Tuesday, February 15, 2005

Why thermal power plants have a relatively low efficiency

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Thermal power plants are the backbone of our electricity system. They convert heat energy into mechanical work, and work into electricity. Their efficiency is typically between 30 and 50%. This means that only half or even less of the heat energy generated in the plant is available as electricity - the remaining heat is dissipated to the environment. As a result, these power plants are judged inefficient and to be replaced as soon as possible by ’better ones’.

But this conclusion is a bit rash. Upon closer examination, one can see that the numerical value for efficiency is based on a combination of the laws of physics with definitions made by man. Special attention should be given to 2 issues: First, according to physics, only a fraction of the energy necessary to produce electricity can be converted into electricity. This fraction depends on the type of the power plant. Secondly, the remaining rest of the energy, that fraction that cannot be converted into electricity, is treated differently in efficiency calculations for different types of power plants. Sometimes, it is taken into
account in the calculation, and sometimes it isn’t. This is mostly due to historic reasons - frequently, the method for determining efficiency was defined to make calculations easy. This simplifies labour with a specific plant, but makes comparisons between different types of power stations difficult. For thermal power stations, this arbitrary agreement is particularly relevant. Their relatively low efficiency is mainly a consequence of the definitions used, and based on these definitions, it is governed by the laws of physics.

Based on these different definitions, it is impossible to determine by that single value which is ’better’: a hydro power station with 85% efficiency, a coal-fired thermal power plant with 45% efficiency, or a solar power plant with 15% efficiency. Which power plant is ’better’ depends on how it fulfills its role to produce reliable, cheap and environmentally sustainable power in the best way. This could very well be a plant with a relatively low efficiency.

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