Hydroelectric Power, what is it?
It is a form of energy . . . a renewable resource. Hydro power provides about 96 percent of the renewable energy in the United States. Other renewable resources include geothermal, wave power, tidal power, wind power, and solar power. Hydroelectric power plants do not use up resources to create electricity nor do they pollute the air, land, or water, as other power plants may. Hydroelectric power has played an important part in the development of this Nation’s electric power industry. Both small and large hydroelectric power developments were instrumental in the early expansion of the electric power industry.
Hydroelectric power comes from flowing water . . . winter and spring runoff from mountain streams and clear lakes. Water, when it is falling by the force of gravity, can be used to turn turbines and generators that produce electricity.
Hydroelectric power is important to our Nation. Growing populations and modern technologies require vast amounts of electricity for creating, building, and expanding. In the 1920’s, hydroelectric plants supplied as much as 40 percent of the electric energy produced. Although the amount of energy produced by this means has steadily increased, the amount produced by other types of power plants has increased at a faster rate and hydroelectric power presently supplies about 10 percent of the electrical generating capacity of the United States. Hydropower is an essential contributor in the national power grid because of its ability to respond quickly to rapidly varying loads or system disturbances, which base load plants with steam systems powered by combustion or nuclear processes cannot accommodate.
Hydropower Works
Hydroelectric power comes from water at work, water in motion. It can be seen as a form of solar energy, as the sun powers the hydrologic cycle which gives the earth its water. In the hydrologic cycle, atmospheric water reaches the earth’s surface as precipitation. Some of this water evaporates, but much of it either percolates into the soil or becomes surface runoff. Water from rain and melting snow eventually reaches ponds, lakes, reservoirs, or oceans where evaporation is constantly occurring.
Moisture percolating into the soil may become ground water (subsurface water), some of which also enters water bodies through springs or underground streams. Ground water may move upward through soil during dry periods and may return to the atmosphere by evaporation.
It is a form of energy . . . a renewable resource. Hydro power provides about 96 percent of the renewable energy in the United States. Other renewable resources include geothermal, wave power, tidal power, wind power, and solar power. Hydroelectric power plants do not use up resources to create electricity nor do they pollute the air, land, or water, as other power plants may. Hydroelectric power has played an important part in the development of this Nation’s electric power industry. Both small and large hydroelectric power developments were instrumental in the early expansion of the electric power industry.
Hydroelectric power comes from flowing water . . . winter and spring runoff from mountain streams and clear lakes. Water, when it is falling by the force of gravity, can be used to turn turbines and generators that produce electricity.
Hydroelectric power is important to our Nation. Growing populations and modern technologies require vast amounts of electricity for creating, building, and expanding. In the 1920’s, hydroelectric plants supplied as much as 40 percent of the electric energy produced. Although the amount of energy produced by this means has steadily increased, the amount produced by other types of power plants has increased at a faster rate and hydroelectric power presently supplies about 10 percent of the electrical generating capacity of the United States. Hydropower is an essential contributor in the national power grid because of its ability to respond quickly to rapidly varying loads or system disturbances, which base load plants with steam systems powered by combustion or nuclear processes cannot accommodate.
Hydropower Works
Hydroelectric power comes from water at work, water in motion. It can be seen as a form of solar energy, as the sun powers the hydrologic cycle which gives the earth its water. In the hydrologic cycle, atmospheric water reaches the earth’s surface as precipitation. Some of this water evaporates, but much of it either percolates into the soil or becomes surface runoff. Water from rain and melting snow eventually reaches ponds, lakes, reservoirs, or oceans where evaporation is constantly occurring.
Moisture percolating into the soil may become ground water (subsurface water), some of which also enters water bodies through springs or underground streams. Ground water may move upward through soil during dry periods and may return to the atmosphere by evaporation.
Water vapor passes into the atmosphere by evaporation then circulates, condenses into clouds, and some returns to earth as precipitation. Thus, the water cycle is complete. Nature ensures that water is a renewable resource.
Generating Power
In nature, energy cannot be created or destroyed, but its form can change. In generating electricity, no new energy is created. Actually one form of energy is converted to another form.
To generate electricity, water must be in motion. This is kinetic (moving) energy. When flowing water turns blades in a turbine, the form is changed to mechanical (machine) energy. The turbine turns the generator rotor which then converts this mechanical energy into another energy form “electricity”. Since water is the initial source of energy, we call this hydroelectric power or hydropower for short.
At facilities called hydroelectric power plants, hydropower is generated. Some power plants are located on rivers, streams, and canals, but for a reliable water supply, dams are needed. Dams store water for later release for such purposes as irrigation, domestic and industrial use, and power generation. The reservoir acts much like a battery, storing water to be released as needed to generate power.
In nature, energy cannot be created or destroyed, but its form can change. In generating electricity, no new energy is created. Actually one form of energy is converted to another form.
To generate electricity, water must be in motion. This is kinetic (moving) energy. When flowing water turns blades in a turbine, the form is changed to mechanical (machine) energy. The turbine turns the generator rotor which then converts this mechanical energy into another energy form “electricity”. Since water is the initial source of energy, we call this hydroelectric power or hydropower for short.
