RENEWABLE ENERGY AND DISTRIBUTED GENERATION
Most forms of renewable energy do not fit happily into this operational structure. When electricity from them is available it must be used as if it were from a base-load plant, but because their output is intermittent, they cannot be relied on in the same way as a base-load plant. As a consequence, the introduction of large quantities of renewable energy into electricity systems that began at the beginning of the 21st century is leading to important changes in the way grid stability is maintained.
While the output of a conventional power plant will, barring accidents, remain steady and predictable, many renewable sources including wind and solar energy are, as just noted, intermittent and often unpredictable. This means that system operators must now manage not only variable demand from customers but also a variable supply. New strategies are being adopted by the sys- tem operators, including the use of highly detailed weather forecasting, so that the output from variable renewable plants can be predicted ahead of time and alternative capacity arranged where a shortfall is expected. The amount of electricity storage capacity included on a grid is expected to grow too as a means of balancing renewable generation.
The growth in the use of solar and wind power has led to other changes. One of these is an increase in the number of power stations that feed power into the distribution level of the grid rather than the transmission system level. This has arisen partly because many renewable power plants are too small to provide power into the backbone of the grid network. However, another important factor is the increased use of electricity generated locally.
Local generation might be a rooftop solar panel supplying power to a single domestic household or a small wind farm providing electricity to the community that owns it. In both cases, the electric power never enters the transmission backbone but travels from generator to consumer either internally or across the distribution system. Distributed generation, as generation at this point in the grid hierarchy is called, is valuable because it means that power does not have to be transported far between the generator and the user, thereby reducing transmission losses. In the extreme case, such as power from a rooftop solar panel being used in a domestic household, virtually all transportation losses are eliminated.
The use of distributed generation allows new capacity to be added in small tranches as demand grows in a particular area. However, it creates difficulties for distribution networks. Traditionally these have been designed as passive net- works that deliver power from transmission networks to customers. They do not have the ability to balance their own network power locally because it has never been necessary for them to have this capability. Distributed generation alters the situation, and it is leading to the development of balancing capability at the distribution level. It is likely that future electricity networks will resemble an inter- connection of such semi-autonomous distribution networks rather than a single hierarchical network.
One of the technologies that will enable this to take place is called the smart grid. A smart grid is an electricity network in which the system operator, the power suppliers, and the power consumers can all interact and communicate in real time. To achieve this it is necessary to build a computer network that runs in parallel to the electricity network. This type of system is beginning to appear in parts of the world and it is likely to become common over the next 10–20 years.
The two-way communication across the network will eventually allow net- work system operators to control electricity-using devices such as washing machines or water heaters in homes, as well as consumption in commercial and industrial organizations, in response to changes in demand. This type of flexibility will help integrate larger amounts of renewable energy into grids without endangering stability, as well as making electricity use more efficient.