Table of Contents
Introduction
Geothermal energy uses heat from within the Earth as an energy resource. To understand how this resource works, it is useful to look at where the heat comes from, how it moves through the Earth, and how that heat becomes accessible at or near the surface. This chapter introduces the basic physical and geological ideas behind geothermal resources, especially the concepts of geothermal gradients, heat flow, and different resource types. More detailed topics about power plants, direct use, and heat pumps are covered in later chapters and are not explained in depth here.
Sources Of Heat Inside The Earth
Earth’s internal heat mainly comes from two processes. First, there is “primordial” heat that remains from the time when the planet formed and differentiated billions of years ago. Second, there is heat from the radioactive decay of elements such as uranium, thorium, and potassium in the Earth’s crust and mantle. As these radioactive isotopes decay, they release energy in the form of heat.
The solid Earth behaves as a very slow conveyor of this heat from the deep interior toward the surface. Because rocks are not very good conductors of heat, the temperature increases with depth. This increase is described by the geothermal gradient.
Geothermal Gradient And Temperature With Depth
The geothermal gradient is the rate at which temperature increases as you go deeper into the Earth’s crust. It is usually expressed in degrees Celsius per kilometer, written as $^\circ\mathrm{C}/\mathrm{km}$.
In many regions with “normal” conditions, the average geothermal gradient near the surface is about $25$ to $30~^\circ\mathrm{C}/\mathrm{km}$. This means that for every kilometer of depth, the temperature rises by roughly 25 to 30 degrees Celsius. If the average surface temperature is $15~^\circ\mathrm{C}$ and the gradient is $30~^\circ\mathrm{C}/\mathrm{km}$, then at 3 km depth you might expect a temperature close to
$$
T \approx 15 + 30 \times 3 = 105~^\circ\mathrm{C}.
$$
This is only an approximate calculation, since actual gradients can vary significantly from place to place and with depth. In many sedimentary basins the gradient might be lower, while in tectonically active or volcanic regions the gradient can be much higher. These high gradients are particularly important for geothermal development, because they mean useful temperatures can be reached at shallower depths and with less drilling.
Key rule: The geothermal gradient is the change in temperature with depth,
$$
\text{Geothermal gradient} = \frac{\Delta T}{\Delta z},
$$
typically expressed in $^\circ\mathrm{C}/\mathrm{km}$.
Heat Flow And Heat Flux
While the geothermal gradient describes how temperature changes with depth, heat flow describes how much thermal energy is moving through a unit area of the Earth’s surface in a given time. It is often expressed in milliwatts per square meter, written as $\mathrm{mW}/\mathrm{m}^2$.
Heat flows from hotter regions in the deep Earth to cooler regions nearer the surface. In many continental regions, typical heat flow values are around $60~\mathrm{mW}/\mathrm{m}^2$, although this can be higher in areas with strong tectonic or volcanic activity.
Heat can move within the Earth’s crust in two main ways. The first is conduction, where heat passes through solid rock due to temperature differences. The second is convection, where hot fluids such as water or steam move and carry heat with them. Geothermal systems that rely on moving fluids are usually more productive resources, because convection can transport large amounts of heat efficiently over shorter distances.
In simple terms, where the heat flow is high and where hot fluids can circulate, the geothermal resource is generally better for energy use.
Types Of Geothermal Resources
Geothermal resources can be described and classified in several ways. A common approach for beginners is to look at the temperature range of the resource and the physical state of the fluid in the underground system.
Low temperature resources are usually taken to mean below about $90~^\circ\mathrm{C}$. These are often used for direct heating applications, such as space heating, greenhouses, and bathing. Intermediate temperature resources typically lie around $90$ to $150~^\circ\mathrm{C}$ and can be used for some types of power generation with special technologies, as well as for heating. High temperature resources, often above $150~^\circ\mathrm{C}$ or even $200~^\circ\mathrm{C}$, are usually needed for conventional geothermal power plants that generate electricity at a significant scale.
Geothermal resources also differ in the type of fluid they contain. Some reservoirs contain liquid water at high temperature, called liquid dominated systems. Others contain mainly steam, called vapor dominated systems, which are relatively rare but very valuable for power generation because the steam is easy to bring to the surface and use in turbines. In many liquid dominated systems, hot water can flash to steam when the pressure drops as it rises toward the surface.
Another important distinction is between hydrothermal systems and so called “hot dry rock” systems. Hydrothermal systems contain naturally occurring water or steam that circulates through porous or fractured rock, thereby transporting heat. Hot dry rock systems, in contrast, consist of hot but relatively impermeable rock with little or no natural fluid circulation. These harder to access systems are the focus of some advanced technologies discussed elsewhere in the course, but they still count as part of the broader geothermal resource base.
Geological Settings Of Geothermal Resources
The best geothermal resources usually occur in specific geological environments. One of the most important settings is at plate boundaries where the Earth’s tectonic plates meet. In regions with active volcanism, such as volcanic arcs associated with subduction zones, magma intrudes into the crust and heats the surrounding rocks and groundwater. This can produce very high temperature reservoirs at relatively shallow depths.
Another important setting is within rift zones, where the crust is being pulled apart. Thinning of the crust and upward movement of hot mantle material can increase heat flow and raise the geothermal gradient. Some well known geothermal fields are located along such rifted regions.
