In This Issue
Spring Issue of The Bridge on Emerging Issues in Earth Resources Engineering
April 14, 2014 Volume 44 Issue 1

Geothermal Energy: An Emerging Option for Heat and Power

Friday, April 18, 2014

Author: Roland N. Horne and Jefferson W. Tester

Geothermal energy could be called an “underground” renewable energy source not only because of its physical origin but also because its importance remains largely unknown to many decision makers and members of the public. Although historically high oil prices in the early 1980s stimulated a substantial expansion of geothermal capacity, there followed a period of very slow growth for almost 20 years. The past several years, however, have been a boom time for international geothermal energy development, with substantial interest and activity in the United States and many other countries.

Background

The geothermal resource base consists of thermal energy stored in the Earth’s crust to a prescribed depth—usually taken to be about 10 km, which is accessible using current drilling technologies. Heat is produced in the form of steam or hot water at the surface from wells that tap into permeable and porous regions of hot rock.

Geothermal energy development uses the heat of the Earth either directly or by conversion to electricity. The energy can be used directly to provide heating (e.g., for district heating, industrial process heat, agriculture, aquaculture, and balneology and spas) or to generate electricity in a power cycle. Electric power is generated from hydrothermal systems using two types of systems: open cycle dry steam (or flash) plants that make use of the natural produced fluids extracting power in a turbine, and binary plants with a closed Rankine cycle that uses an organic working fluid to exchange heat with the geothermal fluid before being expanded in a turbine. In addition, geothermal heat pumps draw on thermal energy in shallow wells, trenches, ponds, and lakes, significantly improving the electrical efficiency of heating and air conditioning by factors of four or more over air-to-air heat pumps.

High-grade hydrothermal systems use natural fluids that either occur in the subsurface region of the geothermal reservoir or have been injected into the reservoir to sustain flows and pressure. The natural replenishment of heat in active reservoirs by thermal conduction and convection with proper subsurface system management enables the sustainable use of geothermal energy. If the region of hot rock has insufficient natural fluids, permeability, and/or porosity, these can be stimulated using enhanced geothermal systems (EGS) technology to create a connected open fracture network that simulates the characteristics of a conventional (natural) hydrothermal system.

Geothermal sources are already being used to provide clean energy in commercial applications in many countries. In 2012 they were the fourth largest renewable source of electricity in the United States (behind hydro, wind, and biomass). Geothermal energy plants, with their underground energy supply, have smaller surface footprints than hydro, wind, or solar and are capable of baseload generation with a low-carbon emission profile and without a need for storage because they are not subject to daily or seasonal intermittency (Tester et al. 2007).

As with any energy source, however, there are environmental concerns: (1) water consumption for hydraulic stimulation and long-term productivity, particularly in arid regions, and (2) risk of induced seismicity, because most operating geothermal plants are located in seismically active regions; geothermal development projects must therefore include careful measurements, monitoring, and management of seismicity levels to mitigate the risk of enhancing or triggering damaging seismic events.

Geothermal energy in the United States varies regionally in importance. Western states are advantaged geologically because they have extensive areas of recent volcanism resulting in high heat flows and geothermal gradients. California and Nevada in particular benefit from this energy source, with about 6 percent of their total electricity generation from conventional geothermal reservoirs in 2010 (Table 1). Geothermal energy also provides electrical generation in Hawaii, Oregon, Utah, Alaska, and Idaho, and accounts for significant direct use in the West (e.g., municipal district heating facilities in Boise, Idaho, and Klamath Falls, Oregon).

Table 1

Recent drilling activity for oil and gas in shale deposits has helped to refine quantitative assessments of geothermal resources in Eastern states, which have lower average heat flows and temperature gradients. While these lower-grade areas are not attractive for electricity generation using conventional hydrothermal conversion technology at today’s energy prices, they could be competitive for direct use and cogeneration applications. In addition, many areas make extensive use of geothermal heat pumps in residences and commercial buildings.

