Intermittent Sources

Power production from intermittent generating technologies is vulnerable to the periodicity of the natural forces upon which it depends. That is, power production from the wind and solar radiation can be achieved only when the wind blows or the sun shines. However, when combined with storage or other integrated system response strategies, these combined resources can reduce the need for other sources of power.

Substantial development of an intermittent resource-wind-has occurred in several utility systems. Photovoltaics (PV) has been successful in reaching the remote power market at thousands of sites worldwide. Thus, intermittent renewable resources are reaching an important stage in acceptance and use. Although it is classified an intermittent, PV provides daytime power that may coincide with utility or customer load profiles. Such a match adds to the value of PV for the user.

Solar thermal systems. Solar thermal systems use concentrated sunlight to generate heat for thermal conversion processes, such as electricity generation. Three types of solar thermal technologies-parabolic-trough systems, central-receiver plants, and parabolic dish systems-are either currently in use or under development. Appropriately configured, any of these can provide dispatchable power and energy.

First, 274 MW of privately funded, grid-connected parabolic-trough generating capacity is operating in southern California[5]. These plants operate in a hybrid mode using auxiliary natural gas to overcome the intermittency of the solar resource as well as to extend generation to better match load. This daytime dispatchability has proven the reliability of the hybrid concept. A project to deliver an additional 80 MW is currently under construction, and there are firm plans for another 300 MW to be built by about 1994. The systems currently provide energy at costs of less than 100/kWh. Further expected improvements in the technology could result in 30% cost reductions, making this technology cost-effective in more U.S. and world markets.

The second major technology is the central-receiver plant. A 10-MW central-receiver power plant was deployed by a joint government/industry team and operated successfully for several years in a grid-connected mode by Southern California Edison Company. Thermal storage in the system would move the technology to the dispatchable category for a utility. Six hours of storage is expected to provide daytime dispatching under variable weather conditions. The average levelized capital and operating costs projected for solar thermal central-receiver stations range between 80 and 12^/kWh for early plants, based on current component and system designs.

Finally, prototype parabolic dish electric systems, totaling about 5 MWe, have been operated in a utility setting in Georgia and in southern California. Prototype dishes with small Stirling heat engines and generators mounted at the focal point of the dish have led to significant increases in system performance and hold the world record for system conversion efficiency from sunlight to electricity (29%). The Stirling engine configurations may be most appropriate for small, stand-alone applications. U.S. industry involvement in this technology is beginning to increase as the technology approaches cost competitiveness in early markets. Germany, Japan and Spain are also working on small dish system concepts for export.

Worldwide interest in solar thermal hybrid systems has increased recently. Plants are planned for India, Jordan, and Israel, and aggressive R.D&D is continuing in Spain, Germany, and Israel to capitalize on these emerging markets, lending the industry a multinational character. The major solar thermal hybrid supplier has both United States and foreign involvements. The U.S. industry hopes to expand to other high-insolation areas of the world.

Wind power. Of the intermittent technologies, wind power is currently the largest contributor, with an installed power-generating base of about 1.5 GW, primarily in California. The many so-called wind farms, or wind power plants, which have been developed by nonutility entities, generated over 2 billion kWh of electricity in 1989[6]. Although early development was closely tied to the availability of federal and state tax incentives, as well as lucrative utility power purchase contracts, cost reductions and performance improvements have been considerable over the last several years. While early systems (arrays of wind turbines) suffered from poor reliability, unit availabilities of the best California wind power plants now approach 95%. The average levelized capital and operating costs for new wind facilities can be as low as 60/kWh at excellent sites.

Wind power's primary current market is in California, where wind resources are relatively good and utilities have offered attractive prices for purchased energy. Since the cost of energy from wind turbines depends strongly on wind speed (power output increases with the cube of the wind speed), wind energy can compete with conventional fuels in the near term at the very best sites. These near-term markets are expected to displace high- and moderate-cost fuels. In the long term, our projections suggest that wind energy can compete against low-cost base load fuels using moderate wind resources such as those of the Great Plains. Because of the operating flexibility of hydropower, areas that have hydropower resources or hydropower storage capability may be able to incorporate greater wind penetration than areas without them.

