Our work aimed to analyze and evaluate the buoyancy-driven ventilation based on the exergy analysis. We took the exergy load as a desired output for this consumer system and used the functional exergy efficiency to evaluate the ventilation performance. Through the numerical case studies for a high-rise building with a tall atrium, we found that the results from the energy and exergy analysis are quite different from each other, but the latter reveals the real essence of energy utilization in ventilation systems. The results showed that the exergy efficiency of the buoyancy-driven ventilation system is very poor, only 16.9% of the exergy input is effectively utilized and the exergy destruction counts for 83.1% of the total input. However, the exergy efficiency of the mechanical ventilation system is 100% because the input shaft work is entirely utilized to undertake the exergy load; no extra exergy losses are produced. We also analyzed the relationships between the temperature difference and the exergy efficiency. Furthermore, we found that the total radiation-to-exergy efficiency is 3.5 and 15% for ventilation systems equipped with solar collectors and solar cells respectively, it is concluded preliminarily that the latter is more efficient to utilize solar energy to create ventilation. (C) 2009 Elsevier B.V. All rights reserved.