General
The primary energy sources for the production of electricity have been based on the combustion of fossil fuels (coal, oil, and natural gas) to produce steam to drive turbines. Alternatively, rivers are impounded to provide water to drive hydraulic turbines. A third principal source is the heat of nuclear reaction by uranium to produce steam to drive steam turbines.
Fossil Fuel Resources
During the early stages of the industrial revolution, most energy was generated by burning wood or coal in a boiler to produce steam to drive reciprocating steam engines, which, in turn, drove machinery by a system of belts and pulleys or was connected to drive wheels for locomotive use. Early electric power generation used the same process except that the belts and pulleys were connected to a generator to produce electricity. A significant advance was the development of the steam turbines.
Multiple units are generally located in one plant in order to achieve economies of scale, as common equipment can then serve more than a single unit. Common equipment includes fuel- and ash-handling equipment, water treatment, support buildings, and computer equipment, electrical equipment inventory for replacement parts, operating and maintenance staff, and transmission line substation equipment.
Bituminous, subbituminous, and lignite are classifications given to coals to indicate the amount of heat content per measure of weight. Transportation costs are significant and thus lignite, which has the lowest heat content, is often burned only in plants located at the fuel source. Experiments to convert coal to gases have been conducted to reduce the cost of coal transportation and have been implemented with limited success.
Part of the sulfur found in coal is converted to sulfur oxides, which are considered pollutants when discharged into the atmosphere. Most of the eastern and all the midwestern coals have high sulfur content, which requires some form of sulfur-removal equipment. Such equipment significantly increases plant capital costs and reduces plant efficiency. Coals with lower sulfur content are located in some western states. Transportation costs to bring this coal to the East and Middle West add significantly to its cost.
Presently, it is not financially feasible to convert coal to gaseous or liquid fuel, but it is an area of increased research and development. These procedures are attractive because they offer the possibilities of sulfur removal before combustion and of providing fuel for combustion turbines as well as steam boilers.
Boilers and precipitators are designed for the specific heat content, and so and so other physical and chemical properties (like sulfur content) of the fuel need to be used. Rising fuel costs have justified the conversion of many units to the use of multiple fuels.
Biofuels are quickly coming to the forefront as the price of oil escalates. Bio-fuels include ethanol, soy diesel, and gases produced from agricultural sources and animal and human wastes. Recent advances in waste processing have led to the building of power plants in conjunction with waste treatment, especially the waste from animal herds. Plans have been announced to convert human wastes into gases in the near future, partly as a response to reduce the environmental impact of waste-water treatment.
Solid waste is currently being used as a fuel and as an additive to coal in conventional power plants. Such combination fuel burning was in response to landfill limitations, but the increasing cost of oil is starting to justify the active use of waste resources.
Combustion turbines use gaseous and liquid fossil fuels that are burned, such that the hot gases can be used to drive a turbine directly. These combustion turbines eliminate the conversion of energy to steam and subsequent conversion to electricity, and thus have lower costs due to this system reduction. Such combustion turbines are less efficient and require more expensive fuels and more maintenance.
The net economic impact is higher operating costs. Recent developments have increased their efficiencies significantly by using the exhaust output of several units as input to a boiler system to create steam as a traditional unit performs.
The output of a combustion turbine is a high heat content exhaust gas. Not only is this gas at a high temperature, but it also contains a considerable amount of unburned fuel. It is economically possible to use the exhaust gas to generate steam either directly in a waste-heat recovery boiler or as preheated combustion air into a conventional boiler with the addition of other fuels. The steam produced can then drive a steam turbine-generator.
This arrangement is called a combined-cycle plant. Internal combustion engines are used to drive electric generators at distributed sites for reliability of supply. Hospitals, airports, emergency facilities, communication facilities, and other infrastructure needs require distributed generation to achieve significantly increased reliability requirements. Due to the operating costs of such facilities, they do not represent a significant part of total power generation at this time.
Residual fuel oil is a significant source of energy for power production. This oil contains the heavier components of crude oil that remain after gasoline and other light hydrocarbons have been removed. Oil fired steam power plants are less expensive to build and operate than coal fired plants. Combustion turbines use lighter oils as fuel.