There are also geothermal resources in stable continental interiors, sometimes related to concentrations of radioactive elements in the crust or to deep sedimentary basins that trap heat. These resources are often lower in temperature, but can still be very useful for heating and for some power generation technologies that work with moderate temperatures.
The presence of faults and fractures is also crucial. Fractures can act as conduits that allow water to circulate, absorb heat from deep rocks, and transport that heat upward. Without this kind of fluid circulation, even very hot rocks may be difficult to exploit.
Reservoirs, Permeability, And Fluids
From a practical perspective, a geothermal resource is useful when there is a reservoir that can deliver heat to the surface at a sufficient rate and over a long enough time. A geothermal reservoir is a volume of rock that contains hot fluids and has sufficient permeability for those fluids to flow. Permeability describes how easily a fluid can move through a rock due to the presence of pores, cracks, and fractures.
The fluids in geothermal reservoirs are most often groundwater that has percolated down from the surface, been heated at depth, and then circulated back up due to buoyancy and pressure differences. Salinity, dissolved minerals, and gas content can vary, which affects both how the fluid behaves in the reservoir and how it must be handled at the surface. In some high temperature fields the fluid may be mainly steam by the time it reaches production wells.
A key feature of successful geothermal reservoirs is the presence of a heat source at depth, a permeable reservoir rock with stored fluids, and a cap rock that is relatively impermeable. The cap rock helps trap the hot fluid and maintain pressure over long time scales.
Resource Assessment Concepts
To understand how much energy can be extracted from a geothermal resource, it is useful to think in terms of stored thermal energy and recoverable energy.
The total thermal energy stored in a volume of hot rock and fluid can be approximated by
$$
Q = m \, c_p \, (T - T_\text{ref}),
$$
where $Q$ is the thermal energy, $m$ is the mass of the material, $c_p$ is its specific heat capacity, $T$ is the temperature of the material, and $T_\text{ref}$ is a reference temperature, often taken as the surface or ambient temperature.
However, only a fraction of this stored energy can be practically recovered. This fraction is sometimes called the recovery factor. It depends on how the reservoir responds to production, how fluids are reinjected, and how long the project is operated. Realistic recovery factors are much lower than 100 percent, because not all the heat can be extracted without cooling the reservoir too much or causing other issues.
Another practical concept is the difference between resource and reserve. The geothermal resource refers to the total amount of heat that exists in the rocks and fluids. The geothermal reserve is the portion that can be economically and technically extracted with current technology and under current conditions. These distinctions are important for planning geothermal projects, but the detailed project development aspects are discussed in other parts of the course.
Important relation: The thermal energy stored in a geothermal volume is often estimated as
$$
Q = m \, c_p \, (T - T_\text{ref}),
$$
but only a limited fraction of $Q$ is technically and economically recoverable as useful energy.
Spatial Distribution And Global Potential
Geothermal heat exists everywhere beneath the Earth’s surface, but it is not equally accessible everywhere. Regions with high geothermal gradients and active tectonics, such as the so called “Ring of Fire” around the Pacific, have particularly favorable resources for electricity generation. Countries located in these belts, such as Iceland, Indonesia, New Zealand, and several countries in East Africa and Central America, have developed large geothermal power industries in specific regions.
At the same time, low and moderate temperature resources are widespread, including in many parts of Europe, North America, and Asia. These lower temperature systems are well suited for direct use in heating networks, spa facilities, greenhouses, and some industrial processes.
Estimates of global geothermal potential depend strongly on assumptions about technology and economics. Under conservative assumptions that focus only on today’s conventional hydrothermal systems, the technical potential for electricity is smaller than that of solar or wind. However, if advanced technologies that access deeper or less permeable hot rock become commercially viable, the long term potential of geothermal energy as a low carbon resource could be much larger.
Sustainability Of Geothermal Resources
A key question is how sustainable a geothermal resource can be over time. The answer depends on how fast heat is extracted compared to how fast heat flows into the reservoir from surrounding rocks and deeper layers. If extraction is balanced with the natural or induced recharge of heat and fluids, then the resource can be used for very long periods with only slow temperature decline.
Production from a geothermal reservoir usually causes some cooling and pressure reduction in the vicinity of the wells. However, with careful management, including reinjection of cooled fluids into suitable parts of the reservoir, the overall resource can often be sustained for decades or longer. The time scale is much longer than typical project lifetimes.
It is important to recognize that geothermal resources are renewable in the sense that the heat originates from ongoing processes in the Earth and continues to flow. At the same time, local reservoirs can be depleted or underperform if they are overexploited without proper management. Understanding the basic physical characteristics of each geothermal system is therefore essential before deciding how to use it.
Conclusion
Geothermal resource basics rest on a few core ideas: the existence of heat inside the Earth from primordial and radioactive sources, the increase of temperature with depth described by the geothermal gradient, the movement of heat through conduction and convection, and the formation of reservoirs where hot fluids can circulate and be extracted. These physical and geological foundations determine where useful geothermal resources are found, at what temperatures, and how much energy can be obtained sustainably. Later chapters in this section build on these concepts to explain how different geothermal technologies tap into this heat for both power generation and direct use.