Wet-Bulb Temperature Performance Maps for Chiller-Water Side Economizer Plants: Guiding Multi-Sequence Control Strategies

Chiller-Water-Side Economizer (WSE) plants are widely adopted in data centers and commercial buildings because they leverage ambient wet-bulb conditions to provide free cooling, offering significant savings compared to chiller-only operation. However, current practice lacks systematic evaluation; design engineers select critical operational sequences (like tower priority versus chiller priority) primarily based on experience. No comprehensive study compares these sequences across multiple ASHRAE climate conditions to provide a consistent guideline for selecting an appropriate sequence as a starting point. To address this research gap, this work proposes developing wet-bulb temperature based performance maps as a decision-support. We utilize a high-fidelity co-simulation environment, modeling the Chiller-WSE plant architecture using Modelica models (encapsulating thermodynamics and control logic) exported as Functional Mock-up Units (FMUs). The methodology involves a cross-climate evaluation across representative ASHRAE climate zones for two sequences: chiller-priority (chillers engage first) and tower-priority (cooling tower maximized for cold water inlet). Simulation outputs capture total HVAC energy consumption (kWh), which is aggregated into designer-ready performance maps that delineate sequence decision boundaries. The results, plotted for climate zones 1A to 4C, show clear distinctions for the optimal control sequence. Specifically, we observed different operational zones of distinction in the range of -4 degrees C to 11 degrees C where tower priority and 11 degrees C to 17 degrees C chiller priority sequence can save energy compared to the other. This case study provides designers with data-driven tools, demonstrating the energy-saving potential achieved through the systematic selection of the optimal control sequence.