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suzzane.jpg (6486 bytes) Suzanne Shelley
Guest Columnist and Freelance Writer
Suzanne Shelley is a Manhattan-based freelance writer specializing in science, engineering and technology (Email: suzanneashelley”at”yahoo.com). A 16-year veteran and former Managing Editor of Chemical Engineering magazine, Suzanne now writes about a broad array of engineering and business topics related to the chemical, petroleum refining, pharmaceutical and related industries, for both corporate clients and technical trade magazines. She currently serves as Contributing Editor to several magazines, including Chemical Engineering, Chemical Engineering Progress, Turbomachinery International, and Pharmaceutical Commerce. Suzanne holds a B.S. in geology (honors) from Colgate University, and an M.S. in geology from the University of South Carolina (Columbia).

Novel Power-Generation Strategies

energy_circle_1.gif (2265 bytes)Greener Power Generation Continues to Advance

Engineering advances related to both coal and biomass gasification, and the use of solar and wind energy, are helping change the face of power generation, by enabling commercial-scale electricity production that has significantly reduced fossil fuel use and environmental impact

People everywhere have an immense appetite for power — the type that flows from the wall socket. As the populations and industrial activities in industrialized and developing nations alike continue to proliferate, citizens everywhere will continue to consume massive amounts of power, a hallmark of modern life.

Today, the engineering community and many governments throughout the world are devoting considerable attention to encourage the commercial-scale deployment of promising alternative power-generation schemes — such as those that rely on renewable feedstocks, including solar power, wind energy, and biomass gasification. Great strides are also being made to improve the commercial-scale viability of coal-based integrated combined-cycle gasification (IGCC) power plants, whose use of coal gasification instead of coal combustion provides significant environmental improvements compared to traditional coal-fired power generation.

The pursuit of non-traditional routes for power generation is fueled by the desire to both slow society’s dependence on crude oil and natural gas (both of which have been subject to extreme price and supply volatility since 2000), and to find more environmentally friendly ways to use coal — which remains the cheapest and most widely abundant, yet the most notoriously polluting, of all of the fossil fuels (the technologies that allow for this are discussed in detail below).

Today’s advanced technologies to convert wind and solar energy to produce electricity, and to convert biomass and coal into electricity via gasification, provide nations with not only marked environmental improvements, but the chance for increased national security as well, by helping them to reduce their dependence on foreign oil imports.

The desire to pursue non-traditional energy sources is no longer reserved for optimistic university researchers and environmental activists. Today, a broad range of companies, universities and government agencies are actively involved in this arena, and in 2005 and 2006, many research-and-development breakthroughs and commercial-development announcements were made in each of these areas.

For instance, in early 2006, the London-based global oil giant BP formed BP Alternative Energy (Sunbury, U.K.; www.bp.com), through which it will pursue alternative and renewable energy technologies related to solar, wind, hydrogen, and IGCC power generation in the years to come. BP plans to invest $8 billion over the next decade to provide its customers with cleaner power sources (thereby reducing greenhouse gas emissions), and the company is projecting that this new division will deliver revenues of $6 billion/yr within the next decade.

Similarly, for GE Energy (Atlanta, Ga.; www.gepower.com), wind energy is a key component in the company’s recently announced “Ecoimagination” initiative, through which its parent company GE will be pursuing more environmentally products and processes. Underscoring both its commitment to the market, and the increasing popularity of this greener power option, GE Power’s 2005 revenue related to wind-based power generation (more than $2 billion) was up by more than 400% compared to 2002, its first year of wind-related operations, according to the company.

AES Corp. (Arlington, Va.; www.aes.com), one of the world’s largest global power companies, also recently announced it would invest $1 billion over the next three years to expand its alternative energy businesses. AES already operates 600 MW of wind facilities but is now developing an additional 2,000 MW of wind projects worldwide, and the company is evaluating other sources of alternative energy, such those based on solar power, biomass gasification and even wave technologies to capture energy from the world’s oceans.

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While solar panels have traditionally been limited to relatively small-scale applications, promising advances are also being made to improve the efficiency and reduce the costs associated with larger-scale systems.

IGCC — a greener way to turn coal into electricity

Coal has traditionally been the primary fuel used by power plants to generate electricity via coal combustion. However, in an effort to reduce the environmental impact associated with coal combustion, the chemical engineering community has made great strides in the development of integrated combined-cycle gasification (IGCC) power plants, which are able to achieve significant environmental improvements by enabling power generation via coal gasification, rather than traditional coal combustion.

In most IGCC facilities, gasification is carried out by injecting coal (typically in a slurry form) into a partial-oxidation gasifier reactor, along with oxygen (of 95-99.5% purity), and high-temperature steam. (Several gasification systems that use air instead of oxygen are also being pursued, mainly to sidestep the high cost of oxygen, but they are not discussed further here). At elevated temperatures and pressures, the feedstock reacts with the steam and oxygen in a reducing atmosphere, producing a synthesis gas (syngas) consisting of CO and H2 with lesser amounts of CO2 and methane.

It is well known that the combustion of coal and other petroleum-based fuels leads to the formation of various airborne pollutants, such as SO2, NOx and particulate emissions (which are to blame for acid rain and ground-level ozone problems), CO2 (which is implicated in long-term climate change and global warming), and mercury emissions (which can have devastating human-health implications). As a result, over the past several decades, coal-fired power plants have been the subject of extensive state and federal regulations that require the installation of costly, capital-intensive pollution-control systems, such as fluegas desulfurizaton (wet scrubber) systems, to meet increasingly stringent thresholds on regulated pollutants.