National and International Expansion

The United States is currently the world leader in both geothermal electrical generation and deployment of geothermal heat pumps, with over a million units in operation. Internationally, geothermal energy has been used to great advantage in places that are geologically suitable. For example, Iceland, the Philippines, El Salvador, Tibet, and New Zealand produce 20 percent or more of their electrical energy from geothermal sources—in Iceland, these resources supply 95 percent of the country’s heating demand and 20 percent of its electrical demand. Iceland also makes use of cascaded systems that deliver energy for a range of uses below the boiling point of water (Fox et al. 2011).

Although slowed somewhat by the financial crisis of 2008–2009, installed geothermal generating capacity in the United States has steadily risen, and between March 2012 and March 2013 147 MWe (megawatts electrical output) of capacity, or 4 percent of the 2012 total of 3,385 MWe, was added (GEA 2013). Similar expansions are occurring in many other countries, with an increase of 1,782 MWe in installed capacity worldwide (from 8,933 to 10,715 MWe) between 2005 and 2010 (Bertani 2010). By 2015, worldwide generating capacity is expected to exceed 18,000 MWe (Figure 1).

Figure 1

This renewed interest is the result of world economic and political forces—mainly increased oil prices and moral preference for renewable energy—combined with advances in technology that make geothermal energy more accessible. There have been three particularly significant innovations in utilization technologies:

1. Increasing use of innovative power plants, often by marrying steam turbine (flash) plants with binary cycle plants or using cogeneration approaches for providing both heat and electric power. The result is an increased recovery of thermal energy in the resource.

2.   Use of fluids of lower temperature, with refined binary cycle power plants. The result is a wider availability of producible resources. A noteworthy example is the 250 kilowatt (kW) organic Rankine cycle plant in Chena Hot Springs, Alaska, which produces electricity from a very low temperature (74°C) geothermal resource (Lund et al. 2010a).

3. Reservoir enhancement techniques. Over the past 35 years, many EGS field projects at various scales have been under development globally. The first commercial EGS plant began operation in Landau, Germany, in 2007 (Schellschmidt et al. 2010), and by 2013 active EGS field projects were operating at three sites in Europe, one in Australia, and five in the United States.

These innovations offer exciting challenges and opportunities in earth resources engineering.

Innovative Plants

For many years geothermal power plants had a degree of uniformity thanks to the general adoption of strategies that had worked in the small number of early flash plants. Based on experience in the dry steam fields at the Geysers in California, the 55 MW plant came to be accepted as “normal” in size, and based on reservoir temperatures common at the time turbine inlet pressures tended to be in the vicinity of 600 kilopascals (kPa).

Recently, however, there has been considerable variation in plant design. A good example is the combined cycle plant at Rotokawa, New Zealand, one of the first built with binary bottoming cycles supplied from the exhaust of a steam flash plant. It combines a back-pressure steam turbine that has a very high inlet pressure (2,550 kPa) with multiple binary cycle plants that receive the exit steam (Legmann and Sullivan 2003). This combined cycle unit has a steam consumption of around 5 kg/kWh (kilowatt hour), which is superior to the steam consumption of about 8 kg/kWh at the Geysers (computed from data in Sanyal and Enedy 2011) or around 9 kg/kWh in Ahuachapán, El Salvador (Handal et al. 2007).

Combinations of binary and flash plants are now found in several other projects and in some cases have been integrated into a range of direct uses and other applications. An excellent example of such integration is the Svartsengi power plant on Iceland’s Reykjanes Peninsula. It provides hot water and CO2 for a range of uses—fish farming, carbon recycling, enhanced crop and algae growth in geothermally heated and lighted greenhouses and photobioreactors, and warm water for the Blue Lagoon spa resort.

There is also interest in the combination of geothermal generation with other sources, as in the geothermal-solar operations in Ahuachapán (Alvarenga et al. 2008; Handal et al. 2007) and Stillwater, Nevada (Greenhut et al. 2010). The combination of geothermal and solar thermal energy provides an opportunity to raise source fluid temperatures and even out the inherent intermittency of insolation. The combination of solar photovoltaic and geothermal sources allows for increased generation in the hot afternoon, when the air-cooled condensers of geothermal binary plants are at their lowest efficiency. Other designs combining gasified biomass and geothermal heat are under consideration (Tester et al. 2010).

Innovation will certainly continue with new hybrid energy combinations for the supply of heat and power and for the possible use of geothermal reservoirs to sequester CO2 generated from fossil fuel power plants (Randolph and Saar 2011).

