Page 45 - Zero Net Energy Case Study Buildings-Volume 3

Post-Occupancy: Natural Ventilation
Perhaps the most significant “lesson learned” in the design and operation of this building pertains to the natural ventilation design strategy. Correctly recognized as potentially one of the most effective methods of reducing energy use for cooling and fresh air in the office and conference spaces—more than half the conditioned space in the building—natural ventilation was planned by the design team to take advantage of the optimal climate for such a strategy. This strategy ordinarily includes use of the outside air for cooling both during the day and also during even warmer weather cycles to pre-cool the building with cooler night air.
However, the strategy was not entirely successful for a number of reasons. First, the decision was taken to make the natural ventilation only user-operated—no system control was included. That effectively eliminated the night cooling operation strategy. Secondly, even with a green light signal to the occupants that the windows could be opened, not all of them are operated. This is because either the occupants are not acting on this information or they are simply not there. Quite often they are in the laboratory wing.
Another factor is that the chilled beams in the office space limit the time when the natural ventila- tion mode can be used. The high relative humidity of the ocean fog conditions, which is prevalent in the morning hours, would create condensation when switching from natural ventilation to the chilled beams for cooling in the system as designed. Thus, use of natural ventilation during these morning hours was technically not possible without creating a condensation problem.
And finally, the natural ventilation openings in the office wing are apparently not large enough op- erationally at the inlet or outlet, so not enough air flow develops to provide the necessary cooling effect for the occupants. The design approach taken was to utilize the stairwells at the “knuckles” in the plan of the office wing as “thermal chimneys”, where the top of the stair shaft would have the air outlet, thus creating the drafting effect. The air inlet would be the occupant-controlled windows in the office space.
But the inlet size (controlled by the occupants), outlet size and possibly the height of the shafts themselves apparently were not adequate to create the necessary air flow. No low-power backup fans at the air outlet were included in the design, so the only alternative was to use mechanical cooling by the HVAC system even when outside air conditions would have allowed free cooling via natural ventilation.
The natural ventilation design feature could not be designed as simple cross ventilation of the narrow floor plate of the office wing because all of the private offices fill one side. For cross venti- lation to work, the office doors would have to be left open (or a vent system would have to open), which would eliminate acoustic privacy. This led to the decision to add the “thermal chimney” design strategy as back-up.
Lessons learned with regard to the design of natural ventilation for spaces like the JCVI office wing in marine climates similar to La Jolla, then, are as follows:
1. Do not rely only on occupant control for natural ventilation to operate successfully as a significant design strategy to save energy. A limited amount of occupant control is desirable for psychological reasons and to enhance the sense of personal comfort, but invest real control with an automatic system tied to the BMS to ensure proper performance.
2. Be sure to employ the “night purge” operation to pre-cool the building when the weather conditions warrant—again, an automatic control system with smart inputs is required. At JCVI, this would mean incorporating forecasts of temperature and humidity into the Building Manage- ment System (BMS) control sequence to avoid unwanted condensation due to humid night air or,
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THE J. CRAIG VENTER INSTITUTE
CASE STUDY NO. 12