Electricity from Renewable Resources Status, Prospects, and Impediments (2010) / Chapter Skim
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3 Renewable Electricity Generation Technologies
3 Renewable Electricity Generation Technologies
Pages 67-132
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From page 67... ...
To tap these sources of energy, renewable electricity generation technologies must be located where the natural energy flux occurs, unlike conven tional fossil-fuel and nuclear electricity-generating facilities, which can be located at some distance from their fuel sources. Renewable technologies also follow a paradigm somewhat different from conventional energy sources in that renewable energy can be thought of as manufactured energy, with the largest proportion of costs, external energy, and material inputs occurring during the manufacturing process.
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From page 68... ...
Disruptive technologies that may underperform today, relative to what users in the market demand, may be fully performance-competitive in that same market tomorrow. Traditional sources of electricity generation at least initially outperform non hydropower renewables.
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From page 69... ...
For wind turbines, it typically includes all the related electronics required to provide the connection to the grid. 2Background description and information on activities of the wind industry can be found on the American Wind Energy Association website at http://awea.org.
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From page 70... ...
4Voltage control ability provides control of wind turbine voltage output. 5Output control ability allows the power produced to be reduced by feathering the blades.
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From page 71... ...
With these new control technologies, wind power plants are better at mimicking traditional generating plants. This capability led to Federal Energy Regulatory Commission (FERC)
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From page 72... ...
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 about integrating wind power into the existing power system and requires wind turbines to provide LVRT, volt age control, output and ramp rate controls, and VAR support.
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From page 73... ...
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 targets: growth at 18–20 percent through 2010.
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From page 74... ...
Although no big breakthroughs are anticipated, continuous improvement of existing components is anticipated, and many are already being actively devel oped. For example, there are advanced rotors that use new airfoil shapes specifi cally designed for wind turbines instead of those based on the design of helicop ter blades.
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From page 75... ...
Development of offshore wind power plants has already begun in Europe (approximately 1200 MW of installed capacity)
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From page 76... ...
It should be noted that chal lenges posed by the greater technical difficulties of offshore wind power develop ment 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)
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From page 77... ...
Summary of Wind Power Potential Wind-power technologies are actively deployed today, and there are no techno logical barriers to continued deployment. Cost reductions will be possible as a result of wider deployment and incremental improvements in components.
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From page 78... ...
x 100% Solar Cell Efficiency (%)
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From page 79... ...
modules achieve, on average, only about 10–15 percent efficiency, which is 50–60 percent of the efficiency of the best research cells. Figure 3.5 includes several PV technologies: single-crystalline Si, thin films, multi-junction cells, and emerg ing technologies, such as dye-sensitized nanocrystalline TiO2 cells, cells based on organic compounds, and plastic solar cells.
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From page 80... ...
. Additionally, thin film deposition technology allows production of large-area solar cells, and though they exhibit lower efficiencies (upward of 10 percent)
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From page 81... ...
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 13–15 percent range; small-area research cells are 20 percent efficient.
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From page 82... ...
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.
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From page 83... ...
Consequently, organic solar cells could lower costs in four ways: low-cost constituent elements (e.g., carbon, hydrogen oxygen, and nitrogen sulfur) ; reduced material use; high conversion efficiency; and high-volume production techniques (e.g., high-rate deposition on roll-to-roll plastic substrates)
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From page 84... ...
For example, new technologies are being developed to make conventional solar cells by using nanocrystalline inks of precursor as well as semiconducting materials. 15Includes nanowires, nanotubes, and nanocrystals, including single-component, core-shell, embedded nanowires or nanocrystals, as either absorbers or transporters.
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From page 85... ...
Even lower costs are possible with plastic organic solar cells, dye-sensitized solar cells, nanotechnology-based solar cells, and other new PV technologies. Long Term: After 2035 Widespread deployment of PV technology will depend on the ability to reach scale in manufacturing capacity and achieve cost reductions using technologies for ultralow-cost module production at acceptable efficiency.
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From page 86... ...
The difference in these technologies is the optical system and the receiver where the concentrated solar radiation is absorbed and converted to heat or chemical potential.16 These differences also define the potential plant size from the smallest (dish-Stirling concentrator) to the largest (parabolic troughs and power towers)
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From page 87... ...
Parabolic trough plants can include solar energy storage capabilities, e.g., concrete, molten salt, and thermocline storage, that can extend generation for several hours. At present, many plants have a backup fossil-fired capability that 17Formore information on the California SEGS design, see http://solar-thermal.anu.edu.au/ high_temp/concentrators/basics.php.
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From page 88... ...
integrates a parabolic trough plant with a gas turbine combined-cycle plant. The ISCCS uses solar heat to supplement the waste heat from the gas turbine to augment power generation in the steam Rankine cycle.
