e-News #69: Chilled Beams
Chilled Beams: Saving Space, Saving Energy
Chilled beams are among the recent energy-saving innovations making their way to the U.S. market. Chilled beam technology, which involves locating a low-temperature radiator at ceiling level to cool the rising warm air, has been utilized in Europe and Australia for more than a decade. Once cooled, the air slowly descends into the occupied zone, providing adequate cooling with minimal air movement and fan power, while providing an unobstructed radiant heat sink above the occupied zone.
Chilled beams have emerged in the United States as an attractive alternative to variable air volume (VAV) systems and have been proven to be effective in conditioning both new and existing buildings. Their flexibility, ease of installation and maintenance, and energy efficiency present a cost effective alternative to more conventional cooling systems.
Types of Chilled Beams
Chilled beams can be classified into three different categories: Passive Chilled Beams, Active Chilled Beams, and Multi-Service Chilled Beams. The distinguishing characteristic of each design is the type of air flow utilized and the means by which fresh air is provided to the space. Common to all types of chilled beams is the radiator element, which uses circulation of chilled water as the source of radiant cooling.
Chilled beam technology is most applicable to interior environments where heat gain in the space - solar radiation, people, or equipment heat - is the primary factor for determining air flow quantities. This refers to spaces where the amount of air required to cool the space surpasses the amount of air required by code to maintain acceptable indoor air quality. Therefore, hospitals and laboratories are facility types thinterat are particularly well-suited to using chilled beams.
Passive Chilled Beams
Passive chilled beams are akin to finned-tube radiators positioned within the ceiling cavity. Perforated metal tiles allow warm air to flow from the occupied zone to up the beam in the ceiling cavity. This passive technology is operated purely from natural or free convection, whereby the warm air rises to the radiator, is cooled, and then descends naturally without any mechanical fans. Passive chilled beams are typically employed to offset perimeter heat gain in a building. They are also often paired with displacement ventilation systems.
Figure 1 - Chilled beam (Photo source: Trox)
Chilled beams achieve their cooling effect by convection, using finned elements through which water at 59°F to 65°F is circulated. Passive beams work with natural convection while active beams offer increased cooling capacity by using forced air to induce convection over the elements. The primary air is then supplied to the room through diffusers built into the beam.
Active Chilled Beams
Active chilled beams incorporate tempered ventilation air, supplied through ducting in the beam itself, and provide ventilation air to a space through induction nozzles. These nozzles create a pressure differential across a cooling coil, "inducing" air flow from the room across the coil and supplying cool air back to the room that is a mixture of the tempered ventilation air and the recirculated room air. The cooled air enters the room via outlet slots on the underside of the beam. Because they rely upon powered air movement, rather than just buoyancy, active beams can also be used for heating by supplying hot water to the heat exchanger.
Multi-Service Chilled Beams
Mutli-Service Chilled Beams are similar in function to active chilled beams, but they may also act as a conduit to supply other building services such as lighting, speaker systems, IT systems, fire protection (sprinklers and detectors), acoustic insulation, Building Automation System sensors, and photocells.
Benefits of Chilled Beams
A number of thermal comfort and energy benefits can be derived from incorporating chilled beams into the design of a building. They include:
Chilled beams can provide potential energy reductions from 20 to 50%, depending on the system design, building details, and climate zone. One particular energy-saving advantage of chilled beams is the ability to use higher chilled water supply temperatures (65°F to 59°F). This allows the chiller system to operate more efficiently and to potentially make greater use of waterside economizer control.
In addition, chilled beams allow fans to operate at lower speeds, meaning that both the fans and chiller are doing less work to achieve the same amount of cooling as in a conventional system.
As mentioned above, designing ventilation rates for occupancy rather than to offset heat gains can reduce the number of air changes per hour in laboratories and other buildings with large internal equipment loads, resulting in further energy savings. For example, in a typical laboratory, ventilation rates can be reduced from 12 to 18 air changes per hour to 6 to 8 air changes per hour.
Smaller Equipment Required, Smaller First Cost
With fewer air changes needed, ductwork, air-handling units, exhaust fans, chillers, and boilers can all be downsized. In new construction, this avoided first cost can help to offset the cost of the chilled beam units and infrastructure. Even in 2005, with contractors relatively unfamiliar with the technology, the savings from downsizing the HVAC components were found to offset the first cost of an entire chilled beam system.
|Chilled Beams Case Study|
Constructed in 2004, the Tahoe Center for the Environmental Sciences utilizes chilled beams to efficiently deliver comfortable working conditions to its occupants. The 40,000 square foot building with 10,000 square feet of laboratory space was awarded LEED® Platinum Certification in the summer of 2007, having achieved energy savings of 60 percent over ASHRAE 90.1- 2004.
click to enlarge
On warm days when outdoor air temperatures exceed 68F, chilled water between 55°F and 60°F is used to supply 68F ventilation air to the chilled beams in order to cool the laboratory spaces.