By comparison, IGCC power plants that rely on coal gasification are inherently less polluting than conventional coal-fired power plans, for several reasons. In particular, with an IGCC system, the unwanted sulfur and mercury are removed from the relatively small-volume, high-pressure (typically 400 to 1,200 psig) syngas stream — before it is combusted in the gas turbine. The syngas stream typically has an overall volume that is less than one-tenth that of the post-combustion fluegas stream leaving a comparably sized, coal-fired power plant. The ability to remove pollutants from this small-volume, more-concentrated syngas stream helps to reduce the capital expenditures and operating costs required for pollution-control systems, compared to those that are required by typical, end-of-pipe wet scrubbers and other pollution-abatement systems that are routinely used to capture regulated pollutants from the post-combustion fluegas produced during conventional coal-fired power generation.

One inherent advantage of IGCC is that the unwanted sulfur and mercury found in coal are removed early in the process from the relatively small-volume, high-pressure (typically 400 to 1,200 psig) syngas stream — before it is combusted in the gas turbine.

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Shown here is the generalized flowsheet for producing coal-derived synthesis gas (syngas), consisting of CO and H2 with lesser amounts of CO2 and methane, for use in an integrated gasification combined cycle (IGCC) power plant.  Because the sulfur and mercury are removed from the syngas stream before it is combusted in the gas turbines, and the overall water usage is reduced by 30-45%, power generation from IGCC facilities has an inherently smaller "environmental footprint" compared to that produced from traditional coal-fired power plants.  Image courtesy of Eastman Gasification Services (www.eastman.com).

By contrast, the end-of-pipe pollution-abatement systems used during coal-fired power plants must be sized to handle the much larger-volume, relatively dilute post-combustion fluegas. The ability to clean the syngas stream, which typically has an overall volume less than one-tenth that of the relatively dilute post-combustion fluegas stream exiting a comparably sized coal-fired power plant, provides distinct advantages, in terms of both increased emissions-removal efficiency and reduced capital expenditures and operating costs, compared to those required by typical end-of-pipe fluegas desulfurization (wet scrubber) systems and other pollution-abatement systems that are routinely used to capture regulated pollutants from the post-combustion fluegas produced during conventional coal-fired power generation.

In addition, traditional coal-fired power plants typically rely entirely on steam turbines. By comparison, in a typical IGCC facility, 60% of the power comes from gas turbines. This allows IGCC facilities to consume roughly 30-45% less water than a comparable coal-fired power plant, making IGCC particularly attractive for regions in which water scarcity is an issue.

To generate electricity, the cleaned, coal-derived syngas is burned in a gas turbine, and the hot exhaust from the primary gas turbine provides energy to a heat-recovery steam generator (HRSG), which supplies steam to a secondary steam turbine that generates additional electricity (hence the term “combined-cycle” power generation).

A flurry of grassroots project announcements made in 2005 and early 2006 demonstrates industry’s keen interest in IGCC, and underscores the commercial-scale viability of this greener approach to power generation. After a 10-year hiatus during which only one IGCC plant was built in the U.S., at least a half-dozen grassroots IGCC plants have been announced in the past year or two — by American Electric Power (Columbus, Ohio; www.aep.com), Duke Energy (Charlotte, N.C.; www.duke-energy.com), which recently merged with Cinergy (Cincinnati, Ohio; www.duke-energy.com), ERORA Group LLC (Louisville, Ky.; www.erora.com), Excelsior (Holman, Minn.; www.excelsiorenergy.com), and others.

Particularly noteworthy is the fact that each of these forthcoming IGCC facility is slated for 600- to 700-MW capacity — more than twice the capacity of the prevailing, first-generation IGCC plants that are still in operation today. For instance, each of the vintage IGCC facilities built around 1995-1996 — such as the Polk IGCC facility in Tampa, Fla., the Wabash IGCC facility in Terre Haute, Ind., the Demkolec IGCC plant in the Netherlands, and the Elcogas IGCC facility in Spain — has 250-300 MW capacity.

To streamline the commercial-scale deployment of IGCC power plants, each of the three primary manufacturers of coal gasification systems —GE Energy (Atlanta, Ga.; www.gepower.com), Shell (Houston; www.shell.com) and ConocoPhillips (Houston; www.conocophillips.com) — has recently partnered with a major engineering-and-construction firm, to offer a bundled IGCC-related package that combines all related engineering, design, procurement and construction services. For instance, GE Energy has partnered with Bechtel Corp. (San Francisco, Calif.; www.bechtel.com); Shell has partnered with both Black & Veatch (Kansas City; www.bv.com) and Uhde (Dortmund, Germany; www.uhde.biz); and ConocoPhillips has partnered with Fluor (Irvine, Calif.; www.fluor.com).

Meanwhile, to advance the state-of-the-art engineering concepts required for clean, coal-gasification-based power generation, the non-profit FutureGen Industrial Alliance (www.futuregenalliance.org) is working to bring online a 275-MW IGCC power plant that will combine “near-zero emissions,” hydrogen co-production and long-term underground sequestration to manage the nearly 2 million m.t./yr of CO2 that will be produced by the facility.

