Retrocommissioning Findings: Leveraging a Chilled Water System Thermal Flywheel to Save Energy and Compressors – Overview

Author’s Note:   Thanks to the folks at CERL via the training I am helping them with, I was able to put together a string of informal, on-demand sort of training videos that look at this topic in more detail.   The first two videos in the series focus on Functional Testing concepts in general.  The last two videos look at how we built the functional test associated with this blog post and provide the details behind the issue as well as a more detailed look at our results.  You will find the videos on our Cx Resources website in the On Demand training page.

Well, I’m on my https://www.av8rdas.com/introduction-to-functional-testing.htmlway home from Denver, Colorado where I attended the 21st National Conference on Building Commissioning.  As always, it was great to see others I know in the industry, make new acquaintances, and learn about new ideas and techniques.

Kent Barber of KBA and I presented together in a session that looked at how to put together a functional test.   Kent focused on how he goes about that process for a new construction project and I looked at it from the existing building commissioning side of things.  I have posted my presentation and related information  on my Google drive and you can access it via the resource links on the right side of the blog’s home page under 03 – Materials from Classes and Presentations – National Conference on Building Commissioning and the looking in the 2013 subfolder.

You will also find it on the conference web site, along with Kent’s and presentations from  the other speakers at the conference.  In fact, if  you explore the conference site a bit more, you will find the complete proceedings from conferences all the way back to 2004 or 2005 as I recall.  And, you can find many of the papers presented in the years prior to that at the California Commissioning Collaborative web site.  These are great resources and a good place to look if you have a question about commissioning practices, processes and trends.

The purpose of this post is to set up a string of posts that will supplement the presentation I did at the conference.  The presentation used a case study as a method to illustrate how you go about developing a functional test in an existing building commissioning process.   Specifically, it focuses on a functional test the team I was working developed to understand how much of a thermal flywheel was represented by the chilled water piping system in Le Conte Hall.

Le Conte is on of the department of physics buildings on the UC Berkeley campus that has research labs and teaching labs on the lower two floors and offices on the upper two floors.  The slide below will give you a feel for the chilled water system, which was the focus of our test.  You can also see AHU2 on the left side of the roof, which plays into the story.

This next graphic is a system diagram for the chilled water system.  A larger version is included with the presentation and related resources on my Google Drive if you are interested in a closer look.

If you study the diagram, you will discover that one of the loads is a typical fan coil unit in a research lab.  This particular load could be very small in the context of the chiller serving the system and when it was brought on line, the system had a tendency to short cycle, even with the compressors running fully loaded and using hot gas bypass.

The short cycling represented a problem on a number of fronts including poor temperature control, wasted energy, and the potential to ruin compressors.  One of the ways to mitigate a problem like that is to create a thermal flywheel by adding a tank to the system.  The tank allows a large volume of chilled water to be generated and stored (i.e. a long compressor run cycle).   The loads can then use this stored capacity to provide cooling at a very low rate with out compressor operation (i.e. a long compressor off cycle).

The team I was working with on the Le Conte project decided a thermal flywheel might be the best answer for the problem there.  But, we got to wondering if the capacity represented by the volume of water in the chilled water piping circuit might  be enough of a flywheel to solve a problem.  The building, of course, knew the answer to that question.  So, we developed a test to ask that question.

My presentation focused on the development of the test, but I thought it might also be useful to provide more detail on the problem itself and its resolution because I have used an approach like this before.  Typically, resolving the problem can generate benefits on a number of fronts including energy savings, reduced operating and maintenance costs, and improved in door air quality.

That discussion will involve a lot of detail, and to make it all more manageable, I decided to spread it out over a number of posts in a manner similar to what I did for the string of posts about the make up air handling system that was simultaneously heating and cooling.

This post gives an over-view of the topic and provides links to allow you to jump to the remaining  content.   The topics will include:

• A description of the refrigeration circuit in the Le Conte Hall chiller and how it works
• Why the chiller compressors were short cycling at Le Conte Hall
• Why short cycling can cause problems for compressors and HVAC systems
• The savings associated with eliminating the short cycling in Le Conte Hall
• Methods used to mitigate short cycling in general
• How we used functional test results to provide an automated flywheel cycle for Le Conte Hall to mitigate the short cycling in that system
• Other opportunities that were identified after we had eliminated the short cycling
• Optimizing the Le Conte chiller compressor run times
• Optimizing the Le Conte chiller hot gas bypass system settings
• Dealing with an unanticipated problem that occurred during the chiller service call

I will add hyperlinks to the bullets above as I create the posts, so check back if there is a  topic of interest with out a link.  It will probably take me a bit of time to work through this, but eventually, I will get it all put together.

Meanwhile, I hope everyone is having a pleasant time of it out there in the rest of the world.  I’m looking forward to a pretty spring afternoon out on the town with Kathy celebrating her birthday when I get home.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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