Emerging Technology: Chilled Beam Cooling
INTRODUCTIONConventional variable air volume (VAV) systems for large commercial buildings use a large fraction of their energy just to distribute energy in air. However, pumping air is much less efficient than pumping water. One alternative is the “chilled beam” system, in which small amounts of outdoor air are entrained into a larger supply of recirculated air, and passed across a radiant-convective chilled beam fixture. One benefit is that the chiller energy is delivered efficiently to the zone by being carried in water. Also, to prevent condensation, much warmer water is used, which helps to reduce energy consumption. These systems can be quieter, and can allow smaller floor-to-floor heights, reducing construction costs. It is estimated that at least 20 percent energy savings and costs competitive with VAV systems are achievable when more experience is gained.
The chilled beam approach was introduced in Scandinavia in the 1980s. The technology is now beginning to be seen in North America.
HOW CHILLED BEAMS SAVE ENERGYChilled beam systems provide savings by replacing fan energy with pump energy. They use pumped chilled water instead of blowing cold air. Water has much higher heat capacity, both by mass and volume. In typical pump and fan arrangements, this has been shown to translate into a reduction in fan energy by a factor of seven.
Chilled beams save energy by moving much less air - only the “primary” or ventilation air is brought to the zone, where it mixes with recirculated room air. VAV systems are typically designed with very high duct resistance, typically in the range of 6 inches water gauge (wg). On the other hand, most of the energy for chilled beam systems is distributed by chilled (or heated) water carried to the beams. The ventilation air requires no more than 1 inch wg in most designs, so there is a large difference in fan energy. In retrofitting a 215,000-square-foot office building in Chicago, combined fan and pump energy dropped from 190 kW at design and 114 kW at 70 percent load for the VAV base case, to 34 kW for the chilled beam system.
In addition, chilled beams generally require a minimum supply water temperature greater than 57°F to avoid condensation forming on the beam surface. Cooling the supply water less saves energy. In fact, for roughly the northern half of the United States and all of Canada, where groundwater is available, the groundwater temperature is less than 55°, so direct cooling with groundwater would suffice for radiant beams.
The warmer supply water also minimizes or eliminates terminal reheat that may account for 20 percent of energy consumption in complex VAV installations such as laboratories and large office buildings - too many lightly-loaded zones require that the chilled air, typically 55°, be reheated before being distributed.
Finally, chilled beam systems reduce the mean radiant temperature of a space by 2° to 4°. This saves energy because the space can be kept at a higher dry-bulb (sensible) temperature.
OTHER BENEFITSChilled beams reduce the floor-to-floor height required, because less space is required for ductwork. Thus, they may become a preferred design for buildings where this is important. For new construction, for example, this may allow five stories at the height normally needed for four, which would offer substantial construction economies. For retrofits, it may allow higher ceilings.
Chilled beams reduce first costs by eliminating up to 50 percent of the ductwork required. Chilled beam systems also don’t require equipment rooms on each floor. This may increase rentable floor area, perhaps as much as 2 percent. Commissioning time and cost may be reduced because the beams are more nearly “plug and play.”
Another important benefit is that maintenance is simpler, less frequent, and less expensive. Chilled beams do not have fans or filters. Because the surface temperature must be held above the dew point, they have no condensate lines or traps. Savings are estimated at approximately $0.75 per square foot per year.
Finally, chilled beam systems have low airflow velocities, which reduce drafts. Low pressures and remote fans also lead to relatively quiet work zones.
CURRENT APPLICATIONSDesigners in the U.S. are beginning to use the technology in two principal applications: laboratories and large office building retrofits. Laboratories are attractive because their ventilation requirements are high. In large office building retrofits, there can be several advantages, starting with the relatively small amount of space (and headroom) required for installing a system. Growth is expected in the office building market as a whole because of the combination of energy efficiency, relatively easy design, reduced maintenance requirements, and perceived amenities (quiet, comfortable, etc.).
FACTORS THAT MAY AFFECT IMPLEMENTATIONChilled beam systems cannot handle super-high internal loads, so they are more or less constrained to situations with cooling demand less than 80 W/m2 (maybe peak at 120 W/m2), and heating loads less than 40 W/m2 (Rehva 2008). Chilled beam systems also require good humidity control. The dew point of the air in the space must remain above the temperature of the beams. This requires that the ventilation air must be dried to relatively low humidity, and internal moisture loads must be reasonable and understood.
Barriers to chilled beam implementation include:
• Lack of familiarity with the technology. Better design tools are needed to provide simulation models and systematic design guidance.
• Chilled beams require a low-infiltration building envelope. This is because: (1) only the ventilation air is filtered so if the system is bringing in dust by ventilation, or if it has large internal sources, there will be dirt-streaking; and (2) high infiltration will lead to loss of humidity control.
Publication date: 05/24/2010