The Energy and Minerals Division of the AAPG focuses on unconventional hydrocarbon energy resources, such as coalbed methane and gas hydrates and alternative energy resources, such as coal, uranium and geothermal energy.
These resources are important to members of AAPG not only because they compete with conventional hydrocarbon resources but also because they often share technologies in exploration and production. High-temperature geothermal resources share many of the temperature challenges that are found in deep oil and gas wells.
This article takes a broader look at geothermal resources in terms of their overall contributions to the energy use equation.
Geothermal is a renewable resource with a useful temperature range from ambient to more than 650 degrees Fahrenheit. In addition, it can be a source of important minerals.
Uses of the geothermal resources fall into three categories. These are, in order of increasing temperature: ground-source heat pumps, direct use and geothermal electricity generation.
Ground-Source Heat Pumps
Ground-source heat pumps, also known as geothermal or geoexchange heat pumps, use the near surface of the earth as a thermal reservoir at ambient or near ambient temperatures. They do not use heat directly from the interior of the Earth, but use a heat pump to move heat from inside a building to the ground in the summer and from the ground to the building in the winter.
A heat pump is the main cooling component in a refrigerator or refrigerated air conditioning. In a refrigerator, heat is transferred from the icebox to cooling coils outside the refrigerator. In refrigerated air conditioning, heat is moved from inside a building to cooling coils outside the building. However, a ground-source heat pump is reversible and can transfer heat out of the building in the summer and into the building in the winter.
Approximately half of the energy consumed in an average home in the United States, and in many governmental, commercial and industrial buildings, is used for heating and cooling. Ground-source heat pumps are very efficient for heating and cooling, operating with 20-50 percent of the energy consumption of conventional systems. Widespread adoption of ground-source heat pumps could reduce electricity demand in the United States by 10-30 percent, with a similar reduction in carbon emissions.
Many relatively small-scale domestic, commercial and industrial uses of fuel and electricity are for low-temperature applications or hot water. These uses include space heating, greenhouse heating, aquaculture, food processing and preparation, and timber drying. The most efficient source of heat and water for these processes may be direct-use geothermal hot water, as no energy conversion is required to provide the heat and water.
Sources of hot water at the surface – thermal springs – are geographically limited and are often further limited by existing users, most commonly spas. However, as is known by all drillers and most members of AAPG, temperatures of 212 Fahrenheit and higher are commonly encountered at typical drilling depths for oil and gas. Where energy prices are high, and multi-story apartment buildings are common, such as in Germany, deep geothermal may be economic to produce hot water for space heating.
These conditions are not common in the United States, but hot produced water is a common by-product of oil and gas production. Wherever this water is likely to have a long lifetime, it could be a useful emission-free energy resource as direct-use geothermal.
Electricity Generation
The highest temperature geothermal resources are used to generate electricity. There are three basic types of geothermal generating systems for different types of geothermal resource. At a few locations (e.g., Larderllo, Italy; The Geysers, California), the resource is dry steam at a temperature greater than 300 degrees. The steam is fed directly to a turbine, which turns a generator to produce electricity. More commonly the resource is water at a temperature of greater than 360 degrees, which is flashed to water and steam at the surface. The steam is fed to a turbine, which turns a generator to produce electricity and the water is injected back into the reservoir to maintain reservoir fluid volume. For both dry-steam and flashed-steam power plants, more than one stage may be connected in series with the steam pressure decreasing at each stage.
For geothermal resources with temperatures lower than about 350 degrees, a binary system is used. In the binary system the geothermal fluid passes through a heat exchanger where a secondary fluid with a lower boiling temperature than water is vaporized. The geothermal fluid is then reinjected back into the reservoir. The vaporized secondary fluid drives a turbine that turns a generator to produce electricity. The secondary fluid is cooled before returning to the heat exchanger resulting in a back-pressure on the turbine. Two types of secondary fluid are used in operating binary power plants, organic refrigerants, (the organic rankine cycle, or ORC), and a mixture of two components, typically ammonia and water (the Kalina Cycle).
Economic geothermal power generation systems are currently sited on geothermal resources with sufficient permeability to produce adequate flow to maintain the energy transfer required for the electricity production. Experimental systems have been explored in a few countries in which permeability has been enhanced by hydraulic fracturing or similar techniques (enhanced geothermal systems, or EGS), but these systems are still in the investigation stage.
Some geothermal waters, especially with magmatic heat sources, have a high mineral content with solutes that can include economic concentrations of silica, zinc, lithium, manganese, gold, silver and some rare earth minerals. After the heat has been extracted from the geothermal fluid, one or more of these minerals may be extracted, increasing the economic return of the geothermal operation.
Direct use of geothermal resources and geothermal power production make valuable contributions to the mix of renewable but they are geographically limited to where high temperatures are near the surface or where the use justifies the cost of drilling.
Ground-source heat pumps may be used at almost any location where space heating and/or cooling, and even hot water and refrigeration are required.
Heat pumps are not an energy source and consume electricity. However, the energy savings that result from replacing most conventional heating and cooling systems with ground-source heat pumps would make a large reduction in the quantity of electricity needed to be generated. The most efficient and cleanest electricity is the electricity that does not need to be generated.
Paul Morgan is chair of the Geothermal Energy Committee of AAPG’s Energy Minerals Division. He is also senior geologist of the Colorado Geological Survey of the Colorado School of Mines, Golden, Colo.