Geothermal energy

GEOTHERMAL ENERGY

Geothermal energy is energy recovered from the heat of the earth's core. In nature, geothermal heat shows up in the form of volcanoes, hot springs and geysers. For thousands of years, humans have used naturally occurring hot springs for bathing. More recently, geothermal energy has been used to generate electricity, and to provide heat for homes and industries. Geothermal energy is a versatile and reliable source of heat and electricity which generally produces none of the greenhouse gases associated with the combustion of fossil fuels. Unfortunately, the best geothermal resources are concentrated in areas of volcanic activity and are not widely distributed. California, Iceland, Italy, New Zealand and Japan are all areas where geothermal energy is used on a significant scale.

The high temperatures in the earth's core are a result of heat trapped during the formation of the earth approximately 4.7 billion years ago, as well as the decay of naturally occurring radioactive elements. The rate of heat flow out of the earth is about 5,000 times smaller than the rate of solar energy reaching the earth's surface. Solar radiation therefore controls the surface temperature of the planet; but a few meters below the earth's surface, temperatures are governed by the internal heat of the earth. Geothermal energy is often considered a renewable source of energy. This is not strictly true, because human uses of geothermal generally remove the heat from a location faster than it is replaced. The magnitude of the geothermal resource is so large, however, that on a human time scale it may be considered as a renewable energy source.

The Geothermal Resource

The temperature of the earth's crust rises as the depth from the surface increases, all over the world. In some places the rate of this increase in temperature, the "geothermal gradient", is higher than in others. These areas tend to be located in regions that are geologically active, where sections of the earth's crust are either colliding or moving apart (Figure 1). Due to this fact, the most promising geothermal resources are located in areas of volcanic activity. The higher the geothermal gradient, the less expensive it is to extract heat from the earth, due to drilling and pumping costs. In the ultimate case, the gradient may be so high that naturally occurring surface waters have been heated to a useful temperature. This is the case with hot springs and geysers.

Geothermal energy can be usefully extracted from four different types of geologic formations. These include hydrothermal, geopressurized, hot dry rock and magma. Each of these different reservoirs of geothermal energy can potentially be tapped and used for heating or electricity generation. Different extraction and processing techniques are required for the different sources of geothermal heat. In addition to the above, heat pumps can be used to extract low temperature heat from shallow depths. Such heat pumps are similar to the air-to-air heat pumps commonly used to heat homes, and will not be examined in detail.

Hydrothermal reservoirs contain hot water and/or steam trapped in fractured or porous rock formations by a layer of impermeable rock on top. Hydrothermal reservoirs have been the most common source of geothermal energy production worldwide. Geopressurized resources are from formations where moderately high temperature brines are trapped in a permeable layer of rock under high pressures. These brines often contain dissolved methane which can potentially be extracted for use as a fuel.

Hot dry rock is another potential geothermal resource. Hot dry rock reservoirs are generally hot impermeable rocks at depths shallow enough to be accessible (<3,000 m). To extract heat from such formations, the rock must be fractured and a fluid circulation system developed. Although hot dry rock resources are virtually unlimited in magnitude around the world, only those at shallow depths are currently economical. The final source of geothermal energy is magma, which is partially molten rock at very high temperatures (>600°C).

The theoretical potential of the world's geothermal energy resource is enormous. There is enough heat in the earth's core to provide all of the world's energy needs for thousands of years. Unfortunately, most of this heat is at such great depths below the surface that it is extremely expensive or impossible to extract. Accessible geothermal energy is also not evenly distributed around the globe. For instance, here in Canada, the only area with geothermal resources that can be economically extracted at the present time is British Columbia.

The result of the above facts is that geothermal energy is currently being exploited only in those regions where heat is available near the surface, such as "The Geysers" in California where 1,866 million watts (MW) of electricity are generated. Other locations with extensive use of geothermal energy include Reykjavikand Rotorua in Iceland where shallow reservoirs of subterranean steam are tapped to provide heating and hot water for many buildings. In all cases, individual reservoirs from which heat is extracted will eventually cool to the extent that they are no longer useful. This forces new or deeper wells to be drilled, increasing the costs of geothermal energy.

Geothermal Energy Technologies

Geothermal energy can either be used directly as heat for a district heating system or industrial process, or, if the temperature is high enough, converted into electricity. Unlike other renewable sources of electricity, geothermal power is not intermittent. It provides a reliable source of electricity 24 hours a day. The technology required to extract geothermal energy depends upon the type of the geothermal reservoir and the end use. Technologies for producing electricity from the four types of geothermal resources outlined above, will be looked at in detail. Using geothermal energy directly for heating involves the same heat extraction stages, but eliminates the need for a turbine and generator.

