Author
InterAcademies Council
Lighting the Way: Toward a Sustainable Energy Future
Research Contractor
InterAcademies Council
Release Date
October 1, 2007
Copyright
2007

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  • 3.3 Non-biomass renewables

    Renewable sources of energy—biomass, wind, solar, hydropower, geothermal, and ocean energy—have helped to meet humanity’s energy needs for millennia.43 Expanding the energy contribution from modern renewable technologies can help to advance important sustainability objectives and is widely considered desirable for several reasons:

    Environmental and public health benefits. In most cases, modern renewable energy technologies generate far lower (or near-zero) emissions of greenhouse gases and conventional air pollutants compared to fossil-fuel alternatives;44 other benefits may involve reduced water and waste-disposal requirements, as well as avoided impacts from mining and drilling.

      Energy security benefits. Renewable resources reduce exposure to supply shortages and price volatility in conventional-fuel markets; they also offer a means for many countries to diversify their fuel supplies and reduce dependence on non-domestic sources of energy, including dependence on imported oil.
      Development and economic benefits. The fact that many renewable technologies can be deployed incrementally in small-scale and standalone applications makes them well-suited to developing country contexts where an urgent need exists to extend access to energy services in rural areas; also, greater reliance on indigenous renewable resources can reduce transfer payments for imported energy and stimulate job creation.

    As with all energy supply options, renewable energy technologies also have drawbacks, many of them related to the fact that the resource being tapped (e.g., wind or sunlight) is diffuse and typically has low power density. A first issue, obviously, is cost—in particular, cost relative to conventional resource options with and without price signals to internalize climate impacts. Without price signals, many renewable energy options remain more costly than the conventional alternatives at present (although some technologies—such as wind—are rapidly approaching or have already achieved commercial competitiveness in some settings).

    The diffuse nature of many renewable resources also means that large-scale efforts to develop their energy potential typically require more land (or water) area than conventional energy development. As a result, impacts on wildlife, natural habitats, and scenic vistas can become a significant issue for some projects. In the case of large hydropower developments, additional concerns may include impacts on human settlements and the potential for offsetting methane and carbon dioxide emissions. In many cases, concerns about land or ecosystem impacts can be addressed through appropriate siting, technology modifications, or other measures; in addition promising opportunities exist to deploy some renewable technologies in decentralized applications (e.g., rooftop solar panels).

    The remainder of this section focuses on non-biomass renewable energy options. (Modern biomass technologies are discussed separately in the next section). In the near to medium term, these resources have the potential to compete with conventional fuels in four distinct markets: power generation, hot water and space heating, transportation, and rural (off-grid) energy.

    Renewable resource contribution
    At present, the contribution from small hydropower, wind, and other non-biomass energy resources remains relatively small, accounting for only 1.7 percent of total primary energy production on a global basis in 2005.45 Recent years, however, have seen explosive growth in several key renewable industries. Table 3.4 shows average annual energy production and production growth rates for different modern renewable technologies for 2001–2005.46 In average, the contribution of modern renewables to the total primary energy supply (TPES) increased by approximately 11.5 percent per year, over the period 2001–2005. Figure 3.7 shows the projected contribution of modern renewables, including biomass, to the total primary energy supply in 2010 and 2020 based on a continued growth of 11.5 percent per year.

    Increasingly common in many countries, government policies—typically motivated by climate-change and energy-security concerns—have played an important role in spurring recent renewable-energy investments.47 Currently, at least 45 countries, including 14 developing countries, have adopted various policies—often in combination—to promote renewable energy (REN21, 2006 and 2005). Chief examples include investment or production tax credits; ‘feed-in’ tariffs (that require utilities to pay a certain minimum amount for renewable power supplied to the grid); portfolio standards or targets (that establish a specific share of

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

    Sources: UNDP, UNDESA, and WEC, 2000 and 2004; REN21, 2006; and IEA, 2006.

    energy or electricity supply to be provided using renewable resources);48 and grants, loans, or other forms of direct support for research, development, demonstration, and early deployment efforts. For example, in March 2007, the member states of the European Union agreed to adopt, as a binding target, the goal of meeting 20 percent of all EU energy needs from renewable sources by 2020. China has adopted a goal of 10 percent renewable electric-generating capacity by 2010 (excluding large hydropower) and 10 percent primary energy from renewables within the same timeframe (Table 3.5).

     

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    Figure 3.7 Modern renewables projections for 2010 and 2020

    Note:
    Projections of modern renewables (including small hydro, excluding large) based on 11.5 percent growth per year, over the period 2001-2005.

