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The current trend is towards taller, more powerful, wind machines. Energy output is greater with taller machines because the energy available rises very quickly with wind speeds (as the cube of the wind speed), and wind speeds generally increase by about 60% for every doubling in tower height. Furthermore, the longer rotors possible on a taller tower greatly increase the area swept by the rotor (square of the rotor length) and, consequently, the energy captured. The advantages of higher towers were sought in the early 1980s through the US-government-sponsored "MOD" series, with MOD -5B machines as large as 3.2 MW with rotors 97 meters in diameter. However, mechanical difficulties with the turbines, support structures, and especially the turbine blades led to a new generation of smaller machines in the 200-400 kW range. Subsequent advances in blade design and variable-speed turbines are now opening new opportunities for machines as large as 1 MW or greater. More flexible blades are also allowing renewed consideration of machines with rotors downwind of the generator and tower. If these can withstand the turbulence introduced by the tower structure, they may allow even larger machines. Other advances being investigated include improved controls and better wind forecasting. These improvements are forecast to bring costs down below $0.03/kWh for the better wind sites.

5.2.2.3 Ethanol from biomass

Currently, fermentation processes are being used to produce ethanol from sugar and starch feedstocks, such as sugar cane in Brazil and corn in the United States. However, in the last few years, hydrolysis processes have been developed with acids and newly discovered enzymes that break down cellu-losic material of plants like poplars and switchgrass into fermentable sugars. A simultaneous saccharification and fermentation process has been developed allowing hydrolysis of both cellulosic and hemicellulosic material. Refinements of this process are currently being pursued that should allow high-productivity conversion of the cellulosic material found in fast-growing short-rotation woody crops and herbaceous switchgrasses. R&D is also under way to improve the production rates for these dedicated energy crops. These include breeding and genetic manipulation for resistance to pests and diseases and for higher yields; efficient propagation, harvesting and handling methods; and improved nutrient management systems. Production has already increased to 10 dry tons per acre (Cassedy, 2000) with cost goals of $35/ton. Although commercial plants are not yet in operation for producing ethanol from cellulosic materials, estimates of production costs today are in the range of $1.05-$1.55 per gallon. Early goals were to achieve costs of $0.67/gallon (Johansson et al., 1993), which would be competitive with gasoline at about $1.00 per gallon.1

5.2.3 Market opportunities

Not only are there many renewable energy sub-technologies, as shown in Table 5.2, but there are a host of different applications for renewables, each with different costs and values. Table 5.3 shows some of the major markets that renewables can address. Some of these opportunities are still in the laboratory stage, e.g. biofuels, while others have already largely saturated their markets, e.g. large hydro and biomass cooking. However, even the saturated markets present new opportunities. For example, research on more fish-friendly hydro may expand the market for this mature technology. Subsidized corn-to-ethanol biofuels are already pervasive in the United States while research continues on ethanol production from cellulosic material.

The point of Table 5.3 is that many market opportunities exist, and for renewable energy to make its maximum possible contribution to the reduction of greenhouse gases, a number of market and technology issues will have to be

1 In a properly designed engine, a gallon of ethanol will yield about 80% of the driving range of a gallon of gasoline, due primarily to the lower energy content of ethanol.

Table 5.3 Market opportunities

Technology Market/technology Market size Value/competitiveness

Table 5.3 Market opportunities

Technology Market/technology Market size Value/competitiveness

Photovoltaics

Consumer electronics

Small

High value

Cathodic protection

Small

High value

Remote water pumping

Small

High value

Village power

Large

Battery and diesel competitors are expensive

Distributed on-grid generation

Large

Provides additional value beyond energy

Central-station power

Large

Lower value

Wind

Village power

Large

Battery and diesel competitors are expensive

Distributed on-grid generation

Small, limited sites

Provides additional value beyond energy

Central-station power

Large

Competitive at best resource sites

Biomass power

Forest/agricultural industry cogeneration

Moderate

Highly competitive; almost 6 GW of wood and pulp liquor power plants exist in the US today

Village power

Large

Battery and diesel competitors are expensive

Industrial co-products

Moderate

High-value, non-energy co-products can make it competitive

Cofiring of coal plants

Small

Economics depend on proximity of quality resource to the coal plants

Central power

Large

Requires improvements in biogasifiers

Landfill gas

Industrial cogeneration

Small

Limited by co-location requirements for landfill and industry

Power only

Small

High value in reducing emissions of methane, a greenhouse gas

Hydro

Large hydro

Large

Environmental issues preclude most additional development

Small-scale hydro

Moderate

Highly competitive, but sites limited

Geothermal Geothermal heat pumps for Moderate space heating/cooling

Industrial process heat Small

Industrial cogeneration Small

Central hydrothermal Small

Concentrating solar Water and process heat with Moderate power troughs

Distributed generation with Small dishes

Central power with troughs Moderate and power towers

Active solar heating Pool heating Small

Cooking Moderate

Hot water heating Large

Space heating Large

Passive solar Space heating Large

Biomass heat Cooking Moderate

Space and water heat Moderate

Biofuels

All transportation fuel needs Large

Competitive where both cooling and heating space loads exist

Limited by resource/industry co-location Limited by resource/industry co-location and temperature requirements Resource limited; economics very site specific

Requires direct sunlight for concentration; high materials cost

Reliability remains an issue; costs of collector and engines still high

Further cost reductions are required

Competitive in non-freezing climates; extends swimming season

Sunlight availability is an issue

Competitive with electric heating, especially in freezing climates

Shell improvements and high-efficiency gas furnaces are currently more economic

Solar tempering combined with shell improvements are effective

Resource depletion is an issue; decreasing with urbanization

Resource depletion is an issue; decreasing with urbanization

Further R&D on crop engineering and conversion process required.

addressed. In many cases, these multiple market opportunities are essential to building up renewable energy markets with initial niche markets, thereby enabling productivity gains, which lead to lower costs and entry to larger bulk markets. In Table 5.3, the more mature niche markets for each technology are listed first, with the less easily penetrated markets near the bottom. Future penetration of these markets will depend heavily on the realization of the anticipated cost/performance improvements summarized in Table 5.1. However, even the mature and stalled markets could expand significantly were heavy reductions in carbon emissions mandated worldwide.

5.2.4 Resources

Unlike fossil fuels that can be mined, pumped, or otherwise collected and transported to their point of use, renewables generally must be consumed at or near the point of the resource or converted there to a transportable form of energy like electricity, gas, or liquid fuel. Fortunately, renewable resources are fairly ubiquitous. Resources like solar radiation, wind, and biomass exist worldwide at varying concentrations. Even geothermal and ocean energy are fairly widely distributed, though more localized than wind and solar.

Not only are renewable resources spread around the globe, they exist in huge, albeit dispersed, quantities. For example, it has been estimated that enough solar radiation reaches the earth every 45 seconds to meet the world's current electricity demand for a year. Similarly, there is enough wind in the state of North Dakota to meet 35% of all current US electricity demand. While such estimates are interesting as upper bounds, they say little about the cost, accessibility, or spatial and temporal variability of these vast resources. We discuss such issues for each of the major renewable resources in the paragraphs that follow. More specific regional resource data is available in the subsequent section on "Regional perspectives".

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