These measures of overall energy mass efficiency (11.6) reveal the influence of capital and labour and the corresponding technologies in all stages of energy conversion.
A second group of efficiency measures is of special relevance for thermal conversion processes and is based on either the concept of exergy or the related concept of entropy. Two energy flows (e.g. low temperature heat and electricity) with the same energy content (e.g. measured in joules) may nevertheless have a different quality (e.g. measured by their ability to provide mechanical work).
A third group of measures may describe the environmental impact of the energy system by evaluating the content of renewable energy in the primary energy mix and the intensity of emissions (e.g. CO2) with respect to a unit of energy services or a unit of energy flows.
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