Members of the voluntary global coalition —including American Electric Power (Columbus, Ohio; www.aep.com); Anglo American Coal (London; www.angloamerican.co.uk); BHP Billiton (Melbourne, Australia; www.bhpbilliton.com); the China Huaneng Group (Beijing; www.chng.com.cn); Consol Energy (Pittsburgh, Pa.; www.consolenergy.com); Foundation Coal (Linthicum Heights, Md.; www.foundationalcoal.com); Kennecott Energy (Gillette, Wyo.; www.kenergy.com), Peabody Energy (Dallas, Tex.; www.peabodyenergy.com), PPL Corp. (Allentown, Pa.; www.pplweb.com), Rio Tinto Energy (London, U.K.; www.riotinto.com), and Southern Co. (Atlanta; www.southerncompany.com) — have voluntarily committed more than $250 million to help fund project development. In 2006, the governments of India and South Korea joined the alliance, pledging funds, as well. The U.S. Dept. of Energy (DOE; Washington, D.C.; www.doe.gov) is set to invest $700 million in the project, which is slated to go online in 2012.

Biomass — the ultimate renewable feedstock

While power generation based on coal gasification provides several advantages to traditional coal-fired power generation, this approach still relies on coal — a non-renewable fossil fuel. By comparison, the ability to power gas turbines using syngas that is produced by gasifying biomass — a diverse category that includes plant materials (such as fast-growing trees and grasses), agricultural residues (such as grains, corn and sugar cane, and even the woody, cellulose-rich leaves and stalks), and paper mill and lumber mill scrap— offers process developers a way to generate electricity using a truly renewable resource.

Meanwhile, when it comes to producing liquid fuels from biomass feedstocks, corn and sugar cane are already widely used in commercial-scale facilities to produce ethanol, while soybeans are already being used to produce biodiesel (these technologies are beyond the scope of this report, and are not discussed further).

As with coal gasification, syngas is produced when biomass is gasified in a partial oxidation reactor (without a catalyst), at temperatures ranging from 900 to 1,400C, depending on whether oxygen or air is used.

While such feedstocks will not likely ever replace fossil fuel feedstocks entirely, the ability to commercialize technologies that convert “bushels into barrels,” as the saying goes, promises to play an increasingly important role, as society continues in its quest to achieve more environmentally friendly, renewable sources of fuels, electricity and chemical feedstocks, and achieve greater overall energy self-sufficiency. Today, a variety of promising processes are being pursued to convert biomass into valuable fuels and other feedstocks.

Generating electricity from biomass

To generate electricity from biomass, renewable feedstocks such as forest residues, agriculture, landfill gases, and municipal wastes are typically used in one of four ways: via direct-firing, co-firing, gasification and anaerobic digestion.

Direct-fired systems are the most widely used. With this approach, biomass is burned directly, in lieu of oil or natural gas in a boiler that produces steam, which drives a steam turbine to generate electricity. However, such systems tend to have relatively low efficiency — on the order of 20% — due to the relatively low heating value of the biomass-based feedstocks compared to traditional fossil fuels.

To attain higher fuel heating values, co-firing systems mix biomass with coal or other fossil fuels, and use this fuel blend in a conventional power plant.

According to a 2006 report by the Worldwatch Institute (Washington, D.C.; www.worldwatch.org), more than 100 U.S. coal-fired power plants are now burning biomass together with coal. Experience has shown that biomass can be substituted for up to 2-5% of coal at very low incremental cost; higher rates — up to 15% biomass — are possible with moderate plant upgrades.

When burned with coal, biomass can significantly reduce the overall emissions of SO2, CO2 and other greenhouse gases produced by the facility. And, by burning biomass that would otherwise be destined for landfills, this approach reduces the amount of organic waste that would ultimately decompose and release methane —a greenhouse gas that is 21 times more potent than CO2.

Biomass gasification systems use a gasification process (described above) to convert the biomass into syngas. Once this syngas (carbon monoxide and hydrogen) is cleaned to remove unwanted pollutants, it is burned (in lieu of fossil fuels) in either a boiler that provides steam for a steam turbine, or in a gas turbine that generates electricity directly. Such power plants are said to achieve efficiency ratings of 60% of better.

The fourth option — anaerobic digestion — uses engineered systems to promote biomass decay, in order to capture the methane (the primary constituent of natural gas) that is produced during the process. The captured methane is then used to power a conventional power plant that generates electricity. Such highly engineered systems rely on a particular blend of bacteria that promotes the decomposition processes in the absence of oxygen, in a closed bioreactor. The bioreactor must be designed and operated in such way as to maintain the precise balance of nutrients and operating conditions (such as pressure, temperature and pH) that is required by this colony of fragile bacteria, in order to sustain the bacteria and maximize the desired reactions.

Lurgi AG (Frankfurt, Germany; www.lurgi.de) and Karlsruhe Research Center (Karlsruhe, Germany; www.fzk.de) are constructing a pilot plant that will use flash pyrolysis at 500C to convert biomass into pyrolysis oil and coke (up to 40 wt.%). This product can then be gasified to produce syngas, which can then be further processed to produce various sulfur-free biofuels.

For instance, researchers at Japan’s National Institute of Advanced Industrial Science & Technology (Tsukuba, Japan; www.aist.go.jp) have developed a new catalyst (1 wt.% rhodium supported on oxides of cerium and silicon) that allows biomass gasification to be carried out at lower temperatures — on the order of 650 to 700C. The resulting synthesis gas (a mix of CO, H2, CH4, and CO2, with solid carbon generation of only 1% and no tar), is suitable to produce power in a gas turbine, or can be used as a chemical feedstock. Scaleup efforts are underway.

