Combining energy efficiency with good air qualityHow can you minimise the energy associated with achieving good indoor air quality? HARRI ITKONEN shares his ideas.Combining good indoor air quality (IAQ) and efficient use of energy is a challenge that always will inspire minds. The challenge becomes even more exciting when it is explored in its entire context — the building fabric and going deeper into the everyday activities and occupancy patterns of the occupants of the building. A displacement system is preferred in spaces where airflow rates are high due to high heat loads, occupancy levels and also in spaces with high contaminant loads — for example in industrial halls and smoking areas. Displacement ventilation is best suited to high spaces with ceiling heights above 3 m, where the stratification improves both thermal performance and contaminant control. Typical applications are lobbies, atria, conference halls, auditoriums, meeting rooms, classrooms, restaurants, kitchens, shopping centres, supermarkets and gyms. The most commonly applied approach to displacement ventilation uses 100% outdoor air to maximise the benefits of supplying air at low level. Warm and eventually contaminated exhaust air is extracted from the upper level of the spaces. Two approaches to air distribution are used in a displacement ventilation system: horizontal low-velocity supply and floor-mounted diffusers. Good acoustic conditions can readily be attained. Heating is provided by separate systems such as radiators/convectors or ceiling panels/floor heating. High exhaust-air temperatures, long operation times and cold/moderate climate conditions make it feasible to recover heat from exhaust air to supply air. Heat regenerating wheels or plate exchangers are typically used. Efficient ventilation for good indoor air quality Ventilation efficiency with displacement ventilation is high due to the distribution of fresh supply air at low velocity directly to the breathing zone. By definition the maximum ventilation efficiency of ideal displacement ventilation is double that of mixing ventilation and in practice 1.5 to 1.8 times better. The good ventilation efficiency enables good air quality with lower airflow rates than using mixing ventilation. When ventilation rate is calculated, several parallel requirements need to be considered. The minimum primary airflow rate is based on occupancy level. The typical minimum ventilation rate is 10 l/s per person. Another calculation approach is to base supply air rate on thermal balance (Fig. 1). Supply and exhaust airflow rates can be roughly estimated from the typical temperature difference between exhaust and supply air. This temperature difference depends on the height and use of the space as well as the surface temperature and location of heat sources. Alternatively, thermal-balance zone models have been developed to make more precise estimates. The room space can be divided in two zones: occupied and upper zone. Energy balance is deduced for those zones. The heat loads (people, lighting, equipment, warm surface and solar load) in the occupied and upper zone should be known. The interaction of the two zones is also taken into account. Displacement ventilation is also a sustainable application for buildings and spaces where heat and occupancy loads vary with time. If airflow rates are controlled by ventilation demand, the energy savings throughout the life of the building are significant. The air distribution also operates well at low airflow rates, and only areas close to supply units suffer reduced comfort. In buildings where people are the main source of heat gain and contamination, the ventilation requirement is well indicated by measuring the concentration of carbon dioxide in the air. Examples of such buildings include assembly buildings, theatres, concert halls, cinemas etc. Airflow can be controlled either with pressure-independent or pressure-dependent dampers combined with constant-pressure ductwork. Carbon-dioxide sensors are positioned in places representing well the ventilation demand in the space. Alternatively, sensors with a shifted set value can be installed in common exhaust duct. Displacement ventilation can save energy in following ways. • Low fan power due to low airflow rates.
• Low chiller power required for cooling due to lower airflows.
• Low chiller power required for cooling due to smaller sensible and latent cooling demand.
• High proportion of free cooling reduces cooling energy for much of the year.
| Fig.2 One of the reasons for the energy efficiency of displacement ventilation is that only in the occupied zone is the temperature maintained at a comfortable level for people.|
| Fig. 3 Displacement ventilation can exploit free cooling for over 40% more of a year than mixing systems.|