The Physics of Energy, page 133
Ground source heat pumps/cooling systems Ground source heat pumps and heat extraction devices exploit the approximately constant ambient temperature several meters below Earth’s surface to drive efficient heat transfer into or out of buildings or other systems. As explained in §6.5.3, it is actually solar energy that maintains the roughly constant near-surface ground temperature. Thus, while often described as a type of geothermal energy, ground source heating and cooling systems are actually a form of solar energy.
Low-enthalpy wet geothermal resources The vast majority of easily accessible geothermal resources contain fluids heated to temperatures below 150 ℃. Such reservoirs occur in a wide range of geological circumstances. Near plate boundaries or hot spots, magma intrusions that reach within several kilometers of the surface can generate low-enthalpy as well as higher-temperature geothermal fluids. Low-enthalpy geothermal resources can also result from heating by rock formations in continental crust with high radionuclide densities, or in regions where porosity and fractures in the rock bring meteoric water – water originating as precipitation – far beneath the surface into contact with hot basement rock, with a circulation pattern that brings the heated water back close to the surface.
Low-enthalpy geothermal resources can be used directly for space and industrial heating. More recently, binary cycle technology has evolved, where heat exchangers are used to run a phase-change cycle based on a working fluid with a relatively low boiling point, as discussed at the end of §32.4.3.
Medium to high-enthalpy hydrothermal resources Regions with geothermal fluids heated to temperatures above 150 ℃ within a kilometer or so of the surface present the most desirable geothermal resources for power production. Such geothermal resources generally only occur in regions with ongoing or recent tectonic or volcanic activity, associated with plate boundaries or hotspots. In optimal high-temperature geothermal locations, temperatures up to 300–350 ℃ can be reached within 2–3 km of the surface. Such resources can be fluid or steam dominated depending upon the pressure and configuration of the location.
Hot dry rock In many locations the thermal gradient is sufficiently steep that temperatures of 200–300 ℃ are attained at depths of 5–10 km in the hot basement rock below the surface of continental crust. This hot dry rock represents a tremendous potential energy resource that may be within reach of the next generation of geothermal technologies, known as enhanced geothermal systems (EGS).
Magma Molten rock at temperatures of 600–1400 ℃ provides the original thermal energy source for most near-surface geothermal energy resources. In some places on Earth’s surface, particularly near active volcanic areas, magma comes within several kilometers of the surface. It has been proposed that directly tapping the thermal energy within such magma intrusions could provide a tremendous source of geothermal energy. The high temperatures involved, however, place such an endeavor beyond the reach of existing technology. If materials are eventually developed that make drilling and extracting thermal energy directly from magma feasible, this could represent a substantial energy resource in the long-range future. Magma comes particularly close to the surface in the vicinity of ocean spreading ridges, but the logistical difficulties of accessing such thermal energy resources are difficult to imagine surmounting at the present time.
Geopressured resources In addition to the types of geothermal resource just described, there are also geopressured resources in locations where heavy sedimentation has compressed a volume of fluid or gas contained within a region bounded by impermeable rock. Such systems were first discovered beneath the Gulf coast of the US near the Gulf of Mexico, where a mixture of hydrocarbons and water are buried beneath clay sediments, with a high thermal and pressure gradient. Such locations represent another potential future energy resource.
It can be rather complicated to develop any geothermal energy system other than the simplest near-surface low-temperature resources. First, exploration must identify feasible locations for geothermal energy exploitation. Second, a systematic assessment of a potential resource must be carried out. The process of assessing potential resources is quite challenging due to the substantial distance beneath the surface at which measurements must be made. Third, a location-specific system must be engineered to exploit the resource. Finally, there are a variety of technical challenges to building and maintaining geothermal systems, including the fact that as substantial amounts of energy are extracted from most underground geothermal systems, pressure, temperature, and fluid concentrations can change, necessitating a dynamical approach to utilizing the resource.
We describe the main types of geothermal energy that can be used for large-scale power systems in somewhat more detail below after a brief discussion of ground source heat pumps in the following section.
32.3 Ground Source Heat Pumps
As mentioned above, ground source heat pumps are often referred to as “geothermal energy,” even though they are primarily powered by solar energy. Just as solar energy is absorbed in ocean surface waters providing a reservoir of thermal energy (§27.6), solar energy is absorbed and stored by the ground, and can be put to use by ground source heat pumps and cooling systems. In §6.5.3, we used the heat equation (6.22) to give a simple estimate of the ground temperature at a depth of a few meters beneath the surface as a function of time. The basic result is that diurnal (daily) fluctuations are damped in 10 or 20 cm of soil, and seasonal fluctuations are damped in several meters of soil. The solar energy absorbed during the summer months at mid-latitudes propagates slowly downward, with an effect that decreases exponentially with depth so that at a depth of 3–6 m beneath the surface the temperature is quite constant year round in most locations and is approximately equal to the yearly average surface temperature (see Figure 6.16). Due to the small flux involved, the flow of geothermal heat upward has very little effect on the temperature of the surface.
