A study of time-dependent responses of a mechanical displacement ventilation (DV) system and an underfloor air distribution (UFAD) system : building energy performance of the UFAD system

As alternative systems for saving cooling energy compared to conventional overhead (OH) air-conditioning systems, mechanical displacement ventilation (DV) systems and underfloor air distribution (UFAD) systems have been widely adopted for commercial buildings. In these alternative systems, supply air is discharged from low momentum diffusers located at lower positions close to the floor in the DV system, and the supply air of the UFAD system is distributed by an underfloor plenum and discharged from floor diffusers. To predict transient vertical temperature responses when the heat source or the ventilation flow rate vary in time, we introduce transient two-layer stratification models of the DV and UFAD systems, non-dimensionalize them by competing the filling box time (Baines & Turner 1969) and the replenishment time in which all the air in the enclosure is replaced by supply air and validate them by laboratory experiments using a salt-water analogy. In various scenarios of the heat source and the ventilation flow rate, the models show a good agreement with the laboratory experiments. Building energy simulation, which predicts cooling and heating demands of a building, has been used for building design, environment, economics, and occupant comfort. EnergyPlus, which is a building energy simulation tools developed by the U.S. Department of Energy, has integrated capability to predict cooling and heating demands as well as the HVAC (heating, ventilation and air conditioning) energy consumption. To evaluate performance of energy savings of UFAD, we developed prototype office buildings adopting OH and UFAD, and the UCSD-UFAD model was used to estimate realistic cooling and heating demands by simulating the stratified temperature profile in a room. In Californian climates, annual electricity consumption of UFAD is always lower than that of OH by up to 20 %, since UFAD has more opportunities to utilize the economizer ̀f̀ree cooling" compared to OH. For electricity demand reduction of various Demand Response (DR) activities, increasing room set point temperature is the most effective DR activity and UFAD has higher peak demand reduction compared to OH by approximately 6-10 % when the room set point temperature is higher than 26 Celsiu