High Performing Buildings - Winter 2008 - (Page 50)

H Y B R I D A N D D E D I C AT E D V E N T I L AT I O N While radiant slabs handle the heating and cooling needs of the building, ventilation air must still be supplied to the building. For this, natural ventilation was considered, again to reduce fan energy consumption. However, a typical natural ventilation system (using operable windows) was not possible for security reasons, as well as because of dust and noise generated from the nearby boulevard and urban highway. The design team eventually chose two means of distribution for this building: a hybrid (i.e., mechanical-natural) ventilation system using tunnels running alongside the foundations and a dedicated outside air system (DOAS). The hybrid ventilation system has two operating modes. During free cooling hours, outside air is first drawn in from intakes located at ground level by a group of propeller fans within, then filtered and supplied to the building using the underground tunnels. The air is distributed to the occupied spaces on the ground and second floors using vertical shafts located on the perimeter of the building. Air circulates in the building horizontally in a displacement ventilation mode and exits through dampers installed at the highest level in the roof monitor. In this mode, the hybrid ventilation system can supply up to 50,000 cfm of outside air to the building (Figure 2). The outdoor air supply is monitored according to indoor temperature and controlled using 16 exhaust dampers located in the roof monitor. Due to the wider range of acceptable indoor temperatures with natural ventilation, free cooling is possible when the outdoor temperature is above 55°F dry bulb and below 65°F dew point (or 75°F dry bulb at 60% rh). In a typical meteorological year, the temperatures during more than 1,500 hours over the 3,200 yearly hours of business operation fall between those limits. If high dry bulb temperature occurs while having low dew point, the mechanical systems compensate by cooling the slabs using geothermal-free cooling. When below 55°F dry bulb, the outside air is considered too cold for free cooling. If the air dew point is above 65°F, condensation may occur on the cool slabs. At that point the hybrid ventilation system closes its outside air dampers, and outside air is supplied by the 6,000 cfm DOAS. This unit injects heated or dehumidified outside air directly into the hybrid system tunnels. The hybrid system then acts as a 25,000 cfm recirculation system, drawing air from the building’s back store and supplying it to the retail area. The DOAS also reduces outside air energy loads through a 75% efficient enthalpy heat recovery wheel (Figure 3). Figure 1 Radiant slabs on the ground and second floor help heat and cool the building. Figure 2 A hybrid ventilation system uses both natural and mechanical ventilation during free cooling mode. Figure 3 During summer/winter mode, the building uses a dedicated outside air system (DOAS). Winter 2008 50 HIGH PERFORMING BUILDINGS

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High Performance Buildings - Winter 2008 Passing On the Gift: Heifer International Headquarters Head of the Class: University of Florida’s Rinker Hall How Far Can You Go? Pearl River Tower The Proof Is Performance: How Does 4 Times Square Measure Up? Lighting the Way: Two Guilford County Schools Montreal’s Retail Example: Mountain Equipment Co-op® (MEC)

High Performing Buildings - Winter 2008

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