Lower Resource Temperatures

In a cross-sectoral study of US energy use, Fox and colleagues (2011) estimated energy consumed as a function of temperature. By integrating the demand from the lowest temperature to higher temperatures, they determined that about 20 percent of US total primary energy (20 exajoules out of 100 exajoules annually) is used at or below the boiling point of water (100oC or 212oF). Much of this energy is supplied by burning heating oil and gas, an incredible waste as higher-grade electricity could be generated from these hot combustion gases before they are used for heating applications at lower temperatures. Direct use of low-temperature geothermal fluids would avoid this large exergy loss as geothermal supply temperatures are closer to temperatures for end-use applications.

Geothermal heat pumps, which use energy stored in rock and soil near the surface in horizontal trenches or vertical wells, are increasing the efficiency of energy use to heat and cool residences and commercial buildings. A key metric of their efficiency is the coefficient of performance (COP), a direct measure of the net heating or cooling achieved per unit of electricity consumed. As a heat source during the winter and a heat sink in the summer, the COP of geothermal heat pumps averages 4 or more, which is substantially higher than conventional air-to-air heat pumps can achieve. For this reason the use of geothermal heat pumps is growing: worldwide there were about 3 million units installed in 2013, over four times the number installed in 2000 (Lund et al. 2010b).

Lower geothermal resource temperatures can also be used to produce electricity in binary power plants. Although not yet common, there are examples of remote or isolated electrical loads such as at Chena Hot Springs (Lund et al. 2010a), which is tens of kilometers from the closest electrical transmission line and would otherwise be dependent on diesel-fueled generation. In fact, there are many off-grid communities in Alaska that could benefit from geothermal electricity generation in place of diesel fuel, which is very costly to supply to these remote areas and often leads to significant local emissions pollution. An active drilling program is under way at Akutan Island in the Aleutian chain (Kolker and Mann 2011), and similar advantages are to be gained in island communities such as in the Caribbean (Huttrer 2010).

As electricity production from lower temperatures becomes more common, another intriguing possibility is the recovery of geothermal energy from coproduced fluids, such as water brought to the surface in oil fields. Pilot projects are in operation in Wyoming (Johnson and Walker 2010) and Huabei, China (Gong et al. 2011). The global oil industry produces as much as 300 million barrels of water per day (540,000 kg/sec) and in many places the temperatures are within the range of operational geothermal power plants. Oil field operations are often also substantial consumers of electrical power, so the generation of electricity local to the operation is of particular benefit.

Figure 2

The importance of resource temperature is somewhat more complex than appears at first. Although in simple terms it is true that hotter is better, there remains a “hole” in resource accessibility because self-flowing steamwater wells in hydrothermal systems drop substantially in productivity at temperatures below a certain range, while in pumped wells the downhole pumps are effective only up to a specific temperature range. This was succinctly described by Sanyal and colleagues (2007), who showed that a gap lies roughly between 190 and 220°C, where neither pumped nor self-flowing wells provide sufficient thermal output (Figure 2). This resource temperature gap represents a technological challenge that is being addressed by the geothermal industry.

Enhanced Geothermal Systems

Although new conventional geothermal reservoirs are being discovered and exploited, the fact remains that the likelihood of major conventional resource discoveries is diminished. The world is not likely to have another resource like the California Geysers or the high-density, high-grade hydrothermal systems found in tectonically and/or volcanically active areas such as Iceland, New Zealand, Italy, and Indonesia. Even the most optimistic estimates of undeveloped hydrothermal systems in the United States would amount to only about 20 to 30 GWe, just a few percent of the current total electric generating capacity of about 1,000 GWe.

The prospect for major expansion of geothermal development lies in EGS when one or more of the three critical ingredients for an operable system are lacking: sufficient reservoir permeability and porosity, sufficient quantities of natural steam or hot water in the reservoir, and sufficiently high temperatures (Figure 3).

Figure 3

A large effort, led by the US DOE with support from private industry, is under way to improve the quality of US geothermal resource information. An important outcome has been the creation of the National Geothermal Data System (www.geothermaldata.org/), which incorporates new data on subsurface temperatures and on new geologic and geophysical information. For example, in New York and Pennsylvania alone, where there has been considerable activity in the past few years associated with drilling for gas in the Marcellus Shale, the number of wells included in the geothermal resource assessment database has increased from about 20 to 7,400 (Aguirre et al. 2013; Shope et al. 2012; Stutz et al. 2012).

