Ashrae Journal - December 2008 - (Page 36) air-handling unit being higher than 50°F (10°C) dry bulb/49.5°F (9.7°C) wet bulb under some conditions (discharge temperature is reset based on controlling building relative humidity), and the near elimination of reheat energy. Although it is also possible to reset the discharge air temperature from the air-handling unit under Systems 1 and 2, a single interior laboratory space, with high heat production from equipment could drive the discharge temperature back down at any time. For this reason, discharge temperature reset is not included in the simulations for Systems 1 and 2. Limitations of the Simulation Results System 1: Typical VAV System (No Energy Recovery) 4,662,300 Total kBtu Percent Variation Maximum cfm Chiller (Tons) Boiler (MBh) 118 100 18,500 System 2: Typical VAV System Energy Recovery With Wheel) System 3: Chilled Beams Dual Energy Recovery (Using Wheels) 18,500 3,563,000 1,160 2,176,500 490 74 77 47 9,500 340 57 Since the hourly analysis program does not specifically recognize the use of chilled beams, these devices have been modeled as two-pipe fan coil units with zero static pressure on the fans (zero horsepower). The program is also limited to inputting a single number for energy recovery efficiency. Since under a dual energy recovery design the efficiency improves greatly under summer operation, separate simulations were performed for preheating and cooling. The variations in recovery efficiency for wheels under intermediate ambient conditions are not captured by the simulation. The air-handling system is run as a “Tempering Ventilation” unit under hourly analysis software, controlling the mechanical refrigeration chilled water coil to maintain building relative humidity at 50% maximum. The results of these simulations should be considered as approximate only, providing a basic indication of the potential benefits in cooling and heating energy reductions for the system. It was not the intention of this analysis to take into account variations in energy use by equipment located within the cooling and heating plant. The results presented here focus on cooling and heating energy use in the rooms and at the central air-handling unit. The chilled water pumping requirements for the beams has not been factored into the results to weigh against the reduction in fan energy at the air-handling system. However, for the same energy transfer the pumping power would be only about one-seventh of the required fan energy use,4 and there is less chilled water required at the air-handling system due to its reduced size. As an additional consideration, during winter operation a chilled water system can operate on a water-side economizer, enabling the mechanical refrigeration equipment to be turned off. In the case of a water-cooled chiller, a plate frame heat exchanger can be used in conjunction with the cooling tower to achieve water-side economizing. In the case of an air-cooled chiller, the chiller can be customized to include a free cooling coil. The chilled beams operate at much higher chilled water temperatures than air-handling systems (59°F or 60°F [15°C or 15.5°C] versus 40°F to 45°F [4°C to 7°C]), so more hours will be available for water-side economizing. It is expected that this benefit will offset the energy considerations for producing chilled water for the beams in winter, when the 100% outside air units in the more traditional designs (Systems 1 and 2 in this analysis) are benefiting from air-side economizing.4 36 ASHRAE Journal Figure 7: Simulation. Percentage Variation from Base Case 100% 77% 47% 53% 58% Reduction in Size of Air Handler None None 50% 35% 25% Total kBtu System 1 System 2 System 3 System 3 (Alternate 1) System 3 (Alternate 2) 4,662,300 3,563,000 2,176,500 2,469,800 2,697,300 Table 3: Simulation—Alternate reductions in size of air handler. The annual energy use for plug loads, (lighting and equipment), are not tabulated in the results, since it was assumed that these loads would remain constant. First Cost Considerations The initial costs of the chilled beams, their associated piping, and the additional energy recovery device in the air-handling unit (i.e., dual energy recovery) can be justified by the reduction in the size of the air-handling unit and ductwork, as well as the chiller and boiler plants. If some reductions in floor-to-floor height can be realized due to the reduction in ductwork, it is possible that the chilled beam system could result in an overall savings in construction costs, as well as in significantly reduced operating costs.4 Initial estimates performed, on the NJEDA Tech IV Building and Project X in Table 1, projected relatively small initial cost differences for the mechanical budget between the chilled beam design and a traditional system (under 5% cost increase on NJEDA and a slight cost reduction on Project X). Since both these projects use existing building structures, no credit was taken for reduced floor-to-floor heights in the estimates. Potential Drawbacks and Limitations Typical concerns or drawbacks include ceiling coordination between the beams and light fixtures, the installation of large quantities of beams, load limitations of the beams, and condenashrae.org December 2008 http://www.ashrae.org
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