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From page 89... ...
19The lack of other strong renewable energy opportunities in the transmission-constrained state of California has pushed solar project bids ahead of wind power projects. 20See http://www.eere.energy.gov/news/news_detail.cfm/news_id=11474.
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From page 90... ...
However, as noted in Chapter 5, CSP consumes at least as much water as some conventional generation technologies. The primary water uses at a Rankine steam solar power plant are for condensate makeup, cooling for
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From page 91... ...
Long Term: After 2035 In the longer term, the use of concentrated solar energy to produce fuels and thus provide storage via a number of reversible chemical reactions is promising. Fuels produced from concentrated solar energy may provide a means of generating electricity during periods of low insolation or at night.
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From page 92... ...
An evolving long-term technology that relies on solar concentration is high-temperature chemical processing. Solar thermochemical production of fuels is a promising mechanism for storage of solar energy.
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From page 93... ...
Hydrothermal power plants are binary or steam. Binary plants are more prevalent than steam plants, because lower-temperature reservoirs suitable for binary plants are far more common than steam reservoirs.
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The heat extraction rate would depend on the site. Technologies for electricity generation from the hot fluid would be similar to those for hydrothermal power plants.
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From page 95... ...
Extensive development of EGS is less certain because of the lack of experience in recovering the heat stored at 3- to 10-km depths in low-permeability rock. The primary technical challenges are accurate resource assessment and understanding how to achieve sufficient connectivity within the fractured rock so that the injection and production well system can yield commercially feasible and sustainable production rates.
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From page 96... ...
Medium Term: 2020 to 2035 As indicated in Table 3.4, the largest source of geothermal energy resides in the thermal energy stored in rock formations that require EGS technology for extrac tion. Implementation of EGS has not been demonstrated at large scale, and there are unanswered questions about the extent of economical power available.
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From page 97... ...
Summary of Geothermal Power Potential Geothermal energy is a renewable energy resource that can provide baseload power without storage. Existing geothermal power plants rely on well-understood power plant technology but are restricted to hydrothermal resources within 3 km of Earth's surface.
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From page 98... ...
. Different categories of hydropower include large conventional hydropower with generating capacity greater than 30 MW, low-head hydropower with a hydraulic head of less than 65 feet and a generating capacity of less than 30 MW, and micro-hydropower with a generat ing capacity of less than 100 kW.
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From page 99... ...
Hydrokinetic Power New technologies to generate electricity from waterpower include those that can harness energy from currents, ocean waves, and salinity and thermal gradients. Many pilot-scale projects are demonstrating technologies that tap these sources, but only a few of them are commercial-scale power operations at particularly favorable locations.
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From page 100... ...
Designs using salinity gradient power would rely on the osmotic pressure difference between freshwater and salt water, although none of these have moved beyond the conceptual stage. In general, even though waves, currents, and gradients contain substantive amounts of energy resources, there are significant technological and cost issues to address before such sources can contribute significantly to electricity generation.
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From page 101... ...
The focus will be on developing and deploying technologies to improve fish passage and water quality, increase turbine efficiencies, and design enhanced tools for monitoring and managing water resources. Environmentally advanced hydropower turbine designs can improve fish survival and improve water quality.24 The Grant County Public Utility District Advanced Hydropower Turbine System program is one example where the need for turbine replacement at a hydropower facility on the Columbia River is resulting in new turbines that have greater efficiencies and improved fish passage survival.25 Other activities, such as those in the Oak Ridge National Laboratory's Environmental Sciences Division, are directed toward improving the balancing of hydropower production with other objectives for which dams are also operated, such as flood control, recreation, and ecosystem benefits, through mathematical modeling of complex hydrological systems.
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From page 102... ...
One of the leaders in the development of ocean wave and current electricity is the United Kingdom. The waters around that country are potentially an abundant source of clean renewable energy that could contribute up to 20 percent of its electricity needs (RAB, 2008)
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From page 103... ...
These technologies exist in little more than conceptual designs, laboratory experimentation, and field trials. Ocean thermal energy conversion (OTEC)
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From page 104... ...
. Biomass is abundant, accounting for almost 50 percent of the national renewable energy resources in 2005, the largest single source of renewable energy (EIA, 2007)
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From page 105... ...
rarely reaching 50 MW, as compared to the 100–1500 MW range of conventional coal-fired power plants. Similarly, LFG power plants have capacities in the 0.5 MW to 5 MW range, whereas those operating on natural gas average about 100 times larger, in the 50 MW to 500 MW range.
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From page 106... ...
As of December 2007, approximately 445 LFG energy projects operated in the United States, generating approximately 11 billion kWh of electricity per year and deliv ering 236 million cubic feet per day of LFG to direct-use applications, amounting to just under 20 percent of biomass electricity generation. Potential Technology Development Short Term: Present to 2020 The Energy Information Administration (EIA)
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From page 107... ...
