Page 28 - Zero Net Energy Case Study Buildings-Volume 2

CASE STUDY NO. 7
DPR CONSTRUCTION OFFICE BUILDING
16
Zero Net Energy Case Study Buildings: Volume 2
Natural Ventilation
As noted, without openings on three sides of the building cross ventilation is not possible and draft-induced ventilation via the roof was deemed too costly for the amount of energy saved. As a result of this analysis during the design and modeling phase, natural ventilation and night flush operation were eliminated as possible low-energy design strategies.
During the first year of building operation, however, energy-use monitoring indicated that the cooling energy demand was higher than that indicated in the modeling. It appears that a night flush operation may have been useful in general as a low-energy cooling strategy. However, the HVAC system is not a standard VAV system (see discussion below), which has the ability to move 100% outside air through the building, so a further retrofit to accommodate night flush is not really practical at this point.
Heating, Ventilating and Cooling Systems
The design approach to the HVAC system follows the general set of strategies common to many low-energy building designs:
• Utilize highest efficiency heating and cooling equipment;
• Minimize electricity-driven fan use;
• Reduce energy use due to sheer amount of air ductwork to and from a building central plant (i.e., employ distributed dedicated outside air system (DOAS) units in lieu of a central plant);
• Utilize low-pressure drop design for the air ducts;
• Decouple air system cooling and fresh air ventilation.
The first strategy is used with the incorporation a single air-source heat pump into a variable re- frigerant flow (VRF) system. The latter includes fan-coil units for each zone, providing fast-acting cooling and heating. The system also enables heat to be transferred from spaces requiring cool- ing to spaces that require heating, allowing for better zoned thermal control of the building overall and higher efficiencies due to this internal heat recovery aspect. This type of system also has excellent partial/low load efficiencies.
The fresh air requirement is met using four dedicated outside air system (DOAS) units. These units use 100%-outside-air and demand-controlled ventilation based on CO2 sensors. This is inherently more efficient than a centralized system because of the tuning of the fresh air require- ment to specific demands of separate areas of the building. In addition, the DOAS units avoid the built-in inefficiencies of the large fan power of a central system and long duct runs through ceiling space.
The DOAS units have the possibility of an even higher level of efficiency in this building applica- tion. They are equipped with an air-to-air plate heat exchanger, transferring heat from exhaust air to supply air, requiring no additional conditioning at the units because of the mild climate of the San Francisco area.
For cost reasons, only one of the four DOAS units is equipped with a variable-speed fan drive (VFD). The other three are constant volume type. During the design phase, energy modeling indicated only very small energy savings would result if all four DOAS units were VFD type, in- cluding daily operation and a potential night flush operation, and the additional cost would not be made up over the period of the 10-year lease. Subsequent building performance measurements now indicate that the building is indeed operating at ZNE or better, so this decision appears to have been justified.