At facilities called hydroelectric power plants, hydropower is generated. Some power plants are located on rivers, streams, and canals, but for a reliable water supply, dams are needed. Dams store water for later release for such purposes as irrigation, domestic and industrial use, and power generation. The reservoir acts much like a battery, storing water to be released as needed to generate power.
The dam creates a “head” or height from which water flows. A pipe (penstock) carries the water from the reservoir to the turbine. The fast-moving water pushes the turbine blades, something like a pinwheel in the wind. The waters force on the turbine blades turns the rotor, the moving part of the electric generator. When coils of wire on the rotor sweep past the generator’s stationary coil (stator), electricity is produced.
This concept was discovered by Michael Faraday in 1831 when he found that electricity could be generated by rotating magnets within copper coils.
When the water has completed its task, it flows on unchanged to serve other needs.
Note: For making small amounts of electricity without building a dam, the small-scale hydroelectric generator is often the best solution, especially where fast-flowing streams on steep slopes are close by. A small-scale hydro system usually consists of an enclosed water wheel or turbine, which is made to spin by jets of high-velocity water. The water is taken from the stream and moved down slope to the turbine through a long pipe called a penstock. Water flowing through the penstock picks up speed, and is directed at the blades of the turbine by nozzles. The turbine spins continuously, as long as there is water to drive it. The turbine is connected to an electrical generator, and the electricity is then available for running appliances or charging batteries. The spent water is returned to the stream. This kind of system is called a “micro-hydro” system, “run-of-stream hydro” or “low-impact hydro.”
This concept was discovered by Michael Faraday in 1831 when he found that electricity could be generated by rotating magnets within copper coils.
When the water has completed its task, it flows on unchanged to serve other needs.
Note: For making small amounts of electricity without building a dam, the small-scale hydroelectric generator is often the best solution, especially where fast-flowing streams on steep slopes are close by. A small-scale hydro system usually consists of an enclosed water wheel or turbine, which is made to spin by jets of high-velocity water. The water is taken from the stream and moved down slope to the turbine through a long pipe called a penstock. Water flowing through the penstock picks up speed, and is directed at the blades of the turbine by nozzles. The turbine spins continuously, as long as there is water to drive it. The turbine is connected to an electrical generator, and the electricity is then available for running appliances or charging batteries. The spent water is returned to the stream. This kind of system is called a “micro-hydro” system, “run-of-stream hydro” or “low-impact hydro.”
Transmitting Power
Once the electricity is produced, it must be delivered to where it is needed our homes, schools, offices, factories, etc. Dams are often in remote locations and power must be transmitted over some distance to its users. Vast networks of transmission lines and facilities are used to bring electricity to us in a form we can use. All the electricity made at a power plant comes first through transformers which raise the voltage so it can travel long distances through power lines. (Voltage is the pressure that forces an electric current through a wire.) At local substations, transformers reduce the voltage so electricity can be divided up and directed throughout an area.
Once the electricity is produced, it must be delivered to where it is needed our homes, schools, offices, factories, etc. Dams are often in remote locations and power must be transmitted over some distance to its users. Vast networks of transmission lines and facilities are used to bring electricity to us in a form we can use. All the electricity made at a power plant comes first through transformers which raise the voltage so it can travel long distances through power lines. (Voltage is the pressure that forces an electric current through a wire.) At local substations, transformers reduce the voltage so electricity can be divided up and directed throughout an area.
While hydroelectric power plants are one source of electricity, other sources include power plants that burn fossil fuels or split atoms to create steam which in turn is used to generate power. Gas turbine, solar, geothermal, and wind-powered systems are other sources. All these power plants may use the same system of transmission lines and stations in an area to bring power to you. By use of this “power grid”, electricity can be interchanged among several utility systems to meet varying demands. So the electricity lighting your reading lamp now may be from a hydroelectric power plant, a wind generator, a nuclear facility, or a coal, gas, or oil-fired power plant . . . or a combination of these.
How power is computed?
Water can generate power when it moves from a high potential energy state to a low potential energy state. We can see how this happens by considering a mass of water Δmwater falling a height h through a gravitational field with constant gravitational acceleration g. The change in potential energy ΔPEwater is
ΔPEwater = (Δmwater) gh (3.1)
The height h is called the elevation or hydraulic head, and g ≈ 9.8 m/s2 on the surface of the earth.
One of the most common examples of hydropower, or power generated
by the change in potential energy of water moving through a gravitational field, is the hydroelectric power plant. Dams with turbines and generators are used to convert the change in potential energy in Equation (3.1) into mechanical kinetic energy. The water behind a dam falls through an elevation h. If the density of water ρwater is considered a constant, the mass of falling water can be written as
Δmwater = ρwater ΔVwater
Water can generate power when it moves from a high potential energy state to a low potential energy state. We can see how this happens by considering a mass of water Δmwater falling a height h through a gravitational field with constant gravitational acceleration g. The change in potential energy ΔPEwater is
ΔPEwater = (Δmwater) gh (3.1)
The height h is called the elevation or hydraulic head, and g ≈ 9.8 m/s2 on the surface of the earth.