    Sources: UNDP, UNDESA, and WEC, 2000 and 2004; REN21, 2006; And IEA, 2006

    Additional incentives or targets and other policies to promote renewable energy are increasingly also being adopted at the state and municipal level. Current research and development spending on renewable technologies by the United States and Europe now totals more than US$700 million per year; in addition, roughly half a billion dollars per year are being directed to renewable energy projects in developing countries.49 Recent developments in the business world reflect the growing enthusiasm for renewable energy: large commercial banks have begun to ‘mainstream’ renewable energy investments in their lending portfolios, and several major corporations have recently made substantial investments or acquisitions in renewable energy enterprises. The 60 leading, publicly traded renewable energy companies, or divisions of companies, now have a combined market capitalization of US$25 billion and new organizations

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

     

    Note: Data updated with new EU-targets. The table presents targets as adopted by different governments. No attempt is made to convert these targets to a single, readily-compared metric, such as electricity production, capacity, share of generation, or share of capacity. The EU decided on its target in Spring 2007; EU member states are expected to elaborate on country-specific policies and regulations.

    Source: REN21, 2005.

    are emerging to facilitate renewable energy investments through specialized networking, information exchange, market research, training, financing, and other assistance (REN21, 2006).

    Current trends are encouraging, but most of the anticipated growth in renewable energy capacity remains concentrated in a handful (five or six) of countries. The challenge is to sustain healthy growth rates in countries that already have ambitious renewable energy commitments and to initiate similar deployment efforts in more countries around the world. That challenge has important institutional and social dimensions, as well as technological and economic ones. Capacity building, for example, has emerged as a crucial issue for the maintenance of modern renewable energy infrastructure in the developing world. Many well-intended renewable energy projects funded by international agencies or foreign governments have failed because of a lack of attention to the concomitant need for competent technicians and managers to maintain these systems. Other factors that have contributed to a disappointing success rate for renewable energy projects in Africa include lack of suitable policies, lack of involvement by target groups, lack of commitment to maintain projects by the governments of host countries, and lack of coordination between donors.

    Issues and hurdles of non-biomass options
    Various issues and market hurdles apply to each of the chief non-biomass ‘new’ renewable energy options: wind, solar photovoltaic (PV), solar thermal, small hydropower, and geothermal. For each energy option, policymakers confront a similar set of questions:

      Is the available technology adequate—in theory and in practice—to support growing demand?
      Are there aspects of the resource—such as the intermittent nature of wind and sunlight—that currently limit its role in the marketplace?
      Can the technology compete economically with other options in an emissions-constrained world (taking into account current subsidies for conventional and unconventional, resources as well as costs and benefits that are currently not internalized in market prices)?
      How can other barriers, including siting issues, market or regulatory barriers, infrastructure constraints, and other barriers be overcome?

    While the specifics of these questions vary for different technologies and resources, several general points are worth noting before proceeding to a more detailed discussion of the different options.

    Resource adequacy is generally not an issue, although some parts of the world hold more promise for certain renewable technologies than others. The rate at which sunlight is absorbed by the Earth is roughly 10,000 times greater than the rate at which human beings use commercial energy of all kinds. Even when practical limitations are factored in, the remaining renewable resource base remains enormous. A recent analysis commissioned for this report suggests that if one considers only those onshore areas that are already economic for commercially available wind turbines (i.e., areas with Class 5 or better winds) and one applies a 90 percent exclusion factor (i.e., one assumes that only 10 percent of these areas are available due to competing land uses or for other reasons), remaining wind energy potential is still theoretically sufficient to supply 100 percent of current global electricity consumption and as much as 60 percent of projected global consumption for 2025 (Greenblatt, 2005).

    The challenges for renewable energy technologies, therefore, are primarily technological and economic: how to capture the energy from dispersed resources that typically have low power-density compared to fossil or nuclear fuels and deliver that energy where it is needed and when it is needed at reasonable cost. Significant cost reductions have been achieved in solar and wind technologies over the past decade, but as a means of generating electricity these options generally remain more expensive per kilowatt-hour of output than their conventional competitors. Other deployment hurdles derive from the nature of the resource itself. Wind and solar energy, because they are intermittent and not available on demand, present challenges in terms of being integrated into electricity supply grids, which must respond instantaneously to changing loads. Intermittency imposes costs on electric power systems—costs that may be substantial at foreseeable levels of wind and solar deployment.

