Initial Evaluation of Displacement Ventilation and Dedicated Outdoor Air Systems for U.S. Commercial Buildings.
Initial Evaluation of Displacement Ventilation and
Dedicated Outdoor Air Systems for U.S. Commercial
Emmerich, S. J.; McDowell, T.
NISTIR 7244; 35 p. July 2005.
Sponsor:Environmental Protection Agency, Washington, DC
commercial buildings; ventilation; evaluations; costs;
technology transfer; air distribution; dedicated outdoor
air systems; displacement ventilation; energy
efficiency; indoor air quality; outdoor air
Conventional heating, ventilation and air-conditioning
(HVAC) systems in commercial buildings meet both outdoor
air ventilation and space conditioning requirements
using air distribution approaches that provide a mixture
of outdoor air and recirculated air with the goal of
achieving good air mixing within the occupied space.
More recently, advanced ventilation approaches have been
proposed, and in some cases installed, that separate the
outdoor air ventilation and space conditioning functions
(dedicated outdoor air systems or DOAS) or that
distribute air to achieve thermal stratification within
the space (displacement ventilation or DV). NIST has
completed an initial evaluation of the potential
benefits and limitations of DV or DOAS in lieu of the
conventional mixing ventilation (MV) approach common in
U.S. commercial buildings.The first task in this effort
was a compilation of information on DV and DOAS systems.
This included a review of the scientific literature from
sources including the American Society of Heating,
Refrigerating and Air-Conditioning Engineers (ASHRAE),
the Air Infiltration and Ventilation Centre (AIVC), and
Indoor Air and other international conferences. There is
a large body of literature on the design and performance
of DV systems including two recent design guides -- one
published in the U.S. and the other from Europe. As
reported in the literature and summarized in the design
guides, DV systems have the potential to improve indoor
air quality (IAQ) in commercial buildings while also
reducing cooling energy use relative to traditional MV
systems. However, a close examination of the literature
also reveals the need for caution in considering these
benefits and care in applying DV systems. Chief among
the concerns is that the IAQ improvement is only for
some types of contaminants from some types of sources,
cooling energy reductions may be offset by heating or
fan energy increases, and thermal comfort is not easily
achieved and maintained. Despite these concerns, DV
systems have promise and are worth pursuing but they
have not yet been proven effective in the U.S.
applications. The literature on DOAS contains far fewer
detailed studies. Advantages of DOAS cited include
improved humidity control, assured delivery of proper
ventilation airflow quantities, and reduced energy use.
Although one field study of DOAS application in Florida
schools supports some of the claimed advantages, the
energy and humidity control performance of these systems
has not been adequately studied in real buildings. Since
the literature contains more analyses of the performance
of DV systems than DOAS, the second task of this effort
was a simulation study of the potential energy impacts
of DOAS systems in a small U.S. office building.
Simulations were performed with a combined airflow
building energy modeling tool linking TRNSYS and CONTAM.
Based on the simulation results, the DOAS showed promise
in reducing energy consumption relative to the baseline
system in all climates studied. Also, a more complex
DOAS did not show significant improvement over a simpler
DOAS consisting of only a preheat coil and enthalpy
wheel. A key outcome of this effort is recommendations
for future research and technology transfer efforts
based on this initial evaluation. These recommendations
include further study of humidity control and other
performance aspects of DV systems, field studies of DV
and DOAS installations in U.S. commercial buildings, and
extension of the U.S. Environmental Protection Agency's
modeling tools to include DOAS capability.