How Buildings, Chiller Plants Work: Discovering the SEE Model

By Kirby Nelson, P.E., Life Member ASHRAE

Since retiring in 2001, Kirby Nelson, P.E., Life Member ASHRAE, has been creating his own modeling tool. He developed the System Energy Equilibrium (SEE) Model to better understand how buildings and chiller plants work. The tool can be used to design and understand how a building or plant responds to changes in weather, equipment or control, Nelson said.

The SEE Model can also be used for commissioning and for benchmarking the energy consumption of buildings on any given day, providing a method to compare the energy consumption of real buildings to standard designs as weather changes from day to day. The SEE Model is presently configured to model a building and plant designed to ANSI/ASHRAE/IES Standard 90.1-2010, Energy Standard for Buildings Except Low-Rise Residential Buildings. Other standards can be modeled, and any design or control parameter can be changed within the model.

“The SEE Model is a concept, not a finished product,” said Nelson. “It is an approach to modeling buildings and chiller plants based on the same equations used to design a building and plant…If the concept is accepted, I believe it will become a college course, and utility companies or others, will provide benchmark data for cities, and a few engineers will become proficient in running and developing SEE Models.”

Nelson discussed the SEE Model and its attributes with ASHRAE Journal.

What is the SEE Model?
The System Energy Equilibrium (SEE) Model is a set of building equations and chiller plant equations solved simultaneously by Microsoft Excel. The SEE Model obeys the first law of thermodynamics: energy in = energy out + change in internal energy (E_in = E_out + DE). Therefore, the SEE Model gives an energy balance for any hour of the day.

The equations are, for the most part, basic HVAC equations, such as those found in 2021 ASHRAE Handbook–Fundamentals’ Chapter 2, “Thermodynamics and Refrigeration Cycles,” and Principles of Heating Ventilation and Air Conditioning.

Users can change and add equations, but they need to be HVAC engineers, competent in Excel and trained on the SEE Model.

What does the SEE Model do?
The SEE Model defines the hourly energy consumption characteristics of a building on any given 24-hour day as well as the hourly energy consumption characteristics of a chiller plant on any given 24-hour day.

I am not aware of other models and software that can perform these actions. To my knowledge, no other model solves for each hour of a 24-hour day. This gives engineers the opportunity to post on the internet how much energy a building that is compliant with versions of ASHRAE Standard 90.1-2007 or newer compliant building should have used the day before, which also allows them to approach building owners with data and estimates of possible reductions in energy use and cost.

How does it work?
Microsoft Excel is a program that iterates to a steady-state condition after a change in conditions. The SEE Building Model is driven by a change in weather that results in a new load to the chiller plant, and the SEE Chiller Plant Model is driven by a change in load on the plant. The building model and plant model are two separate models.

The SEE Model requires energy in = energy out; therefore an energy balance of the building and plant results, providing an analysis tool to compare a real building and plant performance on a given hour or day.

Using the Excel SEE Model, users can chart variables and gain an understanding of how the building and plant work and interact. The SEE Model is for 24 hours, so it is much smaller than the annual, 8,760-hour, analysis programs.

The SEE Model can provide charts of any variable of a building or plant. The program models chillers’ condenser and evaporator refrigerant temperatures, a building’s air and water flows and other characteristics, including outside air infiltration. The SEE Model can also provide a schematic of the building and plant for any hour, and the energy balance feature provides a method to understand and troubleshoot a building and plant on any day utilizing real energy use data on a given day.

Why should engineers know about the SEE Model?
The SEE Model gives engineers another tool for designing and analyzing the energy consumption of buildings and chiller plants. Excel is available to all; therefore, engineers can develop their own SEE Models.

The SEE Model is different because it is set up to model the energy performance of a building and chiller plant on any given 24-hour day. It does not model 365 days. Also, the energy balance feature sets it apart from other models.

How can engineers review the SEE Model?
Go to http://kirbynelsonpe.com to review the SEE Model approach to designing, benchmarking and energy balance analysis of buildings and chiller plants.
The SEE Model cannot be accessed over the internet. It must be transferred to others by download or flash drive, and users must be trained to operate the SEE Model.
Contact Kirby at kirbynelsonpe@aol.com with questions and comment.