Exergy Analysis as an Assessment Tool of Heat Recovery of Dwelling Ventilation Systems

In cold and moderate climates, improvements in building shell insulation and air-tightness imply a shiftin heating loads from transmission and infiltration towards ventilation. Heat recovery from the ventilation airflow plays an increasingly important role in minimising energy needs. Such heat recovery systems rely on the input of electric power (to drive fans, heat pumps, etc.) in order to recover thermal energy. Since electricity input is relatively small compared to the amounts of thermal energy recovered, such systems are efficient from an energy viewpoint. One important yet often overlooked aspect, however, is the difference in quality between the high-grade electricity input and the lower grade thermal energy recovered. This paper analyzes the effectiveness of heat recovery from ventilation airflows from the viewpoint of exergy. The results provide a common basis for evaluating different forms of energy (e.g. thermal and electric), considering their different abilities to produce work in relation to a given environment.