BC: GEOTHERMAL ENERGY PRIMER
..
..
..
..
..
..
..
..
..
..
..
..
..
..
..
..
This renewable energy source needs to be more widely
exploited for both space heating, domestic hot water
and large scale electric generation ..
The thought I had was to assess the geothermal potential
of the oil/gas industries deep "dry holes" .. the industry
still drills several hundred deep "dry holes" every year and
the number/location should be well known ..
Heat from the earth can be used in many ways, from large and complex to small and simple. Geothermal energy is as remote as deep wells in Indonesia and as close as the dirt in our back yards. Tapping that heat is a relatively clean and sustainable way to reduce fossil fuel use.
In California and Nevada, and in other places around the world, geothermal energy produces electricity in large power plants. Geothermal energy provides about 5 percent of California's electricity, and a third of El Salvador's. In Idaho and Iceland, geothermal heat is used to warm buildings and for other applications. In thousands of homes and buildings across the United States, geothermal heat pumps use the steady temperatures just underground to heat and cool buildings, cleanly and inexpensively.
The Geothermal Resource
Under the earth's crust, there is a layer of hot and molten rock called magma. Heat is continually produced there, mostly from the decay of naturally radioactive materials like uranium and potassium. The amount of heat within 10,000 meters (about 33,000 feet) of the surface contains 50,000 times more energy than all the oil and natural gas resources in the world.
The areas with highest underground temperatures are in regions with active or geologically young volcanoes. These "hot spots" occur at plate boundaries or at places where the crust is thin enough to let the heat through. The Pacific Rim, called the "ring of fire" for all of its volcanoes, has many hot spots, including some in Alaska, California, and Oregon. Nevada has hundreds of hot spots, covering most of the northern part of the state.
These regions are also seismically active. The many earthquakes and the movement of magma break up the rock covering, allowing water to circulate. As the water rises to the surface, natural hot springs and geysers occur, such as "Old Faithful" at Yellowstone National Park. The water in these systems can be more than 200 degrees C, or over 430 degrees F.
How Geothermal Energy Is Captured
Geothermal springs for power plants. The most common current way of capturing the energy from geothermal sources is to tap into naturally occurring "hydrothermal convection" systems. When heated water is forced to the surface, it is a relatively simple matter to capture that steam and use it to drive electric generators. Geothermal power plants drill their own holes into the rock to more effectively capture the steam.
There are three designs for geothermal power plants, all of which pull hot water and steam from the ground, use it, and then return it as warm water to prolong the life of the heat source. In the simplest design, the steam goes directly through the turbine, then into a condenser where the low-temperature steam is condensed into water. In a second approach, the steam and hot water are separated as they come out of the well; the steam is used to drive the turbine while the water is sent directly back underground.
In the third approach, called a binary system, the hot water and steam mixture is passed through a heat exchanger, where it heats a second liquid (like isobutane) in a closed loop. The isobutane boils at lower temperatures than water, so as steam it is used to drive the turbine. The three systems are shown in the graphics here.
The choice of which design to use is determined by the resource. If the water comes out of the well as steam, it can be used directly, as in the first design. If it is hot water, it must go through a heat exchanger. Since there are more hot water resources than pure steam, there is more growth potential in the heat exchanger design.
The largest geothermal system now in operation is a steam-driven plant in an area called The Geysers, north of San Francisco. Despite the name, there actually no geysers here, and the heat that is used for energy is all steam, not hot water. Although the area was known for its hot springs as far back as the mid-1800s, the first well for power production was drilled in 1924. Deeper wells were drilled in the 1950s, but real development didn't occur until the '70s and '80s. By 1990, 26 power plants had been built, for a capacity of over 2,000 megawatts. In 1992, the area produced enough power for a city of 1.3 million.
Because of the rapid development of the area in the '80s, and the technology used, the steam resource has been declining since 1988. In the Geysers, the plants use an evaporative water-cooling process to create a vacuum that pulls the steam through the turbine, producing power more efficiently. But this process loses 60 to 80 percent of the steam to the air, not reinjecting it underground. While the steam pressure may be declining, the rocks underground are still hot. Some efforts are under way to remedy the situation, including reinjecting water pumped in through a 26-mile pipeline, and replacing the water-cooled systems with air-cooled.
