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3
Renewable Electricity Generation Technologies
A renewable electricity generation technology harnesses a naturally existing
energy flux, such as wind, sun, heat, or tides, and converts that flux to electricity. Natural
phenomena have varying time constants, cycles, and energy densities. To tap these
sources of energy, renewable electricity generation technologies must be located where
the natural energy flux occurs, unlike conventional fossil fuel and nuclear electricity
generating facilities, which can be located at some distance from their fuel sources.
Renewable technologies also follow a somewhat different paradigm than conventional
energy sources in that renewable energy can be thought of as manufactured, with the
largest proportion of costs, external energy, and material inputs occurring during the
manufacturing process. Although conventional sources such as nuclear- and coal-
powered electricity generation have a high proportion of capital-to-fuel costs, all
renewable technologies, except for biomass-generated electricity (biopower), have no
fuel costs. The trade-off is the ongoing and future cost of fossil fuel against the present
fixed capital costs of renewable energy technologies.
Scale economics likewise differs for renewables and conventional energy
production. Larger coal-fired and nuclear-powered generating facilities exhibit lower
average costs of generation than do smaller plants, realizing economies of scale based on
the size of the facility. Renewable electricity achieves economies of scale at the
equipment manufacturing stage rather than through construction of large facilities at the
generating site. Large hydroelectric generating units are an exception and have on-site
economies of scale, but not to the same extent as coal- and nuclear-powered electricity
plants.
With the exception of hydropower, renewable technologies are often disruptive
and do not bring incremental changes to long-established electricity industry sectors. As
described by Bowen and Christensen (1995), disruptive technologies present a package of
performance attributes that, at least at the outset, are not valued by a majority of existing
customers. Christensen (1997) observes:
Disruptive technologies can result in worse product performance, at least in the
near term. Disruptive technologies bring to market very different value
propositions than had been available previously. Generally, disruptive
technologies underperform established products in mainstream markets. But they
have other features that a few fringe customers value. Disruptive technologies
that may underperform today, relative to what users in the market demand, may
be fully performance–competitive in that same market tomorrow (Christensen,
1997).
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Traditional sources of electricity generation at least initially outperform
non-hydropower renewables. The environmental attributes of renewables are the
initial value proposition that have brought them into the electricity sector.
However, with improvements in renewables technologies and increasing costs of
generation from conventional sources (particularly as costs of greenhouse gas
production are incorporated), renewables may offer the potential to match the
performance of traditional generating sources.
This chapter examines several technologies for generation of renewable
electricity. It discusses the technology associated with each renewable resource, the state
of that technology, and research and development needs until 2020, between 2020 and
2035, and those beyond 2035.
WIND POWER
Wind power uses a wind turbine and related components to convert the kinetic
energy of moving air into electricity and other forms of energy. Wind power has been
harnessed for centuries—from the time of the ancient Greeks to the present. The modern
era of wind-driven electrical generation began with the oil shocks of the 1970s and
accelerated in the 1980s with the passage of the Public Utilities Regulatory Policies Act
(PURPA). Both the development of wind technology and the installation of wind power
plants have grown ever since.
Status of Technology
System Components
A typical wind turbine consists of a number of components: rotor, controls, drive
train (gearbox, generator, and power converter), tower, and balance of system.1 Each of
these components has undergone significant development in the last 10 years, with
improvements integrated into the latest turbine designs. In addition, improved
understanding and better modeling capabilities have contributed to the rapid introduction
of technical improvements. What were initially small clusters of 100 kW turbines in the
early 1980s have grown to clusters of hundreds of machines, including machines of 1.5
MW or more.
In general, wind speed increases with height, and the energy capture capability
depends on the rotor diameter. Figure 3-1 shows the change in rotor diameter and rated
capacity over time. In 2006 the most common installed machine had 275 ft (84 m) hub
1
In general, the balance of system (BOS) is the system between the technologies that convert the
renewable flux (wind, or solar) into electricity and the electricity grid (for power production) or load (for
direct use). The BOS might include the power-conditioning equipment that adjusts and converts the DC
electricity to the proper form and magnitude required by an alternating-current (AC) load. For solar PV,
the BOS consists of the structure for mounting the PV arrays and storage batteries. For wind turbines, it
typically includes all the related electronics required to provide the connection to the grid.
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heights and had a rotor diameter of 220 ft (67 m). Turbines as big as 5 MW have been
installed in offshore locations; these have 505 ft (154 m) hub height and 420 ft (128 m)
rotor diameter (IEEE, 2007a).2 As noted in Chapter 1, the U.S. wind energy industry
installed almost 14,000 MW of capacity during 2007 and 2008. The U.S. wind power
capacity is now more than 25 GW and spans 34 states; the world’s largest wind power
plant, Horse Hollow Wind Energy center with a capacity of 750 MW, recently
commissioned in Texas (SECO, 2008). U.S. wind farms will generate an estimated
52,000 GWh of electricity in 2008, about 1.2 percent of the U.S. electricity supply. As
discussed in Chapter 1, the installed wind power generating capacity worldwide at the
end of 2006 was 75,000 MW.
Electrical Output Controls
Besides the mechanical characteristics, the development of the turbine mechanical
to electrical conversion characteristics have evolved from machines based primarily on
fixed-speed induction generators (Type 1), to variable-speed machines with electronic
control (Type 2), and then machines incorporating vastly different outputs and controls
(Type 3). These Type 3 machines are able to control for low voltage ride-through
(LVRT),3 voltage,4 output5 and ramp rate,6 and volt-ampere-reactive (VAR) support.7
While wind generators have increased in height and rotor diameter, the major changes in
internal operating characteristics are not as apparent. Figure 3-2 depicts the evolution of
the internal operating characteristics. Many perceptions of wind technology’s negative
impact on the electrical system, such as the inability to remain connected to the electricity
grid during voltage disturbances and the draw on the grid’s reactive power resources,
stem from Type 1 machines.
The evolution of control technologies has made wind generators and their
electricity output easier to integrate into the utility system. With these new control
technologies, wind power plants are better at mimicking traditional generating plants.
This capability led to Federal Energy Regulatory Commission (FERC) Order 661-A,
issued December 2005, which deals with machine design and system integration. It calls
for wind facilities of 20 MW or larger to provide the ability to maintain operations,
including LVRT, during disturbances on the electric grid; provide reactive power; and
maintain continuous real-time communications and data exchange with the control area
operator. These power integration capabilities have been incorporated into Type 3
machines. However, wind power generation takes place where and when the wind blows,
and electricity must be used when it is generated. This intermittency has raised concerns
2
Background description and information on activities of the wind industry can be found on the
American Wind Energy Association website at http://awea.org.
3
Under FERC order 661A, low voltage ride-through is the capability to continue to operate down to 15
percent of rated line voltage for 0.626 s and continuously at 90 percent of rated line voltage. This capability
keeps the plant from shutting down as a result of short-term voltage fluctuation.
