School districts in Southern California are looking to increase the number of their schools that have air conditioning in an attempt to improve learning environments. One technology many school districts are interested in is thermal displacement ventilation (TDV), which stratifies air by temperature so only lower levels need to be cooled.
AECOM, a global provider of professional technical- and management-support services, provided consultants to determine whether the use of TDV is feasible in Southern California schools and to provide guidelines on how the technology should be used. AECOM used computational-fluid-dynamics (CFD) software designed for building interiors to simulate the use of TDV in various classroom configurations. The consultants also developed guidelines for classroom suitability and furniture arrangement.
HOW TDV WORKS
TDV introduces cool air at about 65°F — compared with a standard mixed system's 55°F air — at a low velocity at a low level using a raised floor or terminals around a space's perimeter. Cool fresh air passes over heat sources, such as people and equipment. The heat sources warm the air, causing it to rise and accumulate moisture and contaminants, such as carbon dioxide (CO2). This warm, moist, contaminated air collects at a high level — above the breathing zone — where it is removed from the space.
TDV reduces energy consumption because it is necessary only to cool occupied zones. Most high-level heat loads, such as from lights and heat gain through roofs and walls above 7 ft, do not have to be cooled if a 100-percent-outside-air system is used. Because a TDV system's supply-air temperature is higher than a mixed system's, free cooling is available more often. Finally, all contaminants, including CO2, collect at a high level, so the air quality of a well-designed TDV strategy is better than that of a standard mixed system with the same quantity of ventilation air.
CFD SOFTWARE DESIGNED FOR HVAC
The consultants used FloVent CFD software from Mentor Graphics' Mechanical Analysis Division (formerly Flomerics) to analyze TDV use in classrooms.
“We have a number of different CFD codes in our company,” Jim Saywell, an engineer for AECOM, said. “But I use FloVent whenever a project involves building interiors because it is designed specifically for modeling heating, ventilation, and air-conditioning applications, so it is both quicker and easier to use than general-purpose CFD codes.”
In the first part of the study, AECOM addressed whether TDV provided a practical, low-energy cooling solution for Southern California schools. For TDV to be effective, a ceiling needs to be high enough that warm, moist, contaminated air can collect without dropping into the breathing zone. FloVent was used to analyze identical classrooms with various ceiling heights to determine which schools or classrooms could best utilize TDV technology.
“It was decided that the best guidance we could give was never to implement TDV on a classroom with a ceiling lower than 8½ ft, and it was recommended that the ceiling height should be at least 9 ft,” Saywell said. “Some classrooms have sloped ceilings. In these cases, if the average height of the ceiling is above 9 ft, then this will be sufficient height for a TDV strategy to properly function.”
The next step was to find the most effective terminal layout, one that would have minimal impact on how teachers could make use of space. The goal was to take as little perimeter space away from the classroom as possible while still providing enough cooling and fresh air.
“The simulation results showed that providing two terminals with as narrow a width as possible, capable of supplying 600 cfm each, was sufficient to provide acceptable air distribution,” Saywell said. “Placing these terminals in opposite corners of the room is more effective than placing both in corners of the same wall. But this would mean running ductwork along two walls instead of one, possibly taking usable wall space away from the teacher. Placing terminals on opposite corners also might be impossible due to the location of a door or window. For this reason, it was considered acceptable from an air-distribution standpoint — and preferable from a space-efficiency standpoint — that both terminals be placed in two corners of the same wall.”
The next step was to find areas in which vertical temperature gradients or insufficient cooling were likely to cause thermal discomfort. A survey of several example classrooms revealed that most teachers use all available space around the perimeter of a classroom for bookshelves, boxes, and other objects. Thermal gradients were found to be significant when an object, such as a piece of furniture, was placed too close to a terminal, directing airflow toward occupants.
ANSI/ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, suggests that if the difference in temperature from ankle to head height is greater than 5.4°F, people will feel thermal discomfort. Although the supply air of a TDV system is not as cold as that of a standard mixed system, it still is 7°F to 10°F cooler than design temperature. This means certain areas of a space have a vertical temperature gradient greater than the recommended 5.4°F. Students should not be sitting in the areas shown in Figure 1.
Figure 2 shows the results of a CFD simulation in which a bookcase was placed 3½ ft from a terminal. The air was directed over the furniture, forcing it to flow farther into the room. This increased the chance that more students would feel a draft or discomfort because of a vertical temperature gradient. A new guideline instructed teachers to avoid placing objects of significant size, such as bookcases, within 4 ft of a diffuser.
RETURNING HIGH-LEVEL AIR
Returning high-level air can be more energy efficient because it often takes less energy to condition return air than outside air. The schools' design team performed a simulation to assess what CO2 levels would be like in scenarios using 100-percent, 66-percent, or minimum outside air (figures 3, 4, and 5). The outside air was assumed to have CO2 levels of 400 ppm. The American Society of Heating, Refrigerating and Air-Conditioning Engineers recommends that inside air not have CO2 levels of 750 ppm above outside air. The simulation results showed that 100-percent outside air is excellent, 66-percent outside air is acceptable, and minimum outside air is unacceptable. Therefore, maximum CO2 levels should be less than 1,050 ppm in school buildings.
USING HEATING WITH TDV
Heating can be a design challenge when utilizing TDV. In heating mode, supply air is much warmer than room air. Therefore, supply air rises and collects at a high level, where it is exhausted before it can provide heating to the occupied level.
In Southern California, however, little heating is required, usually only during the first hour or two of occupancy. That means it is possible to provide enough heating to an occupied classroom by supplying air that is only 2°F higher than the required room-air temperature, provided the space is preheated sufficiently. Preheating can be achieved by recirculating warm air through the space via a diffuser. One diffuser increases supply velocity, allowing adequate mixing, while two diffusers are needed when a room is occupied to keep air speed within comfortable levels. Figure 6 shows what happens when 70°F air is supplied to an occupied classroom via two diffusers. On a cold day, this continues for the first hour or two, after which cooling is required.
An alternative method can be used in cooler climates if the method shown in Figure 6 is insufficient to achieve occupant comfort. A separate diffuser significantly smaller than the one used in cooling mode is added to the bottom of the terminal. When the system is in heating mode, air is directed through the small diffuser at a much higher velocity than when the system is in cooling mode. This allows sufficient mixing before the warm air rises.
Based on these simulation results, AECOM consultants developed guidelines for use of TDV that could be used in Southern California school districts. AECOM also carried out full designs of TDV schemes in a few pilot schools. The simulation results demonstrated that TDV can provide comfortable classrooms at energy-consumption levels considerably lower than those of conventional air-conditioning systems. With guidelines in place, competitive bidding will be used to select contractors to install TDV in other schools.
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