Integration: Lighting and HVAC systems

04/16/2013


Heat transfer basics

Before modeling the lighting loads for a building, it is important to understand the basics of heat transfer and possible sources of heat gain in a space. The three types of heat transfer that can take place are conduction, convection, and thermal radiation. Conduction occurs from direct contact or interaction of particles with a temperature difference. Convection occurs between surfaces and moving fluids when a temperature gradient exists. Thermal radiation is the transfer of energy through electromagnetic waves. Light energy uses two of the three modes of heat transfer—convection and thermal radiation—while conduction is negligible. Both convective and thermal radiation heat transfer release heat to the space, resulting in 100% of the lighting energy input becoming cooling load. It is important to note that heat that is convective becomes cooling load immediately, while thermal radiation is converted to cooling load over a period of time. This holds true for any type of lighting fixture. Different types of light sources (LED, fluorescent, incandescent, etc.), however, have different amounts of heat gain that result from convection and thermal radiation.

For example, with a 60 W incandescent lamp, approximately 20% or 12 W of the input power is transferred to the space in the form of convection, which becomes a cooling load immediately.  The remaining 80% or 48 W is transferred to the space in the form of radiation.  Most of this radiation is IR, with very little visible light.  The visible light and the portion of wasted energy that is thermal radiation will be absorbed by surfaces in the room such as ceilings, floors, walls, furniture, etc. The heat raises the temperature of the surface and is stored in the mass of that element. The heat will then get reradiated to other surfaces in the space, and some is convected to air in the room, becoming a cooling load. The process of radiating, storage, and convection into the space continues until eventually all of the heat energy is transferred to the space. 

In the case of a comparable 14W LED lamp, approximately 50% or 7 W of the power is transferred to the space in the form of radiation.  Because LEDs generate little or no infrared (IR) or ultraviolet (UV), all 7 W are converted to visible light.  The remaining 50% or 7 W is transferred to the space in the form of convection.  The convected heat is wasted energy and becomes a cooling load immediately.  All of the original 14 W of electrical input power will be converted to cooling load.

Energy modeling

Carrier’s Hourly Analysis Program (HAP) and Trane TRACE 700 Building Energy and Economic Analysis Program are two of the more widely used energy modeling applications in the engineering industry. Each can be used to help model a building and provide accurate heating and cooling loads to help size equipment and determine the loads for each individual space in the building. It is important for mechanical engineers and designers to understand how to accurately input the lighting loads into energy modeling software.

In Carrier HAP one would need to do the following to properly account for the designed lighting:

  • Input the W or W/sq ft for the fixtures in the room.
  • Choose the correct fixture type in the space. This will determine the convective/radiant split for heat gain in the space.
  • Create a schedule for the lights that describes the percent of peak wattage that is in use each hour during the day.
  • If applicable, input the percent of lighting heat gain to the plenum.

The watts or power density (W/sq ft) for the space can be determined by working with the electrical engineer or looking at the electrical drawings. If entering the amount of watts, the input wattage of all the lighting fixtures should be totaled and entered into the software. If it is early in the design process and exact power densities or watts are not known, the Nonresidential Cooling and Heating Load Calculations chapter of the ASHRAE Handbook or ASHRAE Standard 90.1-2007 provides typical power density values for different types of spaces that you can use as a placeholder.

Carrier HAP has three different types of fixtures that can be selected: recessed unvented, recessed vented, or free-hanging. According to Carrier HAP, recessed fixtures only radiate to the walls and floors in the space; free-hanging fixtures radiate to the ceiling, walls, and floors; and vented fixtures tend to have a higher proportion of convective heat gain than unvented fixtures. Since only one type of fixture can be picked, pick whichever fixture type is the majority in the space. When fluorescent lamps are selected, the fixture wattage is increased using the ballast multiplier to account for power consumption from the ballast starter device. If incandescent lamps are selected, the total fixture wattage equals the lamp wattage. Carrier HAP uses ASHRAE defaults to determine how to split the heat gain between convective heat gain and radiant heat gain. The split is mainly determined based on the fixture type according to ASHRAE research. Lighting heat gain parameters for typical operating conditions can be found in the Nonresidential Cooling and Heating Load Calculations chapter of the ASHRAE Handbook.

After the wattage and types of fixtures are input into the software, a schedule should be created that accurately represents the usage of the lighting fixtures. Carrier HAP allows the user to define profiles for hourly and daily variation for lighting use. A “fractional” schedule should be selected as this allows the user to define the percentage of maximum heat gain present on an hourly basis. Once all hourly profiles are created, they should be assigned to their respective days on the “assignments” tab.

The last step in accurately accounting for the lighting in the building is to input the percent of lighting heat gain to the plenum, if applicable. The percent of heat gain going to the plenum can be found by contacting the lighting manufacturer. If the lighting fixtures are not specified at the time, the percent of heat gain going to the plenum can be calculated using the figure above. The plenum heat gain fraction is simply one minus the space fraction (i.e., plenum heat gain = 1 – space fraction). The plenum heat gain fraction can be input into the software for the created systems.