4
Voltage control ability provides control of wind turbine voltage output.
5
Power output ability allows the power produced to be reduced by feathering the blades.
6
Ramp rate management allows the power output to stay within the increase or decrease limits
required by the system.
7
VAR support provides reactive power compensation to aid in electricity grid stability.
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about integrating wind power into the existing power system and requires wind turbines
to provide LVRT, voltage control, output and ramp rate controls, and VAR support.
Integrating Type 3 machines into existing grids is not without its challenges.
Circumstances such as wind fluctuations and overall grid stability are unique to each
particular control area. Thus, even as technologies improve, it will be critical to carry out
site-specific analyses of each control area, which will better aid grid operators in
balancing the system within their control area.
Integration into Utility System Operation
A number of studies on the integration of wind power into a utility capacity and
dispatch structure indicate that wind can be integrated at up to approximately 20 percent
of the total electricity mix without requiring storage, although the exact level depends on
the power system (Parson et al., 2006; ETSO, 2007; DOE, 2008).8 The specifics of these
studies are discussed in this report in the chapters on economics (Chapter 4), deployment
(Chapter 6), and scenarios (Chapter 7). As the studies point out, achieving such levels of
renewables penetration will depend on upgrades to the grid (necessary regardless of the
energy mix) and new transmission lines for more remote sources.
Modern electricity grid systems are designed to handle loss of the largest power
plant without disruption; to have ramp up and ramp down capabilities: and to increase or
decrease generation as demand increases or decreases. However, each system has its own
generating capacity structure, transmission capabilities, and ability to purchase power
outside its own boundaries, making wind power integration somewhat unique for each
utility.
Small Wind Systems
The vast majority of wind power is generated by large wind turbines feeding into
the electricity grid, while small wind turbines generally provide electricity directly to
customers. The United States is the leading world producer of small wind turbines.
These residential turbines are erected and connected directly to the customer’s facility or
to the electricity distribution system at the customer’s site. The manufacture and
marketing of wind-powered electric systems sized for residential homes, farms, and small
businesses have experienced major growth in the past decade. These small wind turbines
(Figure 3-3), defined as 100 kW or less in capacity, have seen significant market growth,
and the industry has set ambitious growth targets: growth at 18 to 20 percent through
2010.
8
A number of studies can be found on the Utility Wind Integration Group (UWIG) website at
http://www.uwig.org.
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Key Technology Opportunities
Short Term: Present to 2020
The key technological issues for wind power focus on continuing to develop
better turbine components and to improve the integration of wind power into the
electricity system, including operations and maintenance, evaluation, and forecasting.
Goals appear relatively straightforward: taller towers; larger rotors; power electronics;
reducing the weight of equipment at the top and cables coming from top to bottom; and
ongoing progress through the design and manufacturing learning curve (Thresher et al.,
2007; DOE, 2008). Table 3-1 summarizes the incremental improvements under
consideration.
Although no big breakthroughs are anticipated, continuous improvement of
existing components is anticipated, and many are already being actively developed. For
example, there are advanced rotors that use new airfoil shapes specifically designed for
wind turbines, instead of those based on the design of helicopter blades. These rotors are
thicker at points of highest stress and reduce loads during turbulent winds by flying the
blades using turbine control systems. Other improvements include the use of composite
materials and advanced drive trains. In particular, gear boxes are a major area of concern
for reliability. Approaches for improving of this component include direct drive
generators; greater use of rare-earth permanent magnets in generator design; possibility
of single-stage drives using low-speed generators; and distributed drive trains using the
rotor to drive several parallel generators. Advanced towers are a major focus for
innovation, given the current need for large cranes and transport of large tower and blade
sections. Concepts under investigation include self-erecting towers, blade manufacturing
on site, vibration damping, and tower drive train interactions.
There is certain to be some development of offshore wind in the United States in
the near term, but it is not expected that this will have a significant impact before 2020.
Nonetheless, there is a near-term opportunity to learn from offshore projects in Europe
and the United States, if offshore wind is going to have an impact in the medium term.
Other near-term opportunities will lie in improving the integration of existing
wind power plants into the transmission and distribution system, which includes using
improved computational models for simulating and optimizing system integration (Ernst
et al., 2007). Chapters 6 and 7 discuss the deployment and integration of wind-generated
electricity.
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TABLE 3-1 Areas of Potential Wind Power Technology Improvements
Performance and Cost Increments
(Best/Expected/Least Percentages)
Annual Energy Turbine Capital
Production Cost
Technical Area Potential Advances
•
Advanced Tower +11/+11/+11 +8/+12/+20
Taller towers in difficult locations
Concepts • New materials and/or processes
• Advanced structures/foundations
• Self-erecting, initial, or for service
•
Advanced (Enlarged) +35/+25/+10 −6/−3/+3
Advanced materials
Rotors • Improved structural-aero design
• Active Controls
• Passive controls
• Higher tip speed/lower acoustics
•
Reduced Energy Losses +7/+5/0 0/0/0
Reduced blade soiling losses
and Improved • Damage-tolerate sensors
Availability • Robust control systems
• Prognostic maintenance
•
Drivetrains (Gearboxes +8/+4/0 −11/−6/+1
Fewer gear stages or direct-drive
and Generators ad • Medium/low speed generators
Power Electronics) • Distributed gearbox topologies
• Permanent-magnet generators
• Medium-voltage equipment
• Advanced gear tooth profiles
• New circuit topologies
• New semiconductor devices
• New materials (gallium arsende
[GaAs], SiC)
• Sustained, incremental design and 0/0/0 −27/−13/−3
Manufacturing and
Learning Curvea process improvements
• Large-scale manufacturing
• Reduced design loads
Totals +61/+45/+21 −36/−10/+21
a
The learning curve results from the NREL report (Cohen and Schweizer et al., 2008) are adjusted from 3.0
doubling in the reference to the 4.6 doubling in the 20% Wind Scenario
SOURCE: DOE (2008).
Medium Term: 2020 to 2035
Mid-term wind technology development will have two thrusts: the movement
toward offshore, and its implications for turbine design; and the development of efficient
low-wind speed turbines. Development of offshore wind power plants has already begun
in Europe (approximately 1200 MW of installed capacity), but progress has been slower
in the United States. Nine projects are in various stages of development in state and
federal waters. In addition to technical risks and higher costs, these projects have been
slowed by social and regulatory challenges (DOE, 2008).
