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Wind sites that are economic today typically have winds that average near the surface (10 m) better than 6.5 m/s (14.5 mph). Below 5 m/s, sites are not considered economic today, but that could change with improvements in technology and changing market conditions. Wind resource quality is not simply a function of wind speed at the ground, but also of turbine height above the ground, air density (cooler, more dense air contains more kinetic energy), and the frequency with which the wind blows. As shown in Figure 5.2, most regions of the world have ample wind resources. It has been estimated that global technical wind potential might be as large as 14000 (IPCC, 1996) - 26000 TWh/yr (IIASA, 1981), or up to twice the 1995 world annual electricity consumption of 13000 TWh (IEA, 1998).

In the last few years, wind systems have made significant advances in penetrating world markets, with installations increasing by more than 35% in 1999 to reach a total of 10 GW of total installed capacity worldwide. Much of this has occurred because of the confluence of technology improvements and cost reductions with increased public concern about local air pollution and global climate change. In particular, the European goal of 4000 MW by 2000 was reached by 1997, doubled in that year to 8000, and exceeded again by the end of 1999 with 8915 MW installed. Consequently, the 2010 goal has now been increased from 25000 MW to 40000 MW (EWEA, 2000).

kWWsq m pef day

m s toss

Figure 5.1 Global horizontal insolation.

WOFUO WOt WINO BWCHOV RESOURCE DISTRIBUTION ESTIMATES

WOFUO WOt WINO BWCHOV RESOURCE DISTRIBUTION ESTIMATES

Figure 5.2 Worldwide wind energy resource distribution estimates.

Darker areas denote higher-speed wind resources

Figure 5.2 Worldwide wind energy resource distribution estimates.

Many of the best wind resources are on remote sites (e.g. mountain ridges) far away from electric load centers (i.e. people don't generally like to live in continuously windy locations). Thus, the availability of transmission lines can be a significant issue. The intermittency of wind is another major issue that impacts system reliability, capacity planning, and transmission. These issues will be discussed in more detail in the "Market issues" section.

5.2.4.3 Biomass

Biomass resources exist in many different forms, including wastes from agriculture, forests, industry, and municipalities; standing growth; and dedicated energy crops. Current biomass use accounts for approximately 15% of world energy use (Johansson et al., 1993). Global resource estimates are scarce because:

1. Resource data is unavailable or uncertain in many areas of the world that currently derive significant fractions of their energy use from biomass.

2. Biomass production can use the same resources as required for food and other crops, e.g., land, water, fertilizer.

3. There is considerable debate over the impact of removing biomass wastes on soil nutrients and erosion.2

4. Like any commodity, there is no single resource amount; more will become available if the demand and price are high enough.

The estimate in Table 5.4 of the potential biomass supply worldwide indicates biomass resources could be recovered at rates close to 300 exajoules per year, approaching today's worldwide energy use of all energy forms of 400 exajoules per year. While residues are significant contributors to the total potential of Table 4, 90% of the resource shown is assumed to come from dedicated energy crops. The leading-candidate dedicated energy crops include short-rotation woody crops like poplar trees, and herbaceous grasses like switchgrass. The dedicated energy crop estimates assume 10% of the land now in forests/woodlands, cropland, and pasture (approximately 900 million hectares worldwide) will be planted in dedicated energy crops and yield, on average, 15 dry tonnes of biomass per hectare with 20 gigajoules per tonne. There is considerable debate as to whether this level of production (and energy content) could be sus-

2 These concerns are exacerbated in dry-land agriculture practiced in the poorest areas of Africa, Asia, and Latin America (Johansson et al., 1993). Residue removal can also be increased through conservation tillage and increased fertilizer use (especially nitrogen). Too many residues left on fields can depress growth, inhibit nitrate formation, and spread crop diseases. Many crops are purposefully removed or burned today (e.g. sugar cane bagasse, rice). Dung residues are a potentially large source that can be converted to biogas through anaerobic digestion with residue nutrients returned to the field. Forests also require that some residues be left for soil fertility, erosion control and biodiversity. Approximately 70% of a tree's nutrients are found in the foliage, twigs, and fine roots, which should be left behind (Johansson et al., 1993).

Table 5.4 Potential recoverable biomass supplies (1018 joules per year)

Region

Crop residues

Forest residues

Dung

Dedicated crops

US/Canada

1.7

3.8

0.4

34.8

Europe

1.3

2.0

0.5

11.4

Japan

0.1

0.2

0

0.9

Australia & New Zealand

0.3

0.2

0.2

17.9

Former USSR

0.9

2.0

0.4

46.5

Latin America

2.4

1.2

0.9

51.4

Africa

0.7

1.2

0.7

52.9

China

1.9

0.9

0.6

16.3

Other Asia

3.2

2.2

1.4

33.4

Total

12.5

13.6

5.2

266.9

Source: Johansson et al. (1993).

