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thermal

Qualitative Analysis

1 Passive strategy: Building envelope for thermal insulation

  • Orientation: The building is oriented to the south, elongated the east–west axis (220 ft./67m) to maximize solar heat gain for heating in the winter and to capture prevalent summer breezes for cooling

  • Windows: The south wall is mostly glass and fixed with wooden louvers (solar shading), which can both harnesses winter sun for heating and shield the summer sun from verheating the interior. Windows use spectrally selective, low-e glazing with wood frames for reduced heat loss and contain an argon gas interior to increase insulation value.

 

South-facing Windows Form

Source: inhabitat.com

  • Wall-floor-roof: Structural insulated panels (SIPs) are used in the walls and roof to improve the envelope’s thermal resistance efficiency. The SIPs with high R-value are composed of oriented strand board (OSB) panels that sandwich expanded polystyrene foam. SIPs have foam cores with a thickness of either 5.5 in. (0.14 m) or 7.5 in. (0.19 m).

  • Mixed-Mode Natural Ventilation System

    Low- and mid-height windows on the south side are grouped in banks of four and operated with hand cranks. Windows located high on the north side have motorized operators that are controlled by the building’s EMS.

      In addition, operable windows on the north (leeward) side of the building are larger than those on the south (windward) side, to promote greater air velocities inside. Inlet windows are total approximately 495 ft2, while outlets are total approximately 822 ft2.

     Temperature and humidity sensors located on the exterior of the building determine when the climate is appropriate for natural ventilation. Upon sensing optimal conditions, the building management system turns off the mechanical heating or cooling system and turns on modified exit lighting fixtures which notify the building occupants that it is ok to open the windows. Simultaneous to this change mechanical operator open the windows in the north-facing clerestory above the second floor to allow a natural draft to begin. Air is then pulled in though the operable windows, rises through the building, and exits through the clerestory.

The North-facing Clerestory above the Second Floor

Source: www.tradelineinc.com

The natural ventilation system is a hybrid or mixed-mode system in that fans are often used to help move ventilation air. These fans exhaust air on the north side. A fan on the second floor exhausts air at a rate of 2,800 cfm (1.32 m3/s). A fan on the first floor exhausts air at a rate of 5,600 cfm (2.64 m3/s).

2 Active strategy: HVAC system for thermal comfort

The Merrill Center uses ground-source heat pumps as the primary source of heating and cooling, combined with a desiccant dehumidification system. Fin and tube radiators connected to a propane boiler provide supplementary heating in the entryways, middle landing of the main stairway, and along the glazed south wall of the main building.

  • Ground-source heat pumpsThe water-to-air heat pumps are chosen to conduct the underground heat transfer. And individual air-handling unit is served by a single groundwater loop. The ground loop that serves the ground-source heat pumps is connected to four circuits; each circuit has 12 vertical geothermal wells or boreholes. Each heat pump package is plumbed in parallel with the ground loop. Water is circulated with redundant parallel pumps. The loop pumps are variable speed and are controlled to maintain a pressure set point determined by the EMS (sensor control). The operator manually adjusts the pressure set point to achieve a high temperature difference across the wells. Outside air is introduced via two air handlers, one in the main building and one in the conference pavilion. The air handlers provide the supply and return air streams for outside and relief air.Previous researches reported that, because of the additional heat added to system to meet capacities during the real operation, the ground-source heat pumps showed higher temperatures than expected, indicating HVAC efficiency is below expectations

The North-facing Clerestory above the Second Floor

Source: www.tradelineinc.com

Ground Heat Exchanger

Source: from NREL

  • Desiccant dehumidification system

The system was designed to remove moisture from the air and reduce the need for mechanical air conditioning. But actually, because of the need to operate a boiler in the summer to regenerate the desiccant wheel, the heat recovery and desiccant systems are not being used.

3 Zonal control with individual heat pumps

  • The locations of thermal zones served by heat pumps in the main occupied areas of the building has been showed. 18 heat pumps and one air conditioner are connected in parallel to the ground loop.

  • The heat pumps are packaged water-to-air units with integral, constant volume air systems. The cooling only air conditioner (ACU-1) cools a computer server room. Two small heat pumps (HP-1 and HP-18) serve unoccupied areas on the ground floor.

Location of Heat Pump Thermal Zones

Source: from NREL

4 Comparison of LEED Buildings with CBE Survey Database for Thermal Comfort​

  • As mentioned survey conducted by CBE, as for the thermal satisfaction, there were several LEED buildings that clustered near the 80th percentile, including the CBF building. Though the ranking was relatively higher, the 20% to 30% who were dissatisfied complained that the building was often too cool in both warm and cool weather. This dissatisfaction was attributed to a lack of accessibility and control of thermostats as well as stratification of warm air to the upper floor and certain rooms that then result in discomfort.

     

Comparison of LEED Buildings with CBE Survey Database for Thermal Comfort

Source: Heerwagen, J. U. D. I. T. H., & Zagreus, L. (2005). The human factors of sustainable building design_ post occupancy evaluation of the Philip Merrill Environmental Center.)

Quantitative Analysis

1 Building Envelope Construction and Nominal Thermal Properties

Building Envelope Construction and Nominal Thermal Properties

Source: from NREL

2 Natural ventilation control is based on sensor data for interior temperatures, interior humidity, and outdoor temperature.​

Natural Ventilation System Control Set Points

Source: Chang, J.-C. (2002). Case Studies of Naturally Ventilated Commercial Buildings in the United States. MSc Thesis. Cambridge, MA: Department of Mechanical Engineering. Massachusetts Institute of Technology.

Qualitative

  • Passive Strategy

  • Active Strategy

  • Zonal control with individual heat pumps

  • Thermal Comfort

 

Quantitative

  • Thermal Properties of Building Envelope

  • Natural Ventilation Control Baseline

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