An increasing number of building automation contractors are encountering thermal storage systems in chilled water plants. Controls for these systems must be integrated with the chiller operating controls, but the sequence of operation can be quite complex due to the many layers of control and modes of operation.

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A fundamental measurement important to any such sequence is the amount of stored cooling energy available. This measurement tracks performance and identifies problems, which makes it essential to predictive demand-control algorithms.

Stored cooling energy in the form of ice is tricky to measure. In fact, the only truly precise way to do so involves separating the ice from the working fluid and weighing it; however, comparing the volume of the ice/working fluid mixture to its volume in the pure liquid state and its volume in the maximum-stored-energy state can make a good approximation. 

Another complication is that there are a variety of ice storage systems, all of which enable storage of cooling energy during off-peak times for use during on-peak times when utility rates are higher. Some storage vessels contain a freezing system with mechanical agitators to produce ice-water slurry; others pass refrigerant through an immersed steel heat exchanger to form ice around the individual tubes. A type growing in popularity because of its simplicity is based on dimpled, water-filled, polyethylene balls that are suspended in a glycol-based cooling loop. Whatever the system, they all share one common physical property – as ice is produced, the volume of the system’s working fluid increases.

The density of water at its freezing point is almost exactly 1.0 gm/cc (62.4 lb/cu ft), and the density of ice at that temperature is 0.917 gm/cc (57.2 lb/cu ft). Thus, the volume of a given quantity of water increases by about nine percent as it freezes. As long as the system is designed to prevent the ice from floating and breaking the surface of the working fluid, the water level will rise as ice forms, and this level will yield a good approximation of stored ton-hours of refrigeration.  

The maximum design value must be known, and some field measurements must be made. In an atmospheric (vented) thermal storage system, the fluid level in the main tank itself is the quantity to measure. In a closed system, the level in the expansion tank will be the appropriate variable.

This level variation can be measured with a wet/wet differential pressure transmitter. For the best use of its span, the transmitter should be installed to expose itself only to the variation in fluid level, not the entire level of the tank. Another option, if the expected change in level is large enough and mounting is convenient, is an ultrasonic level transmitter.

The low level represents all liquid, no ice, and thus no stored energy. The storage system designer can provide the high level, or the level can be measured in the field at commissioning time. The high level represents maximum stored energy, and a simple linear multiplier relates the fluid level to stored ton-hours of refrigeration. For example, if the level rises 10 inches from the no-ice state to the maximum-ice state, and the system is designed to provide storage of 5,000 ton-hours of refrigeration, the approximate level of storage at any given time will be equal to the number of inches above all-liquid multiplied by 500, as shown in the example below.

Example:
An ultrasonic level transmitter with a 48-inch span and 4-20 mA output is installed to measure working fluid level. The field-measured change in level between maximum stored energy (5000 ton-hours) and zero stored energy is 24 inches. The transmitter is positioned 36-inches above the lowest fluid level and is field-calibrated for 4 mA at minimum level and 20 mA at maximum level.

Stored energy (ton-hours) =
((X mA – 4 mA zero) x 5000 ton-hours maximum) / (20 mA – 4 mA) span

At 14 mA, the stored energy would be 3125 ton-hours.

[an error occurred while processing this directive] Less common are systems in which the ice is allowed to float to the surface of the storage tank. In these cases, the same principle can be used in reverse. The level of liquid will actually fall as ice forms, since a portion of the ice will be sticking out above the surface. Here it is advisable to add a sight glass or similar parallel device to the side of the tank and heat trace the connections to prevent them from freezing up. This will provide a nice, ice-free spot to connect a wet/wet differential pressure transmitter to provide level variation data.

Using thermal storage in chilled water plants results in lower costs for electricity and maintenance. For that reason, these types of systems are more and more likely to appear on the radar screens of building automation contractors. With a few measurements, some basic physics, and the right transmitter, the complexity of integrating thermal storage systems and chiller operating controls can easily be conquered.

Special thanks to Cryogel, makers of Ice Balls™ Thermal Storage, for information included in this article.

About the Author
Currently serving as the Product Marketing Manager for Kele, Inc., in Memphis, Tennessee, Dave Weigel has more than 25 years of experience in the HVAC industry. He earned a Bachelor of Science in electrical engineering from Christian Brothers University and a Masters of Business Administration in economics and finance from the University of Memphis.

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