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In the mid-term, offshore turbines will have a larger size and generating capacity
than onshore turbines, but, due primarily to technical and cost concerns, development
will likely lag behind onshore machines. Transmission siting issues with offshore wind
power plants will be simplified because of fewer siting impediments. However,
underwater cables must be carefully constructed, and there will likely be a move to
develop microgrids with HVDC to integrate the offshore resources. Offshore wind
technologies face several transition problems as they move from near-shore, land-based
sites to offshore sites of various depths, and finally floating designs. Assessment tools
for sensitive marine areas, wind loads, and system design are not now ready for offshore
development. Offshore projects must be built to handle both wind and wave loads, and
components must be able to endure marine moisture and extreme weather. Offshore
wind projects have a higher balance of station cost (approximately 2/3 of total costs) than
onshore projects, and thus will rely on cost reductions across the system in order to
become more competitive. All of these developments pose both technological and
organizational problems and will require continuous research and development in order
to be feasible. It should be noted that challenges posed by the greater technical
difficulties of offshore wind power development are being addressed by other countries.
However, political, organizational, social, and economic obstacles may continue to
inhibit investment in offshore wind power development, given the higher risk compared
to onshore wind energy development (Williams and Zhang, 2008).
In terms of onshore development, as the higher wind speed sites are used, wind
power development will move to lower wind speed sites, which will require turbines that
are relatively efficient at lower wind speeds, necessitating larger rotors with lighter,
stronger materials, and increased tower height.
Long Term: After 2035
At present, no revolutionary technology to extract energy from wind has been
proposed, but several designs, e.g., vertical wind turbines or eggbeaters, are again under
consideration. There have been conceptual proposals to access high-altitude winds using
balloons or kites. Component improvements will continue, with additional emphasis on
offshore turbine installation. Floating offshore platforms may gain interest, but first must
come experience from anchored offshore wind facilities.
Summary of Wind Power Potential
Wind-power technologies are actively deployed today, and there are no
technological barriers to continued deployment. Cost reductions will be possible as a
result of wider deployment and incremental improvements in component. No other
enhancing technologies are required for wind power to meet 20 percent and higher of
U.S. electricity demand.
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SOLAR PHOTOVOLTAIC POWER
Solar power involves the conversion of the radiant energy from the sun into
electricity by using photovoltaics (PV) or concentrating devices. When sunlight strikes
the surface of the PV cell, some of the photons are absorbed and release electrons from
the solar cell that are used to produce an electric current flow, i.e., electricity. A solar
cell consists of two layers of materials, one that absorbs the light and the other that
controls the direction of current flow through an external circuit (Figure 3-4). The
absorbing materials can be silicon (Si), which is also used in integrated circuits and
computer hardware; thin films of light-absorbing inorganic materials, such as CdTe or
GaAs, that have absorption properties well matched to capture the solar spectrum; or a
variety of organic (plastic) materials, nanostructures, or combinations.
Status of Technology
The PV industry has grown at a rate greater than 40 percent per year from 2000
through 2008. Much of this growth is the result of national and local programs targeted
toward growing the PV industry and improving PV’s competitiveness in the marketplace.
In 2007, PV modules supplying 3.4 GW were produced worldwide, and approximately
220 MW were installed in the United States.9 Table 3-2 provides a breakdown of PV
module shipments by technology type.
Over the past 30 years, the efficiency of PV technologies has steadily improved.
Figure 3-5 presents the historical progress of the best reported solar cell efficiencies
through 2008 (NREL, 2009). Commercial (or even the best prototype) modules achieve,
on average only about 10 to 15 percent efficiency, which is 50 to 60 percent of the
efficiency of the best research cells. Figure 3-5 includes several PV technologies: single-
crystalline Si, thin films, multiple-junction cells, and emerging technologies, such as dye-
sensitized nanocrystalline TiO2 cells, cells based on organic compounds, and plastic solar
cells.
Flat-Plate PV Technologies
Photovoltaic technologies can be divided into two main types: flat plates and
concentrators. Flat-plate technologies include crystalline silicon (from both ingot and
ribbon- or sheet-growth techniques) and thin films of various semiconductor materials,
usually deposited on a low-cost substrate, such as glass, plastic, or stainless steel, using
some type of vapor deposition, or wet chemical process. Thin film cells typically are 1
to 20 μm in thickness and require one-tenth to one-hundredth of the expensive
semiconductor material required by crystalline silicon (DOE, 2007f). Additionally, thin
film deposition technology allows production of large-area solar cells, and though they
exhibit lower efficiencies (upward of 10 percent) than crystalline silicon PV panels, their
lower production costs can make them an attractive alternative. Even thinner layers are
involved in some of the future generation technologies, such as organic polymers and
nanomaterials (DOE, 2007j).
9
See http://www.solarbuzz.com/Marketbuzz2008-intro.htm.
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TABLE 3-2 PV Cell and Module Shipments by Type, 2005-2007
Shipments (Peak Kilowatts) Percent of Total
Type 2005 2006 2007 2005 2006 2007
Crystalline Silicon
Single-Crystal 71,901 85,627 128,542 32 25 25
Cast and Ribbon 101,065 147,892 181,788 45 44 35
Subtotal 172,965 233,518 310,330 76 69 60
Thin-Film 53,826 101,766 202,519 24 30 39
a
1 1
Concentrator 125 1,984 4,835
b
⎯ ⎯ ⎯ ⎯ ⎯ ⎯
Other
U.S. Total 226,916 337,268 517,684 100 100 100
a
Less than 0.5 percent.
b
Other includes categories not identified by reporting companies.
NOTE: Totals may not equal sum of components due to independent rounding. ⎯, no data reported.
SOURCE: EIA (2008), Table 3.5.
Of the PV modules produced today, nearly 88 percent are based on crystalline
silicon wafer technologies. Of this total, about 30 percent are based on conventional,
single-crystal silicon grown by the Czochralski ingot process,10 60 percent are based on
polycrystalline (also referred to as multicrystalline) ingots cast in a crucible, and
3 percent are from silicon ribbons/sheet produced by various processes. The typical
efficiency of these crystalline PV cells is 12 to 18 percent, and further development is
required to increase the efficiency and to lower the production cost (DOE, 2007e).
Concentrator PV Technology
The key elements of a concentrator PV system are low-cost concentrating
(reflective or refractive) optics, low-cost mounting and tracking systems (to track the
movement of the sun), and high-efficiency III-V11 or silicon solar cells (DOE, 2007g).
The large-scale manufacturing capability for all components has already been
demonstrated, including 27 percent efficient silicon cells and 28 percent efficient GaAs
cells (DOE, 2007g; Surek, 2001). Concentrator systems using point-focus Fresnel lenses
have been routinely fabricated. Module efficiencies of up to 20 percent have been
demonstrated by commercially made 25 percent efficient silicon solar cells (DOE, 2005).
Progress in multi-junction, III-V based solar cells for space applications has led to
evaluating their terrestrial potential in concentrating applications (Bett et al., 1999; DOE,
2007g). An efficiency of 37.3 percent (at up to 600 times the sun’s normal intensity) has
been achieved for a GaInP2/GaInAs/Ge triple-junction structure (King et al., 2004), and
10
A method of crystal growth commonly used to obtain single crystals of semiconductors.
11
III-V compounds (the III and V indicate the column location on the Periodic Table) are the basic
materials for modern optoelectronic devices typically used in high-speed transistors (Bett et al., 1999).
