Creating efficient office lighting
An example of an office that used energy modeling for HVAC load analysis and a lighting optimization simulation analysis.
A remodel of a 17,000-net-sq-ft commercial office space in a circa-1929 office in downtown Houston took advantage of both energy modeling for HVAC load analysis and a lighting optimization simulation analysis to determine the impact on yearly energy consumption. The baseline model, which used the existing space design, was modeled with its existing lighting (2x4-ft recessed lay-in return air T-12 light fixtures) that had previously been retrofitted from to 2-lamp T-8 fixtures. The existing lighting demand was 10 kW (input power). This retrofit design was compared to the same space, using dimmable energy-efficient solid-state high-output T-5 high output and LED luminaires with occupancy sensing control and natural daylight from the existing north-facing palladia windows, resulting in a reduced peak power demand of 8.2 kW.
In comparing the two analyses for the floor cooling and heating load requirements, the existing design had required approximately 7 cooling ton-hrs per year consumption, while the remodeled floor require 6 cooling ton-hrs per year consumption. Peak heating demand increased from 92 kBtuh to 125 kBtuh as a result of the redesign, with energy consumption increased from 500 therms to 592 therms. Replacing the 2-lamp T-8 parabolic fixtures with programmable, occupancy based T-5 high-output lamp pendant luminaires and LED downlights offset with natural daylighting allowed for a yearly reduction in energy consumption of 16%. As a result of decreasing the lighting W/sq ft, the heating consumption increased by 15%.
The majority of load reduction was due to lighting controls and use of daylighting for the space. The existing lighting design was based on manual occupant switching. As occupants left for the day, lights would be switched off; when the cleaning staff switched lights on after-hours, lights would be left on overnight. To reduce lighting consumption, the renovation took advantage of vacancy sensing. With a vacancy sensor strategy, lights are turned off automatically soon after an area is vacated, but occupants must turn lights on when they re-enter the space. In the renovation, lights tied to a vacancy sensor turn off 2 hours after they are energized after-hours. After the cleaning crew has left the space, lighting would reduce to 30% of the design level in the space, rather than remain at 100% design level with switching.
This illustrates the benefit of application of lighting modeling and energy/load modeling in developing a lighting retrofit design that is unique to the building by incorporating daylighting and task lighting, employing new energy-efficient LED technology, and controlling supplemental lighting in order to achieve energy savings, enhanced lighting, reduced equipment cost, and improved comfort and aesthetic benefit for the tenant.
David B. Duthu is board principal at ccrd, where he has more than 37 years of experience in the fields of mechanical engineering design, technical engineering design, and project management. Nolan Rome is associate principal and lead mechanical engineer for ccrd and has been responsible for the design of all types of healthcare facilities including hospital expansions, cancer centers, and imaging centers in multiple states.