Dr. Jacob Klimstra
Modern power supply systems increasingly need distributed generation and, if possible, cogeneration in order to keep systems stable and electricity costs acceptably low. This is, however, not clear to everybody, especially in the case of the average citizen (and most policymakers), who are unfamiliar with capacity factors, utilisation factors and load factors. One often reads in the news, even in technical magazines, that a newly installed wind park produces sufficient electricity to cover the energy demands of, say, 100,000 households. Most people have no idea that such a message is misleading.
Firstly, final electricity use is only 12% of total global energy use. Secondly, households consume, on average, only 20% of the electricity needs of a modern nation. Globally, 46% of electric energy supplied goes to industry and the remaining 34% is for services. Using only households as a criterion for covering electricity demand gives an over optimistic idea of such a wind park’s achievements.
Thirdly, wind parks’ output is variable and cannot be relied on to cover a given demand. One often sees so-called doldrums of up to 10 days, when the wind scarcely blows in a widespread area. And the wind can blow so hard in an area half the size of Europe that 90% of the installed wind turbines spin at maximum output.
Solar photovoltaic (PV) panels also show large output variability. They do not produce at night, and have limited output in the darker seasons. Backup power is required for these renewable sources, and cogeneration and distributed generation are the best options.
For a proper analysis of the need for and benefits of local generators, we must distinguish between capacity factors, utilisation factors and load factors. There is no common agreement yet on the definitions of these factors. In the old days, before the introduction of volatile electricity generators, some ambiguity in definitions did not matter. Today, however, exactness is crucial in order to understand the implications of these factors. As an example, in Germany the average available output of the combined wind turbine portfolio is about 20% of the installed capacity. This means that the output of the installed 32 GW is only 6.4 GW averaged over a year. We can therefore define the capacity factor as the averaged unrestricted output divided by the installed capacity. For solar PV panels in Germany, the capacity factor is around 10%. If wind turbines’ output was noticeably curtailed, their so-called utilisation factor would be lower than the capacity factor. The utilisation factor of a generator is the actual averaged output divided by the installed capacity. The load factor of a generator is the instantaneous output divided by its nominal capacity.
Imagine a case of no renewables in a system, while fuel-based generators have a utilisation factor of 60%. The load factor might vary between 100% and 60%. Now, volatile renewable generators will be connected with the same installed capacity as the fuel-based generation. If the renewable generators have a capacity factor of 15%, and their output is not restricted, the utilisation factor of the fuel-based generators will decrease from 60% to 45%. For large central power plants, this would mean operational and financial disaster.
Distributed generation, however, can offer the required flexibility. Its load factor can vary widely. Because of its relatively low capital costs, even a lower utilisation factor is not a problem. So, it is essential to know all about capacity factors, utilisation factors and load factors.
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