Natural gas was traditionally a fuel for steam power plants located near oil fields where the gas is produced. The clean burning properties of gas have lead to gas firing in coal or oil boilers in other parts of the country as natural gas pipeline capacity is available. As there is a high value of natural gas for chemical and space heating uses, its future use as an energy source for electric generation is limited.
Natural gas is a significant fuel for distributed generation, especially if the heat can be used locally. Such generation includes combustion turbines that readily use natural gas as a fuel, especially when combined with an additional heat recovery system, called a combined cycle plant.
Nuclear Fuel
Nuclear reactors were developed as economical electric power production when the long term storage of the spent nuclear fuel was considered inexpensive. Subsequent studies have led to a mixed conclusion as to whether or not spent fuel can be stored economically over the lifetime of the radioactivity.
No new nuclear units have been built recently in the United States due to the concerns of long term storage and potential run-away reactions. Many other countries have continued to develop nuclear energy, given the increasing shortage of fossil fuels. The long term storage of spent fuel continues to be an unsolved problem. Several existing facilities have presently reached the maximum local storage of spent fuel. Additionally, many existing units in the United States are approaching the end of their useful life cycles.
Natural uranium is the basic fuel for all heavy-water fission reactors. It must be enriched (the content of fissionable uranium increased from the natural value of about 0.7% to about 3%) to be usable. This increase is accomplished by passing the natural product through filters or centrifuges that increase the concentration of fissionable material in part of the output while reducing it in the remainder, which is then unusable as fuel. A breeder reactor converts this depleted uranium back into usable fuel, thereby greatly extending the amount of usable uranium.
Plutonium is a fissionable by product of nuclear reactor operation. It can be mixed into natural or enriched uranium to recover the energy available in the plutonium.
Fusion reactors are expected to use deuterium as fuel. This material exists in large quantities in water but would have to be extracted and converted into a usable form as is presently under investigation as a multinational experiment.
Hydroelectric Power
Natural precipitation as rain or snow provides a continuous source of water at elevations higher than sea level. The flow of water back to lower elevations provides a source of energy by converting the potential energy into kinetic energy using waterwheels. Impoundment of rivers by dams provides a steady energy source and a larger elevation difference to localize the potential energy. The higher elevation of water locally is measured as effective water head. The natural elevation differential Niagara Falls was used for the motive power for the first commercial alternating current (ac) central station.
Hydropower is a renewable fuel resource. However, the traditional harnessing of hydropower is complicated by the need to dedicate a significant part of a river course to form a lake large enough to provide a steady water source. Initial costs for the dam and other construction work are significantly higher than for other types of generation. This higher first cost must be offset by long time fuel cost savings.
Therefore, the justification of hydropower is very sensitive to the replacement of other fuels and the scheduling procedures. Often the cost of a project is divided between multiple uses of water, such as power, navigation, irrigation, recreation, and flood control. These competing uses greatly restrict the availability (and thus the relative cost) of the power.
The use of tidal movement of water to generate power has been proposed in some coastal locations where there are large tides. Because of the relatively low water head provided by tidal action, it was originally thought necessary to impound huge quantities of water. The cost of the impounding structures has been found to be prohibitive. The structures also probably would have a significant environmental impact owing to their great size.
A new alternative is to use wind generators to harness the energy in tidal, river, and ocean currents. Such water generators resemble wind generators but are inverted, suspended from the surface, restricted to locational movement by anchors, and spin as the current flows across the blades, roughly at the speed of a revolving door.
While navigational use of that immediate area is restricted, the impact is considerably different from conventional hydro facilities. It is expected that the low cost of such systems may revitalize many of the abandoned hydro facilities with low head capability
Efforts are being made to develop power from ocean-wave action, but these are experimental and have a significant impact on the aesthetic shoreline use.
Geothermal Steam
At several locations in the world, natural steam is close enough to the surface of the earth that is accessible by using conventional drilling methods to pipe it to the surface. These locations are too few to be of any overall significance to most countries. The expansion of the use of geothermal steam to areas where the heat is not near the surface will require major progress in the development of very deep well drilling technology.
There is a considerable cost to the maintenance of such units as the steam has significant quantities of corrosive and solid materials that reduce the life expectancy of heat transfer equipment. However, the availability of such steam in several locations could be harnessed to generate hydrogen within the near future for export to energy dependent regions.
