Overall Perspectives on the Renewable Technologies

While each of the characterized renewable technologies is discussed in detail in this document, the following summary presents an overview of current status and applications for each.

Biomass: The use of forestry and agricultural residues and wastes in direct-combustion systems for cogeneration o f electricity and process heat has been a well-established practice in the forest-products industry for many years. Us e of these feedstocks in utility electric power plants has also been demonstrated in several areas of the country with access to appropriate fuels, in general with acceptable technical performance and marginal economics. The margina l economics are due to the small size of many of the existing plants and the consequent high operating costs and lo w efficiencies. Also, fuel shortages have often driven fuel prices up and made operation too expensive. The larger-sized plants, in the 50 MWe range rather than the 10-to-25 MWe size range of many projects built in the 1980s, hav e economics that are acceptable when fuel costs are close to $1/MMBtu, or when steam or heat from the direct -combustion biomass boiler is also a valued product. In addition to activity with current technology, development i s proceeding on advanced direct-combustion systems.

One technology can use direct combustion of biomass fuels today without incurring the capital expense of a new boiler or a gasification/combined-cycle system. This technology is biomass co-firing, wherein biomass is co-fired, or burned together, with coal in existing power plants. Though it does not increase total power generation, this mode of operation can reduce power-plant emissions and serve as a productive use for a waste stream that requires disposal in some way. Co-firing can be carried out as a retrofit, often with very low incremental capital and O&M costs. Biomass co-firin g has been successfully demonstrated in a number of utility power plants, and is a commercially available option i n locations where appropriate feedstocks are available.

Biomass gasification and subsequ ent electricity generation in combustion-turbine or combined-cycle plants is also being pursued. This mode of operation can be more attractive than direct combustion because of (a) potentially highe r thermal efficiency, (b) the ability to maintain high performance in systems over a wide range of sizes from about 5 MW to about 100 MW, and (c) increased fuel flexibility because of opportunities to reduce unwanted contaminants prio r to the power generation stage. These systems are in the development and demonstration phase. The key issue requiring successful resolution is sufficient cleanup of the biogas so that turbine damage is avoided. The gas must be cleane d of alkalis to gas-turbine-entrance standards, and this cleanup must take place in an environment that is prone to ta r formation.

Geothermal: Commercial electricity from geothermal steam reservoirs has been a reality for over 30 years in California and Italy. However, steam reservoirs are rare and have already been exploited, at least in the developed countries. Of greater potential in both developed and developing countries are geothermal-hot-water, or liquid-dominated -hydrothermal, resources. A number of hydrothermal plants, perhaps 30 to 40, both developmental and commercial , have been built and are in operation. Some use conventional steam-separation and steam-cycle power-plant equipment, while others employ a binary cycle that takes advantage of working fluids with lower vaporization temperatures tha n water. Commercial attractiveness depends largely on the quality of the hydrothermal resource: temperature of the ho t water, permeability of the rock formation, chemistry of the hot water, and necessary drilling depth. To ascertain thi s quality, wells need to be drilled. Since the outcome is not assured prior to drilling, locating suitable resources presents a major commercial risk.

Another geothermal-power approach is in the research stage. This involves drilling deep holes (one-to-five kilometers) to reach hot dry rock that is close to locations where magma or other hot intrusions from the molten mantle of the Earth come unusually close to the surface. In this context, "dry" rock implies that no natural water source is associated with the hot rock, unlike the situation in the hydrothermal case. Water from a surface source would be injected, heated, used in a steam- or binary-power cycle, and then re-injected for recycling. If successful, this approach could make available a huge resource relative to present geothermal resources. However, technical uncertainties and risks are very high, s o the commercial potential of this approach cannot be estimated accurately today.

Photovoltaics: Photovoltaic power systems convert sunlight directly into electricity through a solid-state-electroni c process that involves no moving parts, no fluids, no noise and no emissions of any kind. These features are attractive from operating, maintenance and environmental standpoints, and have positioned photovoltaics to be the preferre d power technology for many remote applications both in space and on the ground. Relative to conventional grid power, photovoltaic electricity is some five-to-ten-times more expensive. Hence, it is currently used in locations o r applications where utility distribution lines are not readily available. Newer, potentially lower-cost photovoltai c technology is emerging from ongoing industry-government research and development programs, and its use i n commercial and demonstration applications is beginning.