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NREL has recently announced an efficiency of almost 41 percent (at 380 suns intensity)
(NREL Press Release, August 13, 2008).
Concentrated photovoltaic (CPV) plants are composed of many aggregated
photovoltaic modules, as are non-concentrator plants, but the required cell area is reduced
by the concentration factor (DOE, 2005). The concentration ratio12 of one-axis CPV
systems is commonly 10-50. High-concentration PV (HCPV) systems use two-axis
trackers with concentration ratios of 200-500. Concentration makes the use of the most
efficient and expensive PV cells more practical. Mature HCPV systems are projected to
cost about 40 to 60 percent of standard PV systems and to provide 10 to 20 percent more
energy with the same power rating. Projections put the installed costs of CPV with multi-
junction PV cells now under development at about $2/W (DOE, 2007g). The present cost
of single-junction systems from Amonix and Solar Systems Pty Ltd. is, for example,
about $4/W.
Potential Technology Development
Future directions for thin film technologies include multi-junction thin films
aimed at significantly higher conversion efficiencies, better transparent conducting oxide
electrodes, thin polycrystalline silicon films, and organic inks.
Concentrator systems use only direct, rather than diffuse or global, solar radiation;
therefore, their areas of best application (e.g., in the southwestern United States) are more
limited than those for flat plates. There is also ongoing research to improve the long-term
reliability of concentrator systems and to develop standard tests for concentrator cells and
systems. Thus, most of today’s remote and distributed markets for PV systems are not
suitable for concentrator systems.
By far the fastest-growing segment of the PV industry is that based on casting
large, multicrystalline ingots in some crucible that is usually consumed in the process.
Manufacturers routinely fabricate large multicrystalline silicon solar cells with
efficiencies in the 13 to 15 percent range; small-area research cells are 20 percent
efficient. Silicon ribbon or sheet technologies avoid the costs and material losses
associated with slicing ingots. The present commercial approaches in the field are the
edge-defined, film-fed growth of silicon ribbons and the string ribbon process. Full-scale
production of silicon modules based on micron-sized silicon spheres was recently
announced. In this process, submillimeter-size silicon spheres are bonded between two
thin aluminum sheets, processed into solar cells, and packaged into flexible, lightweight
modules. Another approach uses a micromachining technique to form deep narrow
grooves perpendicular to the surface of a 1- to 2-mm thick single-crystal silicon wafer.
This technique results in large numbers of thin (50 μm), long (100 mm), and narrow
(nearly the original wafer thickness) silicon strips that are processed into solar cells just
prior to separation from the wafer. In another technique, a carbon foil is pulled through a
silicon melt, resulting in the growth of two thin silicon layers on either side of the foil.
After the edges are scribed and the sheet is cut into wafers, the carbon foil is burned off,
resulting in two silicon wafers (150 μm thick) for processing into solar cells.
12
Defined as the average solar flux through the receiver aperture divided by the ambient direct normal
solar insolation.
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Thin-film technologies have the potential for substantial cost advantages over
wafer-based crystalline silicon, because of factors such as lesser material use due to direct
band gaps, fewer processing steps, and simpler manufacturing technology for large-area
modules. Thin-film technologies commonly require less or no high-cost crystalline Si.
Many of the processes are high throughput and continuous (e.g., roll-to-roll); they usually
do not involve high temperatures and, in some cases, do not require high-vacuum
deposition equipment. Module fabrication, involving the interconnection of individual
solar cells, is usually carried out as part of the film-deposition processes. The major
systems are amorphous silicon, cadmium telluride,13 and copper indium diselenide14
(CIS) and related alloys (DOE, 2007h). Future directions include multijunction thin films
aimed at significantly higher conversion efficiencies, better transparent conducting oxide
electrodes, and thin polycrystalline silicon films.
Dye-sensitized Solar Cells
The dye-sensitized solar cell (O’Regan and Grätzel, 1991) has its foundation in
photochemistry rather than in solid-state physics. In this device, also called the “Grätzel
cell” after its Swiss inventor, organic dye molecules are adsorbed on a nanocrystalline
titanium dioxide (TiO2) film, and the nanopores of the film are filled with a redox
electrolyte. The dyes absorb solar photons to create an excited molecular state that can
inject electrons into the TiO2. The electrons percolate through the nanoporous TiO2 film
and are collected at a transparent electrode. The oxidized dye is reduced back to its
initial state by accepting electrons from the redox relay via ionic transport from a metal
counter-electrode; this completes the circuit and electrical power is delivered in the
external circuit. Dye-sensitized solar cells are very attractive, because of the very low
cost of the constituent materials (TiO2 is a common material used in paints and
toothpaste) and the potential simplicity of their manufacturing process. Additionally,
sensitized solar cells are tolerant to impurities, which allow ease in scaling up the
production. Laboratory-scale devices of 11 percent efficiency have been demonstrated,
but larger modules are typically less than half that efficient. Stability of the devices (e.g.,
dye materials and electrolyte) while maintaining high efficiency is an ongoing research
issue (DOE, 2007k).
Organic and Nanotechnology Solar Cells
Organic semiconductors hold promise as building blocks for organic electronics,
displays, and very low-cost solar cells. In an organic solar cell, light creates a bound
electron-hole pair, called an exciton, which separates into an electron on one side and a
hole on the other side of a material interface within the device. Polymers, dendrimers,
small molecules and dyes, and inorganic nanostructures are materials that can be used
13
CdTe PVs require a small amount of semiconductor, and the production can be automated, which
can increase its yield.
14
CIS has higher efficiency and has the capability to be made on a flexible substrate, but large-scale
production might be limited to the availability of indium.
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For energy storage, the energy density stored in gasoline is much greater than that
storable in existing technologies for lithium-ion or flow batteries.
Chemical Energy Storage
Chemical energy storage refers to synthetic routes to producing fuels from energy
resources. Depending on its nature, a fuel can subsequently be used for electricity
production via fuel cells or used in conventional combustion systems. By far the simplest
fuel to consider in this scenario is hydrogen, created according to the reaction:
H2O + (renewable) energy → H2 + ½ O2.
Regardless of how hydrogen is produced, the fuel must be stored, which is a daunting
challenge. For example, compressing hydrogen to a pressure of 800 bar incurs an energy
penalty of ~13 percent. At any pressure, the volumetric energy density of methane, a fuel
more familiar to the electricity industry, is more than three times greater than that of
hydrogen stored at an equal pressure (Bossel and Eliasson, 2003). Furthermore, after
storage, hydrogen would be used either in a combustion process or in a fuel cell to
provide electricity, both incurring additional efficiency penalties (~60 percent loss for
combustion and ~30 percent loss for the fuel cell), resulting in a maximum “round-trip”
efficiency of ~60 percent, assuming a 70 percent efficient fuel cell and 87 percent
efficient compression, excluding energy penalties for the hydrogen production itself.
