Modeling building lighting systems
- Explain factors that affect lighting energy consumption and methods to reduce it.
- Recall codes and standards that dictate lighting energy requirements.
- Demonstrate the capabilities of energy modeling in simulating lighting energy consumption.
Lighting energy has always been a significant component in buildings’ energy consumption. With the adoption of the 2010 edition ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings by governing bodies and U.S. Green Building Council’s LEED, capturing daylight and automating lights will become a prerequisite in the design of new buildings and renovation projects.
As energy codes become more stringent, more complex methods of modeling and understanding lighting energy consumption will be required. Energy modelers already have the means to determine how well a proposed design matches up to these higher standards, and to calculate lighting energy consumption and its impact on other energy end uses.
Lighting and its effect on energy consumption
Lighting, at its simplest, consumes energy when electrical current is allowed to pass through resistive elements of a light fixture. Electrical current passes through an engaged switch, controlled either manually by a physical light switch or electronic remote sensor or automatically via sensor within the space or timer. Lighting designers are concerned with achieving proper lighting levels within the space. The minimum amount of light striking a task surface, or illuminance, is normally designed around Illuminating Engineering Society (IES) requirements, which take into account the types of activities done within the space.
While lighting designers follow IES guidelines for minimum lighting requirements, ASHRAE 90.1 defines the maximum electricity consumption on a building-type and space-by-space basis. Current lighting technologies allow building owners to meet and exceed energy codes. Ultimately, the goal of the standard is to engage building owners and designers to continuously push the limits on energy design, targeting net zero energy consumption for building construction adhering to the version of the standard released by 2031.
Lighting controls and building-activity types are only some of the factors that drive lighting energy consumption. Figure 1 demonstrates these drivers. Lighting technology, daylighting, codes, and operating periods all play a role in energy consumption. It is useful to note that while some of these drivers are based on building operation, others can be controlled to reduce lighting energy consumption.
However, is the amount of lighting energy consumption significant? According to the 2003 Commercial Buildings Energy Consumption Survey (CBECS), 1,340 trillion Btus of energy are consumed for lighting energy consumption annually among the current stock of buildings. On average, this is 20% of the total site energy of the current stock of commercial buildings (see Figure 2). For medium-sized commercial buildings, money spent on lighting can be hundreds of thousands of dollars annually.
Techniques for lighting energy reduction
In many cases, lighting designers are able to curb much of the lighting energy consumption through previously tested techniques. At the conception of a building’s design, designers already are considering how the massing effects energy consumption. Daylighting sensors take advantage of natural daylight to light perimeter spaces. This has the potential to save a significant amount of energy because most commercial buildings are occupied in daylight.
Aside from using energy efficiency lamps and daylighting sensors, controllability gives the user the ability to manage lighting levels. Rather than on/off switches, dimming controls allow the user to set the lighting to levels that are preferable and may be lower than what is designed for the space. Automatic controls, vacancy, and occupancy sensors work well in transient spaces such as school corridors where occupancy is predictable under certain operating hours.
For building owners looking to do retrofit projects, swapping out old lighting fixtures can be an economic choice. Payback periods as low as 9 mo for hospitality buildings and 3.5 yr for office areas have been achieved. Table 1 contains data assessing the current stock of commercial buildings: lamp types, efficacies, the average number of lamps per building, the quantity used in buildings currently, and average operating hours per day. We can observe several things from this table:
- Commercially available LEDs are up to eight times more efficient than incandescents. Research is proving this number can increase significantly in the coming years.
- Fluorescents are commonly used in commercial building applications. Incandescents still constitute 22% of all lamps used in commercial buildings and require four times more energy than fluorescents.
- Only one out of three commercial buildings have LEDs installed. Based on this, we can see tremendous savings by switching over to newer technologies.
- Control strategies for dimming or turning off lights when not in use can reduce the number of hours lighting is in operation.
Turning lights on affects the operation of heating and cooling systems as well. When considering lighting strategies, engineers must account for the effect of lighting on heating, cooling, and fan energy. Electric lighting energy dissipates all its energy as heat into the occupied space or directly into the plenum. It negatively impacts cooling energy and the fan energy to transport cooling during the summertime. In the wintertime, it helps with heating the space; however, with no added benefit because heating sources provide heat at the same or higher level of efficiency. As such, lighting design ideally has the goal of consuming the least amount of electricity, while providing light at the minimally required levels. While daylighting is beneficial and decreases lighting energy, the associated increased solar insulation and decreased envelope thermal performance may increase cooling energy.
Lighting, daylighting modeling
Of interest to energy modelers who assist in LEED certification are the changes in modeling lighting in the baseline building between LEED 2009 and LEED v4. The changes pertain to Energy and Atmosphere (EA) prerequisite 2: Minimum Energy Performance and EA credit 1: Optimize Energy Performance. For LEED v4, the proposed building energy model must be compared to a baseline building according to ASHRAE 90.1-2010, which includes a number of new mandatory lighting controls requirements. The updates from the 2007 to the 2010 version for lighting include:
- Automatic lighting controls for interior lighting (excluding 24-hr lighting, emergency care, and lights used to maintain safety and security) require all interior lighting to be controlled automatically with a timer, occupancy sensor, or vacancy sensor.