Researchers at the University of Wisconsin (Madison, Wisc.; www.wisc.edu) are also developing a milder process to convert biomass-derived oxygenated hydrocarbons into a syngas that consists of hydrogen with less than 60 ppm carbon monoxide. Because it uses a nickel (not platinum) catalyst, and is carried out at milder conditions (roughly 225C, versus 600-1,000 deg C), the process is being pursued as a less costly alternative to conventional steam reforming of natural gas to make CO-lean H2 for fuel cells and other purposes.

The use of supercritical water (SCW) is key to the one-step process for making hydrogen from biomass (including black liquor from the pulp-and-paper industry, municipal garbage and paper sludge) that has been developed by researchers at Japan’s Shizuoka University (Hamamatsu, Japan; www.u-shizuoka-ken.ac.jp). The continuous process, which has thus far been demonstrated at bench scale, is said to produce two to fives times more hydrogen than conventional reforming and gasification processes. The researchers are currently seeking commercial partners for scaleup.

Together Shell Deutschland Oil GmbH (Hamburg; www.shell.com) and Choren Industries GmbH (Freiberg-Saxony; www.choren.com/de/) are constructing the world’s first commercial facility to convert biomass into a synthetic fuel called “SunFuel,” using Choren’s Carbo-V three-stage gasification technology. The SunFuel can then be further converted into methanol or diesel using Fischer-Tropsch synthesis. According to Shell, the fuels that will be produced at the 15,000-m.t./yr plant can be used without modification in any diesel engine.

Meanwhile, Neste Oil Corp. (Helsinki, Finland; www.neste.com) is building a 170,000-m.t./yr plant, slated for startup in 2007, as the first commercial-scale demonstration of its NExBTL (“next generation biomass-to-liquid”) process at its Porvoo, Finland, petroleum refinery. The process produces diesel fuel from renewable raw materials, and, according to the company, can be adapted to use many types of vegetable and animal fats. Meanwhile, Neste Oil and Total S.A. (Paris, France; www.total.com) have signed a memorandum of understanding to evaluate the possibility of building a large-scale production plant for biodiesel fuel using the NExBTL process at one of Total’s petroleum refineries.

While many biomass-conversion processes rely on partial oxidation reactors (using either air or pure oxygen), several groups are also developing biomass-conversion processes that rely on pyrolysis in an oxygen-depleted environment. For instance, DynaMotive Energy SystemCorp. (Vancouver, B.C.; www.dynamotive.com) has developed a fast pyrolysis-based process to produce “BioOil” from a wood-residue feedstock. In the process, pulverized biomass is pyrolyzed in a bubbling fluidized-bed reactor that operates oxygen-free at 450-500 deg C.

Once it’s operational, the pyrolysis plant that will process 100 tons/d of plywood residue and produce 70 m.t./d of BioOil (along with 20 m.t./d of char and 10 m.t./d of non-combustible gases), which will to fuel a turbine to generate electricity in a cogeneration plant (up to 2.4 Mwe) operating at Erie Flooring and Wood Products facility in West Lorne, Ont. DynaMotive Energy Systems is also in the process of developing a larger, 200-m.t./d pyrolysis plant for producing BioOil for other clients.

Researchers at the Pacific Northwest National Laboratory (PNNL; Richland, Wash.; www.pnl.gov) are also involved in developing a pyrolysis-based biomass-conversion process for producing both gaseous and liquid fuels.

Winds of Change

The first commercial-scale wind farms were constructed in California in the early 1980s. With wind power increasingly viewed as a more environmentally friendly alternative to traditional fossil-fuel-powered electricity generation, the worldwide wind energy market has been experiencing dramatic growth in recent years. Today, “wind farms” consist of anything from a single turbine to as many as several hundred turbines, and are becoming an increasingly important component of the world’s electricity pool.

U.S. wind activity

According to figures released in mid-2006 by the American Wind Energy Assn. (AWEA; Washington, D.C.; www.awea.org). The total worldwide wind-based power generation capacity at the end of 2005 was 59,322 MW. In the U.S., the total installed capacity for U.S. wind-based energy was nearly 10,000 MW (enough to serve the annual electricity needs of 2.5 million homes), by the American Wind Energy Assn. (AWEA; Washington, D.C.; www.awea.org). About 3,000 MW of new capacity (representing investment of over $4 billion) is expected to be added in the U.S. by the end of 2006.

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Today, "wind farms" consist of anything from a single turbine to as many as several hundred turbines, and are becoming an increasingly important component of the world's electricity pool.

AWEA also notes that wind-based power generation was the second-largest source of new power generation capacity added in the U.S. in 2005, after facilities powered by natural gas, and this trend is likely to continue through 2006.

According to AWEA, the 10,000-MW capacity of the U.S.’s current wind turbine fleet offsets the equivalent of 73,000 tons/yr of SO2, 27,000 tons/yr of NOx, 16 million tons/yr of CO2, every year, as well as mercury and other pollutants (the emissions that would result from the production of 10,000 MW of power using the average U.S. utility fuel mix). In addition, today’s 10,000 MW of wind power saves about 0.6 billion cubic feed per day (bcf/d) of natural gas — about 3.5% of the natural gas currently used in the U.S. to generate electricity.