With these caveats, it is nevertheless useful to consider methods of renewable hydrogen
generation.
If the energy input for splitting water is electricity, the reaction occurs simply by
electrolysis. In the context of renewable electricity, generation is from solar, wind, or
other renewable resources, and the electricity is then directed to a separate electrolysis
cell. Small-scale electrolyzers are commercially available for the production of hydrogen
for technical purposes. However, these systems’ overall efficiency, 65 to 70 percent,
renders them unattractive for large-scale energy storage (Bossel and Eliasson, 2003).
These systems require the use of platinum (Pt) at a quantity that can be estimated from
the platinum used in state-of-the-art polymer electrolyte membrane fuel cells, which
essentially operate in reverse relative to electrolyzers. A DOE target for platinum use is
1 g/kW. Storage for 46 GW average capacity (amounting to 10 percent of the U.S.
average) would require 46 × 103 kg of platinum, which is a relatively small amount
compared to both the known platinum reserves, ~ 7 × 106 kg, and the present rate of
platinum consumption, ~ 250 × 103 kg per year (Wilburn and Bleiwas, 2004). Because
of the inverse relationship between electrolyzers and fuel cells, there has been some
research on electrochemical cells that could operate in either mode, particularly in the
case of high-temperature ceramic electrolyte systems. These dual attributes would be
attractive, because costs would be reduced as a result of the multi-functionality of the
electrochemical cell, and the high-temperature operation would obviate the need for
precious metal catalysts.
In the case of solar energy, direct photo-electrochemical production of hydrogen
is an attractive alternative to the two-step process (renewable energy → electricity →
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fuel). In direct photo-electrochemical production, a semiconductor material, immersed in
water, absorbs light, exciting electron-hole pairs across the band gap of the
semiconductor. These electronic species are then available to perform reduction and
oxidation reactions at the electrodes of the cell. As with the ambient-temperature
electrolysis cell, developing robust and efficient, non-precious-metal catalysts remains a
daunting challenge for this approach. However, the recognition that biological systems
carry out such reactions (i.e., photosynthesis) using base-metal compounds as catalysts
suggests that success could ultimately be achieved. The DOE is attempting to increase
investment in this area, reflecting the potential offered by recent advances in this
approach (e.g. , Muckerman et al., 2008).
Yet another alternative for hydrogen production is the thermochemical cycle. In
this approach, thermal energy, ideally solar-thermal energy, is the renewable input
applied to a material that occurs in oxidized form at low temperatures and undergoes
dissociation/reduction at high temperatures. The process of cycling between these two
states under appropriate gaseous atmospheres releases the desired reduced chemical fuel.
For example, if one considers the FeO/Fe3O4 system, the hydrogen production cycle can
be described as
Fe3O4 → 3FeO + ½ O2 (g) high temperature and
3FeO + H2O → Fe3O4 + H2 (g) low temperature.
The success of the thermochemical approach relies fundamentally on the chemical
thermodynamics of oxide stability. Rapid reaction kinetics and strong coupling of the
solar radiation to the material for effective heating are also essential. There are no
commercial activities in thermochemical fuel production, but there are ongoing large-
scale demonstration plants at Sandia National Laboratories and at ETH Zurich.
Alternatives to hydrogen fuel production are under consideration, because
converting renewable energy to hydrogen fuel merely transfers the energy storage
problem to a different part of the energy delivery infrastructure,. Alternatives typically
employ biological processes to produce alcohols, alkanes, or other carbon-containing
fuels, and can be considered advanced biomass approaches, such as production of
biodiesel from algae. The few synthetic chemistry approaches investigated center largely
on electrochemical reduction of CO2 to CO, whereby the combined carbon monoxide and
hydrogen, or syngas, becomes the input in known industrial processes for the creation of
a more suitable fuel. These approaches, still in the laboratory research stage, focus on
chemical reaction pathways rather than potential scale-up to provide an energy solution.
As components produced electrochemically, it is theoretically possible to react CO and
H2 further to generate methane, a fuel familiar to the electricity industry and thus likely to
have more immediate impact than penetration of renewable electricity. Because natural-
gas peaking plants are often co-sited with solar and wind farms, direct production of
methane using the output of the combustion power plant could provide a closed-loop
system in which methane would not have to be transported.
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Summary of Storage Potential
Analysis of the future for the various storage technologies is beyond the scope of
this panel, but some summary statements are in order. In the near term, diabatic CAES
and various battery technologies, especially sodium sulfur batteries, have found initial
applications in the electricity sector. In the longer term, when penetrations of renewables
in the electricity sector might reach levels requiring energy storage, there may be a
variety of approaches, including adiabatic CAES or the use of renewable energy in the
production of chemical fuels. Advances in ultracapacitors and other short-term storage
solutions may provide additional mechanisms to effectively integrate and stabilize
intermittent resources.
Energy storage is a system resource that should be operated for the overall benefit
of the system. The greatest value of energy storage is realized when it is operated for the
benefit of the entire system, and not dedicated to balancing any particular resource on the
system. Storage tied to smart transmission and distribution grids would become a
valuable component of any power system, and could provide numerous benefits to the
system. Storage benefits the system without renewables, and renewables benefit the
system without storage. The task is to manage variability with flexibility.
Improved Grid Intelligence⎯the Smart Grid
The architecture needed to improve integration of renewables into the electricity
grid would incorporate a variety of technologies, such as advanced sensors; smart meters
(net metering, turn-on/turn-off capability, and the capability to enable time-of-day
pricing); power converters, conditioners, and other power-quality technologies; source
and load controls; improved software, including forecasting and operations models; and
storage technologies (Kroposki, 2007). Most of these technologies are part of the broad
initiative to improve the intelligence of the modern grid.33 The objectives to meet in
modernizing the electricity grid go beyond increasing intermittent renewables, and
include improving security and power quality and creating a more efficient, adaptive
electricity system. Demonstrations are under way in several U.S. cities (e.g., Boulder,
Colorado), but widespread deployment is expected to take decades.34 More details on the
objectives and technologies involved in creating a future electricity grid with increased
capacity and intelligence are presented in the upcoming report of the Committee on
America’s Energy Future (NAS-NAE-NRC, 2009b).
33
The term “Smart Grid” has often been used to describe this initiative. The Smart Grid may be
described as the overlaying of a unified electronic control systems and two-way communication over the
entire power delivery infrastructure. Smart Grid capabilities optimize power supply and delivery, minimize
loss, and enable maximum use of electricity generation resources, energy efficiency, and demand
responses. However, this term suffers from overuse and multiple interpretations. The panel instead uses
the improved term “grid intelligence” to refer to the collection of technologies needed to improve the
integration of renewables into.
34
EISA 2007 authorized the Smart Grid Advisory Committee and Task Force through 2020. An earlier
(2003) DOE plan was called Grid2030; the intention was to have 100 percent of electricity running through
a smart grid by 2030.
