The first goal of this project was to demonstrate the energy savings and control stability of a new method for controlling air handling units (AHUs) and rooftop units’ fan speeds at five Iowa Army National Guard (IAARNG) facilities. The second objective was to generate practical sample control programming codes under different Direct Digital Control (DDC) system platforms. These programming codes would then serve as “templates” for others (i.e., controls contractors, consulting engineers, facility engineers) to emulate and implement at additional future Department of Defense (DoD) sites. The third objective was to analyze the economic benefit and demonstrate the cost effectiveness of applying the proposed method to different DoD building types using a basic life-cycle cost analysis.
Many existing DoD facilities nationwide operate their Heating, Ventilation and Air Conditioning (HVAC) systems at design static pressure set point meant to alleviate building loads during hot summer or cold winter days. However, these design loads are not present most the time. By optimizing static pressure rise in HVAC systems, significant fan energy savings can be achieved. Recognizing this, American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) has moved forward in requiring the supply air static pressure set point be reset at the zone level in new buildings to satisfy the most critical zone. The reset can be accomplished through custom building control software programming, and the state-of-art algorithm is the trim and respond (TR) method. A modified version of the TR method, the tiered trim and respond (TTR) method, has shown promise in reducing air handling unit (AHU) fan energy use while maintaining steadier static pressure control in a lab study and a University campus building pilot study. For this demonstration, the TTR method was implemented at five existing Iowa Army National Guard (IAARNG) facilities to show energy savings and control stability. Comparisons were made by alternating static pressure control modes every two weeks between fixed static control and TTR control over a one year period at these facilities.
In older commercial buildings, HVAC systems are often forced-air, Constant-Air-Volume (CAV) systems. In such a system, supply and return fan airflow rates are manually set to meet the maximum airflow requirements for thermal load and ventilation. The supply air temperature is controlled at a set point to satisfy the zones with maximum load. Reheat coils on constant-volume terminal boxes are controlled by individual thermostats to adjust the zone temperatures.
A different forced-air system design called variable-air-volume system gradually replaced constant-air-volume system because the Variable-Air-Volume (VAV) system is usually more energy efficient. A typical single-duct, multi-zone VAV system schematic highlights key relationships among components. In such a system, AHU supply and return fans are used to deliver air to zones through zone terminal units (or VAV boxes). AHU supply air is heated or cooled to maintain a certain temperature through heating or cooling coils. Terminal unit damper positions are continuously adjusted to provide appropriate air flow in each zone to meet the different cooling loads. AHU static pressure is usually maintained at a fixed set point based on peak load design conditions.
However, most the time these HVAC systems do not operate at peak load conditions. Automatically lowering AHU supply air static pressure at partial load conditions may significantly reduce fan energy used to deliver air to the system. If the operating point can be moved to a different point on the “ideal” static pressure curve while still maintaining 50% of the design air flow, the supply fan power can be reduced by approximately 57% (~12% vs. ~28% of design fan power when running at 100% air flow rate condition.)
The state-of-art in resetting static pressure is the TR method, and it typically uses maximum VAV damper position as an indication of system cooling load and sets a target of 90% to 95% Open for the maximum damper in the system.
The TTR method is an improved version of the TR method. Research done by Dr. Ron Nelson and his students [Nelson, 2011] showed that the target of 95% to 98% threshold value as described in the ASHRAE handbook and other papers might be too high for stable control. At higher damper position ranges, large percentage changes in VAV damper position can only marginally decrease air flow rate due to the flattened curve in that region. On the other hand, the change in VAV air flow set point due to small to modest zone load changes or disturbance could cause a relatively large change in damper command and position. This significant change in damper command or position will affect the set point reset calculation since the damper output itself is usually the result of a Proportional-Integral (PI) control loop output for VAV box cooling and is subject to oscillation if not properly tuned. The PI and Proportional-Integral-Derivative (PID) control methods are standard classical control methods that calculate control output based on the difference between a process variable and a set point. The control performances using these methods are highly subject to proper parameter tuning in the field. Further tests also concluded the trim and respond rate change were not a major factor in control stability, but the reset time interval could be a factor. Too short of a time interval, e.g., one minute, could easily cause the system to be unstable. While a longer time interval, e.g., 15 minutes, increases system stability, it also may save less energy and respond to system changes too slowly.
