ASHRAE Journal - May 2014 - 73

CourteSy of NW ArChiteCturAL PhotogrAPhy

PhotoS CourteSy of NW ArChiteCturAL PhotogrAPhy

2014 ASHRAE TEcHnology AwARd cASE STudiES
nology

These goals were achieved by a combination of building orientation, careful lighting and daylighting design,
and an HVAC strategy that capitalizes on Oakland's
Mediterranean climate with its reliable breezes and significant diurnal temperature swings.

Systems Description
While the data center, clinic, and TV studio use a highefficiency, single-duct VAV HVAC system, the classrooms
do not use compressor-based cooling. Maintaining comfort required a multi-pronged conditioning strategy that
relies on local climate, thermal mass and sophisticated
controls.
The classrooms and offices have operable windows
(with switches that disable HVAC), allowing for occupant
control and passive natural ventilation. When windows
are closed these spaces are cooled by unconditioned
100% outdoor air from central AHUs. Displacement diffusers deliver air to the classrooms, which improves
ventilation effectiveness and IAQ, and also rejects occupant heat from the space rather than mixing it into the
rooms' air mass. Air supply is controlled by demand and
outdoor air temperature, delivering large volumes when
the weather is cool but reducing to ventilation minimum
when weather is warm.
Additional cooling is provided by thermal mass: a 4 in.
(25 mm) concrete floor slab and a 2 in. (25 mm) thick
cement plaster layer on interior walls. Thermal mass is
charged (cooled) at night by a high volume purge cycle,
leveraging a cooling season diurnal temperature swing
of 20°F (11°C) or more. This cycle is controlled based on
outdoor air temperature, room temperature, and thermal mass temperature from sensors embedded in the
floor and walls to minimize fan energy and avoid overcooling. This provides a thermal flywheel effect to maintain comfort even on warm days.

Above And Left Large ceiling fans in each
classroom provide comfort cooling on the
hottest days.

A final element of the classroom cooling strategy is
automatically controlled high volume, low speed (HVLS)
ceiling fans. The fans are off during the first stage of
cooling, when the supply air is cool, so that displacement ventilation provides beneficial stratification in the
occupied zone. As the outdoor temperature (and, thus
SAT) rises, the ceiling fans activate to provide up to 4°F
(2°C) additional effective cooling.
The ceiling fans also assist in heating, which is by parallel fan-powered VAV (FP-VAV) boxes with hot water
coils in each room. Heated air is delivered via the same
displacement diffusers used for cooling, while the ceiling fan operates at low speed to destratify the space and
ensure uniform heating.
These cooling and heating strategies are also applied in
the great room, which has a dedicated single-zone AHU
with economizer to provide heating, ventilation and 100%
outdoor air cooling. However, the great room is periodically subject to high occupant densities, up to 10 ft2 (0.9
m2) per person. This exceeds what outdoor air cooling
and thermal mass alone can support, so a pair of passive
evaporative downdraft towers ("cool towers") is employed
to address these loads.
The towers are 40 ft (12 m) tall and 12 ft (3.7 m) square
with high-pressure fogging nozzles at the top that inject
a modulated volume of very finely atomized water. The
water evaporates and cools the air, which drops under
buoyancy pressure, cooling the space and then relieving
through automatic louver/dampers installed near the
ceiling. Each tower also has a wind scoop that faces west
into the prevailing wind to provide cooling and ventilation without evaporation when the weather is suitable.
Tower and AHU controls are integrated so that the towers provide natural ventilation and respond first to a
high CO2 signal, energizing the AHU for mixed mode

M ay 2 0 1 4

ashrae.org

ASHRAE JouRnAl

ASHRAE Journal - May 2014

Table of Contents for the Digital Edition of ASHRAE Journal - May 2014