One of the most common examples of hydropower, or power generated
by the change in potential energy of water moving through a gravitational field, is the hydroelectric power plant. Dams with turbines and generators are used to convert the change in potential energy in Equation (3.1) into mechanical kinetic energy. The water behind a dam falls through an elevation h. If the density of water ρwater is considered a constant, the mass of falling water can be written as
Δmwater = ρwater ΔVwater
Where ΔVwater is the volume of falling water
ΔPEwater = ρwater ghΔVwater
ΔPEwater = ρwater ghΔVwater
The power generated by the falling water in a time interval Δt is the hydropower
The term ΔVwater/Δt is the volumetric water flow rate qwater, or flow rate in volume of water per unit time. In terms of qwater, Equation (3.4) can be written in the form
Phydro = ρwater gh qwater
The output power depends on the efficiency ηhydro of the hydropower system, thus
Pout = ηhydro Phydro = ηhydro ρwater gh qwater
We see from Equation (3.6) that power output depends on efficiency ηhydro, elevation h, and volumetric flow rate of water qwater.
The height h, or head, in the preceding equations should be replaced by the effective head heff for realistic systems. The effective head is less than the actual head h because water flowing through a conduit such as a pipe will lose energy to friction and turbulence. In this case
Phydro = ρwater gheff qwater
and
Pout = ηhydro Phydro = ηhydro ρwater gheff qwater
Phydro = ρwater gh qwater
The output power depends on the efficiency ηhydro of the hydropower system, thus
Pout = ηhydro Phydro = ηhydro ρwater gh qwater
We see from Equation (3.6) that power output depends on efficiency ηhydro, elevation h, and volumetric flow rate of water qwater.
The height h, or head, in the preceding equations should be replaced by the effective head heff for realistic systems. The effective head is less than the actual head h because water flowing through a conduit such as a pipe will lose energy to friction and turbulence. In this case
Phydro = ρwater gheff qwater
and
Pout = ηhydro Phydro = ηhydro ρwater gheff qwater
The rate qwater that water falls through the effective head heff depends on
the volume of the penstock shown in Figure (3.5). If the penstock volume is too small, the output power will be less than optimum because the flow rate qwater could have been larger. On the other hand, the penstock volume cannot be arbitrarily large because the flow rate qwater through the penstock depends on the rate that water fills the reservoir behind the dam.
the volume of the penstock shown in Figure (3.5). If the penstock volume is too small, the output power will be less than optimum because the flow rate qwater could have been larger. On the other hand, the penstock volume cannot be arbitrarily large because the flow rate qwater through the penstock depends on the rate that water fills the reservoir behind the dam.
The volume of water in the reservoir, and corresponding height h, depends on the water flow rate into the reservoir. During drought conditions, the elevation can decline because there is less water in the reservoir. During rainy seasons, the elevation can increase as more water drains into the streams and rivers that fill the reservoir behind the dam. Hydropower facilities must be designed to balance the flow of water through the electric power generator with the water that fills the reservoir through such natural sources as rainfall, snowfall, and drainage. Typical hydropower plant sizes are shown in Table (3.1).
Peaking with Hydropower
Demands for power vary greatly during the day and night. These demands vary considerably from season to season, as well. For example, the highest peaks are usually found during summer daylight hours when air conditioners are running.
Nuclear and fossil fuel plants are not efficient for producing power for the short periods of increased demand during peak periods. Their operational requirements and their long startup times make them more efficient for meeting baseload needs.
Since hydroelectric generators can be started or stopped almost instantly, hydropower is more responsive than most other energy sources for meeting peak demands. Water can be stored overnight in a reservoir until needed during the day, and then released through turbines to generate power to help supply the peak load demand. This mixing of power sources offers a utility company the flexibility to operate steam plants most efficiently as base plants while meeting peak needs with the help of hydropower. This technique can help ensure reliable supplies and may help eliminate brownouts and blackouts caused by partial or total power failures.
Demands for power vary greatly during the day and night. These demands vary considerably from season to season, as well. For example, the highest peaks are usually found during summer daylight hours when air conditioners are running.
Nuclear and fossil fuel plants are not efficient for producing power for the short periods of increased demand during peak periods. Their operational requirements and their long startup times make them more efficient for meeting baseload needs.
Since hydroelectric generators can be started or stopped almost instantly, hydropower is more responsive than most other energy sources for meeting peak demands. Water can be stored overnight in a reservoir until needed during the day, and then released through turbines to generate power to help supply the peak load demand. This mixing of power sources offers a utility company the flexibility to operate steam plants most efficiently as base plants while meeting peak needs with the help of hydropower. This technique can help ensure reliable supplies and may help eliminate brownouts and blackouts caused by partial or total power failures.
Increasing use of other energy-producing power plants in the future will not make hydroelectric power plants obsolete or unnecessary. On the contrary, hydropower can be even more important. While nuclear or fossil-fuel power plants can provide base loads, hydroelectric power plants can deal more economically with varying peak load demands. This is a job they are well suited for.
