Editing Exergy (section)

===Exergy of heat available at a temperature===
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Maximal possible conversion of heat to work, or exergy content of heat, depends on the [[temperature]] at which heat is available and the temperature level at which the reject heat can be disposed, that is the temperature of the surrounding. The upper limit for conversion is known as [[Carnot efficiency]] and was discovered by [[Nicolas Léonard Sadi Carnot]] in 1824. See also [[Carnot heat engine]].

Carnot efficiency is

{{NumBlk|:|<math>\eta = 1 - \frac{T_C}{T_H} </math>|{{EquationRef|15}}}}

where ''T''<sub>''H''</sub> is the higher temperature and ''T''<sub>''C''</sub> is the lower temperature, both as [[absolute temperature]]. From Equation 15 it is clear that in order to maximize efficiency one should maximize ''T''<sub>''H''</sub> and minimize ''T''<sub>''C''</sub>.

Exergy exchanged is then:

{{NumBlk|:|<math>B = Q \left(1 - \frac{T_o}{T_ \text{source} }\right)</math>|{{EquationRef|16}}}}

where ''T''<sub>''source''</sub> is the temperature of the heat source, and ''T''<sub>''o''</sub> is the temperature of the surrounding.
[[File:Cost and exergy for heating energy in Finland.jpg|thumb|350px|Higher exergy content tend to mean higher energy prices. Here the costs of heating (vertical axis) are compared with the exergy content of different energy carriers (horizontal axis) in Finland. Energy carriers included are district heating (D), ground-source [[heat pump]] (G), exhaust air heat pump (A), bioenergy meaning firewood (B), heating oil (O) and direct electric heating (E). Red dots and trend line indicates energy prices for consumers, blue dots and trend line indicates the total price for consumers including capital expenditure for the heating system.<ref name="Muller"/>]]