In the TTR method, if the maximum damper output or position is within a specified narrow range [Low1, High1], the static pressure set point will not change. However, if the damper deviates from this range, the set point will be adjusted based on three tiers of ranges ([Low1, High1], [Low2, High2], and [Low3, High3]). The rates of change will be based on preset trim rates (TM1, TM2, TM3) and respond rates (RP1, RP2, RP3). The technology is innovative in a sense it recognizes a major factor that causes the instability of static pressure reset control and difficulty in tuning parameters, and provides a solution to alleviate the problem. The approach has better control or adjustment capability for various building types and building mechanical systems. It is a variation and improvement on the state-of-the-art TR method, and it allows a smooth, energy-efficient transition between states. Lower fan speed and more stable control would also result in reduced noise levels compared to constant pressure control and traditional TR method.
Key benchmarks were used to determine the success of the project: fan energy savings of 30% or greater over fixed static pressure (FSP) strategies (based on past studies on the TR method), 6% reduction in overall Greenhouse Gas (GHG) emissions, six-months to one-year simple payback (based on a university campus building TTR pilot study,) and no additional user complaints.
Demonstration results showed that total fan energy savings for the five demonstration sites ranged from 14.4% to 34.8% compared with fixed static pressure control. Reduction in overall GHG emissions at five sites ranged from 0.6% to 4.7%. Simple payback years are 1.7, 4.9, 5, 11.8 and 14.9 years. Users (building occupants and facility engineers) mostly had no additional comfort complaints. The potential reduction in site energy across DoD installations could be 295 Gigawatt hours (GWh) per year, and the potential electricity cost savings could be $29.5 million per year.
Overall, the key energy savings results and user satisfactions met or partially met project objectives, while other targets, such as system economics, fell short of the original project goals. Contributing factors include low local electricity cost, non-ideal mechanical equipment and control operating conditions, and the need to hire control contractors to troubleshoot and solve “rogue zone” problems to make TTR method work effectively.
The factors influencing the energy savings and cost-effectiveness of building controls retrofit projects to convert fixed static pressure control to either TTR or TR method are summarized in the following table:
For this demonstration, the only end-user concern was at one site where occupant experienced significant noise from AHU fans ramping up and down, debris falling from the ceiling and temperature discomfort (due to AHU tripping on high static pressure) when AHU static pressure was running at or close to design setpoint. This was mainly due to HVAC system design flaws and improperly tuned TTR parameters. The facility engineer at this site had previously lowered the normal operating static pressure to a much lower value. There were no other significant complaints from occupants or facility engineers during the one-year demonstration period.
For new construction, static pressure reset is a prescriptive requirement in ASHRAE Standard 90.1 when there is DDC control at the zone level. This requirement may also apply to significant HVAC additions or alterations. Potential DoD fixed installation applications are in existing buildings that have VAV systems with zone-level DDC but are still using the fixed static pressure control strategy. From energy saving and system economic analysis results based on this demonstration, the decision-making factors regarding switching to static pressure reset strategy (either TTR or TR) could include:
From this demonstration project, the energy savings and system economics at the five IAARNG buildings are somewhat lower than previously estimated due to many factors. It is predicted that practitioners can find ways to improve the algorithm to make things work better in real buildings in the future. For example, by allowing some zones to be ignored from the reset strategy, the operator is implicitly sacrificing airflow and potentially temperature control in some spaces for minimizing energy use. Occupants often do not complain when zone temperatures are off a few degrees compared to set points. Automated Fault Detection and Diagnostics (AFDD) could be a useful tool for facility engineers and control contractors to quickly identify rogue zones and fix maximizing energy and cost savings.