Pumped Storage
Like peaking, pumped storage is a method of keeping water in reserve for peak period power demands. Pumped storage is water pumped to a storage pool above the power plant at a time when customer demand for energy is low, such as during the middle of the night. The water is then allowed to flow back through the turbine-generators at times when demand is high and a heavy load is place on the system.
Like peaking, pumped storage is a method of keeping water in reserve for peak period power demands. Pumped storage is water pumped to a storage pool above the power plant at a time when customer demand for energy is low, such as during the middle of the night. The water is then allowed to flow back through the turbine-generators at times when demand is high and a heavy load is place on the system.
Figure (3.8) When demand is high and a heavy load is placed on the system, water is allowed to flow back through the turbine-generators.
The reservoir acts much like a battery, storing power in the form of water when demands are low and producing maximum power during daily and seasonal peak periods. An advantage of pumped storage is that hydroelectric generating units are able to start up quickly and make rapid adjustments in output. They operate efficiently when used for one hour or several hours.
Because pumped storage reservoirs are relatively small, construction costs are generally low compared with conventional hydropower facilities.
Because pumped storage reservoirs are relatively small, construction costs are generally low compared with conventional hydropower facilities.
Advantages
• Hydropower is a fueled by water, so it’s a clean fuel source. Hydropower doesn’t pollute the air like power plants that burn fossil fuels, such as coal or natural gas.
• Hydropower is a domestic source of energy, produced in the United States.
• Hydropower relies on the water cycle, which is driven by the sun, thus it’s a renewable power source.
• Hydropower is generally available as needed; engineers can control the flow of water through the turbines to produce electricity on demand.
• Hydropower plants provide benefits in addition to clean electricity. Impoundment hydropower creates reservoirs that offer a variety of recreational opportunities, notably fishing, swimming, and boating. Most hydropower installations are required to provide some public access to the reservoir to allow the public to take advantage of these opportunities. Other benefits may include water supply and flood control.
• Once the dam is built, the energy is virtually free.
• No waste or pollution produced.
• Much more reliable than wind, solar or wave power.
• Water can be stored above the dam ready to cope with peaks in demand.
• Hydro-electric power stations can increase to full power very quickly, unlike other power stations.
• Electricity can be generated constantly.
• Hydropower is a fueled by water, so it’s a clean fuel source. Hydropower doesn’t pollute the air like power plants that burn fossil fuels, such as coal or natural gas.
• Hydropower is a domestic source of energy, produced in the United States.
• Hydropower relies on the water cycle, which is driven by the sun, thus it’s a renewable power source.
• Hydropower is generally available as needed; engineers can control the flow of water through the turbines to produce electricity on demand.
• Hydropower plants provide benefits in addition to clean electricity. Impoundment hydropower creates reservoirs that offer a variety of recreational opportunities, notably fishing, swimming, and boating. Most hydropower installations are required to provide some public access to the reservoir to allow the public to take advantage of these opportunities. Other benefits may include water supply and flood control.
• Once the dam is built, the energy is virtually free.
• No waste or pollution produced.
• Much more reliable than wind, solar or wave power.
• Water can be stored above the dam ready to cope with peaks in demand.
• Hydro-electric power stations can increase to full power very quickly, unlike other power stations.
• Electricity can be generated constantly.
Disadvantages
• The dams are very expensive to build. However, many dams are also used for flood control or irrigation, so building costs can be shared.
• Building a large dam will flood a very large area upstream, causing problems for animals that used to live there.
• Finding a suitable site can be difficult – the impact on residents and the environment may be unacceptable.
• Water quality and quantity downstream can be affected, which can have an impact on plant life.
• Fish populations can be impacted if fish cannot migrate upstream past impoundment dams to spawning grounds or if they cannot migrate downstream to the ocean.
• Hydropower plants can cause low dissolved oxygen levels in the water, a problem that is harmful to riparian (river back) habitats and is addressed using various aeration techniques, which oxygenate the water. Maintaining minimum flows of water downstream of a hydropower installation is also critical for the survival of riparian habitats.
• Hydropower plants can be impacted by drought. When water is not available, the hydropower plants can’t produce electricity.
• The dams are very expensive to build. However, many dams are also used for flood control or irrigation, so building costs can be shared.
• Building a large dam will flood a very large area upstream, causing problems for animals that used to live there.
• Finding a suitable site can be difficult – the impact on residents and the environment may be unacceptable.
• Water quality and quantity downstream can be affected, which can have an impact on plant life.
• Fish populations can be impacted if fish cannot migrate upstream past impoundment dams to spawning grounds or if they cannot migrate downstream to the ocean.
• Hydropower plants can cause low dissolved oxygen levels in the water, a problem that is harmful to riparian (river back) habitats and is addressed using various aeration techniques, which oxygenate the water. Maintaining minimum flows of water downstream of a hydropower installation is also critical for the survival of riparian habitats.
• Hydropower plants can be impacted by drought. When water is not available, the hydropower plants can’t produce electricity.
