Distributed Genertation: small-scale and grid-independent energy solutions
refers especially to small generating units, say less than 5 megawatts, that are "distributed" around the electrical distribution grid grid. These units could also operate independently of the electric grid. Such a spreading out of generation is in contrast to the usual situation in which generation is concentrated in large central stations, often on the order of 1000 megawatts, from which power flows over long distances by means of transmission and distribution lines.
There are many advantages to distributing generation in small units. In the past, however, these advantages were outweighed by the great economies of scale in central station power plants-the main expense in generation. The larger the units, the cheaper have been the costs of electricity. For most of the last century, this relationship drove the increasing size of electric plants. In the last twenty years of so that situation has changed and with the development of efficient gas turbines the optimum plant size has come down to the order of hundreds of megawatts and below. Such units are built in factories and transported to site, which has lots of advantages. And now even smaller units, in the below-5 megawatt category are beginning to look attractive. The process at work is that the economy of generating plant scale is being replaced by the economy of mass production of small units. I am talking mainly about how things look from the United States, but the ideas have general application.
Consider the advantages of small power plants?
1. Factory production of units is more efficient than field construction
2. Small units can match the customers' increasing needs for power more closely.
3. Distributed generation puts less stress on the grid and improves grid stability. This is important in the US because as a result of the restructuring of the electricity business the future ownership of the electrical grid is uncertain and grid investment has slowed.
4. Some of the technologies have environmental advantages.
5. More recently, in the US at least, there is concern over the vulnerability of the electric system to accidental or deliberate harm. Distributed generation decreases the vulnerability of the electric system.
The principal technologies that fall into the below-5 megawatt category are the following, more or less in order of commercial availability:
a. Reciprocating engines-mainly diesel engines, which have been becoming much more efficient, but also some gas fueled engines.
b. Small gas turbines in the 1-5 megawatt range
c. Microturbines-a simplified category of very small gas turbines (which may also find application in automobiles)
d. Fuel cells-which are basically batteries to which one continually feeds the basic component, or fuel. In practice, this is natural gas, from which an auxiliary unit extracts hydrogen which is the cell's basic fuel that combines with oxygen in air. Some types of fuel cells may have commercial application for automobiles and buses.
e. Other-renewable technologies. In principle, a distributed generation arrangement can accommodate a variety of units including wind generators, etc. There is a difference, however, between technologies whose output is guaranteed or predictable and those for which it is uncertain. The more predictable the generation, the more valuable it is.
A graphic way to illustrate the change in thinking about the role of small generating sources is to consider their possible application to automobiles. It used to be thought-when people were tied in their thinking to central power station model-that we would move to electric cars that would be mostly run on batteries in the daytime and be charged at night from central power stations, which are then less occupied. There is, however, another way to think about electric cars-that it carries its own generator. If the automobile uses a highly efficient small generator, when it is parked-which is most of the time-it can be providing energy for the owner's household needs, or supplying the grid. In this arrangement, instead of the car plugging into the house to charge its battery, the house would plug into the car.
Currently the technologies in use in the small categories are mainly diesel engines, which account for most of the total, and small gas turbines. The worldwide total amounts to about 14,000 megawatts in 2000 and increasing. Much of this is intended for standby power but a good deal is also used for prime power. The sizes of plants are more or less equally distributed among the categories of 20-300 kilowatts, 300 kilowatts-1 megawatt, and 1-5 megawatts.
The power units are spread around the world, with large numbers in the Far East, Southeast Asia, the Indian subcontinent, the Middle East, Europe, and North America. The most popular fuel has continued to be diesel oil but in the long run that is likely to change for environmental reasons, with a shift to natural gas.
Substantial capacity margins in the United States and Europe make it unlikely that there will be a rush toward the smaller plants in the immediate future. But thinking about electrical power and the role of the grid is changing and these plants may take on a larger part of the electricity supply. There is a convergence of thinking among policymakers, manufacturers, fuel suppliers, environmentalists, investors, and consumers. This combination may be powerful enough to change the status quo. The future of the small generating units is likely to be interesting and merely an extrapolation from the past.