Source: Johansson et al. (1993).

tained in the long term, even with best practices and extensive fertilizer use (Trainer, 1996). Yet others have estimated the long-term annual potential from biomass to be as high as 1300 exajoules per year (IPCC, 1996), significantly larger than the estimates of Table 5.4.

Figure 5.3 presents a supply curve for biomass resources in the United States that includes all forest, mill, and agricultural wastes, as well as dedicated energy crops (but excludes municipal solid wastes). The annual production cost of biomass increases with increasing production as less attractive (i.e. less fertile or less accessible to markets) resources are employed. The initial points on the curve of Figure 5.3 are associated with different forms of biomass wastes. The latter, more costly, points (above $2.00/GJ) are mostly associated with dedicated energy crops, grown explicitly for biomass energy purposes. Even these costs continue to rise with higher production levels as less attractive (e.g. less fertile, less accessible to markets) land is brought under production. The highest US production points in Figure 5.3 are considerably smaller than the estimates of potential shown in Table 5.4 for the United States. The discrepancy is partially due to the fact that the Table 5.4 entry includes Canada, but is also indicative of the differences between different reference sources. There is a high degree of uncertainty in all such estimates, both in terms of ultimate resource and in terms of the cost of that resource. In the United States, transportation costs can also limit the use of biomass resources when transport distances are more than 200 km, which is usually considered non-economic. Costs in other countries will depend on local economies, climate, and a host of other factors.

Exajoule/yr

Figure 5.3 US biomass resource costs (Source: Walsh, 1999).

5.2.4.4 Hydroelectricity

Hydropower currently contributes about 6% of the total world primary energy production, second only to biomass among the various renewable energy forms. Worldwide technical potential estimates range as high as 15000 TWh /yr (Johansson et al., 1993) with only about 14% of that developed to date. As shown in Table 5.5, the economic potential (the amount that is cost competitive) worldwide is about half the technical potential (accessible, but not necessarily cost competitive). But at 8300 TWh/yr, the economic potential is still two-thirds of the current world total electricity use of almost 13 000 TWh/yr.

While existing systems range in size from tens of kilowatts to more than 10000 Mwe, more than 95% of existing hydro capacity is larger than 10 MWe. Two thirds of the development has been in industrialized countries, leaving significant potential in the developing world for both large and small systems. Even in the United States, where a high fraction of the total potential has been developed, a recent DOE study estimated that there are 5677 sites with more than 30 GWe of capacity that can be developed today (INEEL, 2000).

Although many existing hydroelectric facilities have been located near load centers, the larger remaining sites are typically far from loads and must be connected via long-distance, high-voltage transmission lines. For distances greater than 500 km, the more economic option is high-voltage direct current (HVDC) lines with silicon-controlled rectifiers capable of converting high voltages and large currents from AC to DC power. Such HVDC lines are the basis for proposals to ship power from large Central African hydro projects on the Zaire

Table 5.5 World hydro* potential

Region

Economic potential (TWh/yr)

Already exploited (%)

Small hydro potential by 2020** (GWe)

Small hydro potential by 2020** (TWh/yr)

North America

801

72

12.9

59.2

Latin America

3281

12

6.6

27.1

Western Europe

641

63

21.7

90.7

Eastern Europe and CIS

1265

21

6.9

28.3

Mid East and North Africa

257

16

0.3

0.7

Sub-Saharan Africa

711

6

1.1

2.8

Pacific

172

22

0.3

1.3

Asia

1166

45

24.9

98.3

Total

8295

27

74.6

* Excludes pumped storage. ** Based on "favorable" case economic potential; technical potential is estimated to be twice as large. Source: World Energy Council WEC, (1993).

Notes:

* Excludes pumped storage. ** Based on "favorable" case economic potential; technical potential is estimated to be twice as large. Source: World Energy Council WEC, (1993).

River to loads 6500 km away in Europe. HVDC lines could also be employed for other remote renewable resources like wind.

The worldwide capacity factor for hydro is relatively low (40%) primarily because plants are often used to meet peak loads. Smaller systems are generally run-of-river systems with little or no reservoir storage. Thus, the availability of these systems depends more on the hydrologic cycle. When water is available, they operate more like base-load systems. A third type of hydroelectric system is pumped storage, which, as its name implies, provides for electricity storage. Pumped storage has no potential on its own to displace fossil fuels nor to reduce carbon emissions, but can be used to facilitate the use of intermittent renewable energy technologies.