In 2005, 2,431 MW of new capacity was installed in the U.S. (across 22 states) making wind-based power generation the second-largest source of new power generation capacity in the U.S, after natural-gas-fired power plants, according to AWEA. Based on total installed capacity at the end of 2005, California (2,150 MW), Texas (1,995 MW), Iowa (836 MW), Minnesota (744 MW) and Oklahoma (475) are currently lead the pack, according to AWEA, although Texas is expected to overtake California as the leader by the end of 2006.

In terms of manufacturers with installed U.S. wind capacity delivered during 2005, GE Energy (Atlanta, Ga.; www.gepower.com) dominated the market with 1,433 MW (representing 60% of total annual capacity), with Vestas (Randers, Denmark; www.vestas.com) in second place with 700 MW (nearly 30% of total installed capacity). Mitsubishi Electric (Tokyo, Japan; www.mitsubishielectric.com) with 190 MW Suzlon Energy Ltd. (Mumbai, India; www.suzlon.com) with 55 MW, and Gamesa (Huelva, Spain; www.gamesa.es) with 50 MW, sharing the top five positions, according to AWEA.

European wind activity

While the installation of wind-based power generation capacity is on the rise in the U.S., most of the wind-based activity has been in Europe. For instance, while the U.S. had 9,150 MW of installed capacity by the middle of 2006, the total installed capacity for wind-based power generation across the European Union (EU) was 40,504 MW by the end of 2005, according to the European Wind Energy Assn. (Brussels, Belgium; www.ewea.org).

In fact, cumulative wind power capacity in the EU grew by an average 32%/year between 1995 and 2005, from a base of 1,638 MW in 1994, and this bullish growth rate helped the EU to achieve the European Commission’s 40,000-MW target five years ahead of time, according to EWEA. Germany, Spain, Portugal, Italy and the U.K currently top the list of individual markets in terms of installed capacity. By 2010,wind power is expected to deliver 33% of all new electricity-generation capacity, and provide electricity for 86 million Europeans, according to a 2004 estimate by the EWEA.

One of the challenges associated with harnessing wind energy is the fact that wind is —by its very nature—unpredictable, and its strength and reliability are dependent on location. Sometimes the wind doesn’t blow, and it often blows in unpredictable gusts. To cope with such variations in wind energy, developers of commercial-scale wind farms tend to install hundreds or even thousands of wind turbines.

However, because of its inherent variability, wind-based energy is not typically used as the sole source of power for a given application (as is often the case for solar-energy-based solutions, which are discussed below). Rather, wind farms more commonly produce electricity that is fed into the power grid.

Wind technology has progressed mightily over the past quarter century. For instance, at a given site, a single modern wind turbine now produces 180 times more electricity, and at less than half the cost per kilowatt-hour (kWh) than an equivalent system of 20 years ago, according to EWEA. Nonetheless, to better understand the fundamentals of wind-based power generation, fundamental and applied research remains ongoing.

For instance, researchers at the U.S. Dept. of Energy’s Sandia National Laboratories (Albuquerque, N.M.; www.sandia.gov) are working to gain better understanding of the effects of wind gusts and turbulence, both of which can significantly reduce the life expectancy of turbine blades and airfoils, and to develop more robust and reliable turbine designs, which can more cost-effectively generate power via wind energy.

While it might seem like a quaint relic of bygone days, wind turbine blades are typically fabricated by hand, by laying down multiple layers of fiberglass cloth and resin in a mold, using the same fabrication techniques that are used in the boat-building industry. However, the imperfections that result from this type of composite structure can lead to premature failure of wind turbine blades. To improve performance, the Sandia researchers are also investigating a variety of advanced materials— carbon fiber and carbon/glass hybrid composites, advanced resins, additional fiber treatments — in order to develop blades that are lighter and less costly while still offering sufficient reach, and more structurally reliable to minimize fatigue-related failures. They are also working to develop more-efficient blade designs and better (automated) manufacturing techniques. The goal is to create rotating blades that encompass the greatest possible sweep area, so as to maximize the amount of electricity that can be generated by the wind energy.

Sandia researchers are also developing a range of computational tools to significantly improve the design and structural analysis of longer, thinner and more durable blade geometries with greater sweep area (to increase energy capture), and to perfect non-destructive testing techniques that can help researchers to evaluate and improve the design of existing blades. The challenge is to design blades that are both stiff and strong to span greater areas, while remaining both lightweight and able to adapt to varying system loads.

In addition, to mitigate the high-frequency loads on wind turbine blades that result from periodic wind gusts and turbulence, the Sandia researchers are also developing small, fast-acting sensors and control devices that can be used to optimize turbine operation, and improve the maintenance procedures for gears and bearings, which are most often to blame for extended outages related to wind turbines.

Proven condition-monitoring sensors and systems are now widely used with wind turbines, to track vibration signatures, temperature, shaft speed, torque, wind velocity and other machinery parameters. As with other condition-monitoring efforts, this allows operators to evaluate the ongoing condition of all rotating parts over time, so that progressive wear and other issues can be watched in realtime, and preventive (and even predictive) maintenance can be undertaken to improve efficiency and reduce failures. In addition, most state-of-the-art wind turbines now routinely include monitoring devices to track the performance of gears and bearings, which are most often to blame for extended outages related to wind turbines.

