Leveraging the BAS
By using the building automation system (BAS) to its fullest extent, a school district was able to control its HVAC, electrical, and lighting systems.
- Understand how to coordinate a building automation system so that facility managers can use it wisely.
- Learn how to monitor various building systems to achieve energy goals.
- Use the strengths of the BAS to optimally size the standby power system.
In planning for most buildings together with an owner’s maintenance staff, engineers find that facility managers enjoy the ability to use the building automation system (BAS) to view the operating status of the HVAC system and adjust setpoints with the click of the mouse. With the continuing improvements to the graphic interface of the BAS, facility managers increasingly want the monitoring and control of the electrical systems integrated into the BAS rather than displayed through stand-alone software that is proprietary to a particular electrical subsystem.
When designing for a new high school for the Oregon Trail School District, Interface Engineering leveraged the BAS to provide multiple benefits to the design, monitoring, and maintenance of the electrical system. These benefits started with the design for the backup generator, continued with master lighting controls of the campus for improved energy efficiency, included the monitoring of energy consumption and production on-site, and featured supervision of large electrical equipment such as building uninterruptible power supplies (UPS) and surge suppressors to alert when repairs and maintenance are required.
Right-sizing the generator
Sandy High School is located by the foothills of the Cascade Mountains, and so is subject to significantly more snow and freezing rain than nearby Portland, Ore. For this reason, winter outages are a more frequent occurrence. While the district wanted to provide emergency lighting and power backup to phones, fire alarm, and other mission critical systems, it also wanted the ability to provide backup power to the administrative wing, gymnasium, commons area, and kitchen, in the event of an outage that lasted for more than a few hours.
This design need by the district needed to be balanced with a fixed budget and limited space for the backup diesel generator. While the sum of connected load would have required at minimum one 2000 kW generator, two strategies were employed to allow the use of a smaller 1250 kW generator. First, the electrical service to which the generator is connected was divided into essential and non-essential branches.
Essential loads included lighting and power outlets in the administrative wing, gym, commons area, and kitchen, as well ventilation fans for the same areas; the central UPS for the district data center; pumps associated with the geo-exchange and boilers for heating; building elevators; and the kitchen freezers and coolers. These essential loads are in addition to emergency lighting, fire alarm, and communications, which are on the life safety branch covered by NFPA 70: National Electrical Code (NEC) Article 700.
Non-essential loads include the building multi-stage chiller, power for the theater and the counseling services wing, and the laundry room associated with the athletic department. The ability to use a master "load shed" signal to reduce power consumption in the event that the generator is more than 90% loaded allowed the 1250 kW generator to be used for a larger connected load, and in effect gave two steps to reduce the load below the targeted maximum: first to reduce the power draw of the building’s multi-stage chiller, and second to shed the non-essential branch altogether. The load shed signal originates from the generator control panel, which is tied to the BAS through a set of dry contacts.
Adding to these benefits the local utility, Portland General Electric, entered the school into its dispatching standby generation program, where it provides funding to the owner in exchange for the owner installing a paralleling switchgear and supervisory control and data acquisition system for the generator. These added features allow the utility to run the generator for up to 100 hours per year for power production at times of peak demand. By adding motorized breakers in the switchgear for the essential and non-essential branches, load can be shed when necessary.
It’s important to note that so far, peak load on the switchgear has remained under 40% of the connected load, which has translated to not needing to use the load shed feature, even during summer power outages. This is a lesson for future projects; while code necessitates designing the generator for connected load, the common occurrence that commercial and institutional buildings typically draw a peak load of 30% to 50% of design load gives room to use a load shed to power.
Interfacing the BAS, lighting controls
While lighting relay panels and occupancy sensors are used for control of lighting in corridors, the commons, and gym (as well as classrooms and offices), the district wanted the ability to have master override control from the BAS to provide a future alternative to the local occupancy sensors. This feature is also included because the district had used occupancy sensors in smaller room spaces, but had less experience with their use in high ceiling areas.
The BAS was programmed with five zones of lighting control output that include the classroom wing hallways, the gym and commons hallways, the gym itself, the commons itself, and exterior lighting. Schedules are then programmed to sweep lighting on/off in each of these areas based on a programmed schedule programmed for weekdays, weekends, and holidays. In addition, because the BAS also receives status information on whether the fire alarm has gone into alarm, the BAS can force lighting in these areas to full on after-hours when needed in an emergency.
After the school had been open for several months, the interior lighting controls from the BAS were left in the disabled configuration, allowing the local occupancy sensors to operate the lights.
While the link between lighting controls and the BAS was fairly simple in this particular case, other features given to the district for future consideration are listed at the end, and point to some powerful features that can be applied for added energy savings.
Feedback for energy consumption and production
Because the high school’s physical plant included layers of energy-saving features, the owner wanted feedback to ensure that power consumption would meet the anticipated targets shown by the building energy model. This feedback helps both with the commissioning performed after construction to confirm that the central plant is operating as designed, and with future maintenance to determine if there is excessive power usage due to either altered control programming or equipment requiring repair.