4. GEOTHERMAL ENERGY
Geothermal energy is the use of heat from the depths of the earth to generate power. It has been around for as long as the earth has existed. “Geo” means earthy and “thermal” means heat. So, geothermal means earth-heat.
The earth’s interior is subdivided into a crystalline inner core, molten outer core, mantle, and crust. Basalt, a dark volcanic rock, exists in a semi-molten state at the surface of the mantle just beneath the crust. Drilling in the earth’s crust has shown that the temperature of the crust tends to increase linearly with depth. The interior of the earth is much hotter than the crust. The source of heat energy is radioactive decay, and the crust of the earth acts as a thermal insulator to prevent heat from escaping into space.
The earth’s interior is subdivided into a crystalline inner core, molten outer core, mantle, and crust. Basalt, a dark volcanic rock, exists in a semi-molten state at the surface of the mantle just beneath the crust. Drilling in the earth’s crust has shown that the temperature of the crust tends to increase linearly with depth. The interior of the earth is much hotter than the crust. The source of heat energy is radioactive decay, and the crust of the earth acts as a thermal insulator to prevent heat from escaping into space.
Have you ever cut a boiled egg in half? The egg is similar to how the earth looks like inside. The yellow yolk of the egg is like the core of the earth. The white part is the mantle of the earth. And the thin shell of the egg, that would have surrounded the boiled egg if you didn’t peel it off, is like the earth’s crust.
Below the crust of the earth, the top layer of the mantle is a hot liquid rock called magma. The crust of the earth floats on this liquid magma mantle. When magma breaks through the surface of the earth in a volcano, it is called lava.
Below the crust of the earth, the top layer of the mantle is a hot liquid rock called magma. The crust of the earth floats on this liquid magma mantle. When magma breaks through the surface of the earth in a volcano, it is called lava.
For every 100 meters you go below ground, the temperature of the rock increases about 3 degrees Celsius. Or for every 328 feet below ground, the temperature increases 5.4 degrees Fahrenheit. So, if you went about 10,000 feet below ground, the temperature of the rock would be hot enough to boil water.
Deep under the surface, water sometimes makes its way close to the hot rock and turns into boiling hot water or into steam. The hot water can reach temperatures of more than 148ºC (300ºF). This is hotter than boiling water (212ºF/100ºC). It doesn’t turn into steam because it is not in contact with the air.
When this hot water comes up through a crack in the earth, we call it a hot spring, like Emerald Pool at Yellowstone National Park pictured on the left. Or, it sometimes explodes into the air as a geyser, like Old Faithful Geyser pictured on the right.
Geothermal energy can be obtained from temperature gradients between the shallow ground and surface, subsurface hot water, hot rock several kilometers below the earth’s surface, and magma. Magma is molten rock in the mantle and crust that is heated by the large heat reservoir in the interior of the earth. In some parts of the crust, magma is close enough to the surface of the earth to heat rock or water in the pore spaces of rock. The heat energy acquired from geological sources is called geothermal energy. Magma, hot water, and steam are carriers of energy.
The heat carried to the surface from a geothermal reservoir depends on the heat capacity and phase of the produced fluid. We illustrate this dependence by considering an example. Suppose the pore space of the geothermal reservoir is occupied by hot water. If the temperature of the produced water is at the temperature Tres of the geothermal reservoir, the heat produced with the produced water is
(4.1)
where ΔT is the temperature difference Tres − Tref, Tref is a reference temperature such as surface temperature, mw is the mass of produced water, and cw is the specific heat capacity of water. The mass of produced water can be expressed in terms of the volumetric flow rate qw, the period of flow Δt and the density of water ρw, thus
(4.2)
Substituting Equation (4.2) into (4.1) gives
(4.3)
The heat produced from a geothermal reservoir in time Δt is the geothermal power, or
(4.4)
The electrical power that can be generated from geothermal power depends on the efficiency ηgeo of conversion of geothermal power Pgeo to electrical power, thus
(4.5)
If steam is produced instead of hot water or in addition to hot water, the heat produced must account for the latent heat of vaporization.
Some of the largest geothermal production facilities in the world are at the geysers in California, and in Iceland. These areas are determined by the proximity of geothermal energy sources. The technology for converting geothermal energy into useful heat and electricity can be categorized as geothermal heat pumps, direct-use applications, and geothermal power plants. Each of these technologies is discussed later in this section.
Deep under the surface, water sometimes makes its way close to the hot rock and turns into boiling hot water or into steam. The hot water can reach temperatures of more than 148ºC (300ºF). This is hotter than boiling water (212ºF/100ºC). It doesn’t turn into steam because it is not in contact with the air.
When this hot water comes up through a crack in the earth, we call it a hot spring, like Emerald Pool at Yellowstone National Park pictured on the left. Or, it sometimes explodes into the air as a geyser, like Old Faithful Geyser pictured on the right.
Geothermal energy can be obtained from temperature gradients between the shallow ground and surface, subsurface hot water, hot rock several kilometers below the earth’s surface, and magma. Magma is molten rock in the mantle and crust that is heated by the large heat reservoir in the interior of the earth. In some parts of the crust, magma is close enough to the surface of the earth to heat rock or water in the pore spaces of rock. The heat energy acquired from geological sources is called geothermal energy. Magma, hot water, and steam are carriers of energy.
