|
Archives
Underfloor Air: Better Models, Better Performance
THE PROBLEM
Underfloor air distribution (UFAD) systems, in which raised access floors
serve as plenums for distributing cooled air through buildings, offer the potential
to reduce energy use under certain conditions (Figure 1). However, computer
modeling
techniques have not yet been accurate enough for designers to consistently
make the right choices when considering the application of UFAD.
THE SOLUTION
Researchers have developed a new model for the U.S. Department of Energy’s
EnergyPlus™ software that is capable of modeling UFAD systems. EnergyPlus™ is
a whole-building simulation software tool. The new EnergyPlus™/UFAD model
can be used by designers to calculate the energy use of UFAD systems and compare
their performance to conventional overhead air distribution systems. Using
data from a multiyear study, the software allows designers to better understand
how
UFAD systems work. That improved understanding will lead to better system design
and increased efficiency.
FEATURES AND BENEFITS
Three factors improve the accuracy of UFAD modeling in the EnergyPlus™/UFAD
software:
Improved representation of room air stratifications (RAS). A two-zone RAS
model, which was developed and validated based on a number of laboratory tests,
allows
EnergyPlus™/UFAD to divide a room into two temperature zones: a lower occupied
zone and an upper zone (Figure 2). The EnergyPlus™ software determines
the height of separation between the two zones and then analyzes how the temperatures
in each zone interact with one another. In addition, the software models how
heat sources such as computers and diffusers affect the UFAD system’s
performance.
Detailed plenum modeling. Because the underfloor air plenum is cooler than
both the slab underneath it and the raised floor panel, heat transferring into
the
plenum becomes an important issue when modeling energy balances. To develop
an accurate underfloor plenum model, researchers conducted tests on plenums
of various
sizes, shapes, and materials. As a result, the EnergyPlux™/UFAD software
can estimate the magnitude of heat infiltrating into the plenum from above
and below.
Additional HVAC modules. Researchers also developed a way to model two HVAC
elements commonly used with UFAD systems: a variable-speed fan coil unit and
a return-air
bypass arrangement. Variable-speed fan coil units are used for supplying air
to the perimeter zones of the UFAD systems. Return-air bypass arrangements
are commonly used to provide humidity control for systems in humid climates.
APPLICATIONS
EnergyPlus™/UFAD is a whole-building simulation program that can be used
by design professionals or others to model any building, whether it is new construction
or retrofit. Buildings located in all climates can be simulated because the EnergyPlus™ database
continuously collects weather data from around the world.
WHAT’S NEXT
In addition to improving the EnergyPlus™/UFAD software, the Center for
the Built Environment is also developing a UFAD cooling airflow design tool that
is separate from EnergyPlus™/UFAD. As a subtask of the UFAD study, the
spreadsheet-based design tool estimates the amount of air needed to handle a
cooling load in a UFAD system. The preliminary version of the design tool is
complete, but additional research will be conducted this year to improve it.
The design tool, in its current form, will be available soon on the California
Energy Commission’s Public Interest Energy Research (PIER) web site.
This project and other recent work have shown that UFAD systems have fundamentally
different heat transfer pathways than conventional overhead systems. As a result,
ASHRAE Technical Committee 4.1 is planning a detailed UFAD load-calculation
project to develop a robust set of tools for UFAD designers.
Reports documenting this project and providing more details may be downloaded
at www.energy.ca.gov/2007publications/CEC-500-2007-050.pdf. The EnergyPlus™/UFAD
program is available at www.energyplus.gov.
To view Technical Briefs, visit
www.esource.com/public/products/cec_form.asp.
Archives
|