Although increasing use could occur more rapidly in some developing countries, grid-competitive photovoltai c electricity is probably ten-to-twenty years off in the developed world. However, interest is growing in a new mode o f photovoltaic deployment, called building-integrated, where the photovoltaic cells or modules become integral t o structural, protective or cosmetic elements of a building such as roofs and facades. In these applications, the high cost of the photovoltaic components is partially masked by the cost of the building elements, and the decision to emplo y photovoltaics is made on the basis of such factors as aesthetics and social conscience rather than cost of electricit y alone. Many believe that this commercial entry strategy will ultimately succeed in reducing photovoltaic costs through production experience to the point where they can approach costs of grid power. Several governments and many communities in the developed world are incentivizing these applications based on this belief. Because of the growin g prominence of building-integrated and other on-site applications of photovoltaics, a section on residential roofto p photovoltaic systems is included in this document.

Another approach to power plants employing photovoltaics uses concentrated sunlight in conjunction with unusuall y high-performance photovoltaic cells. While attractive technical performance has been demonstrated in some instances, an early market for these systems has not materialized. Unlike flat-plate photovoltaic systems that have establishe d themselves in remote power applications, the potentially high-performance concentrator systems have not ye t established a track record in the field. This, coupled with the need to build relatively large systems (at least several tens of kW) to realize their cost advantage and the added complexity associated with required sunlight tracking, ha s seriously hampered market entry up to now.

Solar Thermal: Solar thermal power systems use concentrated sunlight to heat a working fluid that generates electricity in a thermodynamic cycle. Three general approaches have received development attention. The first, called the central-receiver or power-tower configuration, employs a field of mirrors that track the sun and reflect sunlight to a centra l receiver atop a tower. The working fluid is circulated through and heated in the receiver, and is then used to drive a conventional turbine. The fluid and its thermal energy can be stored to decouple the collection of the solar energy and the generation of electricity, enabling this power plant to be dispatched much like conventional thermal power plants . This is an attractive feature to electric utilities and power system managers. Several experimental and demonstratio n power-tower systems have been built; and one, employing thermal storage, is currently under test and evaluation i n California. As yet, the commercial prospects for this approach cannot be accurately projected.

Another approach employs parabolic dishes, either as single units or in fields, that track the sun. A receiver is place d at the focal point of the dish to collect the concentrated solar energy and heat the system's working fluid. That flui d then drives an engine attached to the receiver. Dish systems also have potential for hybridization, although mor e developmental work is required to realize this potential. In contrast to the other two approaches, which are targeted at plants in the 30 MW and higher range, and which use a single turbine-generator fed by all of the solar collectors , each dish-receiver-engine unit is a self-contained electricity-generating system. Typically, these are sized at about 1 0 to 30 kW. Hence, a larger power plant is obtained by employing a number of these units in concert. With som e interruptions due to changing market conditions, dish systems using Stirling engines have been deployed, with bot h public and private support, for experimental and demonstration purposes since the early 1980s. Current development and demonstration activities are aimed at key technical and economic issues that need to be resolved before commercial prospects can be clarified. Stirling-engine development for prospective vehicular applications is also under way. I f successful, transportation sector market penetration would substantially improve the commercial outlook for solar dish-Stirling systems.

The third approach employs a field of sunlight-tracking parabolic troughs that focus sunlight onto the linear axis of the trough. A glass or metal linear receiver is placed along this axis, and a working fluid is circulated through and heated in this receiver. The fluid from a field of troughs passes through a central location where thermal energy is extracte d via a heat exchanger and then used to drive a conventional turbine. This configuration lends itself well to hybri d operation with fossil fuel combustion as a supplemental source of thermal energy.