The heat carried to the surface from a geothermal reservoir depends on the heat capacity and phase of the produced fluid. We illustrate this dependence by considering an example. Suppose the pore space of the geothermal reservoir is occupied by hot water. If the temperature of the produced water is at the temperature Tres of the geothermal reservoir, the heat produced with the produced water is
(4.1)
where ΔT is the temperature difference Tres − Tref, Tref is a reference temperature such as surface temperature, mw is the mass of produced water, and cw is the specific heat capacity of water. The mass of produced water can be expressed in terms of the volumetric flow rate qw, the period of flow Δt and the density of water ρw, thus
(4.2)
Substituting Equation (4.2) into (4.1) gives
(4.3)
The heat produced from a geothermal reservoir in time Δt is the geothermal power, or
(4.4)
The electrical power that can be generated from geothermal power depends on the efficiency ηgeo of conversion of geothermal power Pgeo to electrical power, thus
(4.5)
If steam is produced instead of hot water or in addition to hot water, the heat produced must account for the latent heat of vaporization.
Some of the largest geothermal production facilities in the world are at the geysers in California, and in Iceland. These areas are determined by the proximity of geothermal energy sources. The technology for converting geothermal energy into useful heat and electricity can be categorized as geothermal heat pumps, direct-use applications, and geothermal power plants. Each of these technologies is discussed later in this section.
There are four main types of geothermal field:
1- Hot Dry Rock: the dry rock field is simply an area which has dry hot rocks several kilometers deep inside the earth. These rocks are heated by magma directly below them and have elevated temperatures, but they do not have a means of transporting the heat to the surface. In this case, it is technically possible to inject water into the rock, let it heat up, and then produce the hot water. Figure (4.3) illustrates a hot, dry rock facility that is designed to recycle the energy-carrying fluid. Water is injected into fissures in the hot, dry rock through the injector and then produced through the producer. The power plant at the surface uses the produced heat energy to drive turbines in a generator. After the hot, produced fluid transfers its heat to the power plant, the cooler fluid can be injected again into the hot, dry rock.
1- Hot Dry Rock: the dry rock field is simply an area which has dry hot rocks several kilometers deep inside the earth. These rocks are heated by magma directly below them and have elevated temperatures, but they do not have a means of transporting the heat to the surface. In this case, it is technically possible to inject water into the rock, let it heat up, and then produce the hot water. Figure (4.3) illustrates a hot, dry rock facility that is designed to recycle the energy-carrying fluid. Water is injected into fissures in the hot, dry rock through the injector and then produced through the producer. The power plant at the surface uses the produced heat energy to drive turbines in a generator. After the hot, produced fluid transfers its heat to the power plant, the cooler fluid can be injected again into the hot, dry rock.
2- The dry steam type of geothermal source is the best known because it is the easiest to exploit. It consists of a layers of hot, porous rock capped with a dense layer of impermeable rock. Water seeps into the porous rock and it turned to stream by the heat .The upper layer of rock prevent the stream from escaping forming a natural boiler. When a hole is bored through the top layer of the rock, a tail plume of superheated stream goes roaring into the air under high pressure. The fields at Larderello, Italy; The Geysers, California; and Valle Caldera, New Mexico are of the type.
3- The wet steam type of geothermal field is about twenty times more common than the dry stream type. It contains a large amount of water and relatively little stream under high pressure. When the pressure is relieved by drilling, part of the water flashes into steam, forcing a mixture of steam and hot water to the surface. The discharge is 10-20 % steam, the rest water. The geothermal field at Wairakei, New Zealand; Parantunka, U.S.S.R.; Cerro Prieto, Mexico; and the imperial Vally, California are of this type.
4- The hot water geothermal field is one in which the temperature is not high enough to produce steam. While not directly useful to producing power, the hot water from these sources can be used for home heating and many other purposes. Hungary and Iceland both have extensive fields which they have used for this purpose.
Geothermal Heat Pumps
A geothermal heat pump uses energy near the surface of the earth to heat and cool buildings. The temperature of the upper three meters of the earth’s crust remains in the relatively constant range of 10ºC to 16ºC. A geothermal heat pump for a building consists of ductwork in the building connected through a heat exchanger to pipes buried in the shallow ground nearby. The building can be heated during the winter by pumping water through the geothermal heat pump. The water is warmed when it passes through the pipes in the ground. The resulting heat is carried to the heat exchanger where it is used to warm air in the ductwork. During the summer, the direction of heat flow is reversed. The heat exchanger uses heat from hot air in the building to warm water that carries the heat through the pipe system into the cooler shallow ground. In the winter, heat is added to the building from the earth, and in the summer heat is removed from the building.
Direct-Use Applications
A direct-use application of geothermal energy uses heat from a geothermal source directly in an application. Hot water from the geothermal reservoir is used without an intermediate step such as the heat exchanger in the geothermal heat pump. Hot water from a geothermal reservoir may be piped directly into a facility and used as a heating source. A direct-use application for a city in a cold climate with access to a geothermal reservoir is to pipe the hot water from the geothermal reservoir under roads and sidewalks to melt snow.
