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Different people have different opinions regarding
the construction of green buildings. To some, it means taking steps
to avoid a global warming disaster. To others, it means increased
initial cost with no real benefit. For the purposes of this article
we are going to replace the term “green” with “high
performance.” Why? High-performance buildings make sense in
both green and financial paradigms. Whether you plan to save the
planet or plan to save money, high-performance buildings are where
it’s at.
High-performance buildings need high-performance systems. These
systems reduce operating costs by saving energy. When we save energy,
we reduce greenhouse gas (GHG) emissions. Let’s take a look
at five readily available high-performance technologies: variable
air volume (VAV) systems, enthalpy wheels, rainwater harvesting,
T5 direct/indirect light fixtures, and terminal induction boxes.
Each of these technologies can be part of an overall strategy for
reducing maintenance and utility costs. Additionally, they are environmentally
sound.
Turn Down the Air Volume
Constant volume heating, ventilation and air-conditioning (HVAC)
systems deliver a set volume of air to the spaces they serve. Many
school facilities have constant volume systems throughout, such
as water-source heat pumps, unit ventilators, fan coil units and
some air-handling unit (AHU) systems.
Yet, when using the auditorium, wouldn’t it make sense to
reduce the air flow into the auditorium? Would that reduce the cost
of heating and cooling the auditorium? Yes and yes—and these
answers apply to all large spaces with variable occupancies. Oddly
enough, large spaces have been traditionally served by constant
volume systems.
VAV systems have been in use for more than 25 years now. These systems
consist of central AHUs delivering cold air through a duct system
to a series of terminal units or “air valves.” Cold
air is delivered year-round, and heat is added at the terminal units
when needed. When the space gets too cold, the air valve closes
to a preset minimum position required to maintain air quality in
the space. When the space gets too warm, the air valve opens to
provide more cold air to the space. When air valves close, the fan
in the central AHU slows down, thereby saving energy.
There are two basic VAV terminal unit technologies used to vary
air flow: fan-powered and shut-off. Fan-powered terminal units,
as the name suggests, are equipped with a fan. The function of the
fan is to provide good air delivery to the space. However, it is
an energy user. Shut-off VAV boxes are simply an air valve without
a fan, and have the ability to “shut off” the air. When
considering using shut-off terminal units to eliminate parasitic
energy losses, know that some small spaces will require a fan. The
design engineer will know when this is necessary.
With regard to large spaces traditionally served by constant volume
systems, the cost of variable speed technology has decreased, so
it is now possible to afford variable volume operations for units
serving large single spaces such as auditoriums, gymnasiums, cafeterias
and commons areas. In addition to indoor VAVs, outdoor air intake
can vary in relation to the number of people in the space. Carbon
dioxide (CO2) sensors can provide this function. This strategy,
often referred to as “demand controlled ventilation,”
will save a tremendous amount of energy over time, and the return
on investment can be achieved within one year.
Enthalpy Wheels Save Energy
Unlike most other energy-recovery devices that only recover sensible
energy, an enthalpy wheel also recovers latent energy.
Sensible heat can be felt and measured with a thermometer. Latent
heat is the amount of moisture in the air. So unless the relative
humidity is very high, you will not “feel” latent heat.
High humidity levels contribute to that “stuffy” feeling
sometimes experienced when humidity is not under control. In the
summer, latent energy (moisture) should be removed from incoming
outdoor air, and in the winter it should be added.
An enthalpy wheel has a desiccant coating that gives it the ability
to transfer moisture. It is mounted in a dual path AHU and rotates
between the two paths, transferring energy between exhaust air and
incoming outside air.
During summer, the wheel removes heat from the incoming air and
deposits it into the outgoing air. In winter, it removes heat from
outgoing air and adds it to incoming air. Remember, both sensible
(temperature) and latent (moisture) types of heat are transferred.
Other technologies that recover sensible heat energy are available;
however, only the enthalpy wheel is widely accepted and transfers
both sensible and latent forms of heat.
An enthalpy wheel is a good investment when exhaust volume exceeds
1,500 cubic feet per minute. Any less than that and it is typically
not a good investment. Properly installed and integrated, an enthalpy
wheel system can pay for itself on day one.
Rainwater Harvesting
If you were going to flush 1.2 million gallons of water down the
toilet and evaporate 3.4 million gallons of water for air conditioning—the
amount of water used by an average 200,000-square-foot high school
in one year—wouldn’t it make sense to use rainwater
rather than treated potable water from the municipal water supply?
That’s the idea behind rainwater harvesting. Rainwater is
collected and made available for other uses, thereby reducing the
amount of drinking water required for building operations.
