July 2009

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Advanced Controls Optimization for High Performance Buildings

Strategies that should be considered to optimize the operations of these systems using the control system to dramatically reduce energy usage.

Paul Ehrlich & Ira Goldschmidt
Building Intelligence Group

As published
 

July Issue - Column 

As a regular reader of Engineered Systems you likely have a pretty good understanding of temperature controls and building automation (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.

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Distech Controls

Part I: Air Side Controls

We 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, however, with the use of advanced 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 Reset:
Using a static pressure sensor and a variable frequency drive on a VAV air handler provides for significant energy savings compared to constant volume operation. However controlling to a fixed duct static pressure set-point is problematic. Challenges include both properly locating the sensor as well as determining the correct set-point. 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 set-point? 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 set-point 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%.

J2 Innovations Supply Air Temperature Reset:
In addition to resetting the duct static pressure, strategies can also be utilized to reset the discharge temperature set-point 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 be sure that adequate de-humidification is being achieved.

Demand Controlled Ventilation
In 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: Water Side Control Optimization


About the Authors

Paul and IraPaul and Ira first worked together on a series of ASHRAE projects including the BACnet committee and Guideline 13 – Specifying DDC Controls. The formation of Building Intelligence Group provided them the ability to work together professionally providing assistance to owners with the planning, design and development of Intelligent Building Systems. Building Intelligence Group provides services for clients worldwide including leading Universities, Corporations, and Developers. More information can be found at www.buildingintelligencegroup.com  We also invite you to contact us directly at Paul@buildingintelligencegroup.com or ira@buildingintelligencegroup.com
 

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