Exergy

Also called available energy. It is the energy that can be extracted as work when a system in a certain state undergoes a reversible change until it reaches equilibrium with a reference system. In other words, it is a function used to evaluate the quality of energy from the second law of thermodynamics. If the enthalpy and entropy of a system are H and S , respectively, the reference systems are H ° and S °, and the temperature of the reference system _is T0 , then the exergy of the system is:

ε = H − H ° − T ₀ ( S − S °)

It is defined as follows. Therefore, the value of exergy varies depending on the reference system chosen, but the state in equilibrium with the natural environment is usually taken as the reference system. For example, oxygen is taken to be oxygen (0.203 atm) in saturated moist air at 1 atm and 298.16 K (25 °C), and the exergy value of oxygen in this state is taken to be ε = 0. The exergy of 1 mol of oxygen at 1 atm and 298.16 K is equal to the work of reversibly compressing 1 mol of oxygen at 0.203 atm and 298.16 K to 1 atm at the same temperature, which is 0.9435 kcal (3.948 kJ). Furthermore, the exergy of heat at 298.16 K is ε = 0 regardless of the amount of heat. The exergy of heat at temperature T and amount of heat q can be found from the definition equation as follows:

In other words, it is equal to the work obtained from a Carnot cycle with a high heat bath temperature T and a low heat bath temperature T0 _. The value of the change in exergy that accompanies the change is called the exergy loss. This value is obtained from the algebraic sum of the exergy values ​​of the system before and after the change and the exergy values ​​of heat, work, etc. that the system exchanges with the outside world. The smaller the exergy loss, the closer the change is to a reversible process, and in the case of a reversible change, the exergy loss is 0.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

有効エネルギーともいう．ある状態の系が基準系に対して平衡になるまで可逆変化したとき，仕事として取り出すことができるエネルギーをいう．すなわち，エネルギーの質を熱力学第二法則から評価するための関数で，系のエンタルピーおよびエントロピーをそれぞれH，S，基準系のそれぞれをH °，S °，基準系の温度を T₀ とすれば，系のエクセルギーは，

ε ＝ H － H ° － T₀(S － S °)

により定義される．したがって，エクセルギーの値は基準系の選び方によって異なるが，基準系としては，通常，自然環境と平衡にある状態がとられる．たとえば，酸素は1 atm，298.16 K(25 ℃)の飽和湿り空気中の酸素(0.203 atm)がとられ，この状態の酸素のエクセルギー値をε ＝ 0とする．1 atm，298.16 K の酸素1 mol のエクセルギーは，0.203 atm，298.16 K の酸素1 mol を同温度下で1 atm まで可逆圧縮する仕事に等しく，0.9435 kcal(3.948 kJ)となる．また，298.16 K の熱のエクセルギーは，熱量にかかわらずε ＝ 0である．温度T，熱量qの熱のエクセルギーを定義式から求めると，

となる．すなわち，高熱浴温度T，低熱浴温度 T₀ のカルノーサイクルにより得られる仕事に等しい．変化に伴うエクセルギー変化の値は，エクセルギー損失とよばれる．この値は，変化の前後における系のそれぞれのエクセルギー値と系が外界と授受する熱，仕事などのエクセルギー値との代数和から求められる．エクセルギー損失が小さいほど，変化が可逆過程に近いことを示し，可逆変化の場合には0となる．

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