    To address this issue, large-scale improvements to transmission infrastructure, the addition of more responsive conventional generation and possibly energy storage technologies may enable wind power to supply more than 30 percent of electric generation while keeping intermittency costs below a few cents per kilowatt-hour (DeCarolis and Keith, 2005; 2006). The development of cost-effective storage options, in particular, should be a priority for future research and development since success in this area could significantly affect the cost of intermittent renewable resources and the magnitude of their contribution to long-term energy supplies. Potential storage options include added thermal capacity, pumped hydro or compressed air energy storage, and eventually hydrogen. Large hydropower has the advantage that it is not intermittent and is already quite cost-competitive, but the potential for new development in many areas is likely to be constrained by concerns about adverse impacts on natural habitats and human settlements.

    WIND

    With installed capacity increasing by an average of 30 percent per year since 1992, wind power is among the fastest growing renewable energy technologies and accounts for the largest share of renewable electricity-generating capacity added in recent years. In 2006 alone, 15.2 gigawatts of new wind capacity (representing a capital investment of more than US$24 billion) was added worldwide, bringing total installed wind capacity to 59 gigawatts (GWEC, 2006). Leading countries for wind development are

    Germany (18.4 gigawatts total), Spain (10 gigawatts), the United States (9.1 gigawatts), India (4.4 gigawatts), and Denmark (3.1 gigawatts). This impressive progress is due in large part to continuing cost reductions (capital costs for wind energy declined more than 50 percent between 1992 and 2001) and strong government incentives in some countries (Juninger and Faaij, 2003). Over time, wind turbines have become larger and taller: the average capacity of individual turbines installed in 2004 was 1.25 megawatts, double the average size of the existing capacity base (BP, 2005).

    A simple extrapolation of current trends—that is without taking into account new policy interventions—suggests that wind capacity will continue to grow robustly. The IEA (2004) World Energy Outlook reference case forecast for 2030 includes 328 gigawatts of global wind capacity and 929 terawatt-hours of total wind generation, a more than five-fold increase of the current capacity base. Renewable energy advocates have put forward far more aggressive scenarios for future wind deployment: the European Renewable Energy Council’s Advanced International Policies Scenario, for example has wind generation increasing to 6,000 terawatt-hours by 2030 and 8,000 terawatt-hours by 2040.50 Overall, the potential wind resource is vast though not distributed evenly around the globe. Based on available surveys, North America and a large part of the Western European coast have the most abundant resources, whereas the resource base in Asia is considerably smaller, with the possible exception of certain areas such as Inner Mongolia where the wind potential may be in excess of 200 gigawatts. Looking beyond the continental scale, wind resources in North America are concentrated in the middle of the continent, while Europe’s best resources are found along the Western coast and in Russia and Siberia. Further study is needed to assess the resource base in Africa where it appears that wind resources may be concentrated in a few areas on the northern and southern edges of the continent.

    Intermittency is a significant issue for wind energy: wind speeds are highly variable, and power output drops off rapidly as wind speed declines. As a result, turbines produce, on average, much less electricity than their maximum rated capacity. Typical capacity factors (the ratio of actual output to rated capacity) range from 25 percent on-shore to 40 percent off-shore depending on both wind and turbine characteristics. At current levels of penetration, wind’s intermittency is generally readily manageable: grid operators can adjust output from other generators to compensate when necessary. In these situations grid operators treat wind parks much like ‘negative loads’ (Kelly and Weinberg, 1993; DeCarolis and Keith, 2005). Longer-term, as wind penetration expands to significantly higher levels (e.g., in excess of 20 percent of total grid capacity), the intermittency issue may become more significant and may require some combination of innovative grid management techniques, improved grid integration, dispatchable back-up resources, and cost-effective energy storage technologies.51 Obviously, some of these options—such as back-up capacity and energy storage—would add to the marginal cost of wind power. In addition, new investments in transmission capacity and improvements in transmission technology that would allow for cost-effective transport of electricity over long distances using, for example, high-voltage direct current lines would allow for grid integration over much larger geographic areas and could play a crucial role in overcoming intermittency concerns while expanding access to remote but otherwise promising resource areas.52