By contrast, on cold days where the temperature drops below 55°F, ventilation air is pre-heated to 55°F, and then heated further in each laboratory space, as needed.
This strategy eliminates the need for reheat, and allows for untempered outside air in the space when temperatures are between 55°F and 70°F.
Futhermore, such systems allow for reducing the size of the ducting system and air handlers by one third.
Indoor Air Quality
Depending on the location, air may be re-used locally with chilled beams, so no contaminant mixing occurs. This results in a cleaner, healthier indoor environment, without energy penalties for introducing large quantities of outside air. Although chilled beams are well suited for managing the cooling loads in some laboratories, they can be an unacceptable solution in others due to heightened indoor air quality concerns. In areas with potentially dangerous contaminants, many laboratories prohibit recirculation.
Depending upon the type of installation, chilled beams contain few or no moving parts. The fact that there are no internal fans or filters to repair or clean accounts for a long life expectancy and hassle free operation. In general, all maintenance associated with a chilled beam system will be at the central plant. This compares favorably to HVAC systems that have dampers and/or actuators at each terminal box.
Figure 2 - Active Chilled Beam
Cross-sectional schematic of an active chilled beam. Both fresh ventilation
air (introduced through nozzles) and warm recirculated air (from the occupied
zone) are cooled as they pass by the chilled beams cooling coils.
As with any technology, chilled beams also have some some limitations. Here are a few ways in which chilled beams may require extra consideration:
In humid climates, humidity controls may be a necessary addition to chilled beam systems. If not properly controlled, humidity levels may cause condensation on the surface of chilled beams. To avoid this problem, the internal humidity must be controlled such that the beam temperature is always above the dew point temperature of the air. A rule of thumb states that relative humidity should be kept below 50°F to 55°F dew point. This is equivalent to a maximum relative humidity of 50% to 55% at 72°F. If it is not possible to control the humidity in the space, chilled beams may not be the preferred method for providing space cooling.
Many of the benefits derived from chilled beams come from their ability to recirculate the air within a space. When necessary, fresh ventilation air can be provided via conventional means or as part of an "Active Chilled Beam" system.
Chilled beams can be integrated into a ceiling grid or left exposed. Aesthetic considerations may also include minimizing the visual impact of the beam. Manufacturers may work with design teams to customize the appearance of the chilled beams through molds and extrusions that better adapt to the facility's architecture. In addition, the design and layout of chilled beams may incorporate key infrastructure components, such as lights, sprinkler heads, speakers, sensors, air nozzles, smoke detectors, and voice/data cables.
The design team must consider the location and spacing of chilled beams to ensure adequate coverage of the space. In order to achieve proper interior thermal comfort, the building automation system or building operator can chose to control chilled beams individually, or controlling grouped regions of chilled beams.
Passive beams are virtually noise free, as no fans are incorporated into the system. Active beams utilize relatively low airflow rates-typically around 40 CFM. However, it is recommended to incorporate a suitable acoustic in-fill membrane into the design to enhance sound absorption, without inhibiting air flow to or from the beam.
One 2005 study showed that in one typical 14,100 square-foot laboratory, installation of a chilled beam system was less than the cost of a standard VAV system, coming in at just 84% of the cost of the conventional system. Even a chilled beam installation with integrated lighting cost just 96% of the VAV system installed first cost.
Chilled beams may not be an effective solution for spaces with exceptionally high cooling loads, which would require an impractical number of chilled beams to be installed in the ceiling. Spaces with such loads will likely be better served by a conventional cooling system.
California utilities offer outstanding educational opportunities that focus on the design, construction and operation of energyefficient buildings. Listed here are a few of the many upcoming classes and events; for complete schedules, visit each utility's website.
Implementing Energy Efficiency Projects
Title 24, 2008 is now in effect
San Diego Energy Resource Center: "Preparing for California's New Nonresidential Title 24 Standards."
New PG& E Training Schedule
The Pacific Energy Center and the Stockton Training Center will issue new schedules at the end of January.
Introduction to Life-Cycle Costing
Benchmarking with ENERGY STAR Portfolio Manager
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e-News is published by Energy Design Resources (www.energydesignresources.com), an online resource center for information on energy efficiency design practices in California.
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Energy Design Resources and Savings By Design are funded by California utility customers and administered by Pacific Gas and Electric Company, Sacramento Municipal Utility District, San Diego Gas and Electric, Southern California Edison and Southern California Gas Company, under the auspices of the California Public Utilities Commission.