Common-sense automation strategy for scheduling is being redefined by new approaches and technology
Now required by most energy codes, automatic shutoff of building lighting has proven to be a reliable source of energy savings. For example, the ASHRAE/IES 90.1 Standard, recognized by the Department of Energy as the minimum national energy standard, requires automatic shutoff in buildings larger than 5,000 square feet except for buildings where lighting must be operated continuously. There are basically three choices to satisfy this requirement: occupancy sensors, building automation systems, and lighting control panels.
Even when not required by code, occupancy sensors and lighting control panels can be an economically attractive retrofit for existing building controls. Occupancy sensors are typically optimal when the occupancy pattern is unpredictable. Scheduling using a lighting control panel is typically optimal when the occupancy pattern is predictable. Luckily, both strategies can work in tandem to maximize energy savings.
According to the California Energy Commission, scheduling a building's lighting to automatically shut off at the end of the workday has been documented to generate 5% to 10% kWh and 0% to 5% peak kW savings. Scheduling the lighting to automatically shut off selected lighting circuits and luminaires during peak demand periods has been documented to generate 5% to 15% kWh and 5% to 15% peak kW savings. While still a nascent trend, adding dimming capability to large loads in the building and scheduling this function for energy savings and/or load shedding has been shown to generate 2% to 10% kWh and 5% to 20% peak kW savings.
In large buildings, lighting control panels provide a centralized platform for switching a significant number of loads. Traditionally, these panels, containing high densities of relays with low-voltage inputs from control devices and line-voltage outputs to the load, were centralized in the electrical room near the electrical panel. In some cases, the control panel can include controllable breakers; therefore, it can replace the electrical panel. In other cases, the control panel can contain dimming modules instead of, or in addition to, switching modules.
For scheduling to occur, the panel must be intelligent — that is, it must contain a time clock that enables scheduling. These intelligent control panels are most suited to applications where the granularity of control stops at the branch circuit level. Using a computer interface, branch circuits can be individually controlled, grouped together into larger control zones, or reconfigured into new control zones in the field. Intelligent panels are digital in construction and offer a number of advantages, such as monitoring and alarm features.
The scheduling system becomes the backbone of the building's control system, and can be supplemented by other controls such as occupancy sensors, photosensors, and dimmers. The simplest application of scheduling is to shut off the entire building's lighting system at the end of the workday, with local overrides for users who need to occupy the space after hours. The result is energy savings by eliminating waste.
Distributed control. The traditional control system is centralized (Fig. 1); all control devices are line-voltage wired to the control panel in the electrical room (i.e., home run). The control panel polls connected control devices for input that is then filtered through its logic circuit to determine the output. In a building requiring control of very large zones, this approach can be effective. If the building requires greater granularity of control with smaller zones, then another approach — distributed control — should be considered.
Distributed control (Fig. 2) distributes panel-level control function across the facility. In its simplest form, the approach is already quite common. An example is when you install a control panel on each floor of a multi-story building for networked control from a master panel or convenient access and individual stand-alone control by each floor's occupants.
This strategy can be taken further using small control panels — typically with only two to four control outputs each — that are installed in the immediate area of the loads they control, typically above the ceiling. Manufacturers may also call these distributed control panels “automatic relay packs,” “remote relay packs,” or some other term. A completely distributed system would have no control panel in the electrical room, which can save space. The distributed panels are typically networked via low-voltage cabling to share information and implement local and global commands based on a shared protocol, which increases information, flexibility, and scalability. For centralized scheduling, these distributed panels can connect back to a system time clock.
One big advantage of this type of approach is installation cost savings due to a reduction in required input/output wiring. There are lower labor costs because installation is often less complicated and less costly without the typical line-voltage wiring requirements. When inputs and outputs are located within close proximity, the distributed architecture will reduce the total installed cost of the project and provide more information to the user. But perhaps the biggest advantage of distributed control is the ability to meet prevailing energy codes, which increasingly call for individual control for smaller spaces.
However, there can be a downside to a distributed control design. If not designed properly, a distributed system can be somewhat difficult to maintain — as there is no single central location. Issues to consider include the serviceability of the devices, as they are located above the ceiling. Servicing these devices can be more intrusive to occupants.
Another major controls trend is distributed intelligence, which is an advanced option available in distributed control systems — a subset of distributed control. In a traditional centralized-intelligence control system, the control panel uses a processor to assign its switches to control zones, and polls connected devices for inputs that will result in responsive switching outputs. If the processor fails, the entire control system fails.
In a distributed-intelligence system, each control device has its own processor, which enables networking of devices using any configuration that the application may require. The devices communicate directly with each other instead of using a central processor as the intermediary. Each device has the ability to make decisions independently. If a processor fails, that particular device fails but the rest of the system will not be affected, which increases reliability. Other advantages include faster response times because decisions can be made locally, greater flexibility in wiring, future capability for reconfiguring a space, and generation of more information for monitoring and decision-making.
Lighting control panels can be an economical option for satisfying growing demand for energy-saving strategies that help owners comply with new energy codes. In some cases, specifying intelligent control panels can provide basic lighting automation. In recent years, distributed control and distributed intelligence — both of which can add value through greater capabilities and problem solving — have gained in popularity.
DiLouie is the communications director for the Lighting Controls Association and principal of Zing Communications in Calgary.