In the early 1980s, federal and California-state financial incentives were established to encourage the commercia l deployment and use of emerging renewables. Two technologies were in a position to benefit from these incentives : solar thermal troughs and wind turbines. Trough systems were deployed on a commercial basis in the 1980s and early 1990s, and continue to operate today. In addition to the government-tax-credit incentives, these plants were partially supported by above-market energy payments that are no longer available. Hence trough systems have not been offered commercially since 1991. Should conventional energy costs rise to the above-market support levels of the late 1980 s (when significant increases in oil prices were being projected), or should significant incentives for renewable energ y arise in the near future, trough technology would be available to play an important role in areas with good sunlight . In addition, efforts are underway to revive this technology for use in developing countries that have urgent needs fo r new electric power sources, such as India and Mexico.

Although the solar-thermal trough (and wind) systems fielded in the early 1980s experienced considerable technica l difficulti es, the overall result of the deployments of the 1980s and the associated experience and technical development was that both trough systems and wind systems (see wind discussion below) had achieved technical and commercia l credibility by the early 1 990s. Energy costs from these systems were approaching the competitive range for grid power. Trough-energy costs were somewhat higher than wind-energy costs; but, owing to hybridization with natural gas, th e trough plants were dispatchable. Hence their energy had higher value in some instances. Wind energy, in contrast , was available only when the wind blew.

Wind: As mentioned above, wind power systems progressed substantially as a result of the 1980s governmen t incentives, with a steady trend of cost reductions throughout the 1980s. Since 1990, the cost of energy from the wind has continued to decline, due to continued deployment and to public-private development programs in the U.S. and , to an even greater extent, in Europe. Wind power is now on the verge of becoming a commercially established an d competitive grid-power technology. Although expansion of the U.S. wind market has been slowed since the onset o f electric-sector restructuring in 1995, the wind markets in Europe and elsewhere in the world have continued to grow, led by firms in Denmark and Germany. The growth of wind in Europe has been fueled, in part, by aggressive goal s for renewable power deployment in response to strong public and political support for clean energy and growin g concern over global climate change. And there are signs that the pace of wind deployment in the U.S. is again on th e rise.

With the exception of the Southeast, most regions of the U.S. have commercially attractive winds. In addition to wind resource quality, other issues that need to be considered, as with most commercial power plants, are transmissio n requirements and potential environmental impacts. Most U.S. wind facilities installed to date are wind farms wit h many turbines interconnected to the utility transmission grid through a dedicated substation. There is growing interest in distributed wind facilities, with a small number of turbines connected directly to the utility distribution syste m without a substation. Such installations account for more than half of the over 4,000 MW of wind in Europe, but th e U.S. to date has little experience with this mode. Hence this document focuses on central-station wind applications .

The great majority of wind power experience has been obtained with the traditional wind turbine configuration, i n which the rotor revolves about a horizontal axis. In addition, several development programs of the past twenty year s have focused on turbines with rotors that turn about a vertical axis (sometimes called "egg-beater" turbines). Although the case cannot be considered completely closed, the weight of experience indicates strongly that the vertical axi s machines will not show a performance or commercial advantage relative to the horizontal axis machines. Henc e development of the vertical axis units has all but halted, and this document focuses entirely on horizontal axis turbines.

Energy Storage: Recent advances in batteries and other storage technologies have resulted in systems that can play a flexible, multi-functional role in the electricity supply network to manage power resources effectively. The curren t electricity market offers a number of opportunities for energy storage technologies in which storage of a few second s to a few hours of electricity is valuable. These systems can be located near the generator, transmission line, distribution substation, or the consumer. Improved, low-maintenance, spill-proof, relatively compact lead-acid batteries ar e commercially available today.

Energy storage systems are used beneficially today in a variety of applications. Examples include mitigation of power-quality problems and provision of back-up power for commercial/industrial customers, utility substations, an d transmission-line stability. In addition, energy storage can play an important role in enabling the increased utilizatio n of intermittent renewable energy sources such as wind and photovoltaics. In grid-connected applications, the storag e system can be charged from the renewable source or from the utility grid, whichever is economically preferred.

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