    Meanwhile, as has already been noted, options for low-cost energy storage on the scale and over the timeframes required (i.e., multiple hours or days) merit further exploration. Potential storage options for wind and other intermittent renewable resources include pumped hydroelectric storage, compressed air energy storage, and hydrogen. Pumped hydro requires two reservoirs of water at different heights, whereas compressed air storage—in the two commercial projects of this type that exist to date—has entailed using a large underground cavern. Compressed air storage may also be feasible in more ubiquitous underground aquifers. While pumped hydro may be preferable when a source of elevated water storage is nearby, compressed air storage can be sited where there is suitable underground geology. It is worth noting, however, that compressed air must be heated in some way before it can be directly used in an air turbine; hence the usual assumption is that compressed air storage would be integrated with a gas turbine. Longer term, hydrogen may provide another promising storage option for intermittent renewables. When wind or solar energy is available, it could be used to produce hydrogen, which could in turn be used for a variety of applications—including for electricity production, as a primary fuel source, or in fuel cells—once appropriate distribution infrastructure and end-use technologies are developed.53

    Longer term, other innovations have been suggested that could further improve wind’s competitive position. Potential R&D frontiers include ‘derating’ techniques that allow turbines to operate at lower wind speeds (thereby reducing capital costs and energy storage requirements); specialized turbines and other infrastructure to access deep offshore resources; or even systems designed to capture the vast wind resources that exist in the free troposphere, several kilometers above the earth’s surface.

    SOLAR PHOTOVOLTAIC

    Solar PV technologies use semiconductors to convert light photons directly into electricity. As with wind, installed capacity has increased rapidly over the last decade; grid-connected solar PV capacity grew on average more than 60 percent per year from 2000 to 2004. This growth started from a small base however. Total installed capacity was just 2.0 gigawatts worldwide by the end of 2004; it grew to 3.1 gigawatts by the end of 2005 (REN21, 2006). Solar PV has long had an important niche, however, in off-grid applications providing power in areas without access to an existing electricity grid. Until recently, solar PV has been concentrated in Japan, Germany, and the United States where it is supported by various incentives and policies. Together, these countries account for over 85 percent of installed solar PV capacity in the OECD countries (BP, 2005). Solar PV is also expected to expand rapidly in China where installed capacity—currently at approximately 100 megawatts—is set to increase to 300 megawatts in 2010 (NDRC, 2006). Increasingly, solar PV is being used in integrated applications where PV modules are incorporated in the roofs and facades of buildings and connected to the grid so that they can flow excess power back into the system.

    Estimates of solar energy’s future contribution vary widely and, as with all projections or forecasts, depend heavily on policy and cost assumptions. As with wind, the potential resource base is large and widely distributed around the world, though prospects are obviously better in some countries than in others. To the extent that PV modules can be integrated into the built environment, some of the siting challenges associated with other generating technologies are avoided. The main barrier to this technology in grid-connected applications remains high cost. Solar PV costs vary depending on the quality of the solar resource and module used, but they are typically higher than the cost for conventional power generation and substantially higher than current costs for wind generation.

    Another significant issue, as with other renewable options like wind, is intermittency. Different economic and reliability parameters apply in non-grid applications where solar photovoltaic is often less costly than the alternatives, especially where the alternatives would require substantial grid investments.

    Achieving further reductions in the cost of solar power will likely require additional technology improvements and may eventually involve novel new technologies (such as die-sensitized solar cells).54 Near-term cost-reduction opportunities include improving cell production technology, developing thin-film technologies that reduce the amount of semiconductor material needed, designing systems that use concentrated solar light, and substituting more efficient semiconductors for silicon. In the mid- to longer-term future, ambitious proposals have been put forward to construct megawatt-scale solar PV plants in desert areas and transmit the energy by high voltage transmission lines or hydrogen pipelines.55  Even more futuristic concepts have been suggested. Meanwhile, solar photovoltaic is likely to continue to have important near-term potential in dispersed, ‘distributed generation’ applications, including as an integral part of building envelope design and as an alternative to other non-grid-connected options (like diesel generators) in rural areas.

    SOLAR THERMAL

    Solar thermal technologies can be used to provide space conditioning (both heating and cooling) in buildings, to heat water, or to produce electricity and fuels. The most promising opportunities at present are in dispersed, small-scale applications, typically to provide hot water and space heating directly to households and businesses. Solar thermal energy can be effectively captured using ‘passive’ architectural features such as sunfacing glazing, wall- or roof-mounted solar air collectors, double-façade wall construction, air-flow windows, thermally massive walls behind glazing, or preheating of ventilation through buried pipes. It can also be used as a direct source of light and ventilation by deploying simple devices that can concentrate and direct sunlight even deep inside a building and by exploiting pressure differences that are created between different parts of a building when the sun shines. In combination with highly efficient, endues energy systems, as much as 50–75 percent of the total energy needs of buildings as constructed under normal practice can typically be eliminated or satisfied using passive solar means.