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A truly intelligent modern grid would anticipate the fluctuations in the power
output from intermittent renewable energy sources and maintain absolute supply/demand
equivalency on a given transmission or distribution circuit, while requiring less
compensating backup power and storage capacity. Instantaneous electronic control of the
grid would allow each transmission line to operate at a higher load factor without risking
thermal overload than is now feasible on the electromechanically controlled transmission
system. This level of coordinated control would require improved communications and
seamless connectivity, or interoperability,35 which would make the grid a dynamic,
interactive infrastructure for the real-time exchange of power and information. Open
connectivity architecture would create a plug-and-play environment that would securely
network grid components and operators. The current lack of uniform interconnection and
operations codes and standards as well as the acceptance of standardized open
communications architecture is restricting the timely implementation of the modern grid.
A system-wide integrated cyber security capability is also an important dimension of this
communications architecture.
The Smart Grid’s emphasis today is primarily on creating interstate high-voltage
transmission capabilities to facilitate bulk wind power access. While important,
transmission is only one element of the nationwide grid modernization effort needed to
realize the potential benefits of renewable energy. The electronic modernization of the
local electricity distribution network is equally essential to incorporating distributed
renewable energy technologies such as photovoltaics and wind power. One critical
objective of smart distribution grids is to enable the seamless, uninterruptible balancing
of electricity supply and demand, which could allow distributed renewable power
generation to be broadly dispatchable. Dispatchability would improve intermittent
renewables’ compatibility with the reliability and operational requirements of the bulk
power system. The result could help transform buildings into power plants and provide a
more reliable, efficient and clean electricity supply system.
Advanced Metering
Advanced metering—the use of electricity meters that provide detailed
consumption profiles⎯is one technology for improving the intelligence of the grid that
would be particularly important to increasing the use of distributed renewables. Unlike
conventional metering, advanced metering would couple the cost of electricity generation
with the price to the consumer. In the context of renewables integration, the ability to do
time-of-day pricing and net metering would better enable the deployment of renewables,
especially solar PV. Such meters also could communicate real-time information to the
consumer for billing and pricing purposes. Because solar PV generation peaks close to
the late-afternoon price peak, meters allowing time-of-day pricing could improve the
cost-competitiveness of solar PV at the consumer end. Advanced metering also helps to
create incentives to use energy at off-peak times when possible, thereby reducing
demands on the transmission and distribution systems. Chapter 4 discusses the use of
real-time pricing to encourage the development of renewables.
35
Seamless, end-to-end connectivity of the hardware and software throughout the transmission and
distribution system to the electrical energy source.
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Furthermore, advanced metering technologies would enable net metering for
those with on-site renewable generation. Net metering improves the integration into the
grid of distributed renewable resources such as solar PV installed at residential and
commercial facilities. It measures both the consumption of electricity and the excess
energy produced on-site, and at least partly credits the consumer for excess generation
produced by consumer-owned solar PV or other renewable electricity technologies.
Software/Modeling Support
New grid operating tools are also needed to incorporate renewable energy
resources, including operating models and system impact algorithms that address the
transient behavior of renewable energy; improved operators’ visualization techniques and
new training methodologies; and advanced simulation tools that can provide an accurate
understanding of grid behavior. These grid operating tools would also assist system
planners in designing reliable power systems for this new environment. Better
forecasting algorithms would allow better use of temporally varying resources such as
wind energy. The objective of this work is to improve the forecasting of wind and its use
in electricity markets (Ahlstrom et al., 2005; Hawlins and Rothleder, 2006; Smith, 2007).
Reactive Dynamic Power
The demand that some renewables place on ancillary services, such as reactive
power and dynamic voltage control, also must be considered.. Reactive power is the
portion of electricity that establishes and maintains the electric and magnetic fields of
alternating current (AC) equipment. Because wind and solar power produce direct
current (DC), reactive power must be provided in the DC-to-AC conversion process, a
requirement that is complicated by the variable/intermittent nature of these renewable
energy sources: the reactive power must be equally dynamic to keep pace. Many early
wind machines were induction generator wind turbines with a constant frequency and so
required reactive power to be supplied from the grid. While newer machines have solved
this problem, voltage stability remains an issue. The European Transmission System
Operators (ETSO) recently completed a study on the ancillary services required by wind
power as the amount of installed wind capacity in Europe increased from 41 GW in 2005
to an expected 67 GW in 2008 (ETSO, 2007). In particular, the ETSO study looked at
the effects of variable power output on the electricity grid and the ability of various wind
turbine types to provide system service needed for the stable operation of an electricity
grid. Another study describes technologies used to provide reactive power for a large
wind farm and the interactions of the wind farm, reactive power compensation, and the
power system network (Muljadi et al., 2004).
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FINDINGS
The most critical elements of the panel’s findings on renewable electricity
generation technologies are highlighted below.
Over the first time frame through 2020, wind, solar photovoltaics and
concentrating solar power, conventional geothermal, and biopower technologies are
technically ready for accelerated deployment. During this period, these technologies
could potentially contribute a much greater share (up to ~10 percent of electricity
generation) of the U.S. electricity supply than they do today. Other technologies,
including enhanced geothermal systems that mine the heat stored in deep low-
permeability rock and hydrokinetic technologies that tap ocean tidal currents and wave
energy, require further development before they can be considered viable entrants into
the marketplace. The costs of already-developed renewable electricity technologies will
likely be driven down through incremental improvements in technology, “learning curve”
technology maturation, and manufacturing economies of scale. Despite short-term
increases in cost over the past couple of years, in particular for wind turbines and solar
photovoltaics, there have been substantial long-term decreases in the costs of these
technologies, and recent cost increases due to manufacturing and materials shortages will
be reduced if sustained growth in renewable sources spurs increased investment in them.
In addition, support for basic and applied research is needed to drive continued
technological advances and cost reductions for all renewable electricity technologies.
In contrast to fossil-based or nuclear energy, renewable energy resources are more
widely distributed, and the technologies that convert these resources to useful energy
must be located at the source of the energy. Further, extensive use of intermittent
renewable resources such as wind and solar power to generate electricity must
accommodate temporal variation in the availability of these resources. This variability
requires special attention to system integration and transmission issues as the use of
renewable electricity expands. Such considerations will become especially important at
greater penetrations of renewable electricity in the domestic electricity generation mix. A
contemporaneous, unified intelligent electronic control and communications system
overlaid on the entire electricity delivery infrastructure would enhance the viability
and continued expansion of renewable electricity in the period from 2020 to 2035.
Such improvements in the intelligence of the transmission and distribution grid could
enhance the whole electricity system’s reliability and help facilitate integration of
renewable electricity into that system, while reducing the need for backup power to
support the enhanced utilization of renewable electricity.