5.2.4.5 Geothermal

Geothermal energy systems take advantage of the fact that the earth's crust is generally at higher temperatures than the surface temperatures. The crust temperature increases at an average rate of 30° to 35°C per kilometer as one descends from the surface. However, at the edge of the earth's crustal plates, where fluids flow from geysers, natural spas, hot gases, or volcanic lava, the

Figure 5.4 World geothermal resource areas.

temperature gradients are generally higher. In these "hot spots", the economics of extracting this natural heat are favorable enough to allow exploitation to a maximum depth of about 5 kilometers. Figure 5.4 shows the regions of the world where plate tectonics create the more favorable geothermal energy opportunities.

It is estimated that the theoretical amount of heat that could be captured within 5 km of the earth's surface is 140 x 106 exajoules. However, much of this is too low in temperature to be useful. Total useful resources have been estimated to be 5000 exajoules (Johansson et al., 1993), an amount roughly equal to 12 years of total world consumption of all energy forms. Annual long-term technical potential has been estimated to be greater than 20 exajoules/year (IPCC, 1996). Today's economic reserves may be only one-tenth as large as the total useful resources, but technology improvements and higher costs for future energy supplies will increase reserve estimates.

As shown in Table 5.6, geothermal resources have been employed in a number of applications ranging from low-temperature bathing (up to 50°C) to high-temperature (above 100oC) electric power generation. Worldwide, the total non-electric use of geothermal energy is almost 14 MWt.

The largest use of geothermal energy is for producing electricity. There is currently more than 8000 MWe of geothermal capacity throughout the world, including more than 2800 MWe in the United States, as shown in Table 5.7. All geothermal electricity generation comes from hydrothermal (hot water/steam) resources. With their continuous operational requirements and high availabilities, geothermal power plants provide largely baseload power.

Table 5.6 Direct geothermal heat use (MWt)

Region

Space and water heating

Greenhouses, aquaculture

Industry

Balneology

Other

Total

N. America

936

129

427

284

1776

W. Europe

1770

191

105

682

65

2813

E. Europe

119

761

70

719

446

2115

CIS

429

395

220

360

1404

Pacific

91

52

143

4501

153

4940

China

133

146

91

25

395

Others

122

115

44

179

86

546

Total

3600

1789

1100

6750

750

13989

Source: World Energy Council (1993).

Source: World Energy Council (1993).

Current power plants for generating electricity from hydrothermal resources can be divided into two general types: steam and binary. Steam plants are typically used with higher-quality, liquid-dominated resources to produce either a liquid or two-phase fluid, which is depressurized to generate steam. With lower-quality, liquid-dominated resources, it is usually more cost-effective and efficient to transfer heat from the geothermal fluid to a volatile working fluid. As a closed-cycle system, dual working fluid or binary plants release no emissions to the atmosphere.

Approximately 1000 MWe of capacity has been curtailed, largely in the United States, due mostly to resource depletion concerns

5.2.4.6 Ocean energy Ocean energy exists in the form of tides, waves, and thermal and salinity gradients. Each of these resources is extremely large. For example, the solar energy absorbed by the oceans each day is equivalent to 250 billion barrels of oil per day, or enough to meet more than a thousand times the world's current annual energy demands.

Tidal power: Total worldwide potential has been estimated to be 500-1000 TWh/year, but only a fraction of this is likely to be economic in the foreseeable future (Johansson et al., 1993). The distributed nature of this resource and its isolation from large load centers significantly increases the cost of its capture and reduces the size of the useful resource. Today, only the more mature tidal power option is being exploited using commercially available technology, and that at only a relatively modest number of sites, as shown in Table 5.8.

Table 5.7 Geothermalpower capacity by nation (MW)

Country

1990

1995

1998

Argentina

0.67

0.67

0.00

Australia

0.00

0.17

0.40

China

19.20

28.78

32.00

Costa Rica

0.00

55.00

120.00

El Salvador

95.00

105.00

105.00

France (Guadeloupe)

4.20

4.20

4.20

Greece

0.00

0.00

0.00

Guatemala

0.00

0.00

5.00

Iceland

44.60

49.40

140.00

Indonesia

144.75

309.75

589.50

Italy

545.00

631.70

768.50

Japan

214.60

413.70

530.00

Kenya

45.00

45.00

45.00

Mexico

700.00

753.00

743.00

New Zealand

283.20

286.00

345.00

Nicaragua

70.00

70.00

70.00

Philippines

891.00

1191.00

1848.00

Portugal (Azores)

3.00

5.00

11.00

Russia

11.00

11.00

11.00

Thailand

0.30

0.30

0.30

Turkey

20.40

20.40

20.40

United States

2774.60

2816.70

2850.00

Totals

5866.72

6796.98

8240.00

Source: www.demon.co.uk/geosci/wrtab.html, April 17, 2000.