Since the ground temperature at several meters depth is roughly constant through the year, it can be used effectively as a source or sink for thermal energy to drive a heat pump. Recall from §10.6 that a heat engine run in reverse can be used to transfer thermal energy from a cooler region to a warmer region. This makes it possible, for example, to warm a house in the winter using thermal energy from the ground with a much smaller input of electrical energy than would be required by electric resistive heating. The coefficient of performance (CoP) of a heat pump (10.15) is the ratio of the heat output over the work done. This is bounded above by the Carnot limit . For example, to keep a house at 20 ℃ when the ground is at a temperature of 10 ℃, the Carnot bound on the CoP is around 30. While realistic heat pumps cannot come close to this performance, it is possible to achieve CoPs of 2–5 for commercial systems; this is significantly better than the ratio of one achieved by an electric heater. Ground source cooling systems work in a similar fashion, by pumping heat from the warmer above ground environment down into the cooler ground in the summer.
Several approaches have been developed for ground source heat pumps. A standard configuration currently in use employs a closed-loop system in which water (mixed with an antifreeze agent such as monopropylene glycol) is circulated through a horizontal system of pipes (usually polybutylene, chosen because of its high thermal conductivity) under the ground at a depth of 2–4 meters. Thermal energy flows from the ground into the fluid in the pipes. The energy is then transferred to a phase-change working fluid in a second loop and is extracted by a heat pump that deposits the thermal energy in the space to be heated. The principal limitations to this form of heating system are the thermal conductivity and heat capacity of the soil (or rock) through which the pipes run. The lower the heat capacity of the soil, the larger the field from which energy must be drawn. If the soil’s thermal conductivity is low, heat cannot be captured from far away and the temperature of the soil in the immediate vicinity of the pipes falls, decreasing the CoP of the heat pump.
If the ground has substantial water content, its thermal conductivity and heat capacity are higher, and the system is more effective. In some cases, a body of water can be used as the primary thermal energy source. Variations on the approach just described include open systems – where water is extracted from and returned to an underground channel – and deeper vertical systems extending to a depth of 30–200 m. Vertical systems are sometimes used when insufficient horizontal area is available. For both open- and closed-loop systems based on water as a heat transfer fluid, a separate loop is needed for the refrigerant used in the heat pump itself. Simpler direct exchange systems circulate the refrigerant itself through the ground in copper pipes.
The key to a successful ground source heat pump installation is ensuring that the volume of ground from which energy is extracted is sufficient to cover the needs of the system. To illustrate this we perform a simple estimate in Example 32.3 of the area that must be covered by the pipe loop to heat air and water in a typical single-family house in a location with relatively cold winters. In general, the area becomes prohibitively large unless the ground has sufficient moisture content and/or groundwater flow. These issues are explored further in the problems.
Example 32.3 Ground Source Heat Pump in Boston
To illustrate the need for a large ground area and adequate moisture content we perform a back-of-the-envelope calculation to spec a ground source heat pump with CoP in a mid-latitude location such as Boston, where ground temperatures at 6 m are around 10 ℃. Consider trying to use a ground source heat pump for air and water heating in a single-family house that requires a total of 24 GJ (6600 kWh) of thermal energy in the month of February when outside temperatures average around 0℃. With a CoP of 4, 18 GJ must be taken from the environment. If the ground is dry and sandy, the volumetric heat capacity is roughly MJ/m3 K. So, providing the required energy for the month of February would require lowering the temperature of 2000 m3 of ground by around 7.5 K. The heat capacity of the ground increases with moisture content, which can reduce the volume needed by a factor of 2–3. Since even with moist ground this is a very large volume, thermal conductivity is needed to draw thermal energy from the ground away from the immediate vicinity of the pipes and reduce the quantity of piping needed. For dry sandy earth, the thermal conductivity is around 0.3 W/m K. Covering a horizontal area of 200 m3 with a piping network, a temperature gradient of 10 K/m below the pipes would give a thermal energy flow rate of 600 W towards the pipes. This is an order of magnitude less than the 10 kW average needed for the house in February. (Assuming the horizontal piping is installed at a depth of 3–6 m, thermal energy would soon cease to flow from above; if the piping network lay in a vertical plane, the flow rate estimated here would be doubled as thermal energy would come in from both sides.) Soil with increased moisture content can have a thermal conductivity of 1–3 W/m K, making the deployment of an adequate piping system for a ground source heat pump a more tractable proposition. The situation is much easier in locations with some flow of groundwater, where thermal energy transport in the ground is greatly enhanced.
This illustrates some issues involved in designing and implementing ground source heat pumps. Clearly, in locations with cold winters and dry soil, good home insulation plays an important role in reducing the total energy needed to make a ground source heat pump system tractable.
