GEOTHERMAL ENERGY SECOND EDITION, RENEWABLE ENERGY AND THE ENVIRONMENT

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            t is a well-known fact that the interior portions of the globe are very hot, the temperature rising, as observations show, with the approach to the center at the rate of approximately 1 degree C. for every hundred feet of depth. The dificulties of sinking shafts and placing boilers at depths of, say, twelve thousand feet, corresponding to an increase in temperature of about 120 degrees C., are not insuperable, and we could certainly avail ourselves in this way of the internal heat of the globe. In fact, it would not be necessary to go to any depth at all in order to derive energy from the stored terrestrial heat. The supericial layers of the earth . . . are at a temperature suficiently high to evaporate some extremely volatile substances, which we might use in our boilers instead of water.

            As the above quote demonstrates, the vision of utilizing the earth’s internal heat to beneit the world is not new. It is obvious to even the most casual observer that energy, in the form of heat, is present below the earth’s surface—volcanoes spew scorching lava from their summits and water in hot springs bubbles up from below—attesting to the fact that the interior of the earth is hot. But the nature of that heat remained, for many years, a source of mystery. Why volcanoes are unevenly distributed over the continents and why hot springs are abundant in some places and nonexistent in others seemed inexplicable. But, over the last 200 years, intensive research by cadres of earth scientists has provided a clearer understanding of the source, distribution, and properties of this massive thermal reservoir.

            It is now well understood that heat constantly radiates from the earth’s surface into space. Some of that energy is solar energy that has been absorbed by soil and rock and re-radiated as infrared radiation. But, using a variety of measurement techniques, earth scientists have been able to establish that on average about 1% of the total energy that radiates into space is heat that comes from the interior of the earth itself. However, it may seem insigniicant that 1% of the radiated heat represents a miniscule fraction of the amount of heat energy the earth contains. In fact, the amount of energy, in the form of heat, that is present a few thousands of feet below the surface is more than enough to satisfy the energy needs of every nation of the world many times over.That heat energy is geothermal energy. It is remnant heat derived from the formation of the planet 4.5 billion years ago, as well as heat from the radioactive decay of naturally occurring radioactive isotopes.

            That heat is suficient to power plate tectonics, which is the slow movement of the continents and the ocean loor that make up earth’s crust, and the upper mantle. It provides the energy to drive mountain building processes that occur when continents and oceans collide. It is also suficient to melt rocks, generate volcanoes, heat water to form hot springs, and keep base-ments of buildings at a constant temperature. It is a perpetual, renewable, and inexhaustible energy resource.
With a few important exceptions, geothermal energy did not play a signiicant role in the energy mix associated with electrical power generation or other applications until the latter half of the twentieth century. At that time, growing interest in the environmental, economic, and social aspects of energy generation and use encouraged exploration of energy sources that would diminish reli-ance on fossil fuels. This chapter will discuss the context of those changes and their implication for the development of geothermal energy. The remainder of this book will consider speciic topics that, if taken together, provide a comprehensive body of knowledge for informed consideration of geothermal energy use.

GEOTHERMAL ENERGY AS A RENEWABLE ENERGY SOURCE NO FUEL, FEW EMISSIONS, AND REDUCED VOLATILITY

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          As noted in Section “Historical Role of Fuel”, the historical use of energy has relied on fuel to make heat. The sources of fuel have traditionally been wood, coal, oil, and natural gas. The changes in use of these fuels over time within the United States, as well as the contribution of hydroelectric and nuclear generation, are shown in Figure 1.1. Coal, natural gas, and oil account for more than 85% of the energy used in the United States. Although these fuels may, in combination, provide decades to several hundred years of supply, the combination of greenhouse gas emissions and competition for the petroleum-based fuels that are implicit in Figure 1.2 have made problematic reliance on these energy sources. In addition, these fuels are derived from a resource base that is not being replen-ished. Instead, these resources are being extracted from geologically derived materials that took millions of years to form. As a commodity, these fossil fuels are not renewable and will become more and more dificult, and hence more costly, to extract.

          Resource depletion is a characteristic of nonrenewable energy resources. However, how deple-tion is measured is a matter of considerable debate. This issue is discussed in detail in Chapter 8, but several facets of this topic are relevant for consideration here. One aspect of this issue con-cerns the accessibility of fossil fuel energy resources, such as oil or natural gas or coal. Oil, for example, occurs in underground reservoirs (oil “ields” or “pools”). Some of these reservoirs are within a few hundred meters of the ground surface and occur in very porous rocks through which the oil can low relatively easily. Other oil reservoirs are present at much deeper levels (thousands of meters below the surface) in rocks that have very little interconnected porosity, thus making it dificult for the oil to low. The shallow reservoirs are easy to access through standard drill-ing techniques and the oil is easily and readily pumped out of them. The cost of producing such oil is relatively low. The converse is true for the deeper reservoir—drilling costs will be high
and production from a given well will be low. Thus, as long as the price per barrel of oil that a customer is willing to pay is low, the shallow reservoir will be developed and considered an oil “reserve,” whereas the deeper reservoir may be a resource but it will not be possible to develop it and consider it a reserve because the cost of doing so would be greater than a potential customer would be willing to pay.