Power monitoring communication with the BAS appeared cost effective with simple 4 to 20 mA pulse output where we had six or fewer power meters. However, the larger number of meters necessitated consideration of a way to group signals together. For this, the engineering team chose using a Modbus protocol for the meters to tie back to the BAS, as there were multiple manufacturers that could provide this option. More recently, we have seen that several manufacturers are now also providing a BACnet transmission control protocol/Internet protocol option to facilitate connection to the BAS, as this has become more commonplace.
Power submeters were added to monitor the data center UPS load, the life safety branch, the essential and nonessential switchboards connected to the generator, the rooftop photovoltaics (PV) system through the inverters, and the classroom wing branch panels. In addition to energy consumption (kWh) and peak power demand (kW), the energy meters can also provide a record of reactive energy usage (kVARh) and peak demand apparent power (kVA), so the owner has a record of power factor and its effect on utility demand charges.
The monitoring of the PV panels year over year has been particularly helpful for maintenance. Depreciation of the PV panels due to accumulated dust is revealed as consistently reducing energy production when comparing year-over-year data. The district then uses this data as an important reminder of when to rinse off the panels and restore production levels.
In addition to being used for maintenance, the power meters have had an important role in helping interpret the building’s energy consumption and production to the public, including the school’s own staff and students. The building dashboard is available through kiosk displays located throughout the high school, and through a Web interface available to the public. From these displays, the public can learn about the energy-conserving features of the school, and view graphically the power production and consumption data.
Monitoring large electrical equipment
Due to the prevalence of ice storms in the vicinity of the high school, power surges related to falling power lines is not uncommon, and so surge suppressors play a vital role in increasing the longevity of power electronics. In this case, the power electronics include everything from the controllers for the elevators, to variable frequency drives for large motors in the central utility plant, to electronic ballasts for fluorescent lighting and drivers for LED lighting. In short, there are many solid-state devices beyond the personal computers and smart boards used in the classrooms.
For this reason, levels of surge suppression are used at the high school-main surge protection devices (SPDs) at the main switchgear, as well as branch devices at panels serving mechanical loads for the central utility plant, at the switchboard serving the elevators and the UPS in the data center, 208/120 V switchboards serving plug-in loads, and life safety branch and panelboards for building lighting.
However, when engineers specified SPDs with integral alarms and event counters without remote monitoring, if the SPD is located in an electrical room, even if the audible alarm goes off, it may not be noticed by maintenance right away. Even when it is noticed, if maintenance overrides the audible alarm without noting the device failure condition, replacing the SPD (or its modules in the case of a main device) may be forgotten.
Instead, each SPD at the high school is connected to the BAS via a set of dry contacts to provide a "failure" signal, reducing the time between a failure occurrence and when the necessary maintenance is provided.
Because the district’s data center is also located at the new high school, the UPS is monitored for when it is running on its battery source and when it has been taken off-line. In addition, master signals from the generator control panel are sent to the BAS, including when the generator is running, a general warning signal if maintenance needs to check the generator, and an alarm when the generator has shut down. This is in addition to the load-shed signal explained above.
Planning for the future
Another key feature of the school is the use of digital lighting controllers in the classrooms. These intelligent relay boxes give the ability to combine low-voltage inputs from daylighting sensors, occupancy sensors, and wall dimmers on a common signal bus using Category 5 plug-and-play patch cables. The controller can then both switch room lighting through relay outputs, and raise and dim lighting with 0 to 10 Vdc control wiring.
However, a powerful added feature of these controllers emerges when they are networked together with a common communications bus, again using networking cable. From a common master signal, the controllers can be instructed to collectively dim connected lighting based on a demand response signal. This is currently required by California Title 24 energy code as a future capability of building lighting control systems. While the controllers at the high school were not networked together to both simplify the design and reduce installation cost, performing such networking in the future could provide additional energy savings.
By tying the BAS to such digital controllers, a means of reducing energy consumption from lighting becomes much easier, especially since dimming lighting by 5% to 10% can be done with a minimum perceived change in the output of the lighting, in the case of fluorescent lighting. LED lighting promises even greater savings in such conditions because power reduction is in more direct proportion to reduction of light output. This also has implications for reducing energy consumption at building startup because lighting can typically be dimmed by 5% to 10% from maximum while luminaires have not yet accumulated dust or wear reducing light output (lumen depreciation).
The BAS has a well-established track record for optimizing the energy usage of building HVAC. However, the ability of the BAS to leverage greater energy efficiency for building lighting, as well as improved monitoring and maintenance of building electrical systems, is just beginning to be explored. For instance, LED luminaires with varying color temperature output can, in the future, be directed through the BAS to change their output to a cooler temperature during daylight hours and to a warmer temperature at night. Such greater integration with digitally controlled lighting and solid-state luminaires points the way to greater potential savings.
David Chesley is associate principal/senior electrical engineer at Interface Engineering. His expertise includes building integration, renewable energy systems, telecommunications infrastructure, backup power systems, variable frequency drives, and energy metering and building dashboards.