We are particularly interested in studying cases with a high cooling load. Our survey in several buildings in the greater Boston area shows that the cooling load can be as high as 40 Btu/(h⋅ft2) or 120 W/m2. Some investigators suggested that the maximum cooling load a displacement ventilation system can handle is about 13 Btu/(h⋅ft2) (40 W/m2) without increasing ventilation rate. In order to design displacement ventilation for U.S. buildings, the ventilation rate must be increased because of the high cooling loads. A displacement ventilation system may maintain a comfortable environment with a cooling load up to 40 Btu/(h⋅ft2) (120 W/m2) with increased ventilation rate.
We have studied a case with such a high cooling load in a small office. As shown in Figures 5.8 and 5.9, the PD and PPD in the occupied zone can still be less than 15%. The air temperature difference between head and foot levels is 3°F (1.6°C). In this case, the ventilation rate is 15 ach. The high ventilation rate would require a large fan, duct, and air-handling unit. It may not be economically feasible compared to Figure 5.7 The ventilation effectiveness distribution in the classroom: (a) in
mixing ventilation. In other words, energy efficiency and cost might limit the maxi- mum cooling load that will be discussed in the next chapter. In addition, the available area of the walls for installing supply diffusers will limit the maximum cooling load. In winter conditions, downward flow near the exterior window and wall might bring polluted air from the upper zone to the lower zone. A heating device placed near the exterior wall at the floor level may prevent the downward flow. If the displacement diffuser is used to supply warm air, the air will move upward. The advantage of displacement ventilation for better indoor air quality will disappear.
Figure 5.8 Distribution of the percentage of dissatisfied people due to draft in a small office with a cooling load of 40 Btu/(h⋅ft2) (120 W/m2): (a) at ankle level and (b) at head level.
Figure 5.9 Distribution of the predicted percentage of dissatisfied people for thermal comfort in a small office with a cooling load of 40 Btu/ (h⋅ft2) (120 W/m2): (a) at ankle level and (b) at head level.
Figure 5.10 illustrates the computed flow pattern with a baseboard heater with a capacity equal to the heating load from the exterior wall and window, while Figure 5.11 shows the flow pattern with a heating capacity equal to 80% of the heating load. It can be seen from the figures that the heating capacity should be at least equal to the heating load.
Figure 5.10 Flow pattern with a heating capacity equal to the heating load from the exterior wall and window.
Figure 5.11 Flow pattern with a heating capacity equal to 80% of the heating load from the exterior wall and window.
5.4 CONCLUSIONS
With proper design, displacement ventilation can maintain a thermally comfort- able indoor environment. The air velocity is smaller than 40 fpm or 0.2 m/s. The temperature difference between the head and foot level of a sedentary occupant is less than 3.6°F (2 K). The percentage of dissatisfied people due to draft (PD) and the predicted percentage of dissatisfied are less than 15%.
Compared with mixing ventilation, displacement ventilation provides better indoor air quality when the contaminant sources are associated with the heat sources. Thermal plumes bring the contaminants to the upper zone and the contaminant concentrations in the lower zone are lower. The mean age of air is younger and the ventilation effectiveness is higher in a room with displacement ventilation than with mixing ventilation.
A high cooling load in a room would require a high ventilation rate that may limit the application of displacement ventilation.
Energy and Cost Analysis
Proper design of displacement ventilation requires information about its energy consumption and first costs. A good ventilation system should save energy and be cost-effective. This chapter presents energy and cost analysis of displacement venti- lation. The investigation uses a mixing system for comparison.
Energy simulation methods range from manual to detailed computer simulation methods. Manual methods, such as degree-day and bin methods (ASHARE 1997), are still widely used in practical design, although they are not accurate. The model- ing strategy used in building energy simulation is in a sequence of load, system, and plant (Sowell and Hittle 1995), no matter whether a manual or a detailed computer simulation method is used. Degree-day uses only one value of temperature, while the bin method calculates energy over several intervals (bins) of temperature. However, detailed methods often calculate energy in an hour-by-hour interval. Although the manual methods are simple, they could not, for example, be used for the comparison of energy consumption by displacement and mixing ventilation systems. A detailed computer simulation can consider the difference between displacement ventilation and mixing ventilation. That method is also powerful for analyzing a number of alternatives in order to make an optimal design for an HVAC system. Therefore, the present investigation uses a detailed computer simulation method.
The detailed methods calculate cooling and heating loads hour-by-hour for an entire year for a building. Then the secondary systems are simulated to calculate the required energy flows at the air handlers or other equipment supplied by the central plant. The next step is to calculate the source energy requirements in the central plant. Finally, one would calculate the costs of the source energy, and sometimes introduce capital and other costs for a complete life-cycle economic analysis.