Hydrothermal reservoirs containing high temperature steam are the simplest source of geothermal electricity. Two holes or wells are drilled into the formation containing the steam. The steam is drawn out of one of the wells (the "production well") and allowed to pass through a standard turbine such as those used at thermal electricity generating stations (Figure 2). After passing through the turbine and thus turning a generator, the steam is condensed and returned to the rock formation through the second, "injection well". Returning the condensed liquid into the ground maintains a supply of geothermal fluids in the reservoir.

Hydrothermal and geopressurized reservoirs containing very hot water rather than steam are exploited in a similar fashion; but the hot water must first be "flashed" into steam as its pressure is reduced above the ground. Pumps are required to extract the water from hydrothermal reservoirs, while geopressurized systems often do not require a pump. Variations on the above technology include systems where steam is passed through two successive turbines, called a "double flash". The more expensive double flash systems capture more of the energy of the geothermal fluid and are therefore 10 to 20 percent more efficient than single flash plants.

Electricity production from lower temperature (<190°C) hydrothermal and geopressurized sources is generally accomplished through the use of "binary cycle technology". Binary cycle geothermal power plants pass lower temperature geothermal fluids through a heat exchanger to heat a working fluid such as iso-butane. Iso-butane has a lower boiling temperature and quickly vaporizes to power a turbine. Although this type of system is less efficient and more expensive than the single and double flash technologies, lower temperature geothermal resources can be exploited and the geothermal fluids are never released into the atmosphere. Methane can also be extracted from some geopressurized brines and used to generate additional power.

Hot dry rock (HDR) geothermal reservoirs are tapped by drilling two long boreholes and then fracturing the rock at whatever depth it is hot enough to provide useful amounts of energy. Water is pumped down one hole and comes up the other at an elevated temperature (Figure 3). Most HDR resources provide water at moderate temperatures (200°C), suitable for heating or use in a binary cycle power plant. Magma, or molten rock geothermal resources are very high temperature sources of geothermal energy. Although there is currently no existing technology for recovering heat from magma, it is a source of large amounts of energy when the magma is at reasonable depths.

FIGURE 3: As Hot Dry Rock (HDR) Geothermal Systems Concept for Low-Permeable Formations (90K).

Environmental Concerns

Although geothermal energy generally results in negligible greenhouse gas emissions, it is not without environmental impacts. The main environmental concern with geothermal energy is the result of natural contaminants dissolved in the water or brine extracted from the ground. Silica, sulfates, sulfides, carbonates, silicates and halides present in geothermal fluids present problems for both equipment and the environment. Hydrothermal and geopressurized water and brines are often very corrosive. This complicates the choice of materials for pipes, pumps and turbines. Dissolved compounds also tend to precipitate out of solution when these fluids are flashed into steam, clogging up the system.

Most geothermal power plants attempt to keep the working geothermal fluids within a closed system that returns them to the original reservoir after useful energy has been extracted. Despite this, many geothermal power plants release small amounts of these fluids into the surface environment. This is a result of having to vent steam that has reached excessive pressures, or mechanical breakdowns such as broken pipes. The major gaseous discharge from geothermal plants is hydrogen sulfide (H₂S), which smells like rotten eggs and can be toxic or fatal at high concentrations. The release of acidic geothermal fluids into surface water is also a concern, as it can damage aquatic ecosystems or contaminate drinking water supplies. Finally, locations with potential geothermal resources have often become tourist destinations due to attractions like hot springs and geysers. Producing energy from these resources can eliminate these naturally occurring features, hurting tourism and altering natural processes.

Conclusions

Geothermal energy represents a potentially huge source of reliable heat and electricity. The technology exists to exploit this resource in an environmentally acceptable manner, although only a few sites are cost effective at the present time. The best geothermal resources are not evenly distributed around the world; but more costly sources of geothermal energy such as HDR are widely available. The amount of geothermal energy utilized in the future will depend upon the cost and environmental concerns associated with traditional sources of energy, rather than the limits of the geothermal resource. As supplies of fossil fuels dwindle, or the impacts of global warming and acid rain become more severe, geothermal energy will become an attractive option for supplying heat and electricity in the future.