Renewable Energy Sources Rising: Beyond Fossil Fuels

This trend is not confined to the United States, however; countries all over the world are working to increase use of renewable energy, and many countries are ahead of the U.S. in this effort. With both government money and businesses/utilities moving in this direction, industries that make or use valves and actuators should look at what opportunities the situation presents. This article will examine how valves and actuators are used in solar, wind and geothermal energy.

RENEWABLES GROWING EVERYWHERE

China has been building coal-fired power plants at a frantic pace for some time now, but in recent years, the country has also ramped up alternate energy. A Nov. 18, 2009 article in RenewableEnergyWorld.com reports China plans to spend $2 trillion over the next 20 years to restructure the way it produces and consumes energy, which could represent a great opportunity for American companies. Meanwhile, Germany, which already has a substantial installed base of renewables, plans to have half of its primary energy consumption come from renewables by 2050, according to Germany Trade & Invest. In Canada, federal energy minister Lisa Raitt recently reported that 73% of her country’s energy already comes from renewable resources and that by 2010, 90% of electricity will be generated from clean, renewable sources.

Meanwhile, here in the U.S., the Energy Information Administration reports that just 7.1% of electric power comes from conventional hydroelectric while other renewables (biomass, geothermal, solar and wind) and other miscellaneous energy sources currently generate 3.6%. (The rest comes from coal, petroleum, natural gas, other gases and nuclear).

In the solar industry, 2009 market research from Solarbuzz reports that 5.95 GW (gigawatts) of new photovoltaic capacity was installed worldwide in 2008, an increase of 110% over 2007. Solarbuzz said solar cell production in China and Taiwan reached 3304 MW (megawatts) in 2008 followed by Europe at 1729 MW and Japan at 1172 MW in 2008. U.S. manufacturers, meanwhile, contributed 375 MW in 2008.

Until recently China believed solar power could not compete with coal, according to Guardian.co.uk, but it is now moving ahead rapidly on solar power as part of an effort to increase renewable energy to 20% of domestic consumption by 2020.

As far as wind energy, installations worldwide and here in the United States are growing rapidly. A U.S. Department of Energy Report (20% Wind Energy by 2030: Increasing Wind Energy’s Contribution to U.S. Electricity Supply), addresses how this country might achieve the goal in that title. In China, ChinaFAQs reports that in just the first three quarters of 2008, 5.59 GW of wind power projects were commissioned, and guardian.co.uk says plans call for 100 GW of wind capacity by 2020.

The world leader in wind power on a per-capita basis is Denmark. There, wind power supplies more than 20% of electricity consumption, and that number will be 50% by 2025, according to the Danish Wind Power Association.

Meanwhile, the geothermal industry may be helped along in the U.S. by recent government activity. Last fall, the U.S. Department of Energy announced up to $338 million in Recovery Act funding for exploring and developing new geothermal fields and research into advanced geothermal technologies. These grants will support 123 projects in 39 states, with recipients including private industry, academic institutions, tribal entities, local governments and DOE’s National Laboratories. The grants will be matched more than one-for-one with an additional $353 million in private and non-federal cost-share funds.

Biofuels, particular corn-based ethanol, have recently suffered from criticism that they are driving up the price of food, followed by a crash in the industry caused by overinvestment. But biofuels are not going away any time soon. According to the Energy Information Administration’s Annual Energy Outlook 2009, overall consumption of marketed renewable fuels grows by 3.3% per year, much faster than the 0.5% annual growth in total energy use.

Researchers also regularly report progress on new ways to produce biofuels and biomass energy. For example, biofuel company Qteros recently reported success in using municipal sewage to produce ethanol, and an American Chemical Society magazine has featured articles on converting coffee grounds to biodiesel and producing biodiesel from feather meal, a byproduct of the poultry industry.

Actuators and valves stand to play an important role in all of these alternative forms of energy.

SOLAR: HEADED IN THE RIGHT DIRECTION

In the valve industry, people tend to think of actuators as a tool for operating valves. But they are also used to move the mirrors and collectors in utility-scale solar power applications.