Fuel Cells
Fuel cells generate low level direct-current (dc) power as a result of a chemical reaction between a hydrocarbon fuel and oxygen. Development has progressed to the point where practical devices are available, even with the use of natural gas and other biogases. However, the costs have not yet been reduced to the point where fuel cells can be considered as competitive with other conventional power sources except in special applications where highly reliable power sources are required or in remote locations.
Primary Batteries
Primary batteries use a chemical reaction between two components of the battery to produce dc power. The battery components are depleted up in the process. At present, the cost is prohibitive for large scale applications.
Solar Electric Power
Electric power can be developed from the sun’s rays in two ways: solar cells that produce low levels of dc power as a result of the sun’s rays striking certain materials and solar boilers that consist of a system of mirrors that concentrate the rays from a large area onto a vessel containing water.
Practical use of solar electric power must overcome two fundamental problems: (1) the sun’s energy is so diffuse that very large earth surface areas must be covered by the mechanism used to collect and convert the energy; and (2) practical energy output is limited to part of the daylight hours on cloudless days. The practical locations in the United States are in the southwestern deserts, which are relatively far from power consuming areas as to require major transmission lines to deliver the power.
The use of solar collectors to power a conventional boiler have been constructed and demonstrated. Maintenance costs are high as the reflective surfaces are easily contaminated and abraded in such environments. Research into more resistant materials may soon render it possible to justify the conversion of solar energy to steam, solely on the basis of the fuel that is not consumed.
Wind Power
It is practical to generate power from propeller-driven generators. Recent developments in the capability of equipment and the advanced controls to cope with the variable nature of the wind and demand have lead to a major shift to use wind as a primary source of electricity.
The European Union and several U.S. investors have committed to major wind development investments. Several European countries have shifted to a high penetration of wind generation, as high as 61% in the Netherlands, due to expected scarcity of fossil fuels in their regions. Costs have been significantly reduced, while the equipment reliability has been dramatically improved. The use of wind generation is easily justified for remote areas.
Distributed Generation
There are two generic types of distributed generation. Distributed generation is inherent when renewable resources are the fuel, such as biofuels, solar, and wind. Distributed generation is also justified when heat or steam can serve other uses.
Several companies have developed small gas-fired generating units that are intended to be located in small groups scattered throughout the distribution system. The first units were designed to be 50 kW in size. Such systems have been installed and justified when the heat is also used for environmental heating or manufacturing processes.
In remote locations where electrical systems do not exist, it is expected that one or two extra units can be installed if biofuels are available. Such systems are used extensively as backup and for unplanned expansion. Many of these are operated as stand alone systems.
It is necessary to have alternative power sources to supply the load when the sun is not shining, the water flow is reduced in dry season, and the wind is not blowing at the proper speed. These alternative resources include energy storage and demand–side management, as well as the use of conventional power plants.
Thus, many of the renewable energy systems (wind, water, biofuels, etc.) require alternative sources, such as conventional power systems, or local storage. Local storage can include heat storage as well as hydrogen based fuel cells.
There are industrial processes that require large amounts of heat at temperatures and pressures below those at which boilers generate steam. When such combined demands are served by an integrated power plant, it is possible to obtain low cost power by generating steam at a higher temperature and pressure and running it through a turbine, subsequently exhausting the steam in the condition required by the industrial process.
This arrangement for multiple uses is called a cogeneration unit (traditionally called a topping unit) as such a joint service capability provides economical energy for the following reasons:
- The additional construction cost for the higher-temperature and higher pressure boiler plant is not significantly higher than the cost of a boiler plant built to supply the industrial process demand only.
- The required additional fuel generating higher temperature and higher pressure steam is less than the fuel cost for generating steam for industrial demand only. One principal reason for this is that for a conventional generating unit, the steam must be condensed back into water to obtain good overall efficiency. The condenser used for this purpose must be supplied with cooling water that absorbs most of the heat in the steam exhausted from the turbine. This heat is then dissipates to the atmosphere. There is far less condenser heat loss because the exhaust steam is used for process heat.
- Cogeneration units are installed in many facilities requiring high reliability for industrial processes.
Another method of producing by product power is the use of an extraction turbine, which has openings at one or more points to allow steam to be removed after it has passed partway through the turbine. This steam is at a lower temperature and pressure than the inlet steam and can be used as process steam. As with a topping unit, the extraction steam does not lose heat to a condenser; therefore, its generation efficiency is very high.
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