Summary:

In the technical field the energy balances are derived from the 1st thermodynamic law, which covers merely the instantaneous quantitative aspects of heat flows. In terms of energy conservation law these balances are expressed in the form of constant sums of exergy and anergy. And that is why they cannot, or can only to a very limited extent, be used to define the transformation of part of the energy (exergy) to the less valuable energy (anergy). Using the 2nd thermodynamic law we have a chance to trace the energy transformation through entropy, a quality indicator of every heat process and an accompanying symptom of process irreversibility.
The plate air-to-air heat exchanger, whose exergy analysis is in the spotlight of our interest, operates, in terms of the laws above, in irreversible changes, while the change of entropy induced by the heat flows depends on the construction and surface of the heat exchange areas.
The article analyses the influence of air inlet temperatures on the thermal exergy efficiencies of the heat exchanger and the heat loss exergy. Assessment is based on extensive measurements of the temperature, humidity and flow rate of the heated and cooled air in the plate heat exchanger. Both air flow rates were defined constant and equal during all measurement, so the volume flow ratio of the cooled to heated air is Vi / Ve=k=1.02.
Theoretical analysis presents the process leading to the final exergy balance of heat flows E (6) and definition of particular exergies (7). These equations are employed for assessment of heat loss exergy E and three exergy efficiencies. To be able to evaluate the heat exchanger we computed the exergy efficiency of the heat transfer from cooled to heated air ηex,p (8), the exergy efficiency of the utilization of the heat from the cooled air ηex,i (9) and the total exergy efficiency ηex,c (10). Thermal efficiency (11) is defined as usually.
From the results (fig.2,3) is clear that the exergy efficiency of the utilization of heat from cooled air hex,i increases with the difference Dti,e1. The behaviour of the curve hex,i = f(Dti,e1) confirms to Fourier's and Newton's laws of heat conduction and convection. The build-up of heat gradient Dti,e1 results in an increase in the heat flow transferred between the cooled i and heated air e, a decrease in the exergy of heat flow Ei2 and pursuant to the equation (9), an increase in the efficiency hex,i. With the increasing temperature gradient Dti,e1 the exergy efficiency of the heat transfer from the cooled to heated air hex,p decreases. When the exergy of the heat flow Ei2 decreases, so does the efficiency hex,p.
The total exergy efficiency of the heat exchanger hex,c decreases with the increase in the temperature gradient Dti,e1. As can be seen from the behaviour of the curves the decrease is lower than that of the efficiency hex,p. From both the chart (fig.2) and the computations it follows that the equation hex,c = ht - 0,2 holds for the heat exchanger.
The diagram (fig. 3) shows the percentage distribution of the exergies of heat flows in the heat exchanger. The heat loss exergy DE accounts for the biggest portion