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A better environment that includes favorable
light, sound, and temperature
can help students learn better.
In many cases, improving these attributes
can also reduce energy use. Studies show
that daylighting, which uses the sun to produce
high-quality, glare-free lighting, can
improve academic performance by as much
as 20 percent.
Because it requires little or no electrical
lighting, which can increase cooling loads, daylighting is also a key strategy for achieving
energy savings. Quality lighting systems
include a combination of daylighting and energy-
efficient electric lighting systems. These
complement each other by reducing visual
strain and providing better lighting quality.
For electrical lighting to be used efficiently,
spaces must have light-colored finishes,
according to ASHRAE, the American
Society for Heating, Refrigeration and Air
Conditioning Engineers. Ceiling reflectance
should be at least 70 percent (preferably 80
to 90 percent), which in general means using smooth white acoustical tile or ceiling paint.
ASHRAE’s “Advanced Energy Design
Guide for K-12 School Buildings” says the
average reflectance of the walls should be
at least 50 percent, which in general means
using light tints or off-whites for the wall
surface, as the lower reflectance of doors,
tack surfaces, and other objects on the walls
will reduce the average.
Floor surfaces should be at least 20 percent,
for which there are many suitable surfaces.
In addition, take the shape and finish
of the ceiling into account. A flat painted or
acoustical tile ceiling is the most efficient;
sloping ceilings and exposed roof structures,
even if painted white, may significantly reduce
the effective ceiling reflectivity.
Lighting systems with indirect components
are recommended, but if the ceiling
cavity includes exposed structures or exposed
ductwork, a higher percentage of downlight may be required. Make sure the
ceiling and all components are painted a
high-reflectance white.
Linear Fluorescent Lamps
T8 lamps and electronic ballasts are the
standard commercial fluorescent lighting
system in the United States. Light-source efficacy
and LPD requirements can be
achieved as long as the more efficient versions
of T8 lamps and ballasts are used.
To evaluate the efficacy (lumens per watt)
of a lighting system, the mean lamp lumens
in typical manufacturers’ catalogs are divided
by the ballast’s rated input power. In
these catalogs, the mean lumens are lower
than the initial lumens. Mean lumens represent
the average light output of the lamp
over its rated life, which better characterize
actual performance.
Also, the mean lumens vary according to
color temperature and between standard series
(SP) and premium series (SPX) lamps.
Low-mercury fluorescent lamps are available
from the major lamp manufacturers and
have become the standard for sustainable
design projects. The color-rendering index (CRI) is a
scale measurement identifying a lamp’s ability,
generally, to adequately reveal color
characteristics. The scale maximizes at 100,
which indicates the best color-rendering capability.
Lamps specified for ambient lighting
should have a CRI of 80 or greater to
allow the occupants to effectively examine
the color characteristics.
Ballasts
Next, select the ballast. This is not trivial,
as there are several choices:
• Standard “generic” instant start electronic
ballasts are the most common and least
expensive ballast; the typical input power
for a two-lamp normal light level (0.87
ballast factor [BF]) is about 59 W. If you
do not specify the ballast, this is what you
will receive.
• Low-light-level standard ballasts are similar
to the standard ballast; this version operates
at 0.78 BF and has input power of
about 54 W for a two-lamp ballast.
The resulting light level is
about 10 percent less than the standard ballast, but the
watts are 10 percent lower.
• High light level version of standard ballasts.
Similar to the standard ballast, this version operates at 1.15–1.20 BF and has
input power of 74–78 W for a two-lamp
ballast.
The resulting light level is about 32 percent
higher than the standard ballast, but the
watts are 32 percent higher.
• Program start ballasts are available in low
power and normal power models. They
use an additional watt per lamp to perform
programmed starting, which makes lamps
last longer when frequently switched.
• Dimming ballasts. Dimming ballasts are
also rapid start, which is less efficient than
either instant or programmed starting. At
0.87 BF, most dimming ballasts require
62–64W for two lamps. The added power
is used to add extra heat to the lamp cathodes
to permit proper dimming operation.
• High-efficiency versions of all of the
above. Efficient electronic ballasts are now
available for almost every type listed
above. Better electronics require 1–3 fewer
watts per lamp to deliver similar performance,
but they cost more and are less
common.
To determine the system efficacy, multiply
the lamp mean lumens by the number
of lamps and the BF, then divide by the
ballast input watts. Low-wattage (“energy-saving”) T8 lamps
may also be considered, but may result in
lower ambient light levels or an increased
number of fixtures or lamps to achieve
recommended light levels.
Because they cannot be dimmed and have
other limitations, these lower wattage lamps
are not recommended for new construction.
Use occupancy sensors in all classrooms,
offices, mechanical rooms, restrooms, and
special use spaces like music practice
rooms. The greatest energy savings are
achieved with manual on/automatic off occupancy
sensors if daylight is present. This
avoids unnecessary operation when electric
lights are not needed and greatly reduces
the frequency of switching.
In non-daylighted areas, ceiling-mounted
occupancy sensors are preferred. In every
application, the occupant should not be able
to override the automatic off setting, even if
it is set for manual on. Unless otherwise
recommended, factory-set occupancy sensors
should be set for medium to high sensitivity
and a 15-minute time delay (the
optimum time to achieve energy savings
without excessive loss of lamp life). ❑
Source: ASHRAE Advanced
Energy Design Guide for K-12 School Buildings. |
