Factoring lighting into cooling loads
How to select, design, and redesign lighting systems so they complement HVAC systems.
- Understand how lighting choices affect a building’s overall energy efficiency.
- Discover key factors of lighting design that impact a building’s cooling and heating loads.
- Learn issues to consider when evaluating alternative lamp technologies.
Some federal, state, and city energy codes, standards, and guidelines now restrict building lighting power density (LPD) to as low as 0.60 W/sq ft. This restriction is requiring the architectural and engineering design teams to fully comprehend and evaluate the contribution of lighting to the building cooling and heating loads for retrofit applications. Designing lighting systems so that they complement the HVAC systems design to a net reduction in building energy use requires close interaction between the lighting designer, architect, and project mechanical and electrical engineers. It is the challenge of the team to develop a lighting layout that not only provides quality illumination to the space, but also reduces overall energy consumption.
One challenge the design team faces in developing a lighting strategy is to incorporate components that can be accurately modeled by HVAC cooling load and energy analysis software. With a renewed focus on sustainability and energy conservation, the financial support for product development in lighting technologies has spawned a wide array of new lamps and control devices available to design architects and engineers. However, performance data may not always be presented in an equivalent manner regarding energy usage and quality of light. As design teams take advantage of this new technology, it will be critical that correct comparable performance data be obtained and incorporated into not only building cooling and heating load analysis and energy modeling, but also into photometric software programs that allow the design team to study, test, and implement lighting designs.
According to the U.S. EPA Energy Star Building Upgrade Manual, lighting is typically the largest source of waste heat, representing approximately 35% of electricity consumed in commercial buildings. That waste heat translates into heat gain, which significantly impacts the building cooling and heating loads. Although other factors also influence the final cooling/heating load analysis, the lighting system contributes a major portion of internal heat gain. This internal heat gain, for certain climates or building configurations, can be useful when the building is in the heating mode. When the building is in cooling mode, however, lighting heat gain can be detrimental, due to the increase in the cooling load and the capacity of the cooling equipment required to maintain space thermal comfort conditions.
In many retrofit applications, reduced building lighting loads and corresponding reduction in the cooling requirement may result in reduced full-load operations of the HVAC systems. This can save significant amounts of energy used for lighting and cooling the building, lower energy cost and may prolong the service life of existing HVAC components. An additional benefit is that the resulting excess cooling capacity could be used to serve future cooling load requirements, provide redundant capacity for existing critical loads, or allow replacement cooling equipment capacities to be lower, i.e. right sized for the lower loads, further reducing operating expenses.
New energy codes and standards
In the past few years, several revised energy codes and standards have been released. Understanding the basic requirements of the standard applicable to the specific project is imperative in designing and modeling lighting systems that provide optimal performance and are cost effective on a lifecycle basis.
Recently implemented codes and standards that have an impact on the design of lighting systems for retrofit applications include the Energy Policy Act of 2005 (EPAct 2005), 2012 International Energy Conservation Code (IECC) and ANSI/ASHRAE/IES Standard 90.1-2010, the State of California 2013 Building Energy Efficiency Standards, Title 24, Part 6 (and Associated Administration Regulations in Part 1), and the City of New York City LL85: Energy Conservation Code and LL88: Lighting Upgrades & Sub-Metering Code.
These codes and standards as well as voluntary sustainability programs—such as the U.S. Green Building Council (USGBC) LEED certification program, the Green Building Initiative (GBI) Green Globes program, ASHRAE Standard 189.1-2011, and the U.S. EPA Energy Star program—represent a paradigm shift in the way architects and design engineers have to consider not only the initial impact of lighting in the calculation of heating and cooling loads, but also ongoing operating conditions and retro-commissioning of existing systems. In fact, some code authorities and the U.S. Army (UFC 1-200-02 High Performance and Sustainable Building Requirements) have adopted all or portions of these standards for some specific building types and/or locations within their jurisdiction.
For design team members, these changes highlight the importance of collaboration in selecting the lighting technology to employ on a specific project. Design of today’s energy-efficient and innovative lighting systems requires a total effort by the design team in evaluating alternative lighting system impacts on interior space planning, lighting fixture layout, furniture and fixed equipment layout, and lighting controls, resulting impact on HVAC cooling and heating loads, and ultimately the energy use and utility costs to operate the building.
Building configuration and load calculations
Calculating space cooling and heating loads requires many aspects of the building design to be considered. Factors that affect the heating and cooling loads include:
- Type of building
- Building configuration and floor area
- Wall to window ratio
- Building orientation on the site
- Thermal performance of the building envelope
- Impact of external shading devices or adjacent buildings
- Ground reflected solar radiation
- Climate conditions
- Indoor design requirements
- Internal heat gains, including plug and process loads
- Building occupancy schedules
- Energy consuming equipment operating schedules
- HVAC system types
- Sequences of operation of the HVAC systems.
For retrofit applications, engineers must know the size and efficiency of existing heating and cooling systems and how the building equipment is operated in order to accurately predict energy consumption and peak demand. Typically, large high-rise buildings are dominated by high internal loads, and consume more air conditioning and heating than most low-rise applications due to the size and density of the building occupants and equipment heat gain. According to the EPA, high-rise buildings present the best opportunity for energy savings. Each kWh of reduction in annual lighting energy use yields an additional 0.4 kWh of annual reduction in HVAC energy.