Solar energy installations fall into two general classes: photovoltaic and solar thermal. Photovoltaics create electricity directly from sunlight. They are expensive to construct on a per-kilowatt basis, and even larger projects are fairly modest in size. The largest utility-scale photovoltaic installation is the 60 MW Olmedilla plant in Spain; and recently, First Solar signed an agreement with China for a 2 GW installation to be built in the Mongolian desert in 2019.

The largest systems currently are solar thermal systems, which concentrate sunlight using mirrors to generate steam for turbines. The largest currently in operation is the Solar Electric Generating System (SEGS) installation, a group of nine plants spread over 1,000 acres in the Mojave desert, which have an installed capacity of 354 MW. SEGS uses 400,000 trough collectors, long parabolic mirrors that focus sunlight onto tubes; the tubes are filled with a heat-transfer fluid (a synthetic oil) that is pumped to steam generators that feed turbines. Single-axis actuators keep the collectors pointed at the sun.

In the other main type of solar thermal plant, a field of movable mirrors called heliostats focus sunlight onto a collector atop a central tower containing a boiler or heat collector. California-based PG&E is currently under contract with BrightSource Energy to install such a system in California’s Mojave Desert that will have an aggregate capacity of 1310 MW. One such installation, planned for Ivanpah, CA, will occupy 6 square miles and produce 400 MW. Resistance from environmental groups has led to the cancellation of another BrightSource project elsewhere in the desert, and the Ivanpah site itself may run into similar difficulties.

Besides the actuators that move the collectors in solar systems are the valves and actuators that help to control the hot oil pumped in solar processes. An example is the Andasol 1 plant in the Spanish province of Granada, which has 510,000 square meters of collector surface and an output of 50 MW. The plant uses intelligent electric actuators (from Rotork) with digital control in all areas of the generating process—on-off valve control using multi-turn and quarter-turn electric actuators, and for the 10 control valves on each of the networks, modulating actuators with proportional controllers and current position transmitters operating from a 4-20 mA control signal.

Electric vs. Hydraulic

Heliostats and photovoltaic installations tend to be electric, while many trough systems are hydraulic. Rich Nagel, business unit manager, Parker Hannifin, says the Kramer Junction solar trough plant, which is part of the SEGS installation and was built in the mid- to late-1980s, uses a variety of actuator systems: cable drums, gear boxes and hydraulic push/pull. Hydraulics offer the advantage that they can be built with a fair degree of compliance to withstand impact loads using a crossover relief circuit and sometimes an accumulator, Nagel explains. Meanwhile, an electric drive involves gears with metal-to-metal contact where high impacts can break teeth, he says.

The other advantage of a hydraulic system is the power it can deliver. Troughs are getting longer and wider, so there is a lot of area affected by wind, Nagel explains. Normal tracking torque, which might assume 40 miles per hour headwind, would mean about 300,000 and 500,000 inch-pounds (in-lbs); the torque to get to a -30 degree stowed position would be similar. However, the torque required in a high wind can reach 3 million in-lbs, which is generally provided by a mechanical clamping system to keep from overloading the actuator.

The range of motion is also significant: It takes 180 degrees to track the sun, 30 degrees to move to the stowed position, and sometimes an additional 30 degrees to reach a maintenance position.

Hydraulically, this can be done in two ways: a rack-and-pinion actuator or a push/pull system. The rack-and-pinion is the quickest to install. “You have a drive pylon that’s sunk in the ground and connected to a concrete piling and that actuator bolts with 4, 8, 16 bolts. You attach your torque arms, which are the plates that secure the mirror to the actuator, and you’re up and running in a couple of minutes,” Nagel explains.

The other method—push/pull—involves two long-stroke cylinders; one pushes part of the way; the other takes over for the rest of the rotation. “You do have to have a fairly long stroke actuator, and you have to push the cylinder half way on the torque arm. Once you get half way, the other side has to take over,” says Nagel.