Researchers at the National Wind Technology Center (NWTC; Boulder, Colo.; www.nrel.gov/wind), a world-class research facility managed by the U.S. Dept. of Energy’s National Renewable Energy Laboratory (NREL; Golden, Colo.; www.nrel.gov), are also working to reduce the cost of wind energy through ongoing research and development of state-of-the-art wind turbine designs. For instance, NWTC has been working with The Wind Turbine Company (WTC; Bellevue, Wash.; windturbinecompany.com), which has focused its R&D efforts on developing so-called “downwind” turbines.

WTC’s design features two rotor blades, which are oriented on the downwind side of the tower. By contrast, conventional upwind turbines feature three rotor blades that are oriented on the upwind side of the tower.

According to WTC, its patented lightweight design allows the units to be constructed using 40-50% less materials compared with similarly rated, conventional “upwind” turbines. With both lower price and lower operating cost, WTC’s wind turbines also produce electricity for 30% less than today’s most economic units, according to the company. TWC is currently testing its prototype 750-kilowatt (kW) wind turbine at DOE/NREL’s National Wind Technology Center. Testing on earlier 250-kW and 500-kW prototypes has already been completed.

To improve the preventive and predictive maintenance of wind turbines, and assist in both diagnostic efforts and root-cause failure analysis, Bently Nevada Corp. (Minden, Nev.; www.bently.com) has developed a variety of condition-monitoring systems, which monitor vibration signatures, temperature, shaft speed, torque, wind velocity and other machinery parameters.

In May 2006, Siemens Power Generation (Erlangen, Germany; www.siemens.com) announced that it would be supplying 17 new wind turbines for three “repowering” projects in Germany. Specifically, the company will be providing larger, more robust units to replace some of the smaller, aging wind turbines that have been in operation for several years. For instance, in one project —the Marienkoog project operated by Buergerwindpark Galmsbuell GmbH — Siemens will provide seven 3.6-MW machines, to replace 15 older units. With an installed capacity of more than 50 MW, this project will become the largest wind farm in Germany once one it begins commercial operation by mid-2007. Buergerwindpark has also ordered seven 2.3-MW wind turbines for its Norderhof Wind Park.

Meanwhile, in late 2006, Siemens received an order worth 350 million Euros, for what it claims will be Europe’s largest commercial wind farm (slated for 322-MW capacity upon completion). The installation will include 140 2.3-MW wind turbines for the Whitelee Wind Farm south of Glasgow, Scotland. Completion is set for the summer of 2009.

GE Power has installed more than 8,500 wind turbines, with a total rated capacity of 7,600 MW worldwide, according to the company. In 2006, the company expanded its line of “multi-megawatt” wind turbines by launching a new platform in Europe, which will be followed by similar product rollouts in the U.S. and Asia in 2008, says the company. Evolving from GE’s proven 1.5-MW unit (which are typical sold in multiples to gain the needed generating capacity at a given site), GE’s newer machines will include a 2.5-MW unit (with a 100-meter rotor diameter) and a 3-MW unit (with either 90- or 94-meter rotor diameters). The increased rotor sizes will offer higher energy capture, says the company, and the units also include a number of other industry innovations, including a highly efficient, permanent magnet synchronous generator (enabling higher efficiency at lower wind speeds), a modular converter with full power conversion (which allows for more effective power quality control), improved bearing and lubrication system design, and advanced control technologies (which provides, among other things, improved pitch regulation, power/torque control, and load-dampening capabilities).

Meanwhile, one of the early projects being undertaken as part of BP’s expanding alternative-energy portfolio (mentioned above), is a 9-MW wind farm currently being built at the company’s oil terminal in Amsterdam, whose electrical output (enough to power 5,000 homes) will eventually be sold into the Dutch grid. BP also operates the Nerefco wind project at the (jointly owned) BP/Chevron Texaco (jointly owned) refinery near Rotterdam. That project, which began commercial operations in 2003, produces nearly 23 MW of electricity, enough to power 20,000 homes.

Harnessing the power of a sunny day

The desire to cost-effectively harness the sun’s energy to produce electricity is certainly not new. Today, steady improvements are being made in the production and use of both solar (photovoltaic) cells, which are made of semiconductor materials such as silicon (both crystalline and amorphous silicon), and more advanced materials, and convert sunlight directly into electricity, and solar modules that combine multiple, interconnected photovoltaic cells into a single, electricity-producing unit.

According to the U.S. Dept. of Energy (DOE; Washington, D.C.; www.doe.gov), today’s commercial photovoltaic systems generally have an efficiency of about 7-17%, although some experimental systems have been able to convert nearly 40% of the energy in sunlight to electricity. For comparison, DOE says a typical fossil fuel power generation system has an efficiency of about 28%.

To improve the efficiency of solar cells, they are often paired with complex mirror arrays, Fresnel lenses and even holographic films, to maximize energy capture by focusing sunlight more efficiently, either to capture solar energy for space-heating applications or to heat water (i.e., to produce steam, which can then be used to produce electricity via a steam turbine), or to focus the sun’s energy on arrays of photovoltaic cells, which use semiconductor materials to convert sunlight directly into electricity.

According to DOE, these so-called Concentrating Solar Power (CSP) systems can be small (involving, for instance, Stirling engines as small as 10 kilowatts to help a small village meet its power needs). Or, they can also be much larger, generating up to 100 MW of power for use in utility-grid-connected applications.

The three basic mechanisms for concentrating the sun’s rays in CSP systems involve the use of parabolic troughs, power towers, and solar dish-engine systems:

Parabolic troughs use curved, trough-like collectors that concentrate sunlight onto a receiver. The receiver is essentially a pipe containing oil or some other heat transfer fluid, which runs along the inside of the curved surface of the trough. The heated fluid is then used to run a conventional steam generator for electricity production. Such systems typically include a natural-gas-fired system to supplement the solar energy at night or when it’s cloudy.

