Assessing the performance of hybrid and natural ventilation systems: a review of existing methods

Natural and Hybrid ventilation systems, by using exclusively or partially natural driving forces, help to reconcile building energy sobriety and good Indoor Air Quality (IAQ). However, in France, Building Regulations restrict the use of natural ventilation by imposing minimum airflows in buildings. Natural ventilation, whose driving forces are atmospheric conditions, has an efficiency depending on the climatic region. Concerning climatic regions where natural ventilation is not likely to provide sufficient airflow, the ventilation system may be featured with a mechanical assistance, which is called Hybrid ventilation systems. Hybrid ventilation systems, taking advantage from natural driving forces when the induced airflow is strong enough, and assisting it otherwise, allow using natural ventilation while verifying airflow requirements. The assessment of mechanical systems performance has already been questioned by the French protocol PROMEVENT. The absence of a similar protocol restricts the development of natural and hybrid ventilation systems. Such a protocol would widen the use of natural ventilation in favourable climatic regions and enable the improvement of the control strategy of hybrid ventilation systems. However, the plurality of openings, variable airflows, and unstable flow patterns make the measurement of the performance of natural and hybrid ventilation systems a challenging task. It represents the objective of the French research project VNAT. 
This paper is issued from the first task of the VNAT project: the state of the art on the assessment of natural and hybrid ventilation systems. It presents a comprehensive review of existing studies and protocols regarding the measurement of airflow and Air Change Rate (ACR). It appears that direct measurement method of airflow may interfere with the flow pattern, which is troublesome for natural ventilation. Thus, the direct measurement is not likely to be representative. Indirect methods allow measuring Air Change Rate (ACR). They may be conventional tracer gas methods or occupant-generated CO2 methods. Tracer gas methods are numerous and they are based on assumptions which can differ from a method to another. Two of them, called the constant injection and the concentration decay methods, are widely used to characterize the performance of ventilation systems thanks to their ease of implementation. The violation of assumptions leads to important measurement uncertainties. Occupant-generated CO2 methods depend on the occupation rate and on the CO2 emission rate, which induces uncertainties too. These measurement methods as well as tracer gas methods are compared regarding their accuracy and their limits. Modelling methods are also discussed.  
Results from this paper will help to build a new protocol more suited to the assessment of the performance of hybrid and natural ventilation systems under real conditions.