. Medium Term: 2020 to 2035 In the medium term, it is likely that new biopower capacity, if pursued, will incorporate a pretreatment step in which the biomass is converted to a gaseous or liquid fuel more suitable for power generation, rather than direct-firing as is the norm today.
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From page 108... ...
. Figure 3.11 presents a comparison of the efficiencies of the three generation technologies operated on biomass as a function of power plant size (Bridgewater, 1995)
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From page 109... ...
Consequently, they can be well matched to biomass power plants. High-temperature fuel cells have chemical-to-electrical conversion efficiencies of ~50–60 percent, and, as with the gas turbine, the high-tempera ture fuel cell exhaust can be supplied to a steam engine for even higher system efficiencies.
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From page 110... ...
As noted in Chapter 2, the solar-to-biomass conversion in typical plants is only ~0.25 percent; subsequent conversion from biomass to electricity proceeds with another efficiency penalty of at least 50 percent. Thus, solar-to-electric energy conversion efficiency is on the order of 0.1 percent, which is far below the 10–20 percent efficiency achievable with state-of-the-art photovoltaic and concentrating solar power sys tems.
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From page 111... ...
, which would again favor biomass for use in power systems. ENHANCING TECHNOLOGIES FOR ELECTRICITY SYSTEM OPERATION There are a host of technologies, operational modifications, and system upgrades that could enhance renewable energy resource use.
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From page 112... ...
Elec tricity consumption varies over the course of the day, whereas coal, nuclear, and hydropower electricity plants are generally designed to provide baseload electricity at some optimal level of generation.28 Renewable resources such as solar and wind are intermittent by nature, and that intermittent supply can be mismatched with demand. Thus, neither baseload nor intermittent electricity generation technolo gies supply electricity in alignment with demand.
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From page 113... ...
However, at moderate penetrations, up to at least 20 percent in the case of wind power, studies indicate that the existing management approaches suffice, and storage is not an immediate necessity for successful integration of renewable resources. These studies are dis cussed in Chapters 6 and 7.
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From page 114... ...
Energy storage in the form of chemical fuels, including biomass and batteries, has direct implications for transportation and underscores the likelihood of increasing overlap between the electricity and transportation sectors in future years. Pumped Hydropower Energy storage via pumped hydropower involves the use of electrical energy to move water into an elevated hydropower reservoir by operating the generator as a motor and running the hydroturbine in reverse.
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From page 115... ...
New approaches to diabatic compressed air storage are directed toward microscale systems that use smaller volumes and capitalize on underground natural gas storage or storage in depleted gas fields. Adiabatic CAES systems eliminate the need for combustion fuels by storing not only the mechanical energy of compression, but also the thermal energy 32In diabatic storage the heat produced during the compression of air escapes to the atmosphere and is wasted, whereas in adiabatic storage the heat produced during compression is also stored.
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From page 116... ...
Beyond the technical challenges of constructing and operating CAES power plants, it is of value to consider the storage volume (geologic) requirements for maintaining compressed air energy storage at a scale that would be significant compared to present-day electricity consumption.
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From page 117... ...
Renewable Electricity Generation Technologies Power Out Electrolyte Flow Ion Exchange Membrane Electrode Electrode Electrolyte Tanks FIGURE 3.14 Schematic of a flow battery. R 3.14 per-megawatt flow rate required for electricity generation from a compressed air cavern would result in a total generation capacity of 26 GW, which amounts to ~5.5 percent of the U.S.
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From page 118... ...
It is unclear where and how fundamental breakthroughs can bring revolutionary advances in battery technologies. For energy storage, the energy density stored in gasoline is
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From page 119... ...
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.
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From page 120... ...
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 → fuel)
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From page 121... ...
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.
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From page 122... ...
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 pen etrations 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.
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From page 123... ...
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.
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From page 124... ...
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.
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From page 125... ...
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/intermit tent nature of these renewable energy sources: the reactive power must be equally dynamic to keep pace.
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From page 126... ...
In contrast to fossil-based or nuclear energy, renewable energy resources are more widely distributed, and the technologies that convert these resources to use ful 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 elec tricity must accommodate temporal variation in the availability of these resources.
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From page 127... ...
2004. Advanced adi-adi abatic compressed air energy storage for the integration of wind energy.
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From page 128... ...
2007k. National Solar Technology Roadmap: Sensitized Solar Cells.
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From page 129... ...
2007. Renewable energy interconnection and storage.
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From page 130... ...
2006. The Future of Geothermal Energy: Impact of Enhanced Geothermal Systems (EGS)
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From page 131... ...
SERI (Solar Energy Research Institute)
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