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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 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 societys 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). Todays
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
companys 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 Powers 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. (
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.
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
industrys 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 1,400°C, 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. 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 500°C 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 Japans 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 700°C. 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 225°C, 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 Japans 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 worlds first commercial facility to convert biomass into a
synthetic fuel called SunFuel, 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 Totals 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
its 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 According
to figures released in mid-2006 by the American
Wind Energy Assn. (AWEA;
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, todays
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, 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 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 Commissions 40,000-MW
target five years ahead of time, according to EWEA. 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
Energys 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 Energys 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. WTCs
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, WTCs wind turbines also produce
electricity for 30% less than todays most economic units, according to the company.
TWC is currently testing its prototype 750-kilowatt (kW) wind turbine at DOE/NRELs
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 Europes 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 GEs proven 1.5-MW unit
(which are typical sold in multiples to gain the needed generating capacity at a given
site), GEs 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 BPs expanding alternative-energy portfolio
(mentioned above), is a 9-MW wind farm currently being built at the companys 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 suns 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),
todays 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 suns 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, The
three basic mechanisms for concentrating the suns 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 its cloudy. Since
the 1980s, nine plants, totaling 354 MW, have operated in 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 suns 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 worlds 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 worlds 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
Californias Imperial Valley. Sandia National Laboratory (Albuquerque, N.M.;
www.sandia.gov) is also working with Sterling
Energy Systems to test Sterlings solar dish-engine systems for making electricity.
In 2004, five new systems were installed at Sandias 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 Belgiums 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.
Solarmundos 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 cells
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 Energys
National Renewable Energy Laboratory (NREL; Golden, Colo.; www.nrel.gov) to speed
the commercialization of advanced solar cells that are manufactured using NRELs
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 Energys 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 1,500°C, 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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