Hydraulics are not noted for energy efficiency, but that’s not a problem in solar applications because the duty cycle over the course of a day is less than 5%. It takes all day for the troughs to swivel from one horizon to the other. “A pulse can be anywhere from 200 to 500 microseconds, and you can increment anywhere from 30 seconds [of angle] to 2 minutes, depending on wind load [and] internal leakage in the hydraulic system,” says Nagel.

Heliostats are made up of thousands of individual mirrors, each moving in two axes and each pointing in a slightly different direction with enough precision to keep the sun’s reflection centered on a space not much larger than the mirror itself. Because the mirrors are small and great precision is needed, they tend to use electric actuators.

The more familiar type of solar installation is photovoltaics. Any solar collector works best if it’s kept aimed at the sun as it moves across the sky, and the actuators used tend to be electric, often powered by direct current with battery backup. These systems—like with hydraulic systems—minimize overall energy use; each individual actuator runs for just a moment at intervals of about 10 minutes. Operating the actuators one at a time minimizes the required power.

Challenges and the Future

Solar systems are expected to last about 30 years, and they tend to be installed in deserts, frozen areas and other inhospitable places. Because of this, they’re challenged by sand and dirt, tremendous rainstorms, and high humidity and salt. “If you’re using cylinders, you have to deal with ingression of contaminants coming in through the rod seal,” explains Nagel. Rotary positioners suffer less because “everything is redundant sealed, so everything is self contained,” he explains.

Other problems include the number of connections and the loss of pressure in accumulators. Even normal maintenance can be a challenge, both because the number of units to be serviced is so large—a 250 MW field may have close to 4,000 drives—and because someone has to check the seals, monitor the hydraulic fluid and fix cylinders. Many of these systems are installed in places such as rural Spain where it can be difficult to find trained personnel.

Still, solar will continue to grow in size as better, stronger systems are put into place.

ACTUATORS CONTROLLING WIND TURBINES

Most people are familiar with modern wind turbines. These units are steadily increasing in size, with blades between 100 and 150 feet long, weighing between 7 and 14 tons, according to Dheeraj Choudhary, business unit manager, Global Wind Energy, Parker Hannifin. Some of the newer units have blades more than 200 feet long and electrical outputs greater than 5 MW per turbine.

Wind turbines have two axes of control—yaw and pitch—plus braking systems. Yaw control keeps the turbine pointed into the wind, while pitch control continuously adjusts the angles of the blades to ensure as high and constant power output as possible, while protecting the equipment from being damaged by overspeed. Most of the larger units have adjustable pitch, though some turbines manage to get by with fixed-pitch blades, using clever design to ensure that if wind speed gets too high, the blades will go into aerodynamic stall and lose lift.

Pitch control can be electric or hydraulic. In electric control, a motor with an attached brake is coupled to the pitch-control mechanism through a gearbox. A set of backup batteries ensures that the blades can be moved to a safe position in case of power failure. Hydraulic control uses push-pull cylinders that extend or retract to control blade angle. These cylinders tend to be large, says Choudhary, up to a meter or a meter-and-a-half stroke, “and you’re looking at bore sizes of anywhere north of 10 cm for each blade,” he says. Backup power is provided by a hydraulic accumulator, which also helps reduce the duty cycle of the hydraulic power unit. An electric system must use electric power every time it adjusts blade pitch, but “in a hydraulic system you charge up the accumulators and then you can keep using those accumulators until they discharge down to almost 40%, and then you just start your hydraulic power unit, charge it back up to 100% and go dead,” he says.

There are two sets of brakes on a wind turbine, one for yaw and one to stop the rotor from turning during an emergency or during servicing. These brakes are generally spring-applied and either electrically or hydraulically released, because it’s better not to use power while the turbine has no output and because a spring-applied brake is inherently failsafe.

Both electric and hydraulic systems have advantages and disadvantages for wind turbines. Hydraulic systems have to include rotary unions, and it’s difficult to prevent these from leaking. Electric systems don’t leak, which means not only a cleaner installation but also nothing slippery will drip onto the brake pads or cause environmental problems.

