  The increased attention to energy savings has contributed to more widespread
use of energy recovery systems for building ventilation. We investigate the
efficiency of such systems under different outdoor conditions using exergy
analysis and nonequilibrium thermodynamics. This analysis makes it possible to
assess performance in terms of loss of work potential, to account for the
different quality of energy and to localize and compare the different sources
of loss in the system. It also enables the use of exergy efficiency as a single
performance parameter, in contrast to the several indicators that are commonly
used. These more common indicators are difficult to compare and relate to each
other. Further, since there is no obvious optimal trade-off between them, it is
challenging to combine them and develop a global performance indicator that
allows for a sensible comparison of different technical solutions and different
types of recovery devices. We illustrate the concepts by applying the analysis
to a heat recovery ventilator (HRV) and to a structurally similar membrane
energy recovery ventilator (MERV) that can exchange both heat and moisture. We
show how the exergy efficiency can be used to identify the range of operating
conditions for which the recovery ventilator is not beneficial as the energy
cost is greater than the energy recovery. This is not trivial using traditional
performance parameters, yet it is a natural outcome of exergy analysis. In
addition, we identify the mechanism by which work potential is lost, which can
help the eventual optimization of both the recovery process and the auxiliary
systems present in ventilation systems.