Harvesting rainwater requires a cistern in the form of a concrete,
fiberglass or steel tank installed above or below ground. Cisterns
range in size from 3,000 gallons to 500 million gallons or more.
They are typically installed underground, to keep the water dark
and cool, because light will cause algae and other organisms to
grow in the water.
An underground installation also allows roof water to drain into
the cistern much easier. Above-ground installations can be problematic
if you can’t drain the water into them by gravity, and pumping
rainwater at the rate it can fall is not practical.
A system of buffer tanks, filters and pumps deliver the water from
the cistern to the building systems. Pipes carrying this water are
labeled non-potable, as it cannot be used for drinking.
Uses for rainwater include toilets, urinals, irrigation and cooling
tower water supply. (A cooling tower evaporates water to reject
heat from the air conditioning system.)
For example, in a 200,000-square-foot high school, a cistern could
provide nearly all of the water required for flushing and air conditioning.
At $3 per 100 cubic feet, an annual savings of approximately $13,000
could be achieved. The cistern would be about 200,000 gallons. Total
cost for the installed system would be about $400,000, and payback
is in the range of 30 years.
While this may not sound attractive, there are compelling reasons
for implementing this technology. One, for example, would be a project
on a site where a good water supply is not available. A cistern
could provide water storage for fire protection, eliminate some
portions of stormwater management systems, and alleviate upgrades
to the municipal water supply, thereby reducing cost.
T5 Direct Indirect Lighting
T5 fluorescent lamps first came to market about four years ago.
They are smaller in diameter than the T8 lamps in wide use today.
Due to the number of existing T8 lamps and the stock of replacement
tubes already in place, this technology is expanding more slowly
than when T8 lamps replaced T12s.
Why use T5 lamps, and what is direct indirect? T5 lamps provide
a higher-intensity light in a smaller package than T8 lamps. A direct
indirect fixture shines some light downward (direct) and some upward,
where it is diffused and reflected back in the downward direction
(indirect). This type of fixture provides very-high-quality, even
light distribution. The small diameter and high-intensity light
provided by T5 lamps makes the design of cost-effective direct indirect
fixtures possible.
Classrooms are typically required to be provided with a lighting
level of 70 foot candles. Using T5 lamps can reduce that to 50 foot
candles in some jurisdictions. This technology provides a 30-percent
reduction in light energy, requires fewer light fixtures and reduces
the air-conditioning load. This allows the purchase of smaller central
plant equipment. Properly integrated into a design, this technology
will reduce costs even though the fixtures are slightly more expensive
on a one-to-one basis.
A word of caution: Replacing a T8 system with the same number of
T5 tubes is not recommended. There will be an increase in lighting
levels and energy costs. Be sure to consult an electrical engineer
and have the system properly designed.
Terminal Induction Boxes
Terminal induction boxes provide a new twist on an old technology.
This device induces airflow in a room using high-velocity chilled
air. It was available long before VAV systems. The original unit
emitted a noticeable hissing sound and was generally mounted on
the wall near the floor.
The new induction unit nozzles are larger and operate quietly. A
100-percent outside air unit equipped with an enthalpy wheel and
sized to provide the minimum amount of outside air required for
each space provides cold air to the terminal induction boxes. Nozzles
within the box accelerate the air across the face of a coil, inducing
room air to flow through the coil. Cool water and warm water are
available to the coil. If the space gets too cool, warm water is
directed through the coil to warm the space. If the space gets to
warm, cool water is directed through the coil to cool the space—and
efficient comfort is achieved.
One aspect of this system is that air is not circulated between
rooms while the building is occupied; no germs going from space
to space. Of course that won’t help Jamal when Sally sneezes
on him, but at least the air quality is being maintained.
The entire latent load is handled by the central air handling unit,
so there is no condensation on the coil serving the classroom. No
water, no mold. There is no fan, no filter and no moving parts (save
a few control valves) to maintain—and they are quiet.
These systems work very well in high-occupancy buildings such as
schools because 15-20 cubic feet per minute of outside air volume
is required for each person, and that is enough to make the system
work.
That being said, the redesigned terminal induction box is a relatively
new technology, which has only just begun to make its way into HVAC
designs. It is a high-performance technology well worth investigating.
All of these systems are ready to begin conserving energy in your
school today.
When consulting the maintenance staff on any system decision, it
is critical that they understand the systems they will be tasked
with maintaining. No matter what systems are chosen, to obtain true
high performance for the life of the building and protect the health
and welfare of its occupants, proper design, construction and, most
important of all, continuous preventative maintenance must be provided.
Wes Bonafé has more than 25 years of engineering experience
and is a vice president and director of engineering at Moseley Architects
in Virginia. He can be reached at wbonafe@moseleyarchitects.com. |