          However, once the shallow reservoir is depleted (it contains a inite amount of oil), the deeper reservoir will become economic to develop if there is a shortage of oil and the price a customer is willing to pay increases suficiently. At that point, a new reserve is now possible to develop and oil availability increases. If the size of that deeper reservoir is larger than the shallow reservoir, the total available oil inventory will then increase, even though the original reservoir is totally depleted, provided customers remain willing to pay the higher cost for the fuel. In other words, the size of a resource depends, in part, on economic considerations. Generally, as markets increase the value of a commodity, as that commodity becomes more dificult to access, the price of the commodity increases.

          Recently, it has become important to also consider the amount of energy that is required to produce an energy resource (e.g., Murphy and Hall 2010). This measure, called energy returned on energy invested (EROEI or EROI), is the ratio between the amount of energy that is expended in developing, producing, and processing an energy source, such as oil, and the amount of energy that is obtained in using that fuel. EROEI is dificult to establish in an absolute sense, because it is dependent on which factors one chooses to include in the analysis. For example, with oil pro duction, the energy invested includes the energy expended in drilling the well, pumping the oil.

Introduction

          out of the ground, transporting the oil to a reinery, processing the oil in the reinery, etc. What et cetera might include would be the energy cost of delivering the product, the energy expended for environmental maintenance and cleanup once production ceases, energy expended in mar-keting, and energy expended during initial exploration for the resource. Obviously, each analysis that is conducted will provide different results, depending upon how the analysis is done. One example of such an analysis that was recently done (Figure 1.6) shows how the EROEI for some
fossil fuels has dropped signiicantly as easily accessed fossil fuel resources are depleted and more challenging resources become economically attractive. These results show that, over time, less net energy is available from the extracted resource, as we expend more energy to obtain it. This analytical approach to the economics of energy extraction and use is discussed in more detail in Chapter 14.

          It is for these reasons that there is growing interest in inding sources of energy that reduce reliance on fossil fuels to generate heat to produce work and that can reduce or eliminate produc-tion of greenhouse gases. In addition, more attention is being given to innovative direct uses of heat in applications where a process can be accomplished without burning a fuel to obtain heat. For example, fruit drying can be accomplished either by generating electricity or burning fuel to heat a drying oven, or by using hot waters obtained directly from natural hot springs to heat
a drying oven.

Criteria that are generally used to establish the viability of an energy source that would supplant or displace reliance on fossil fuels are the following:

• It is suficiently abundant to meet a signiicant percentage of the market demand.
• It can be obtained at a cost competitive with existing energy sources.
• Its use will reduce or eliminate greenhouse gas emissions.
• It is self-replenishing (i.e., renewable).

GEOTHERMAL ENERGY IS A FLEXIBLE ENERGY RESOURCE

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          Another signiicant attribute is that geothermal heat occurs in diverse ways, making it possible to use it for different purposes. One such application is for heating and cooling buildings, as mentioned in Section “No Fuel, Few Emissions, and Reduced Volatility”. Every square meter of land surface has heat lowing through it. Although the temperature of the upper few meters of soil and rock luctuates with the effects of local weather patterns and solar insolation, the temperature at depths of three to ten meters is usually constant because of the low of heat from the interior of the earth. That heat can be used as a source of energy for HVAC purposes in buildings using geothermal heat pumps (GHP; Chapter 11). Because of their high eficiency, GHP use diminishes electrical demand. When combined with programs that improve building eficiency, such applications of geothermal energy can displace a signiicant percentage of electricity generation and natural gas usage required to meet the HVAC load.

          In many areas, modest heat low is available to allow the use of geothermal energy for indus-trial applications that currently rely on fossil fuels. These direct-use applications include food processing, drying materials, agricultural activities and greenhouses, aquaculture, and paper manu-facturing. Although such applications have been developed and successfully used throughout the world, they remain relatively unknown and vastly underutilized. Chapter 12 discusses many of these applications.