Another problem with open systems like the ones at the Geysers is that they produce some air emissions. Hydrogen sulfide, along with small amounts of arsenic and minerals, is released in the steam. At a power plant at the Salton Sea reservoir in California, a significant amount of salt builds up in the pipes and must be removed. While the plant initially started to put the salts into a landfill, they now reinject the salt back into a different well. With closed-loop systems, such as the binary system, there are no emissions; everything brought to the surface is returned underground.
Direct use of geothermal heat. Heat from geothermal springs can also be used directly for heat. Hot spring water is used to heat greenhouses for plants, to dry out fish and deice roads, for improving oil recovery, and to heat fish farms and spas. In Klamath Falls, Oregon, and Boise, Idaho, geothermal water has been used to heat homes and buildings for over a century. New housing developments in Reno, Nevada, are using geothermal heat from a well to heat homes.
In Iceland, virtually every building in the country is heated with hot spring water. In fact, Iceland gets 45 percent of its energy from geothermal sources. In Reykjavik, for example (population 145,000), hot water is piped in from 25 kilometers away, and residents use it for heating and for hot tap water. In the photo below, people bathe in the hot springs that supply a power plant in Iceland.
Hot dry rock. Geothermal heat occurs everywhere under the surface of the earth, but the conditions that make water circulate to the surface are found only in less than 10 percent of the land area of the earth. An approach to capturing the heat in dry areas is known as "hot dry rock." The rocks are first broken up by pumping high pressure water through them. Water is then pumped from the surface down through the broken hot rocks. After the water heats up, it is brought back to the surface through a second well and used to drive turbines or to provide heat.
Researchers at the Los Alamos National Lab in New Mexico have studied hot dry rock since 1974. The Fenton Hill plant involves a well drilled 11,500 feet into rock at 430 degrees F. Water pumped down the well at 80 degrees returned to the surface at 360 degrees F. The plant has produced as much as 5 megawatts of power, proving the technical feasibility of hot dry rock.
However, a number of barriers must be overcome before hot dry rock can become a commercial source of power. The wells must be quite deep, deeper than for conventional geothermal plants. Also, the flow of heat through dry rock is slow, which means the heat removed through the well will be slow to be renewed. Finally, the most promising sites for hot dry rock are in dry areas of the West, which means that water may be hard to come by.
Ground-source heat pumps. A much more conventional way to tap geothermal energy is by using geothermal heat pumps to provide heat and cooling to buildings. Also called ground-source heat pumps, they take advantage of the constant year-round temperature of about 50 degrees F just 5 to 10 feet underground. Either air or an antifreeze liquid is pumped through pipes that are buried underground, and recirculated into the building. In the summer, the liquid moves heat from the building into the ground. In the winter, it does the opposite, providing prewarmed air and water to the heating system of the building.
In the simplest use of ground-source heating and cooling, a tube runs from the outside air, under the ground, and into a house's ventilation system. More complicated but more effective systems use compressors and pumps, like in electric air conditioning systems, to maximize the heat transfer.
In regions with temperature extremes, such as the northern United States in the winter and the southern United States in the summer, ground-source heat pumps are the most energy-efficient and environmentally clean heating and cooling system available. A study by the Environmental Protection Agency found that they are as much as 72 percent more efficient than electric heating and air conditioning systems. The Department of Energy found that heat pumps can save between $300 and $800 a year in energy costs for a typical home, with the system paying for itself in three to eight years.
About 150,000 ground-source heat pumps had been installed by 1992, with the number growing by about 30,000 each year. While this is significant, it is less than 1 percent of the heating and cooling market. Heat pumps have higher up-front costs. Installing them in existing buildings can be difficult, since it involves digging up the yard around a house (provided it has a yard). Finally, many heating and cooling installers are just not familiar with the technology.
Ground-source heat pumps are catching on in some areas though. In rural areas without access to natural gas pipelines, homes must use propane or electricity for heating and cooling. Heat pumps are much less expensive to operate, and since buildings are widely spread out, installing underground loops is not a problem. Underground loops can be easily installed during construction of new buildings as well, resulting in savings for the life of the building.
|
