Save energy by integrating lighting controls, HVAC systems

Integrating lighting controls and HVAC systems is complex, and can offer significant energy savings in a high-performance building.

04/20/2014


Learning Objectives

  1. Learn how integrating lighting controls and HVAC systems can benefit a building and its energy efficiency.
  2. Identify which codes and standards pertain to lighting and HVAC integration.
  3. Understand challenges of integrating HVAC controls and lighting systems.

This article has been peer-reviewed.Historically, integration between lighting and HVAC control systems has been limited due to complexity, program language differences, and cost of these systems. However, with advances in lighting control systems, there is greater opportunity to add a considerable amount of functionality to them to help conserve energy in two of the largest consumers of energy in buildings.

With a trend toward high-energy performance buildings through organizations like U.S. Green Building Council (USGBC) and its rating system, energy consumption has become a major focus for many facilities. ASHRAE/IES Standard 90.1-2013 and the International Energy Conservation Code (IECC) continue to become more stringent and comprehensive compared to previous versions. As jurisdictions begin to mandate these newer codes and standards, lighting control systems are becoming more complex. In addition, HVAC building monitoring and controls is a more common baseline standard in more facilities due to economizers and minimum efficiencies.

Figure 1: This depicts the percentage of energy used in commercial buildings by various loads. Lighting and HVAC account for the two largest internal consumers of energy in commercial buildings. Note: This chart uses an adjustment factor (*) used by the EIA to reconcile to data sets. Courtesy: Dept. of EnergyAs more advanced lighting control systems make their way to the market, the level of integration between lighting controls and the BAS could become a more common practice. While the greatest challenge in integrating these systems during design and construction will continue to be the level of coordination needed to make them both user friendly and operational, the potential energy savings makes it worthy of further investigation.

Providing an efficient building requires both thermal and visual comfort, and this starts with lighting and HVAC controls. Traditional BAS have used occupancy sensors and time-of-day schedules for operational efficiencies. Historically, there has not been a need to tie these systems together. With the cost premium and the technical challenges associated with integrating lighting control systems with BAS, there have been a limited number of applications. Due to evolving code requirements, the number of applications for integration continues to grow.

For example, to comply with lighting codes, a user interface is needed for turning on lights while still requiring automated off functionality. ASHRAE 90.1-2013 also includes a much more comprehensive table that prescribes a higher level of lighting control than previous editions. Using the same sensors that the lighting system uses for occupant control allows for one system to control both HVAC and lighting, but may result in inherent issues related to how overlapping control strategies—such as daylight reductions, manual overrides, and manual-on—affect occupant satisfaction.

Figure 2: This shows the airflow requirements and air change rates required for typical patient rooms based on FGI Guidelines. Courtesy: ccrd partnersDesigning a control strategy

With HVAC and lighting control systems of this level, there may be a lot of overlap in what they need to accomplish to save energy and help a building perform more efficiently. Integrated systems will allow many benefits, such as including improved efficiency from common scheduling, occupancy control, and thermal and visual optimization. Information from lighting and HVAC control systems can be used to analyze energy usage for verification of systems operating within specified ranges.

A common energy-saving strategy employed in HVAC design has been that of nighttime setback or the use of an unoccupied mode. In this strategy, the HVAC systems’ temperature or airflow setpoint is adjusted to let the space dictate the occupied comfort conditions and thus save on energy by supplying less air, reducing the temperature of air supplied in heating mode, or increasing the temperature of air supplied in cooling mode.

The unoccupied or nighttime setback mode is most common in buildings with regular occupancy patterns, such as educational buildings, office buildings, shopping centers, and places of worship. These buildings have a predictable time of use that makes them good candidates for HVAC setback. While it is true that night setback and unoccupied modes can save energy when used properly, the design team must take several factors into account as they develop a control strategy.

Measurements versus perception

In buildings that have a regular occupancy time and use night setback, it is important to consider the effect of building mass. The mass of the building includes its envelope and furnishings, which are altered during the setback period. In a heating mode, the envelope and furnishings are cooled down during the setback period. While a typical setback strategy uses a building warmup mode to bring the space temperature back to comfort conditions before the occupants arrive, it is crucial that the warmup period be long enough to provide adequate time for the building mass to also recover. As the envelope and furniture are much denser than air, they take much longer to recover. While the building air temperature may be recovered in most spaces within 1 or 2 hours, recovery of the mass objects’ temperature may take longer depending on their materials. Occupants will perceive the space temperature as a rough average between the radiant surface temperature of the objects that they contact and the air temperature. This means that if an occupant sits in a cold chair, he or she will not perceive that the building is comfortable regardless of what the HVAC control system measures for the space temperature. Therefore, compensation strategies need to be investigated during planning to handle this possible occupant dissatisfier.

In buildings where occupancy sensing or manual devices—such as a push-button station on the room thermostat—are employed to tell the HVAC control system to take a zone in and out of occupied mode, it is important to consider the occupant‘s satisfaction as well as potential energy savings.

When the occupant is out of the room or zone, the space is allowed to leave the typical comfort conditions to save energy. As the occupant reenters the space and the system is commanded to occupied mode, the system begins to work to satisfy typical comfort conditions as well as handle the newly introduced thermal load—the occupant. During this initial time period, the occupant is using the space before comfort conditions have been met.

While some occupants will tolerate a certain amount of discomfort for a short period of time, if the occupant’s stay is short, say only a few minutes in her office between meetings, then the occupant will never experience a comfortable office if the system cannot recover in less than 5 to 10 minutes. This is one of the typical complaints of occupants in demand-controlled environments, especially those that use occupancy modes during typical business hours. Therefore, the mechanical system designer must take into consideration the time that the system will take to recover.

Other complications associated with occupancy controlled HVAC involve room zoning. Due to economic and space constraints, most buildings have multiple rooms grouped on a HVAC control zone. In a variable air volume (VAV) system, several offices may be on the same terminal box, and thus one office in the group has the thermostat or thermal sensor. As long as the spaces present similar thermal profiles, orientation, and sizing, this is rarely a concern.

However, if occupant demand-control strategies, such as occupancy sensors or push-button interfaces, are being considered, this introduces another level of complexity into the system design. Each room would need to have the ability to put the zone into occupied mode. This will add cost for additional occupancy devices. This cost must be weighed against the projected energy savings over the owner’s established payback period to establish if the strategy meets the engineering and economic requirements of the project.


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