Development and validation of a generic evaporator model of chillers

Chillers consume approximately half of a building’s total energy, and variables within a chiller evaporator are crucial for system operation, control, and design of the secondary chilled water loop. Although detailed physical models like computational fluid dynamics are used to study evaporators, their complexity and high computational cost make them impractical for Heating, Ventilation, and Air Conditioning (HVAC) systems with limited data in a building automation system.

This paper presents a grey-box model for chiller evaporators under steady-state conditions, integrating both physical and data-driven approaches. The model development starts with an analysis of the evaporator energy balance and heat transfer on both the water and refrigerant sides. It is then simplified into a practical equation with a simple format, requiring a minimal number of model input variables that are usually available in building automation system.

The proposed model targets to estimate the chilled water temperature difference across the chiller evaporator. The case study of a real institutional building with a dataset at 15-minute intervals is used to validate the model performance. Results indicate it achieved a high accuracy with a coefficient of variance of root mean square error of 3.9%. The proposed model can be used to study HVAC operation optimization, fault detection and diagnosis, ultimately contributing to improved energy efficiency and system reliability.