Factoring lighting into cooling loads
Reduction in LPD
For lighting retrofits in commercial office building spaces, the LPD load analysis should include all luminaires that are added, replaced, or removed. Lighting alterations that involve only the replacement of lamps and ballasts must also comply with the LPD requirements. This cooling load analysis should include the wattage of line voltage luminaires containing remote ballasts, transformers at the labeled maximum wattage of the luminaire, or the combination of values for auxiliary manufacturer’s literature or a nationally recognized testing laboratory.
In the case of lighting power calculations for ballasts with adjustable ballast factors, calculations for load impact should be based on the ballast factor that will be used in the space, provided the ballast factor is not user-adjustable. In commercial applications using specialty lighting for display or architectural purposes, such as line voltage track or plug-in busway, localized load impact should be considered in the overall load calculations.
Updates to energy codes and standards have already caused the retrofit and new building markets to transition from less efficient lamps and luminaires to more efficient devices. Energy consumption can be further reduced by taking advantage of the new technologies to control lighting systems that use high-tech lamps and ballasts.
Automatic lighting controllers can dim or switch lighting based on time of use, occupancy, daylighting level, or a combination thereof. Lighting systems in commercial office buildings are often left on for long periods of time due to low occupancy in a space or a cleaning crew working into the evening. Having the ability to control lighting by turning off lights that are no longer required or that are left on in unoccupied spaces, or using daylighting when available can present additional energy saving opportunities. Some lighting control strategies currently being used by designers include:
- Vacancy or occupancy control (lights are turned on and off or dimmed according to occupancy)
- Scheduling (lights are programmed to turn on and off according to work schedules)
- Daylight harvesting (electric lights are automatically dimmed or turned off in response to the presence of daylight)
- Demand response (power to electric lighting is reduced in response to utility curtailment signals or to reduce peak demand power charges to a facility)
- Tuning (light output is reduced to meet the occupants’ needs)
- Adaptive compensation (light levels are lowered at night to take advantage of the fact that occupants need or prefer less light than during the daylight hours).
Improved power quality
Poor power quality is a concern in buildings because it wastes energy, reduces electrical capacity, and can harm building and tenant equipment. In some cases, it can negatively impact the building’s electrical distribution system itself.
Power quality is a condition of the power supplied to equipment. The power supply may contain transients and other short-term under- or over-voltage conditions that may result from switching operations, faults, motor-starting, lighting disturbances, switching of capacitors, electric welding, and operation of heavy manufacturing equipment that may contain harmonic content. Harmonics are integral multiples of the fundamental (line) frequency involving nonlinear loads or control devices, including electromagnetic devices (transformers, lighting ballasts) and solid-state devices (rectifiers, thyristors, phased-controlled switching devices).
Upgrading lighting equipment with new, high-power-factor and low total harmonic distribution characteristics can help improve power quality in an existing electrical system and possibly free up electrical capacity. In many cases this benefit can justify the cost of a lighting upgrade. For instance, the measured watts of low-power-factor ballasts are approximately the same as the measured watts of the high-power-factor (above 90%) type when connected to the same load. The low-power-factor type draws more current from the same power supply and, therefore, larger supply conductors may be necessary. The use of high power-factor ballasts permits greater loads to be carried by existing wiring systems. Many public utilities have established penalty clauses for the use of low power-factor equipment.
The use of natural daylight to provide up to 140 lumens (lm) of light compares favorably with the 90 lm/W from most electric lighting systems. Systems that take advantage of daylight to supplement electric lighting present one of the best ways to reduce building lighting energy consumption by balancing loads and peak demand and creating a more desirable indoor environment for occupants. In designing daylighting into a retrofit building condition, there are four basic criteria to consider:
1. Harvestable light: Amount of light that can be brought into the space for effective use via skylights, light shelves, clerestory windows, or light pipes.
2. Interior material and color impact: Balancing the use of specialized reflective materials and interior colors to use the light’s benefits.
3. Glare: Direct sunlight into a space can cause uneven luminance ratios that are distracting to the occupants and cause not only irritation, but also hot-spots in the space. Bouncing light or allowing diffuse daylight from certain exposures such as north, can aid in glare reduction.
4. Control of electric lights: For daylighting to be most effective, lighting controls are required to maximize performance. Automatic sensing control presents an approach that ensures electric lighting will be reduced when enough ambient daylight is available to illuminate the space. The application reduces the opportunities for over-dimming, under-dimming, and/or rapid-cycling of the lighting devices, thereby assisting in reducing the cooling load and energy savings.
Another daylight control opportunity is to enable the use of automatic window shade control as part of the space lighting plan. HVAC load analysis typically incorporates the benefit of window shading devices commonly modeled in cooling load analysis software programs. In calculating the value of daylighting benefit, items to be evaluated include the time of day, season, available light, and controls that can raise or lower shades to optimize daylight contribution.
Efficient lighting sources
With the advent of efficient lighting sources such as linear fluorescent lamps, solid state LEDs, and high-intensity discharge (HID) lights, it is critical to be able to assess the actual impact of the lighting device performance. The combination of lamp, ballast, and heat extraction fixtures helps maximize efficiency while balancing the considerations of lighting quality and quantity.
The designer has the option available to choose from a variety of types and manufacturers for each application depending on efficacy, color quality, and service life within the HVAC load analysis. Whenever possible computerized energy modeling can be used to assess the HVAC load components of a given lighting design allowing the designer to overlay and model the results of lighting performance within the designed space.
The use of light modeling software enables the architectural/engineering design team to preview the design of lighting within a BIM model and quickly understand the impact on the user space. In most cases more than one simulation may have to be developed to identify the optimal lighting arrangement for a space. Once the modeling layout is agreed upon by the team, load data can be imported to the HVAC load analysis program.
The importance of accurate cooling load analysis and modeling of lighting systems is key to optimizing overall HVAC system performance. Taking advantage of computer-based modeling and light simulation programs has enabled the design team to consider lighting options that can provide significant benefit to the project. New lighting technology, when incorporated with enhanced lighting control, does present potential initial increase in first costs to the project. However, it is incumbent on the design team to develop designs that ultimately provide financial benefit, on a life cycle cost basis, as well as HVAC system performance optimization while providing the occupants with a space that is comfortable.
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.