In the third time period, 2035 and beyond, further expansion of renewable
electricity is possible as advanced technologies are developed, and as existing
technologies achieve lower costs and higher performance with the maturing of the
technology and an increasing scale of deployment. Achieving a predominant (i.e., >50
percent) penetration of intermittent renewable resources such as wind and solar into the
electricity marketplace, however, will require technologies that are largely unavailable or
not yet developed today, such as large-scale and distributed cost-effective energy storage
and new methods for cost-effective, long-distance electricity transmission. Finally, there
might be further consideration of an integrated hydrogen and electricity transmission
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system such as the “SuperGrid” first championed by Chauncey Starr (Starr 2002), though
this concept is still considered high-risk.
REFERENCES
Ahlstrom, M., L. Jones, R. Zavadil, and W. Grant. 2005. The future of wind forecasting
and utility operations. Power and Energy Magazine, IEEE 3(6):57-64.
Bain, R.L. 1993. Electricity from biomass in the United States: Status and future
direction. Bioresource Technology 46:86-93.
Bett, A.W., F. Dimroth, G. Stollwerck, and O.V. Sulima. 1999. III-V compounds for
solar cell applications. Applied Physics. 69:119-129.
Beaudry-Losique, J. 2007. Biomass R&D Program & Biomass-To-Electricity.
Presentation at the First Meeting of the Panel on Electricity from Renewables,
September, 18, 2008. Washington, D.C.
Bossel, U., and B. Eliasson. 2003. Energy and the Hydrogen Economy. Methanol
Institute, Arlington, Va. Available at
http://www.methanol.org/pdf/HydrogenEconomyReport2003.pdf.
Bowen, J.L., and C.M. Christensen. 1995. Disruptive technologies, Catching the wave.
Harvard Buisness Review 73(1):43-53.
Bridgewater, A.V. 1995. The technical and economic feasibility of biomass gasification
for power generation. Fuel 74:631-653.
Bullough, C., C. Gatzen, C. Jakiel, M. Koller, A. Nowi, and S. Zunft. 2004. Advanced
adiabatic compressed air energy storage for the integration of wind energy.
Proceedings of the European Wind Energy Conference, EWEC 2004, November
22-25, 2004, London, U.K. European Wind Energy Association, Brussels,
Belgium. Available at http://www.2004ewec.info.
Christensen, C.M. 1997. The Innovators Dilemma: When New Technologies Cause Great
Companies to Fail. HBS Press, Cambridge, Mass.
DOE (U.S. Department of Energy). 2005. Basic Research Needs for Solar Energy
Utilization: Report on the Basic Energy Sciences Workshop on Solar Energy
Utilization. Washington, D.C.
DOE. 2007a. Workshop for Enhanced Geothermal Systems Technology Evaluation.
Summary report, Workshop for Enhanced Geothermal Systems, Technology
Evaluation., June 7-8, 2007. Washington, D.C.
DOE. 2007b. Enhanced Geothermal Systems Reservoir Creation Workshop. Summary
Report, Enhanced Geothermal Systems Reservoir Creation Workshop. Houston,
TX.
DOE. 2007c. Enhanced Geothermal Systems Reservoir Management and Operations
Workshop. Summary Report, Enhanced Geothermal Systems Reservoir
Management and Operations Workshop San Francisco, CA.
DOE. 2007d. Enhanced Geothermal Systems Wellfield Construction Workshop.
Summary Report, Enhanced Geothermal Systems Wellfield Construction
Workshop. San Francisco, CA.
DOE. 2007e. National Solar Technology Roadmap: Wafer-Silicon PV. Energy Efficiency
and Renewable Energy. Washington, D.C.
88
OCR for page 89
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
DOE. 2007f. National Solar Technology Roadmap: Film-Silicon PV. Energy Efficiency
and Renewable Energy. Washington, D.C.
DOE. 2007g. National Solar Technology Roadmap: Concentrator PV. Energy Efficiency
and Renewable Energy. Washington, D.C.
DOE. 2007h. National Solar Technology Roadmap: CdTe PV. Energy Efficiency and
Renewable Energy. Washington, D.C.
DOE. 2007i. National Solar Technology Roadmap: CIGS PV Energy Efficiency and
Renewable Energy. Washington, D.C.
DOE. 2007j. National Solar Technology Roadmap: Organic PV. Energy Efficiency and
Renewable Energy. Washington, D.C.
DOE. 2007k. National Solar Technology Roadmap: Sensitized Solar Cells. Energy
Efficiency and Renewable Energy. Washington, D.C.
DOE. 2007m. National Solar Technology Roadmap: Nano-Architecture PV. Energy
Efficiency and Renewable Energy. Washington, D.C.
DOE. 2008. 20 percent Wind Energy by 2030⎯Increasing Wind Energy’s Contribution
to U.S. Electricity Supply. Energy Efficiency and Renewable Energy.
Washington, D.C.
EIA (Energy Information Agency). 2001. Biomass for Electricity Generation.
Washington, D.C.
EIA. 2007. Renewable Energy Annual, 2005. Washington, D.C.
Ernst, B., B. Oakleaf, M.L. Ahlstrom, M. Lange, C. Moehrlen, B. Lange, U. Focken, and
K. Rohrig. 2007. Predicting the wind. IEEE Power & Energy Magazine 5(6):78-
89.
EPA (Environmental Protection Agency). 2008. An Overview of Landfill Gas Energy in
the United States. U.S. Environmental Protection Agency Landfill Methane
Outreach Program. Washington, D.C.
EPRI (Electric Power Research Institute). 2007a. Assessment of Waterpower Potential
and Development Needs. Palo Alto, Calif.
EPRI. 2007b. Compressed Air Energy Storage (CAES) Scoping Study for California.
Sponsored by California Energy Commission. Sacramento, Calif.
ETSO (European Transmission System Operators). 2007. European Wind Integration
Study (EWIS) Towards a Successful Integration of Wind Power into European
Electricity Grids. ETSO, Brussels, Belgium. Available at http://www.etso-
net.org/upload/documents/Final-report-EWIS-phase-I-approved.pdf.
Fletcher, E.A. 2001. Solar thermal processing: A review. Journal of Solar Energy
Engineering 123:63-74.
Gyuk, I. 2008. Energy Storage for a Greener Grid. Presentation at the Third Meeting of
the Panel on Electricity from Renewables, January 16, 2008, Washington, D.C.
Hawlins, D., and M. Rothleder. 2006. Evolving role of wind forecasting in market
operation at the CAISO. Pp. 234-238 in Power Systems Conference and
Exposition, 2006 (PSCE ‘06). IEEE, Washington, D.C.
IEEE (Institute of Electrical and Electronics Engineers). 2005. November/December
Issue: Working with wind⎯Integrating wind into the power system. IEEE Power
& Energy 3(6).
IEEE. 2007a. November/December Issue: Wind integration, driving policies, and
economics. IEEE Power & Energy Magazine 5(6).
89
OCR for page 90
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
Jones, A.T., and W. Finley. 2003. Recent developments in salinity gradient power. Pp.
