Integration: Lighting and HVAC systems
By using building energy modeling software, engineers can determine how to size HVAC systems to balance the heat given off by lighting systems, particularly energy-efficient lighting fixtures.
- Learn about the interaction between lighting and cooling loads.
- Learn how to use building energy modeling software to incorporate lighting system design and properly size the HVAC systems.
The use of an energy-efficient lighting design not only provides significant lighting savings, but also can reduce the cooling requirements for a building. Engineers should use building energy modeling software to incorporate lighting system design and properly size the HVAC systems.
Building energy modeling software is widely used in the industry for a number of purposes including determining energy savings, HVAC design, or as a compliance path for U.S. Green Building Council LEED certification. There are hundreds of different building energy modeling applications available, and each has its strengths and weaknesses. The U.S. Dept. of Energy (DOE) publishes a comprehensive list of building energy software tools on its website.
While there are many important factors in creating an accurate building energy model (building area, orientation, amount of glass, etc.), internal heat gains from people, lights, and equipment in the space contribute to the majority of the cooling load in many buildings. If engineers can develop more accurate energy models, HVAC systems can be optimally sized, resulting in energy-efficient systems with improved thermal comfort for building occupants and satisfied owners.
According to the U.S. EPA Energy Star Building Upgrade Manual, lighting is typically the largest source of waste heat, also known as heat gain, inside commercial buildings. Approximately 18% of the electricity generated in the United States is consumed by lighting loads, with another 5% being used to cool the waste heat generated by the lighting. As shown in Figure 2, lighting constitutes 35% of a building’s electricity use. Because lighting represents the largest portion of a commercial building’s electricity consumption, it also presents a great opportunity for energy savings by using energy-efficient lighting systems and lighting controls. This applies to both existing and new buildings.
Interactive effects of lighting on heating and cooling
The type of lighting systems installed can have a large impact on the HVAC requirements. Reducing the energy used for lighting affects the heating and cooling that will be required. As more efficient lighting systems are installed in buildings, cooling loads will be reduced while heating loads can be expected to increase. On a new building designed with efficient lighting systems, the smaller cooling loads, in turn, allow for a building’s cooling system to be sized smaller (and therefore less expensive to purchase and operate). On an existing building where lighting systems are upgraded to be more energy-efficient, the smaller cooling loads can allow for the existing cooling systems to serve future additional loads or to be replaced in the future with smaller units.
Most buildings are made up of several systems, including lighting, HVAC, and control systems. In order to design for optimal system performance, all building systems must be considered as a whole. When designing a new building or major renovation, interactions between the lighting and HVAC systems should be considered to ensure that equipment is sized properly for real-world conditions. Similarly, for lighting efficiency upgrades, engineers and owners alike should understand and be able to account for the potential heating and cooling load net impacts that various upgrades would create.
Figures 2 and 3, adapted from a 1998 Lawrence Berkeley National Laboratory report, Interactions Between Lighting and Space Conditioning Energy Use in U.S. Commercial Buildings, written for the DOE, illustrate the interactions between lighting and space conditioning energy use for commercial buildings in the United States. For a one-unit (kWh) reduction in lighting energy, the corresponding heating and cooling load changes are shown. Note that in large office, large hotel, and hospital building types, the average increase in heating load is offset by four or more times as much of a reduction in cooling load. For small retail and school building types, the heating load energy increase is similar in size to the cooling load energy reduction.
Figures 2 and 3 represent average figures for each building type across all geographical areas of the United States. Actual changes in energy usage for a particular building would be influenced by several other factors including climate, operating conditions, building characteristics, and the efficiency of the HVAC systems. Quantifying the net impact can be difficult; there are software tools to assist with these calculations. A building energy model (computer simulation) can help engineers determine the overall energy impact of lighting systems, including interactive effects with HVAC systems, for a particular building.
One of the inputs for an HVAC load calculation or building energy model is the lighting input power watts (W) or power density (W/sq ft). Table 1 represents an example of how much this input power can be reduced by retrofitting existing inefficient T12 lighting systems in a building with various T8 efficient lighting system options. Using standard T8 systems results in a 26% energy savings compared to the baseline case, while high-performance T8 systems result in a 42% savings. Retrofitting T12 lighting fixtures with high-performance T8 lamps and ballasts, new lenses and mirrored specular reflectors can allow half of the lamps to be removed resulting in a 71% energy savings while still maintaining the same illuminance levels. Also, incorporating occupancy sensing and daylight dimming controls will provide additional energy savings. Note that this table does not account for the additional energy savings that may be realized by decreased cooling loads.