Analysis of Exergy of Moist Air and Energy Saving Potential in Hvac by Evaporative Cooling or Energy Recovery

Of evaporative cooling or energy recovery potential in HVAC, the conventional thermodynamic analysis is based on the enthalpy of moist air. In fact, the available energy of moist air is its exergy and the available energy saving potential ought to be the exergy. Isoexergic lines on psychometric charts associated with different ambient states persuade us that exergy and enthalpy are completely independent of each other. Thus, the enthalpy analysis is at least imperfect. Selecting the saturated state of moist air at ambient temperature (T0) and pressure (p0) as the dead-state, this paper analyses the exergy of moist air. It is broken down into three components: thermal, mechanical and chemical, so that each of them may be represented with a special devised diagram applicable to various conditions and ambient states among HVAC applications. Two guidelines for such applications are extracted from the diagrams as these: lower dead-state temperature (T0) with the same difference of the humidity ratio (ω ω0S) is related with higher chemical exergy, and hence, the recovery of latent heat availability will probably be more valuable in winter than in summer; the exergy loss due to air flow resistance in a heat exchanger is usually small when compared with the potential of thermal and chemical exergy gaining. Further, the exergies of outdoor and exhaust room air are calculated and tabled for some Chinese cities and different types of air conditioning based on some statistic average parameters. Some general conclusions are drawn. For most districts on average room air parameters, the exhaust room air exergy is greater in winter than in summer. In winter, when exhaust room air temperature is low, the exergy of sensible heat is the main component of available energy saving potential. On contrast, the latent heat usually makes the prevailing contribution in summer, while at some dry locations, the exergy of outdoor air is even greater than that of exhaust room air.