2.4 Displacement Ventilation

Displacement Ventilation (DV), a space conditioning technology in use in Europe since the 1970’s, has the ability to reduce energy usage in buildings due to a number of energy saving strategies not found in conventional overhead mixing systems. The fundamental principle involved in a DV system is to supply significantly warmer supply air temperatures during cooling mode, typically 63°F to 68°F. With the use of higher supply air temperatures comes the ability to operate in economizer mode many more hours each year. When producing the higher supply air temperatures, chilled water systems have the ability to operate at much higher chilled water temperatures, thus resulting in a significant increase in the chiller efficiency when producing chilled water. In addition, for systems that will be requiring reheat, additional heating and cooling energy is saved since they will be reheating air that is cooled to only 65°F versus a conventional system that has cooled the air to 55°F.

By not mixing the air in the room, the DV system results in more of a stratification effect. Thus, much of the heat in the space will rise towards the ceiling, where it will be exhausted by the high return air register. A portion of the cooling load in the space, including occupant heat gain, lighting and equipment, never appears as a cooling load.

Some software tools have built-in system types that allow for the direct modeling of DV systems while others will require the user to make approximations of some of the effects associated with DV, in particular, the stratification of room loads. In all cases, the supply air temperature of the system will need to be set to the higher design value of the system, and in most cases the control strategy for the cooling coil will be to reset the leaving temperature based upon the warmest zone requiring cooling.

For modeling the actual zone, the idea is to account for the stratification of air in the zone. One technique is to model the lower 6 feet of the zone as a conventionally conditioned zone and to model the upper portion of the zone as a return air plenum. Loads associated with the wall, roof, lighting and some of the equipment will then be modeled as being part of the return air plenum, rather than part of the space. This has the effect of modeling the stratified, non-mixed air in the zone, placing much of the heat gains into the return air. When the system is operating in economizer mode, these loads will simply be exhausted out of the building, rather than appearing as a cooling load in the space.

Assuming the system is VAV with reheat, each zone will be modeled with a conventional VAV terminal box to account for heating. Note that once modeling has been completed, it is important to verify that the higher supply air temperatures associated with the DV system will in fact meet the loads. Output reports from the software should be checked for unmet load hours. In addition, if working in a humid climate, it is unlikely that the higher temperatures will satisfy the latent loads. One solution is to design the system with a bypass on the cooling coil so that a certain portion of air will be cooled to 55 degrees and dehumidified, with the rest bypassing the coil, resulting in a mixed air temperature of 63°F to 68°F.

Depending upon the strategy used for the coil temperature, the chilled water temperature may also be modeled at a higher temperature. Assuming the bypass strategy is not used, the chilled water temperature should be set based upon the higher design, resulting in additional savings at the chiller.
The baseline building is not modeled with displacement ventilation.