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Distributed generation - arguments for and against

Research in the US and Canada is revealing that the strategic use of relatively small electricity production units at or near customers holds many economic and system operation advantages. Distributed generation, as the concept is known, is slowly gaining credibility -- with wind power figuring as an ideal candidate for boosting generating capacity at the edge of a supply system close to consumers.

There are several arguments in support of distributed generation. By meeting loads locally and thereby reducing the need for bulk power generation and transmission, distributed generation (DG) could cut traditional energy losses. At the same time transmission and distribution upgrades and extensions could be delayed, or their high costs diminished. Moreover, in some cases DG systems can match load growth better than large central stations. Wind plants, for example, can be quickly and easily expanded with minimum debt and risk to capital. Such advantages suggest that the worth of DG may substantially exceed the value of central station generation, according to recent studies by the Electric Power Research Institute (EPRI) in the US.

There are problems, however, requiring research on technical solutions. Until now, most independent power units have been connected directly to transmission systems and not to distribution lines. But the idea behind DG is that small units would supply power to the grid at the primary feeder level, along radial distribution lines which feed electricity to consumers. Experience with wind energy in Europe has shown that all too often the distribution grid cannot handle this kind of penetration. This is because traditional electricity supply networks have been designed for one way power flow, from central stations to local loads. In the case of a wind plant, a windy day would reverse this power flow, requiring the installation of safety and power control devices.

These devices are available at transmission level. Utility dispatchers rely on sophisticated computerised systems that monitor demand, communicate with generation units, and adjust power to maintain a stable frequency. But these automatic control systems do not track generation at the level of distribution. Neal Balu at EPRI says that no technology is available today to automatically record and adjust for such small power fluctuations. While utilities have been able to accommodate small amounts of DG because their effects are within the normal variation of loads, "a massive implementation of these units is a different story," he says. "If you've got 50 of these small, non-dispatchable units connected to the same distribution line, there is a potential for overloading the line." Such conditions would require transmission and distribution upgrades. While a 1% penetration in a relatively small area could be readily accommodated, a 20% penetration in the same area would be much more problematic.

Despite these problems, DG is still interesting to utilities. For reasons of economy, the environment and security of supply, the utility model is moving away from very large power stations (Windpower Monthly, November and September 1993). Some researchers believe that DG will achieve from 10-30% penetration of the new generation market by 2010. "Distributed generation does not do well meeting big, urban load centre needs, but is really focused at the individual customer or group of customers. It becomes even more economic where the loads diminish or move away from centres towards the transmission and distribution periphery," says Carl Weinberg, former research director with Pacific Gas and Electric in California (PG&E). With first hand experience in obtaining power from several hundred sources, including many DG units, PG&E is pioneering the concept among US utilities.

Wind is one of several possible alternatives within the DG concept -- and it has some very specific advantages. Wind power can match certain weather-related loads. California desert thermals turn wind turbines and match local cooling needs and in northern regions with strong winter winds, wind power output fits some heating needs. To make the best use of wind energy, though, ways have to be found of managing the excess power produced by turbines on windy days. Battery storage is not viable, but, if pump storage is not available on the system, other options range from reducing turbine output, storing the energy in hot water tanks, or producing hydrogen using electrolyses. In this way wind plant could achieve far greater capacity credit on power supply systems, doing away with some of the need for new fossil fuel plant -- and the pollution they also cause.

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