| dc.description.abstract | Commercial buildings account for 35 percent of electricity consumption in the U.S., of which 30 percent is used by the heating, ventilation, and air conditioning (HVAC) system. Despite the significant role of the HVAC control systems in energy efficiency, its design, commissioning, and retrofit have long been an intricate and complicated issue, considering that only diffuse and fragmented information on system operation is available for decision making in most of the scenarios. Due to this limitation, designers and control contractors can only rely on ad-hoc control sequences for system operation in practice, which is one of the major reasons why buildings are operated sub-optimally. To provide standardized and high-performance rule-based HVAC control sequences, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has developed the Guideline 36 (GDL36) High Performance Sequences of Operation (SOO) for HVAC Systems to maximize energy efficiency. Although GDL36 was considered the most advanced rule-based HVAC control sequences in this era, most of the proposed controls are still under development and its actual performance remains largely unknown. Up till now, only a few field studies have been conducted to verify the overall effectiveness of GDL36 after its publication, and these studies only focused on the energy saving potential. There is a practical need to benchmark the SOO in GDL36 in different aspects.
To address these gaps, this research aims at enhancing the existing standardized high-performance control sequences (GDL36) by conducting a comprehensive evaluation in terms of energy efficiency, fault robustness, ventilation performance, and grid ancillary service compatibility. The target HVAC systems in this research are multi-zone variable air volume (VAV) systems, which are one of the most popular HVAC system configurations in U.S. commercial buildings.
First, a Modelica model of a five-zone VAV system that follows both airside and waterside SOO was developed and verified. This building model serves as the virtual testbed for the following intelligent controller evaluation and comprehensive fault impact analysis.
Second, the energy saving potential of the high-performance rule-based controls was compared with that of the state-of-the-art intelligent controls (deep reinforcement learning (DRL)-based control (DRLC) and optimization-based control (OBC)) in two typical cooling weeks. Two supervisory control loops in the airside GDL36 SOO (e.g., supply air temperature and duct static pressure) were replaced by DRL and OBC controller. The results show that the GDL36 has a comparable energy performance (within a 3% deviation) with DRLC in scenarios under both high and mild cooling loads. GDL36 also has a comparable energy performance (within a 3% deviation) with OBC in scenarios with high cooling load, but it consumed 7% more energy in the shoulder week. In terms of thermal comfort, the GDL36 was found to have slightly more zone air temperature violation in all scenarios compared to the other two intelligent controllers (i.e., DRLC and OBC).
Third, a comprehensive fault impact analysis of the GDL36 was conducted to assess its fault robustness. How these sequences handle and adapt to various types of common faults was evaluated through a large-scale fault simulation. The results show that a vast majority (~90%) of fault scenarios have a fault impact ratio (FIR) of less than 6% for energy consumption and energy cost. Besides, the results of FIR distributions also indicate that GDL36 SOO only has limited influence on key performance indexes (KPIs) such as the supply air temperature control quality, thermal comfort, ventilation performance, and peak power load.
Fourth, considering that the HVAC system configuration of multiple zone VAV systems with multiple recirculation paths has long been neglected in literature, a CO2-based demand control ventilation (DCV) was developed and quantitatively investigated in this study in terms of energy and ventilation performance. The proposed DCV control sequences were tested in four typical ASHRAE climate zones and proved to achieve considerable energy savings while maintaining an acceptable indoor air quality compliant with ASHRAE Standard 62.1.
Lastly, an experimentally validated frequency regulation (FR) control scheme was integrated with the GDL36 SOO for air handling unit (AHU) fans from the perspective of the building providing ancillary service in the future. The impacts on the energy efficiency and thermal comfort were assessed and potential control conflict was identified when the VAV system provides frequency regulation using the GDL36 SOO.
In summary, this dissertation developed a Modelica-based virtual testbed and evaluated the GDL36 SOO for multi-zone VAV systems in a holistic view. For energy efficiency, the GDL36 SOO achieved a comparable performance in terms of energy efficiency and thermal comfort with two intelligent supervisory controls in both high and mild cooling load conditions. For the fault robustness, it demonstrated that there were only minor fault impacts over different KPIs for the system with GDL36 SOO through a large fault simulation. From the ventilation aspect, the proposed DCV SOO for multi-zone recirculating path systems showed its energy efficiency and ventilation compliance and could be readily merged into GDL36. Lastly, when the AHU fan provides the FR service, the FR control could be integrated with GDL36 SOO with limited impacts on the HVAC system. Following prerequisites need to be met. First, the time-varying FR capacity must be correctly estimated. Second, an anti-saturation control scheme needs to be developed to avoid the fan power surge and ensure a smooth transition to post-FR operation. | |
