Performance assessment via exergy method

Second law analysis is a useful tool for the identification of irreversibilities and, in result, improvement potentials of thermal energy systems. Exergy enables the quantification of a system’s potential to perform work when brought in equilibrium with the environment. As opposed to energy, exergy can be destroyed. The exergy analysis of a thermal system therefore helps identifying the loss and destruction of available energy, identifying where the “true” potential to perform work is not exploited.

1.1 Exergy equations for the evaluation of HVAC cycles

The application of the exergy method to moist air processes such as air-conditioning, drying and wet cooling tower processes was first described by Szargut [4]. A pathbreaking publication for the application in HVAC was later written by Wepfer [5]. Processes of evaporative cooling [6,7] and rotary type desiccant systems [8,9] were later both covered. Equation 3 is the generally applied moist air specific exergy equation written on a per mass of dry air basis [4,5]:

image085

[J/kg] (equation 3)

where the three terms give the thermal, mechanical and chemical exergy of moist air. An important issue for the application of exergy to moist air is the selection of the dead state. In the literature, both selecting either ambient conditions or saturated air at ambient temperature is discussed. The latter approach discussed by Chengqin [7] is followed in this paper, considering that unsaturated air still has a potential to perform work as it undergoes a temperature drop when humidified. Ideally, a Carnot engine could then be driven between the ambient air and the humidified air. According to equation 4, the moist air specific exergy is falling monotonously with rising moist air humidity ratio.

The exergy of water used for evaporation is generally described by equation 5 and is derived from analyzing a process where water is condensed from ambient air [5]:

= h(T) — h(To) — To[(T) — s(To)] ■+ [p — PSat (T)V(T) — RvT0 In Po [J/kg] (equation 5)

When choosing ambient temperature and humidity ratio as reference conditions, the last term is dominating. However, when choosing saturated air at ambient temperature as the dead state as followed here the last term drops out. For evaporative cooling schemes, the latter approach was found to give more reasonable results by the authors as the evaporation of water would otherwise be insensibly penalized.