- An automatic daylighting controls requirement for primary sidelighted areas and for toplighting (skylights and rooftop monitors); minimum requirements are a two-step control, which can step down to approximately two-thirds and one-third of peak power as daylighting becomes available. By definition, the primary sidelighted area is a function of window head height, window width, and any 5-ft or higher obstructions within the area. Definitions for daylight area under skylights and rooftop monitors differ, but both are a function of ceiling height, glazing dimensions, and its proximity to obstructions and other daylighted areas. The technical definition should be referenced from the standard.
An energy model can be used to model the new controls requirements. It is a powerful tool that also can help users understand a number of things—the effects of lighting on heating and cooling, and how daylighting can reduce lighting energy consumption. There is a variety of acceptable energy modeling programs that have their own proprietary simulation engines, but all must follow minimum simulation requirements in energy-modeling software standards. Some are available for free while others require purchase or license. The following list is not exhaustive:
Energy models can be constructed to gather all the components that affect lighting—glazing percentage, building orientation, daylighting sensors, glazing constructions, automatic control sensors, and lighting profiles—and compare lighting energy with other energy end uses. Combined with historical weather and solar data for a location’s climate, clients can make informed decisions for their building projects.
Energy modeling studies show that buildings with automatic lighting controls can obtain total site energy savings of up to 10% over commercial buildings without lighting controls.
The amount will vary depending on location, window size and characteristics, building geometry, and other building energy systems. Although we can achieve decreased energy consumption from daylighting controls in highly glazed areas, it is understood that building energy consumption always increases with larger window-to-wall ratios. Thus, minimizing glazing is a key first step in reducing overall energy consumption. Lighting controls should not be a replacement for passive design strategies.
New code regulations
Modeling the new code regulations has added a layer of complexity for the energy modeler. Without daylighting modeling, the procedure is quite simple. The current standard of practice for modeling lighting is to develop weekday and weekend schedules, apply them to every hour of the calendar year, and sum it up to determine consumption. ASHRAE has completed research and provides default lighting schedules for a range of common building usages, such as offices, schools, manufacturing facilities, and other building types.
For example, as modeled by ASHRAE, office lighting adheres to a schedule that follows a typical workday. The lights turn on to 90% of the peak load by 8 a.m., during which most employees enter the building. A small dip in the lighting load occurs while folks step away from their desks for lunch. The lights remain at 90% for the afternoon until 5 p.m., and slowly ramp down to 5% of the design load to represent employees slowly leaving the office for the day. An office designed at 1,000 W of lighting fixtures will be calculated to have an electrical demand of 900 W at 8 a.m., 300 W at 6 p.m., and so on.
To model automatic sensors, a profile is superimposed on the lighting load. Normally, default lighting schedules track the occupancy within a space—with the exception of a small partial load during off-hours to account for potentially anyone remaining in the office, accidentally leaving the lights on, or emergency lighting. The lighting schedule must be adjusted such that the lighting profile is set to zero for every hour a space is unoccupied. To understand the real effect of this code regulation is quite onerous, because the savings generated from having automatic sensors for lighting control will vary by building depending on how many occupants tend to leave the lights on in an unoccupied room. Alternatively, ASHRAE 90.1 allows the user to take credit for automatic lighting controls by adjusting the lighting power from 10% to 15% for spaces where the control is not already a requirement.
To model daylighting controls, a dimming profile must also be superimposed on the lighting schedule. Most of the popular energy-modeling packages have built-in capabilities to calculate daylighting. For example, within the IES-VE software, one can locate and orient sensors, simulate daylighting, and control lighting power in a realistic manner with an "if" statement for stepped control or a ramping equation for continuous dimming. Maximum and minimum lighting in the space is settable as well. Software packages that do not include this functionality, such as Carrier HAP, will require manual manipulation of the lighting schedules or post-processing lighting consumption.
Lighting energy consumption is a significant portion of total site consumption for many buildings today and can be reduced with proper light fixtures, building configuration, and lighting controls. As codes continuously upgrade to the latest lighting energy standards, techniques to conserve lighting energy start to become a requirement rather than exemplary performance. The interplay between the building façade, daylighting, heating, and cooling is important, and energy models can help us understand this interplay and direct the design of new construction and retrofits for lighting-installation projects. Innovation in lighting technology and design will continue as we strive for reduced energy consumption in all buildings.
Christian Paunon is a mechanical engineer at Arup and enjoys working on projects that deliver high value for clients. His experience in energy modeling includes developing new-building and renovation energy models for energy-code compliance and LEED. His work extends throughout the building lifecycle, from providing preliminary energy studies to aiding in the design process to constructing energy models for use in understanding energy-conservation measures in existing buildings.