Modeling building lighting systems
Familiarity with building energy modeling techniques and software is crucial for designing code-compliant buildings. Lighting designers should consider the various tools available when designing new- or existing-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.