Evaluating post-occupancy performance
Innovative building systems are rarely evaluated after occupancy due to lack of resources, lack of interest on the part of the owner, or concerns regarding inconvenience to the occupants, impositions on privacy, or liability or defamation of the owner's or architect and engineer's reputation. On-going real time measurement and feedback on energy use and occupant satisfaction as part of building operations is not yet standard practice. And yet, post-occupancy data are invaluable to the building industry, providing factual, non-anecdotal feedback on whether innovative systems work as claimed: delivering energy and demand savings to the degree predicted, meeting occupant requirements, and running smoothly under facility management. Without well documented performance data, opportunities to maximize the impact of the lessons learned are lost.
In 2011, five years after initial occupancy in July 2007, the Times Company agreed to collaborate on a post-occupancy study of their building in partnership with the US Department of Energy's Commercial Building Partnership program. CBP aims to demonstrate validated, cost-effective low-energy technologies for both new construction and retrofit projects in the commercial buildings market. Additional support was provided by the California Energy Commission through its Public Interest Energy Research (PIER) Program.
Together with the Lawrence Berkeley National Laboratory (LBNL) and the Center for the Built Environment (CBE) at the University of California, the Times Company participated in a monitored evaluation of three installed energy efficiency measures – dimmable lighting, automated interior roller shades, and an underfloor air distribution system (UFAD).
- The energy performance of the lighting system was monitored directly using sensors on the circuit panels supplemented with control status data from the manufacturer to determine the strategy in effect at the time.
- Automated and manual operations of the interior motorized roller shade were logged by the manufacturer.
- HVAC energy use was determined by combining a series of measurements and operations data with a calibrated EnergyPlus model. HVAC energy savings are therefore presented as an aggregated value not broken down by energy efficiency measure since all three energy-efficiency measures worked together to produce the resultant thermal environment and therefore HVAC energy use in the zone.
- Total annual energy use was compared to a baseline defined by the mandatory and prescriptive code in effect at the time of design: ASHRAE 90.1-2001.
- Survey data gathered by the Sustainable Energy Partnerships under a separate study funded by NYSERDA were used in the LBNL analysis to assess occupant satisfaction, comfort, and acceptance of the energy efficiency measures.
Significant findings were as follows:
Annual lighting energy use savings from the lighting control strategies (occupancy, setpoint tuning and daylighting) was 56 percent (3.94 kWh/ft2-yr) across a 40 foot deep perimeter zone compared to a code-compliant building with only the standard scheduled lighting controls.
The survey results report that 57% of the occupants responded with greater than neutral satisfaction with the automatic lighting controls (occupancy sensors, dimming in response to daylight conditions). 78% of the occupants were very satisfied with the overall quality of the lighting in their workspace (average rating was 5.53 on a 7-point scale where a value of 4 is "neutral").
The automated shading enabled lighting and cooling energy use reductions, and reductions in peak electric demand. Energy savings due to the shading system alone could not be determined in isolation but the reduction in annual electricity use due to the combination of all three systems was estimated to be 24% (2.58 kWh/ft2-yr) across a typical tower floor compared to a code-compliant building. Annual heating energy use was reduced 51%. Peak electric demand was reduced by 25%. The Times Company's investment in advanced energy-efficiency technologies was estimated to yield a 12% rate of return on their initial investment.
Occupants on all floors overrode the automated shading system infrequently. For motors overridden at least once, 80% of these motors were overridden an average of 18 times per year (1.5% of the year) for an average total time of 38 hours per year during primary work hours. 41% of the occupants responded with greater than neutral satisfaction with the automatic window shades, with an average rating on all 20 floors of 4.12 on a 7-point scale.
An underfloor air distribution (UFAD) system distributes air from the plenum under the floor to ventilate the space above. A UFAD helps save air conditioning energy by maintaining comfortable conditions in the lower occupied zone of the space, while allowing warmer and less comfortable conditions to exist in the higher space elevations. The air conditioning setpoint can be higher (therefore lowering the airflow and reducing cooling) while still allowing the average conditions in the occupied zone to remain comfortable.
Temperature measurements in the interior zone showed a reasonable amount of air stratification (2-3°F difference between standing head height (67 in.) and ankle height (4 in.)) in the occupied zone—an indicator of good UFAD cooling performance.
The time and care that the Times Company took to design, engineer, and follow-through on the design intent behind the energy efficiency measures and the actual performance of the technological measures themselves led to post-occupancy survey data indicating that a significant fraction of the building occupants were satisfied to very satisfied with the overall building and that, compared to other buildings, the overall level of satisfaction was greater than the norm of surveyed buildings.
The lesson for replicating the success of this building on a large scale is that such factors as paying attention to details such as procurement of building equipment, and verifying the performance claims of the equipment after it is installed matter. The Times Company did its homework well before the construction of the building: they constructed a 4500 ft2 full-scale mockup and evaluated the shading and daylighting technologies prior to installation in the actual building. After some fine-tuning of the systems for a year after occupancy, Patrick Whelan, the Times Company's facilities director, said that maintenance and operations needs for the three systems were surprisingly minimal given the advanced technology incorporated into the three systems.
Detailed results from this study can be found in the Publications section.