Figure 4

New data and other enhancements have led to updated geothermal resource maps for heat flow, gradients, and temperature at specific depths. Figure 4 provides an example for the continental United States of temperatures at a depth of 5.5 km, which is well within accessible drilling depths using conventional drilling technology. Substantial portions of the East and Midwest also have attractive geothermal gradients that could be used if technology were in place to extract the energy. Such a development would fundamentally change the belief that geothermal energy is found only in regions with high-grade hydrothermal systems. Using EGS technology, the nation as a whole could enjoy the 5–6 percent geothermal electricity production found in California and Nevada today.

EGS provide a means of using geothermal energy when hydrothermal conditions are not ideal, that is, when natural conditions in the host rock do not provide sufficient fluid content and/or connected permeability. The idea behind EGS is to emulate what nature provides in high-grade hydrothermal reservoirs at depths where rock temperatures are sufficient for power or heating applications. A fractured reservoir is stimulated hydraulically and connected to an injection and production well separated by sufficient distances to yield a sustainable system for extracting the stored thermal energy in the rock.

EGS research started in the 1970s with the Hot Dry Rock (HDR) project, led by the Los Alamos National Laboratory with the support of the Energy and Development Administration (predecessor of the US Department of Energy). This first HDR reservoir, at the Fenton Hill site near a large hydrothermal system in the Valles Caldera of northern New Mexico, had very low permeability and porosity in crystalline rock at depths ranging from 3 to 5 km. In Figure 3, the Fenton Hill site would be located in the conduction-dominated region, with rock matrix permeabilities less than a microdarcy and porosity with an average gradient of about 60oC/km. During the past 35 years a number of large-scale EGS demonstration projects followed, in Rosemanowes, UK; Soultz, France; Cooper Basin, Australia; Landau, Germany; Basel, Switzerland; and Japan (two sites); and the United States added sites at Newberry Caldera in Oregon, the Geysers and Clear Lake in California, and Desert Peak, Nevada.

These demonstration tests have done much to establish the technical feasibility of the EGS concept, showing the feasibility of directional drilling to 5+ km and 300oC, of hydraulically stimulating and seismically mapping 2+ cubic km regions of rock, and of producing sustained flow and heat extraction between injection and production wells in the stimulated region.

But there remain several important challenges before EGS will be ready for commercial development: an increase in production rates by a factor of 2 to 4 to reach levels comparable to those of commercial hydrothermal reservoirs, the achievement of sustained production with sufficient reservoir thermal lifetimes, and demonstration of the effective application of EGS technology over a range of geologic conditions. An MIT-led study (Tester et al. 2006, 2007) and recent IPCC report (Goldstein et al. 2011) provide detailed evaluations of the technical and economic requirements and deployment status and potential of EGS.

Conclusion

Geothermal energy has experienced a renaissance in the past ten years as many new technologies and countries have joined the industry. The technology for generating electricity and deploying district heating from high-grade hydrothermal systems is relatively mature and reliable. Technologies for geothermal heat pumps are also mature and are being deployed at increasing rates in the United States and Europe. The use of innovative hybrid and combined heat and power plants, lower resource temperatures, and enhanced reservoir stimulation methods are making geothermal energy accessible in a much greater variety of places. At a number of field test sites in the United States and elsewhere, EGS technologies are being demonstrated at a scale that is approaching commercial levels and, if operated long enough to prove sustained production, would enable the deployment of a substantially increased fraction of the huge geothermal resource base, which for the United States amounts to about 14 million exajoules (Tester et al. 2007).

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About the Author:Roland N. Horne (NAE) is Thomas Davies Barrow Professor of Earth Sciences, Department of Energy Resources Engineering, Stanford University, and past president of the International Geothermal Association (2010–2013). Jefferson W. Tester is Croll Professor of Sustainable Energy Systems, director of the Cornell Energy Institute, and associate director for energy at the Atkinson Center for a Sustainable Future, Cornell University.