          In regions where geothermal energy occurs at higher density, as discussed in Chapters 2 and 10, temperatures are high enough to allow the generation of electrical power. Figure 1.8 shows a comparison of CO2 emissions for fossil-fueled power generation compared to geothermal plants. Geothermal power generation in appropriate settings can effect a very significant reduction in greenhouse gas emissions. As discussed in Chapter 2, current geothermal power generation technology has application in speciic geological settings and thus is not currently able to provide electrical power in all settings. For this reason, its application is restricted to about 30% of the geographical area of the United States. However, as discussed in Chapter 13, the development of new technology that allows the drilling and use of deeper wells and geothermal reservoirs will expand deployment of geothermal generation capabilities to most regions of every continental land mass. It is currently

          Deployment of geothermal energy technologies will be most effective where the attributes of the technology are matched to the characteristics and needs of the energy sector they are designed to serve. In the following section, the use of geothermal power in the electricity generation sector is discussed, as an example of how resource attributes and sector needs can be matched for greatest eficiency.

ELECTRICAL DEMAND AND THE CHARACTERISTICS OF GEOTHERMAL ENERGY

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GENERATING ELECTRICAL POWER FOR THE GRID

          The modern electrical grid has evolved over the last century to reliably supply power to an increas-ingly complex market. The grid is a network that links together power generators and power users through a system of transmission and distribution lines. In principle, a sophisticated electrical grid can allow a customer on one side of a country to purchase power from a generator on the other side of a country, or even another country, and reliably have that power provided on demand. For historical and economic reasons, however, power grids are commonly segmented into regions supplied and administered by operators and regulators. Such a system allows responsiveness to local and regional needs, while reducing transmission losses, and provides a measure of security from failures in the grid that could be catastrophically transmitted through the network if the grid were not segmented.

          Such a system has several important characteristics and limitations. One characteristic is that the demand or load on the grid will vary during the day. In a given region, the demand will be lowest in the earliest morning hours and greatest at some point during the day or early evening. Seasonal variability will inluence the timing of the load, and unusual weather can cause spikes in demand at nontraditional times. Because of this variability, the concepts of baseload, peak load, and load following have evolved and play an important role in designing grid components.

          Baseload is the minimum amount of  power a supplier must make available to its customers. The amount of baseload power can vary from hour to hour, depending upon region and the power demands that exist there. Usually a utility or power administrator will have an historical record and contractual obligations on which to establish what the baseload demand will be.

          The peak load is that load placed on the grid by the immediate conditions that are being expe-rienced at that moment that exceed the baseload. Peak load can vary from day to day and month to month. It can be strongly affected by such things as extremes in weather or local emergencies. The capacity to meet this temporary increase in load that exceeds the baseload is called peaking capacity. A power provider uses historical records to estimate what the likely peak demand will be. It is on this basis that the maximum required generating capacity in a region is determined. Generally, a utility or other provider will design the capacity of the local or regional system to exceed by a few percent the estimated maximum possible peak load.
Load following is the ability to respond to changes in demand for power.

          Load following requires the ability to increase power output on a timescale of minutes to tens of minutes. This can be accom-plished by either having generating plants capable of relatively rapid changes in power output or having in place contractual agreements to buy power on short notice from suppliers who can quickly
respond to demand changes.

          Managing transmission and distribution for a power grid have historically been accomplished manually through centralized facilities that can locally or remotely call up generation capacity, or reduce it, as needed. However, recent technological developments have made it conceivable that an electrical grid can be monitored and managed electronically using sophisticated computer systems that quickly compute demand, load variation, and changes in wind and solar generation resulting from local climate conditions. This has given rise to the concept of a smart grid that, essentially, manages itself. Although some years away from full implementation, the concept and framework for putting in place such a capability is moving forward in various parts of the world. Such a capability will reduce transmission losses, manage generation facilities more eficiently, and allow dispatch-ability of the most cost-effective generation methods, and do so in a way that takes into account
local and regional needs.

          Geothermal power plants have historically been viewed and operated as baseload power plants. In this sense, they are different from solar and wind power generating facilities, which are inter-mittent. Although all of these facilities have the advantage that they do not require a fuel supply infrastructure to provide energy, power generation from solar or wind facilities is interrupted (i.e., is intermittent) on a diurnal and/or seasonal cycle and thus cannot provide baseload capability. In contrast, geothermal energy never stops being supplied from the earth’s interior and can thus be part of a baseload supply system. However, there is no theoretical reason why a geothermal reservoir could not be managed in a way that allowed it to be load following, that is, dispatchable.

GENERATING ELECTRICAL POWER FOR LOCAL USE

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          Electrical power generation as described in Section “Generating Electrical Power for the Grid” is the primary means, whereby residential, commercial, and industrial customers obtain electrical power. However, renewable energy resources readily lend themselves to situations where power generation can be accomplished with modest generators that supply power to a limited number ofcustomers in a restricted area. Such distributed generation capabilities are growing in number. The
compact nature of new generation binary geothermal generators, for example, has the potential to supply a few hundred to a few hundred thousand kilowatts of power from a single well. Such a facil-ity can supply power to an industrial site, a small community, or any other type of operation that requires power at that scale without accessing the power grid. Such facilities require minimal operational oversight and can be cost-effective.