    Active solar thermal systems can supply heat for domestic hot water in commercial and residential buildings, as well as for crop drying, industrial processes, and desalination. The main collector technologies—generally considered mature but continue to improve—include flat panels and evacuated tubes. Today, active solar thermal technology is primarily used for water heating: worldwide, an estimated 40 million households (about 2.5 percent of total households) use solar hot water systems. Major markets for this technology are in China, Europe, Israel, Turkey, and Japan, with China alone accounting for 60 percent of installed capacity worldwide.56 Active systems to provide space heating are increasingly being deployed in a number of countries, notably in Europe. Costs for solar thermal hot water, space heating, and combined systems vary with system configuration and location. Depending on the size of panels and storage tanks, and on the building envelope, it has been estimated that 10–60 percent of combined household hot water and heating loads can be met using solar thermal energy, even at central and northern European locations.

    At present, solar thermal energy is primarly used for water heating. Technologies also exist, however, to directly use solar thermal energy for cooling and dehumidification. Cost remains a significant impediment, though cost performance can sometimes be improved by combination systems that provide both summer cooling and winter heating. Simulations of a prototype indirect-direct evaporative cooler in California indicate savings in annual cooling energy use in excess of 90 percent. Savings would be less in a more humid climate, though they can be enhanced using solar-regenerated liquid desiccants. Finally, systems that actively collect and store solar thermal energy can be designed to provide district heating and cooling to multiple buildings at once; such systems are already being demonstrated in Europe—the largest of them, in Denmark, involves 1,300 houses.

    A number of technologies also exist for concentrating solar thermal energy to supply industrial process heat and to generate electricity. Typically, parabolic troughs, towers, or solar-tracking dishes are used to concentrate sunlight to a high energy density; the concentrated thermal energy is then absorbed by some material surface and used to operate a conventional power cycle (such as a Rankin engine or low-temperature steam turbine). Concentrating solar thermal electricity technologies work best in areas of high direct solar radiation and offer advantages in terms of built-in thermal energy storage.

    Until recently, the market for these technologies has been stagnant with little new development since the early 1990s when a 350-megawatt facility was constructed in California using favorable tax credits. The last few years have witnessed a resurgence of interest in solar-thermal electric power generation, however, with demonstration projects now underway or proposed in Israel, Spain, and the United States and in some developing countries. The technology is also attracting significant new investments of venture capital. Longer term, the potential exists to further improve on existing methods for concentrating solar thermal power, particularly with respect to less mature dish and mirror/tower tracking technologies. Methods of producing hydrogen and other fuels (e.g., solar-assisted steam gasification of coal or other solid fuels) and other means of utilizing dilute forms of solar heat (e.g., evacuated tube collectors, solar ponds, solar chimneys, and use of ocean thermal energy) are also being investigated.

    HYDROPOWER

    Hydroelectricity remains the most developed renewable resource worldwide: it now accounts for most (85 percent) of renewable electricity production and is one of the lowest-cost generating technologies available.

    Worldwide, large hydropower capacity totaled some 772 gigawatts in 2004 and accounted for approximately 16 percent of total electricity production, which translated to 2,809 terawatt-hours out of a total 17,408 terawatthours in 2004 (IEA, 2006).

    As with other renewable resources, the theoretical potential of hydropower is enormous, on the order of 40,000 terawatt-hours per year (World Atlas, 1998). Taking into account engineering and economic criteria, the estimated technical potential is smaller but still substantial at roughly 14,000 terawatt-hours per year (or more than 4 times current production levels). Economic potential, which takes into account societal and environmental constraints, is the most difficult to estimate since it is strongly affected by societal preferences that are inherently uncertain and difficult to predict. Assuming that, on average, 40 to 60 percent of a region’s technical potential can be utilized suggests a global economic hydro-electricity potential of 7,000–9,000 terawatt-hours per year.