(Image credit: Mark Johnson)
Ground Source Heat Pumps
Ground source heat pumps use the ground several meters below Earth’s surface as a thermal energy source/sink for a heat pump that heats or cools buildings or water. Substantial soil volume is needed, and water content in the ground improves heat transport to replace energy in the region utilized. The coefficient of performance (CoP) for a ground source heat pump-based heating system can be in the range 2–5.
32.4 Hydrothermal Energy
The first geothermal electric power plant went into operation in Larderello, Italy in 1904, where by 1913 a 13 MW power plant was converting geothermal steam into electricity. Geothermal electric power production developed slowly in the first half of the twentieth century, but over the last several decades has expanded to a capacity of over 10 GWe. Most of the present electricity production from geothermal power taps fluid-dominated hydrothermal resources at medium to high temperatures, but the use of binary cycle systems to produce electricity from low-enthalpy geothermal systems has been rising.
32.4.1 Hydrothermal Resources
Optimal hydrothermal resources occur in locations where the following features are present:
Steep thermal gradient The most easily exploited hydrothermal resources have temperatures around 300 ℃ occurring within a kilometer or so of the surface. It is easy to drill to this depth using well-established technology. Such a steep thermal gradient, compared to the typical value of 30 ℃/km, occurs only in locations such as faults or hotspots where magma has come close to the surface.
Water resource To bring the thermal energy to the surface, water must be present in the geothermal field. In most situations the water is under sufficient pressure that it exists in liquid form even at high temperature. The fluid in the geothermal field often contains significant chemical impurities that can complicate the process of extracting energy.
Permeable rock The rock in the geothermal field must be sufficiently permeable to allow water to flow through the reservoir. Water must be able to flow from throughout the reservoir to the production well where the water is extracted. The reservoir must also be sufficiently permeable to allow either recharge of water from natural sources or re-injection from the geothermal plant.
Cap For the water to flow easily to the surface through the production well, the reservoir must be maintained at a high pressure. This requires an impermeable rock cap over the permeable rock that contains the fluid in the reservoir.
An idealized hydrothermal resource is depicted in Figure 32.13.
Figure 32.13 An idealized hydrothermal resource, containing a geothermal field at moderate depth composed of porous rock containing fluid water at high temperature and pressure. Water is kept at high temperature by a magma intrusion into the crust relatively near the surface, and at high pressure by an impermeable rock cap above the reservoir. Meteoric (rain) water recharges the reservoir. (Credit: Geothermal Energy: Utilization and Technology, edited by Mary H. Dickson and Mario Fanelli (United Nations Educational, 2003))
Geothermal regions where some of these features are absent, or only present in limited form, may still be productively exploited in some cases. For example, if there is not a natural system for replenishing the geothermal fluid, fluid can be artificially pumped down into the reservoir through an injection well. In many geothermal systems, after some time the fluid in the original resource becomes depleted, and the pressure drops, complicating extraction. In some such locations, such as the Geysers geothermal field in California, waste water is pumped back into the system to replenish the fluid.
Identifying a geothermal region suitable for economically viable energy extraction involves substantial research and development before wells are drilled. A variety of approaches, many based on geophysical methods, are used to assess a potential geothermal resource. A simple survey of the surface can identify obvious features such as steam vents, fumaroles, or geysers, which indicate a highly active geothermal region. But more detailed assessment of the configuration below ground requires a number of indirect methods. The volume of fluid in the reservoir, temperature, pressure, permeability of rock, and chemical composition of the fluid must all be evaluated. A hydrologic survey, taking into account inputs to the region from rainfall and surface water flow, and analyzing chemical impurities in surface fluids, gives some information about the properties of the geothermal fluid. Measurements of near-surface heat flow and rock conductivity give some estimate of the thermal profile of the region. The electrical resistivity of the ground, measured by passing current through a high voltage between electrodes spaced several kilometers apart, can indicate the water content of the rock down to a depth of over 1 km, with lower resistivity indicating higher water content. Salinity and higher temperature of the geothermal fluid decrease the resistivity. Imaging methods using electromagnetic and seismic waves can also give useful information about the structure of the underlying formation, as can local variations in the gravity field associated with the density of subsurface material. Finally, if the initial assessment is positive, drilling exploratory wells gives more direct information about the potential resource. Many of these methods, particularly seismic surveys and well logging, have been extensively developed in the oil and gas industry and are discussed in more detail in the following chapter.
Once a promising geothermal resource has been identified, a great deal of physics and engineering is involved in designing a system to optimize the utilization of the resource. Experience in the oil and gas industry has led to a sophisticated set of theoretical and practical tools for modeling fluid flow through complex underground rock formations, drilling production and injection wells, and for constructing above-ground systems that connect the wells. In a typical fluid-dominated reservoir, permeability of the rock plays a crucial role in the reservoir dynamics. For a fluid of viscosity η flowing in a porous medium and subject to a pressure gradient , the permeability K is defined through Darcy’s law
