The Use of Heat Exchangers - Exergy and Pinch Technology

Exergy may be defined as the quahty of heat. More precisely, it is the work or the abihty of energy to be used for work or converted to work. Potential energy as classically 

The exergy of a system is measured by its displacement from thermodynamic eqni-librium. Since 9G = VdP SdT + mdrii, where V and P are volume and pressure, respectively, and /tt, is the chemical potential of the i h chemical component with a number of molecules, n, it follows that the exergy change of a system going between an initial state and a reference state (subscript o) is given by 

The symbol E for exergy used here should not be confused with the same symbol used for energy in classical thermodynamics, nor for fuel cell voltage It is plain that the higher the temperature T, the greater the exergy. This link with temperature and the ability to do work can also be approached less generally via consideration of the Carnot limit. 

If we consider the case in which both the SOFC and PEM systems have the same power output and efficiency, then the heat, which is the enthalpy content, of the exhaust streams produced by both systems will be the same. However, the heat produced by the SOFC will be at a higher temperature and therefore has a higher exergy than that from a PEM fuel cell. We can, therefore, say that the heat from an SOFC is more valuable. The heat that is liberated in a PEM fuel cell is around 80°C and has limited use both within the system and for external applications. For the latter, it may be limited to air space heating, or possibly integration with an absorption cooling system. The heat that is produced by an SOFC on the other hand is exhausted at temperatures of around 1000°C and is clearly more valuable, as it can also be used, as we have seen, in a bottoming cycle. 

The importance of the concept of exergy is that with high-temperature fuel cell systems, the system configuration should be designed in such a way that exergy loss is minimised. 

---