Even though Earth’s interior is a gigantic thermal engine, geothermal power supplies just 0.4 percent of the world’s electricity. To help this energy source grow, geothermal power is one of the focus areas in the U.S. Department of Energy’s research and development programs.
A recently published book entitled Geothermal Power Generation: Developments and Innovation gives the latest survey of success and challenges in geothermal power around the world. This week, we will look at a few resource geoscience takeaways from the book.
Rasoul Sorkhabi
Editor, Core Elements
Geology of Geothermal Concepts
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First, let’s break down a few key concepts within the geology of geothermal. A description by Elders and Moore in the book provides the following helpful explanations:
Heat sources: Earth’s internal heat comes from residual primordial heat from Earth’s accretionary and molten formation, as well as radiogenic heat from decay of radioactive elements.
Heat flow:
Total heat flow from Earth’s interior to the surface is 43–49 terawatts.
Average crustal heat flows are 71 milliwatts per square meter for the oceanic crust and 105 milliwatts per square meter for the continental crust.
Geothermal assets are classified into three types based on reservoir temperature:
Low-temperature resources of less than 125°C, which are used directly for heating
Medium-temperature resources of 125–180°C, which can generate electric power through pumping and binary technology (using hydrocarbon vapors to generate steam for turbines)
High-temperature resources of more than 180°C, which have thermo-artesian flow and directly generate electricity
Systemsare also divided into groups based on tectonic controls:
Andesitic volcanoes at convergent plate margins such as Indonesia and the Philippines
Rhyolitic (felsic) volcanoes, such as New Zealand
Rift or deep sedimentary basin reservoirs such as East Africa
Thin-crust extensional systems such as the Basin-and-Range
Mid-oceanic ridge or hotspot systems such as Iceland
Fluid types: Finally, systems are divided into liquid-dominated (common) and vapor-dominated (less common but with high enthalpy steams sent directly to the turbines).
To be considered unconventional, the geothermal system must:
Be an enhanced geothermal system, in which directional stimulated wells are produced from tight basement rocks, or
Have co-produced heat from oil and gas wells, or
Be a supercritical (have high temperature and high pressure) water system
Geologists employ the following techniques for exploration and development of geothermal systems: Satellite imagery, geologic mapping, petrography, mineralogy, including XRD, fluid inclusion analysis, electron probe analysis, stable isotopes, and radiometric dating.
Here’s a look at the current geothermal power capacity:
The world’s installed geothermal power capacity is more than 16 gigawatts. One gigawatt of electricity meets the electric demands of about 300,000 people in a U.S. city.
The top ten geothermal countries are the United States, Indonesia, Philippines, Turkey, New Zealand, Mexico, Kenya, Italy, Iceland, and Japan.
Join us for a discussion on Argentina's Vaca Muerta shale and how we can apply its key principles to enhance domestic production. We will explore strategies for a more productive energy future.
Now that we’ve covered some of the basic notes around the geology of geothermal power, let’s look at some examples of geothermal at work.
The Geysers in California
Sanyal and colleagues describe the Golden State’s most celebrated geothermal field, The Geysers, which has been supplying continuous commercial electric power since 1960.
Today, it is the world’s largest geothermal field in areal extent at 28,447 acres.
It is a dry vapor-dominated field that generates about 6,300 gigawatt-hours annually.
The field is operated by about 400 active steam wells and more than 70 active injection wells. The average depth of the well is 2,600 meters, with the deepest being 3,900 meters.
The steam average production per well is 5,400 kilograms/hour, at a temperature of 190°C.
A future significant development in The Geysers would be deeper brine reservoirs at ultrahot rock approaching the supercritical point, which is speculated to exist in the field but has not been explored.
Italy’s Larderello
Larderello in Tuscany is the world’s second largest vapor-dominated geothermal field.
Parri and colleagues offer a detailed description of the historical and technological development of Larderello.
Even though the hot springs at Larderello were known for centuries, Prince Piero Ginori Conti made the pioneering drilling effort in the early 1900s. He drilled a steam well near Larderello that powered a piston engine to light up five small bulbs.
Laraderello hot springs are nestled in shallow hot granite and have subsurface steams of up to 220 °C.
Larderello power plant has eight units, with a combined turbine power of 275 megawatts and generative power of 340 megavolt-ampere. One third of Italy’s geothermal power comes from Larderello.
Kenya’s Geothermal Success
Kenya is situated in the East African System with tectonic valleys and lakes oriented nearly north-south. It is home to at least 15 geothermal fields.
The first geothermal field, Eburru, dates back to 1939, when a 200-meter-deep well was drilled and connected to pipelines, bringing steam to a farmland to dry flowers and grains.
The largest geothermal well in Kenya is the Greater Olkaria geothermal field, which began in 1981. Three hundred wells have been drilled in this field to date.
Kenya generates 972 megawatts of geothermal power, ranking seventh in the world and the largest in Africa. Geothermal power supplies 31 percent of Kenya’s electricity.
Geothermal in the Philippines
Pennarroyo discusses the rise and fall of his country’s position in geothermal market.
The first geothermal field was developed in 1977.
Six years later, the Philippines became the second largest geothermal producer in the world. It held this title until 2018, when it was replaced by Indonesia.
In 2019, geothermal power accounted for 13 percent of the country’s electric generation, but this is expected to decrease to eight percent in 2030 and four percent in 2040, unless new investments are made.
The high cost of development activities is slowing down the Philippines’ growth in geothermal production.
The country’s location, sitting on an oceanic-plate subduction setting, has huge proven geothermal reserves.
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