Since the 1980s, nine plants, totaling 354 MW, have operated in California’s Mojave Desert using parabolic trough technology, according to a 2006 report by the Worldwatch Institute. Solargenix Energy LLC (Raleigh, N.C.; www.solargenic.com) is in the process of constructing a 64-MW trough plant in Nevada, which should be operational by early 2009.

Power towers are made up of many large, sun-tracking mirrors (heliostats), which focus sunlight on a receiver at the top of a tower. The sunlight heats up a heat transfer fluid in the receiver, which then is used to generate steam, which is then used in a steam turbine to produce electricity. According to the U.S. Dept. of Energy, early power towers used steam as the heat transfer fluid. However, current designs use molten nitrate salt, because of its superior heat transfer and energy storage capabilities.

Solar dish-engine systems use a dish, or solar concentrator, to collect the sun’s energy and concentrate it on a thermal receiver. The reflective surface of the concentrator is made of glass mirrors, which reflect roughly 92% of the sunlight that strikes the surface.

The thermal receiver is located at the focal point of the dish, and converts the sunlight to heat, which is then transferred to an engine/generator, which produces electricity. According to DOE, the thermal receiver is typically comprised of a bank of tubes filled with either hydrogen or helium, which function as a heat transfer medium. Thermal receivers can also be pipes in which an intermediate fluid boils and condenses to transfer the heat to the engine.

The most common type of heat engine in dish-engine systems is the Stirling engine, which uses heat from an external source (i.e., the sun) to create mechanical power that in turn drives a generator to produce electricity.BP Solar (Frederick, Md.; www.bp.com), currently the world’s third-biggest maker of solar cells, had global solar manufacturing revenue of $500 million in 2005, and expects that figure to double by 2008, with particularly strong demand coming from China, South Korea and Japan. The company plans to double its solar-panel manufacturing capacity to 200 MW/year by the end of 2006, at manufacturing facilities in Spain, Australia, the U.S. and India. In December 2005, the company formed a joint venture in China with Beijing-based China Xinjiang SunOasis Co., with the aim of tapping a market that is projected to multiply 50-fold in the next 15 years. A variety of technical advances being pursued by BP Solar are also discussed later in this article.

Kyocera Corp. (Scottsdale, Ariz.; www.kyocerasolar.com), Sharp Corp. (Huntington Beach, Calif.; www.solar.sharpusa.com) and Conergy AG (Hamburg, Germany; www.conergy.us) also have well-established product offerings related to solar-based power generation.

In mid-2005, two of the world’s largest solar projects were announced by Stirling Energy Systems (Phoenix, Ariz.; www.stirlingenergy.com). Once completed, the projects will provide as much as 800 to 1,750 MW of peak power to two Southern California utilities. In each of these installations, multiple mirrored dishes, each 35 feet across, will concentrate sunlight onto one end of a 25-kW Stirling engine (these engines are sealed systems filled with hydrogen). The focused sunlight heats hydrogen inside the engine. The expansion and contraction of the hydrogen drives pistons, which create mechanical energy, turning a generator and producing electricity. The unit has been rated at about 30% conversion efficiency — roughly twice as high as typical photovoltaic cells on the market, says the company.

One of the projects, contracted by Southern California Edison, calls for 20,000 of these dishes to be erected on 4,500 acres in the Mojave Desert. The second, for San Diego Gas and Electric, will involve the use of another 12,000 dishes on 2,000 acres in California’s Imperial Valley.

Sandia National Laboratory (Albuquerque, N.M.; www.sandia.gov) is also working with Sterling Energy Systems to test Sterling’s solar dish-engine systems for making electricity. In 2004, five new systems were installed at Sandia’s National Solar Thermal Test Facility, and linked to a prototype system already there to generate 150 kW of grid-ready electricity – enough to supply 40 homes.

A group headed by Belgium’s Solarmundo NV (Antwerp, Belgium; solarmundo.be) has developed technology that is said to reduce the capital cost of a solar-thermal energy plant by 30%, compared to conventional solar-thermal plants. Commercial solar power plants typically use an array of tough-shaped mirrors, each of which reflects solar radiation onto an absorber tube under vacuum. A heat transfer fluid in the tube is circulated to a heat exchanger to generate steam, which drives a turbine to produce electricity. Solarmundo’s cost-saving approach involves the use of flat mirrors instead of parabolic troughs, which are expensive to manufacture. Also, the absorber tube is made of coated steel that does not require vacuum insulation, according to its developers. The boiler water flows directly into the tube, thereby eliminating the need for a heat transfer fluid and heat exchanger.

Similarly, engineers at both Nanosolar (Palo Alto, Calif.; www.nanosolar.com) and Energy Innovations (Pasadena, Calif., www.energyinnovations.com) are working to develop motorized mirrors that can effectively track the movement of the sun, to maximize the efficiency of their solar cell’s energy-harvesting capabilities.

In terms of developing advanced solar (photovoltaic) cells, there are also a variety of competing efforts under way. For instance, as an alternative to traditional silicon-based solar cells, HelioVolt Corp. (Austin, Tex.; www.heliovolt.com) is working with the U.S. Dept. of Energy’s National Renewable Energy Laboratory (NREL; Golden, Colo.; www.nrel.gov) to speed the commercialization of advanced solar cells that are manufactured using NREL’s patented process for making thin films of copper indium gallium diselenide. Solar cells made from these materials can be embedded directly into various building materials, or bonded sheets of glass and other surfaces, according to the process developers.