2284-2287 in OCEANS 2003: Celebrating the Past, Teaming Toward the Future.
Marine Technology Society, Columbia, Md.
King, D.L., W.E. Boyson, and J.A. Mratochvil. 2004. Photovoltaic Array Performance
Model. Sandia National Laboratories, Photovoltaic System R&D Department,
Albuquerque, N.M.
Kroposki, B. 2007. Renewable Energy Interconnection and Storage. Presentation at the
First Meeting of the Panel on Electricity from Renewables, September, 18, 2008.
Washington, D.C.
Mancini, T., P. Heller, B. Bulter, B. Osborn, S. Wolfgang, G. Vernon, R. Buck, R. Diver,
C. Andraka, and J. Moreno. 2003. Dishing Stirling systems: An overview of
development and status. Journal of Solar Energy Engineering 125:135-151.
McKenna. 2005.
Miles, A.C. 2008. Hydropower at the Federal Energy Regulatory Commission.
Presentation at the Third Meeting of the Panel on Electricity from Renewables,
January 16, 2008. Washington, D.C.
Mills, D., P. Le Lievre, and G.L. Morrison. 2004. Lower temperature approach for very
large solar power plants. Proceedings of the 12th International Symposium on
Solar Power and Chemical Energy Systems (SolarPACES ’04), Oaxaca, Mexico.
Available at http://www.ausra.com/pdfs/LowerTempApproach_Mills_2006.pdf.
Minerals Management Service. 2006. Wave Energy Potential on the U.S. Outer
Continental Shelf, Technology White Paper. Renewable Energy and Alternate
Use Program, U.S. Department of the Interior, Washington, D.C.
MIT (Massachusetts Institute of Technology). 2006. The Future of Geothermal Energy:
Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st
Century. Cambridge, Mass.
Muckerman, J.T., D.E. Polyansky, T. Wada, K. Tanaka, and E. Fujita. 2008. Water
oxidation by a ruthenium complex with noninnocent quinone ligands: Possible
formation of an O−O bond at a low oxidation state of the metal. Inorganic
Chemistry 47(6):1787-1802.
Muljadi, E., C.P. Butterfield, R. Yinger, and H. Romanowitz. 2004. Energy storage and
reactive power compensator in a large wind farm. Paper presented at 42nd AIAA
Aerospace Sciences Meeting and Exhibit, January 5-8, 2004, Reno, Nevada.
AIAA 2004-352. American Institute of Aeronautics and Astronautics, Reston, Va.
Available at
http://pdf.aiaa.org/preview/CDReadyMASM04_665/PV2004_352.pdf.
NAS-NAE-NRC (National Academy of Sciences-National Academy of Engineering-
National Research Council). 2009a. Liquid Transportation Fuels from Coal and
Biomass: Technological Status, Costs, and Environmental Impacts. The National
Academies Press, Washington, D.C.
NAS-NAE-NRC. 2009b. America’s Energy Future: Technology and Transformation.
The National Academies Press, Washington, D.C.
NREL (National Renewable Energy Laboratory). 2008. About Geothermal Electricity.
Golden, Colo. Available at http://www.nrel.gov/geothermal/geoelectricity.html.
NYISO (New York Independent System Operator). 2008. Forecasts Sufficient Electricity
Supply for Summer 2008. NYISO Press Release. Rensselaer, N.Y.
90
OCR for page 91
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
O’Regan, B., and M. Grätzel. 1991. Low-cost, high-efficiency solar cell based on dye-
sensitized colloidal TiO2 films. Nature 353:737-740.
Parson, B., M. Milligan, E. DeMeo, B. Oakleaf, K. Wolf, M. Schuerger, R. Zavadil, M.
Ahlstrom, and D.Y. Nakafuji. 2006. Grid impacts of wind power variability:
Recent assessments from a variety of utilities in the United States. Conference
Paper NREL/CP-500-39955, July. National Renewable Energy Laboratory, U.S.
Department of Energy, Washington, D.C.
Perkins, C., and A.W. Weimer. 2004. Likely near-term solar-thermal water splitting
technologies. International Journal of Hydrogen Energy 29:1587-1599.
Perkins, C., and A.W. Weimer. 2009. Solar-thermal production of renewable hydrogen.
AIChE Journal 55(2):286-293.
Rastler, D. 2008. Electric Energy Storage Briefing. Presentation at the Fourth Meeting of
the Panel on Electricity from Renewables, March 11, 2008. Washington, D.C.
RAB (Renewables Advisory Board). 2008. Marine Renewables: Current Status and
Implications for R&D Funding and the Marine Renewable Deployment Fund.
United Kingdom.
SECO (State Energy Conservation Office). 2008. Texas Wind Energy. Available at
http://www.seco.cpa.state.tx.us/re_wind.htm.
SEIA (Solar Energy Industries Association). 2004. Our Solar Power Future—The U.S.
Photovoltaic Industry Roadmap Through 2030 and Beyond. Washington, D.C.
SERI (Solar Energy Research Institute). 1989. Ocean Thermal Energy Conversion: An
Overview. Golden, Colo.
Smith, C. 2007. Integration of Wind into the Grid. Presentation at the Second Meeting on
Panel on Electricity from Renewables, December 6, 2007. Charles Smith
Executive Director UWIG . Washington, D.C.
Starr, C. 2002. National energy planning for the century: The continental SuperGrid,
Nuclear News 45(2):31-35.
Steinfeld, A. 2005. Solar thermochemical production of hydrogen: A review. Solar
Energy 78(5):603-615.
Surek ,T. 2001. Photovoltaics: Energy for the new millennium. Physics and Society
30(1). Available at
http://www.aps.org/units/fps/newsletters/2001/january/aaajan01.html.
Thresher, R., M. Robinson, and P. Veers. 2007. To capture the wind. IEEE Power &
Energy Magazine 5(6):34-46.
WGA (Western Governors’ Association). 2006a. Clean and Diversified Energy Initiative
Solar Task Force Report. Washington, D.C.
WGA. 2006b. Clean and Diversified Energy Initiative: Geothermal Task Force Report.
Washington, D.C.
Wilburn, D., and D. Bleiwas. 2004. Platinum-Group Metals—World Supply and
Demand. U.S. Geological Survey (USGS) Open-File Report 2004-1. USGS,
Washington, D.C.
Williams, R.H., and E.D. Larson. 1996. Biomass gasifier gas turbine power generating
technology. Biomass and Bioenergy 10:149-166.
Williams, M., and M. Zhang. 2008. Challenges to Offshore Wind Development in the
United States. MIT-USGS Science Impact Collaborative, Cambridge, Mass.
91
OCR for page 92
PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION
Wiltsee, G. 2000. Lessons Learned from Existing Biomass Power Plants. National
Renewables Energy Laboratory, Golden, Colo. February. Available at
http://www.nrel.gov/docs/fy00osti/26946.pdf.
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