For smaller, exterior envelope-dominated buildings, the net impact of a lighting retrofit may result in a net HVAC penalty, particularly for buildings in cold climates. This means that for each kWh in lighting energy reduced, the building HVAC system net energy consumption may rise as a result of additional annual heating energy used. In other words, a reduction in lighting load may result in an increase in building heating load, which results in no net change or an increase in total energy consumption if the reduction in energy used for cooling is less than the additional heating energy required over the course of the year. Empirical data shows that or the majority of scenarios, lighting upgrades are more likely to reduce cooling costs and increase heating costs.
When calculating building cooling loads, the designers must consider the components that comprise the heat gain due to lighting. These factors can vary such that at any given moment, the heat equivalent of power supplied instantaneously to the lighting is not necessarily that which equates to the instantaneous cooling load. Of the three basic types of heat transfer, convection and thermal radiation are the major contributors to lighting heat gain while conduction is negligible. Both convection and thermal radiation transfer heat to the space, which results in 100% of the lighting power becoming cooling load. However, it is important to recognize that the convective component represents an instantaneous heat gain, while heat gain due to thermal radiation is delayed because the heat is stored within the surfaces in the room such as ceilings, floors, walls, furniture, etc. This is true for all types of lighting technologies (LED, fluorescent, incandescent, etc.), although the fractions attributable to radiation vs. convection will differ.
The 2013 ASHRAE Handbook —Fundamentals presents a detailed discussion of the various parameters which influence the calculation of cooling loads due to lighting heat gain. Of those presented, key factors encountered repeatedly in the new energy codes and standards are the fractions of heat gain and the special allowance factor (SAF).
The fractions of heat gain consider the assignment of the components heat output by the luminaire. Cooling loads typically account for heat generated by in-ceiling (or recessed) luminaires, which are made up of two key parameters:
- Ceiling plenum fraction: The fraction of the lighting power that heats the return air that is directed through the light fixture (zero for surface mounted and pendant hung light fixtures and task lights)
- Space fraction: The fraction of the lighting power converted to heat gain in the conditioned space.
In lighting retrofit upgrades for commercial office buildings, the design typically includes recessed fluorescent lighting fixtures, which will release heat to the space and direct heat to the return air plenum or to the ceiling cavity. It is important to distinguish these components even though total cooling load imposed on the cooling coil remains the same. The larger the fraction of luminaire heat output that is picked up in the return airstream for air return luminaires and directed back to the cooling coil, the better the overall energy performance and interior comfort due to the reduced fraction of heat that goes to the conditioned space. This channeling of the lighting heat to the ceiling plenum helps reduce the room cooling load, thereby reducing the supply airflow (and resulting fan energy) required for space conditioning.
The SAF is the ratio of lighting fixture total power consumption, including lamps and ballast, to the nominal power consumption of the lamps, which includes the luminaire lamps and ballasts (for fluorescent fixtures). As a reference, an incandescent lamp has an SAF of 1.0. To demonstrate the progress made in the development of energy-efficient fluorescent lighting over the past several years, an historical comparison is made between the 1977 and 2013 ASHRAE Fundamentals Handbook. In the 1977 Handbook, the SAF for fluorescent luminaires was as high as 2.19 for 32-W single lamp T-12 high-output fixtures. For a rapid-start 40-W T-12 lamp fixture, allowance factors vary from a low of 1.18 for two lamps to a high of 1.30 for one lamp. These SAF values accounted for the losses in the magnetic ballasts, then commonly used in the fixtures.
Recent ASHRAE research has found that the SAF ranges between 0.87 and 0.90 for T-8 luminaires with electronic ballasts and between 0.98 and 1.02 for with other lamp types. Electronic ballasts can lower electricity consumption below the lamps’ rated power requirement, which represents a significant advance in lighting technology and provides a valuable tool for the designers reduce cooling loads and enhance energy performance. Electronic ballasts operate the lamps at a higher frequency (>20,000 Hz), offer additional controls options for lamps, and consume less power than magnetic ballasts.
The current ASHRAE Handbooks present little data on LED lighting, no doubt due to the rapid development in solid-state LED technology. However, a review of recently published manufacturers’ data for solid-state LED lighting indicates the SAF is further impacted to the positive when considering the light output per unit and quality of light provided to the object area.
Recent developments in solid-state LED technology show superior performance for both the space fraction and radiative fraction numbers. As compared to incandescent fixtures, the fraction of heat transfer due to radiation vs. convection is typically much higher for LEDs resulting in more of the lighting load being stored delaying its conversion to room cooling load. Also, because LEDs emit little or no infrared (IR) or ultraviolet (UV) radiation more of the radiated energy is in the form of visible light. Considering that not all published data on LED technology is equivalent at this time, the design engineer has to carefully evaluate data such that the values are normalized for proper input into the cooling load analysis and energy modeling software. It is necessary for the design engineer to calculate the heat generation performance for each component of the luminaire as a fraction of the total lighting heat gain by using judgment to estimate heat-to-space and heat-to-return percentages.