It is important to recognize that electricity is not mined or harvested, it must be manufactured. And since it's not easily stored in quantity, it must be manufactured at time of demand. Electricity is a form of energy, but not an energy source. Different generating plants harness different energy sources to make electric power. The two most common types are "Thermal Plants" and "Kinetic Plants".

However, fossil-fuel plants can create serious environmental problems. Burning these fuels produces sulfur-dioxide and nitric-oxide air-pollution requiring expensive scrubbers. Wastewater from the used steam can carry pollutants into water-sheds. Even with very good pollution controls, there is still waste material produced. Carbon-dioxide gas, and ash are the current concerns.

Also, fossil-fuels are not renewable. They took millions of years to make, and at some point they will run out. Extracting and transporting them for use has created environmental problems. Strip-mining of coal and oil-spills at sea can produce catastrophic impacts on ecosystems.

Rather than simply condensing and exhausting waste-steam after it has passed thru the turbine, "top-cycle" cogenerators pipe this usable commodity to nearby customers. "Bottom-cycle" cogenerators operate in reverse and use the waste steam from industrial processing to drive turbines. By reusing steam, thermal-efficiency at cogeneration plants can exceed 50%.

Recently developed cogeneration plants use new materials and designs to improve reliability, and control both thermal and atmospheric pollution. Since these new technologies are designed into plants from the start, they are less expensive to install. The economy and capability of cogeneration technology allows many plants to return to burning coal without exceeding air-quality standards. "Circulating Fluidized-Bed" boilers, "Selective Catalytic (and Non-catalytic) Reduction", and "Zero-Discharge" water treatment systems are examples of technologies being used to control various environmental problems.

In addition to these innovations some thermal plants are being designed to burn "biomass." (Shown is a biomass plant in Florida, image copyright US Generating). The term applies to waste wood or some other renewable plant material. For example, Okeelanta Cogeneration Plant in Florida burns bagasse waste from surrounding sugar-cane processing operations during one part of the year, and waste wood during the growing season.

However, the drive to field nuclear power in the US faltered in the face of public concerns over safety, environmental, and economic issues. As more safety mechanisms were specified, construction costs and system complexities grew. Also, plants have shown some unexpected quirks, such as boiler tubes wearing out prematurely. Nuclear engineers contend that early problems with nuclear plants are subject to technical fixes, and are working on new "inherently safe" plant designs. Opponents argue that simply using uranium and plutonium as fuel creates too many problems and risks, not worth any benefits the technology might have.

So far, one problem which has not been solved is that of disposing of spent fuel cores and contaminated accessories which may remain dangerous for thousands of years. Permanent burial in geologically stable locations is the plan being pursued at this time, though this is still very controversial.

High-profile accidents at Three Mile Island in 1979 and Chernobyl in 1986 were, for the nuclear industry, public-relation disasters. Continuing economic problems have made nuclear plants much less attractive investments. Even though it produced 22% of America's electricity in 1996, nuclear power's future in this country is uncertain and hotly debated.

Most hydro-electric plants use a reservoir to compensate for periods of drought, and to boost water-pressure in the turbines. These man-made lakes cover large areas, often creating picturesque sport and recreational facilities. The massive dams required are also handy for controlling floods. In the past, few questioned the common assumption that the benefits outweighed the costs.

These costs stem from the loss of land submerged by the reservoir. Dams have displaced people, and destroyed wild-life habitat and archeological sites. A dam-burst can be disasterous. Some environmental costs can be avoided by thoughtful design; using fish-ladders to permit fish to travel around a dam is one good example. However, other costs remain, and protests against some recent hydro-power projects have become as angry as anti-nuclear protests.

A special type of hydro-power is called "Pumped Storage". Some non-hydro plants can take advantage of periods of low demand (and low costs) by pumping water into a reservoir. When demand rises, some of this water is channeled through a hydro-turbine to generate electricity. Since "peak-load" generating units (used to meet temporary demand surges), are generally more expensive to run than "base-load" units (which run most of the time), pumped-storage is one way to boost system efficiency.

Like run-of-river hydro-plants, there are a limited number of suitable locations where the wind blows predictably. Even in such sites, turbines often have to be designed with special gearing so that the rotor will turn at a constant speed in spite of variable wind speeds. Some find less technical problems with installations that can turn a scenic ridge or pass into an ugly steel forest, or that can take a toll on birds.

By tapping this naturally-created thermal energy, geothermal plants provide electricity with low levels of pollution. There are several different varieties of plants, and the product from a geothermal site is used for heating as well as electricity production. Finding suitable sites can be difficult, although as technical innovations occur, more sites are made practical. Tapping geothermal sources can also have the effect of "turning off" natural geysers, and this possibility must be taken into account during the planning stage.

Solar panels have been relatively expensive to make, and of course they won't work at night or in foul weather. Some of the processes needed to manufacture them have recently been called into question environmentally. Not all of the sunlight striking a solar cell is converted into electricity, and boosting efficiency has been slow work. Yet, the idea of harnessing all of that free sunlight remains a powerful driver for solar power.

Fuel cells have generally been expensive to make and not well suited to large installations. However, they represent a "modular" technology in that capacity can be added in small increments (5 - 20MW) as needed, allowing utilities to reduce both capital expenditures and construction lead times. Research seems to show promise; one test installation in Yonkers, NY, can produce 200kw by using gas created in the operations of a wastewater treatment plant. Also, fuel-cell plants are being used for central power in Japan.

In the early 20^th Century, small generating companies consolidated and independent plants slowly disappeared. It simply became more economical to purchase power from a centrally-located utility rather than generate it on-site. Large regional power pools grew, as companies interconnected their transmission systems and shared reserve capacity. "Economy of Scale" became the watch-words.

This may change in the 21^st Century. As the technology of electrical generation improves, and environmental concerns rise, the very concept of large centralized generating stations is coming into question. In most cases, for example, it is not economical to heat homes and businesses from a central location. Individual furnaces provide heat for separate buildings, with fuel provided by an associated transportation and distribution systems. Gasoline or diesel powered generators provide decentralized power to buildings in emergencies, though they are not economical for full-time power. Continued technical improvements in fuel-cells or photo-voltaics may change these economics. This possibility is especially attractive considering the cost of and objections to building large power lines.