Nanotechnology-related advances are also being pursued to improve solar-energy conversion systems. For instance, researchers at Berkeley Lab (Berkeley, Calif.; www.lbl.gov) are pursuing novel approaches that use nanostructured materials (such as metal oxide and metal sulfide semiconductors) to improve the absorption of light, and maximize the conversion of solar radiation into energy. Together with researchers at the University of California (Berkeley, Calif.; www.berkeley.edu), this group has developed what it claims to be the first ultrathin (abut 100 nm) solar cells made entirely of inorganic nanocrystals.

In this work, rod-shaped, nanometer-sized crystals of two semiconducting materials – cadmium selenide and cadmium telluride —are synthesized separately, then dissolved in solution and spin-cast onto a conductive glass substrate. By comparison, most commercial cells are based on silicon, and need to be fabricated under complex, controlled conditions, such as high vacuum and temperatures between 400-1,400 deg C.

Although solar cells made from hydrogenated amorphous silicon absorb light more efficiently than those made of crystalline silicon and are more cost-effective to make, the amorphous ones tend to suffer from efficiency losses over time. To improve the reliability and efficiency of solar cells, researchers at the U.S. Dept. of Energy’s Ames Laboratory (Ames, Iowa; www.ameslab.gov) and the Microelectronics Research Center at Iowa State University (Ames, Iowa; iastate.edu) are pursuing the development of solar cells made from clusters of nanocrystalline silicon embedded in an amorphous matrix.

Most commercial solar cells are based on crystalline silicon wafers or thin-film semiconductors, which have efficiencies of 15% (silicon) and 3-7% (thin-film). Promising prototypes of a new photovoltaic cell being developed by Konarka Technologies Inc. (Lowell, Mass.; www.konarka.com) have shown 7% efficiency (and a theoretical efficiency of 20-25%) for converting sunlight into electricity, according to the company. They are said to be lightweight, flexible and more versatile than existing products. The new cells feature a novel, dye-sensitized process that combines an organo-ruthenium dye complex and titanium dioxide nanoparticles to mimic photosynthesis.

In August 2006, BP Solar announced that it had developed a new silicon-growth process to produce its new Mono2 wafers, which have significantly increased energy-conversion efficiency — producing 5-8% more power — compared to traditional, multi-crystalline-based solar cells. The company is in the process of incorporating its new technique at its existing facility in Frederick, Md., and is aiming for commercial-scale production of the Mono2 wafers in 2007.

Meanwhile, BP Solar is also working with the Institute for Crystal Growth (Berlin, Germany; www.ikz-berlin.de) to develop a process to deposit silicon onto low-cost supports, such as glass, in an effort to reduce the total amount of silicon required to produce solar cells, and thereby reduce the costs associated with them. As recently reported in Chemical Engineering (October 2006, p. 16) the researchers have developed an advanced, two-step process to produce a continuous, multi-crystalline layer of SI with a thickness of less than 0.04 mm. First the glass substrate is nucleated with Si crystallites, at regularly distributed sites. These Si seeds are then enlarged by crystallization using a metallic solution.

In the fall of 2006, JFE Steel Corp. (Tokyo, Japan; WEB) started up a new production plant, which produces 100 m.t./yr of solar-grade silicon from crude Si metal, and is planning to build a similar facility to produce 500-1,000 m,t./yr of the materials in 2008. As reported in Chemical Engineering (October 2006, p. 13), JFE has developed a new Si-refining process, which involves melting crude Si (95 wt.%) under vacuum at 1,500C, and removing phosphorus impurities are vaporized by an electron beam. Direct-solidification (DS; a process to align the Si crystals in one orientation by controlled thermal flow) is then carried out to remove metal impurities, followed by a plasma melting and oxidation process to remove carbon and boron impurities. A second DS step is then used to remove trace metals, and the purified Si is then remelted and solidified into ingots by DS. The refined Si is said to exhibit commercially competitive light-to-energy conversion rates.

GE Energy (Atlanta, GA; www.gepower.com), through its Ecoimagination initiative, is also pursuing advanced photovoltaic cells and solar systems.

Researchers at the University of Toronto (Toronto, Ont.; www.utoronto.ca) have developed a new type of hybrid photovoltaic cell made from polymer with a thin layer of quantum dots. These nanometer-sized semiconductor crystals are made from lead sulfide, which can be “tuned” to absorb longer wavelengths of sunlight, thereby improving the amount of electric current generated by the solar cell. The group is claiming that the new cell can convert 30% of radiant energy into electricity (a marked improvement over the industry-average conversion efficiency of 6% that is typical of other polymer-based solar cells).

In a move that has both practical and symbolic implications, Google (Mountain View, Calif.; www.google.com) announced in October 2006 that it would build a large solar electricity system at hits headquarters, to provide about 30% of the electricity used each day at its 1-million-ft2 headquarters complex. Once online, the system, which will use 9,200 solar cells (enough to power 1,000 average California homes) and is being constructed by E1 Solutions, a division of Energy Innovations (Pasadena, Calif.; www.energyinnovations.com), is expected to become one of the largest corporate solar installations.

 

By: Suzanne Shelley, Guest Columnist and Freelance Writer

 

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