Like other renewable technologies, wind power's early significant advances in this country have led to a worldwide technology development effort that far surpasses current domestic expenditures. In Europe, seven countries and the Commission of European Communities are each spending as much or more on wind energy R,D&D than we are in the United States. A large Japanese manufacturer has also made a major commitment to penetrate the California market with the installation of hundreds of turbines in 1989. Only a handful of U.S. concerns remain participants in the field.

Photovoltaic^. Photovoltaic systems have been used for more than 30 years to power spacecraft. The reliable performance of solar cells in space has established PV as a dependable technology. Costs have come down dramatically since the first solar cells were deployed for space applications, opening up a terrestrial market of approximately 42 MW per year (1989). This market has three major segments: consumer products, remote power, and bulk power generation.

The current consumer market is characterized by millions of small, milliwatt-sized systems powering calculators and watches. This market, which has been steady at about 5 MW per year, is now expanding to larger systems such as batten/ charging and walkway lighting. The largest use of PV today is for remote power. The remote-power market encompasses stand-alone applications to power telecommunications, highway lighting and call boxes, navigation aids, security systems, water supply pumping systems, cathodic protection, vaccine refrigeration, remote monitoring, rural housing, and small villages. Perhaps 20 MW of capacity exists in thousands of remote, stand-alone applications. Bulk power applications of PV are currently limited. Three megawatt-scale plants were installed in the early 1980s that continue to operate reliably. Electric utilities are currently investigating potential uses for PV systems ranging from distribution system applications to bulk power generation. A recent survey^71 conducted by the Electric Power Research Institute (EPRI) identified 219 grid-connected PV systems with a total combined peak power rating of 11.6 MW (including 9.4 MW for the three large plants).

The current levelized cost of energy from PV is 300 to 350/kWh, too high to be competitive in today's bulk utility power market. But at today's costs there is an untapped remote market of 200 to 300 MW in the United States and abroad.

The most significant energy contribution for PV is nevertheless expected to come in the utility power market. Under baseline assumptions, smaller high-value applications should begin about 1995 followed by major market entry for peak power supply beginning about 2005. Achieving costs of 40 to 70/kWh in the 2010 to 2030 period should increase penetration significantly.

The private PV industry is a mix of some 40 firms, 90% of which are small- to medium-sized firms involved in manufacturing, distribution, or service. In general, the industry, especially manufacturing, has not been profitable. U.S. companies, which were world leaders in the early 1980s, have encountered serious competition from foreign-owned companies. The U.S. PV market share has severely declined, from 65% in 1981 to 35% in 1989. The world's largest PV company, California-based ARCO Solar, has been purchased by a German-based multinational company. Although the market for PV is growing at a 20% annual rate, and industry margins are improving, this economic turnaround has yet to become fully reflected in a strong investment climate in the United States.

Transportation: Blofuels

Transportation needs consume more than a fourth of the primary energy used in the U.S. economy^, primarily in the form of petroleum-based liquid fuels. The conversion of biomass resources represents the primary renewable energy-based pathway to transportation fuels production. Although there are several process concepts for producing these so-called biofuels, only ethanol from corn is produced today in commercial quantities. Eight hundred and fifty million gallons of ethanol are produced annually, primarily by fermentation of corn, for blending into gasoline. These ethanol blends are used in approximately 8% of the gasoline in the United States today. At a 10% blend, ethanol thus represents 0.8% of the motor gasoline market.

The U.S. ethanol industry is dominated by 25 plants operating in the Midwest with corn as the feedstock. This industry is principally made up of corn processors that produce a variety of corn products, including sweeteners, corn oil, animal feed, and ethanol. Continuous operation at a profit has been so difficult that smaller plants and those using older technologies have not survived. Nevertheless, the production of ethanol has grown by more than tenfold since 1981, leveling off in 1989. The U.S. cost of ethanol from corn is currently about $1.28 per gallon; however, the viability of com-to-ethanol conversion is currently based on tax incentives and on present prices for corn in the food and feed markets. If available corn cropland were used, the annual production of ethanol from corn could easily exceed 10 billion gallons or approximately 10% of current gasoline usage in the United States.

