Analyzing the energy efficiency benefits of advanced control strategies in district cooling systems: a real-world case study comparing ASHRAE Guideline 36-2021 with setpoint control optimization

Building controls are essential for ensuring energy-efficient building operation. To address this imperative, the American Society for Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) developed Guideline 36, a standardized set of advanced control sequences to improve building performance while reducing energy use. However, the application of Guideline 36 has been largely limited to commercial buildings with conventional cooling systems. Most large commercial and industrial buildings are now conditioned with centralized district cooling systems that are more energy efficient and environmentally sustainable. Moreover, previous studies found that earlier versions of Guideline 36 decrease building energy consumption, but few analyses have been conducted on G36-2021.

This study evaluates the energy saving potential of the ASHRAE G36-2021 (G36-2021) standard against existing controls for a real-world district cooling system (DCS) in Boulder, Colorado with and without condenser water setpoint optimization. To the authors’ knowledge, this is one of the first case studies that evaluates this standard in a real-world DCS application.

We compare optimized control sequence and G36-2021 energy savings against a calibrated baseline to evaluate if optimizing current controls in an existing DCS can save comparable amounts of energy to G36-2021. The results of this study will help inform district cooling system operators if they should default to ASHRAE control sequences. To conduct this study, we execute four main tasks. First, we developed, calibrated, and validated a model of University of Colorado Boulder’s DCS in Modelica. Second, we optimized the condenser water supply temperature using the Dymola Optimization Library to improve energy efficiency. Third, we will simulate the G36-2021 control sequence models developed by Lawrence Berkeley National in the case study model. Finally, we will compare the energy use from each scenario.

With existing controls, the calibrated district energy system model used 3239.2 MWh of energy annually. Optimizing the condenser water supply temperature setpoint with a constant approach temperature and a constant monthly condenser water supply temperature saved almost 13% and 9% annual energy, respectively, which suggests that using advanced G36-2021 control can also improve DCS design and operation sequences.

In summary, this work presents an example using a real-world case study of the energy effectiveness of G36-2021 compared to setpoint-optimized HVAC control strategies. It is important for improving advanced control sequences and encouraging industry adoption of high-performance control sequences.