Minerals that are present in the geothermal water will be transported with the hot water into the pipe system of the direct-use application. Some of the minerals will precipitate out of the water when the temperature of the water decreases. The precipitate will form a scale in the pipes and reduce the flow capacity of the pipes. Filtering the hot water or adding a scale retardant can reduce the effect of scale. In either case, the operating costs will increase.
3- The wet steam type of geothermal field is about twenty times more common than the dry stream type. It contains a large amount of water and relatively little stream under high pressure. When the pressure is relieved by drilling, part of the water flashes into steam, forcing a mixture of steam and hot water to the surface. The discharge is 10-20 % steam, the rest water. The geothermal field at Wairakei, New Zealand; Parantunka, U.S.S.R.; Cerro Prieto, Mexico; and the imperial Vally, California are of this type.
4- The hot water geothermal field is one in which the temperature is not high enough to produce steam. While not directly useful to producing power, the hot water from these sources can be used for home heating and many other purposes. Hungary and Iceland both have extensive fields which they have used for this purpose.
Geothermal Heat Pumps
A geothermal heat pump uses energy near the surface of the earth to heat and cool buildings. The temperature of the upper three meters of the earth’s crust remains in the relatively constant range of 10ºC to 16ºC. A geothermal heat pump for a building consists of ductwork in the building connected through a heat exchanger to pipes buried in the shallow ground nearby. The building can be heated during the winter by pumping water through the geothermal heat pump. The water is warmed when it passes through the pipes in the ground. The resulting heat is carried to the heat exchanger where it is used to warm air in the ductwork. During the summer, the direction of heat flow is reversed. The heat exchanger uses heat from hot air in the building to warm water that carries the heat through the pipe system into the cooler shallow ground. In the winter, heat is added to the building from the earth, and in the summer heat is removed from the building.
Direct-Use Applications
A direct-use application of geothermal energy uses heat from a geothermal source directly in an application. Hot water from the geothermal reservoir is used without an intermediate step such as the heat exchanger in the geothermal heat pump. Hot water from a geothermal reservoir may be piped directly into a facility and used as a heating source. A direct-use application for a city in a cold climate with access to a geothermal reservoir is to pipe the hot water from the geothermal reservoir under roads and sidewalks to melt snow.
Minerals that are present in the geothermal water will be transported with the hot water into the pipe system of the direct-use application. Some of the minerals will precipitate out of the water when the temperature of the water decreases. The precipitate will form a scale in the pipes and reduce the flow capacity of the pipes. Filtering the hot water or adding a scale retardant can reduce the effect of scale. In either case, the operating costs will increase.
Figure (4.4) Geothermal heating system
Geothermal Power Plants
Geothermal power plants use steam or hot water from geothermal reservoirs to turn turbines and generate electricity. Dry-steam power plants use steam directly from a geothermal reservoir to turn turbines. Flash steam power plants allow high-pressure hot water from a geothermal reservoir to flash to steam in lower-pressure tanks. The resulting steam is used to turn turbines. Another type of plant called a binary-cycle plant uses heat from moderately hot geothermal water to flash a second fluid to the vapor phase. The second fluid must have a lower boiling point than water so that it will be vaporized at the lower temperature associated with the moderately hot geothermal water. There must be enough heat in the geothermal water to supply the latent heat of vaporization needed by the secondary fluid to make the phase change from liquid to vapor. The vaporized secondary fluid is then used to turn turbines.
A geothermal power plant is like in a regular power plant except that no fuel is burned to heat water into steam. The steam or hot water in a geothermal power plant is heated by the earth. It goes into a special turbine. The turbine blades spin and the shaft from the turbine is connected to a generator to make electricity. The steam then gets cooled of in a cooling tower.
Another use of geothermal heat is to increase agricultural productivity in colder regions by warming the soil and preventing forest. Geothermal steam can be used directly by industry. In Kawerau, New Zealand, geothermal steam is used in paper mill. In Japan, geothermal steam evaporates sea water to produce table salt. Geothermal brine may one day be an important source of minerals. For many years, borax was extracted from geothermal water at Larderello; Italy. It has been suggested that geothermal brines in the U.S. could yield lithium and precious metals. Not only the minerals, but the water itself is of value. The heat in geothermal brine can be used desalinate it, resulting in water that can be used for irrigation. It has been estimated that the Imperial Valley in California could yield a billion acre feet of water and power as well. Presently the department of the Interior’s Bureau of Reclamation desalinated geothermal water will be used to replenish the waters of the Colorado River.
Geothermal power plants use steam or hot water from geothermal reservoirs to turn turbines and generate electricity. Dry-steam power plants use steam directly from a geothermal reservoir to turn turbines. Flash steam power plants allow high-pressure hot water from a geothermal reservoir to flash to steam in lower-pressure tanks. The resulting steam is used to turn turbines. Another type of plant called a binary-cycle plant uses heat from moderately hot geothermal water to flash a second fluid to the vapor phase. The second fluid must have a lower boiling point than water so that it will be vaporized at the lower temperature associated with the moderately hot geothermal water. There must be enough heat in the geothermal water to supply the latent heat of vaporization needed by the secondary fluid to make the phase change from liquid to vapor. The vaporized secondary fluid is then used to turn turbines.
