Ashrae Journal - November 2008 - (Page 39)

year-round water flow at a minimum of about 50 gpm (3.2 L/s) to support the aquatic organisms, fish, and plants—similar to a natural underground water spring. An adjacent children’s water sprayground requires 75 gpm (5 L/s) during summer operating hours for its automated water spray toys and ground sprays. Water is used for such diverse purposes as maintaining building comfort, environmental education at the kettle pond and wetlands, and fun at the sprayground. However, the water use story does not end here. Water is recaptured for irrigation at the adjacent golf course or directed to seepage beds, which return the water to the ground. Ultimately, the water returns to the aquifer from which it originated, and the cycle can begin again. Building Design Challenges With all sides of the building capable of being viewed by the public either on their way into the building or as they head out onto the nature trails, the designers insisted that any outdoor mechanical equipment be hidden from view. Also, noise from such equipment had to be minimized. After all, who wants to listen to mechanical equipment when exploring their natural environment? Outdoor equipment such as chillers, cooling towers, and condensers needed to be avoided because of noise and aesthetic concerns. As with most buildings, keeping first costs and operating costs to a minimum was a design goal. With the building located in a remote portion of the park, the “traditional” natural gas heating source was not readily available. Propane (in tanks) was considered but rejected by the design team and owner due to aesthetic and operating concerns. An all-electric HVAC system was desired, but the high operating cost associated with direct electric heating needed to be avoided. Most importantly, the building and its systems needed to demonstrate Metropark’s commitment to the natural environment. Those goals include providing a facility that will support environmental education and nature interpretation, showcase water resource management, and incorporate sustainable design practices. Tackling the Challenges Cost effective system selection. The design team immediately recognized that having a continuous flow of groundwater provided opportunities for the building’s HVAC system. Coupling the groundwater used for the kettle pond, sprayground, and wetlands with water source heat pumps seemed like a great solution. The continually flowing groundwater provides stable water temperatures (at about 50°F [10°C]) that allow water source heat pumps to operate efficiently and without cooling towers or boilers that otherwise are required on systems that are not ground-coupled. In contrast to ground-coupled systems with expensive well fields, the water was already flowing by the building. It was cost effective to simply route piping into the building and back out on its way to either the sprayground or kettle pond. All the HVAC equipment could be located inside and the desired all-electric system provided. A centrally located ground coupled heat pump system was selected. Building cooling and heating is provided by two water- to-water heat pumps with a combined 40 ton (141 kW) cooling capacity and 365 MBH (107 kW) heating capacity. These heat pumps provide chilled and warm water to fan-coil units located near each space and to the larger multipurpose room air-handling unit. The heat pumps use the well water on the way to the kettle pond or sprayground as a heat sink. A 100% outside air-handling unit provides ventilation air to each fan-coil unit. By centrally locating the water-to-water heat pumps in a mechanical room, the number of compressors were kept to a minimum. This complied with the owner’s desire to keep equipment with a possibly shorter service life in the main mechanical room, where it could be readily serviced by an outside maintenance organization without disturbing building functions. Four-pipe fan coil units located in closets next to the conditioned spaces operate quietly and are easily accessed by the owner’s maintenance staff for the infrequent and usually simple maintenance they require. However, using groundwater as a heat sink created additional challenges. The site groundwater is hard and directly using it would quickly foul equipment. If the HVAC demand for water is greater than that for the kettle pond, wetlands, and/ or sprayground, then water used in excess of these amounts would be considered wasteful. Therefore, water use for the HVAC system had to be restricted—but not overly so—because the biologists did not want the water temperature manipulated above 80°F (27°C), nor could it become too cold and approach the freezing point. It became clear to the design team that careful matching of loads, equipment types, and sizing was needed. Hard Water. The site’s well water is not directly used in the HVAC systems due its hardness and concerns about scaling impacts on equipment life. Softening and/or chemical treatment was not an option because of high operating costs and potential impacts on the downstream ecosystems. Cleanable plate and frame heat exchangers are used to keep the well water separate from the treated HVAC water systems. Plate and frame heat exchangers were selected because of their close approach temperatures and ability to resist scaling through turbulent water flow. After the completion of the project, we still found that the heat exchanger quickly fouled. The heat exchanger was originally selected for a 75 gpm (5 L/s) flow rate and a low pressure drop (such that downstream equipment would still have enough pressure to operate). However, most of the time the sprayground does not operate and the flow is only 50 gpm (3.2 L/s). The flow simply was not turbulent enough to stop the scaling. To remedy this, the heat exchanger was taken apart, cleaned, and it was converted from a two pass arrangement to a four pass arrangement. A pump and circulation loop was added to provide constant turbulent flow and to accommodate the additional pressure losses. Minimizing peak demand/loads and ener gy conservation. Early in the design process, the building designers and architects worked with the mechanical engineers to provide a building that meets the functional requirements and goals while reducing energy ASHRAE Journal 39 November 2008

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Ashrae Journal - November 2008 Contents Commentary Industry News Meetings and Shows Feature Articles Reducing Data Center Energy Consumption Building Sciences Feature Articles Sustainable Nature Center Emerging Technologies Special Products Washington Report Special Supplement: BACnet® Today BACnet® Goes Green Seeing the Light with BACnet® BACnet® on Campus BACnet® for Net Zero Analyzing BACnet® BACnet® at Xerox Campus BACnet® in Europe BACnet® for Green Library BACnet® at the University of Minnesota: Flexibility, Interoperability InfoCenter Product Showplace Classified Advertising Advertising Index

Ashrae Journal - November 2008

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