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green@work : Magazine : Back Issues : Mar/Apr 2007 : Building

Building: An 'A' for Efficiency
Whether the intent is to save the planet or plan to save money, high-performance buildings make good sense.

by Wesley S. Bonafe, PE


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.


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