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What is exergy

Energy is never lost as stated by the first law of the thermodynamics. However, something is lost during an energy conversion and this is the quality of energy. To quantify the quality of energy, the concept of exergy is introduced. Exergy is defined as the maximum work potential of a material or of a form of energy in relation to its environment. The exergy of a spinning wheel or electricity is equal to its energy. The exergy of heat, however equals only a share of the energy of the heat. This share depends on the temperature difference of the heat stream with the environment. 

The use of the concept of exergy is still limited, although the concept was introduced in 1956. The example below, will show that exergy has a strong edge over energy when analyzing a process.

A good example to show the added value of the use of exergy is the case of space heating. The thermal efficiency of a condensing boiler is 100% (on lower heating value). This high efficiency may lead one to believe, that the boiler is an ideal system. However, the reality is different, as an analysis on exergy shows.

 
The condensing boiler has an exergetic efficiency of only 8%. This low efficiency is the result of the conversion of a high quality fuel, natural gas, into low temperature heat. A graphical presentation is shown in the figure below.

This clearly sheds a different light on the efficiency of the condensing boiler. 92% of the exergy in the natural gas is lost. Theoretically this is not necessary. This lead to the question, if a practical method exists to heat a building with a higher exergetic efficiency. Indeed, several solutions exist to provide space heating with a higher exergetic efficiency. One of these solutions is an electrical heat pump. The figure below shows the chain efficiency starting with a natural gas fired power plant. The exergetic efficiency almost doubled. The exergy of the environment is not visible in the figure, because, the exergy of the environment is by definition zero.  



 

 
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