    In Western Europe and the United States, approximately 65 percent and 76 percent, respectively, of technical hydroelectricity potential has been developed, a total that reflects societal and environmental constraints. For many developing countries, the total technical potential, based on simplified engineering and economic criteria with few environmental considerations, has not been fully measured while economic potential remains even more uncertain. Current forecasts anticipate continued growth in hydropower production, especially in the developing world where large capacity additions are planned, mostly in non-OECD Asian countries. Elsewhere, concerns about public acceptance (including concerns about the risk of dam breaks); environmental impacts (including habitat loss as well as the potential for carbon dioxide and methane emissions from large dams, especially in tropical settings); susceptibility to drought; resettlement impacts; and availability of sites are prompting a greater focus on small hydro resources. In 2000, a report issued by the World Commission on Dams identified issues concerning future dam development (for both energy and irrigation purposes) and emphasized the need for a more participatory approach to future resource management decisions (WDC, 2000).

    Today, worldwide installed small hydro capacity exceeds 60 gigawatts with most of that capacity (more than 13 gigawatts) in China.57 Other countries with active efforts to develop small hydro resources include Australia, Canada, India, Nepal, and New Zealand. Small hydro projects are often used in autonomous (not grid-connected) applications to provide power at the village level in lieu of diesel generators or other small-scale power plants. This makes them well suited for rural populations, especially in developing countries. Worldwide, the small hydro resource base is quite large, since the technology can be applied in a wide range of streams. In addition, necessary capital investment is usually manageable, the construction cycle is short, and modern plants are highly automated and do not need permanent operational personnel. The primary barriers are therefore social and economic rather than technical. Recent R&D efforts have focused on incorporating new technology and operating methods and further minimizing impacts on fish populations and other water uses.

    GEOTHERMAL

    Geothermal energy lying below the earth’s surface has long been mined as a source of direct heat and, within the last century, to generate electricity.58 Geothermal electricity production is generally practical only where underground steam or water exists at temperatures greater than 100 degrees Celcius ; at lower temperatures (50–100 degrees Celcius) geothermal energy can be used for direct heat applications (e.g., greenhouse and space heating, hot water supply, absorption cooling). A different kind of application altogether involves heat pumps that effectively use the earth as a storage medium. Ground-source heat pumps take advantage of the relatively stable temperatures that exist below ground as a source of heat in the winter and as a sink for heat in the summer; they can provide heating and cooling more efficiently than conventional space-conditioning technologies or air-source heat pumps in many parts of the world.

    Global geothermal electric-generating capacity is approximately 9 gigawatts, most of it concentrated in Italy, Japan, New Zealand, and the United States. The potential for further geothermal development using current technology is limited by available sites, but the available resource base could be significantly affected by improved technologies.59 The hottest hydrothermal fields are found at the Pacific Ocean rim, in some regions of the Mediterranean, and in the Indian Ocean basin. Worldwide, more than 100 hydrothermal fields are thought to exist at rather shallow depths of 1–2 kilometers with fluid temperatures high enough to be suitable for power production. According to the IEA (2006) World Energy Outlook reference case, geothermal power capacity and production can be expected to grow to 25 gigawatts and 174 terawatt-hours, respectively by 2030, accounting for the roughly 9 percent of the total new renewable contribution. Technology improvements that would reduce drilling costs and enable access to geothermal resources at greater depths could substantially expand the resource base. In addition, technologies that could draw heat from dry rocks instead of relying on hot water or steam would significantly increase geothermal potential. Such technologies are not yet developed but are being explored in Europe. An existing EU research program, for example, is pursuing the use of hot dry rock geothermal energy for power production (EEIG, 2007).

    The potential resource base for direct-heat applications of geothermal energy is much larger. In fact, direct-heat utilization nearly doubled from 2000 to 2005, with 13 gigawatt-thermal added over this time period and at least 13 countries using geothermal heat for the first time. Iceland leads the world in existing direct-heat capacity, supplying some 85 percent of its overall space heating needs using geothermal energy, but other countries—notably Turkey—have substantially expanded their use of this resource in recent years. About half of current global capacity is in the form of geothermal or ‘ground source’ heat pumps, with some 2 million units installed in over 30 countries worldwide (mostly in Europe and the United States).

    In summary: Non-biomass renewable options
    In the future, continued improvement in energy conversion, storage, and transmission technologies could further improve the cost-competitiveness of renewable energy options, help to address the reliability concerns that may arise at higher levels of penetration, and expand the number of sites that are suitable for renewable energy development. Ensuring that progress continues at the rate needed to support a major role for renewable energy resources within the first half of this century will require, however, that governments worldwide maintain a strong commitment to implementing policies and funding investments that will accelerate the development and deployment of renewable technologies. Meaningful carbon constraints, especially in industrialized countries, are clearly part of the picture and will be essential in creating opportunities for new renewable alternatives to compete with the conventional technologies that currently dominate world energy markets.

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