Trane TRACE 700 is similar to the Carrier Hourly Analysis Program, but there are some differences. The steps taken to provide an accurate lighting representation are the same as in Carrier HAP but with steps 1 and 4 combined in TRACE 700.

The lighting heat gain should be entered in the different spaces that have been created. This value can be entered in Btuh, Btuh/sq ft, W, W/sq ft, or W/m2—whichever the user prefers. Similar to Carrier HAP, TRACE 700 has a default library for lighting fixtures. The lighting fixtures that can be selected include the following:

  • Fluorescent, hung below ceiling, 100% load to space
  • Incandescent, hung below ceiling, 100% load to space
  • Incandescent, hung below ceiling, 60% load to space
  • Incandescent, hung below ceiling, 75% load to space
  • Recessed fluorescent, not vented, 50% load to space
  • Recessed fluorescent, not vented, 80% load to space
  • Recessed fluorescent, vented return, 20% load to space
  • Recessed fluorescent, vented supply & return, 80% load to space
  • Task lighting fluorescent
  • Task lighting incandescent.

Figure 4: The TRACE 700 Internal and Airflow Loads Library allows users to create custom fixtures. Courtesy: TraneIf the default libraries do not have exactly what the user is looking for, TRACE 700 has the ability to create and define custom fixtures. To create a new fixture, click the “libraries” drop-down at the top of TRACE 700 and select “internal and airflow loads.”

Figure 4 shows all the inputs for creating a custom fixture in TRACE 700. First, the description of the fixture should be entered to help the user differentiate it from the default fixtures. The lighting fixtures that can be selected include the following:

  • ASHRAE1 or RECFL-NV*: Recessed fluorescent – not vented. Supply and return below the ceiling with supply airflow less than 0.50 cfm/ sq ft.
  • ASHRAE2 or RECFL-NV*: Recessed fluorescent – not vented. Supply and return below the ceiling or through ceiling grill and space. Supply airflow greater than 0.50 cfm/ sq ft.
  • ASHRAE3 or RECFL-RA*: Fluorescent – vented. Supply through ceiling or wall diffuser with supply airflow greater than 0.50 cfm/ sq ft. Return around lighting fixtures and through plenum space.
  • ASHRAE4 or RECFL-RS*: Fluorescent vented or free-hanging in airstream with possible ducted return. Vented to supply and return airflows.
  • SUSFLOUR*: Fluorescent hung below the ceiling.
  • INCAND or SUSINCAN*: Incandescent hung below the ceiling.
  • Task Lighting: Lighting typically located at desk level.

*For these fixture types, if the CLTD-CLF (ASHRAE TFM) is selected, the lighting room load is based on weighting factor coefficiencts used by the DOE 2.1 computer program. If a TETD (time averaging room load cooling load methodology) is used, all fluorescent fixture types are assumed to have a radiant fraction of 50% and incandescent fixtures 80%.

After choosing the lighting fixture type, a value should be entered into the percent load to plenum field. Again, it is recommended that the user obtain these values from the lighting manufacturer or contact a lighting product representative to assist in determining these values. For combinations of different lighting fixtures in a room, the percent lighting load to return air should be weighted based upon the number of each fixture type. If at the time of creating the energy model the exact lighting fixtures have not been specified, the percent load to the plenum can be selected from the ASHRAE Handbook. The ballast factor is a multiplier for fixtures, which requires a different amount of input energy than the rated lamp wattage due to the use of a ballast or power supply. The user can obtain this value from the ballast cut sheet. The last two inputs for a custom defined fixture are the longwave radiant fraction and the shortwave radiant fraction. The longwave radiant fraction + shortwave radiant fraction = total radiant fraction. The default value for the shortwave radiant fraction in TRACE 700 for all lighting fixture types is 0%. The shortwave radiant fraction field should be ignored unless either the RTS (ASHRAE Tables) or RTS (Heat Balance) methodology has been selected on the “change load parameters” screen. The other built-in TRACE 700 methodologies have a default assumed value for this field.

Once the custom lighting fixture has been created, the lighting fixture can be selected in each applicable room. Last, a lighting schedule should be created to represent lighting use in the building. Trane TRACE 700 has several default schedules that can be selected, or a custom schedule can be created. The schedules allow the user to define 24-hour profiles for the lighting loads.

Summary

The accurate modeling of lighting systems is an important part of HVAC sizing as lighting can be a significant source of heat gain in commercial buildings. Building energy modeling software is a valuable tool used by engineers, and it is important to understand the inputs selected in the application. With a better understanding of how to model lighting systems, engineers can more accurately determine the proper size for the HVAC systems, resulting in optimal energy efficiency and operational performance.


Justin Schultz is lead electrical engineer at Metro CD Engineering and serves as education chairman and board member for his local Illuminating Engineering Society (IES) section. He is a 2011 Consulting-Specifying Engineer 40 Under 40 award winner and an active member of the USGBC Central Ohio Chapter. Brian Johnson is a mechanical engineer at Metro CD Engineering. He serves as a member of the USGBC Central Ohio Emerging Professionals Group and Advancement Committee as well as the Young Engineers in ASHRAE in Columbus, Ohio.  


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