Primary Energy and Exergy

TL;DR It has been common to represent global energy usage in terms of Primary Energy – that is the energy potential of the various sources. However, this can be easily criticized because not all energy is equal and what we really want to understand is the “Useful Energy” that does work on our behalf at the end of any transformation process. The conversion of energy from one form to another (thermal combustion to electricity for example) is subject to the Second Law of Thermodynamics and there are always irreversible “losses” in conversion. Thus, depending on the efficiency of the conversion process(es), much less Useful Energy is used than available Primary Energy. Viewed from end-use perspective, one can argue that much less energy is required globally if we have more efficient sources of primary energy. Solar and Wind which generate electricity directly without any thermal combustion are significantly more efficient. Ergo, we don’t need to replace all of the current Primary Energy, only a fraction of it, to have the same amount of Useful Energy. Unfortunately its not that simple, and understanding the limitations of this argument shed light on why the energy transition is far more complex than the usual sound bites beloved of headline writers.

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