Table 5.8 Existing tidal plants

Site

Installed capacity (Mwe) Date in service

La Rance, France

240.0

1966

Kislaya Guba, Russia

0.4

1968

Jiangxia, China

3.2

1980

Annapolis, Canada

17.8

1984

Various in China

1.8

Although entirely predictable, tidal power is intermittent, yielding one or two pulses of energy per tide. The tides occur every 12 hours and 25 minutes and thus move in and out of phase with peak loads. The result is a low annual plant capacity factor in the range of 25% to 35 % (WEC, 1993), with no particular correspondence to peak loads. Pumped storage is possible, but capital costs are high and throughput efficiency is in the range of 75%. Tidal energy technology is relatively mature, and dramatic cost reductions are not expected.

Wave energy: Ocean waves are created by the interaction of the wind with the sea surface. The winds blow most strongly in the latitudes between 40° and 60°, and the wave energy potential is highest in this region, with additional potential where regular tradewinds prevail (around 30°). Coasts exposed to prevailing winds with long fetches like those of the United Kingdom, the west coast of the United States, and the south coast of New Zealand have the greatest wave energy density. The wave power dissipated on such coastlines has been estimated to be in excess of 2 TW. Estimates of the development potential by 2020 range as high as 12 TWh/yr (WEC, 1993).

Many different wave-energy devices have been developed at the prototype scale. Some are buoyant structures that are moored at or near the sea surface; some are hinged structures that follow the contours of the waves; some are flexible bag devices that inflate with air with the surge of the waves; others enclose an oscillating water column that acts like a piston to pump air; and some are shaped channels that increase wave amplitude to drive a pump or a fill a land-based reservoir. Prototypes of such devices have been deployed around the world - Japan, Russia, China, Sweden, Denmark, the United States, Taiwan, the United Kingdom, Canada, Ireland, Portugal, and Norway. The technology is relatively immature and should continue to decrease in delivered energy costs, which generally exceed $0.10/kWh today. Given the intermittent nature of the resource, wave energy systems generally cannot be credited with firm capacity and operate at capacity factors below 40% (WEC, 1993).

Ocean thermal energy conversion (OTEC): The difference in temperature between solar-heated surface waters and cooler waters at 1000 m can exceed 20 oC in the tropics and subtropics. This temperature difference can be used to produce power through a thermodynamic cycle similar to that of a conventional generator, where the heat of the surface water is used instead of fossil fuel combustion, and the cooler deep water is used to condense the working fluid after evaporation. The relatively small temperature difference between the surface and deep water implies a maximum theoretical efficiency of only 6.8% at 20°C and 9% at 27°C. In practice, throughput efficiencies are closer to

www.nrel.gov/otec/what.html)."/>
Figure 5.5 Ocean thermal energy conversion resource (Source: www.nrel.gov/otec/what.html).

3%-4% (WEC, 1993). OTEC is a firm source of power that can also provide additional benefits in the form of mariculture and desalinated water supplies. Current costs are in the range of $0.12/kWh (US). OTEC costs are inherently high and are not expected to decline substantially because of the low efficiency associated with the relatively small temperature differences found in the ocean and the consequent large volumes of water that must be handled to produce power.

OTEC is the largest of the ocean energy resources. The area for possible exploitation is 60 million km2. However, as shown in Figure 5.5, most of this area is offshore potential and inaccessible for power production. Estimates of the potential developable by 2020 have ranged as high as 168 TWh/yr (WEC, 1993), but are likely to be much smaller.

5.2.4.7 Total renewable resources

Table 5.9 summarizes the above renewable resources discussion by placing the potential estimates for each form of renewable energy in similar units. Table 5.9 reinforces the well-known fact that total renewable energy resources are formidable, especially for solar and biomass. It also shows a relatively small potential for geothermal and ocean resources to address global climate change.

The long-term technical potential of renewable energy, 450 X 103TWhe/yr, is massive. This number is more than 10 times global primary energy consumption in 1996 (EIA, 1999, p. 142). The economic potential by the year 2025, 14 to 24 103TWhe/yr, is impressive as well. The high end of this range is about one-

Table 5.9 Summary of renewable energy resources (103 TWhe/year)

1990

2025 economic

Long-term

Annual total

Resource

consumption

potential

technical potential

resource flow

Hydro

2

4-6

>14

>40

Geothermal

<1

<1

>2

>85

Wind

<1

1

>14

>21 000

Ocean

<1

>2

>32

Solar

2

>280

>300000

Biomass

6

8-15

>140

Total

8

14-24

>450

>300000

Source: converted from IPCC (1996).

Source: converted from IPCC (1996).

third of the projected global primary energy consumption in 2020 (EIA, 1999a, p. 142).

Clearly, renewable energy has vast potential. What Table 5.9 does not show is that the resources are dispersed and economic access to them can be constrained. The factors that constrain such access will be discussed in the next section.

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