The largest potential for ethanol from biomass exists in the use of abundant cellulosic (woody and herbaceous) biomass materials, which can reduce feedstock costs by 50% or more compared with com. One current focus of R,D&D is to develop techniques to biochemically convert a larger portion of a cellulosic feedstock and material to ethanol. In the last decade, research focused on bioengineering has reduced the cost of wood-derived ethanol to $1.35 per gallon. Current research plans estimate that a goal of $0.60 per gallon could be attained, which would in the future provide ethanol that is economically competitive with the projected price of gasoline without tax incentives.

Biomass resources can also be used to produce methanol, another viable transportation fuel product. Methanol from biomass is made by first gasifying the feedstock and converting it to methanol over commercial catalysts. Improvements are needed primarily in the gasification process to increase conversion efficiencies, provide better cleaning and conditioning of raw gas, and improve the reliability and scalability of the gasifiers. While not yet commercial, current laboratory technology suggests a present-day cost of $0.75 per gallon. A projected research target cost of $0.55 per gallon by 1995 assumes improvements in gas cleanup and a reduction in feedstock costs.

Other pathways being investigated for production of liquid fuels are synthetic hydrocarbon fuels (gasolines) made by pyrolysis of biomass feedstock and diesel oil production from aquatic and terrestrial oil-producing plants. Expanded research will probably be needed to reduce the costs of such renewable fuels to levels competitive with conventional gasoline and fuel oil.

Buildings/Industrial and Other Stationary Uses

Buildings and industrial applications account for the remaining 40% of U.S. primary energy use[2]. In buildings, fuel is used primarily for space conditioning and water heating. In industry, fuel provides process energy and feedstock materials. Renewables currently provide a substantial amount (2.79 quads) in the buildings, industrial, and other stationary use sectors.


In buildings, renewable energy sources in the form of wood and solar energy heat space and water. In 1987, an estimated 5 million households used wood as their primary space heating source[2]. Another 17.5 million households used wood as a supplementary heating source, either in wood stoves or fireplaces. The total energy contribution from wood is estimated to be around 0.9 quad, or more than

10% of the total energy used for residential space heating. Wood fuel use in commercial buildings is much smaller, about 2% of the residential use (0.02 quad).

It is estimated that more than a million active solar heating systems installed in the United States today supply approximately 0.04 quad of primary fuel displacement. These systems chiefly use low-temperature collectors for pool and water heating. Most of them were installed in the late 1970s and early 1980s in response to rising energy prices and federal and state tax incentives. When federal tax credits expired at the end of 1985 and fossil fuel prices declined at about the same time, U.S. production of active solar collectors dropped from about 18 million ft2 per year (between 1980 and 1985) to less than 5 million ft2 per year in 1987. The number of collector manufacturers also declined from more than 300 in 1979 to less than 80 in 1987. Imports of collectors have remained constant, but these amount to only 600,000 ft2 per year. Most of the remaining U.S. production is marketed in the southern United States for pool heating.

An estimated 250,000 to 300,000 U.S. homes employ some type of passive solar design features, displacing about 0.01 quad of primary energy. Passive solar includes a variety of building designs and technologies that rely on natural solar-based processes to heat, cool, or light buildings. Passive solar design is particularly attractive because many materials and techniques can be incorporated into new buildings at little additional cost.

The passive solar buildings industry has been affected by expiration of the federal energy tax credits to a lesser extent than its active solar counterpart because many passive measures did not qualify for credits. Reductions in oil and gas prices have no doubt reduced consumers' and builders' interest in conservation and passive solar, but impacts on market penetration are hard to quantify. Many recent advances in windows, daylighting, ventilation, and storage technologies are economic now in new buildings, but not as retrofits to existing buildings. These advances will begin to penetrate the market as the building stock turns over and builders and homeowners become more aware of recent advances in passive solar technologies. This increase in builder awareness is essential to the increased deployment of solar buildings technologies.

Geothermal sources can supply energy to buildings and industrial processes in the form of direct heat. In these applications, which include district heating systems, low-temperature hydrothermal resources are tapped using conventional hot water handling equipment. About one-fourth of installed direct-use geothermal capacity is used in building applications, supplying an estimated 0.005 quad.

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