As a regular reader of Engineered Systems, you likely have a pretty good understanding of temperature controls and BAS. This includes the design and operation of VAV air-handling systems as well as chiller plants and rooftops. Over the next two columns, we are going to look at strategies that should be considered to optimize the operations of these systems using the control system to dramatically reduce energy usage.
Part I: AirSide ControlsWe can characterize the traditional control of VAV systems as using closed loop control for delivery of a constant pressure and temperature of air, with a consistent percentage of ventilation and a variable flow based on VAV terminal demand. It is possible, with the use of advance control algorithms, to significantly improve the efficiency of these systems resulting in many dramatic reductions in energy usage, and improved comfort. How is this possible? The answer is systems optimization. Let’s look at some examples of common strategies.
Static Pressure ResetUsing a static pressure sensor and a VFD on a VAV air handler provides for significant energy savings compared to constant volume operation. However, controlling to a fixed duct static pressure setpoint is problematic. Challenges include both properly locating the sensor as well as determining the correct setpoint. On many project sites, VAV systems provide more static pressure then is needed, resulting in both higher energy use and problems with box control and fan noise.
So what is the correct static pressure setpoint? The answer is that it doesn’t matter – the fan needs to produce only enough pressure to serve the most extreme box but not too much pressure as to possibly damage the ductwork. The static pressure reset algorithm looks at VAV box valve position to determine the optimal static setpoint for proper system operation. Allowances are typically made for disregarding “extreme zones” that may have undersized boxes. Using this strategy allows for greater flexibility in static sensor placement, can help reduce fan noise, and is estimated to reduce total HVAC energy usage by 2% to 4%.
Supply Air Temperature ResetIn addition to resetting the duct static pressure, strategies can also be utilized to reset the discharge temperature setpoint of the air handler based on the temperature of the individual zones. Increasing the discharge temperature does reduce the amount of energy used by the chiller plant; however, it will typically result in an increase in fan energy. Caution needs to be used to both balance the energy savings (through the use of energy modeling) and also to ensure that adequate dehumidification is being achieved.
Demand Controlled VentilationIn order to maintain a healthy ventilation level, ASHRAE Standard 62 specifies a fixed ventilation level for removal of contaminants and an additional amount of ventilation to disperse the CO2 impact of the occupants. During normal operation, the building is kept ventilated based on the anticipated design occupancy.
The standard does, however, allow for an alternate control strategy which measures the actual occupancy (typically through the use of CO2 sensors) and measures and adjusts ventilation to reflect actual occupancy. This strategy most importantly results in a healthy indoor environment and also can provide significant energy savings. Energy savings will vary depending on the building location and occupancy and can be calculated using broadly available tools. HVAC energy savings for office buildings are in the range from a low of 3% to 20% or higher.
Next month: Waterside control optimization. ES
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