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Distributed generation

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Distributed generation generates electricity from many small energy sources. It has also been called also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy.

Currently, industrial countries generate more than 99% of their electricity in large power plants, the majority of which burn coal. Some countries also use efficient generators burning natural gas, nuclear reactors or hydropower.

These plants have excellent economies of scale, but usually transmit electricity long distances. Coal plants do so to prevent pollution of the cities. Nuclear reactors are thought too unsafe to be in a city. Dam sites are often both unsafe, and intentionally far from cities. The coal and nuclear plants are too far away for their waste heat to be used for heating buildings.

Low pollution is a crucial advantage of combined cycle plants that burn natural gas or that gasify coal. The low pollution permits the plants to be near enough to a city to be used for district heating and cooling.

So-called inherently safe nuclear reactors such as the pebble bed reactors and molten salt reactors have no proven safety advantage: They are unlikely to be safe enough to be deployed near cities, nor to be used for process waste heat generation either.

Distributed generation is another approach. It reduces the amount of energy lost in transmitting electricity because the electricity is generated very near where it is used, perhaps even in the same building. This also reduces the size and number of power lines that must be constructed.

Typical distributed power sources have low maintenance, low pollution and high efficiencies. In the past, these traits required dedicated operating engineers, and large, complex plants to pay their salaries and reduce pollution. However, modern embedded systems can provide these traits with automated operation and clean fuels, such as sunlight, wind and natural gas. This reduces the size of power plant that can show a profit.

The usual problem with distributed generators are their high costs.

The one exception is probably microhydropower. A well-designed plant has nearly zero maintenance costs, and generates useful power indefinitely.

One favored source is solar panels on the roofs of buildings. These have high construction costs ($2.50/w, 2007). This is about fifty-fold higher than coal power plants ($0.047/w, 2007) and 40-fold higher than nuclear plants ($0.06/w, 2007). Most solar cells also have waste disposal issues, since solar cells often contain heavy-metal electronic wastes. The plus side is that unlike coal and hydropower, there are no pollution, mining safety or operating safety issues.

Another favored source is small wind turbines. These have low maintenance, and low pollution. Construction costs and total safety are also manyfold ($0.80/w, 2007) more per watt than large power plants, except in very windy areas. Wind towers and generators have substantial insurable liabilities caused by high winds, but good operating safety.

Distributed cogeneration sources use natural gas-fired microturbines or reciprocating engines to turn generators. The hot exhaust is then used for space or water heating, or to drive an absorptive chiller for air-conditioning. The clean fuel has only low pollution. Designs currently have uneven reliability, with some makes having excellent maintenance costs, and others being unacceptable.

Cogenerators are also more expensive per watt than central generators. They find favor because most buildings already burn fuels, and the cogeneration can extract more value from the fuel.

Some larger installations utilize combined cycle generation. Usually this consists of a gas turbine whose exhaust boils water for a steam turbine in a Rankine cycle. The condenser of the steam cycle provides the heat for space heating or an absorptive chiller. Combined cycle plants with cogeneration have the highest known thermal efficiencies, often exceeding 85%.

See also

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