This summer, we did a series of columns inEngineered Systemsand a webinar on how to optimize buildings with the use of BAS. The focus was on a series of strategies that could be readily applied to new system designs to optimize energy efficiency and occupant comfort.
A recent project has given us the opportunity to evaluate a group of existing buildings for improved efficiency. Not surprisingly, each building presented a unique set of challenges with different systems, energy performance, schedules, and goals. The controls also ran the gamut from new to what could kindly be called vintage. While each of the buildings was professionally maintained, they all showed potential for improved efficiency. Improving efficiency can be fairly complicated and expensive, but many changes can be made simply by tuning up the systems that are already in place. Here are a few areas to consider that have little or no cost to implement and are likely to result in the most energy savings:
Occupancy-Based SchedulingIdeally, systems including HVAC and lighting should be scheduled to operate only when spaces are occupied. For buildings that have intermittent occupancy such as schools and offices, this should be fairly straightforward with the building schedules following normal building occupancy hours. In most buildings, though, we have observed the operating hours are typically twice the scheduled hours. This is done to allow for flexibility for staff who arrive early or stay late, as well as to accommodate cleaning crews and special events.
In most buildings, leaving these systems in occupied mode results in significant energy usage as full ventilation and space conditioning is being provided. Reducing operating hours through more diligent scheduling, perhaps augmented with the flexibility to allow occupants to override systems if necessary, offers the single largest potential for energy savings with no impact on occupants. Even buildings that have continuous occupancy, such as hospitals, have areas that are only used during certain hours, and these areas do not need to be lit or fully conditioned when not in use.
Economizer ControlIn many climates, we can use cool, dry, outside air as a dominant source of cooling for much of the year; remarkably, economizers are often either not provided or properly operated. The general rule is that as long as the temperature and humidity (i.e., enthalpy) of the outdoor air is lower than that of the indoor air, we should be using it as a cooling source, prior to the use of mechanical cooling. Tuning up economizer control and potentially upgrading units for more accurate sensing is another strategy that has low cost and a high return in both comfort and energy savings.
SetpointsThere are certain system setpoints that should be automatically or manually adjusted based on space or outdoor conditions. For example, the chilled water setpoint may need to be set at 42° during hot, humid weather so that cooling coils can adequately remove humidity. During cooler, dryer weather, a setpoint of 45° (or higher) may be adequate. Ideally, the reset schedules for these setpoints is automated; however, it can also be done manually. Other setpoints that are candidates for reset include hot water heating, cooling tower, and fan static.
Ventilation ControlIt is critical that buildings be properly ventilated and follow the recommendations in ASHRAE 62, but in most buildings, the amount of ventilation is constant whenever systems are in the occupied mode. This means that in many cases, we are using excess energy to heat, cool, and dehumidify ventilation air that doesn’t provide benefit for occupants. Using a control strategy such as demand controlled ventilation allows us to measure ventilation and air quality levels and automatically adjust ventilation as defined under ASHRAE 62. Retrofitting existing buildings for demand controlled ventilation is a good idea and should be carefully considered. Properly scheduling systems as described above is another simpler method to match ventilation to occupancy.
These fairly simple changes can readily be made to any existing system. Providing this type of tune up has the potential to improve occupant comfort and reduce energy usage with a nominal investment of time and money.ES
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