On the other hand, hydraulic systems don’t use gears, says Choudhary, while electric ones do. The turbines and the wind itself are not static “so as these blades are vibrating, they are causing fretting issues. The bigger/heavier the blade, the bigger the fretting issues,” he points out. In areas where low temperatures or salt air are a problem, the actuators are more subject to corrosion and poor battery performance. Hydraulics also have greater power density than electric so Choudhary predicts newer generations of larger turbines will increasingly use hydraulics.

GEOTHERMAL: CORROSION/EROSION CHALLENGES

When compared to tapping wind energy, which goes back to prehistoric times, geothermal sources may seem like newcomers; the first recorded geothermal electric generating plant was established at Larderello in Italy’s Piedmont region in 1904. Today, geothermal provides significant amounts of energy in locations where it’s available, and its use is increasing. In Iceland, for example, geothermal provides 24% of the nation’s electric power and 87% of its domestic heat and hot water.

Still, according to the Geothermal Energy Association, the United States is the world leader in use with a total installed capacity of 3152.72 MW as of October 2009. In that same month, Nevada Geothermal Power Inc. dedicated its 49.5 MW Blue Mountain “Faulkner 1” Geothermal Power Plant in Humboldt County, NV, which will add to this nation’s capacity. (The company also has leases in three other areas in the Western U.S. in various stages of development.)

Utility-scale geothermal energy production involves tapping volcanic heat to create steam—either by capturing existing steam, flashing superheated water into steam or pumping water into hot rock formations to produce steam in situ. Pressures are frequently in the vicinity of 5000 psi and temperatures get up to about 800º F, says Dave Clark, business development manager for refining in the Valve & Measurement division of Cameron. This doesn’t present much of a challenge for valves, but corrosion and erosion do. What comes out of the earth is generally far from clean—the steam at the Larderello facility, for example, has heavy concentrations of boric and sulfuric acids, and the plants in California’s Salton Sea area must contend with salt.

“One of the hardest things to control in metallurgy and valves is the pH factor of water,” explains Clark. “If the pH factor is off, it causes very severe corrosion.” Sometimes all wetted surfaces must be inlaid with 625 Inconel for protection—in some cases, Clark continues, “all the way out to the ring groove on the flange.” This does a good job, according to Byron Paris, product manager of gate valves in the Valve & Measurement division, Cameron, but it makes the valve more difficult to manufacture, he says, and more expensive. On the other hand, “they’re really bullet-proof valves…

I went out and witnessed a few of them; they took them out of the line [after four or five years of service] and they were as clean as a whistle.” WCC carbon steel doesn’t last very long under these conditions, he points out. “[We] took out a couple of carbon steel valves where they didn’t have that protection and there wasn’t a whole lot left.”

Cameron makes a line of wellhead assemblies, including equipment for geothermal applications. Clark says a simple slab gate valve is the best choice for geothermal applications, “because an expanding gate valve’s got angles on the back sides of these gates, and when corrosion gets back there and starts doing a number on it, it can freeze up on you.”

Along with the corrosion, says Clark, geothermal must deal with particulates, “picking up rocks and sand and grit, and the seating area of the valve is subject to some pretty severe damage.” A 625 or tungsten carbide inlay can help here as well, and “if we put valves out in sandy service, the customer usually asks for Stellite 6,” says Paris.

The outside of a valve in geothermal service doesn’t face especially difficult conditions as a rule, and can be protected with methods similar to those used for offshore service. The bolts generally have to be stainless steel, says Paris, and as for the body, Clark, says: “You can do a three-part paint system, a sandblast inorganic zinc primer and then a middle coat and a top coat.”

A BRIGHT FUTURE

It’s hard to keep up with what’s happening in the field of alternative energy today because it’s happening fairly rapidly. The world’s focus on trying to find ways to create energy that don’t involve oil or gas, coupled with individual countries’ desires to become more energy independent mean developments aren’t likely to slow anytime soon. Whatever happens, though, we can be assured that valves and actuators will be an important part of the picture.

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Peter Cleaveland, a contributing editor to Valve Magazine, writes extensively about issues related to the flow control industry. Reach him at pcleaveland@earthlink.net.