

The main contractor for the Vopak Skarvik 1 project is the Swedish construction group NCC. Valve and pipework installation is being performed by the specialist contracting company NVS, who ordered the actuators from Rotork's Swedish agent Alnab Armatur AB. Alnab is providing local support for installation, commissioning and after-sales service.

Rotork also enjoys a long association with the Canadian oil sands industry, which is recognised as the largest single petrochemicals resource on the North American continent. Over the years thousands of Rotork valve actuators of all descriptions have been supplied to the industry for the processing and transportation of this valuable resource.

The company's latest contracts involves 180 GP and CP range heavy duty pneumatic actuators supplied to Mogas Valves for the first phase of the Shell Scotford upgrader expansion project.

The Scotford upgrader, which is situated next to Shell Canada's Scotford refinery, near Fort Saskatchewan, Alberta, uses hydrogen-addition technology to upgrade the high viscosity heavy crude oil (bitumen) from the Muskeg river mine into a wide range of synthetic crude oils.

The process has significant environmental advantages, introducing dramatically lower levels of sulphur dioxide emissions and producing synthetic crude oils which enable refiners to manufacture clean, high quality products, including petrol and diesel, with low levels of aromatics, particulates and sulphur. The expansion project will create a 60 per cent production capacity increase when completed in 2010 as well as significant new employment opportunities in the area.

Meanwhile, Emerson Process Management has risen to the demands of its customers with the launch of the new Fisher GX 3-way control valve and actuator system. The new GX 3-way has the ability to accurately control the temperature of water, oils, steam, and other industrial fluids. Applications include heat exchangers and lubricating skids (Fig.1).

The new system came into being because Emerson's customers were demanding a number of innovations, including an anti-corrosion finish, stainless steel bolting, easy to use positioner, multi-language display, push button positioner and emission control packing.

The flow cavity of the GX 3-Way valve body has been engineered to provide stable flow and reduce process variability. This linear stability is perfectly suited for temperature and pH control applications.

The GX 3-way valve is multi-faceted in its ability to cover both flow mixing (converging) and flow splitting (diverging) applications with no extra parts needed.According to Emerson, unlike other 3-way valves, it features both side-port and bottom-port common trim while its robust design addresses the space limitations of the OEM industry.