A geothermal power plant is like in a regular power plant except that no fuel is burned to heat water into steam. The steam or hot water in a geothermal power plant is heated by the earth. It goes into a special turbine. The turbine blades spin and the shaft from the turbine is connected to a generator to make electricity. The steam then gets cooled of in a cooling tower.
Another use of geothermal heat is to increase agricultural productivity in colder regions by warming the soil and preventing forest. Geothermal steam can be used directly by industry. In Kawerau, New Zealand, geothermal steam is used in paper mill. In Japan, geothermal steam evaporates sea water to produce table salt. Geothermal brine may one day be an important source of minerals. For many years, borax was extracted from geothermal water at Larderello; Italy. It has been suggested that geothermal brines in the U.S. could yield lithium and precious metals. Not only the minerals, but the water itself is of value. The heat in geothermal brine can be used desalinate it, resulting in water that can be used for irrigation. It has been estimated that the Imperial Valley in California could yield a billion acre feet of water and power as well. Presently the department of the Interior’s Bureau of Reclamation desalinated geothermal water will be used to replenish the waters of the Colorado River.
Advantages:
1. Geothermal technology is simple. It does not require the complex and sophisticated technology that nuclear power does.
2. The cost of geothermal energy compares very favorably with fossil fuel plants.
3. Geothermal energy does not produce any pollution, and does not contribute to the greenhouse effect.
4. The power stations do not take up much room, so there is not much impact on the environment.
5. No fuel is needed.
1. Geothermal technology is simple. It does not require the complex and sophisticated technology that nuclear power does.
2. The cost of geothermal energy compares very favorably with fossil fuel plants.
3. Geothermal energy does not produce any pollution, and does not contribute to the greenhouse effect.
4. The power stations do not take up much room, so there is not much impact on the environment.
5. No fuel is needed.
Disadvantages:
1. The big problem is that there are not many places where you can build a geothermal power station. You need hot rocks of a suitable type, at a depth where we can drill down to them. The type of rock above is also important, it must be of a type that we can easily drill through.
2. Fixed in certain site which may be far away from the power users.
3. It also resembles oil in that it is a depletable resource. Eventually the underground water is depleted or the temperature falls, then a new hole must be drilled. To be economically successful, a field must be able to supply steam for at least 30 years. The expected lifetime of the power plant.
4. Geothermal power plant must operate at much lower temperature that a fossil fuel plant. In consequence it is less efficient and generates more waste heat per megawatt of electricity produced. Getting rid of this heat may be a problem in the desert and semi-desert areas where many geothermal sources are located. Cooling water from rivers and lakes is seldom available and the high local air temperature makes air cooling difficult. One possible solution is to evaporate part of the brine brought up with steam.
5. Geothermal fields contain small quantities of hydrogen sulfide gas (H2S). This gas, with its rotten egg smell, should be familiar to any one who has ever visited a volcanic hot spring.
6. In some cases, removing geothermal water can cause the ground to subside. The obvious solution is to reinject water. The oil industry already does this when necessary and for the same reason. It has the further advantage of replenishing the water needed to produce steam. A question that remains unanswered is whether subsidence or a change in the underground heat balance might trigger earthquakes. Geothermal sources are located, after all, in earthquake-prone areas. So far there is no reason to suspect that any of the geothermal projects presently in operation have caused earthquakes.
1. The big problem is that there are not many places where you can build a geothermal power station. You need hot rocks of a suitable type, at a depth where we can drill down to them. The type of rock above is also important, it must be of a type that we can easily drill through.
2. Fixed in certain site which may be far away from the power users.
3. It also resembles oil in that it is a depletable resource. Eventually the underground water is depleted or the temperature falls, then a new hole must be drilled. To be economically successful, a field must be able to supply steam for at least 30 years. The expected lifetime of the power plant.
4. Geothermal power plant must operate at much lower temperature that a fossil fuel plant. In consequence it is less efficient and generates more waste heat per megawatt of electricity produced. Getting rid of this heat may be a problem in the desert and semi-desert areas where many geothermal sources are located. Cooling water from rivers and lakes is seldom available and the high local air temperature makes air cooling difficult. One possible solution is to evaporate part of the brine brought up with steam.
5. Geothermal fields contain small quantities of hydrogen sulfide gas (H2S). This gas, with its rotten egg smell, should be familiar to any one who has ever visited a volcanic hot spring.
6. In some cases, removing geothermal water can cause the ground to subside. The obvious solution is to reinject water. The oil industry already does this when necessary and for the same reason. It has the further advantage of replenishing the water needed to produce steam. A question that remains unanswered is whether subsidence or a change in the underground heat balance might trigger earthquakes. Geothermal sources are located, after all, in earthquake-prone areas. So far there is no reason to suspect that any of the geothermal projects presently in operation have caused earthquakes.
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